Implications of cometary water: deep impact, stardust, and Hyabusa

1. Implications of cometary water: deep impact, stardust, and Hyabusa Rob Sheldon, Richard Hoover SPIE SanDiego August 15, 2006

2. Talk Outline • At SPIE 2005 we presented the “wet comet” model that predicts that a pristine, long-period comet (Whipple’s dirty snowball) will melt on its first pass within the orbit of Mars. This causes an irreversible phase change in the properties of the comet, distinguishing long & short period comets. • We developed this theory with pre-s/c data, and confirmed it with 3 s/c flyby visual inspections. • In 2005-6 we had 3 “in situ” comet observations that can potentially falsify our model: Stardust, Deep Impact and Hyabusa. Did they?

3. Why is this important? • Liquid water is critical for all known life, and hence the search for it on Mars, Europa, Ganymede, etc. • If comets have LW, they provide a transporter between all these water-bearing planets. Not only does it ameliorate the harsh environment of space, but it crucially permits organisms to repair damage in transit, greatly increasing their survival likelihood. • Given the relative abundance of comets, one might even talk of the biosphere extending from Venusian clouds to Jovian moons. • Recent comet data supports this enlarged biosphere.

4. A Comet’s Life: “Wet” Model a) c) b) Ice Liquid Vapor Spin Axis Spin Flip Melting snowball Pristine Splitting Cement d) g) f) e) Prolate tumbler Rubble pile Eggshell Polar jet

5. Birth: Density of comets Albedo-Area Kuiper Belt vs Oort Aphelion vs Perihelion Life: Spin rate Shape aspect ratio Brightness vs radial distance Active area, jets New vs. Old comets Outbursts Tail Shedding Death: Earth crossing asteroids Fireballs vs chondrites Tidal Force Breakup Issues with Dirty Snowball Model pre-s/c era

6. Issues after s/c visits to P/Halley, P/Borrelly & P/Wild-2 • Albedo: .02-.03 darker than soot! (weird dirt) • Shape: very prolate! (not oblate spheroid) • Dust distribution across limb, size. (big dust grains moving tangentially to surface) • Small active area jets: dayside, geyser-like (no sublimation cooling needed for snow) • Temperature: 300-400K (too hot for snow) • Pinnacles, cliffs, craters, patterned ground (too rigid for snow)

7. Motive for Deep Impact • To resolve these “hot” anomalies in the comet observations, the modellers proposed a deep layer of dust on the surface of comet that would insulate it from the heat, provide low albedo, and maybe explain jets and geysers. • A’Hearn proposed excavating a crater on a comet with a high-speed impact to determine what was under the dust layer, possibly exposing pristine material. • The expectation was a meter of dust excavated and then ice would evaporate with the ~200 TW pulse.

8. Pre-Deep Impact Lots of water had been seen in outbursts Outbursts 41 hours OH-line in UV

9. Deep Impact Too little gas evolved Mostly fine dust lifted Old cratering visible New crater too small Too little impact light IR saw clay, carbonate, PAHs, Hi-T silicates IR saw 273K temperature UV saw ice crystals Density calculations don’t match Stardust Hi-T silicates Hyabusa Rubble-pile asteroid Issues in 2005-6 “in situ”

10. 1.1 Plume Gases From a slit near the impact site, A’hearn took fast spectra of the plume. Expected volatiles for pristine ice didn’t appear: H20, CO (100x!), CO2… Organics did. Acetonitrile?? Spectrometer slit Before/After A’Hearn

11. 1.2 Fine Dust (SUBARU Telescope) • Sugita saw a submicron dust curtain, >100deg wide, consistent with gravitational cratering, but center filled with out-gassing lifted dust. • All dust speeds & angles consistent with “normal” comet outbursts seen before impact. • E.g. 100TW impactor looks like a typical outburst! Sugita

12. 1.3 Surface cratering • Belton saw craters and “layered” terrain, as if erosion had removed onion-like layers from comet. • Inconsistent with snow or dust! Belton

13. 1.4 Impact Crater Size? 50ms frames Impact site • A’hearn’s betting pool on the size of the impact crater was never seen. 10 meter thick blanket of dust? No ices, dry, micron size silicates? • This is inconsistent with: craters, scarps, coma water, organics, and accelerated dust. A’hearn

14. 1.5 Too Little Light? • Intensity was 1/10,000 expected. • Obscured by dust? 180 deg? Ernst

15. 1.6 IR Saw Carbonates, Clay?? • Lisse reports a best fit of IR to clay and carbonates in the coma. • They need liquid water! Lisse

16. 1.7 IR Temperature map 273K? • Most of comet hovers just above freezing point ice Sunshine

17. 1.8 UV saw coma ice grains • Expected, but consistent with HRS not seeing any ice? >273K surface? • Where did it come from? Deeper? Schulz

18. 1.9 Density determination • Based on gravitational settling of dust plume, A’hearn estimates an average density 60% that of ice. • The patterned surface appears to be higher density (or rigidity) • The cratering models see much smaller density. A’hearn

19. Wet Comet Model Explanations • Surface is rigid shell, wet deposited w/bio-minerals, but interior is empty  inhomogeneous density • Impactor “punched through” shell, into a “steam geyser”, without liberation of much liquid. Looks about like a natural outburst. Ice from interior. • Dust levitated by escaping gas fills coma, but too slow for +2s plume spectrum to show gas, just organic volatiles • R-T stable prolate surface shows low thermal inertia, quite dry bio-minerals & silicates. • Equator of prolate surface is R-T unstable, wet & cold

20. Evacuation of 9P/Tempel-1 • From the wet comet=critical period calculation, T=41hr, D=20kg/m3. That’s really fluffy snow! And completely inconsistent with cratering data. • But that assumes uniform density. If the comet has vapor pockets, then RT instability still operates. If pristine comet has D=200 kg/m3, we estimate 90% of the interior is vapor, 10% pristine. R-T R-T g R-T r

21. 2.1 Stardust ~10micron dust Crystal only forms at >1400K. Not Oort Cloud? Awaiting isotope analysis… Forsterite olivine Brownlee

22. Biosphere Heliocentric Radii • Input 1.4 kW/m2 /AU2 is solar radiation • Albedo 4% • Most cooling on sunlit side = s T4 • AU2 =s T4 / (0.96*1400W)  2.1 AU (Mars) • If we assume a poor IR emitter and/or a surface topography (crevice) gives 45o view of sky: 1/6 emissions = 5.1 AU (Jupiter) • And R-T would move this heat into the comet.

23. Conclusions • The Wet Comet Model continues to supply alternative explanations for cometary mysteries, especially the cratering/density conundrums raised by Deep Impact • The lateral heat conduction via R-T steam or liquid water, lowers the overall temperature of the comet, reduces radiative losses, and increases the heliocentric radius at which liquid water can remain. Thus the biosphere may extend to nearly the orbit of Jupiter (cometary outbursts seen), powered by R-T. • Stardust demonstrates that comets have crystalline silicates, which may enable the model to have an end-state much like the Itokawa rubble pile.