Implication of Martian Deuterium for Water in the planets recent past

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Implication of Martian Deuterium for Water in the
planets recent past
Max K Wallis Cardiff Centre for Astrobiology
EGU07 Vienna April 2007
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Mars geophysics

• Episodic flooding
• water gt carbonates
• km-deep polar cap
• seasonal, obliquity cycle
• Meteorite impacts
• small scale gardening
• Asteroid / comet impacts
• rare large craters
• Dust blow
• Volcanism lava flows
• few 100 Myr old

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Meteorites from Mars

• Antarctic collection - Allan Hills
• Nakhla

Deuterium enriched ALH84001 E 1.6
(3.9 Gyr) Shergottites E 2.3 (few 100 Myr)
Carbonates, little H2O gt episodic flooding
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1200 km wide 1-3 km thick up to 1 km
ravines volume half of Greenland cap -
1.2 M km3
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Frozen Elysium Sea Mars Express Feb.05
5 Myr old few cm dust cover on ice ?
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Can Deuterium tell us about early Mars water ?

• Atmospheric H2O very little 2 Gt 10 µm
• the H is lost to space in 2 x 104 yr
• 2Gt gt 1 km comet - one per few Myr

Elysium flood gt 5000 Gt 5 Myr
ago Volcanos 1000 Gt gt 10 Myr North Polar
cap 1015 Gt 0.9 m H20 Crustal water
1018 Gt 1 km H20
Albedo changes Mars global warming decades
? Polar Cap changes with orbital obliquity
22o -gt 60o - last high 30o 0.4 Myr
ago Sun changes active 3 Gyr atmosphere
loss v. high
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current D enriched E 5.2 x SMOW

• Fractionation in atmosphere
• Exchange with reservoir eg. ground-ice
• - uncertain reservoir E 2 as meteorites
  
• Impact excavation of ground-ice 104 yr
• - comet or asteroid cratering - mass x 104
• Remnant of large flood Elysium 5 Myr

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Yung et al. Icarus 1988 Photo-driven D
reactions In Mars atmosphere
Effusion speeds as V (T)
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fractionation in escape R

• Yung et al. ? R 0.32
• Jeans escape R 0.05 gaussian tail
• Jeans averaged over solar variations
• R 0.1 0.2

Non-thermal escape exosphere photochem ? D gt
0.25eV (5 km/s) Solar wind sweeping
scavenging via charge exchange ionosphere
bubbles Energetic O ? O ? knock-on D, HD
sputtering
all have R ? 1 gravity collisions ? R ? 0.8
- 0.9
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Main photochemical escape channels
thermal escape 8300 /cm2s
2300-7400 6000 5-10 000

• Photodissociation
• h? HD ? H D 1.5eV, 0.75eV
• CO2 HD ? CO2H D 1.1 eV
• CO2 HD ? CO2D H
• CO2D e ? CO2 D 8 eV
• O HD ? OH D 0.57 eV
• O HD ? OD H
• OD CO2 ? CO2D O ? D of 8 eV

Energetic O knock-on D and HD 500
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Modolo et al. (2004) Picked up coronal O / cm3

• From Solar Wind
• energetic ions/neutrals
• O accelerated
• Charge exchange
• ? fast O keV
• sputter
• from exobase level
• supra-thermal popn
• plus several escapees

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Numbers depend on exospheric theory
hard-sphere collisions integrals through
exobase Planet Space Sci 26, 1978 ?
dµ ? dz e-(z/Hi z/He) exp-1/µ ez/H

• What is surface ice enrichment Esurf ?
• If simple steady state loss
• Yungs R 0.32 plus R 0.78 vapour-ice
• ? Esurf 5.2 x 0.78 x 0.32 1.3
• for R 0.7 ? Esurf 5.2 x 0.78 x 0.7 2.8

Could this happen post 0.4 Myr high obliquity?
at loss rate of 1 Gt / 104yr size of mobile
ice reservoir Vo E 1/(1-R) 160 Gt
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number of craters / km2
saturation line
Scaled from lunar crater counts Hartmann
Neukum 2001
age 10 ky
1 My
1 Gy
4 Gy
Diameter km
m
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Cratering function ? gt gardening by small
impacters

• Formation of craters larger than size ? km
• ? 5 x 10-13 ?-3.8 / km2.yr
• from ? 1 m
  to 1 km
• Excavated volume 0.05 p? ? ?3 d ?/d ? d ?
• 1.5 x 10-8 m3 / m2.yr
• gardening at 1.5 cm / Myr
• gt cover by 1 metre craters in 100 Myr

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Apply to Mars

• Atmosphere slowing of small impacters
  1 of Earths 10g/cm2 gt limit ?
  1 m
• Lower impact speeds on Mars
• not 15-25km/s but 10 -20 km/s
• Impacts into ice 20 x more excavated mass
• 2.5 x larger craters

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Large Craters can be significant

• Craters ?gt 1-2 km fit to a shallower crater
  function
• ? 3 x 10-14 ?-1.8 / km2.yr for ?
  gt 1 km
• The rate of crater formation gt ?1 over the whole
  martian surface is
• 1.4 x 108 km2 ? d?/d? d? 1.4 x 108 km2
  ?( ?1)
• 50-70 / Myr for 1 km craters
• and 0.7 / Myr for 10 km craters

Cratering in ice/permafrost excavates 20 x
more covers 10 of Mars gt60o latitude
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number of craters / km2
Last big crater in icy terrain in 105 yr ?
0.9km 4.105 yr ? 1.4km Volume excavated
2.3km3 8.8km3 volumes from small
craters 6-21km3 22-85km3
saturation line
105 yr
4 Gy
?
10 My
1 Gy
Diameter km
m
ice of 1 km3 0.9 Gt H2O lost 10 Gt in
105 yr
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conclude D / H2O over the last Myr

• Cratering of icy terrain replaces several times
  the loss of H and D from atmosphere
• Small cratering supplies more than occasional
  large craters unless ice is deep gt 100m
• Large crater inputs may not be negligible
• ? stochastic changes
• We understand why E is low
• . much water-ice in the surface
• . but not why E is as high as 5.2
• Suggests mobile ice has E ? 3
• . more than meteorites E ? 2