Lecture 2: Mars

A. P. Ingersoll

api@gps.caltech.edu

1. The three main elements of the weather are CO2 , water, and dust. CO2 is the main
constituent of the atmosphere, and unlike nitrogen and oxygen on Earth, itfluctuates by 30% every year due to condensation at the poles. Water, especiallyliquid water, is of potential biological interest, but the total pressure is so low thatwater boils at 0 C. Dust accounts for the largest changes in the weather, and is highly
unpredictable.
2. Here you can see ice clouds, which are probably water, and the northern ice cap.
Light and dark markings are permanent, but they get covered by dust sometimes.
3. The same face of Mars before and during a global dust storm. The dust storms arelike El Nino; they happen at a certain time of year (southern spring for the duststorms), but they don’t happen every year. Unlike Earth, there is no ocean to explain
the interannual variability.
4. Topography of Mars. The mountains are higher than on Earth because the gravity
is lower – the strength of the rocks is the same. The topography is almost as large as
the atmospheric scale height, making the atmosphere like an ocean basin.
5. Like Venus, the atmosphere is mostly CO2, but there are small amounts of othergases: nitrogen 2.7%, argon 1.6%, CO 0.1% (these numbers add up to 99.7%). The
mean surface pressure is only ~7 mbar, which is the vapor pressure of CO2 ice at Tof 148 K. The polar caps get this cold in the winter, and CO2 condenses out [figure isfrom de Pater and Lissauer]
6. The north residual polar cap – the way it looks in mid summer after the seasonal
covering of CO2 has gone away. The temperature is much higher than 148 K (~ 205K), indicating that it cannot be CO2 ice. The spectral features of water are prominent,
and the atmosphere is saturated with respect to water at ~205 K, so the residual cap
is water.
7. The south residual polar cap. The composition is not so obvious. The temperature
is lower, the water spectral features are less obvious, and the atmospheric water isless abundant in southern summer compared to northern summer.
8. The classic model is by Leighton and Murray [Science 153, 136‐144 (1966)], butthere are problems. They assumed that most of the CO2 on Mars is in the polar caps.
The temperature of the polar caps determines the vapor pressure of CO2 in theatmosphere. The polar caps are the reservoir that buffers the atmosphere. The
problem is that the seasonal frost disappears faster than their model would predict.
The observations suggest that it is gone by the first day of summer (Ls is the angular
position of Mars in its orbit. Ls = 0 is the first day of northern spring). If the residual
polar caps were CO2 , they would be losing net mass from one year to the next.

9. The growth and retreat of the polar caps is reflected in the fall and rise of theatmospheric pressure. Viking lander 1 was at a higher elevation than Viking lander
2, so its pressure is lower. There are two minima per year because there are twopolar caps that are out of phase with each other.
10. The polar cap mass balance is mostly just latent heat release as the cap grows,
which is balanced by the loss of heat as longwave radiation minus the gain of heatfrom absorbed sunlight (sunlight is zero during the winter). By adjusting the albedo
and emissivity of the CO2 frost, it is possible to match the seasonal pressure curves,
but then the cap is losing mass from one year to the next. This is the same problem
revealed in the visible observations of the growth and retreat of the cap. This
problem can be summarized as follows: (1) The residual CO2 ice cap is losing mass,and (2) there is an extra heat source that is causing less CO2 to form during thewinter or is causing it to evaporate faster during the spring. The authors have anexplanation for (2), which is heat stored in the water ice below the CO2 seasonal
frost cover, but they don’t have an explanation for (1). [Haberle etal Planet andSpace Sci, 58, 251‐255 (2008)]
11. Another group fitting the CO2 pressure data from the Viking landers. Bothgroups use a full general circulation model (GCM). Kuroda etal have the sameproblem: The observed retreat of the polar cap from visual observations is fasterthan the model would predict. [Kuroda etal, J. Meteor Soc. Japan, 83, 1‐19, (2005)]
12. A close look at the residual southern cap, the one that might be CO2. The sun is tothe upper left. These are circular depressions. The flat topped surfaces are thehighest. Called the Swiss Cheese Terrain because it has holes.
13. Change from year to year. The holes are growing in diameter (at the rate ofseveral meter per year) but not in depth. All the flat topped surfaces will be gone in
100‐300 years, which is very strange. It looks as if the flat topped surfaces werecreated just 100‐300 years ago.
14. Dust may be involved. The sun is to the upper left. The image on the right isearlier (southern spring). The image on the left is later (southern summer). Notice
the dark material on the sides of the depressions during summer.
15. Another example of changes from spring to summer. The flat topped surface has
almost disappeared as its walls evaporate. This material must be CO2 ice. It cannot
be water ice because water evaporates too slowly at these low temperatures.
16. Warmer temperatures indicate water ice at the bottoms of the circulardepressions. Maybe the flat topped material is CO2 (it is only 8‐10 m thick), and theremaining 3 km of the south residual polar cap is water ice [Byrne and Ingersoll,
Geophys Res. Lett. 30, 1696‐1699 (2003)]

17. Sunlight seems to control the shape of the depressions. They often have a “C”
shape with the opening pointing north (toward the sun). The uniform size suggests
they all started growing at the same time (80‐400 terrestrial years ago). It seems theLeighton and Murray model is wrong: There is no large reservoir of CO2 at the poles.
Instead it is a layer 8‐10 meters thick, which is going to disappear in a few hundredyears. This will raise the atmospheric pressure by a small amount (5%). Why does
Mars have so little CO2 compared to Earth and Venus? [Byrne and Ingersoll, Science299, 1051‐1053 (203)]
18. Change the subject. Measure the atmospheric temperature and compute thewinds from the thermal wind equation, assuming the wind is zero at the surface.
The winter pole gets very cold (upper left). [Zurek etal, in Mars, H. H. Kieffer etal,
Eds. U Arizona, 1992]
19. Model the winds with a GCM. They do a good job with the zonal mean. [Kurodaetal, J. Meteor Soc. Japan, 83, 1‐19, (2005)]
20. Models do a good job at simulating clouds. These are water ice clouds at theequator This is the NASA/Ames GCM. [Malin et al, Icarus 194, 501‐512 (2008)]
21. Water ice clouds over volcanoes. The air is lifted by the topography andcondenses as it cools.
22. Except for dust storms, the weather is very repeatable. The diurnal cycle
dominates because the atmosphere is so thin. Baroclinic waves are less important
than they are on Earth.
23. Dust storm and ice clouds together.
24. Evidence that water once flowed in the distant past. The impact craters indicatethat the surface is > 1 billion years old.
25. Further evidence that water once flowed in the distant past.
26. Gullies suggest water flowing from an underground source in the recent past (nocraters, unstable hillside)
27. More evidence. This subject gets a lot of debate. One problem is that water at 0°C(pressure of the vapor = 6.1 mbar) is close to the boiling point. Even if it doesn’t boil,
it will evaporate very fast because the lighter water molecules will be buoyant in theheavier CO2 gas. Any frost that forms overnight will sublimate without melting assoon as the sun hits it [Ingersoll, Science 168, 972‐973 (1970)]

28. Water vapor in the atmosphere vs season (Ls = 0‐90 is northern spring). There is
uncertainty about whether the water comes from the frost cap or from the dark areaaround the cap, perhaps left over from the previous winter.
29. Dust devils photographed from a lander on the surface of Mars
30. Dust devil tracks
31. Sand dunes. The question is, why do global dust storms occur every few years?
Why not every year?
32. El Nino involves a movement of warm water from the western Pacific to theEastern Pacific. The period of this oscillation is longer than a year, so El Ninos do notoccur every year. The analogy breaks down, because Mars has no ocean.
33. Dust has a positive feedback on the atmospheric circulation. The dust absorbs
sunlight and heats the air by a large amount because the atmospheric density is solow. The heated air rises and generates pressure gradients, which drive the windsand pick up more dust. [Basu etal, J. Geophys. Res. 111 (E9), 2006]
34. Another figure from Basu etal (2006). This dust storm started in a deep circularbasin (Hellas), and spread quickly in longitude (1st and 3rd columns). The source ofthe dust remained mostly in Hellas, although one or two secondary lifting centersdeveloped (2nd and 4th columns).
35. Another figure from Basu etal (2006). The model has a threshold wind speed forpicking up dust. If the threshold is too low (τ = 0.040 Pa), you get a dust storm everyyear (top panel). If it is too high (τ = 0.058 Pa), you never get a dust storm (bottompanel; year 1 is the exception because it depends on the initial conditions).
Interannual variability occurs only in a narrow range of the threshold wind speed.
Why should Mars be within this range? Maybe it organizes itself.
36. A simple 12‐parameter model of a Hadley circulation with dust. This is an
example of self‐organized criticality. The threshold wind speed depends on therecent history – it is high after a major dust storm, and it takes more than a year forthe threshold to fall to the point where dust storms occur again. The threshold windspeed is high when the windy places are bare, and it is low when the windy places
are covered with dust. In this example, the initial conditions are dusty, and there is adust storm every year until a balance is obtained where the time between dust
storms is equal to the time for the threshold to fall to the point where dust storms
occur. [Pankine and Ingersoll Icarus 170, 514‐518 (2004)]
37. Same model, but now the initial value of the threshold is too high and no dust
storms occur until the threshold falls to the point where dust storms occur. They do
not occur every year because it takes more than a year for the system to recoverafter a major dust storm. The bottom panel shows that the number of years between
dust storms is a random variable. Weather noise creates the randomness. The
behavior in the balanced state is independent of the initial conditions.