The Climate of Mars

1. The Climate of Mars Stephen Wood University of Washington

2. Why Study Mars? • Why study mice? • For climate science in particular, Mars is especially valuable because: • Its climate system is relatively simple and predictable • No oceans = no long-term “memory” • Thin atmosphere = less influence on surface temperature • Its climate is highly variable over time • Huge variations in obliquity • Evidence for very different climate regimes in the past

3. Mars Climate Predictability This storm system occurred in the same place at the same time of year 4 years in a row!

4. Mars Climate Variations Artist’s representation of Mars at high obliquity (credit Brown University)

5. State of Mars knowledge before 1900

6. Telescopic Observations of Mars Opposition: When an outer planet is on the opposite side of the Earth from the Sun Future Mars Oppositions

7. 1877 brought the ideal opportunity in the form a particularly favorable opposition of Mars.... Schiaparelli prepared for it almost like a prize fighter, avoiding "everything which could affect the nervous system, from narcotics to alcohol, and especially ... coffee, which I found to be exceedingly prejudicial to the accuracy of observation." Giovanni Schiaparelli (1835-1910) Brera Observatory Milan, Italy Telescope: 8.6” refractor ...mapped “cannali”, meaning “channels”, but was translated as “canals”

8. Percival Lowell (1855-1916) Lowell Observatory Flagstaff, Arizona Telescope: 24” refractor Lowell produced intricate drawings delineating "canals“... He concluded that the bright areas were deserts and the dark were patches of vegetation. He further believed that water from the melting polar cap flowed down the canals toward the equatorial region to revive the vegetation. Lowell thought the canals were constructed by intelligent beings who once flourished on Mars.

11. excerpt from Mars by Percival Lowell (1895) Chapter 3 The Polar Cap After air, water. If Mars be capable of supporting life, there must be water upon his surface; for, to all forms of life, water is as vital a matter as air. On the question of habitability, therefore, it becomes all- important to know whether there be water on Mars. On the 3d of June, 1894, the south polar cap stretched, almost one unbroken waste of white, over about 55 degrees of latitude. A degree on Mars measures 37 miles; consequently the cap was 2,035 miles across. ... the cap was already in rapid process of melting; and the speed with which it proceeded to dwindle showed that hundreds of square miles of it were disappearing daily. As it melted, a dark band appeared surrounding it on all sides...it was the darkest marking upon the disk, and was of a blue color...a deep blue, like some other-world grotto of Capri. But the most significant fact about the band was that it kept pace with the polar cap's retreat toward the pole. As the white cap shrank it followed pari passu so as always to border the edge of the snow. It thus showed itself not to be a permanent marking of the planet's surface, since it changed its place, but a temporary one, dependent directly upon the waning of the cap itself ... ...instantly suggested its character, namely, that it was water at the edge of the cap due to the melting of the polar snow. Before going further we will take up here at the outset the question of the constitution of these polar caps...the possibility that instead of ice we have here snow-caps of solid carbonic acid gas (carbon dioxide).

12. Volatiles: Molecular compounds which experience phase changes within the range of planetary surface temperatures (and pressures) Ices: Any volatile in the solid phase Phase Stability Temperatures at P = 1 bar solid liquid gas

13. Pluto Titan Earth solid liquid gas

14. Radiative Equilibrium Temperature Fin = Fout Incident - Reflected = Emitted Iin(πR2) - AIin(πR2) = sTE4(4πR2) Iin (1 - A) (πR2) = sTE4(4πR2) Iin (1-A) / 4 = s TE4 TE = { Iin(1-A) / 4s }1/4 Iin/4 (Iin /4)A s TE4 planet

15. Radiative Equilibrium Temperature TE = { Iin(1-A) / 4s }1/4

16. Radiative Equilibrium Temperature TE = { Iin(1-A) / 4s }1/4 Inverse Square Law I = Fsun / d2 Fsun = IEarth dEarth2 Fsun = IMars dMars2 IEarthdEarth2 = IMarsdMars2 IMars = IEarth (dEarth /dMars )2 IMars = IEarth (1.00/1.52)2

17. Radiative Equilibrium Temperature TE = { Iin(1-A) / 4s }1/4

18. Mars (est.) Pluto Titan Earth solid liquid gas

19. Percival Lowell’s argument for the polar caps being H2O ice instead of CO2 ice: The dark band that surrounds the retreating polar cap in spring must be due to the ice melting into a liquid phase, and CO2 cannot exist as a liquid at the probable temperatures and atmospheric pressure of Mars. Although H2O also is not liquid at the temperatures we calculated, Lowell estimated that Mars is “as warm as the south of England”

20. Percival Lowell’s argument for the polar caps being H2O ice instead of CO2 ice: The dark band that surrounds the retreating polar cap in spring must be due to the ice melting into a liquid phase, and CO2 cannot exist as a liquid at the probable temperatures and atmospheric pressure of Mars. Although H2O also is not liquid at the temperatures we calculated, Lowell estimated that Mars is “as warm as the south of England” • Our best guess for the composition of the polar caps based on pre-1900 information and calculations: • The seasonal polar caps are probably not H2O ice, because Mars’ surface temperatures are too cold for H2O ice to melt (or sublimate) • If the dark band around the cap is due to melting, then the most likely composition is NH3 (ammonia) ice • If the dark band has a different cause, then the composition could also be CO2 ice

21. 1937

22. 1937 Gerard Kuiper uses spectroscopy to measure the amount of CO2 in Mars’ atmosphere – finding it has ~2 times as much as Earth’s atmosphere. 1947 Large yellowish cloud observed over the Hellas-Noachis region of Mars, and in less than a month, spreads to cover the whole planet Gerard Kuiper suggests that seasonal removal of the dust by wind currents can explain the "wave of darkening." 1956

23. 1959

24. 1959 • H. Spinrad, G. Münch, and L. D. Kaplan make improved spectroscopic observations of Mars’ atmospheric composition with the 100-inch reflector at Mount Wilson, CA. • Their findings: • Water vapor: ~1% of the amount in Earth’s driest deserts • CO2 partial pressure: 4 mb • Total surface pressure:  25 mb 1963

25. 1965 - Mariner IV Mars fly-by mission

26. Key Results of Mariner IV • First close-up images of Mars • cratered, moon-like surface • clouds and hazes • First direct measurement of Mars’ atmospheric density • surface pressure ~ 4 mb Therefore... Total surface pressure = 4 mb (Mariner IV) CO2 partial pressure = 4 mb (Earth-based spectroscopy) Mars’ atmosphere is ~100% CO2

27. Carbon Dioxide

28. North polar cap Summer Fall Winter Spring Latitude Winter Spring Summer Fall South polar cap Days CO2 Vapor Pressure / Frost Pt. Temp. 4 mb 145 K

29. 1969 - Mariner 7 fly-by Near-infrared spectra of south seasonal polar cap confirms that composition is CO2 ice Region I - cap edge: Dirt + CO2 ice + H2O ice Region II – fine-grained CO2 ice (1 cm) Region III – large-grained CO2 ice (>10 cm!) Region IV – transparent CO2 ice (+ Dirt) (Calvin & Martin, 1994)

30. 1976 – Viking Mission 2 Orbiters and 2 Landers Meteorology Instrument Package (pressure, T, wind) Camera Carl Sagan

31. Viking Lander 1 pressure measurements (1976-1982)

32. Viking Lander measurements (1976-1982) Leighton & Murray 1966 prediction !

33. Comparison of CO2 Amounts and Pressure Variations on Earth and Mars • Mars’ atmosphere has 50 times more CO2 than Earth • The total atmospheric pressure at the surface of Mars is 150 times less than it is on Earth • The variation in surface pressure on Earth associated with weather systems is ~ 20 mb, which is 2% • The seasonal variation in surface pressure on Mars is ~2 mb, which is 29% ! • On Earth, a 29% decrease in air pressure would be equivalent to going to the top of Mt. Rainier. • The surface pressure on Mars is equivalent to the pressure at 100,000 ft. altitude in the Earth’s atmosphere Earth’s CO2 cycle Mauna Loa Atmospheric Observatory

34. Dust

35. Sahel Dust Storm T. Schoennagel

36. 500 km wide Mars dust storm southern high latitude spring

37. 1971-1972 – Mariner 9 orbiter mission • First spacecraft to orbit another planet • Arrived during a global dust storm, which totally obscured the surface for months • Operated for 349 days in orbit • Transmitted 7,329 images, covering over 80% of Mars' surface. revealing: • river beds • volcanoes • canyons • erosion and deposition by wind and water • weather fronts, clouds, and fogs

38. as the dust settles...

39. Water & Past Climate

40. Valles Marineris • Vast canyon system named after Mariner 9 • 4,500 km long, 200 km wide, and up to 7.7 km deep (~ 10x bigger than the Grand Canyon) • Apparent source location for large outflow channels carved by catastrophic floods, billions of years ago (similar to Channeled Scablands of E. WA)

41. Evidence for catastrophic floods on early Mars (2-3 billion yrs. ago)

42. Possible ancient river valley

43. Evidence of persistent water flow on early Mars: a river delta 10 km

44. Eberswalde Crater delta formation “fossilized” riverbed meander MGS MOC Release No. MOC2-1225 Malin Space Science Systems

46. Residual North Polar Cap in northern summer season • Composed of H2O ice • 500 km in diameter • 2.5 km thick • Contains fine-scale layering which may record past climate variations • Spiral trough features are still unexplained • Ice evaporates in summer, providing primary source of global atmospheric water vapor • Surface shows very few impact craters, therefore is very young (< 1 million yrs)

47. Viking Orbiter image (60 m/pixel) Fine-scale layering in the North polar residual water ice cap MGS MOC image (3 m/pix) NASA/MSSS

48. Morning frost (H2O Ice) at the Viking Lander 2 landing site (48°N) • identified as H2O ice because surface temperatures were too warm for CO2 ice

49. Summary Slide 1 Atmospheric Composition and Temperature Range

50. Summary Slide 2 • In comparison to the Earth, Mars has a more variable, dynamic, and predictable climate – thus is valuable for testing climate physics and models • CO2 • Mars’ atmosphere is 95% CO2, with a surface pressure of 6-8 mb • Vast seasonal deposits of CO2 ice form each winter in the polar regions • 1-2 m thick, and extend down to 50 deg. latitude • decrease total atmospheric mass by ~25% • H2O • Tiny amount of water vapor in the atmosphere (10 precipitable microns) • Lots of water ice clouds, and some water frost forms at higher latitudes • Residual polar caps consist of water ice (and dust) • up to 2500 m thick in central portion • ~500 km in diameter • contain fine-scale layering which may record past climate variations