ASTR 330: The Solar System

ASTR 330: The Solar System Make-up exam #2 scheduled for Thursday. Extra credit term paper: choose subjects and register with me by today! Topics must be science-focused. Papers due December 5th class. Announcements Dr Conor Nixon Fall 2006

ASTR 330: The Solar System Lecture 22: Large Satellites II: Io, Titan and Triton Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL

ASTR 330: The Solar System Io is the most volcanically active object in the solar system, showing extensive re-surfacing between the Voyager images of 1979 and the arrival of Galileo in 1995. The bright orange oval surrounds the volcano Pele. Io: Jupiter’s Volcanic Moon Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL - Galileo

ASTR 330: The Solar System Io is the innermost of the four Galilean moons of Jupiter. Io’s density of 3.3 g/cm3, higher even than Europa, tells us that it has little or nowater. It is a very similar size and density to the Moon. The surface is quite reflective, but completely unlike the pale Europa, being covered instead by brightly colored patches, caused by constant volcanic activity. Io is small enough that it should have cooled long ago: what energy source could be powering the volcanism today? The answer of course is the huge tidal force of Jupiter. Io is about the same distance from Jupiter as the Moon from the Earth, however Jupiter is 300 times more massive! Io’s bulk density and composition Dr Conor Nixon Fall 2006

ASTR 330: The Solar System The tides of Jupiter cause Io to be pulled into a elongated shape (similar to an egg), with a tidal bulge several km high. If Io was able to keep the same face towards Jupiter (like the Moon does to the Earth), then no heating would occur. However, Io’s fate is not so simple. The next outer moons, Europa and Ganymede exert enough gravitational force on Io to perturb its orbit from a purely circular one. Following an elliptical orbit, like Mercury, Io travels at different speeds as it gets closer to and further from Jupiter. Hence, it is unable to ‘choose’ a rotational state which matches its orbital speed at all times, and so it twists and turns as Jupiter pulls on Io’s tidal bulge from different directions. This causes enormous heating. Tides on Io Dr Conor Nixon Fall 2006

ASTR 330: The Solar System We can estimate a lower limit on the amount of energy produced by the tidal heating, by adding up the heat emitted by its volcanoes. We calculate by this method that at least 100 million megawatts (1014 watts) is generated: 10 times the present energy consumption of all human activity! The energy source is the spin of Jupiter, which therefore must be gradually slowing down very slightly over time. This continuous heating has driven off many volatiles over geologic time: no water, and most carbon and nitrogen compounds are gone. Sulfur and its compounds are the most volatile substances remaining, and which are the source of the many colored surface terrains. Energy of Tidal Heating Dr Conor Nixon Fall 2006

ASTR 330: The Solar System The silicate interior of Io is largely liquid, with a solid crust of about 25 km thick. The surface is constantly changing, and any geologic features will not long survive. Io Interior Dr Conor Nixon Fall 2006 Picture credit: Calvin Hamilton/solarviews.com

ASTR 330: The Solar System Io shows no surface craters at all. This is of course due to the constant volcanic activity, resurfacing the crust at a rate of 100s of meters per million years. Notice the trend toward fewer and fewer craters as we have traversed the Galilean moons inwards: the opposite to what we originally expected! What are we to make of the dramatic colors of Io’s surface? The only compound that has been unambiguously detected to date is sulfur dioxide (SO2), a gas on Earth, but which may freeze as a white frost on Io. Io Surface Dr Conor Nixon Fall 2006

ASTR 330: The Solar System The most likely culprit for the colorful yellows, oranges, browns and blacks is elemental sulfur, which may form different colored solid states depending on how it has cooled. This Galileo image shows the colorful deposits around the Pele volcano. Surface Colors Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL Galileo

ASTR 330: The Solar System • Instead of craters, Io exhibits a full range of volcanic features including: • layered lava plains with flow fronts • volcanic mountains and calderas • volcanic vents and lava flows. • Io has several low shield volcanoes and mountain ranges taller than those on Earth (the triangular peak, right-top is 11 km high). • The surface has collapsed in several places, apparently due to magma being removed from beneath. The dark grey region Hi-Iaka Patera (right) is probably caused by rifting, not collapse. • Tectonic pressures have produced tilt blocks and even upthrust mountains. Volcanic Features Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL Galileo

ASTR 330: The Solar System Io’s volcanoes are named for fire gods in various cultures: e.g. Pele (Hawaiian) and Loki (Norse). Plume eruptions, first observed by the Voyager twins, can reach 300 km above the surface. The Galileo image (right) shows two plumes: one on the limb above Pillan Patera (130 km high), and one erupting from Prometheus in the center of the image. Volcanic Eruptions Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL Galileo

ASTR 330: The Solar System These 2 Galileo images show the Pillan Paterabefore and after the eruption of the previous slide: the new dark spot is 400 km across. The red circle surrounds the Pele volcano. Plume eruptions occur when hot liquid rushes to the surface, becomes gaseous as the pressure drops, and then solidifies when it reaches the cold of space, becoming ‘snow’. These eruptions resemble geysers on the Earth such as Old Faithful. However, the liquid is notwater, it is sulfur or sulfur dioxide. Io’s plume eruptions release 100,000 tons of material every second: enough to re-cover 1000s of km2 in a matter of weeks. Resurfacing Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL Galileo

ASTR 330: The Solar System Most of the material from the eruptions just rains back down on Io, but about 0.01%, 10 tons per second, actually escapes into space. SO2 is quickly broken up into S and O atoms and ions by the action of UV light and impacts from other magnetospheric particles. Hence part of Jupiter’s magnetosphere is due to eruptions from Io. This Galileo image shows sunlight being scattered from the plume of the volcano Prometheus (the bright spot). The diffuse yellow glow is due to scattering of sunlight by sodium atoms from eruptions which form a vast cloud around Io. Eruptions and Jovian Magnetosphere Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL Galileo

ASTR 330: The Solar System Galileo images of Io showed familiar volcanic eruptions to the Earth: fire fountains, lava flows and lakes. But these were all silicate lava (normal molten rock) events: not molten sulfur or SO2. What about the plumes then? We believe that the plumes occur when a lava flow meets a deposit of frozen SO2 on the surface. The SO2 explosively heats up and vaporizes. We see some similar events when lava flows meet ice or water on Earth. Io’s gravity is so low however that the plumes can reach great heights. Are the plumes from volcanic vents? Dr Conor Nixon Fall 2006

ASTR 330: The Solar System This figures illustrates the role of sulfur in Io’s surface coloration. Sulfur gas (S2) is ejected from surface deposits. Sulfur gas lands on the surface, solidifies, and re-arranges its chemical structure into red S3 and S4 molecules. Much later, the atoms further re-arrange into the most stable configuration, yellow S8. Sulfur Cycle Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL Galileo

ASTR 330: The Solar System Tvashtar Volcanism Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL Galileo

ASTR 330: The Solar System “It is a riddle wrapped in a mystery inside an enigma” - Winston Churchill, 1939 (on the subject of Soviet intentions in WW II) Titan Until January 2005, the surface of Titan was arguably the most enigmatic ‘planetary’ surface in the entire solar system, representing the largest unexplored, unmapped area. Cassini/Huygens has now changed all that! Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL Voyager

ASTR 330: The Solar System We now leave the Jovian system and come to Saturn’s giant moon Titan. Titan has a density of 1.9 g/cm3, much like Ganymede and Callisto. Titan was originally believed to be the biggest moon in the solar system, (hence the name) but we later discovered that the visible disk is just the orange-colored atmosphere, not the surface. The atmosphere was later found to be immense: much bigger than the Earth’s, which caused the misleading size estimate. The surface of Titan is completely obscured at most visible wavelengths by the ubiquitous orange-brown haze, however we shall see that there are tricks we can use to see in from outside. Titan Overview Dr Conor Nixon Fall 2006

ASTR 330: The Solar System In 1944Gerard Kuiper discovered emission features of methane gas from the atmosphere in the infrared spectrum. Close analysis of the methane lines showed that the gas was in contact with another gas which was much more prevalent. At the time, nitrogen (N2) was guessed to be the mysterious unseen gas, but little further progress was made until the 1970s. By the time of the Voyager Saturn encounter, several hydrocarbon gases had been discovered, but the major component of the atmosphere was still unknown. Even worse, the surface pressure was unknown: estimates ranged widely from a few millibars to 10s of bars. Titan Atmosphere: Background Dr Conor Nixon Fall 2006

ASTR 330: The Solar System Voyager 1 made a close fly-by of Titan in November 1980, the only spacecraft visitor prior to Cassini arrival in mid-2004. Voyager 2 did not pass close to Titan, as it was deflected to Uranus and Neptune. The Voyager 1images were disappointing: they were not able to reveal much in the way of atmospheric features. However, the technique of radio occultation was used to greatly improve our estimate of the atmospheric temperatures and pressures. In this technique, the spacecraft flies behind the moon as seen from the Earth, and scientists ‘listen’ to the radio signal as it slowly fades through the atmosphere, then is blocked by the surface, before re-emerging on the other side. In this way, the atmospheric density profile is measured. Voyager 1 Titan Encounter Dr Conor Nixon Fall 2006

ASTR 330: The Solar System By assuming a mainly nitrogen composition and using the density profile measured by radio occultation, a surface pressure of 1.5 bar was determined. Titan therefore has a more massive atmosphere than any terrestrial planet except Venus. The surface pressure of 1.5 bar is not much higher than the Earth’s; however, the gravity is much less. So, it takes much more atmosphere to exert the same surface pressure. Calculations show that there is ten times as much atmosphere above a square meter on Titan as above a square meter on the Earth. The atmosphere also stretches ten times further into space. Size of Titan’s Atmosphere Dr Conor Nixon Fall 2006

ASTR 330: The Solar System The infrared spectrometer on Voyager 1 was able to provide detailed information the composition of gases in the atmosphere. Many of the gases are hydrocarbons (C and H) species; some are nitriles (C, H and N) but there are few oxygen species. Titan Infrared Spectrum Dr Conor Nixon Fall 2006 Figure credit: Andrew Fortes/UCL

ASTR 330: The Solar System Is this atmosphere like the Earth’s in terms of composition? Could we breathe on Titan? What gases are found in the Earth’s atmosphere but not on Titan. Why? Table of Titan gases Dr Conor Nixon Fall 2006 Table credit: NSS

ASTR 330: The Solar System • Titan’s atmosphere is now known to be largely nitrogen (like the Earth), with methane and possibly argon as the most significant minor gases (unlike the Earth). • The nitrogen and methane are broken up in the upper atmosphere into C, H and N atoms and ions, by the action of: • solar UV radiation (‘photodissociation’) • bombardment by charged particles from Saturn’s magnetosphere • cosmic rays • These fragments re-combine to form the hydrocarbons and nitriles detected in the infrared. • Oxygen must come from water, probably supplied by comets. With some O atoms present in the mix, formation of amino acids (precursor to life) becomes possible. Atmospheric Composition and Chemistry Dr Conor Nixon Fall 2006

ASTR 330: The Solar System This image shows a Voyager image of the ‘limb’ of Titan: the edge of the planet where we see the atmosphere highlighted against space. This image has been greatly enhanced to bring out the faint contrast detail. • The ‘detached’ blue haze layer lies several hundred km above the main haze layer. • HCN molecules can link together in a chain called a polymer. • The reddish brown haze we see lower down is probably a mixture of HCN polymer, and other condensed organic compounds. Smog On Titan Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL Voyager

ASTR 330: The Solar System Cassini-Huygens was launched in 1997 and arrived at Saturn in 2004, after a flybys of Venus, Earth and Jupiter. Cassini has since performed over 30 orbits of Saturn, photographing all the major moons and making many discoveries. Cassini carries 12 scientific instruments. Cassini-Huygens Dr Conor Nixon Fall 2006 Figure credit: NASA/JPL

ASTR 330: The Solar System • This image dramatically shows the improvement in detail seen by Cassini in 2004, compared to Voyager 24 years earlier (previous slide). • Many individual haze layers are now seen, showing a complex situation which must involve not just chemistry, but also vertical and horizontal wind movements to cause layers to separate. Multiple haze layers Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL/Space Science Institute

ASTR 330: The Solar System Titan’s atmosphere is constantly evolving, yet stays the same! How? Methane is constantly being destroyed by the photochemical reactions in the upper atmosphere. Once CH4 is broken into pieces, some of the hydrogenescapes to space. The remaining pieces now have proportionately more carbon compared to hydrogen than before, and molecules such as C2H2 and C3H8 result. The atmosphere is cold enough that these molecules can condense and form liquid hydrocarbon ‘seas’ or ‘lakes’ on the surface. Other atmospheric compounds such as CO2 and the other nitriles can actually solidify into ices. Chemical Evolution of Titan’s Atmosphere Dr Conor Nixon Fall 2004

ASTR 330: The Solar System The solid ices would sink rather than float on the hydrocarbon sea, because their densities are greater than the liquids. Titan is evolving, but very slowly due to the extreme cold. Very likely, we are seeing the same conditions as 4 billion years ago. We have evidence that this evolution is occurring. The form of methane containing the heavy isotope of hydrogen (CH3D) is enhanced over the normal form of methane (CH4): exactly as we would expect if the lighter, normal hydrogen isotope (‘H’) is gradually escaping, allowing the heavier isotope (‘D’) to accumulate. Note that Titan’s surface is only 94 K - far too cold for liquid water. Water will be almost totally lacking from the atmosphere, excepting any that drifts in from cometary debris. However, scientists now know that methane can condense into clouds and ‘rain’ out under certain conditions. Atmospheric Evolution (contd) Dr Conor Nixon Fall 2006

ASTR 330: The Solar System • The sequence of three Cassini images below show how an initial image can be improved by the application of specialist techniques such as ‘stacking images’ (center) and edge-sharpening (right). • Broad geographic differences can be seen near the top: light and dark terrains, and right over the south pole, clouds which change on timescales of just a few hours. South Polar Clouds Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL/Space Science Institute

ASTR 330: The Solar System • This figure summarizes all the processes we have discussed in Titan’s atmosphere: • photodissociation of molecules • haze formation • chemical reactions • methane clouds and rain. Atmospheric Structure Dr Conor Nixon Fall 2006 Figure credit: Andrew Fortes/UCL

ASTR 330: The Solar System Several impact craters have now been confirmed on Titan, by using both special camera filters, and also radar to peer through the hazy ‘veil’. The view (right) is a radar reflection map revealing a large (60 km) impact crater. Brighter areas indicate rougher terrain: note how the ejecta blanket is rougher than the surroundings. The darker ‘plains’ also seem to show linear ‘dune’ features… Impact Craters Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL

ASTR 330: The Solar System Here is another radar view of Titan, showing much different terrain. The dark linear markings have been dubbed ‘cat scratches’ and appear to be wind-blown dunes, perhaps of dirty ice crystals. We also see an apparent drainage system in the lower right, with channels 1-2 km wide and up to 200 km long. These seem to be associated with a circular basin feature 450 km in diameter and have partly eroded the basin rim. Titan Dunes and Channels Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL

ASTR 330: The Solar System The riddle of Titan’s methane supply (to make all that haze) may finally have been solved by Cassini’s VIMS instrument. Volcanoes? This feature, dubbed the ‘snail’ may be a cryovolcano (a volcano of ice lava). This may be how fresh methane escapes from inside the moon to re-supply the atmosphere, compensating for destruction by sunlight. Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL/University of Arrizona

ASTR 330: The Solar System The Huygens probe was carried on Cassini for three orbits of Saturn, before being released on a collision trajectory with Titan. The probe coasted in sleep mode for 22 days, and then woke up 15 minutes prior to entering Titan’s atmosphere. Huygens was slowed by a heat shield and then two chutes, and descended through the atmosphere for 2.5 hrs, finally landing on the surface. The science data was relayed to the orbiter, and thence back to Earth. Huygens Dr Conor Nixon Fall 2006 Picture credit: NASA/ESA

ASTR 330: The Solar System • The Huygens landing of January 2005 was the first ever landing in the outer solar system, and the first landing on the Moon of another planet (other than Earth). • Despite some technical glitches, Huygens was a great scientific success, and made many important in situ measurements including: • winds • gas composition and isotopes • aerosol measurements • surface characteristics Huygens Descent and Landing Dr Conor Nixon Fall 2006 Picture credit: NASA/ESA

ASTR 330: The Solar System This image mosaic has been reconstructed from 30 individual snapshots taken by the Huygens cameras looking in different directions during the descent; ranging from 13 to 8 kilometers in altitude and showing a 30 km -wide area. Huygens Descent Images Dr Conor Nixon Fall 2006 Picture credit: NASA/ESA/University of Arizona

ASTR 330: The Solar System Left images from Huygens show the view from 16 km (top) and 8 km (bottom). The upper image shows apparent drainage channels reaching a shoreline of some type. The right-hand image shows the view from the surface. Icy pebbles are seen in the foreground, which are 10-15 cm across. Work continues to combine Huygens data with Cassini results to assemble the ‘big picture’ of Titan. Reaching the surface Dr Conor Nixon Fall 2006 Picture credit: NASA/ESA/University of Arizona

ASTR 330: The Solar System This image of Neptune’s moon Triton (not to be confused with Titan!) must have been taken by which spacecraft? The mottled surface in the upper part of the image is called ‘cantaloupe terrain’ due to its appearance. Triton Dr Conor Nixon Fall 2006 Picture credit: NASA

ASTR 330: The Solar System Triton at 2705 km diameter is just over half the size of Titan. Triton orbits Neptune the ‘wrong way’ - in the opposite sense to the motion of the planets around the Sun, and in which most moons around their parent planets. Hence, we suspect that Triton was captured by Neptune’s gravity, rather than forming with the planet. Triton must have formed as an independent planetesimal, much like Pluto. However, Triton has been able to synchronize its rotation by tidal forces so that the same side always faces Neptune. Triton’s density of 2.1 g/cm3 indicates an ice-rock mixture, but with more rock than ice. In both density and diameter, Triton is very similar to Pluto. Triton - Overview Dr Conor Nixon Fall 2006

ASTR 330: The Solar System Spectroscopy from the Earth has revealed the signature of methane, carbon dioxide, carbon monoxide, and nitrogen ices on the surface. Triton has no large impact craters and few small ones. Its surface therefore appears to be very young, perhaps as young as Europa. Triton is quite reflective, so that it absorbs little of the faint solar radiation reaching its surface. It is therefore extremely cold: 37 K. The volatile ices we have mentioned (CO, CO2, CH4, N2) overlie a frozen water ice crust. The surface seen by Voyager is mostly a flat plain, with some dark streaks. It appears to be a polar cap around the south pole. The dark material seems to be blown around by a thin wind. Triton’s Surface Dr Conor Nixon Fall 2006

ASTR 330: The Solar System Triton’s thin atmosphere has a pressure of just 16 microbar (16x10-6 bar). However, the atmosphere was not devoid of activity. The Voyager cameras photographed haze and clouds. The haze is probably produced in a similar way to the Titan haze, and much is already condensed out on the surface. The most interesting features were dark plumes of material rising from the surface and flowing downwind. Triton - Atmosphere These ‘cryo-geysers’ show a similar appearance to campfires on the Earth. We do not yet know what causes them. Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL

ASTR 330: The Solar System Largest Moons, Smallest Planets Dr Conor Nixon Fall 2006 Picture credit: solarviews.com

ASTR 330: The Solar System Summary: Galilean Satellite Interiors Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL Galileo SSI

ASTR 330: The Solar System Summary Galilean Satellite Surfaces Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL Galileo

ASTR 330: The Solar System • Compare Io to Europa. Which is (a) denser (b) brighter? • What causes the massive volcanic activity on Io? • Why can Io not synchronize its rotation to minimize tidal forces? • What happened to most of Io’s volatiles? • Which gases and compounds have been detected on Io? What causes the different hues? • Why do we see fewer craters as we traverse the Galilean moons from Callisto to Io? • What causes an Ionian plume eruption? Quiz-Summary Dr Conor Nixon Fall 2006

ASTR 330: The Solar System • What are the two major components of Titan’s atmosphere? • How was the surface pressure measured? • How are hydrocarbons and nitriles formed in Titan’s atmosphere? • What might we expect to find on the surface of Titan? • Which space probe is going to land on Titan? When? • What similarities are there between Triton and Pluto? • What do the orbital characteristics of Triton lead us to believe about its origin? • What ices do we see on Triton’s surface? Quiz-Summary Dr Conor Nixon Fall 2006

ASTR 330: The Solar System