On August 5, 2011, NASA launched the Juno Mission to Jupiter on board the Russian-designed Atlas V rocket at the Kennedy Space Center, shown here in this NASA image.
If you have read my articles on other planets such as Mars, Mercury and Saturn, you may have sensed that, as we learn more about the planets in our solar system, new questions, many of which we would never have thought to ask even a few years ago, present themselves. These evolving questions help direct us on future missions as we seek to understand the mysterious workings of our universe. The Juno mission is one such mission that has been designed to explore such big questions as planet formation and the deep structures within them.
This is a large and expensive mission, expected to cost about $1 billion. It was postponed from an earlier 2009 launch due to budget restrictions. This mission also requires patience, for it will take about 5 years to reach Jupiter and insert itself into a polar orbit around the planet. This is what Juno's trajectory will look like.
The ticks mark 30-day intervals, EFB means Earth flyby, DSMs means deep space maneuvers, and JOI means Jupiter orbital insert. The spacecraft is intended to make 33 highly elongated orbits, passing very close, within in 4300 km, of Jupiter's poles and then far out beyond the moon Callisto's orbit, almost 2 million km from the planet. This unusual orbit will help protect the crafts sensitive electronics from Jupiter's powerful radiation belts, which will be discussed in more detail shortly. In mythology, Juno is the wife of the god Jupiter. She has a special ability to peer through Jupiter's veil of clouds to see his true nature.
What is the true nature of Jupiter?
In this composite image of Jupiter taken by the Cassini spacecraft in 2000, you can see a famous storm at the lower right, called the Great Red Spot, and the shadow of one its moons, Europa, at the lower left.
Jupiter is huge. Its radius is about 1/10 that of the Sun. Its density is similar to that of the Sun as well, suggesting that, like a star, it is composed mostly of hydrogen and helium. Computer modeling, using what we know of the planet, suggests that Jupiter is likely as large as any planet with similar composition and evolutionary history can be. At about 1.6 Jupiter masses, a planet would shrink, despite its increased mass, due to increased compression under greater gravitational force. This shrinkage would continue with increased mass until nuclear fusion is ignited, at around 75 Jupiter masses (even though the smallest red dwarf star is only about one third more massive than Jupiter - a current puzzle, leading some astronomers to consider Jupiter a failed star).
We don't know if Jupiter was formed in a process similar to those in which multiple star systems are formed but we do know that Jupiter was much hotter and about twice its current diameter when it was very young. It has been cooling and shrinking slowly ever since, although it still generates more heat internally than it receives from the Sun through a process called the Kelvin-Helmholtz mechanism: When the surface of a planet cools, its pressure drops as a result and the planet shrinks. The compression, in turn, heats the core. Compressed gases contain more kinetic energy, and therefore more heat. An additional source of heat is a phase transition from gas to superfluid to metallic liquid, which is thought to occur deep inside Jupiter. In a planet, internal heat represents a transfer of gravitational energy into kinetic energy and it has nowhere to go, or dissipate, except through gradual surface radiation. Jupiter radiates infrared energy (heat) into space.
Jupiter completes one orbit around the Sun every 11.86 Earth years. This is two-fifths the orbital period of Saturn, resulting in a 5:2 orbital resonance between the two most massive planets. It has the fastest rotation of all the planets, one Jupiter day lasting a mere 9.9 hours. Its resultant equatorial bulge can be seen through an ordinary telescope.
Atmosphere
If you were to descend into the atmosphere of Jupiter, you would first encounter the thermosphere/ionosphere where layers of ions and electrons are bombarded by solar wind particles, creating phenomena like permanent polar aurorae, airglow and powerful X-ray emissions. Temperatures here reach up to 1000 K. For comparison, temperatures in Earth's thermosphere can reach up to 2800 K. The exterior of your spacesuit would not even feel warm in this layer because the there are so few atoms present to transfer heat.
Two main ingredients in the Jovian atmosphere are molecular hydrogen and helium, with trace amounts of various chemical compounds such as water, ammonia and hydrogen sulfide. Jupiter's atmosphere is organized into various zones and bands, optical effects caused by differences in the opacity of its clouds. The exact nature of the chemicals that make up the colourful bands is not known. Separating these bands are wind jets as fast as 360 km/h. Their vertical extent is unknown. The dynamics of the atmosphere are still unknown in general and no model yet exists to explain such phenomena as the narrow stable bands and jets and the origin and persistence of large storm vortices such as the Great Red Spot.
This is a simplified image of the bands and zones as well as the two largest storms (please ignore all the labels here).
Notice that the bands are concentrically symmetrical with respect to the equator (the reason for this is also unknown).
Juno will be able to penetrate the global structure and motions of the atmosphere below the cloud tops for the first time. We can see some of Jupiter's deep cloud structure using the Very Large Array Radio Telescope here on Earth but powerful radiation belts around the planet create radio noise that severely limits the quality of the images. Juno will be able to probe the atmosphere at low frequencies to test for the presence of various compounds. For example, water absorbs microwave frequencies (this is how a microwave oven works ? the energy of the microwave radiation is absorbed by water molecules and that energy, or molecular vibration, is heat) that should differ slightly depending on atmospheric depth. Juno's radiometer will measure these frequencies and assemble a three-dimensional model of water abundance in the atmosphere.
The Great Red Spot on Jupiter was first seen by Robert Hooke through one of the first simple telescopes in 1664, and ever since it has been a great mystery. What we do know is that it is a persistent anticyclone so large that two to three Earths could fit inside its boundary that is confined by two oppositely traveling jet streams. It is colder, and therefore peaks at higher altitude, than most other clouds on Jupiter. Currents around its edges can travel as fast as 432 km/h while there is often little movement at the center. Its colour has ranged from deep brick red to pale salmon and no one is sure what chemicals contribute to this colouration. The smaller pink oval storm depicted in the diagram above is officially called Oval BA. It was first seen in 2000 after the collision of three small white storms, similar to thunderstorms on Earth, and has intensified since then. It began to turn red in 2005, and the why of this too is a mystery. All the storms on Jupiter are associated with lightning, which is on average more powerful, but les frequent, than lightning on Earth. This time-lapse sequence taken by Voyager 1 as it approached Jupiter in 1979 shows how the various bands, zones and storms move relative to each other. Click on the image to begin its animation.
As you pass deeper into the stormy atmosphere of Jupiter you will (pretending you will not be crushed) encounter a gradual shift, as pressure increases, from a transparent inner atmosphere composed of molecular hydrogen gas into supercritical fluid hydrogen at a depth of around 1000 km. Pressure and temperature increase steadily as you descend toward the core. Hydrogen acts as both a gas and a liquid here. Eventually, another hydrogen phase transition occurs from superfluid into metallic liquid, at around 10,000 K and 200GPa. This thick layer, making up about 78% of the planet's radius, extends inward toward a possible core consisting of various dense elements. You should keep in mind that this interior description is based on modeling that admits a large degree of uncertainty. Again, it is hoped that the Juno mission will contribute much needed information about the inner composition of this planet. This is a diagram of what the current model looks like.
Liquid metallic hydrogen is a fascinating physical state of hydrogen in which individual protons and electrons are unbound and exist in a liquid system. The unbound electrons behave like conduction electrons in metals, and that is why this highly compressed state behaves like a metal. It is an excellent heat and electrical conductor whereas molecular hydrogen is very poor at conducting heat and electricity. The exact pressure at which hydrogen metalizes has yet to be experimentally verified and when it is, models of the interiors of Jupiter (and Saturn) will need to be adjusted. How close metallic hydrogen approaches these planet's surfaces will, in turn, affect how close to the surface these planet's powerful magnetic fields are created. That, in turn, will affect magnetic field and atmospheric models of these planets.
Jupiter's Magnetosphere
As mentioned above, Jupiter exhibits a powerful magnetic field, the most powerful in the solar system except for the Sun. Like Saturn, flowing electric currents within Jupiter's thick inner layer of metallic hydrogen are believed to be the source.
Jupiter's magnetic field creates an enormous cavity, called a magnetosphere, in the solar wind passing by it. Jupiter's magnetosphere is a bit different from Earth's. Our magnetosphere is shaped primarily by the solar wind, whereas Jupiter's magnetosphere is influenced by the interaction of large amounts of gaseous sulfur dioxide emitted by volcanoes on one of its moons, Io, as well as its own rotation. These sulfur dioxide gas emissions form a large torus around Jupiter. Jupiter's magnetic field causes it to co-rotate with the planet, and, in the process, the accelerated torus of ionized gas loads the magnetic field with plasma, which generates permanent aurorae around the poles as well as powerful radio emissions and intense belts of radiation thousands of times stronger than Earth's Van Allen belts. This intense radiation affects the surfaces of Jupiter's moons and Jupiter's tenuous and faint ring system. It also presents a serious hazard to spacecraft and any potential future orbiting human explorers. Juno will study these radiation belts. By necessity, it is one of the toughest spacecraft ever built, equipped with a protective carapace designed to protect the sensitive instruments it is carrying. This is a 2010 artist's concept of what it will look like orbiting Jupiter.
Jupiter has 64 named moons, 47 of which are insignificant, less than 10 km in diameter. Jupiter's four largest moons, named after the lovers of the Greek god Zeus (Greek predecessor to the Roman god Jupiter), are Io, Europa, Ganymede and Callisto. They were discovered in1610 by Galileo Galilei.
Jupiter's moons exhibit a huge range of orbital periods and orbital distances. Their orbits vary from near perfect circles to highly eccentric and inclined, with some orbiting opposite the direction of Jupiter's spin (retrograde). Eight of Jupiter's moons, including among them all of the Galilean moons, exhibit regular circular prograde orbits and are believed to have formed from the protoplanetary disc during Jupiter's formation. The others are likely to have been asteroids captured into orbit. Many would have collided with each other or broken up by the stress of capture, creating the wide variety of satellites we see today. Of the Galilean moons, Io, Europa and Ganymede exhibit a 1:2:4 orbital resonance that stabilizes and protects their orbits.
Ganymede
Ganymede, named after a divine Greek hero described by Homer as the most beautiful of the mortals, is the largest moon in the solar system, about 8% larger than Mercury (but less than half of that planet's mass - remember that Mercury is believed to have been struck early in its life, stripping away most of its crust and mantle and leaving mostly its dense metallic core). This is what Ganymede looks like.
Ganymede has a tenuous and thin oxygen atmosphere, not caused by life, as it is on Earth, but by the splitting of water ice on the surface into oxygen and hydrogen by radiation (a process called radiolysis) and the more rapid loss of much lighter hydrogen into space.
The ESA Orbiter also plans to study Ganymede's magnetic field in detail. Ganymede creates a small magnetosphere imbedded within Jupiter's much larger one. Scientists are unsure how a liquid metal core could have persisted this long within such a small body but it is believed to be a dynamo set up by convective electrical currents that is the source of the magnetic field (this is also how Earth's magnetic field is created). Past orbital disturbances mentioned above may have helped to keep the core hot and molten. It's also possible that Ganymede's relatively weak magnetic field comes from remnant magnetization within its silicate rock in the deep mantle from a dynamo-generated field in its past.
Ganymede was believed to have formed rapidly as Jupiter itself was forming from a disk of hot gas and dust, probably taking no more than 10,000 years to form. Accretional heat didn't have much time to dissipate as it formed, so it started off as a molten ball with denser metals sinking into a well-differentiated core. In contrast, Callisto, another Galilean moon, looks much different from Ganymede, and that difference is most likely because it formed much more slowly and as a result did not melt and differentiate into core, mantle and crust as Ganymede did. This had a large impact on Callisto's later evolution as we will see.
Callisto
Callisto, like Ganymede, was discovered by Galileo in the 1600's. It is named after a mythological Greek nymph, one of Zeus' lovers. It is a heavily cratered dark moon, composed of roughly equal amounts of rock and ices. But it does not participate in the resonance of the other three Galilean moons and, as a result, does not undergo tidal heating. This is what it looks like.
Despite its lack of tidal heating, magnetic field studies of the moon indicate the possible presence of a salty ocean from 50-200 km thick beneath a layer of ice as thick as 150 km. If does exist, dissolved ammonia or some other antifreeze might prevent it from freezing. This does not mean that Callisto's ocean would be warm by our standards. The melting temperature of ordinary ice decreases with pressure. At 2000 atmospheres it can exist as a liquid at -22C. Even 1-2% ammonia would decrease its melting point substantially lower. If this ocean exists it is probably heated only through radioactive decay within its rocky material. Scientists place the probability of life on Callisto lower than Ganymede or Europa (which we will explore next), because Callisto experiences no heat flux from its interior, and its ocean doesn't appear to have much contact with rocky material, a vital source of organic compounds. Beneath this ocean lies an only partially differentiated core composed of a mixture of rocks and ices. Unlike Ganymede, Callisto has no internal dynamo and no magnetic field. It has never been heated to the point of melting during its evolution. It probably formed by slow (between 0.1 and 10 million years) accretion of gas and dust surrounding Jupiter after it was formed.
Callisto has a tenuous atmosphere like Ganymede but it is composed of carbon dioxide rather than oxygen, most likely replenished by sublimation of frozen carbon dioxide on the surface.
Although Callisto itself might not seem terribly exciting, it could serve as a future outpost from which to explore Europa, an extremely interesting moon as we will soon discover. A conceptual study done for NASA, called Human Outer Planets Exploration, explores the possibility of a human base on Callisto. It's geologically stable and it has low radiation. A gravity assist from Jupiter makes travel to Europa and other bodies in the outer solar system relatively doable, so a spacecraft port there could conceivably be very useful. Perhaps it will look like this artist's conception.
Europa
Europa, slightly smaller than our moon and one of the Galilean moons, has become an object of intense interest. This is an image of it taken by the Galileo spacecraft in 1996. Europa is named after a mythical Phoenician noble woman and another of Zeus' lovers.
Europa has a very stable orbit thanks to its resonance with two other moons. However, its orbit does have some eccentricity, enough to periodically stretch it, causing tidal stress (a frictional force) to heat its interior, keeping its ocean liquid enough to drive convection and possibly driving tectonic-like forces on its surface.
Europa is made of similar stuff as Earth; it is composed primarily of silicate rock. Unlike Earth however, it is entirely covered by an outer layer of water and ice, which may be as thick as 100 km. This water is probably salty, based on tests of its induced magnetic field, the result of interaction between a subsurface conductive layer and Jupiter's magnetic field. Whether Europa has a molten metallic liquid core or not remains unknown, but if Europa does have a dynamo driving a magnetic field, it appears to be overshadowed by Jupiter's magnetic field which reverses in Europa's position every 51/2 hours. Europa's magnetic north pole, located near its equator reverses into a south pole every 51/2 hours.
In late 2008, a new kind of tidal force on Europa was suggested. Large slow-moving tidal waves, called Rossby waves, moving only a few kilometers per day may move through Europa's ocean. Though slow, such waves would generate a great deal of kinetic energy. This energy could dissipate and heat the ocean.
Europa has a tenuous atmosphere composed of molecular oxygen, produced in much the same way as Ganymede's thin oxygen atmosphere. However, and this is intriguing, some of Europa's oxygen produced by radiolysis of its surface water may make it into its ocean (based on surface studies by the Galileo spacecraft and assuming that some interaction occurs between the surface and subsurface ocean, perhaps through the tectonic-like activity suggested above). Oxygen, rich with energy stored in its chemical bonds, could fuel life processes. Some new research models do indeed suggest that Europa's ocean might have more than enough oxygen to support even fish-sized life. This possibility makes Europa number one in our solar system for potential life, aside from Earth. The ingredients are there: liquid water, organic compounds, and a supply of chemical energy. These simple ingredients support microbial life here on Earth in deep ocean hydrothermal vents and in lake Vostok. No sunlight is needed in order for life to flourish in either environment. In fact, macrobiotic organisms such as giant tube worms, clams and mussels have found niches in hydrothermal vents.
Future Missions to Europa
The Europa Jupiter System Mission will focus on studying Europa and Ganymede for signs of alien life. This mission is slated for launch in 2020 (but, as mentioned, it may have to be altered or postponed).
This video from NASA illustrates this mission's objectives:
This mission will be the first one of its kind to employ two separate spacecraft, each operated independently by its own organization and each gathering its own data, working toward a common set of scientific goals. Many features of the Jovian system will be explored with special focus on Europa and Ganymede. They will be studying not only the habitability of these two moons but also how the gas giant, Jupiter, evolved. This is a conceptual drawing of what the two craft may look like.
NASA announced in April this year that the joint mission might not happen because of budgetary cuts but ESA has recently suggested that it might proceed with a European-led mission. The Russian Federal Space Agency is considering adding an independent segment to the mission, a Europa Lander, which would drill into Europa's surface ice and study its composition (as I understand it, it will not be designed to fully penetrate the ice into the ocean below - this will have to await some unannounced future mission).
Io
Last, but certainly not least, Io, named after a mythological priestess of Hera, another of Zeus' many lovers, and the fourth of the Galilean moons of Jupiter, is the single most geologically active object in the solar system. The Galileo spacecraft took this image of the Jovian moon. It passed within 200 km of Io, a hazardous journey even as a flyby because the spacecraft received three times the radiation it was designed to withstand, as Io is within Jupiter's intense plasma torus.
Sulfurous compounds on the surface colour this moon yellow. Sulfur-containing compounds at different temperatures and of different compositions also impart greenish, reddish, brown and black regions to the moon, inspiring some to call it the "pizza moon." All the pockmarks are not impact craters but rather volcanoes. There are over 400 active volcanoes on it right now. Io is the innermost of the four Galilean moons, orbiting a mere 422,000 km from Jupiter, so close that it is continuously and brutally stretched and distorted by Jupiter's intense gravitational pull. This internal frictional force, along with tidal forces from Ganymede and Europa, drives its extreme volcanic activity. If you look at the dark spot just to the right of center you can see a volcano erupting, with its plume of sulfur and sulfur dioxide reaching as high as 500 km into space and a ring of bright yellow newly deposited sulfur surrounding it. Material from the many volcanoes contributes significantly to the large plasma torus around Jupiter, and it produces a thin atmosphere around Io itself. The images below of Io's surface taken by the Galileo spacecraft in November 1999 (left) and again in February 2000 (right), show the progress of an active lava flow, this lava being hotter (1800?C) than lavas produced here on Earth (most are around 800?C). Most of Io's lavas are made of basalt, similar to lavas on Earth, but a few lavas consisting of sulfur and sulfur dioxide (such as the one noted above) have also been observed.
The white sections above on the left represent areas of lava so intensely hot that they overpowered the camera's detector.
A cloud of neutral sulfur, oxygen, sodium and potassium atoms also surround Io at a distance of up to 6 Io radii from the moon surface, a space where Io's gravity is stronger than Jupiter's gravity, although some atoms continuously leak into Jupiter's plasma torus. When Io crosses Jupiter's powerful magnetic field lines, the lines couple Io's atmosphere and neutral cloud to Jupiter's upper polar atmosphere. When this happens an electric current of up to 3 million amps, called a flux tube, produces an aural glow at Jupiter's poles, called the Io footprint, and aurorae in Io's atmosphere (as well as lightning in Jupiter's upper polar atmosphere). Because of where Earth is situated with respect to Jupiter and Io, we receive extra powerful radio transmissions (produced by the flux) from Jupiter when Io is visible to us. As this occurs, Jupiter's powerful magnetic field strips off as much as 1000 kg of material from Io every second, generously seeding Jupiter's plasma torus. This doughnut-shaped cloud of intense radiation inflates Jupiter's magnetosphere to twice the size it would be based on Jupiter's magnetic field alone.
Jupiter's magnetic field lines that get past Io's ionosphere induce an electric current, and resulting induced magnetic field, within Io. It is believed to be generated inside a partly molten silicate magma ocean about 50 km below the moon's surface. The Juno mission headed to Jupiter will explore these interactions between Jupiter and Io in more detail.
Io's orbital resonance with Europa and Ganymede keep it stable in an eccentric orbit that fuels its tidal heating. Without the other moons, Io would eventually have stabilized in a much more circular orbit, and would not be nearly as active.
Io is a bit larger than our moon. Its composition more closely resembles the inner rocky planets than other outer solar system moons. Those moons tend to be a mix of water ice and silicate rocks, whereas Io is unusually dense, even denser than our moon. It is believed to be highly differentiated and made up of a substantial iron-rich core surrounded by a mantle and crust of silicate-rich magma and rock. Io's core does not produce a dynamo, so it is not convective. Whether it is molten or not is unknown, but the tidal forces exerted on Io point to at least partial melting.
Io's atmosphere is very thin and composed mostly of sulfur dioxide, with some sulfur monoxide, sodium chloride, and atomic sulfur and oxygen. Io's volcanoes contribute most of the sulfur dioxide, but its atmospheric concentration is also maintained by sunlight-driven sublimation of frozen sulfur dioxide on the surface. Despite its many volcanoes around which the ground literally sizzles, the surface of Io is very cold, averaging around -150?C. The negligible atmosphere does nothing to trap in heat. The plumes from its many volcanoes probably freeze almost instantly and drift back down as sulfur dioxide snow. It is interesting and unusual for a volatile compound such as sulfur dioxide to pile up and blanket Io's entire surface when it would be expected to be lost in space or perhaps concentrated in Io's cold polar regions. On moons and on rocky planets with exospheres, volatiles such as light gases eventually escape into space. Even though much sulfur dioxide is spewed from Io's volcanoes and lost in space, a significant amount of it also snows back to the surface and accumulates there. In Earth, sulfur is just as abundant as an element as it is in Io, but here on Earth most of it is bound up in sulfide and sulfate minerals. In a way, volcanism has turned Io inside out. Over millennia, the moon has been extracting its sulfur in a process that seems to resemble the ancient Sicilian process, in which sulfur-rich rocks from volcanic regions were piled up and heated in hilltop kilns, allowing sulfur deposits to melt and run down the hills. Io may be maintaining a process similar to what happened on Earth and our Moon billions of years ago, when both likely had similar liquid magma oceans and extremely hot sulfur-spewing volcanoes, but on Earth and the Moon, these processes eventually quieted down while Io, in a tug of war between Europa and Ganymede, has been locked in this hellish state. Considering that Io very likely originated from the same protoplanetary disc as Europa and Ganymede, it is fascinating to see how different it is from these water ice/rock moons.
0 comments:
Post a Comment