Max Wallis

1
Sub-crustal lakes for cometary biology
Max Wallis Chandra Wickramasinghe Janaki
Wickramasinghe
Royal Astronomical Society 2005 May 13 meeting
on The Origin and Distribution of Life in the
Solar System
2
Liquid water an impossible idea

• Only the core - if radiogenic heating - in first
  few Myr
• Paradigm of iceball surface at 200K
• very low pressure ltlt 6mb critical
  pressure
• Cometary habitats? Phil. Trans.
  A325, 615-617, 1988
• Accumulation and pyrolysis of crust EMP 72,
  91-97, 1996
• IPDs and carbonaceous chondrites source of
  geo-chemical structures (aqueous alteration
  micro-fossils)
• Sub-crustal pools or lakes first proposed in
• Hoover et al. SPIE proceedings
  2004

3
Halleys Comet 1986

• Giotto picture, nucleus hardly seen (3 albedo)
• Vega infra-red Halley at 0.8 AU 350-400 K

4
Deep Space 1 Image of Comet Borrelly 22 Sept
2001 at 1.4 AU
Dark Icy Nucleus (10 km Long) with jet of gas
dust
L. Soderblom These pictures have told us that
comet nuclei are far more complex than we ever
imagined. They have rugged terrain, smooth
rolling plains, deep fractures and very, very
dark material - 3 albedo.
Infrared imaging --gt T 300 -340K
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Hot Black- insulating- organic- highly
porous
Comet Wild-2 from Stardust

• Cooked in sunlight
• Gases percolate, thicken the crust
• asphaltlike regolith
• space weathering dust impacts UV etc.

6
Halley tumbles 90 hrsurfaces sunlit for 20
hrgassing just on dayside

• gassing from holes, cracks
• 10 of surface
• self-sealing

7
Asphalt-like Crust
Thermal capacity 0.8 J/cm3K Conductivity ?
1.7 W/m .K Thermal skin depth 30-40cm
Burnt surface 1-2cm ? 0.17 W/m .K irregular
and porous on the sub-µm scale very low visual
albedo A 3 IR emissivity ?IR 0.90 0.98
Napier et al. MN 355, 191 2004

• heat conduction estimate heat flux through
  crust
• first explore via
  steady-state solutions valid
  for z lt thermal skin depth
• At subsolar position
• ? ?IR T4 (1-A) Fo / r2 - ?down ?down
  ? dT/dz
• where solar flux at r 1 AU is Fo
  1.4 kW/m2

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1-D model temperature in the cometary crust
Halley post perihelion, 0.7AU
273
340
500
T
Burnt asphalt
20mm d1
Crust
50mm d2
Water
numbers giving high heat flux ie. high
conductive cooling thinnest asphaltic crust
with burnt crust formed near 0.6 AU
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If the water is mixing and convecting, solutions
satisfy ? ?1(To-Ti )/d1 ?2
(Ti-273o)/d2 ie. all heat flux goes towards
melting ice on the pool bottom Outer temperature
To satisfies To TBB ?IR-1/4 (Fo/r2
- ?)1/4 TBB 396K at 1 AU Set
interface Ti 340o at perihelion to give burnt
crust limit
. solutions with ? 0.5-1 kW/m2 are
plausible
Balance of day/night heat fluxes lt ?downgt
lt?upgt (Tday 273o) (273o Tnight) - need
to factor in day heating night cooling times,
?IR is lower at low T ice
formation at night
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Concluding
Why asphalt? high carbon in IDPs and CCs high
molecular weight - also in Stardust
and Halley dust impact spectra
Is the asphalt crust strong enough, 5 cm thick
? pressure gt 10mb likely to leak,
crack organic gases help seal it, strengthen
and thicken it
What about DEEP IMPACT ?
.. blast away the crust allow us to see a new
crust accumulate /consolidate