Lecture 16: The Climate of Early Mars

1
Lecture 16 The Climate of Early Mars
Meteo 466
2
MER Rovers

• Two rovers Spirit and Opportunity
• Arrived at Mars in early 2004
• Still operating!
• Design lifetime was for 3-6 months

Courtesy of Joy Crisp
3
(No Transcript)
4
Endurance crater (Opportunity)
Courtesy of Darren Williams
5
(No Transcript)
6
(No Transcript)
7
Blueberries
From Opportunity rover at Meridiani Planum
8
Spherules (Blueberries) on Mars
(False color image)
9
Opportunity roverMeridiani Planum

• Lots of hematite, in the form of blueberries
• Also see evidence for sulfate evaporites
• Both are evidence for standing water
• Doesnt tell you much about surface temperature,
  though, because sulfate solutions have low
  freezing points

Courtesy of NASA
10
Sulfuric Acid as Anti-Freeze
Courtesy of Andy Knoll
11

• Blueberries are made of hematite (Fe2O3)
• Liquid water is required for their formation

12
Crossbedding?

• Standing liquid water (no ice cover) is probably
• required to form these patterns (John
  Grotzinger, MIT)

13

• The evidence for liquid water on Mars surface is
  not new, however. It goes back to Mariner 9
  (1971) and then Viking (1976) ?

14
Martian Outflow Channel (Viking)
200 km
From J. K. Beatty et al., The New Solar System,
4th ed
15
200 km
Ares Vallis (from Viking)
From J. K. Beatty et al., The New Solar System,
4th ed.
16
200 km
Nirgal Vallis (Viking)
From J. K. Beatty et al., The New Solar System,
4th ed.
17
200 km
Warrego Vallis (Viking)
From J. K. Beatty et al.,The New Solar System,
4th ed
18
River channel
Nanedi Vallis (from Mars Global Surveyor)

• Grand Canyon required several millions
• of years to form
• The same should be true for Nanedi Vallis
• The water that cut this valley was probably
• fresh, like Colorado river water

3 km
19
Evidence for Persistent Flow
Malin and Edgett, Science, 2003
20
Fan in Holden NE Crater (MOC image)

• Cutoff channel meander
• Cross-cutting channels
• (inverted relief in both cases)

Malin and Edgett, Science, 2003
21
How warm did early Mars need to be to produce the
observed fluvial features?

• Outflow channels caused by catastrophic floods ?
  no obvious limit on surface temperature, but lots
  of water!
• Other valleys (Warrego, Nirgal, Nanedi) and
  features such as alluvial fans require a
  long-lived hydrologic cycle
• Answer depends partly on how much water was
  present water-rich planets (like Earth) have
  bi-stable surface temperatures

22
Present Earth (Ts 15oC)
Snowball Earth (Ts -50oC)
Caldeira and Kasting, Nature (1992)
23

• Conclusion
• A water-rich planet (like Earth) must have a mean
  surface temperature near or above the freezing
  point in order to have an active hydrologic cycle
• A planet with much less water (early Mars?) might
  be able to maintain an active hydrologic cycle at
  lower temperatures because ice-albedo feedback
  would be less powerful

24
Old theory for warming early Mars(Pollack et
al., Icarus, 1987)

• Do it with a dense CO2 atmosphere
• Volcanism and impacts should have generated lots
  of CO2
• CO2 removal rate would have been slow as long as
  surface temperatures were below freezing

25
The Carbonate-Silicate Cycle
(metamorphism)
26
However, the Pollack et al. calculations (which
Were done by me as a postdoc) neglected CO2
condensation. This changes the picture
27
Mars T profiles (Present solar luminosity)
CO2 condensation region
Ref. J. F. Kasting, Icarus (1991)
28

• Can still get the mean surface temperature above
  freezing today with CO2
• but
• Solar luminosity was 25-30 lower prior to 3.8
  Ga, when most of the valleys are thought to have
  formed
• ? causes problems

29
J. F. Kasting, Icarus (1991)
30
Mars Flux calculations at Ts 273 K
A 1- 4 FS/S0
SEFF FIR/FS
J.F. Kasting, Icarus (1991)
31

• A gaseous CO2/H2O atmosphere cannot have warmed
  early Mars above freezing (global average)
  because
• Condensation of CO2 reduces the tropospheric
  lapse rate, thereby lowering the greenhouse
  effect
• CO2 is a good Rayleigh scatterer (2.5 times
  better than air) ? increase in albedo outweighs
  the increase in the greenhouse effect
• There is also a problem with carbonates where
  are they?

32
Other ideas for keeping early Mars warm

• Scattering greenhouse effect of CO2 clouds (F.
  Forget and R. Pierrehumbert, Science, 1997)

33
15 ?m
CO2 optical properties in the thermal IR
CO2 ice cloud (?ext 10)
E
R
T
Reflectivity
Transmissivity
Emissivity
Ref. Forget and Pierrehumbert, Science (1997)
34
Scattering greenhouse effect

• CO2 ice crystals are expected to be 10-50 ?m in
  size, comparable to thermal-IR wavelengths
• Outgoing thermal-IR radiation is therefore
  backscattered more effectively than incoming
  (visible/near-IR) solar radiation
• ? surface warms

35
MARS 2-bar CO2 atmosphere S/S0 0.75
100 fractional cloud cover
Wet
Wet
Dry
Ref. Forget and Pierrehumbert, Science (1997)
36

• Problems with the scattering greenhouse
  hypothesis
• Need near 100 cloud cover
• Low (or thick) clouds can cool, as they do on
  Earth (Mischna et al., Icarus, 2000)
• CO2 clouds create localized heating which, in
  turn, makes them disappear (T. Colaprete et al.,
  JGR, 2003)
• Other possible solutions
• A CH4 greenhouse? An SO2 greenhouse? (Halevy et
  al., Science, 2007)

37

• The idea that Mars early atmosphere may have
  contained CH4 is strengthened by (putative)
  measurements of CH4 in Mars present atmosphere

38
Is there CH4 in Mars present atmosphere?
M. Mumma, DPS mtg., Fall 03
39
Methane on Mars?(From Mars Express Planetary
Fourier Spectrometer)
0 ppbv CH4
10-50 ppbv CH4
H2O
H2O
H2O
CH4 (3018 cm-1)
Solar
Formisano et al., Science Express (28 Oct., 2004)
40
CH4 Spatial Variability
High CH4
Med CH4
Low CH4
Formisano et al., Science Express (28 Oct., 2004)
41
Conclusions

• Early Mars (gt 3.8 Ga) was probably warm, i.e.,
  its surface temperature was near or above the
  freezing point of (nearly pure) water
• Higher CO2 levels were probably part of the
  story. CH4 may also have helped to warm the
  planet. The story makes more sense if early Mars
  was inhabited
• Sending an orbiter to Mars to look for CH4 and
  study the upper atmosphere and its interaction
  with the solar wind is a good idea!