Our Solar System Part 4: The Goldilocks Zone

Now that we have explored Earth, Venus and Mars in some detail, it is very tempting to conclude that Venus is too close to the Sun so it is too hot for life as we know it. Mars is too far away from the Sun so it is too cold, and, as the Goldilocks fairytale goes, Earth is just the right distance from our Sun, neither too hot nor too cold to support life. This is the essence of the concept of the Goldilocks Zone, a specific distance from a star in which a planet receives just enough energy to support liquid water on its surface, and thus, life.

However, if we think about these three planets for a moment, we might notice some complications to this idea. First of all, of these planets, only Earth has a significant magnetic field that protects its atmosphere from solar wind. Why? It is this atmosphere that moderates Earth’s temperatures and allows liquid water to exist, between 0°C and 100°C. On Mars, with no magnetic field, any atmosphere that may have once existed has been stripped away over the eons by solar winds. Its surface is now ravaged by ultraviolet and other solar radiation. Earth is also the only planet that exhibits plate tectonics and this means that nutrients and gases can cycle into different forms, keeping their atmospheric concentrations buffered within certain extremes. For example, carbon dioxide is moderated by the carbon cycle on Earth and this prevents a runaway greenhouse effect, which seems to have occurred on Venus, sending its surface temperatures skyrocketing up to about 460°C.

Does distance from the Sun have anything to do with internally created core dynamos or plate tectonic movement? Does how a planet form impact its later habitability? Do catastrophic events early on in a planet’s life affect its later habitability as much or more than its distance from its star? For example, did Earth’s surface water come from within Earth or did it come from comet impacts during the Heavy Late Bombardment? Let’s see what the Goldilocks Zone theory has to offer in terms of answering these questions.

What the Goldilocks Zone Is

The Goldilocks Zone or habitable zone is, technically, the intersection of two regions in space that must both be favourable to life. One region is confined to the planetary system of interest and the other region is where this system exists within its galaxy. The zone around a star, also called the circumstellar habitable zone or ecosphere, is where the star’s energy output allows for water to exist as a liquid rather than freezing or boiling away. The galactic habitable zone is a hypothesis that is met with a bit more skepticism than the ecosphere zone. The idea here is that the center of the universe acts in much the same way as a star does in the ecosphere zone. The favourable zone must exist close enough to gather enough heavy elements to form a habitable planet but far enough away to be protected from radiation from the galactic centre. Planets or moons could exist within this intersected region and possibly support carbon-based life. However, it is important to remember that planets within the Goldilocks Zone may not all be habitable. For example, gas giants in this zone are unlikely to support life.

The Goldilocks Planets Of Gliese 581

The red dwarf star, Gliese 581, is of particular interest to scientists studying Goldilocks planets because it has a planetary system and one planet in particular, that lie within the Goldilocks Zone. This image produced and copyright by the European Southern Observatory (Gliese 581’s planetary system is described in detail on this Wikipedia page) compares Earth’s and Gliese’s Goldilocks planets.

(copyright attributed to European Southern Observatory)

You can see that both Earth and Mars are within the Goldilocks Zone with Venus just on the inner edge of it. Gliese 581 g exists well within the habitable zone around its star, with Gliese 581 c and Gliese 581 d straddling the zone. Scientists currently hold all three exoplanets as possibly life-supporting, with reservations. Gliese 581 g appears to be a perfect candidate for alien life. However, recent evidence indicates that it may be tidally locked, meaning that one side always faces its star. And I should also note that both planets g and f are listed as unconfirmed because they have not been detected by new spectrograph analysis. In both solar systems, planets such as Gliese 581 f and Jupiter do not receive enough solar radiation to make up for radiative losses, and surface water freezes. Planets such as Gliese 581 e and Mercury, on the other hand, absorb too much solar radiation and any surface water simply boils away. Keep in mind that the Goldilocks Zone must be calculated for each star system based on the energy output of that star. The Goldilocks Zone for Gliese 581 is much closer to the star than our Sun’s habitable zone. Gliese 581 has only 0.2% of the visual luminosity of our Sun (but while our Sun radiates mostly in the visible spectrum, Gliese 581 radiates mostly in the near infrared, meaning that although it is much fainter than the Sun it gives off a much higher percentage of its radiation as heat than the Sun does). This NASA image gives you an idea of how the Goldilocks Zone (shown as a green belt) is affected by the host star’s energy output.

Kepler Space Mission Results

The Kepler Space Observatory launched by NASA in 2009 is designed to discover Earth-like planets orbiting within the habitable zones of other stars in the Milky Way galaxy. This is Kepler’s targeted star field, only 1/400^th of the night sky, courtesy of NASA.

We do not yet have the capacity to directly observe these planets but we can detect them indirectly by monitoring fluctuations in brightness of other main sequence stars. Fluctuations in brightness indicate one or more planets and/or moons crossing the star’s surface. This NASA image shows how this works.

In February 2011, the Kepler Space Observatory Mission Team released a list of 1235 possible exoplanets, with 54 of them being both Earthlike in size and existing within the Goldilocks Zone. This 10 - minute NASA video introduces us to the Kepler Mission.

This is a very exciting time for astronomy. These results have allowed scientists to estimate for the first time that about 6% of all stars host Earth-size planets and 19% of all stars host multiple planets. Astronomer Seth Shostak believes that, based on the Kepler findings, there are at least 30,000 habitable worlds within a thousand light years of Earth!

New Missions To Study Goldilocks Planets?

We need to confirm the existence of the Kepler Mission exoplanets and to study them in more detail and for that we will need a new generation of observational missions tailored to study exoplanets. Several such missions have been proposed and await funding. The Darwin, a European Space Agency cornerstone mission, was proposed but scrapped in 2007, because of technical and funding problems. It would have detected exoplanets and then carried out more detailed analysis of their atmospheres, looking specifically for the presence of oxygen. Finding just atomic oxygen does not necessarily mean life, however. Europa, one of Jupiter’s moons, has a tenuous oxygen atmosphere that is produced by the radiolysis of water molecules (this means that solar radiation breaks water molecules into ionized atoms) high up in its atmosphere. To find oxygen produced biologically through some kind of photosynthesis-like process, astronomers must look for the simultaneous presence of ozone, water and carbon dioxide. Oxygen produced at high altitude as it is on Europa immediately attacks atmospheric ozone and prevents its accumulation. If oxygen is produced low in the atmosphere, say through photosynthesis, and little water gets high into the atmosphere, then there are no ions that can attack ozone. Therefore, scientists now believe that ozone, water and carbon dioxide (required for photosynthesis) along with oxygen comprise a reliable biosignature in an alien atmosphere. This mission would have looked for this signature, with the Gliese planets being good first candidates for study.

Other missions that have been proposed and scrapped or put on hold include the New Worlds Mission, PLATO, the Space Interferometry Mission, the Terrestrial Planet Finder and the Transiting Exoplanet Survey Satellite, all of which were planned at least in part by NASA except for PLATO, which was another planned European Space Agency mission. Right now, the Kepler Mission, still active, is our best exoplanet explorer and it has been very successful so far. As our technology improves, new missions to study these worlds will undoubtedly be planned and we will move forward in this exciting new field of research.

Goldilocks Zone – A Concept In Infancy

In the meantime, it is important that we refine our search for habitable planets. The Goldilocks Zone concept is in its infancy and it will be refined and expanded upon as we learn more about what makes planets habitable. We are doing that right now as we study the mysteries of Mars, Venus and Earth right here in our own stellar neighbourhood. We also need to question and refine our definition of life itself. As we discover new forms of life never before believed possible in extreme environments right here on Earth, where there is no oxygen or temperatures are well below freezing are well above the boiling point, we must open ourselves to new possible extreme habitats in which life finds a foothold. And we must explore the possibilities of biochemistries that may not be based on carbon or water at all. Why is liquid water so important for life to exist? Carbon compounds dissolve in water to form the basis of all life as we know it, from enzyme functions to the building of cells, tissues and more complex structures. But are other biological solvents possible? A potential biosolvent should exist as a liquid over the range of temperatures an alien organism might encounter, and pressure must be accounted for. For example, while hydrogen cyanide has a very narrow temperature range as a liquid at 1 atmosphere (the surface pressure on Earth), it can exist as a liquid over a wide temperature range on Venus where the surface pressure is almost 100 times greater. Observations from NASA’s Spitzer telescope hint that planets around cool stars such as M-dwarfs and brown dwarfs, which are widespread around the Milky Way, might offer a prebiotic chemical soup that is different from that of our young Earth. The disc chemistry around these cool stars is different from that around our Sun, containing significant amounts of hydrogen cyanide, for example. Hydrogen cyanide is an active molecule that can combine to form adenosine, an essential building block of DNA. Perhaps it could function as a building block in the biochemistry of some kind of alien life, perhaps on a young planet such as the one in this NASA artist’s conception.

Methane, hydrogen fluoride and perhaps even molten salts could also theoretically be used as biosolvents. As well, rather than carbon, life could use silicon atoms. Silicon is chemically similar to carbon and it is far more abundant on rocky planets than carbon is. However it cannot create as many diverse functional groups as carbon can and it can’t readily form the double and triple bonds important to carbon-based biochemistry. But it may be useful under temperatures and pressures different than those of Earth or it could be used in roles less analogous to carbon. These kinds of studies will affect what we refer to as the Goldilocks Zone and perhaps even do away with the concept all together.

Here are some intriguing articles to explore:

Life As We Didn’t Know It – An ecosystem that thrives in complete darkness

A New Form Of Life – A new extremophile is discovered in California’s exotic Mono Lake

‘Goldilocks Zone' Bigger Than Once Thought - To find worlds within the "Goldilocks" zone, where conditions to support life are just right, look no further than our own solar system

Habitable or Goldilocks Zone  - an article from an excellent space science blog called Weirdwarp

Cruising the Goldilocks Zone - The Search for "Earth's Twins" - this article outlines a comprehensive shopping list for the ingredients to make a habitable exoplanet

Our Solar System Part 5: Mercury

Mercury, named after Mercurius, the Roman messenger and god of trade, is a small mysterious planet. It is relatively bright because of the reflection off of its rocky surface, like the moon, but it orbits so close to the Sun that it is usually lost in the Sun's glare. You might get a chance to see it at twilight along the horizon close to where the Sun comes up or sets.

Try just after sunset in late winter and in early spring and just before sunrise in late summer and early autumn, with clear skies.

This is a photo sequence of Mercury (the tiny white dots) exactly 33 minutes after sunset on 12 consecutive evenings taken in April 2004 in the United Kingdom by Tony Cook.

Gaps in the sequence were because of cloudy evenings. This progression in Mercury's position across the sky hints at a very eccentric orbit. In fact, Mercury has the most eccentric orbit of all the planets.

This is what Mercury looks like close up. 

This is a false colour image of the planet taken by MESSENGER during its flyby in 2008. NASA's MESSENGER probe just moved into orbit around Mercury a few days ago, March 17. We will explore this current NASA mission in a moment.

As first glance, Mercury seems to be nothing more than a small broiled and cratered dead planet, nothing much to see. Upon closer investigation, however, it reveals some fascinating mysteries about how the solar system came to be and about its own history as well.

Let's Start With Mercury's Orbit

Mercury's distance from the Sun varies between 46 and 70 million kilometers. Its orbit looks like a cross section of an egg with Mercury orbiting where the shell is and the Sun located in the yolk. It actually orbits faster when it is near perihelion, which means closest to the Sun, thanks to greater gravitational pull. Mercury is in a 3:2 spin orbit resonance, which means it rotates on its axis 3 times for every 2 orbits around the Sun. Because of the planet's slow rotation and fast orbit, a day on Mercury lasts twice as long as a year. A day, from one sunrise to the next, lasts 176 Earth days, while it only takes 88 Earth days to make one orbit around the Sun, one Mercury year. This arrangement is stable only because Mercury's orbit is so eccentric (non-circular) and, to make matters more complicated, the eccentricity of Mercury's orbit gradually varies between zero (a perfect circular orbit) and 0.45 (a nearly perfect elliptical orbit) over a period of millions of years. Computer simulations suggest that the gravitational influence of Jupiter causes Mercury's eccentricity, and that it's eccentricity may gradually increase to the point where Mercury may someday collide with Venus, sometime within the next 5 billion years.

What is perhaps even more intriguing is Mercury's advancing perihelion. What this means is that Mercury's orbit around the Sun is not really a stationary ellipse but a flower petal shape. The orbital path itself rotates gradually over time, as shown in this diagram.










Mercury is the small brown globe and the Sun is the fiery red large globe. This diagram is obviously not drawn to scale and the advancement is greatly enhanced.

This kind of planetary movement can usually be explained by changing gravitational forces exerted by other planets, motions governed by Newton's laws of classical mechanics. In Mercury's case, however, the advancing perihelion can only be satisfactorily explained by Einstein's theory of general relativity. This is how it works: 

Einstein Gravity Versus Newton Gravity

General relativity explains a gravitational field as a curvature in space-time. Gravity is not a force per se but rather it's a geometry where the source of the curvature is the stress-energy tensor (representing mass). The stress-energy tensor describes the density and flux of energy and momentum in space-time. It can be an attribute of matter, radiation or non-gravitational force fields. In other words, the stress-energy tensor is the source of the gravitational field in general relativity, just like mass is the source of the gravitational field in Newtonian physics. The stress-energy tensor, or stress-energy-momentum tensor as it's sometimes called, generalizes the Newtonian stress tensor. It covers mass as well as conditions other than mass, territory where Newtonian gravity can't go. An example of this territory is the observed gravitational pull on massless photons (units of electromagnetic radiation such as visible light), a phenomenon called gravitational lensing.

This 10-minute video clipping from the PBS series, "The Elegant Universe," is helpful with visualizing space-time gravity:

Mercury's Orbit Defies Newtonian Physics

The orbit of Mercury is an example of a two-body (or Kepler) problem in general relativity, where we are trying to describe the motion and gravitational fields of two bodies interacting with each other. The body in question can range from an individual photon to a galaxy. This is the kind of problem that is solved when calculating how light bends around a massive star or black hole, or the amount of energy that is gradually lost through the orbiting of two binary stars with each other. At first thought, you might think these should be simple calculations but they are not. The solutions to these equations are nonlinear and that means they can only yield a close approximation in terms of predicting these kinds of motion (most physical "real-life" situations are nonlinear in nature - the weather is an example, where simple changes in one part of the system produce complex and often chaotic effects throughout). In our case with Mercury, one solution, called the Schwarzschild solution, almost exactly solves the Kepler problem because the Sun is so much more massive (and its spin is relatively slow) than the planet orbiting it, that the Sun's gravity can be considered for these equations to be the sole contributor to the gravitational field (obviously this is not really true because Jupiter - and other planets to some extent - also impact Mercury as mentioned and Mercury itself has a gravitational impact, but here Mercury is relatively so small we can ignore all of that) around Mercury. When you solve the equations you get a geodesic motion for Mercury that is in agreement with careful observations of its orbit. The advancing perihelion of Mercury, along with the gravitational bending of light, are often used as supporting evidence for the validity of general relativity. The Schwarzschild solution means that Mercury takes the shortest path between two points in curved space-time (space-time is four-dimensional so it's pretty well impossible to visualize). This is what is meant by geodesic, and it means that Mercury's orbit "inches forward" very gradually, at a rate of 5600 arc seconds per century. To give you an idea of how miniscule this rate of movement is, there are 60 arc seconds per 1 arc minute and 60 arc minutes in 1 degree and 360 degrees in a full circle. It takes over 12 million orbits to make one whole "flower," or about 3 million years.

Its orbit is not the only unusual thing about Mercury.

Mercury's Unusual Composition

Mercury looks much like the moon does. But looks can be deceiving. Although Mercury is small, smaller than any other planet and even smaller than the moons Ganymede and Titan, it is the second densest planet in the solar system, with Earth being the densest of all planets. If the effect of gravitational compression is factored out (Earth is so much more massive than Mercury that its interior is squished more by gravity), it is actually far denser than Earth as well. The source of this density is a relatively very large metallic core that makes up almost half of Mercury's volume. Earth's core, by comparison, makes up about 17% of its volume. Mercury has a metal to silicate ratio much higher than that of the meteorites from which it formed. The other rocky planets have ratios that are very close to the ratio of meteorites. Why it would have such a different composition than other rocky planets is a source of debate.

When the inner solar system (inside 4 astronomical units or AU) was very young, it was too hot for volatile molecules like water and methane to condense, so the planets that formed in this region, Mercury, Venus, Earth and Mars, could only form from chunks of rock composed of high melting-point compounds like iron, nickel and aluminum. These rocks, or asteroids (or planetisimals if you want to call them that at this early point in solar system's evolution), were made almost entirely of metals and silicates. Silicate minerals are what make up most rocks. 90% of Earth's crust is silicate mineral. These minerals contribute most of the mass of the other rocky planets too, as well as the moons and asteroids. Metals are quite rare in the universe. They made up only about 0.6% of elements in the gas cloud from which the solar system formed, so these rocky planets are therefore quite small, compared to the much larger gas and ice giants (Saturn, Jupiter, Neptune and Uranus) that formed farther out. The rocky protoplanets that first formed were very small, about 1/25 of Earth's mass, but this region was littered with asteroids so the protoplanets grew as asteriods bombarded them and the pieces stuck together.

There are currently three theories about Mercury's strange composition:

Theory #1

Proto-Mercury might have been quite a bit larger than it is today. It may have been struck by a large asteroid, several hundred kilometers across. According to computer models, this impact would have stripped away much of the original crust and mantle, while leaving most of the core behind.

Theory #2

Mercury may have formed before the Sun's energy output had stabilized. In this scenario, Mercury was originally about twice its current size. As the newly formed Sun contracted, it could have emitted so much energy that the space around it which enveloped Mercury could have reached temperatures as high as 10,000 K (Kelvins). This heat would have vapourized much of Mercury's surface rock and carried it away on the powerful solar wind.

Theory #3

The rotating gas cloud from which the Sun and the planets were forming could have created enough drag on all the particles and debris, which were sticking together and forming Mercury, that lighter elements simply blew away before they could stick to the planet, leaving a disproportionate amount of heavy elements behind.

Each of these hypotheses predicts a different surface composition of Mercury. Astrophysicists hope that MESSENGER, the probe currently in orbit around Mercury, will be able to test these theories.

MESSENGER

In 2004, NASA launched the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) probe to study the chemical composition, geology and magnetic field of Mercury. This is an artist's rendering of what it looks like orbiting Mercury.

This is the second probe to reach Mercury. The first one was Mariner 10, which reached the planet in 1974. MESSENGER made three flybys of the planet before entering into orbit around it on March 18 of this year. Its flight path from Earth was complicated because a probe can't be sent straight from Earth to Mercury. It would be so strongly accelerated by the Sun's immense gravity on the way to Mercury that it would fly past Mercury far too fast to enter into orbit, even if an enormous amount of fuel could be spent. In addition to this, other probes can break in a planet's atmosphere using friction (aerocapture) to slow down without using too much fuel but Mercury has essentially no atmosphere. MESSENGER flew by Earth once, by Venus twice, and by Mercury three times, so that it could be positioned with the right velocity to enter a stable orbit. It had to travel 8 billion km to visit a planet only 92 million km away. This is what its complicated trajectory looks like.

It captured some beautiful images of Earth as it made its gravity-assist swingby in 2005, shown here.

MESSENGER started filming about 66,000 km above South America and the last image was taken 436,000 km away the next day (farther away from Earth than the Moon).

MESSENGER is the first probe to study Mercury in detail. It has two cameras (MDIS) that are mapping the planet's landforms. A laser altimeter (MLA) will create a map of the 3-dimensional topography of the planet surface. A radio science experiment will use the Doppler effect to assess the distribution of mass and the thickness of the crust. An X-ray spectrometer (XRS) and a gamma ray/neutron spectrometer (GRNS) will identify elements in the crust and look for signs of any water ice in permanently shadowed craters in the planet's far north and south poles. An atmospheric and surface spectrometer (MASCS) will search for any gases in Mercury's thin atmosphere and look for surface minerals. Finally, an energetic particle and plasma spectrometer (EPPS) will study the magnetosphere around Mercury and a magnetometer (MAG) perched on a 3.5 metre boom will map the magnetic field. The mission will end sometime in 2012 when the probe runs out of the fuel it needs to maintain its orbit.

It will take several months of data analysis to get a global picture of Mercury. Meanwhile, click here to view the latest images from MESSENGER as they come into NASA.

What We Already Know About Mercury

Mercury's surface is very similar to the Moon. There are extensive plains and heavy cratering suggesting that there haven't been any volcanoes or other geological activity for billions of years. All this cratering records a period of heavy bombardment by comets and asteroids, about 4.5 billion years ago, during Mercury's formation and probably again around 3.8 billion years ago during the late heavy bombardment. It was during this period that Mercury was volcanically active, forming the smooth plains much like the maria visible on the Moon today.

The temperature on Mercury's surface ranges from 100 K to 700 K, with a steep temperature gradient from the equator to the poles. The sunlight striking its surface is between 5 and 10 times more intense than what strikes the top of Earth's atmosphere. In spite of this, we know there is a lot of radar reflection at the poles and that could be a sign of water ice. It would be covered by a layer of regolith, preventing its sublimation and loss into space. This water could have come from either outgassing from the planet's interior or from comet impacts (these are also the two probable sources of Earth's water).

Mercury doesn't have enough mass, and therefore gravity, to hold onto any atmosphere for long. But it does have a thin unstable exosphere containing hydrogen, helium, and other gases. These atoms are continuously lost and replenished. Hydrogen and helium probably come from the solar wind and are deposited on the surface. Radioactive decay might supply more helium as well as sodium and potassium. MESSENGER has already found relatively plentiful calcium, magnesium, oxygen and other elements as well. It also found water vapour and water-related ions like O⁺, OH^- and  H₃O⁺. The heavier elements that were found may come from the vapourization of the surface crust by micrometeorite impacts.

Mercury's magnetic field is strong enough to deflect solar wind, thus creating a magnetosphere, a protective envelope. Its magnetic field is a dipole, like Earth's magnetic field, and scientists think it arises from a dynamo effect caused by the circulation of its liquid iron-rich outer core. Mercury's eccentric orbit would cause strong enough tidal effects to keep its outer core liquid, even though, based on its small size, its core should have cooled and solidified long ago. MESSENGER discovered on its 2008 flyby that Mercury's magnetic field is also very "leaky." When magnetic fields carried by the solar wind connect with Mercury's magnetic field, they twist up into tornado-like vortices and form holes in the magnetosphere through which solar wind can enter and directly strike the planet's surface. This is called magnetic reconnection and it actually happens on Earth too. Our planet also has a leaky magnetosphere although, luckily for us, these leaks are very small and happen only rarely, during the most powerful solar storms.

Finally, enjoy this 8-minute video. It sums up what we know about this small but fascinating planet:

Our Solar System Part 6: Neptune

Neptune is a beautiful and violent blue planet, named after the Roman god of the sea.

This is a composite image taken by Voyager 2 as part of its study of the outer solar system.

Neptune is very far from the Sun, about 4.5 billion km away. Earth, in comparison, is about 150 million km from the Sun, that's about 30 times closer than Neptune is. In July 2011, it will have completed just one orbit, one Neptune year, since its discovery in 1846. As a result of its distance it receives only a tiny fraction, about 1 thousandth, of the Sun's energy that we receive here on Earth.

Neptune's Layers

What's most interesting about Neptune is what it's made of, and thanks to Voyager 2's information, we know quite a bit. It's an ice giant, consisting mostly of water, ammonia and methane ices, similar to Uranus. But Neptune's atmosphere is a more vivid azure blue than Uranus's milder cyan colour because it contains a larger trace of methane, which absorbs red light. Neptune likely has clouds of different compositions depending on the altitude. 

Bands of high altitude clouds of condensed methane droplets cast shadows on a lower cloud deck as shown here in this NASA image taken by Voyager 2.

Below the high-altitude cirrus-like clouds are clouds of ammonia, hydrogen sulfide, ammonium sulfide and even clouds of water ice that lie deep within the atmosphere where the temperature approaches 0°C. If a space probe attempted to land on Neptune, it would be buffeted by supersonic winds of up to 2000 km/h as it begins its plunge through a gas atmosphere thousands of kilometers thick. It would then gradually sink through a slushy mantle, all the while enduring pressure that is increasing from the hard vacuum of space to pressure millions of times greater than what we experience on the surface of Earth, where atoms themselves begin to be crushed into each other and exhibit strange physical properties! On its way, it would have to withstand temperatures ranging from -218°C in the cloud tops to up to 5000°C deep within the planet's mantle. If it could survive all of that, it would finally reach a relatively small Earth-size core of superheated iron and nickel. 

The probe would pass through a very strange electrically conductive super-dense water-ammonia ocean. Then it would be bombarded by diamond-hard methane crystal "rain" and finally it would pass through superionic water that glows bright yellow and is as hard as steel. Any probe we could build would be crushed and melted.

The Great Dark Spot

A series of ever-changing dark spots on Neptune are visible through the Hubble telescope. The Great Dark Spot is actually a series of anticyclonic storms that form and dissipate every few years. 

This image of a Dark Spot, taken by Voyager 2, was about the same size as Earth in diameter, around which winds blew up to 2400 km/h, the fastest wind speed ever clocked in the solar system.

The Great Dark Spot is thought to represent a hole in the methane cloud deck that generates large white clouds of frozen methane crystals. These storms may be tied to Neptune's seasons, each one lasting 40 years (Neptune has an axial tilt which creates its seasons, similar to that of Earth). In addition to its extreme winds, Neptune also experiences strong latitudinal wind shear because of the differential rotation of its atmosphere (strong enough to instantly rip apart any manmade probe).

A long-standing question is why Neptune has such ferocious winds and storms even though it receives very little energy from the Sun. Whereas Earth's storms are driven by the Sun's energy, Neptune is too far away to draw much energy. The energy source of its storms comes instead from deep within. In fact, Neptune radiates more than twice the energy it receives from the Sun. There are several theories about this phenomenon, one of which relies on intense radioactivity within the small core. Another theory proposes that, as methane is squeezed under great pressure, it separates into two components: liquid metallic hydrogen and (carbon) diamonds, and as these diamonds seasonally rise and sink, extra energy through friction is created.

Magnetosphere and Rings

Neptune's magnetosphere is also mysterious. It has a lot in common with Uranus's magnetosphere in that it is about the same magnitude (both of these magnetospheres are far larger than Earth's) and it probably comes from the movement of some kind of conductive material, maybe the highly pressurized water within its slushy shell rather than from a molten iron core, as it does in Earth, or from liquid metallic hydrogen as it does in Jupiter and Saturn (Neptune just isn't big enough to generate the immense pressure required to make liquid metallic hydrogen).

Like Uranus, Neptune's magnetosphere is oriented at an extreme tilt, almost 50 degrees. The configurations of Neptune's magnetosphere and the resulting aurora are so complex they are very difficult to model. Mathematical models, in fact, suggest that Neptune's rings may complicate the motion of the particles in its magnetosphere. Neptune appears to have three separate plasmaspheres (inner magnetospheres) instead of one large one (as Earth does).  

Earth's singular plasmasphere is shown here to give you an idea of where it's located.

Neptune's magnetosphere generates faint aurora as well as radio emissions. Voyager 2 "heard" them as a radio hiss as it passed by the planet in 1989.

Neptune has five principle rings, shown here, along with some of its moons.

The rings are named after astronomers that contributed important research on the planet. 

They are much less dense and fainter than those of Saturn and more closely resemble those of Jupiter. Unlike the highly reflective icy chunks of Saturn's rings, these rings consist of dark organic compounds and dust. Like all planetary rings, they are stabilized by resonant interaction with the planet, with each other, and with the moons. They may be the result of a long ago collision with an inner moon that fragmented.

The gravity of Neptune shapes the Kuiper belt, a ring of small icy chunks that extends from Neptune's orbit to significantly past that of Pluto, in much the same way that Jupiter's gravity dominates the asteroid belt.

Formation and Early Jostling Around

Neptune and Uranus are too large to be explained by core accretion alone, a process in which the collapsing mass of a molecular gas cloud forms a protoplanetary disk, shown here, from which the Sun, planets, moons, asteroids and other bodies formed.

There simply wasn't enough matter density that far away from the disk center to account for the mass of these ice giants. A model called the Nice model, based on three papers published in Nature in 2005, is now the most widely accepted explanation of Neptune's early history. It suggests that the giant planets initially formed much closer to the Sun than they are today, and they migrated outward long after the protoplanetary gas disk itself dissipated. This model also successfully explains other solar system phenomena such as the Late Heavy Bombardment of the inner solar system, and the formation of the Oort cloud, the Kuiper belt and other bodies.

This is how they think it worked: Over time, many planetesimals, large chunks of rock and ice, at the outer edge of the protoplanetary disk, approached the outer giant planets, Neptune and Uranus, which were then relatively close to the Sun, between 5 and 17 astronomical units (AU) away (Neptune is now 30 AU away from the Sun). As the planetesimals approached the giant planets, they were scattered in toward the Sun, like sling-shots, by the planet's immense gravitational fields. By doing so they exchanged angular momentum with the giant planets so that the planets gradually moved outward in response. This preserves the total angular momentum of the system. Jupiter, in contrast, and far more massive than Neptune or Uranus, threw these planetesimals into highly eccentric orbits or right out of the solar system altogether rather than inwards, so this planet moved inward slightly instead, again conserving the system's angular momentum.

This process of orbit adjustment took place gradually, over several hundreds of millions years, with these planets exerting all the while what is called mean-motion resonance on each other and further altering each other's orbitals until they eventually stabilized into what we see today. This resonance also scattered what was left of the primordial disk, removing 99% of its mass. The planetesimals that were thrown into the inner solar system created the Late Heavy Bombardment, around 4 billion years ago, 600 million years after the solar system formed, a period of intense impacts on the inner rocky planets, as evidenced by the many impact craters we can see today on our pockmarked moon. The remaining rock and ice chunks organized themselves into the asteroid belt and the Kuiper belt.

Triton

Neptune has 13 known moons. The largest one by far, and the only one massive enough to form a proper sphere, is Triton, named after a Greek god, the messenger of the sea, and son of Poseidon. Triton is the only large moon in the solar system with a retrograde orbit, and this suggests that it was captured rather than formed in place. It was probably once a dwarf planet within the Kuiper belt. 

This is a Voyager 2 mosaic image of Triton.

Triton, far from being an orbiting dead rock, is alive with tectonic activity. It's covered with a thick bright ice composed of frozen nitrogen, water and carbon dioxide which is rhythmically squeezed and pulled like the ocean waters of Earth, a tidal interaction that is very gradually degrading Triton's orbit so that it will eventually either collide with Neptune or break up and form a new ring system about 3.6 billion years from now. After Triton was captured by Neptune's gravity, it remained entirely liquid for a billion years thanks to a very eccentric orbit which caused a great deal of tidal heating of the moon's interior. Triton's orbit eventually stabilized into a near perfect circle and as it did so it froze.

A solid core of metal and rock is surrounded by a mantle of water, which may be liquid and convective thanks to the core's radioactivity. Surrounding the mantle is a surface layer of frozen nitrogen, water ice and dry ice (carbon dioxide) and above this is a very thin nitrogen atmosphere with some wind and a few faint condensed nitrogen clouds.  It's extremely cold on the mostly flat surface, about -237°C. Instead of molten rock volcanoes on Earth, Triton spews regular water flows from its cryovolcanoes as well as nitrogen geysers with plumes up to 8 km high. These geysers are not associated with volcanic activity as they are on Earth. Instead, they occur when subsurface pockets of nitrogen heat up and expand from solar heating. Very little sunlight strikes Triton but only a 4°C rise in temperature could create enough pressure for nitrogen gas to erupt in the enormous geysers observed. These "summer" geysers likely last for many decades as Triton's polar regions take turns facing the Sun for 82 years at a stretch. 

Here is a simulation of what Neptune might look like from Triton. 

The atmosphere is so thin the sky appears black with only a thin rim of haze visible at the horizon. Neptune's other moons are so small they would not even be visible in the sky.

Future Missions

Because Triton may have a liquid water mantle, there is a possibility that life may have gotten a foothold there. This connection between the presence of liquid water and the possibility of life will be explored fully in a future article. A future mission called Neptune Orbiter was proposed in 2005 to launch in 2016, taking just over 10 years to get to Neptune. It would study both Neptune and Triton in detail. An orbiter would analyze Neptune's magnetosphere and image the planet. Two probes would also go into Neptune's atmosphere. Similar to the Galileo probe that studied Jupiter's atmosphere, the probes would study the atmosphere in detail before continuing to descend, sending back information until they were inevitably crushed.  As well, one or two probes would land on Triton to analyze the surface, the interior and atmosphere and to search for liquid water and signs of microscopic life, a plan similar to but more advanced than the Cassini/Huygens mission which studied Saturn and its moon, Titan. Unfortunately  this mission is no longer in the works at NASA, at least for now.

Our Solar System Part 7: Saturn

Saturn, an enigmatic gas giant, is the second largest planet in the solar system, after Jupiter. It is named after the Roman god, Saturn, a god of contradictions.

The planet Saturn is a mysterious body cloaked in thick cloud and ringed by an icy disc of extraordinary structure and beauty. This is an ultraviolet image of the rings, taken by NASA's Cassini spacecraft during its orbital insertion in June 2009.

The colours of these rings reveal that there is more ice toward the outer part of the rings than the inner part, and this graduation of ice may reveal clues about their origin.

Although the ongoing Cassini-Huygens mission has revealed an enormous amount of data about Saturn, its rings and its many moons, Saturn remains a world of mystery. This real colour image of Saturn shadowed by its rings shows a gradual polar shift in colour from gold to azure, which is still not fully understood, but it may be related to seasonal temperature changes (Saturn has an axial tilt of about 27°).

Below is an extraordinary panoramic view of Saturn with its rings, published in October 2006 and created by combining 165 Cassini images. Here, Saturn is sheltering Cassini from the Sun's glare, revealing faint outer rings never before seen.

The Composition of Saturn

Saturn is enormous, almost one hundred times the mass of Earth, even though it is only about 1/8 Earth's density. It is composed almost entirely of hydrogen with about 3% helium and some trace elements, some of which contribute to a relatively tiny rocky core (scientists believe) surrounded by a layer of hydrogen that is under so much pressure it exists in a metallic liquid state. This is the result of a phase transition into a degenerate state of matter. It is degenerate because the electrons and protons are unbound to each other at pressures of around 400Gpa (400 billion pascals - standard air pressure on Earth is about 100,000 pascals). At least this is what is believed; liquid metallic hydrogen has yet to be made in the lab although techniques are currently being developed to create pressures of up to 500 Gpa. The creation of liquid metallic hydrogen will be an exciting breakthrough in high-pressure physics. Liquid metallic hydrogen is believed to exist to varying extents within all four of the gas giants, shown as a dark purple layer here.

Surrounding this liquid metallic layer is a thick liquid layer of hydrogen and, to a lesser extent, helium molecules. The outermost 1000 km of Saturn's 60,000 km radius consists of a gaseous atmosphere.

The rocky and liquid metallic core of Saturn is intensely hot, reaching almost 12,000°C. This means that Saturn radiates 2.5 times more energy than it receives from the Sun. This energy is believed to be what fuels massive storms on the planet. We will explore these in a moment. The mechanism for this heat production is not entirely worked out but it is believed by many scientists to be the result of at least two mechanisms: slow gravitational compression as well as friction, as heavier helium droplets “rain out” through hydrogen deep within the liquid interior. This latter helium rain theory is not without its controversy, however.

Saturn's Atmosphere

Like the rest of the planet, Saturn's atmosphere is mostly hydrogen with about 3% helium and trace amounts of other substances including water, nitrogen and methane. Despite its exotic features, Saturn is also known to have water snow, water rain, winds, lightning and storms just as Earth does. Well, not quite like Earth. Saturn's thick bands of clouds are pale orange because there is some trace sulfur in them and its winds are as high as 1800 km/h at the equator. Large white storms lasting up to a few months circle the planet. The outermost clouds, which we can see, are made up of ammonia at a very chilly -250°C. Below this deck resides a band of ammonium hydrosulfide clouds, about 170 km below the top layer. The temperature here is about -70°C. The lowest cloud deck is warmer still and is made up of water clouds. Here, temperatures approach 0°C. As we go further inward pressure and temperature increase and, at some point, hydrogen gas condenses into liquid. Below this level, helium eventually condenses into its liquid state as well. And eventually, of course, we reach a layer of liquid metallic hydrogen.

Saturn's atmosphere does not approach the violence of that of Jupiter but it does support a periodic storm called a Great White Spot, an enormous white oval storm, which tends to form once every Saturn year, equivalent to 30 Earth years, during the northern hemisphere's summer solstice. This false-colour infrared image from Cassini shows large ammonia ice crystals dredged up by the storm (yellow).

During this storm, ammonia gas is dragged more than 50 km upward where it reaches the upper cloud deck and freezes into large crystals

Saturn also supports an eerie hurricane-like storm, about 8000 km wide, locked in place rotating around its south pole. The images below were taken by the visual and infrared mapping spectrometer onboard the Cassini spacecraft. The infrared image, right, shows "leopard spots" blocking the heat radiating form the planet. These are substorms rotating around the eye.

Hurricanes on Earth are fueled by warm water and sunlight, both of which are in very short supply on Saturn. And, yet, this enormous hurricane-like structure complete with a well-defined eye (unlike Jupiter's famous Red Spot and Saturn's White Spot, which are eyeless) and ringed by an eye-wall of clouds up to 5 times higher than any on Earth with a rotation of about 350 km/h, seems to be a semi-permanent feature. It's been howling around the south pole since it was first detected in 2003. No one is quite sure what drives this storm but its period of rotation coincides with that of Saturn's radio emissions. This, along with the fact that the storm seems locked in place at the southern pole, provides a clue that the structure may be caused by a standing wave pattern in the atmosphere. Saturn's south pole is warmer than the rest of the surface of the planet by about 4°C. This may not seem like much but models suggest it is enough to cause the atmosphere across the planet from its north to south poles to sink, compress, heat up, and rise, maintaining this weird hurricane-like structure. The bigger picture of what is ultimately driving this phenomenon is currently of great interest to researchers involved in the ongoing Cassini-Huygens mission. It may ultimately be linked to Saturn's southern summer season, which is occurring right now. Images taken over time will help scientists figure out if it is indeed seasonal in nature.

Saturn's Magnetosphere

Saturn boasts a very powerful magnetosphere, second in strength only to Jupiter. This Hubble photo reveals Saturn's double aurorae (bright rings at the bottom and top of the planet), a consequence of its powerful magnetosphere.

Charged particles streaming from the Sun race along the planet's magnetic field lines into the upper atmosphere where they collide with, excite, and ionize gas atoms, making them glow. This is the same process that occurs on Earth, as explored in my article, Aurora Borealis. Refer to this wikilink for a brief explanation of the auroral mechanism.

Saturn's magnetosphere is filled with plasmas (ionized atoms). These come from both Saturn itself and its moons, especially the moon, Encelades, which ejects incredible volumes of water vapour from huge geysers erupting from its south pole (this moon is explored in detail later on). Saturn's magnetic field is generated by a fluid dynamo within a layer of circulating metallic hydrogen in the outer core. It is a dipole, much like Earth's magnetic field, except that its magnetic poles (like Jupiter's) are opposite that of Earth. Saturn's magnetic field is actually slightly weaker than Earth's magnetic field but its magnetic moment is almost 600 times larger. You may be wondering how this is so. If so, I attempt to explain the general concept in the asterisked (*) aside at the end of this article. A technical explanation of magnetic field and magnetic moment is explained here.

Saturn's rings and moons strongly affect its magnetosphere. The plasma in the magnetosphere co-rotates with the planet so its particles are continuously colliding with, and being absorbed by, slower moving moons and components of the ring system. Three moons, particularly Enceladus, also contribute new plasma particles to the magnetosphere. These interactions create large gaps in the radiation belts that surround Saturn as well as a low-radiation zone close to the planet

The Ring System

What makes Saturn so unique is its intricate ring system. These rings extend from about 7000 km to about 120,000 km from the planet and average only about 20 km in thickness. The particles that comprise the rings consist almost entirely of water ice (93%). The rest is a mixture of mostly carbon and a tiny amount of tholin impurities (tholins are made when UV radiation from the Sun acts on simple organic compounds like methane. They don't exist naturally here on Earth but they are found in abundance on the surfaces of icy bodies in the outer solar system, giving them a reddish brown appearance). The ring particles can range in size from dust-sized specks to the size of a car. The rings may have come from either the destruction of an earlier moon or from uncondensed material leftover from the formation of Saturn itself. In this case, some of the material that formed Saturn's protoplanetary disk was within what is called the Roche limit so it could not coalesce to form moons.  The Roche limit in this case is the distance within which a potential Saturnian moon, held together only by its own gravity, will disintegrate when Saturn's tidal forces acting on it exceed its gravitational self-attraction. A third possibility is that a moon that once existed was struck by an impact, not large enough to destroy the moon outright but enough to cause it to exceed the Roche limit. Some researchers believe that planetary rings are inherently unstable and dissipate over time, say tens or hundreds of millions of years. However, new dating techniques suggest that Saturn's rings may be as old as the planet itself. Some ice in the central rings comes from the water expelled from Enceladus' geysers. Strangely, a faint ring far from the planet, about 12 million km away, also exists. It is called the Phoebe ring and it is titled at an angle of 27° to the other rings, which all lie in the same plane, and it orbits in a retrograde fashion, opposite the other rings. Not much is known about the origin and eventual fate of any of these rings or why only some planets have them. Our solar system has four ringed planets: Saturn, Jupiter, Neptune and Uranus. They are all gas giants residing in the outer solar system.

Saturn's Moons

So far I have mentioned Saturn's moon, Enceladus. You may have heard of other Saturnian moons such as Titan and Mimas. Saturn actually boasts a roster of more than 60 moons. Most of these moons are insignificant; some being downright tiny, less than 50 km wide. However, the moon Titan, aptly named, contributes more than 90% of the mass orbiting Saturn, and it is a mystery unto itself.

Titan

Titan, larger than Mercury, shown here in this Cassini composite image along with the tiny moon, Epimetheus with Saturn's rings in the foreground, is especially fascinating for two reasons.

First, it is the only moon known to have a significant atmosphere and, second, it is the only object other than Earth known to have stable surface liquid. This liquid is not water, however, but liquid methane. Methane clouds and nitrogen-rich smog envelope a surface that exhibits many of the same features as those created by liquid water on Earth - rivers, lakes and shorelines, all dominated by seasonal weather patterns. Titan orbits straight above Saturn's equator, so it experiences seasons along with Saturn, due to Saturn's 27° axial tilt. The catch is that all this takes place at a very frigid average temperature of -179°C. The reason Titan is so cold is that Titan's haze has an anti-greenhouse effect, reflecting the sun's energy back into space. This mechanism is similar to that of nuclear winter and it cools Titan even though some methane in its atmosphere contributes to a small greenhouse effect. The greenhouse effect is what keeps Earth warm. Carbon dioxide, methane and water vapour absorb the sun's energy and re-radiate it in all directions. Part of this re-radiation is back to the surface and it is this energy that contributes to atmospheric warming. Infrared images of Titan gathered by Hawaii's Keck observatory suggest that methane rains down to the surface in huge drops, 1000 times larger than water drops, from giant clouds, and collects in lakes that eventually evaporate back into the atmosphere, creating a methanological cycle, similar to Earth's hydrological cycle. As part of the Cassini/Huygens Mission, NASA sent the Huygens Probe to the surface of Titan, shown here in this artist's rendition based on images sent back from Huygens shortly after it landed.

To see what its cameras observed as it descended and landed on a methane-moisted sandy riverbed in 2005 (and much more!), watch the 25-minute video at the end of this article

Titan has no magnetic field to protect its atmosphere from being stripped away by solar wind. However it is almost always enveloped within Saturn's powerful magnetosphere and during brief periods when it is exposed it retains protective remnants of Saturn's magnetic field. Even with powerful protection from Saturn's magnetosphere, solar radiation should have converted all of the methane in Titan's atmosphere into more complex hydrocarbons within as brief a period as 50 million years. So how is this moon replenishing its methane? Most researchers believe it is replenished through eruptions of volcanoes, which spew water and ammonia into the atmosphere, and which are perhaps fueled by internal radioactive heat or tidal forces exerted on the moon by Saturn. No volcanoes have yet been positively identified on Titan (except possibly very recently), however, leading some researchers to wonder if Titan is, in fact, a dead moon and that the methane is replenished simply through slow diffusion from its cold interior. A few bold researchers have questioned whether there might be a biological source for the methane. Titan lacks liquid water, an organic solvent thought by many researchers to be essential to life. However, its atmosphere is rich in organic compounds and it is known to be chemically active (which seems to me to be an argument against the dead moon theory). As of June 2010, scientists analyzing data from the Cassini/Huygens mission have found anomalies in Titan's atmosphere, which could be the products of methane-producing organisms. But they could also be due to abiotic chemical or meteorological processes as well. There is significant debate in the scientific community over whether methane could function as a biological solvent, making life, even on frigid Titan, possible.

If you enjoy imagining what it might be like to personally visit Titan, as I do, consider this intriguing tidbit: Titan's extremely weak gravity (0.0113 g at the surface) combined with its thick atmosphere means that you could (protected in a spacesuit with air supply of course) simply attach some fabricated wings, flap your arms, and fly! Perhaps you could carry with you instruments to investigate the moon's suspected methanological cycle, setting up base camp in a laboratory-equipped research balloon similar to the one proposed for the now-delayed Saturn System Mission, shown here.

Or, perhaps as suggested in an interview with Dr. Ralph Lorenz at NASA's Jet Propulsion Laboratory, you could set off exploring Titan's unique atmosphere and surface in a helicopter!

An interesting future career possibility don't you think?

Enceladus

Now that we've explored what we know (and don't know) about Titan, let's focus now on another Saturnian moon I have mentioned in this article. Enceladus is the sixth largest moon in the Saturn system, only a tenth the size of Titan. Both Voyager 1 and Voyager 2 flew by this moon but neither spacecraft was designed to slow down and achieve orbit like the Cassini/Huygens craft did. The two earlier spacecraft determined that Enceladus is water ice-covered, highly reflective and covered with smooth regions suggesting a youthful, and therefore active, surface. It appears as a bright white sphere deeply embedded in Saturn's E-ring, shown here.

Scientists had to wait for data from the current Cassini mission to answer why this moon was so active (it shouldn't be – based on its tiny size it should have cooled off and frozen solid long ago) and whether it might be the source of Saturn's E-ring (this is the outermost diffuse ring in the composite image of Saturn – the fourth image in this article).

Cassini performed four close flybys of Enceladus and found out some extraordinary things (curious mission specialists have now planned even more flybys, some as close as 25 km from the surface). The moon's south pole ejects massive amounts of water ice from what are called cryovolcanoes into space.  As well, these geyser-like jets contribute to a faint ionized water exosphere concentrated at the south pole where the plumes are located. This 2005 Cassini image dramatizes these incredible plumes.

Cassini was able to fly through Enceladus' enveloping gas cloud and confirm that it has the same chemical composition of minute trace elements as the E-ring, confirming that it does in fact contribute to it.

Close inspection of the southern surface of Enceladus reveals blue ice “tiger stripes,” shown here.

Scientists believe that the blue walls of fracture lines crossing the region expose outcrops of coarse-grained ice that appear blue compared to powdery surface ice covering the flat terrain. The coarse crystals are young (less than a thousand years old) and have recently been thermally altered. There are also some simple organic compounds present in them. Scientists think that water plumes from this fractured region come from pressurized subsurface chambers, and there is recent infrared spectrometer evidence from Cassini that these chambers may be as warm as -116°C. This finding suggests that the interior of the moon is a heat source, and this heat must come not only from the radioactive decay of heavy elements within the core but also from the continuous strain of tidal forces from Saturn. Enceladus is cyclically stretched and deformed as it orbits the planet, creating frictional forces acting all through it.

Warm low-density material rises from within the moon to the surface, within its icy shell (yellow) and/or its rocky core (red). Somehow, Enceladus rolled or rotated to place this area of low density at the south pole, where heat escapes within water plumes. No one is sure what caused this area of low density, perhaps an earlier impact.





Some scientists wonder if the escaping water comes from a subterranean water ocean, and whether this ocean could harbor life.

The highly successful Cassini/Huygens mission, emphasizing Saturn's mysterious moons, Titan and Enceladus, as well as some of Jupiter's moons, is explored in this excellent 25-minute video.



*A Brief Aside To Explore Magnetic Fields and Magnetic Moments

This is how it works: The magnetic moment of a magnet (in this case Saturn is a gigantic magnet) is the torque exerted on that magnet in a magnetic field. It's a vector force, just like the magnetic field force, but in this case it is the product of the strength of the magnet and the distance between its poles, in this case the gigantic diameter of Saturn itself. Perhaps it will help to think of it this way: Each pole of a magnet is a source of magnetic force. Like electrical force and gravity, it weakens with distance. Magnetic poles, however, are unique – they always come in pairs, equal in strength and opposite in type. Their forces interfere with each other so that while one pole attracts the other one repels. Intuitively you might guess that this interference is greatest when the poles are close to each other, for example, when the magnet is short, and you would be right. The magnetic force produced by a magnet at any given point in space depends on two factors – on the strength of its poles and on the distance separating them. This force is, in fact, proportional to the product of these two factors, and this is how we get to the magnetic moment. This is why Saturn's large diameter can mean a very large magnetic moment and a relatively small magnetic field.