Lecture 3: Small Objects

A. P. Ingersoll

api@gps.caltech.edu

1. Some of these objects are planets, some are satellites of planets. Some of these
atmospheres are thick (mean free path much less than the atmospheric scaleheight), and some are thick. Except for Titan, all the atmospheres are controlled by
the vapor pressure of a frost on the surface.
2. The last column gives the gravitational binding energy divided by the thermalenergy of a molecule = (escape velocity/thermal velocity)2. The units are arbitrary,
but it gives a crude measure of which objects are more likely to retain anatmosphere and which are less likely. Ganymede should have an atmosphere if Io,
Triton, and Pluto do, but it doesn’t have one. Mercury and Earth’s moon have
exospheres – atmospheres so thin that the molecules don’t collide with each other;
they just collide with the surface.
3. Start with Titan – the largest moon of Saturn. This is the Voyager view in visible
light. Covered with clouds and haze. You can see the surface at 0.5 microns, although
you can see it at 0.95 microns.
4. Titan with the Sun behind the object. The gravity is low, so the scale height islarge, and the atmosphere does not have a sharp limb – the edge looks “fuzzy.”
5. Mostly molecular nitrogen, N2 (90 – 97%), argon Ar (0 – 6%), and methane CH4(0.5 – 4%). The range for nitrogen and argon is an uncertainty. The range formethane is temporal and spatial variability.
6. Vertical structure. Temperatures were measured by Voyager radio occultation(sending a radio beam tangentially through the atmosphere) and by the Huygens
probe, which parachuted to the surface and measured T, P, composition, images.
7. Methane is being destroyed by UV light from the Sun. The hydrogen escapes fromthe gravity field of Titan, and the remaining C and H atoms form higherhydrocarbons. The photodissociation rate of CH4 and the escape rate of H have beenmeasured. This raises two problems: (1) Titan’s surface should be covered withhydrocarbon lakes 100 meters deep. (2) There must be a subsurface source ofmethane that is still active. These problems have not been solved.
8. Titan has a hydrologic cycle, but the condensable gas is methane. There are twomajor differences from Earth. (1) No ocean, so large parts of the surface should dryout, unless there is a global aquifer that moves the methane around. (2) The latent
heat stored in the methane vapor is large and the power supply is small, so thevapor residence time is very long. M is the mass/area of condensable in theatmosphere, ML is the latent heat/area. Fsurf is the solar power/area at the surface.
τv is the vapor residence time – the time it would take to re‐charge the atmosphere ifit should all rain out. Heavy rain should be infrequent, or else you have nothing but
mild drizzle (mist).
9. There is evidence of erosion, so there must be heavy rains. We don’t know how
old these features are. They could be hundreds or thousands of years old.
10. Looks like Los Angeles as you fly in over the ocean. The smooth dark areas are
not liquid. These images were taken by the Huygens lander as it parachuted to thesurface.
11. Rounded boulders (of water ice at 92 K). They look like boulders from thebottom of a stream bed ‐ evidence of erosion.
12. Synthetic aperture radar (SAR) can see through the clouds. The radar beam is
not looking straight down, so a smooth surface reflects the beam off in a different
direction. Liquids look dark (colored blue in this image), and rough surfaces lookbright (colored yellow in this image).
13. This is a more honest image, because it has no color, but it is just as interesting.
14. These are images from Earth – the Keck telescope with adaptive optics. Theyshow clouds near the summer pole over a 2 Earth‐month period. The cloud activity
stopped one Titan month after summer solstice. Titan’s year is the same as Saturn’s:
30 Earth years.
15. These are images taken by the Cassini orbiter as one of the cloud outbursts wasending. They look like cumulus clouds, which is consistent with moist convection.
16. Sand dunes on the surface as seen by the radar from the Cassini orbiter. The
material of the dunes may be water ice contaminated by hydrocarbons.
17. More sand dunes. Morphology and GCM estimates of wind suggest the dunes are
longitudinal – they run parallel to the wind (mostly east‐west).
18. Dry lakes. The liquid either evaporates or soaks into the underground aquifer.
19. Evaporation minus precipitation for Earth, measured in cm/year. The excess
precipitation that falls in the tropics and high latitudes flows into the oceans, which
supply the evaporation in the sub‐tropics (10 ‐ 30° latitudes).
20. Results from a GCM of Titan’s atmosphere. They assumed that liquid methane
was present on the surface at all latitudes. This could be a bad assumption if there isno underground aquifer. Another problem with this model is that the global
precipitation exceeds the global evaporation. The hydrologic cycle does not balance.

In fairness to the authors, they published the model before the Cassin‐Huygens
reached the planet and returned data [Tokano etal, Icarus 153, 130‐147 (2001)].
21. Wind profile measured by Huygens using the Doppler shift of the radio signal. At
40 km altitude the speed is 20 m/s; at 140 km the speed is over 100 m/s. This isfaster than the jet streams on Earth, even though the solar power is 1% thatreaching the Earth.
22. GCM results for wind and temperature – much less activity than the realatmosphere.
23. There is a cross‐equatorial Hadley cell with rising motion in the summer
hemisphere. Because the rotation is slow (one Titan day = 16 Earth days), theHadley cell extends to high latitudes.
24. Talk now about objects with thin atmosphere. Mercury and the Moon have
exospheres – probably molecules sputtered off the surface by energetic particles.
Pluto and Triton have nitrogen atmospheres controlled by frost on the surface. Io
has a SO2 atmosphere on the day side; it is controlled by surface frosts and byvolcanoes.
25. This is Mars, which has a CO2 atmosphere that is controlled by surface frosts.
The atmosphere is massive enough that it only drops by 30% during winter in eachhemisphere.
26. This is a model of Pluto’s N2 atmosphere. Shaded areas in the lower curve arefrost. The white areas are frost‐free. Pluto’s year is ~200 Earth years. The sinusoidal
line is the latitude of the Sun as seen from Pluto’s surface. Perihelion (Pluto closestto the Sun) occurs during northern spring, so the rising portion is steeper than thefalling portion. Pluto’s obliquity (tilt of its spin axis) is 60°, so the frost caps reachmuch lower latitudes than they do on Earth. The upper panel shows theatmospheric pressure; it varies by orders of magnitude during the year. The
atmosphere collapses twice per year. This is for case when the surface has low
thermal inertia. It heats up and cools down on a short time scale – short comparedto the seasonal cycle. [Hansen and Paige, Icarus 120, 247‐265 (1996)].
27. This is for a high thermal inertia. The surface doesn’t respond to the seasonal
cycle. There is a permanent frost band around the equator. Averaged over the year,
the poles get more sunlight than the equator because the obliquity is so large. Oncethe ground is frost‐free, its low albedo keeps it warm, and frost cannot get started.
The atmospheric pressure is nearly constant.
28. This is Triton – the largest moon of Neptune. The spin is right handed about thesouthern pole, which is the one that you see. When Voyager took this image, it wassummer in the southern hemisphere. The streaks probably left over from thesouthern spring season. They are the remains of active vents of hydrocarbons andnitrogen vapor that were blown downstream by the wind as the frost wasevaporating. The diagonal orientation is consistent with equatorward flow (upwardin this image) in an Ekman layer. The flow developed a component opposite to thedirection of rotation, which is to the right in this image. So you have streaks thatslope upward and to the right. [Ingersoll, Nature 344, 315‐317 (1990)].
29. Another look at the streaks. The pole is off to the left, and the spin iscounterclockwise around that pole.
30. Io is the volcanic moon of Jupiter. Voyager discovered SO2 in the atmosphere, but
the vapor pressure is so low that SO2 vapor can only exist on the day side and undera volcanic vent.
31. The particles and vapor are thrown up to altitudes of 500 km
32. The lava lakes are hotter than most molten silicates on Earth (1800 K), implying
high magnesium content.
33. The satellites Europa and Ganymede do not allow Io to settle into a circularorbit. This means that Io is constantly being stretched and pulled, which generates ahuge amount of internal heat for a small body of its size.
34. Horizontal flow from the day side to the night side involves a gas expanding intoa vacuum. Like a rocket nozzle, the gas can convert all of its initial enthalpy into
kinetic energy, reaching supersonic speeds if it goes through a choke point. In a
rocket nozzle the choke is the narrowest part of the conduit – where the flowchanges from contraction to expansion. On Io the flow is contracting near the sub‐solar point because the surface is adding mass to the flow (evaporation rate E > 0).
The flow is expanding beyond this point because the surface is removing mass fromthe flow (E < 0). [Ingersoll etal, Icarus 64, 375‐390 (1985)].
35. Radar images of Mercury’s north and south poles. The poles are visible from
Earth (at a low incidence angle) because Mercury’s orbit is tipped 11° relative to theEarth’s orbit. The high radar reflectivity is characteristic of ice or other transparent
material.
36. The bright areas coincide with craters at the pole. Mercury has zero obliquity, sothe Sun never shines into these craters. Their temperatures are always less than 90K. At these temperatures, water ice is stable for the age of the solar system. Current
thinking is that a comet strikes the surface of Mercury and vaporizes. The molecules
hop around the hot surface. Many of them get ionized and carried away in the solarwind, but a few hop randomly into these cold traps. There they accumulate for long
periods of time. Although the Sun never shines in these craters, they do get struck bymeteorites. The frost may be in steady state balance between production and loss.