Max Wallis Chandra Wickramasinghe Janaki Wickramasinghe

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 << 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. Halley’s 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 Borrelly22 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 --> T = 300 -340K

5. Comet Wild-2 from Stardust Hot + Black- insulating- organic- highly porous • Cooked in sunlight • Gases percolate, thicken the crust • asphalt–like 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. Thermal capacity 0.8 J/cm3K Conductivity  =1.7 W/m .K Thermal skin depth 30-40cm Asphalt-like Crust 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 heat conduction – estimate heat flux through crust – first explore via steady-state solutions – valid for z < 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 Napier et al. MN 355, 191: 2004

8. 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

9. 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 < down> : <up> ~ (Tday – 273o) : (273o – Tnight) - need to factor in day heating : night cooling times, IR is lower at low T ice formation at night

10. 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 > 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