Curiosity On Mars

The Curiosity Rover on Mars, shown below, made headlines in the last few days.

The image above is a mosaic self-portrait taken by the rover's imager at Rocknest, an area in Gale Crater on Mars, taken in October 2012.

What we know about Earth, Mars, other planets in our solar system and even other planets in our universe is increasing every day but deep mysteries remain. Mars offers a tremendously valuable natural laboratory in which scientists can explore fundamental questions about how life on Earth came to exist.

The image above compares the size of Earth and Mars in true colour.

Understanding Mars (and Earth) will help researchers figure out what to look for as they look for signs of life in the universe. I doubt there is a human alive that hasn't wondered, are we alone? We are asking not only if there is life elsewhere but also how life got its start here on Earth.

Humans have been listening to the cosmos for decades and we haven't heard a peep from anyone or anything, leaving the question of whether we're alone or not unanswered. Meanwhile, there are many questions about the nature of life that might have answers here on our own planet or in our own solar system. What defines life? What does life need to exist in terms of environment, nutrients, energy, and basic bodily building blocks or molecules? Is liquid water or oxygen necessary for life? What about radiation? What chemical reactions are absolutely required for life? Must it be based on DNA, protein, or even carbon, like life on Earth? These questions help astronomers look for possible signs of life in the universe. And they help scientists know what questions to ask and what to look for on Mars. Mars is a rocky planet similar to Earth, and it seems very likely to have had liquid water during some period in its past. The image below is an artist's impression of what a wetter Mars might have looked like billions of years ago, based on geological data.

(Ittiz;Wikipedia)

Mars should have had many of the same raw materials that Earth once did when the first simple unicellular organisms began to evolve here. Figuring out what the raw ingredients for life were on Earth and then looking for signs they once existed, or still exist, on Mars is a logical step in answering the question, are we alone, and it is exactly what Curiosity is designed to do.

What Curiosity Should Look For On Mars

Scientists are using what we know about life on Earth as the starting point, so they are looking for signs of organic (carbon-based) life. That search began with looking for obvious signs of life on Mars such as fossilized organisms like bacteria, as well as atmospheric and geological signatures of living organisms. For example, oxygen in our atmosphere is a signature of abundant photosynthetic plant life on Earth. Molecular oxygen (O₂) gas is highly reactive. It quickly disappears from the atmosphere by reacting with other gas molecules and oxidizing rock. If all photosynthesis suddenly stopped, almost all the free atmospheric oxygen on Earth would gradually be depleted. Although oxygen is necessary for multicellular life on Earth, Mars could have harboured life even though it has an oxygen-deficient atmosphere. The first simple organisms on Earth, called anaerobes, evolved when Earth had almost no oxygen in its atmosphere. In fact, a gradual oxygen build-up in the atmosphere by cyanobacteria colonizing at the time was toxic to them, causing Earth's first major extinction event.

No obvious signs of life on Mars have been found but that doesn't mean that some pre-life organic molecules didn't form. Biochemists have made progress toward understanding how very simple life can evolve from the right mix of complex organic molecules in a favourable environment, but no one has yet been able to replicate the millions of years of natural and spontaneous organic chemistry that led to the first life on Earth.

Any sign of organic molecules that could be building blocks of life on Mars might give us a clue that life can and will form given the right ingredients, environment and time. Current geological evidence suggests that billions of years ago, Earth was a tumultuous place where volcanoes raged, filling the sky with lightning. It was almost completely covered by a shallow warm sea, rich in dissolved carbon dioxide, methane, ammonia, hydrogen sulphide and hydrogen cyanide. Most researchers believe the environment began to stabilize quickly and nucleotides, which are basic components of RNA (shown below right), formed and replicated spontaneously followed by ribosomes and proteins. These complex organic structures were eventually enclosed in a primitive membrane and developed a way to reproduce themselves, forming the first simple unicellular organisms.

LIke DNA, RNA is a remarkable biological molecule that codes and decodes information and regulates the expression of genes. The molecule itself is an enzyme and a chemical catalyst, which means it can self-duplicate and it can  enhance the creation of other molecules such as helping to build peptides from amino acids, both important properties of life. Unlike DNA, RNA is a single strand, like the hairpin strand shown right. It's used as a messenger molecule in our bodies, helping to turn the genetic information in our DNA into proteins. Because RNA is simpler than DNA it may have predated it, being the first genetic material to be function inside simple virus-like and bacteria-like organisms.

If scientists could find signs that a similar process at least began on Mars, we could begin to answer a fundamental question about the universe - is life inevitable? This very human question makes the current Curiosity Mission so compelling.

Past Rover Missons To Mars Found Evidence Of Liquid Water

The Curiosity rover is a car-size robotic rover that was launched in November, 2011 It landed in Gale Crater on Mars in August this year (2012). It was originally on a two-year mission but that mission was extended indefinitely just a few days ago. Curiosity (the one to the right) is the latest of three generations of Mars rovers, shown below.

The examples above are test rovers, all from NASA's Jet Propulsion Laboratory. The smallest rover, front center, is the flight spare of the Sojourner, which landed on Mars in 1997. To the left is the test rover for Spirit and Opportunity, which both landed on Mars in 2004 for a planned 90-(Martian) day mission. Spirit became stuck in 2009 and ceased communications in 2010 but Opportunity is still active on Mars, moving, gathering information and reporting back to Earth. Whereas Opportunity is a solar-powered rover, Curiosity is fueled by a radioisotope thermoelectric generator. It converts the heat generated by radioactive plutonium into electricity. The plutonium decays slowly (half-life of 87.7 years) so the electricity available 14 years from now will be only slightly reduced from 125 watts of power to 100 watts, its minimum expected lifetime.

All the rovers were designed to test the rocks, soil (also called sand but properly called regolith) and atmosphere of Mars in a hands-on way. After seeing what looked like dry riverbeds and other large water-related structures on the surface of Mars (shown below), scientists wanted to look for signs of past water activity such as precipitation, evaporation and sedimentation and to look for minerals that are known to be created only in the presence of water.

The image left shows streamline islands in Maja Vallis on Mars, taken by Viking. The image bottom left shows intricately branched channels, also taken by Viking. For more Viking images and information, try the online publication by the Viking Orbiter Imaging Team.

The identical Viking 1 and Viking 2 landers carried out the first experiments to look for signs of life on Mars in the late 1970's. They carried out gas chromatography, a gas exchange experiment, a labelled release experiment and a pyrolytic release experiment (all described in the preceding link). Organic compounds are common on asteroids, meteorites and comets so researchers expected to find them on the surface of Mars too, but they didn't find anything organic except chloromethane and dichloromethane. One theoretical explanation is that the surface of Mars, exposed to strong ultraviolet (UV) radiation, has built up a strongly oxidizing layer of regolith. One of these oxidants is perchlorate which breaks organic molecules apart, leaving chloromethane and dichloromethane as products. Perchlorate was later discovered on Mars in 2008 by the Wet Chemistry Lab onboard the Phoenix Mars Lander. Most researchers found the Viking organic molecule findings inconclusive, spurring the beginning of Mars rover exploration, where samples can be taken from a variety of geological sites.




All life as we know it requires liquid water, at least at some stage. If direct evidence of past liquid water was found, researchers could focus on determining if there was ever a life-conducive environment on Mars. Meanwhile, the Sojourner rover along with the Pathfinder lander, shown below, was launched in 1996 to test the idea of sending a robotic rover and to explore the Martian atmosphere and surface. It had cameras, a meteorological station to investigate the Martian atmosphere and an X-ray spectrometer to analyze soils and rocks.

Workers at the Jet Propulsion Laboratory are shown above closing up the metal petals of the Pathfinder Lander, enclosing the Sojourner rover inside, visible on the nearest "petal" before its launch.

The Sojourner data suggested that Mars did in fact have a warmer wetter past with a thicker atmosphere and liquid water. The Spirit and Opportunity rovers were then designed to expand on the successful Sojourner rover prototype and test the hypothesis of a once wet and warm Mars further. Below, one of the rovers is shown inside its lander's petals.

These rovers have a robotic arm, panoramic cameras, three different spectrometers to test rock and soil, a rock abrasion tool to remove dust and examine fresh material underneath, and a microscopic imager. The rovers were designed to travel widely and test many different geological sites. They found that most of Mars' rock appeared to be volcanic in origin and its soil, or regolith, comes from the weathering of these rocks. They found significant nickel in some soils, suggesting that the regolith came from meteoric impacts as well. Chemical analysis showed that the volcanic rocks have been slightly altered by tiny amounts of water and that coatings and cracks in the rocks contain water-deposited minerals. They also found that dust on the planet, which covers all surfaces, is magnetic because it was shown to contain the mineral magnetite. Some component in the dust, possibly sulphate minerals, also contains chemically bound water. They found additional chemical confirmation of water in water-specific minerals such as goethite and carbonates. In a region called the Columbia Hills, they found clear evidence of weathering caused by liquid water.

Knowing that liquid water was once present on Mars, the Curiosity mission is designed to go one step further by looking specifically for evidence of chemistry linked to, or a possible precursor of, organic life on Mars.

How To Get A Big Robotic Science Lab To Mars

This rover is big (the size of a small car and weighing about 900 kg), so it could not be placed in a lander. Instead, it had to be put directly inside its aeroshell (the protective container on the spaceship that protects the unit from space). It also couldn't be slowed down and cushioned upon landing on Mars using airbags, like the Pathfinder mission and the Mars Exploration Rover mission used. This landing had to be precisely guided and soft, and it had to be pre-programmed in advance because of the time delay between Earth and Mars.

First, the aeroshell containing the rover separated from the cruise stage of the rocket, which provided power, communications and propulsion during the long flight to Mars. Thrusters on the aeroshell then fired to place it within a 20 by 7 km landing ellipse and align its heat shield. A supersonic parachute deployed when the aeroshell slowed down enough by friction with the Martian atmosphere. All the previous rover landings used similar parachutes but the Martian atmosphere is so thin these parachutes are not enough to slow the aeroshell sufficiently. The returning command module used in the Apollo missions could rely on atmospheric braking and parachutes alone because Earth's atmosphere is much thicker. The aeroshell's heat shield separated and fell away and the rover (attached to a descent stage platform) dropped out of the aeroshell. Variable thrust rockets on the descent stage platform above the rover further slowed the descent, using a radar altimeter feeding data to the rover's flight computer as it navigated itself down. Soon afterward, a sky crane lowered Curiosity under the descent stage while Curiosity transformed from its stowed configuration to its landing configuration, locking its wheels in place. The sky crane slowed to a complete stop as Curiosity touched down and soon after freed itself and flew away to crash land elsewhere. This new procedure, shown below, is an important advance in lander technology, allowing more delicate testing lab equipment to get to far-off planets and study them.

Curiosity's landing, coined "the seven minutes of terror," is discussed by NASA engineers this five-minute video:


             
Curiosity Is A Complete Geological Testing Lab

Curiosity needs to be able to obtain various kinds of surface and internal rock and soil samples from various regions and test them in various ways. What really sets Curiosity apart from its predecessors is its instruments. It is essentially a mobile geologic testing lab. First it uses high-resolution cameras to look for geological features of interest. Gale Crater is a good place to look. Curiosity's landing site is marked by the tiny green dot in the image (created by combining data from three Mars orbiters) below.

In the crater's center is a huge mountain informally called Mount Sharp and from it large outflow channels, possibly carved by once-flowing water, extend into the plains below. This is where researchers hope to find signs of organic material and possibly even signs of extinct primitive life, which might have had a chance to evolve in this once-wet environment. When Curiosity finds a feature of interest here, it vaporizes a tiny part of it's surface with a laser and examines the emission spectrum from it, telling it what elements are present. If the composition is interesting, the rover can swing over a microscope and an X-ray spectrometer to examine it more closely. An X-ray spectrometer uses an X-ray to excite the electrons in the atoms of the material. The spectrum given off by the excited electrons is specific to each element present and provides a confirmation and better detail of the elemental makeup of the material. Finally, Curiosity can drill into the material and place a powdered sample into one of two mobile labs, the SAM or the CheMin.

SAM (Sample Analysis at Mars), shown below, is a suite of instruments - a mass spectrometer, a gas chromatograph and a tuneable laser spectrometer.

These instruments can identify gases present in the rock as well as any organic material present. The tuneable laser spectrometer can precisely measure isotope ratios in carbon and oxygen in any carbon dioxide or methane present in the sample. This can tell the researchers if the organic material has a biological or geochemical origin. Carbon has two stable isotopes, carbon-12 and carbon-13. Carbon-12 makes up 99% of all the carbon on Earth but it is even more concentrated in biological material because biochemical reactions favour carbon-12 over carbon-13, so an overabundance of carbon-12 in a material suggests it has a biological origin, at least a biology familiar to us on Earth.

CheMin (Chemistry and Minerology Instrument) consists of an X-ray powder diffraction instrument and an X-ray fluorescence instrument. It is shown below being installed into Curiosity. The inlet funnel for samples is sticking out at the bottom.

These instruments can identify and quantify minerals present in rock samples and by doing so they can assess whether water was involved in their formation or deposition or whether water has altered the rock at any point in its history. CheMin is especially focused on looking for any possible bio-signature minerals in rock samples. These include minerals that are created only through biological processes. Examples here on Earth are coal, oil, chalk, limestone, pearls and amber. While scientists don't expect to find these materials, they can look at the abundance and isotopic composition of various metals involved in redox reactions common in biology such as iron, chromium, and some rare earth elements as well as sulphur and oxygen isotope ratios in minerals that suggest biological activity. Of course researchers are also on the lookout for any microfossils such as tiny microscopic objects that resemble spores or bacteria. Such a find could be a definitive sign of past life on Mars.

Using these instruments, Curiosity is focused on looking for additional geological signs of past or present liquid water on or near Mars' surface as well as geological signs of past or present life in an area of Mars (Gale Crater) that is most likely to have harboured it. Right off the mark, Curiosity has kept researchers busy, sending back evidence of an ancient riverbed in Gale Crater, monitoring dust storms, measuring radiation levels and, most importantly, analyzing its first sample of Martian soil. Below is an image of the results of the first analysis of soil from the CheMin's X-ray powder diffraction instrument.

This X-ray diffraction image reveals the presence of crystalline minerals such as feldspar, pyroxenes and olivine mixed with amorphous material likely to be volcanic glass. X-rays beamed at the sample are scattered by the atoms in it. Each mineral shows up as a unique scattering pattern, a set of rings. The colours, right, represent the intensity of the X-ray beam, red being most intense in the center. X-ray diffraction gives scientists not only the chemical composition of rocks and sand but it also reads the mineral's internal structure, how its crystals are arranged. It tells them much more about what's present. For example, the presence of carbon could mean diamond or graphite and they have very different structures (and properties), which this X-ray diffraction instrument can distinguish. Knowing what minerals are present can reveal much about the geological evolution of the rocks and sand, and scientists suspect they will be able to piece together a collection of younger and older mineral samples, which could show a gradual transition from a wet environment to a very dry one. Olivine is especially interesting because, in the presence of water, it weathers into a material called iddingsite, which is a combination of clay minerals, iron oxides and ferrihydrites. Comparing the presence of iddingsite with olivine could tell researchers how much water was once present and the rate at which it disappeared.

In this five-minute video, Curiosity chief scientist and geologist, John Grotzinger, describes how Curiosity took and analyzed its first sample:



The first mineral sample taken by Curiosity is similar to volcanic (basalt) soils in Hawaii. Feldspar is a very common mineral in Earth's crust (60% of it) that crystallizes from magma. Pyroxenes and olivine are also very common. Together they make up most of Earth's upper mantle (where olivine is protected from weathering by water). Seeing them on the plains of Gale Crater on Mars was not unexpected.

Life as we know it is built from carbon-based compounds called organic compounds. They make up proteins, carbohydrates and DNA, for example. Below are some very simple building blocks of these compounds. Below left is glucose, a simple sugar and part of many complex carbohydrates. Below right is an amino acid, part of a protein polymer. 20 different amino acids make up proteins in living organisms. "R" stands for a functional group that makes each amino acid unique. Far below left is adenosine, one of four nucleotides of DNA.

These kinds of molecules are especially important to scientists looking for signs of life on Mars. They all contain carbon backbones, as well as oxygen and hydrogen. These elements are present in most planetary atmospheres as well as in sand and rock. It is their distinct chemical arrangements that set them apart as organic compounds, something a good X-ray diffraction device can discern. Neither these molecules nor any organic molecules that are involved in their formation have been definitely discovered yet on Mars, a surprise to researchers. But that doesn't mean there aren't any on Mars.

While the chemical bonds in complex organic molecules, especially DNA, can be broken by ultraviolet (UV) radiation, UV radiation as well as the energy from lightning, likely to have been plentiful thanks to significant early volcanic activity on both planets, might have provided the energy needed to create the first simple amino acids and sugars.

Meteorites - A Bonus Sample Set From Mars

Scientists have a few tantalizing clues about what kind of environment ancient Mars might have been. Little bits of the planet have been raining down on Earth for millions of years, originating from ancient Martian impacts. Most known Martian meteorites have been radiometrically aged to be between 0.5 million and 1.5 billion years old. This is the time they were dislodged from the planet and shot into space. The rock inside them, however, can be much older, on the scale of billions of years old. One meteorite in particular caused a big stir in the scientific community when it was discovered in 1985. It was found in Alan Hills in Antarctica and it is called ALH 84001. This meteorite appears to have been ejected from Mars about 16 million years ago and arrived on Earth 13,000 years ago. Cracks in it are filled with carbonate materials that imply the presence of liquid water. These minerals have been aged to between 4 and 3.6 billion years old. They also found evidence for polycyclic aromatic hydrocarbons (PAHs) and tiny tubular and ovoid structures that some but not all researchers think could be microfossils of something called nanobacteria. Unfortunately this meteorite hasn't proved that ancient life existed on Mars. PAHs are atmospheric pollutants on Earth. Although the concentration of PAHs in the meteorite seems to be higher away from the surface that was exposed to air, it still could be of Earth origin. If the PAH's are of Mars origin, they might hold more promise. These complex organic compounds, detected in interstellar space, consist of multiple aromatic carbon rings that might be created in carbon/hydrogen rich cores of nebulae. They may be hydrogenated, oxygenated and hydroxylated into even more complex compounds such as amino acids and nucleotides when they are exposed to the conditions of interstellar space. Some researchers thinks fullerenes of these molecules created in nebulae might have seeded Earth with the raw materials of life. The idea that the tiny ovoid structures in the Martian meteorite might by nanobacteria is controversial. The existence of nanobacteria on Earth is widely disputed and many researchers think they may be too small to house RNA, DNA's smaller simpler cousin.

While the argument for ancient Martian life is ongoing, the argument for a once-wet Mars is very strong based on a variety of data that includes this meteorite. In 2011, researchers completed isotopic analysis on the rock that indicates its carbonates precipitated at a temperature of 18°C from water that contained dissolved carbon dioxide from the atmosphere. The isotopic ratios suggest a sequential deposition of carbonate from a gradually evaporating body of shallow water.

Other Martian meteorites also have what appear to some researchers to be microscopic fossilized life forms and most tested meteorites contain organic molecules. The latest testing comes from Andrew Steele et al., of the Carnegie Institution for Science. They showed that complex organic molecules containing reduced carbon - carbon bonded to hydrogen or itself - are present inside and throughout the meteorites they've tested and they are of Martian origin rather than from contamination in our biosphere. However, the formation of these particular organic molecules was most likely part of a volcanic process that traps carbon in crystals of cooling magma, a non-biological origin. Complex organic molecules are precursors to life on Earth and researchers have shown that Mars not only had warm shallow water at some point in the past but it was also doing some organic chemistry on its own, creating complex organic molecules, at least while water was present. Using this Martian meteor data, Curiosity will focus on finding and studying pools of reduced organic carbon on Mars to learn more about how it is created and how to distinguish it from organic molecules of biological origin.

What Curiosity Found So Far

The hunt for signs of life on Mars is not as easy as it might seem at first glance. Without obvious fossils or other evidence of life on Mars, researchers have to rely on the biochemistry and geology of all potential biomarkers on Mars, which makes up a complex list of what to look for. Added to that is uncertainty about how to extrapolate from Earth's almost-lost evidence of pre-life chemistry (and steps from non-life to life that still aren't understood) to what might be almost lost on Mars billions of years after the fact. What Curiosity found when it scooped up its first sample of Martian sand and tested it clearly excited the scientific community, but it left some of the rest of us feeling underwhelmed, partly because this sample is just a small piece of a huge and complex puzzle.

So far Curiosity's onboard lab is working very well and it has identified a complex chemistry in the first windblown sand samples it's tested, located in an area called Rocknest, a fairly flat part of Gale Crater. It found perchlorate, Ca(ClO₄)₂, an oxidizing molecule I mentioned earlier which the Phoenix Mars Lander identified. It is also an energy-rich molecule, a component of rocket fuel, and in theory it could be used as an energy source for microbes if they exist or existed. Curiosity has also found chlorinated methane compounds, which are organic molecules of non-biological origin. It identified four gases that were released when it heated its first Rocknest sample: water vapour, oxygen gas, sulfur dioxide gas and carbon dioxide gas. The oxygen gas could be from the breakdown of perchlorate in the sample. The water it detected does not mean the sample was damp at all. It is water molecules chemically bound to dust and sand grains. The levels of water, however, were higher than the researchers expected. So far, its analysis of Rocknest sand reveals several kinds of chlorinated methane compounds such as CH₃CL, CH₂CL₂ and CHCl₃, as well as sulfur compounds such as hydrogen sulfide. The carbon-containing compounds are technically organic molecules but they are not biological organic compounds in and of themselves. The presence of CH₂Cl₂ however is potentially interesting because it is an intermediate step involved in organic chain propagation, shown below, where carbon backbone chains can be created in the presence of UV radiation.

UV radiation (which bombards the surface of Mars and once bombarded Earth's surface before the ozone layer formed) cleaves chlorine gas into two free radicals. These radicals react with methane creating a longer carbon chain, shown right. This process can continue, creating long and complex carbon chains

It is possible, however, that the carbon detected may be from Earth and carried by Curiosity to Mars. The SAM detector is extremely sensitive. The chlorine, however, is almost certainly Martian. NASA now claims it has had no definitive detection of methane on Mars (yet). Methane (CH₄) itself is an interesting molecule connected to life. 90% of the methane on Earth comes from the biological processes of life. It is not a stable compound, reacting quickly with other gases and breaking down in the presence of UV radiation. Methane is not expected to be present in any quantity on Mars if life does not exist there. Very trace amounts could come from comet impacts or chemical reactions underground between rocks and hot water. Volcanoes can pump out significant amounts of methane but none have been active on Mars for billions of years. Some experiments on Mars have detected higher than expected, but transient, methane levels in Mars' atmosphere, making researchers curious. Could there by a microbial source of methane, perhaps underground? On Earth, biologically produced methane tends to come with ethane while non-biological volcanic methane usually comes with sulfur dioxide. This might help provide some clues while exploring Mars's ongoing methane mystery.

Meanwhile we can be justified feeling a little let down that there are no definitive signs of any life or pre-life chemistry on Mars (yet). Scientific exploration can sometimes advance a rate much slower than we'd like it to. NASA and various other research teams around the world are successfully building a foundation for the future of Mars exploration that could get much more interesting when they eventually send humans to the planet. Also, Curiosity is just a few months into its two-year mission and it has not reached its main destination yet. Curiosity's main mission is to go to Mount Sharp, the huge mountain in the center of Gale crater. Mount Sharp, rising 5.5 km from the crater floor, appears to be an enormous mound of eroded sedimentary layered rock. These layers are especially interesting, considering that, being composed of sedimentary layers, they must come from a wet environment where they were sequentially deposited over a long period of time, about two billion years. This suggests that the crater may have once been filled with water, forming a large lake in the distant past. Sand and rock studies here will hopefully tell researchers whether it was once a lake or not.

Currently two rovers are operating on Mars - Curiosity and Opportunity - and three orbiters are surveying the planet - Mars Odyssey, Mars Express, and the Mars Reconnaissance Orbiter. NASA plans to send a new orbiter called MAVEN next year to analyze the atmosphere in greater detail, hoping to understand better Mars' dramatic climate change and its loss of water and most of its atmosphere. The European Space Agency (ESA) plans to send a Phoenix-like lander, called the ExoMars rover, to Mars in 2018. It will be equipped with drilling equipment that could drill about two metres deep into Martian rock for samples, looking for signs of bioorganic molecules using, among other instruments, an organic molecule analyzer. This instrument will have two operating modes - laser desorption mass spectrometry and gas chromatography mass spectrometry. Gas chromatography will identify all the volatile gases that are released as a sample is heated to 900°C. The gases will then be analyzed further in a mass spectrometer. In the other  mode, a special laser will ionize part of the sample surface and a mass spectrometer will analyze those ions. Researchers hope to determine the isotopic composition and the chirality of any organic molecules they identify, which will help determine if they come from a living or nonliving source. I mentioned the isotope connection to life earlier. Chirality is another interesting quality of all building blocks of living organisms. These molecules all have the same handedness. Amino acids are left-handed and the sugars in nucleotides are right-handed.  For example, below are two forms of alanine, an amino acid.

These two forms, called optical isomers, are mirror images of each other. Only one isomer is found in almost all living organisms - the L-isomer (some bacteria are a rare exception, having the mirror-image D-alanine in their cell walls). L-alanine is the one on the left, above. Why they are this way remains a mystery. Although Mars's surface is probably far too hostile for any life (too cold and dry with intense ultraviolet radiation), microbes might conceivably survive underground in protected rock crevices, for example, and this rover will be designed to find them if they are, or were, there.

NASA just announced it plans to send a new robotic science rover to Mars in 2020. It will utilize much of the successful technology developed for Curiosity, keeping costs and risks down. There will be an open competition for the payload and instruments on the new rover and a team will be set up to outline the new mission's scientific objectives.

The Curiosity mission was expensive. It cost NASA about 2.5 billion dollars. NASA, the ESA and other space agencies around the world are facing the pressure of tightening budgets and financially uncertain futures. Though NASA has plans to send a new rover, mentioned above, several future NASA Mars exploration mission plans have been cancelled or postponed. However, there still seems to be a healthy desire to explore Mars, to understand its evolution as a planet and to look for signs of life there. On the bright side, tight budgets and challenges often force ingenuity on researchers. The question, like a bright juicy carrot, remains - are we alone?

Higgs Boson

NOTE: This article has been rewritten from the original August version. My husband, a curious scientist in his own right, told me quite correctly I dropped the ball on this one. The original article is just too damned confusing and impossible to follow. So, please bear with me as I try to make the pesky Higgs boson accessible to us (and hopefully make certain I understand it myself!). So here goes:

The Higgs boson, discovered on July 4, 2012, is, according to some researchers, the science breakthrough of the century. It has even captured the imagination of laypeople, like me. A lot of us, however, feel like we don't quite get it - what exactly is a Higgs boson and what's so great about it? What makes it  the "God Particle" described by the media? It's not easy to get in on the excitement when even the subject - theoretical physics - feels intimidating.

This discovery is kind of a neat story about how science is evolving. People want to be involved in the process, and they want to understand the buzz around the latest breakthroughs in physics, and that has made this discovery one of the fastest transitions of scientific knowledge from laboratory to general public ever. It topped the list of trending twitter topics for weeks, beginning with a flurry of rumours showing up on various physics blogs. You could even find trendy articles on how to talk about the Higgs boson at barbeques and sound smart this summer.

There is a bit of a catch when we try to discuss the Higgs boson, however. We may understand it is a particle in physics, a particle that gives other particles mass, and that kind of makes some sense, but when we ask a few simple questions such as why or how it does this and why it is so important, we start to find ourselves waist-deep in complex theory, an unsettling feeling to say the least. And it's usually here where curious minds run aground and give up.

I'm going to try to understand the Higgs boson right along with you. I'll try to put this discovery in some context with other important discoveries in physics. Things are (still) about to get complicated, but I hope to get us to the end feeling comfortable with a working understanding of the Higgs boson. Let's start with the basics and then build on that:

A Beginner's Sketch of the Higgs Boson:

As I mentioned, you may have heard that this boson is responsible for giving other particles mass. Right here is an interesting breakthrough in itself when you think about it - it means that particles don't inherently have mass - they have to acquire it somehow. How do they do that? According the the theory behind the Higgs boson, particles with mass somehow get mired down in a "sticky" Higgs field, a kind of force field like gravity or electromagnetism, that permeates the universe. Even completely empty space is filled with various force fields like these.

Particles, like electrons and protons inside atoms of matter for example, acquire mass as they interact with this Higgs field, and because they do, they have to slow down. They can't travel at light speed like massless photons of light can. Photons are particles that don't have mass. They don't interact with the Higgs field. They're invisible to it. And because they don't interact, they can zip right along at the speed limit of the universe. The reason why the speed of light is the speed limit of the universe is whole other story; check out this introduction to special relativity, if you'd like to explore that a bit further. This whole mass acquisition business works through a mysterious Higgs mechanism. Now, if you are like me, a bunch of new questions pop into your head when you read this. How does it really work? Why do some particles have mass and not others? Where does this Higgs particle originate? Where is it now and why was it so hard to find? The Higgs mechanism is one of the most difficult concepts to understand in physics but it is also one of the most rewarding. If you can get a handle on the Higgs mechanism, you will gain some deep insight into the how our universe began, and how it came to be the way it is today.

The Higgs boson lives in a weird and fascinating world of particle physics.  Let's get acquainted with this world with this concept-in-a-nutshell introduction by Ian Sample, science correspondent at guardian.co.uk:



Why Were Physicists Looking for The Higgs Boson?

We're going to need to build a little background to understand why an enormously expensive technologically challenging piece of hardware called the Large Hadron Collider was built, in large, just to find this particle.

The foundation of modern particle physics is what is called the Standard Model. This theoretical model, is the result of theories and discoveries made by literally thousands of physicists over the past century, and it is very useful. It provides us with a great deal of insight into the fundamental nature of matter and forces in our universe. Our current understanding is that we have 12 basic building blocks of matter and they are governed by four fundamental forces. The matter particles are the purple and green blocks and the force particles are shown in red:

Source: MissMJ (Wikipedia)

Don't worry about knowing all the names, like muon and so forth. This image is just to give you the idea that all of physics is governed by particles of force and matter.

The Standard Model above contains three generations of matter. The matter particles on the left make up all stable matter. These are the particles of everyday atoms and neutrinos. They are called first generation particles. Heavier unstable particles belong to second and third generations, which quickly decay into second and then first generation particles. These heavier unstable particles, muon neutrinos and bottom quarks for example, are briefly found in extreme high energy environments like particle accelerators or in cosmic rays.

The Standard Model explains how subatomic particles behave and how they are related to one other. However, as successful as it is, there has always been a deep underlying mystery as to why some particles, such as electrons and quarks (quarks make up the protons and neutrons inside the nucleus) inside atoms have mass and others, such as photons of light, do not. You could say, well they just do, but curious physicists are not satisfied with that answer. As I mentioned earlier, photons and some other particles without mass zip around at light speed, Einstein's speed limit of the universe, while others, those with mass, seem to slog through space as if they were slopping through molasses in rubber boots. Why?

We know that gravity acts on mass. Photons and other massless particles do not interact with gravity at all. And here, I should clarify something you may already know about photons. You may know that black holes, objects of infinite mass and gravity, "suck" light into them. What's really happening here is that the photons are not interacting with gravity at all BUT they are moving through space-time. Gravity bends space-time, and where a black hole lies, space-time is bent into a funnel with an endlessly long bottom point to it. The photons simply follow that bent space-time and they get trapped and can't get back out. This geometric concept of gravity is Albert Einstein's general theory of relativity.

What quality gives rise to mass in the first place?

In 1964, physicist Peter Higgs predicted a mechanism by which particles could attain mass. Some particles do not interact with this mechanism - photons for example - and others do - electrons and quarks for example. Particles gain mass by interacting with a field that permeates all of space, called the Higgs field.

Where Higgs theory starts to get interesting is when we talk about force particles. Some of them are massless like the photon, but others are not. Take a look at the particle diagram once again. You will see 4 different force particles there on the right - the photon, the gluon and two kinds of boson. Do you notice the GeV/c² in some of the red blocks? That's a fancy measure of mass that physicists use. The bosons have mass but the photon and gluon don't. How can that be? Weren't all the fundamental forces created right at the beginning to the universe when the laws of physics took shape? What kind of events took place to give only some of the force particles mass? If a force field took shape back then to give mass to particles, why didn't all of them get painted with the same brush? That question has perplexed physicists for decades and it's what made the whole Higgs adventure so interesting, like reading a great detective mystery.

Let's dive a little deeper into the force particle story:

Now, according to the Standard Model, each fundamental force has a particle associated with it. This means that there are particles of matter and there are particles of force. A force particle mediates or carries out a force. It is also the smallest whole (or quantum) unit of the force possible. For example, massless particles called gluons carry out the strong force. This is one of the four fundamental forces in the universe. The strong force holds quarks together inside protons and neutrons. Gluons also hold the protons and neutrons themselves together inside the nucleus. Gluons, like photons, are massless and because of that they travel at light speed. Physicists don't deal directly with gluons much. Their range of influence is so short that it is confined to the insides of atoms. But gluons are very powerful, as implied by the name "strong" force. It isn't easy to tear apart the nuclei of atoms, as illustrated by the enormous energy released by a fission bomb (and that's just prying protons and neutrons loose, not ripping them apart into quarks). The strong force is about 100 X stronger than the electromagnetic force, another of the 4 fundamental forces.

Photons mediate the electromagnetic force, which means they are the particles that carry out all the forces between electrically charged particles, such as electrons and protons inside atoms. Photons are the force particles behind all of chemistry, electricity and magnetism. Almost all the phenomena we experience day to day is due to the electromagnetic force.

Physicists call all the force-mediating particles, including the photon and the gluon, gauge bosons. In addition to photons and gluons there are other gauge bosons in the universe. For example, particles called W and Z bosons really mystify researchers because they have mass. Unlike the photon and the gluon, these particles interact with the Higgs field and, as a result, they cannot travel at light speed. Their masses are about 80 and 90 GeV* respectively, so they are influenced by gravity, while photons and gluons are "invisible" to gravity. W and Z bosons, together, carry out the weak fundamental force. This is the force responsible for radioactive decay and the fusion of hydrogen into helium inside stars.

*A GeV is a unit of energy, a giga (billion) electron volts to be exact. To give you an idea of how much energy that is, when two protons collide with each other at the Large Hadron Collider (LHC) at CERN (shown below), they can release an energy of 14 TeV (14 thousand billion electron volts). That's about 12 thousand times more energy than a Z boson has. A lot of that energy is kinetic energy that's released in the collider. But it's also in the mass of various newly created particles that shoot off in all directions. In particle physics, it is easier to describe the masses of particles in terms of energy instead of something like grams, because particles are so small. The mass-energy equivalence Einstein described as E = mc² plays out over and over in collision-type experiments. Inside colliders, particles are annihilated, releasing energy, and they are created, consuming energy, as a matter of course.

Credit: Julian Herzog (Wikipedia)

You might have noticed I said 4 fundamental forces. I left out the fourth force, gravity, on purpose. Gravity is a real mystery to physicists - they haven't yet found the particle that carries it out IF there is a particle. Some physicists think gravity might not be force per se at all. It might just be a property of space-time itself. The stretching of space-time simply looks like a force to us. Others think that gravity, rather than being an out and out particle, might hint at a mysterious deeper reality of physics, perhaps some kind of fundamental string-based universe, where gravity and all the other forces and particles are strings, each vibrating at a particular frequency.

A motley crew of massive and massless force particles in the universe present physicists with a real puzzle. How did just some of them get mass? Let's take a closer look at the Higgs field and how it might offer us some clues to solving this puzzle.

What is the Higgs Field?

Thanks to Higgs' and others work, the Standard Model in physics predicts a field. This force field has energy, even in its ground (or lowest possible energy) state. Even in the vacuum of empty space, this field implies a small but definable lowest possible energy exists there. This energy is what physicists often refer to as the Higgs mechanism.

The Higgs Mechanism Tells Us About the Very Mysterious Beginning of Our Universe - And It's All About Symmetry

If we measure the current expansion of the universe, we can extrapolate backwards to its beginning. The universe exploded from a perfect single point. The laws of physics imply that all the forces acting on this point should be equal all around. This is just a way of saying that the baby universe should have been a perfectly even, homogenous, expanding sphere. It should, in fact, have created equal amounts of matter and antimatter. And if it did, then both matter and antimatter would have annihilated each other right off the bat and our universe would have nothing in it today, no stars, no gas, nothing. The fact that matter somehow won out gave physicists a clue to how the universe unfolded. If we look at the laws of thermodynamics, the universe must have cooled as it expanded, thanks to the conservation of energy. And when it cooled, physicists think it went through some phase changes. These phase changes were not that much different from the phase change of water freezing into ice, and we can think of them in the same way.

Here we get to a common theme underlying the story of how forces and particles came to be in our universe. New forces and particles come into existence when basic symmetries break. These symmetries are often referred to as gauge symmetries and there is a whole complicated gauge theory behind them. This is why we call the fundamental force particles gauge bosons. They can all be described using the mathematics of gauge theory. In order to understand the Higgs (another gauge) boson, we don't need to get all muddled up in complex gauge theory. We can think of symmetry-breaking as a phase change, and I'll show you soon how this concept works for us when we try to explain why some particles have mass and others don't.

For now, lets make sure we get the idea of symmetry-breaking: a glass of water, after cooling sufficiently (ie. after it loses enough energy) begins to freeze. At this point the symmetry, the uniformity of the water if you will, begins to break into two different sections: water and ice. Uniformity breaks into two non-uniform parts. (If you are curious about how the universe got more matter than antimatter, take a look at my article on antimatter. I also talk about matter/antimatter symmetry-breaking when I explore Our Universe in the same-titled series of articles elsewhere in this blog.) Similarly, the universe "broke" into different forces as it began to expand and cool just after the Big Bang.

Higgs: Looking for a Needle in a Haystack?

From a starting point of infinite energy, the universe began to cool after the Big Bang. As it did so, several fundamental symmetries broke. For example, physicists have documented that two fundamental forces, the electromagnetic force and the weak force ultimately came from a once unified force called the electroweak force. Very soon after the Big Bang, the energy of the universe "cooled" to about 100 GeV. Today the energy of the universe, the cosmic background radiation in other words, is around 0.235meV.  That's a difference on the order of about 10¹². At around 100 GeV, the electroweak force broke into the two forces we know today - electromagnetism and the weak force. That was a breaking of symmetry analogous to a phase change. And just like a water phase change, physicists can reverse the process by adding energy back into a system. Tremendous energies can be achieved inside particle accelerators, and it is here where physicists have observed evidence for the unified electroweak force.

It is here too where we are about to find out how our motley crew of massive and massless fundamental force bosons came from one single (homogenous if you want to call it that) progenitor force particle.

When you create a collision with enough energy (100 GeV) the W bosons (there are 2 of these, each with an opposite charge), the Z boson and, remarkably, the photon all revert into one indistinguishable particle, which mediates a single force - the electroweak force. This is right where the needle in the haystack can be found. It is here where somehow a particle breaks into massive and massless particles. But how? This is where the Higgs mechanism comes in.

High-energy experiments from the 1980's up to today at the Large Hadron Collider have produced millions of particles behaving the way we would expect if the electromagnetic and weak forces were unified. It seems when you first think about it that the mystery of the boson's masses has been all sewn up. These massive and massless bosons come from electroweak symmetry breaking into the weak force and the electromagnetic force. But with this success came an unexpected problem. The electroweak theory can explain what happens at energies where the force first unifies (100 GeV), but if higher energies are inserted into the mathematical gauge theory describing these particles, the results become nonsense. Some new force field, some new specific energy was needed to "fix" the theory at very high energies. And this is, once again, where the Higgs mechanism comes in. Remember that baseline energy of the Higgs field I mentioned earlier. It comes into play now.

In physics, fundamental forces are described as fields, and each is associated with a particle that mediates the field. The particle, a gauge boson, is a quantum unit of the force. At high enough energies you can "shake" these particles out. What I mean is, you can see evidence for them inside colliders, where there is tremendous energy. Physics at "everyday" energies does not encounter them. You won't stumble into a Higgs boson while walking down the street. I'll go back to my gluon example to explain what I mean. As I mentioned earlier, this particle mediates the strong fundamental force. It acts like a glue that holds quarks together inside protons and neutrons and neutrons and protons together inside atomic nuclei. It is the field that we indirectly experience as atomic nuclei holding themselves together inside atoms. The gluons are there in every-day atoms but only in "force form." Only at very high energies can we "experience" the gluon particle itself. Gluons were first found in 1978 in the collider called DORIS. We could argue that gluons are hidden inside atoms so that's why we can't "see" these particles. But that's not the whole story: We can't see W, Z or Higgs bosons either, not because they are hidden or confined somewhere but because they aren't there. At ordinary energies, we can experience and test only the forces they convey. To "see" the particles themselves in action, we need to create the high-energy environment in which they "shake out." So, as we go up in energy, force particles shake out, and the forces they mediate come together.

If you are wondering where the electrons and quarks that make up atoms fit into all this, I can give you a short version of the story, and here you will see that in contrast to energy particles, matter particles "shake out" as you go down in energy, or as the universe cooled.  As the electroweak force and strong force broke from a once single unified fundamental force, a kind of proto-matter made its first appearance in the universe. It was a disordered quark-gluon soup that, as the universe cooled further, became seeded with electrons and neutrinos. The LHC is now focused on studying quark-gluon plasma now that the Higgs discovery was made. The universe had to further cool before quarks could slow down enough to bind to each other and create protons and neutrons. As the universe continued to cool, these first simple nuclei attracted electrons to create the first simple atoms - hydrogen and helium. Check out my articles, Our Universe Parts 6, 7 and 8, for a more detailed description of how ordinary matter came to be.

Why The Higgs Mechanism is so Important

Getting back to our Higgs story, when the electroweak force broke into the electromagnetic force and the weak force, four subatomic particles "shook" out as a result of the "phase change." These particles all mediate forces. They are the W- and W+ bosons, the Z boson and the photon. The W bosons are charged and have a different mass than the neutral Z boson, while the photon has no charge or mass. How do you get from a single homogenous state, one force with one associated electroweak boson, to this? And why do some of the products have mass while one does not? The Standard Model, as good as it is, could not account for this until the Higgs mechanism was introduced.

Researchers think that the Higgs mechanism is what spontaneously broke the electroweak gauge symmetry when the universe was very young, somewhere between 10^-35 seconds and 10^-12 seconds old. When this happened, the Higgs boson itself made its appearance in the universe. The Higgs boson is a sign, if you will, of the Higgs mechanism happening. Physicists do not yet know how the mechanism was triggered but its triggering is associated with the sudden appearance of a pervasive new field, the Higgs field, which is mediated by a particle, the Higgs boson. Remember that a boson is a quantum unit of a field, in the same way a gluon, for example, is a quantum unit of the strong force holding quarks together in neutrons. Before this, no particles had mass. Remember too I mentioned that quarks and electrons made their first appearances just after the Higgs mechanism was triggered, and force bosons with mass (the W and Z bosons) appear simultaneously with the mechanism. The Higgs boson is itself is a massive boson, and most researchers think it attained its mass the same way other particles did. After this symmetry broke, leptons and quarks as well as the W and Z bosons and the Higgs boson itself (and possibly neutrinos too if they are proven to have mass) all attained mass, while some bosons did not, such as the photon and gluon.

A Mathematical Formula Describes the Higgs Mechanism

According to Standard Model theory, which itself is an elegant mathematical formulation, the Higgs field consists of four component fields. Two of these fields are neutral and two are charged. Two of the charged fields and one of the neutral fields are mediated by what are called Goldstone bosons which in theoretical models act like W+, W- and Z bosons, all of which have mass. The last neutral field component is predicted to be mediated by a neutral boson of mass - the Higgs boson. Remember that the photon does not interact with the Higgs field. It is not described by these four component fields. To give you an idea of what the math looks like, here is an excerpt from Wikipedia which mathematically describes the symmetry breaking of the electroweak force which gives rise to the W and Z boson masses, fermion (quark and lepton) masses as well as an energy value for the Higgs field, formally called the vacuum expectation value:

You certainly don't need to understand all this math but I hope it gives you a feel for how physicists can  use it to describe how one (electroweak) boson can turn into four unique new bosons, some making their appearance with a new quality called mass for the first time in the universe. And the quality that gives them mass is the same thing that triggered their appearance in the first place.

The Standard Model  has been in development since the 1950's and many brilliant minds have gone into its making, both on the theoretical front and on the experimental front. Physicists use this important theoretical framework to refine their search for new particles. Quarks and neutrinos were theorized well before they were discovered in collider experiments. The Higgs boson is a key building block in the Standard Model. This boson was first theorized in 1964 in a series of papers on symmetry breaking. This gauge theory symmetry-breaking extends the Standard Model by incorporating the models of the grand unified theory. This theory is based on symmetry-breaking processes. These grand unified theory models seek to unify the fundamental forces into one grand force thought to exist when the universe first exploded into being - before gravity, the strong force and the electroweak force broke into existence, when they were all unifed into one "mother" force. This mother force implies some kind of mother boson but it is almost impossible to imagine what kind of mother particle could mediate such a force. What does seem clear is that the energy required to create such a particle would be far beyond our current technology, if it even exists. It seems to me if any particle should merit the name "God particle," it should be this one.

Meanwhile, the Higgs discovery puts some very essential pieces of the Standard Model and our understanding of cosomolgy and particle physics in general into place for us. It not only gives us powerful verification of our current understanding of how particles and forces work, it gives us our first glimpse of how mass first arose in our universe. To me the best part of the Higgs story is the symmetry-breaking part - the Higgs boson allows us to understand how our current (low energy) unsymmetrical universe, filled with some massive particles and other massless ones, arose from a once perfectly symmetrical point of origin, the Big Bang.

How To Catch a Higgs Boson

How did physicists finally catch this elusive high-energy massive particle? The Higgs boson itself, as mentioned, lives only in a very high-energy environment. The math in Higgs theory predicts the Higgs boson should have a very high mass. There is more than one mathematical solution to the theory, so what you get is a range of possible masses to look for in the collider. The LHC can attain energies sufficient to create particles with masses around what the Higgs boson is expected to have, by smashing two protons together at tremendous speed. That part of the hunt for the Higgs wasn't all that terribly difficult. The problem is the Higgs boson is likely to be created only rarely during these collisions and it doesn't last long - it immediately decays into other lighter particles. The trick is to accurately predict what its decay particles should be and look for them instead. It's not an easy process.

Physicists at the LHC were looking for the Higgs boson within a mass range of 120 to 140 GeV. The LHC has more than enough energy to create particles within this range but it gets difficult to find the Higgs boson as you approach the lower end of the range. If it's over twice the mass of the W boson, for example, it should decay into two W bosons, and they can be spotted. If it's not quite massive enough to do this, it could decay into two Z bosons, which are a bit less massive. This would be easy to spot too because the two Z bosons would quickly decay into 4 leptons that are easy to detect. But, if it is lighter than this it could decay into two heavy bottom quarks. This would result in a bunch of hadrons being detected (hadrons are combinations of quarks bound to each other; protons and neutrons are examples) and hadrons are so common as decay products in collisions that it would be very difficult to find a signature of a Higgs boson in all that hadron debris, even given that you would expect many more Higgs bosons to be created with the same energy than if it were more massive, say with a mass equivalent of two W bosons for example.

The big questions here then is: if you find, for example, a spray of two Z or W bosons as well as perhaps many other lighter and massless particles as your result, how do you know for sure a Higgs boson left its particular signature in it?

The Elusive Higgs Signature

The Higgs boson, according to Standard Model theory is expected to create a particular pattern based on something called its coupling energy. This is the energy with which it couples to the Higgs field. All force-mediating particles couple to their force fields. In this case, actual results are complicated by two things. First, systematic errors - is the detector working correctly, is the device calculating the energies correctly, etc. Second, every subatomic event plays out in the strange environment of quantum mechanics, so there is a random component to identical repeated collisions. These reasons are why the Higgs boson is described as being discovered with a 5-sigma accuracy. This accuracy scale which physicists use is equivalent to a 1 in 3.5 million chance of being wrong. Given the challenges involved those are astoundingly good odds! Here is one possible Higgs signature according to CERN:

In this simulated data, two protons have just collided, creating two jets of hadrons and two electrons. The lines reperesent possible paths of particles produced and the energy of these particles is shown in light blue.

All this is why the Higgs boson has been so hard to find and why physicists have been carefully running many tests to see that it recreates the expected result, and to attain almost irrefutable odds that they actually found it.

The great announcement you heard in the news was when physicists discovered evidence of a particle right in the range where the Higgs mass should be. That is the Big Breakthrough in a nutshell. Like other particles, the Higgs boson is a quantum particle, and that means it not only has mass but it has other qualities too like spin and charge.  For example, the Higgs boson should have zero spin, zero charge and no colour charge. Physicists still have to verify that this particle matches all these qualities before they say with 100% certainty it is the Higgs boson.

The Standard Model has so far been extremely accurate in predicting particles that have later been discovered experimentally (the W and Z bosons, gluon, top and charm quarks). The predicted Higgs boson rounds out the theory - it is the last missing puzzle piece to add.

What is the Higgs Mechanism Precisely? More Questions

We've learned in this article that the Higgs field switched on and when it did so, it "cracked" the electroweak field into photons (mediating particles of electromagnetism) and into W and Z bosons (mediating the weak field), imparting mass on the W and Z bosons (as well as on quarks and electrons and the Higgs boson itself) but not photons (or gluons). We know when the Higgs mechanism switched on, but we are left wondering how it imparts a new quality (mass) on some particles, making them suddenly "visible" to gravity. We're left to wonder how this new quality of mass "hooks into" or interacts with gravity. Gravity and the Higgs field have one thing in common - an interaction with mass.

Can we connect gravity with the Higgs mechanism?

The Higgs Mechanism Has a Mysterious Connection to Gravity

I've hinted at what gravity is earlier in this article. Now we need to really understand it in detail. Isaac Newton thought that masses attract each other, and that attraction is experienced as gravity. Einstein, in his theory of general relativity, describes gravity in terms of geometry, as the curvature of space-time. Space-time is a four-dimensional framework of the universe, consisting of three spatial dimensions plus one dimension of time. Mass curves spacetime. What Newton describes as mutual attraction of masses to each other can be described by general relativity as two masses falling into each other's gravity wells.

Now, if we look back at our description of particle mass, we will remember that mass and energy are equivalent. So energy too must curve spacetime and therefore must also be a source of gravity. This means too that when we consider the mass of a molecule, for example, we are measuring not only the combined mass of its quarks and electrons but also its binding energy, chemical bond energy, its temperature, its momentum (a quality closely associated with energy), and its pressure and even tension. All these qualities tell spacetime how much to curve. They are all components or facets of a general description called the stress-energy tensor. So, we can describe our molecule mathematically in terms of a stress-energy tensor and we can calculate just how much it should bend spacetime (it won't be much!). This works in reverse too. The geometry of spacetime also tells our molecule how much it can move. For example, the molecule will be accelerated in an external gravitational field, just like a ball falling to the ground after you throw it up in the air.

Hopefully this gives us a good understanding of how gravity and mass work on each other. Space, time and mass are all intimately intertwined through general relativity. But notice that we have not yet incorporated the Higgs mechanism. There is a good reason, but I warn you it won't be satisfying. The Higgs mechanism is a quantum phenomenon, as we currently understand it. Gravity is understood only as a relativistic phenomenon. Physics doesn't yet have a way of getting these two theoretical frameworks to talk to each other. Their mathematics will not work together without giving us nonsense answers. I'd like to add a little clarification here: We can describe the Higgs field as a sticky surface, where the motion of particles with mass is impeded, but massless particles are perfectly slippery to it and zip right through at light speed. This impediment of motion is not the same as the effect of gravity on mass - you need to apply energy to accelerate our molecule because it has inertia. That energy has nothing to do with the Higgs field. Instead it is the energy described by general relativity. Sometimes in the literature I've noticed these two concepts get mixed up.

There is no way yet to bridge the gap between the Higgs mechanism and gravity. We will need a quantum description of gravity in order to relate the two phenomena to each other. They have the common denominator of mass and maybe through that, there might be a way to explore the quantum nature of gravity.

The Higgs Boson Wasn't All They Were Looking For: Big Questions Remain

If you think the mystery of the universe has been wrecked by the Higgs discovery, don't fret one bit. Besides the Higgs boson, there are other high mass particles physicists are hunting for, and they are hoping one or more of them might explain pesky dark matter. Dark matter has mass. In fact, that is how physicists know it's there. Though it neither emits nor absorbs any electromagnetic radiation, meaning it is undetectable, its gravitational effects on visible ordinary matter, radiation and on the motion of galaxies betray its existence. Based on these effects, about 84% of all the matter in the universe is calculated to be dark matter. Is there a particle with mass that has not yet been discovered, one that interacts only through gravity and perhaps through the weak force, but not with electromagnetism? This rules out almost all ordinary (baryonic) matter, that is, matter made of atoms. It doesn't necessarily rule out non-emitting atoms in things such as non-luminous gas and condensed objects like black holes, neutron stars, and brown dwarfs. These very faint-to-invisible objects have mass but physicists don't believe there is nearly enough of them to account for so much matter in the universe. This leaves us with nonbaryonic matter, such as neutrinos (if they have mass), hypothetical particles with mass called axions (which are not part of the Standard Model but instead are a theoretical solution to an unrelated problem in theoretical physics) and something called supersymmetric particles. That means none of the Standard Model matter particles is a candidate, with the possible exception of neutrinos, and right now neutrino mass is standing on pretty iffy ground. Therefore, many researchers have turned to an extension of the Standard Model called supersymmetry theory. This idea comes from a theoretical extension of the Standard Model that embraces at least some string theory.

There is an elegant mathematical framework that describes this symmetry; in fact it provides possible solutions to several current theoretical problems in particle physics, including those in string theory. It suggests that there is a symmetry underlying all force-carrying particles and particles of matter. In other words, all bosons (all of which have whole integer spins and are force-carrying particles) have supersymmetric partner fermions (all of which have half or multiples of half integer spins and tend to be particles of matter) and vice versa.

For example, the photon would have a corresponding mirror particle called the photino, with a spin 1/2, and a mass between 10 and 100 times that of a photon. There is as yet no direct evidence for supersymmetry but the photino is one of a handful of possible candidate particles for dark matter. It would be the lightest supersymmetric particle produced, and therefore it itself cannot decay, so it would be expected to exist in our current low energy universe. Supersymmetry, if it were verified, would fatten up our current standard model by doubling the number of particles in the universe. The promise of these particles as candidates for dark matter is their mass. Each of these supersymmetric particles would have an expected mass much higher than its partner, most falling between 100 and 1000 times more than that of the proton. The LHC can muster enough energy to produce some of these particles, particularly the photino, and it is looking for them now. The fact that photinos haven't been found yet in fact makes supersymmetry theorists a bit nervous.

Supersymmetric particles fall into five classes with funky names such as squarks, gluinos, charginos, neutralinos and sleptons. Their expected interactions and decays are described in detail by the minimal supersymmetric standard model, giving physicists characteristic signatures they can look for in the collider. Here's an example:

(image captured from Wikipedia: Minimal Supersymmetric Standard Model)

The photino, not shown in the above chart, has an expected mass of between 500 MeV to 1.6 GeV. Physicists are currently attempting to work out the photino's relic abundance in the universe to see if it could be a suitable dark matter candidate.

The idea of supersymmetry suggests that the universe might have gone through a very brief state in which matter and energy were unified before splitting into supersymmetric particles and their partners, and at that time, energy may have first existed as a single "mother" force before it too split into the fundamental forces today. Almost all of these supersymmetric particles would have decayed soon after they formed, thanks to their high mass-energy values. They would be strictly high-energy entities. The Higgs boson fits right in the middle of this early and unimaginably energetic slew of particles, suggesting that it or some relative particle of it (perhaps the Higgsino?) was around along with its mysterious mass-imparting mechanism to impart mass on these supersymmetric particles.

In terms of explaining the Higgs mechanism, the dark matter/supersymmetry trail also leaves us unsatisfied. Somehow the Higgs boson, or perhaps a Higgs superpartner, interacts with these supersymmetric particles, if they exist, and contributes all the mysterious mass of dark matter. However, it does not get us any closer to understanding exactly how mass is inferred upon particles. How does this Higgs field interact with particles exactly? And how do the various specific masses of the particles arise? In other words, why do some (heavy) particles interact with the Higgs field more strongly than other (light) particles? Saying the field acts like a sticky (3-dimensional) surface gives us a way to visualize what might be going on but it doesn't tell us how that happens.

I wonder too if the Higgs mechanism was some kind of inevitable physical process (perhaps analogous to magnetic field lines setting up in cooling rock) or was it an historical accident, without which our universe would have no mass today. It would have remained a frothy mess of particles instead, all of them zipping around at light speed. There would be no clumping of matter, so no planets, stars or galaxies. This doesn't preclude gravity's existence however. Our current understanding is that gravity is either the first force expected to have broken from the universe's initial perfect state of symmetry, or it is an artifact of the fabric of spacetime itself. But without mass, gravity would have nothing to "hook" itself to. Every particle would be invisible to its effects.

Finally, it doesn't seem intuitively obvious that a force with a single boson should break into two forces, one governed by three bosons, two of which are the same mass but opposite charges, along with a third neutral slightly less massive particle, and another force governed by a massless particle. There's no obvious pattern to these particles. One splitting into two with opposite charges would somehow be much more satisfying. This randomness isn't confined to these particles either. If you look once again at the particle diagram earlier and compare energies between generations of Standard Model particles you get the sense someone blindly picked them out of a bag. And yet, the fact that matter comes in different particles at different energies might be a clue to how mass works regarding the Higgs boson.

Conclusion: Is the Higgs Boson the "God Particle"?

Theoretical physicist Peter Higgs, from Edinburgh University, now 83, is enjoying the rare pleasure of being alive to see the experimental confirmation of a mathematical theory he began working on 48 years ago.

(credit: Gert-Martin Greuel (Wikipedia))

Some of the theoretical significance of this discovery, however, is yet to come. It completes the Standard Model, but already it is presenting physicists with many new questions about the nature of mass, matter and gravity in our ever-enigmatic universe. The Higgs boson is a significant piece snapped into the great physics puzzle, but "God particle"? The name, coined by a publicist and disliked by many physicists, doesn't seem appropriate.  The particle itself seems less revolutionary than the process to which it speaks, that is, symmetry-breaking. Symmetry-breaking does nothing short of telling a good chunk of the story of how the universe evolved from the Big Bang to present day, and the Higgs boson discovery does much to confirm that story.

Playing Around With Higgs

With the Higgs discovery comes some intriguing possibilities. For example, if we could figure out how the Higgs field "hooks" onto some particles but not others, maybe we could manipulate that process and, let's say, unhook a whole spacecraft. We could slip through space invisible to gravity, at light speed! Or, if the Higgs boson imparts mass to particles by bouncing off of them, perhaps some kind of Higgs boson imager might be possible, analogous to an electron microscope in which electrons are bounced off objects providing us with high resolution images. Here, Higgs bosons could be beamed into space (at very high energy before they have a chance to decay; in fact it would probably require enormous energy to accelerate these massive beasts to near light speed), illuminating any object with mass, regardless of its electromagnetic luminosity, so dark matter could be visualized and mapped. The interior of our and other planets and stars could be mapped too, telling us what really is inside a neutron star, for example.

None of these fantasies is anywhere near realizing. In the meantime I hope not only that the Higgs boson now makes some sense to us but that we can also see how it's a discovery in progress. It hints at a much larger story about the amazing evolution of matter and energy that took place when the universe was just a tiny fraction of a second old.