The enigmatic polar caps of Mars

1. The enigmatic polar caps of Mars • Brief summary of history of observations • Theory of seasonal cap behavior • Residual (permanent) polar caps and evidence for climate change 105 to 107 years 10 – 200 years • Future space observations

2. Christian Huygens

3. Sir William Herschel • Discovered Uranus • Discovered IR radiation • Deduced disc galaxy • Recognized that polar caps were seasonal; used observations to measure Mars’ obliquity (1784)

4. G. Johnstone Stoney • Suggested term “electron” for unit of charge in 1891 • Applied kinetic theory of gasses to planetary atmospheres. Studied helium in earth’s atmosphere. • Used this to suggest that Martian polar caps are CO2.

5. Composition of Seasonal Cap • Scientists (except Stoney) assumed that the caps were water ice snow • This assumption was crucial to Percival Lowell’s canal theory, since he assumed the melting polar caps were the source of water. • Kuiper used reflection spectra to identify water ice in cap. He also measured CO2 in atmosphere.

6. Mariner 4 (1965) • Radio occultations  Martian atmospheric pressure is ~ 600 pascals, over an order of magnitude less than most conservative previous estimates. Therefore, pATM ~ pCO2.

7. Robert Leighton and Bruce Murray

8. Polar energy balance • Absorbed insolation + net energy advected into region + conduction from subsurface + IR radiance from atmosphere + latent heat released by subliming CO2 = energy radiated by surface • L & M (Science, ’66) showed that CO2 will condense and that seasonal polar caps are carbon dioxide

9. Viking Landers: 1976-1982 Pressure

10. 1980’s-1990’s • Hiatus in space exploration of Mars • Modeling of polar caps using the Viking pressure curves as the primary constraint • One D models gave way to GCM models based on primitive atmospheric equations • Curves can be fit to pressure and predicts mass of CO2 condensed

11. CO2 Condensed Mass • During polar night the latent heat should be roughly equal to the radiation • Unphysically low emissivities required to avoid having too much CO2 condense leading to large amplitude pressure curve • Is there an additional source of energy (in addition to CO2 latent heat) in polar night?

12. Conduction from sub-surface • The ability of the surface to store energy is determined by thermal inertia = √KTρcP • Thermal inertia of surface traditionally determined from diurnal temperature observations that sample ~ 1-10 cm. For that inertia, conduction is unimportant • However, the seasonal penetration is much greater and samples ~ 10 cm – 1 m

13. Gamma Ray Spectrometer on Mars Odyssey

14. GRS determination of water

15. Thermal storage in surface • GRS discovered that in the polar regions there is nearly pure water ice just beneath the surface • This enhances the conduction storage term and reduces CO2 condensation to match pressure

16. Residual (permanent) Caps at both poles

17. Buffering • Suppose that CO2 remains at one of the poles all year. • In equilibrium, the energy absorbed by the cap = Energy radiated by cap = σT(p)4 • Energy absorbed ~ sinα (obliquity) • In pure CO2, sublimation temperature is a function of pressure • So if there is big enough block of CO2 at poles, p will change with obliquity

18. Obliquity • The obliquity of Mars changes greatly over relatively short time scales due to the effects of other planets. • Pressure change would bring about different climate (Sagan & Malin, ’73)

19. One of main goals of polar orbiting Viking Orbiter 2 was to determine composition of the much larger residual north polar cap. VO2 measured water vapor concentration (MAWD) and surface temperature (IRTM) of 220 K during summer. Result: residual north polar cap water ice IRTM later showed that smaller residual south cap is CO2 ice because its temper-ature remains at ~ 150 K all summer.

20. Polar Layered Terrain • Viking discovered that ground underlying the caps is composed of many layers • Possibly responds to variation of orbital parameters with T ~ 105 – 107 years • Layers composed of various mixtures of dust and water ice

22. Mars Orbiter Camera on MGS • Two wide angle (140˚ FOV) cameras make daily global map in red and blue wavelengths • High resolution camera can resolve features as small as ½ meter at nadir; minus blue filter

23. Earth and Moon from MarsMars Odyssey from MGS

24. Residual South Cap: MOC Color images of the residual south polar cap at LS=306º on (A) February 22, 2000, (B) January 9, 2002, and (C) November 28, 2003.

25. MGS – Viking – M9 RSPC

26. “Swiss cheese” terrain

27. One Mars Year Change

28. Changes since Mariner 9 (16 MY)

29. 2001 Dust Storm

30. Mountains of Mitchel 1999/01

31. Effects of Atmospheric Dust on Sublimation

32. Flux redistribution by dust • Visible flux at surface CO2 frost decreases with increasing dust optical depth • However, infrared flux increases with increasing optical depth because of emission by hot dust

33. Albedo / Emissivity of CO2

34. Effect of dust on sublimation • Region with 0% dust sublimes more rapidly with increasing optical depth • Region with large dust content and low visible albedo sublimes more slowly • Effect on sublimation small for typical areas in the seasonal cap

35. RSPC Albedo • Measurements from HST HRC at 2003 opposition • Dashed lines are albedos assuming τ = 0; solid lines τ = 0.2 • Albedos sufficient to stabilize residual cap • Dust will increase sublimation rate

36. Conclusions • The RSPC is a unique feature totally unlike other portions of the polar caps • The RSPC is dynamic on time scales of years. Stratigraphy suggests short deposition periods separated by longer periods of erosion • The timeline, together with Mariner 9 B images, suggest that the last period of deposition was somewhere around 1970 • Late season dust storms could effect removal of RSPC units • May also be connected with H2O ice distribution

37. Mars Reconnaissance Orbiter • HIRISE: hi res with some color • MARCI 180˚ FOV 5 vis + 2 uv bands • CTX: 5 meters / pixel • CRISM: imaging spectrometer .4 - 4μm • MCS: atm profiles • SHARAD: 15 meter depth resolution

38. Polar Observations • 3pm orbit (compare 2 pm for MGS 5 pm Odyssey) • Periapsis over south pole at 255 KM • Apoapsis over north pole at 320 Km • After one (earth year) < 5 km between ground tracks at equator • 12 orbits cross the poles every day

39. MARCI Polar Science • Acquire albedo maps of the poles in five bands and two UV channels • Study behavior of dust storms and condensate clouds associated with the frost boundaries of both poles • Search for interannual variability of and within seasonal caps • Diurnal behaviors of storms and clouds • Study frost phase functions at various wavelengths

40. Polar Observations • CTX is ideal for monitoring temporal changes in “Swiss cheese” features in RSPC, spiders, dark spots, etc. in the South Polar Region • Similarly, CTX should be useful for monitoring albedo features and specific areas in the north polar region. • CTX should reveal details of polar dust storm and cloud structures