Geology Exam 3

Two different models for the internal structure of Europa:

Most scientists believe Europa has an ocean of liquid water beneath a hard icy crust that is a few kilometers up to ~30 km thick. However, it is possible that the interior of Europa is warm enough to soften the ice below the surface, but not cause it to melt. Solid-state convection of soft ice could produce the grooved features observed at the surface.

Pluto

Pluto is too distant for us to learn much from even the best telescopes. Prior to New Horizons, we could tell very little about its surface. We knew that the surface varies in terms of brightness and color, so distinct terrains are present. Pluto has a reddish color, probably due to organic molecules on the surface that form due to exposure of methane and ammonia ice to UV radiation. This model image was composed using data from the Hubble Space Telescope. Before the New Horizons mission, this was pretty much all we could say about the surface of Pluto.

Pluto and Other Kuiper Belt Objects

Pluto is far from unique. Other Kuiper Belt Objects similar in size to Pluto have been discovered. The largest known Kuiper Belt Object is Eris, which has a diameter of ~2300-2400 km. The large number of objects with similar sizes and orbits to Pluto is one reason that Pluto has been demoted from "planet" to "dwarf planet". If we let Pluto in the club, everyone will want to join. So, we'll have to make do with just eight planets instead of nine, right?

Extremophiles

Thermophylic (heat-loving) bacteria in a hot spring Halophylic (salt-loving) bacteria in evaporation ponds.

Scientists think there is liquid water beneath the surface ice layer on Europa, but how do we detect something we cannot see?

Another way of probing the interior of these moons is to measure how they interact with Jupiter's magnetic field. The magnetic moment of Europa requires that the planet contain a layer of electrically conductive material below the surface. Ice is a poor conductor of electricity, but salt water is a good conductor, so Europa's magnetic moment supports the existence of a liquid water layer.

Saturn Atmosphere

Like Jupiter, Saturn's atmosphere has a banded structure, although the bands are less pronounced than on Jupiter. The atmosphere is also very dynamic, constantly changing as storms develop. Giant storms, like the one shown to the left, are driven by convection , and transport heat from the interior. This storm caused an increase in temperature of the "surface" of 150 K, equivalent to going from Fairbanks, Alaska in the winter to the Mojave Desert in summer.3

Plutos Atmosphere

Like Triton, Pluto's atmosphere is mostly made of nitrogen. It has several haze layers and is much more complex that was expected.

Galilean Moons Summary

1)Outer moons have low density, are composed of mixtures of rock and ice, whereas inner moons are mostly made of rocky material. This variation reflects variations in temperature in the disk of material from which the moons formed. 2)Strong radiation produced by Jupiter's magnetic field plays an important role in the evolution of the surfaces of the moons, leading to breakdown of surface water ice, production of oxygen atmospheres for the icy moons, and Io's plasma torus. 3)The degree of internal differentiation and surface reworking of each moon is controlled by the amount of tidal heating. The inner moons are heated to a greater extent. Callisto receives the least tidal heating because it is not locked into an orbital resonance with the other moons.

Models for the age and origin of Saturn's rings:

1)The rings are an ancient feature (~4.5 Ga) resulting from destruction of a planet or moon that entered the Roche Limit around Saturn. Saturn's moons, like Jupiter's moons, should be made of a mixture of rock and ice. If a moon disintegrated, we should see both the rocky and icy components preserved in the ring material. 2)The rings are an ancient feature composed of material left over from the original disk of material that surrounded Saturn during accretion. The moons are mixtures of rock and ice because that is what was present in the disks surrounding Jupiter and Saturn during their formation. If the rings are leftover disk material, where is the rocky component? 3)The rings are an ancient feature resulting from partial or complete disruption (destruction) of one of Saturn's moons due to a giant impact. 4)The rings are a young (less than a few 100 million years) feature resulting from partial or complete disruption (destruction) of one of Saturn's moons due to a giant impact. If the moon had differentiated into a rocky core and icy mantle, and if the impact ejected material from the mantle portion into the ring (similar to formation of our own moon), then this would be consistent with the icy composition of the rings. Complete disruption, however, would result in rocky material present in the rings, which we don't see.

Graben

A graben is a geologic structure that forms when the crust is pulled apart. In contrast, if the crust is pushed together, we tend to see ridges, folded terranes, and mountains. Think of what a rug would look like if you pushed it. On several icy worlds like Ariel we see lots of evidence for grabens(crustal extension), but less evidence for landforms created by compression. How can the crust be under tension everywhere? In winter, freezing water in pipes can cause the pipes to burst due to the expansion of ice.The expansion of water that occurs when a sub-surface ocean freezes can cause the solid crust of an icy moon to "burst" in the same way.

Cassini Huygens Missions

Although both Voyager 1 and Voyager 2 made fly-bys of Saturn, much of what we know about the planet, it's moons and rings is the result of the Cassini Huygens mission, a joint NASA-ESA-ASI mission launched in 1997. The spacecraft went into orbit around Saturn in 2004 and studied the planet and its moons until September, 2017. Saturn is too far from the sun for spacecraft to be powered by solar panels. To provide the energy needed for Cassini and other deep space probes, engineers use RTGs, or radioisotope thermoelectric generators. The Cassini RTG used pellets of plutonium to produce heat, which is converted into electricity.

Preliminary answers to some of our questions

Are planets common or rare around other stars? Definitely fairly common. At least 20% of stars likely have planets. How many planets are out there?The number of planets in our galaxy probably is well over 100 billion (maybe several hundred billion). How typical is our solar system? Does the process of planet formation/evolution differ around other stars?Too early to say. However, there definitely are systems that look very different than ours (e.g., giant inner planets). How many planets in our galaxy might be hospitable for life?Too soon to tell, but the fact that we are already finding potentially habitable planets, despite the fact that our methods are not well suited to finding such planets, suggests the number could be very large, ie., 10s of billions in our galaxy. Is life rare or common in the Universe? How can we look for life? We'll discuss this next. The short answer is we really have no idea, but we do know that the basic building blocks for life (water, organic molecules) are relatively common. How easy it is for these building blocks to come together to produce life is impossible to tell at the moment.

Extrasolar Planets

Are planets common or rare around other stars? How many planets are out there? How typical is our solar system? Does the process of planet formation/evolution differ around other stars? How many planets in our galaxy might be hospitable for life? Is life rare or common in the Universe? How can we look for life? We are only now beginning to develop the techniques that will allow us to answer these questions. The next few decades should see significant progress towards answering each of these questions.

Drake Equation

Are there any other "technological civilizations" out there that we could potentially detect, perhaps by picking up the alien equivalent of "I love Lucy"?The Drake Equation was proposed by Frank Drake as a way of thinking about the different variables involved in estimating the likelihood of detecting alien civilizations. The Drake Equation is more thought experiment than scientific hypothesis, because many of the variables involved are completely unknown, so you can put in any numbers you want and conclude either that intelligent life is very abundant, or that we are completely unique.

Grand Tack Model

As the outer planets formed, they initially drifted inward due to interaction with the solar nebula. At a later stage, Jupiter and Saturn entered into a 3:2 resonance, which pushed them and the other outer planets outward. Computer models suggest that as the outer planets formed, Jupiter and Saturn evolved into a 1:2 resonance that disturbed the orbits of the other giant planets. The orbits of Uranus and Neptune migrated outward, scattering small planetesimals that existed in the outer regions of the solar system. A fraction of these planetesimals ended up in the Kuiper Belt, a region extending beyond the orbit of Neptune that is home to Pluto and many other icy worlds and the source of short-period comets.

Uranus and Neptune shouldn't exist!

At the distance from the sun where Uranus and Neptune are today (~20 and 30 AU) there should have not been enough material available to form these planets. Scientists now think that Uranus and Neptune formed closer to the sun, and migrated outwards due to gravitational interactions with Jupiter.

So how did we go from a pre-biological soup of amino acids and other random organic compounds, and end up with a collection of self-regulating, self-replicating systems (e.g., living organisms)?

At the moment, we don't know, but there are some hypotheses (some more colorful than other). There are lots of problems in nature where we don't yet have an answer. That is how science works. The surfaces of clay minerals can act as catalysts to speed organic molecule polymerization. Early Earth was likely rich in simple organic compounds. Turning these compounds into more interesting building blocks for life (proteins, nucleic acid chains) requires linking the simple compounds together, a process called polymerization. The surfaces of clay minerals can act as catalysts to speed organic molecule polymerization. Several natural processes may have helped concentrate organic compounds and aided polymerization. Hydration/dehydration cycles in evaporation ponds or free/thaw cycles have been shown to be effective at triggering polymerization. Once complex molecules form, they may begin to "self-organize". For example, phospholipids have a natural tendency to form membrane-like structures due to their dual hydrophylic/hydrophobic structure. When we reduce life to it's component metabolic parts, the boundary between biology and chemistry becomes very blurry, and we can identify a number of abiotic mechanisms for generating structures that today we associate with life. We don't know the complete recipe (yet), but we do have a good idea of what some of the basic steps are. The truth is that the line between biology and chemistry is a lot more blurred and gradual than we might like to think. Amino acids => proteins Nucleic acids => RNA Phospholipids => membranes Put the right combination of these together, and you have a simple cell. We don't know how life emerged from an initially lifeless "soup" of organic molecules, but the "Tree of Life" may provide some clues. Many of the most primitive life forms are "extremophiles", living in very hot or very salty environments. Does this provide a clue to the environment where life started?

Io Atmosphere

Because of Io's volcanism, the moon has a very thin atmosphere composed mostly of sulfur and sodium compounds. These components are ionized by radiation from Jupiter, and swept up in Jupiter's magnetic field. The result is a plasma torus composed of charged particles that flows between Io and Jupiter. Basically, these two worlds are bound by an electric current of several million amps. Up to 1000 kg of material is removed from Io by this process every second! Temperatures within the torus are hotter than the sun! Because of interaction between Io and Jupiter's magnetic field, the surface of Io receives ~3600 REM (roentgen equivalent in mammal) of radiation daily. The lethal dose for a human is ~400-450 REM! Radiation exposure at Io for more than ~2 hours would be fatal (assuming the lack of oxygen, etc. didn't kill you first).

Pluto and Triton Structure

Both Triton and Pluto are thought to have similar, differentiated internal structures. Based on their density, these bodies are probably composed of ~50-70% rocky material, with the rest being icy material. In addition to water ice, CO2, CO, and N2 ice are also present. Layer 1: Frozen nitrogen Layer 2: Water ice Layer 3: Rock and iron Radioactive decay of naturally occurring elements like potassium, uranium, and thorium generates heat. These elements are concentrated in the rocky portions of planets and moons. Depending on the initial thermal structure of Pluto, the heat might be enough to melt a layer of water at the base of Pluto's icy mantle.

A search for life on Earth from the Galileo spacecraft

CARL SAGAN*, W. REID THOMPSON*, ROBERT CARLSON†, DONALD GURNETT‡ & CHARLES HORD§ In its December 1990 fly-by of Earth, the Galileo spacecraft found evidence of abundant gaseous oxygen, a widely distributed surface pigment with a sharp absorption edge in the red part of the visible spectrum, and atmospheric methane in extreme thermodynamic disequilibrium; together, these are strongly suggestive of life on Earth. Moreover, the presence of narrow-band, pulsed, amplitude-modulated radio transmission seems uniquely attributable to intelligence. These observations constitute a control experiment for the search for extraterrestrial life by modern interplanetary spacecraft.

Callisto

Callisto (shown here) has a radius larger than our moon (2403 km vs 1738 km), though it is much less massive because of its low density (1.85 g/cm3), reflecting its composition of a mixture of rock and ice. Its surface is heavily cratered, revealing that Callisto has been inactive for a long time. Measurements of Callisto's moment of inertia by the Galileo spacecraft suggest that Callisto is not fully differentiated. It may have a very small rocky core, but most of the interior consists of a mixture of rocky and icy material, with the ratio of rock to ice slowly increasing with depth. Crater erosion on Callisto may also be driven by gradual removal of surface water via both sublimation and chemical destruction. Many craters are unusually shallow and have ragged, ill-defined rims, giving the planet a "knobby" appearance. Some of these features may have formed at a time when Callisto's interior was still relatively warm and soft, not cold, hard, and brittle as it is today. However, others probably formed by sublimation of water ice exposed at the surface. As the ice sublimates, the rocky material is left behind. This same process accounts for variations in surface albedo. Knobby terrains on Calisto reveal many spires that likely are remnants from crater rims. Spires are bright and icy. Darker material fills low-lying areas.

Umbriel

Diameter ~1172 km Density ~1.39 g/cm3 Lots of craters Meh.

Earth

Earth is shaped like an onion (sort of).Meaning, it is an oblate spheroid. Earth's radius as measured from the center to the poles is slightly smaller than measured from the center to the equator. The reason for this is the centrifugal force created by Earth's rotation, which is strongest at the equator and weakest at the poles. It turns out that Pluto is almost perfectly spherical in shape (like a basketball). It doesn't look like an onion at all.

Oxygen and Methane

Earth's reflectance spectra varies over time, and not just because of the waning and waxing of ice cover. In winter, parts of North America not covered by snow appear brown. In summer, they look green. No known mineral reactions can do this, but chlorophyll can. Simultaneous presence of methane and oxygen in our atmosphere is a result of biologic activity. Earth's atmosphere contains significant levels of methane, which is chemically out of equilibrium with the high atmospheric oxygen levels. The reason for this is that biologic processes constantly replenish the methane in our atmosphere. The higher levels in the tropics are due to high levels of biologic activity.

Enceladus

Enceladus' surface reveals a complicated array of cross-cutting fractures. In 2005, the Cassini spacecraft detected geysers emitting water plumes from near the south pole of Enceladus. The geysers erupt from a series of fissures similar to those in this photo. Geysers erupting from the southern "grooved terrain" of Enceladus. In 2005, the Cassini spacecraft detected plumes of icy particles emanating from grooved regions on the moon's surface. These are thought to derive from "cold geysers" powered by tidal heating. In 2009, Casinni detected ammonia, which may help explain the cryovolcanism. Ammonia acts as an antifreeze, allowing water to stay liquid down to much lower temperatures than pure water. Recent spectral analysis of the ice surrounding the geyser vents on Enceladus indicate the presence of salt. Salt has also been detected in the icy plumes ejected from the vents. This indicates that the liquid water beneath the surface is salt water Salt water suggests the water has reacted with rocky material and leached certain minerals out of the rocks. This suggests that Enceladus' "ocean" is located at the boundary between it's outer icy layer and rocky core. Enceladus' plumes also appear to contain a mixture of simple and more complex organic compounds, including methane (CH4), propane(C3H8), acetylene (C2H2) and formaldehyde (CH2O). These compounds may form as a result of water/rock chemical reactions. Enceladus is locked in a 2:1 orbital resonance with Dione, another of Saturn's moons. This resonance produces tidal heating ,which may explain Enceladus' cryovolcanism. Enceladus experiences tidal heating due to a 2:1 orbital resonance with Saturn's moon Dione. However, calculations suggest this tidal heating should produce <1.5 gigawatts of power. Thermal images of the south polar region of Enceladus reveal much greater heat output.

Europa

Europa is denser than Ganymede and Calisto (2.99 g/cm3), suggesting it is made primarily of rock rather than ice. However, it has an icy surface that is only lightly cratered and which is covered with a very complex network of cracks, ridges, and grooves. Young (sparsely cratered), complex terrains suggest Europa's surface may have been reworked by cryovolcansim or by tectonic processes. Definitive evidence for active cryovolcanism on Europa has not yet been found, but a number of structures certainly look "suspicious", such as the blister-like features seen below. Europa's proximity to Jupiter and gravitational interaction with the other Galilean satellites results in tidal heating, which warms Europa's interior. Europa's icy crust may be quite thin and hide an ocean of liquid water several hundred km thick. Some portions of Europa's crust resemble icebergs embedded in a frozen sea. Compare the picture above of the Conamara Chaos region on Europa with the satellite photo to the right of the Wilkins ice shelf here on Earth.

Does this planet support life?

Evidence from the Galileo spacecraft suggests that this may be likely. Evidence includes: 1)Strong absorption of red wavelengths of light (especially over continents => chlorophyll); 2)Detection of significant O2 (and O3) in atmosphere; 3)Concurrent detection of methane (CH4) in atmosphere 4)Detection of modulated narrow-band radio waves with no known natural source. Strong chemical disequilibrium is often considered a "biomarker". E.g., the simultaneous presence of oxygen (ozone) and methane in the atmosphere.

Ganymede

Ganymede is the largest Galilaean satellite (radius = 2634 km). It is similar in composition to Callisto (e.g., mixture of rocky and icy material. Although parts of Ganymede's surface are heavily cratered, others are less so. In addition, massive fractures and grooves cut across the planet, suggesting a much more active past than observed for Calisto. Unlike Callisto, Ganymede appears to be fully differentiated, with an iron core surrounded by layers of rock and then ice. Ganymede may also have a salt-water "ocean" 200 km below the surface, sandwiched between layers of ice.

Europa

Future missions may look for organic compounds in the ice of Europa and Enceladus in places where liquid water has recently been brought to the surface. Although many non-biological processes can generate organic components, the types and ratios of these compounds may serve as "biomarkers".

Ganymede Magnetic Field

Ganymede is the only moon in the solar system known to possess its own internally-generated magnetic field, although Europa might have a weak field as well. Ganymede's magnetic field is thought to be generated by convection in its molten iron core, just as Earth's magnetic field is generated. The Galileo spacecraft discovered that Ganymede not only has a magnetic field, it also has a very tenuous atmosphere composed of oxygen. In fact, the combination of a magnetic field and an atmosphere results in aurora on Ganymede. On Earth, our atmosphere is rich in oxygen due to photosynthesis. Does this mean there is life on Ganymede? No. On Ganymede, oxygen is released from the surface by the photolysis of water: 2H2O => 2H2 + O2.The hydrogen escapes into space, but the oxygen escapes more slowly, resulting in a very thin atmosphere.

Although the jury is still out, from what we know about the basic chemistry of life and the speed with which life originated on Earth, my personal opinion is that life is probably fairly common. But what kind of life?

Given the history of life on Earth, primitive, single-celled organisms are probably a lot more common than multi-cellular organisms. For most of Earth history, life on Earth has been dominated by simple, single-celled prokaryotes like bacteria and Archaea. Stromatolites like these shown from Shark Bay, Australia are common in the Proterozoic fossil record. For nearly 2 Ga after the origin of life, bacteria and archaea ruled the world (in many ways, they still do). Stromatolites (right) are mounds of bacterial that grow in intertidal zones. Fossilized stromatolites have been found in 2.7. billion year old rocks that look almost identical to stromatolites living today.

Gliese 581

Gliese 581, a tiny red dwarf star about 20 light-years from Earth (that's practically next door by galactic standards) is possibly one of the most exciting places in our galactic neighborhood, and is currently the subject of intense scientific scrutiny. Why? Size comparison of Gliese with our own sun. Observations of Gliese 581 reveal it has at least six planets! Furthermore, at least one and possibly two of these planets appear to orbit within the habitable zone, the region around a star where a planet with the proper atmosphere could have liquid water (necessary for life to evolve). Gliese 581 g has been described as "the most Earth-like goldilocks planet yet discovered." Distance = 23.6 light-years (close) Mass = 4.9x Earth mass Radius = 1.9x Earth radius Surface temp. (estimate) = 27 0C Orbital period (year) = 28 days Age (estimate) = <2 Ga

Mars

Have we discovered life on Mars? No, at least not yet. Was there once life on Mars, or is there life there still today? We don't know. But, the basic requirements of life: water, organic matter, and energy sources (in the form of minerals and chemical compounds that can be broken down to release energy) are all present.

Iapetus

Iapetus ,with a diameter of 1470 km, is Saturn's 3rd largest moon. Iapetus is characterized by a "two-tone" appearance (one side is dark, one side bright) and prominent equatorial bulge. Why is one side dark and the other bright? As Iapetus orbits Saturn, it passes through a disk of dusty material known as the Phoebe ring. The leading face of Iapetus accumulates this dust, which darkens the surface. The darker face absorbs more sunlight and warms up relative to the brighter trailing face. Ice sublimates from the warm face and refreezes on the cooler trailing face. This leaves a lag of darker material on the leading face and fresh, bright ice on the trailing face, producing a positive feedback. The other prominent feature observed on Iapetus is a large equatorial bulge, shown above. Several theories have been proposed for the origin of this bulge, though none are fully satisfactory. One leading theory suggests that the bulge formed at a time when Iapetus was rotating more rapidly than it is today, forming the bulge through centrifugal effects.

Pluto and Charon Liquid Ocean

If Charon once had liquid water in its interior, then Pluto probably did as well, but can we prove it? And, is it still there? To figure this out, we need to discuss why Earth is shaped like an onion, and Pluto is not. Pluto and Charon are tidally locked, meaning the rotation period of both exactly matches the orbital period of Charon around Pluto (or, more accurately, Charon and Pluto around their shared center of mass). But what does this have to do with Earth being shaped like an onion? Although Pluto and Charon are tidally locked today, they initially would have had a much higher rate of rotation, which would have produced a tidal bulge. However, if Pluto was completely solid and rigid when it slowed down, it would have preserved a "fossil" bulge. The lack of a bulge may indicate that Pluto was (or still is) partially soft or liquid inside, so it could reform into a sphere as its rotation slowed. Extensional features and evidence for cryovolcanism also support the idea that Pluto may have had a liquid ocean in its interior for much of its history. This image shows a mountain with a large pit in the middle, and smoother terrane along the northern flank. This is thought to be a cryovolcanic vent. Other streak-like features elsewhere on Pluto may have been produced by geysers.

Predictions:

If no ocean ever present:equatorial bulge should exist from early rapid rotation (like Iapetus)mostly extensional tectonic features present on surface If ocean present beneath icy layer:no equatorial bulge mostly compressional surface features (e.g., wrinkle ridges)(melting of ice causes volume contraction, compression) If there used to be a subsurface ocean, but isn't anymore:mixture of compressional and extensional features no equatorial bulge. Figuring out if Pluto (and Charon) have or had an internal ocean was one goal of the New Horizons mission.

Other planets with water

If we want to look for life elsewhere, a good way to start is to look for worlds that have (or had) liquid water available. Two top candidates in our own solar system are Mars (past water) and Europa (water beneath the surface) The Curiosity rover on Mars has detected highly variable "bursts" of methane detected by the rover, suggesting a sporadic local source. Recently, NASA also reported the detection of more complex organic compounds contained in ancient sedimentary rocks from Mars. Methane and organic compounds don't necessarily indicate life, but they leave open the possibility that life on Mars might once have existed (and just possibly still does).

Galileo

In 1610, Galileo Galili discovered the four largest moons of Jupiter using a small telescope, and deduced from the apparent movements of the moons that they orbited Jupiter. Evidence that any celestial bodies orbited an object other than the Earth was a severe challenge to the Ptolemaic theory that dominated at the time. Galileo recorded the positions of several bright stars near Jupiter. He found that, whereas all other stars had "fixed" positions, these points of light remained close to Jupiter as it wandered through the night sky, but their relative positions to Jupiter and each other changed. In 1616 the Catholic Church condemned heliocentrism as "false and contrary to Scripture", and Galileo was warned to abandon his support of it. When he later publicly defended his views, he was tried by the Roman Inquisition and found "vehemently suspect of heresy". He was forced to recant his views and spent the rest of his life under house arrest. It took nearly 400 years for Galileo to be "rehabilitated". In 1979 Pope John Paul II called for theologians, scholars and historians to re-examine Galileo's case, and in 1992 Pope John Paul II publicly endorsed Galileo's philosophy.

Neptune's Atmosphere

In comparison to Uranus, Neptune's atmosphere is much more varied and active, with log-lived storm systems and white cloud layers. The atmosphere's of Neptune and Uranus are very similar in composition, so why is Neptune so much more active than Uranus? The answer may lie in the interiors of the planets. We saw before that the convection of Jupiter's (and Saturn's) atmosphere is driven by the release of internal heat. Jupiter, Saturn, and Neptune all release more heat than they absorb from the sun.

Direct Imaging of Planets

In order to observe the light emitted (or reflected) by a planet, light from the star must first be subtracted from the spectrum. The "residual" spectrum can provide information on the presence or absence of a planetary atmosphere, composition, and temperature. Planet "Corot2b" orbits its home star once every 2 days. Because it is so close to the star, it's surface and atmosphere are extremely hot (>2000 K). It is literally "red hot". Currently, we can observe light reflected (or emitted) by very large, very hot planets orbiting close to their parent star. However, in the future we will be able to study the surface and atmospheres of more "Earth-like" planets. What would we like to look for? By very carefully measuring how different frequencies of light are observed as a planet occults (passes in front of) it's star, we can determine what species are present in the planetary atmosphere. Recently, measurements using the Hubble space telescope confirmed the presence of water vapor in the atmospheres of 5 exoplanets. However, all of these planets are so-called "hot Jupiters".

Future Studies

In the future, spectrographic analysis of the atmosphere of planets like Keppler 22b may indicate not only if these planets are habitable (e.g., by measuring water and CO2 concentrations), but may also reveal if they contain chemical signatures of biological activity.

How could we look for life on Europa?

In the future, we may develop probes that can melt or drill through the icy crust, and submersibles that can explore Europa's ocean. However, for now, Europa's surface may provide the clues we need. The grooves on Europa mark areas where warm ice that forms at the top of Europa's ocean pushes up to the surface. If life exists in the ocean, this ice could contain chemical clues (e.g., complex organic molecules) to its existence. A sample return mission could help answer this question. Another (maybe easier) way to explore Europa's interior would be to examine material brought to the surface by cryovolcanism. In 2012 the Hubble Space Telescope detected water plumes near Europa's south pole. If Europa's seas contain organic molecules, these should be present in young material brought to the surface through cryovolcanism. An orbiting spacecraft or a lander could collect and study this material.

Io

Io is the most volcanically active body in the solar system. Io's high density (3.53 g/cm3) indicates a rocky composition. No ice is present like for the other Galilean moons No impact craters. Io's surface is constantly being reworked by volcanism. Eruption of Pele volcano, Io. Volcanic plumes from Io's volcanoes can extend hundreds of km into space. Because of Io's volcanism, Io has a tenuous atmosphere composed primarily of sulfur dioxide. Volcanism in Io had been predicted before the Voyager fly-by based on the amount of tidal heating the planet experiences. Volcanism on Io includes both effusive and explosive volcanism. The lava lake shown to the left is over 200 km across! Thermal image of Io (right) reveals numerous volcanic "hotspots". The effects of a large explosive eruption are evident in these two images. Io's interior is fully differentiated, with an iron core and a silicate mantle. Surface heat flow is very high, indicating that the mantle must be quite hot. In fact, Io's mantle may be 10-20% molten.

Tidal Heating

Io's interior is fully differentiated, with an iron core and a silicate mantle. Surface heat flow is very high, indicating that the mantle must be quite hot. In fact, Io's mantle may be 10-20% molten.

Why is Titan's atmosphere so fascinating?

It may help us to determine how the "primordial soup" of amino acids and other organic components necessary to create life were formed in the early solar system. In 1952, the Miller-Urey experiment showed that amino acids could be produced from a N2-CH4 atmosphere. The surface of Titan is a natural experiment conducted on a grand scale. Recipe for (pre-)life: 1) Mix water, methane, and nitrogen. 2) Add energy (electrical spark, UV radiation, etc.) 3) Wait. Result? Lots of amino acids and other essential building blocks for life.

Neptune and Uranus

Just as Earth and Venus can be considered "twins" because they are similar in size and composition, Uranus and Neptune are also often considered sister planets. Uranus is just barely visible to the naked eye, but it wasn't recognized to be a planet until observations made by William Herschel in 1781. Herschel initially thought he had discovered a comet, but calculations of Uranus' orbit revealed it to be nearly circular, like a planet. The discovery of Uranus led to the later discovery of Neptune. Deviations of Uranus' observed orbit from theoretical predictions suggested that another planet was gravitationally interacting with Uranus. In 1843, John Couch Adams predicted the orbit of a hypothetical 8th planet required to explain the orbit of Uranus, and in 1845 Urbain le Verrier made a similar but more accurate prediction. Subsequent astronomical observations discovered Neptune within 1o of where le Verrier had predicted.

Kepler 186f

Kepler 186f also sits within the habitable zone, and is the most Earth-like planet known in terms of its size. It come from a star system with at least 4 other suspected planets. The star is ~490 light years from Earth.

Kepler 22b

Kepler 22b sits near the inner edge of its star's "habitable zone". If it's atmosphere is the right thickness and composition, it could have an average surface temperature of a balmy 31 C. Distance = 536 light-years; Mass = 6.4x Earth mass; Radius = 2.1x Earth radius; Surface temp. (estimate) = 31 0C; Orbital period (year) = 290 days; Age (estimate) = ???

Moons of Saturn May Be Younger Than the DinosaursThursday, March 24 2016 - 9:00 am, PDT

MOUNTAIN VIEW - New research suggests that some of Saturn's icy moons, as well as its famous rings, might be modern adornments. Their dramatic birth may have taken place a mere hundred million years ago, more recent than the reign of many dinosaurs. "Moons are always changing their orbits. That's inevitable," says Matija Cuk, principal investigator at the SETI Institute. "But that fact allows us to use computer simulations to tease out the history of Saturn's inner moons. Doing so, we find that they were most likely born during the most recent two percent of the planet's history."

A possible scenario for Saturn's rings:

Moons collide, are destroyed and form rings of debris Rings clump together through gravitational attraction and accretion leads to formation of new moons Moons gravitationally interact and orbits become disturbed Moons collide, are destroyed and form rings of debris.Wash, rinse, repeat.

Where are Neptune's other moons?

Neptune has at least 12 additional moons besides Triton. However, they are all tiny in comparison, totaling <<1% of Triton's mass. In contrast, Uranus, Saturn and Jupiter all have several large moons that are comparable in mass to one another. Why is the mass distribution of Neptune's system of moons so lopsided? The answer lies in the capture origin of Triton. After it's initial capture by Neptune, Triton would have had a highly elliptical orbit that would have crossed the orbits of other "normal" moons. These smaller moons would have been destroyed by collision with Triton of flung out into space by gravitational interactions with Triton.

Is there anybody out there (and I don't mean slime mold)?

Okay, I think that life might be common in the universe, and I think we will actually be able to answer this question in the next few decades. But, enough about slime mold. What about ET? What about complex, intelligent life? Absolutely no idea. Zero data (for now) = no way to make quantitative, testable predictions.

Why do we care so much about whether Europa has a hidden ocean?

On Earth, deep-sea hydrothermal vents team with life, and may have been home to the earliest life forms. Could similar vents beneath Europa's icy crust provide a habitat for life? Future missions may use cryobots to melt through Europa's outer ice layer and explore the ocean beneath. The technology for these missions can be developed and tested in sub-glacial Antacrtic lakes such as Lake Vostok. NASA is developing cryobot technology to study Antarctic sub-glacial lakes and the ice sheets of Antarctica and Greenland. In the future, cryobots may be used to examine ice deposits on Mars or to drill through the icy surface of Europa.

Shear Localization

One possible mechanism for generating sufficient local heating to generate liquid water pockets is "shear localization". Warm ice is softer than colder ice. Tidal deformation tends to get concentrated in the warmer, softer areas, which leads to greater heating, which leads to further localization of deformation and heating - a positive feedback that leads to local melting of the ice. Enceladus and Europa are two worlds that could potentially harbor life. In both cases, water in the interior is thought to come in contact with rock layers. Water/rock reactions provide a number of potential energy sources as well as vital building blocks for life, and is thought to be a prerequisite for life.

Big events in the history of life on Earth

Origin of life:>3500 Ma Origin of oxygenetic photosynthesis: ~2700 Ma Origin of Eukarya (cells w/ nucleus and mitochondria):~2700-1600 Ma Origin of animals: ~665-610 Ma (maybe 1000 Ma) Cambrian Explosion: Sudden appearance of diverse, large lifeforms: ~545-530 Ma

Pluto and Triton

Pluto and Triton are very similar in terms of size (radii of 1153 and 1353 km, respectively) and inferred composition (densities of ~2.0 and 2.06 g/cm3). These similarities are shared with other Kuiper Belt Objects, and Triton is believed to represent a Kuiper Belt object that was captured by Neptune.

Pluto facts

Pluto is a Kuiper Belt object. Most Kuiper Belt objects are so small and so far away that even in the most powerful telescopes they appear as little more than specks of light. Pluto is the most famous and one of the largest Kuiper Belt Objects (KBOs). Over 70,000 KBOs >100 km in diameter are thought to exist, and several dwarf planets similar in size to Pluto have been found. The high eccentricity and orbital inclinations of KBOs distinguish them from planets and are the result of gravitational scattering by Neptune. Neptune's mass was initially believed to be too small to account for deviations in Uranus' orbit . In 1906, Percival Lowell began a search for "Planet X". In 1930, a young assistant at the Lowell Observatory, Clyde Tombaugh, discovered Pluto by comparing thousands of photographs taken a week apart and looking for any "movement". Pluto rotates very slowly. A day on Pluto is 6.4 Earth days.

Charon

Pluto's moon Charon has a vast network of canyons that cuts across the entire visible face of the moon. The region to the north of these canyons is very rugged, but the southern portion of the moon is much smoother. Many of the canyons on Charon resemble grabens. What does that tell us about the stresses the crust has experienced? The smooth plains of Charon (right) contrast with the rugged highlands. On the moon, the relatively smooth mare were generated by outpourings of basalt, which filled in surface features. The smooth plains of Charon may reflect ancient cryovolcanism. we have evidence for crustal extension in the form of graben-like features, and evidence of past cryovolcanism in the form of smooth plains.

Pluto Orbit

Pluto's orbit is very different from the orbits of the planets. Most of the planets have low-eccentricity orbits (Mercury is an exception with an eccentricity of 0.205). In contrast, Pluto has a highly elliptical orbit, with an eccentricity of 0.25. In addition, Pluto's orbit is tilted relative to the plane of the ecliptic by 17o. Direct measurement of Pluto's mass was impossible until it was discovered in 1978 that Pluto has a moon, Charon. In 2005, the Hubble Space Telescope discovered two new small moons of Pluto, Nix and Hydra.

Ultima Thule

Rotating Cloud of small, icy bodies starts to coalesce in the outer solar system Eventually 2 large bodies remain the two bodies slowly spiral closer until they touch, forming the bilobed object

Saturn Rings

Saturn's rings are very "clean". However, the solar system is a dirty place, with lots of dark chondritic material floating about. If the rings were as old as the solar system, they should have accumulated a lot of chondritic "soot", which would make them much darker. Some scientists (not all) therefore feel that the rings may be the result of a relatively recent (last few 100 million years) catastrophic impact on one of Saturn's icy moons. SAN FRANCISCO, Calif. - New observations by NASA's Cassini spacecraft indicate the rings of Saturn, once thought to have formed during the age of the dinosaurs, instead may have been created roughly 4.5 billion years ago, when the solar system was still under construction.

Undiscovered Planet

Scientists recently reported that several Kuiper Belt objects with perihelion distances > 30 AU (so outside the sphere of influence of Neptune) have non-random argument of perihelion. This is most easily explained if some other massive object is influencing their orbits. An undiscovered ninth planet with a mass >10x Earth's mass and a semi-major axis of ~700 AU could explain the curious orbits of these Kuiper Belt objects. Several searches are underway to hunt for this hypothetical giant ice planet.

Is this how/where life began?

Seawater driven by heat from intruding basalts forms hydrothermal springs. Hot water dissolves material, and with cooling, deposits metal-rich clays and sulfides. Entirely new ecosystems were discovered living at these vents living off of chemicals rather than sunlight Possible model for early life - life in an extreme environment

How do we look for planets around other stars?

Several methods have been developed, including the Doppler method, Transit method, Astrometry, and Direct Imaging. Of these, the Doppler Method was the first to detect extra-solar planets. The Doppler and astrometry methods involve measuring the "wobble" of a star produced by the gravitational pull of a large orbiting planet. The star and planet orbit around a shared center of mass, causing the star to "wobble". The "wobble" of a star produced by the gravitational pull of a planet causes the frequency of light emitted to shift, much as the pitch of a train whistle changes depending on whether the train is moving towards or away from an observer. Measurement of this change in the frequency of light emitted allows the orbits of the star and planet to be deduced. Bearing in mind that the doppler method for finding exoplanets is more likely to detect large planets and planets that orbit close to their star, what have we learned so far? One important finding is that "hot Jupiters" are very common. These are giant planets that orbit very close to their star. Current models for planet formation indicate these planets cannot have formed in their current position-they must have formed further from their star and spiraled inward.

Liquid Water Layers

Several of the Galilean moons (Europa, Ganymede, and possibly even Callisto) are thought to have a layer of liquid water at various depths beneath the surface. This is because liquid water is denser than water ice at low pressures, but less dense at higher pressures Water ice has many different forms or crystal structures depending on pressure and temperature.

Searches for life

Several searches are underway looking for signs of alien intelligent life. The SETI (Search for Extraterrestrial Intelligence) organization uses data processing software to look for non-random radio signals from distant stars using radio telescopes. So far, nothing has been found, so at the very least we can say that civilizations similar to our own are not a dime a dozen in our own galaxy. The Fermi Paradox was posed by physicist Enrico Fermi in 1950. Even given the great distances separating stars, an advanced species should be able to explore/colonize the galaxy in geologically short period of time (< 50 million years). So, why aren't they here? Either intelligent life is very rare (or short lived), or our assumptions about what such species might do are incorrect. Our search for alien civilizations necessarily involves assumptions based on our own development of what such civilizations might look like or what signs they might leave behind.

How do we find it?

So, the building blocks of life are common in the Universe, and there's reason to believe that life itself may be relatively common. But, most of the time, we're talking slime mold and not ET. How do we find it? We saw last time that we're starting to find exoplanets that are potentially habitable, but they are too far away to travel to. How do we look for life at places like Gliese 581g? By very carefully measuring how different frequencies of light are observed as a planet occults (passes in front of) it's star, we can determine what species are present in the planetary atmosphere. Recently, measurements using the Hubble space telescope confirmed the presence of water vapor in the atmospheres of 5 exoplanets. However, all of these planets are so-called "hot Jupiters".

Frozen Lakes on Pluto

Some of the old, dark terranes of Pluto have features that resemble frozen lakes. These lakes made of frozen nitrogen, but are thought to have once been liquid. This suggests that Pluto once had a much thicker atmosphere than it does today, because liquid nitorgen cannot exist at current atmospheric pressure (~10-5 atm). Below ~0.1 atm, nitrogen is only stable as a solid or a gas. In order for Pluto to once have had liquid nitrogen at the surface, the atmosphere would need to be 10,000 times thicker than it is today.

Sputnik Planum

Sputnik Planum, surrounded by older, darker, rougher terrane. The "coastline" of Sputnik Planum. The darker terrane is much older (notice the impact craters). The jumbled terrane is likely made of water icebergs floating in the frozen nitrogen. Along the margins of Sputnik Planum, there is evidence for glaciers of nitrogen ice shaping the older water ice terrane and flowing into the "sea". There is also evidence for convection of the soft nitrogen ice in the form of polygonal features observed in the ice. Sputnik Planum appears to be a vast frozen sea of nitrogen ice. The troughs visible in the ice may be convective downwellings caused by convection in the soft ice. Hills may be water icebergs that get swept up and concentrated where these downwellings occur.

Habitable Zone

The Habitable Zone defines the region around a star where liquid water can exist on the surface of a planet with a suitable atmosphere. Our nearest neighbors, Mars and Venus, exist just outside and inside this zone. Note that the thickness of this zone is subject to debate. For example, Mars could be habitable if it had a thicker atmosphere. Some estimates suggest that up to 10% of all stars have at least one planet in their habitable zone. Given >100 billion stars in our galaxy, that's over 10 billion potentially habitable worlds. Over two dozen planets within their stars' habitable zone have been detected, and there are many other potential candidates. It is estimated that there are 10-40 billion potentially habitable planets in our galaxy.

James Webb Space Telescope

The James Webb Space Telescope is scheduled for launch in 2021(?). A major mission of the JWST is to study the atmospheres of extra-solar planets, especially potentially habitable planets, to determine their composition, temperature, and look for signs of life. Direct imaging of planets around other stars is currently extremely difficult, but it potentially can provide us with the most information about atmosphere composition and temperature. The problem is that stars are vastly brighter than their companion planets, so the starlight tends to mask the planet. It's like trying to see a firefly while staring directly into high-bean headlights. The image above shows a planet orbiting Beta Pictoris. This method currently is best at finding large, very hot planets, such as might exist shortly after planet formation.

Kepler Mission

The Kepler mission, launched in 2009, was specially designed to continuously measure the brightness of over 145,000 stars, looking for periodic fluctuations that would indicate an orbiting planet. One disadvantage of this method is that we can only detect planets this way if we observe the orbital plane edge-on. Otherwise, the planet will not pass directly in front of the star. Kepler was designed to look for Earth-like planets, and was incredibly successful from 2009-2013. Unfortunately, instrument problems have put a halt to its planet-hunting days.

Kuiper Belt

The Kuiper Belt is a region of space extending roughly from the orbit of Neptune (~30 AU) to around 55 AU. This region is the source of most short-period comets (periods < 200 years). The objects in the Kuiper Belt did not form there-they were "cast out" during the outward migration of Neptune. Kuiper Belt Objects are left-over planetesimals and planetary embryos. They can tell us a lot about how the giant planets formed.

New Horizons

The New Horizons spacecraft was launched in 2006, and flew past Pluto in 2015. New Horizons has revolutionized what we know about Pluto. We are just beginning to figure out what all of the information we received means. Because Pluto is so far away (it takes signals nearly four and a half hours to be beamed from New Horizons to Earth at the speed of light. New Horizons flew past Pluto in July, 2015. The spacecraft was in a race against time. Pluto passed its perihelion in 1989 and is now traveling away from the sun. Scientists think that Pluto's atmosphere is a seasonal phenomena-it will begin to freeze onto the surface as Pluto travels further from the sun. Because New Horizons was a fly-by mission (there are no brakes in space), we were only able to see one side of Pluto at close range. This is the last, best image of Pluto's "far side". It actually looks very different to the side we saw up close. One thing that New Horizons was able to do was measure the thickness of pluto's atmosphere, by observing how it refracted sunlight is the spacecraft passed into Pluto's shadow.

Roche Limit

The Roche Limit is the distance from a celestial body (e.g., a planet) within which a smaller object held together by gravity will be torn apart by the tidal forces exerted by the larger body. Any moon that wanders within this limit will be destroyed.

How did life start?

The basic requirements of life (on Earth) are 1)Water 2)Organic compounds 3)Energy All three are fairly common. We've seen several worlds in our own solar system either have or had liquid water (either at or below the surface) Astronomers have also found amino acids and other organic compounds in interstellar gas clouds. These compounds are necessary for life. Amino acids and many other organic compounds necessary for life can be created by mimicking conditions that were widespread in the early solar system (little free oxygen, abundant CH4 and NH3). We know these compounds did form, because we see them in primitive meteorites.

What are icy worlds in the outer solar system made of?

The composition of the icy satellites and Kuiper Belt objects varies with distance from the sun. With greater distance, temperatures drop and new phases begin to freeze out in the solar nebula. Whereas Jupiter's satellites are mostly rock and water ice, Kuiper Belt objects contain greater quantities of ammonia, methane, and nitrogen.

Galilean Moons

The density of the Galilean moons decreases systematically with increasing distance from the planet. Does this suggest that the moons were captured, or that they formed in situ? As we discussed earlier, planetary moons may derive through: 1)Gravitational capture of other objects; 2) Accretion of ejected material; 3) Co-accretion (e.g., from disks of material surrounding planets). The trend in composition of the Galilean moons along with their nearly circular orbits and very minor inclination to Jupiter's equatorial plane suggests that the moons formed "in place" from a disk of material surrounding the growing planet. Callisto (far away) would have formed more slowly, and Io (closer to Jupiter) more quickly. This in turn influenced how much heat from the accretion process each moon retained after its formation. Jupiter has over 39 known satellites. The four largest (Callisto, Ganymede, Europa, and Io), are sometimes called the "Galilean satellites".

Cyrovolcanism on Pluto

The evidence for crustal extension and for cryovolcanism on Pluto may indicate that Pluto's subsurface ocean is starting to freeze. As water freezes, it expands, fracturing the crust and forcing liquid water up to the surface.

Pluto's near side.

The heart-shaped region in the lower right is called "Sputnik Planum". It appears to be a vast "sea" of frozen nitrogen, surrounded by craggy mountains of water ice.

Increase in Oxygen

The increase in atmospheric oxygen appears to have occurred in stages. Around 2.5 billion years ago, oxygen rose to ~10% of modern levels. This is too low to support complex multicellular life forms. Around 700 million years ago, oxygen levels rose to close to modern levels. Not long afterwards, complex life took off. Complex life arose fairly quickly once the necessary conditions were met. Banded Iron formations like the sample shown to the left formed when iron dissolved in (low-O2) seawater reacted with oxygen to form insoluble Fe2O3. These formations mark the rise of atmospheric oxygen ~3-2 billion years ago. Atmospheric oxygen levels rose in a number of steps, and it wasn't until ~600 million years ago that oxygen rose to close to its current level. Coincidentally(?) this is also when complex organisms first evolved. Late Precambrian fauna like those shown below and to the left didn't arise for 3 billion years after life first arose. The first animals appear to have evolved around 650 Ma. The rise of oxygen made more complex life possible.

Orbital Resonances

The innermost three Galilean moons Io, Europa, and Ganymede are locked in a 4:2:1 orbital resonance. For every orbit of Ganymede, Europa orbits Jupiter twice, and Io four times. The gravitational tugs produced by this resonance increase the eccentricity of the orbits of these moons. Increased eccentricity = greater libration = more tidal heating. Callisto is not part of this resonance, so it's orbit is nearly circular and it receives very little tidal heating.

Saturn's Rings

The rings of Saturn are what we think of first when we think of Saturn. Although Jupiter and Uranus also have thin rings, they are nowhere near as spectacular as those of Saturn. What are Saturn's rings made of, and how did they form? Discovery of Saturn's Rings: In 1610 Galileo became the first person to observe Saturn's rings, but he could not see them well enough to discern their nature. Instead, it looked like Saturn was accompanied by two smaller worlds. In 1655, using a much better telescope than available to Galileo, Christaan Huygens observed Saturn and proposed that the planet was surrounded by a thin, flat disk. Saturn's rings are composed of countless small icy particles, ranging in size from ~1 meter to < 1 micrometer. The main rings range from 7,000 up to 80,000 km above Saturn's equator. They are incredibly thin-in many areas no more than ~10 meters thick. The rings are composed mostly of water ice (~93%). This may provide a clue to how they formed. An important clue to the origin of Saturn's rings is their composition-they are almost pure water ice (93%), with most of the remaining material some form of carbon (7%). What is missing from the rings? Are all of the proposed mechanisms for forming the rings consistent with this composition?

Is anybody out there?

The search for life in the Universe is hampered by the fact that we only have one known example to base our theories on-life here on Earth. Therefore, if we want to constrain where and how to look for life, we need to start by reviewing a few features of the history of life on our own world. Life has existed on Earth since at least 3.5 Ga, and probably much longer. The picture below shows fossil cyanobacteria. Cyanobacteria (photosynthesis) have been around since at least 2.7 Ga. The presence of ancient fossil bacteria indicates that life arose on Earth fairly quickly once suitable conditions (e.g., liquid water at the surface) were present. This suggests that life may form fairly easily given the right conditions and materials.

What other chemicals are contained in these lakes, and can they constrain how life got started on Earth?

The surface of Titan contains "lakes" of methane/ethane. These images were taken by the Huygens probe as it descended through the Titan atmosphere. Radar imaging of Titan by the Cassini spacecraft reveal a surface dotted with dozens of "lakes", thought to be composed of liquid ethane, methane, and propane. The lakes appear dark in radar images because liquids do not reflect radio waves well. One of the outstanding mysteries about Titan is the source of methane in its atmosphere. Chemical reactions in response to UV light should quickly destroy methane and produce other organic compounds like acetylene. The presence of methane therefore requires a constant source. Possibilities include hydrothermal or volcanic input and the presence of a sub-surface water-ammonia lake. Most scientist believe a biologic origin is unlikely, but it can't be completely ruled out.

Transit Method

The transit method measures the drop in luminosity that results as a planet passes in front of a star, blocking a part of its light. The degree of reduction is a function of planet size, and the periodicity with which it occurs is a function of orbital period, and therefore distance.

Uranus

This breathtaking picture of Uranus, taken by Voyager 2 in visible light, reveals the dramatic banding and complex structure we've come to expect in the atmospheres of the giant planets. Uranus has a mass ~15x that of Earth, slightly smaller than Neptune (~17x Earth). Uranus is unusual in that it has an axial tilt of 98o. This very large tilt is thought to derive from a giant impact with an Earth-sized object during or shortly after formation of the planet, "knocking" Uranus on its side. Uranus isn't as bland and featureless as it first appears. Like Jupiter and Saturn, it displays latitudinal banding and some storm features, but these are much more visible in infrared light than visible light. However, it is the lack of activity in comparison to the other giant planets that is significant. In contrast, Uranus releases very little internal heat, so convection in the atmosphere is much less vigorous. It is unclear why Uranus lacks an internal heat source like the other giant planets, although some theories link this to the giant impact that knocked Uranus on its side.

Titan

Titan is the second largest planetary moon in the solar system (after Ganymede). It is by far the largest moon of Saturn, with a diameter of 5150 km and a mass nearly twice that of our own moon. It's low density (1.89 g/cm3) indicates it is composed of ~50:50 mixture of rock and ice. Titan has a dense atmosphere composed of nitrogen and methane. Titan has a dense atmosphere composed mostly of N2 (~95%) and CH4(~5%) in the troposphere. The presence of large quantities of methane in Titan's atmosphere is a mystery, because methane should be quickly converted into other compounds by exposure to solar radiation. Titan's atmosphere is "hazy" or opaque, due to the presence of organic compounds that form through reactions involving methane and nitrogen. These organic compounds slowly rain down to the surface.

Exoplanets

Today, there are nearly 4000 known exoplanets. At least 20% of main-sequence stars have at least one planet (and many probably have several), so the total number of planets in our galaxy probably is hundreds of billions. Not all planets have near-circular orbits, but many do. Some systems with high apparent eccentricity my actually be multi-planet systems.

Why does Ganymede have a differentiated structure whereas Callisto is only partially differentiated? Ganymede's interior must have been heated to a greater extent than Callisto, softening it and allowing dense material to sink, but why?

Two things are probably related to the greater heating of Ganymede: 1)Faster accretion, which "buried" heat in the interior; 2)Greater tidal heating The different internal structures of Callisto and Ganymede may result from the speed of formation. Slow accretion (Callisto) allows heat from impacts to escape to space, so the interior never warms up enough to cause differentiation. Fast accretion (Ganymede) heats the interior of the moon, allowing differentiation to occur and possibly melting part of the interior. The surface of Ganymede is divided into two different types of terrain: a very ancient dark terrain that is thought to be ~4.5 Ga old, and a lighter terrain that is younger, but still very ancient. The presence of distinct terrains with different ages indicates that the surface (and interior) of Ganymede was active at periods in the past. The lighter, younger terrain contains many cross-cutting grooves. Although earlier theories suggested that these grooves marked locations where liquid water welled to the surface (e.g., a form of cryovolcanism), most scientists now believe the grooves are formed by tectonic processes, e.g., the faulting and movement of different blocks of crust in response to convection within the interior. In addition to having a fully differentiated structure (evidence for this comes from Ganymede's low moment of inertia), Ganymede may also have a liquid ocean of salt water ~200 km below the surface, sandwiched between layers of ice.

Uranus and Neptune Structure

Uranus and Neptune have an internal structure that is very different from Jupiter and Saturn.The layers of Neptune and Uranus include 1&2) Layers of hydrogen, helium, and methane gas and liquid 3) A "mantle" of water, ammonia, and methane "ice" 4) A core composed of rock and iron. Uranus and Neptune have much thicker "ice" layers, and thinner layers of hydrogen and helium than Jupiter and Saturn.

Albedo

Variations in color and albedo reflect variations in composition of the surface of Calisto. Lighter = younger; Darker = older. This variation may reflect gradual removal of surface ice through sublimation or dissociation via UV radiation.

Triton

Very few craters = young surface. Plumes of dark, dusty material transported by nitrogen geysers. On Europa and Enceladus, cryovolcanism was driven by tidal heating. However, Triton receives very little tidal heating, because there are no other large moons of Neptune. Voyager 2 imaged portions of the surface of Triton during it's Neptune fly-by. What do you see in the above image? What don't you see? All of Triton's geysers are seen in areas close to the subsolar point (where the sun is overhead, suggesting that solar heating plays an important role. The surface temperature of Triton is only ~38 K (-236 oC), just below the freezing point of nitrogen. Sunlight warms the surface enough to cause nitrogen ice to sublimate, converting to gas. The gas pressure builds up and nitrogen gas breaks through the icy surface, carrying a stream of solid particles with it. Sublimation of nitrogen ice produces Triton's very thin atmosphere. This Voyager 2 photo shows thin clouds of nitrogen ice crystals that form ~1 to 3 km above the moon's surface. Pluto also forms a nitrogen atmosphere as it approaches perihelion, but the atmosphere freezes when Pluto approaches aphelion. Triton is a captured Kuiper Belt Object. Neptune's moon Triton is the only major moon in the solar system with a retrograde orbit, which indicates it cannot have formed in place but instead was captured by Neptune during the final stages of solar system formation.

Ariel

~1158 km diameter 1.59 g/cm3 Now that's more like it. Ariel has a complex surface with scarps, valleys, and ridges. Much of the surface viewed by Voyager is crisscrossed by grabens.

Oberon

~1500 km diameter Density ~1.65 g/ cm3 Lots of craters Boring.

Titania

~1577 km diameter ~1.7 g/cm3 Lots of craters Some scarps near the horizon. Too bad we cant peak around the other side. Still, not doing much for me.

Miranda

~472 km diameter 1.2 g/cm3 Weird. Just weird.