Impacts Facilitating Elementary Life on Mars

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Impacts Facilitating Elementary Life on Mars
Max WallisJanaki Wickramasinghe Cardiff Centre
for Astrobiology
UKAC06, Canterbury April 2006
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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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Home Plate
Spirit panorama
Gusev crater
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Meteorites from Mars

• Antarctic collection - Allan Hills
• Nakhla

Carbonates, but very little H2O gt episodic
flooding
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Mars Surveyor north polar cap
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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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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 mass 0.05 p? ? ?3 d ?/d ? d ?
• ? x 10-7 m3 / m2.yr
• gardening at 0.3 m / Myr
• gt cover by 1 metre craters in 1 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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Polar troughs / ravines
Pelletier, Geology 2004


sun-facing slopes warmed for ice-organisms
impact-fragments spread by gravity and winds ..
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Archaea living in low-T ice R(T) exp ( - A/T)
no cut-off at T as low as - 40oC water is
available for cell processes
Tung et al. PNAS 2005
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Microorganisms that may survive on Comets, Europa
Mars Polar Caps

• Microbial Extremophiles Live on Earth wherever
  Water and Energy are found
• Psychrophilic psychrotrophic Archaea,
  Bacteria, Fungi, Algae are dominant Life-forms of
  Earths
• Polar Caps Icy Brines Cryoconite
  ecosystems
• We consider them excellent candidates for Life
  that may inhabit Comets, Europa, Ganymede, or the
  Permafrost polar Ice Caps of Mars

Hoover et al. Proc. SPIE 2004
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North polar cap Ice microbial life at 230 270
K Resistant to cooling in shade and nightside
Impact gardening 0.6 m / 100 kyr
mobilises the micro-organisms carried by
winds to new potential habitats
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Frozen Elysium Sea Mars Express Feb.05
Estimated age 5 Myr Few cm dust cover on ice ?
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Number of craters / km2
saturation line
Scaled from lunar counts Hartmann Neukum
2001
age 10 ky
1 My
1 Gy
4 Gy
Diameter km
m
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Cratering Function ? smaller index -1.8 for
large craters

• Formation of craters larger than size ? km
• ? 3.3 x 10-13 ?-1.8 / km2.yr for
  ? gt 10 km
• Index of -1.8
• gt largest sizes dominate excavated volume area
• Over whole surface of Mars
• ? x Area 37 ?-1.8 / Myr
• Crater events of size ? gt 10 km every
  2 Myr
• size ? gt 100 km every 100 Myr

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Impact Melt

• Following an impact a melt area forms at base of
  the crater with diameter 0.2 ?
• Assuming 1 of impact energy goes to melt
  permafrost, depth of lake z
• z / ? 1 x 10-4 (V/20km/s)-2/3 V2/L
• 0.6 1.5 x 10-2 for V10-20km/s

For 1 km impacter, at base of 10km crater,
lake has radius 1km and depth 100 m
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Initial Lake Conditions

• Start with mix of water and sediment (dust)

• Speedy freezing of surface occurs

• Sublimation then leads to growth of dust/dirt
  layer at the surface

• Diurnal T at surface is little different from
  Mars regolith range 180 240 K

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Sublimation of surface ice

• Ts Temperature at top of dust layer
• T Temperature at dust-ice interface

Using Clausius-Clapeyron equation for ice
sublimation rate, and integrating over time t
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Dust thickness accumulating from sublimation of
dirty ice (0.1dust). Sublimation chokes off
at about x 3cm
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Skin depth for thermal response in ice
The temperature at depth x is described by
infinite half-plane heat conduction equation
Thermal capacity C 0.8 J/cm3K Ice conductivity
? 1.7 W/m .K Thermal diffusivity a ? /C?
0.003 m2/hr Thermal skin depth v(a ?) 30cm
For ice depth lt a few times skin depth,
quasi-steady solution valid
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Quasi-Steady State Temperature Profile
Heat Loss es?4
T180K-240K
Dust thickens sublimation
Dust
Dirty Ice
Ice thickens Rate ? dT/dz
Water
T273K
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Rate of Ice Formation
energy flux at water/ice interface gt -? ?T/?z
L?(dz/dt)
?thermal conductivity of ice Llatent heat of ice
Dust
Ice
Water
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Growth of Ice Layer over 2 Days
Liquid water lake would persist for days enough
time for microbial replication
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Smaller impacts
happen much more frequently, distributing
amplified microbes globally

• Atmosphere slowing of small impacters
  10g/cm2 gt limit ? 1 m
• Impacts into ice 20 x more excavated mass
• 2.5 x larger craters

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Conclusion
Four billion years ago conditions on Mars were
favourable for life Spores dating back to this
epoch may still be present within permafrost and
relic lakes Every 2 Myr, 10km impact crater
forms, and consequently a 1km lake Micro-organism
s replicate for days/weeks Much more frequent
smaller impacts gt spore-bearing fragments
picked up in dust storms, transported to poles
and accumulate in polar ice These could await
favourable light and heat conditions for revival.