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

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implications of cometary water deep impact stardust and hyabusa

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

Rob Sheldon,

Richard Hoover

SPIE SanDiego

August 15, 2006

talk outline
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?
why is this important
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.
a comet s life wet model
A Comet’s Life: “Wet” Model







Spin Axis



Melting snowball








Prolate tumbler

Rubble pile


Polar jet

issues with dirty snowball model pre s c era

Density of comets


Kuiper Belt vs Oort

Aphelion vs Perihelion


Spin rate

Shape aspect ratio

Brightness vs radial distance

Active area, jets

New vs. Old comets


Tail Shedding


Earth crossing asteroids

Fireballs vs chondrites

Tidal Force Breakup

Issues with Dirty Snowball Model pre-s/c era
issues after s c visits to p halley p borrelly p wild 2
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)
motive for deep impact
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.
pre deep impact
Pre-Deep Impact

Lots of water had been seen in outbursts


41 hours

OH-line in UV

issues in 2005 6 in situ
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


Hi-T silicates


Rubble-pile asteroid

Issues in 2005-6 “in situ”
1 1 plume gases
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



1 2 fine dust subaru telescope
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!


1 3 surface cratering
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!


1 4 impact crater size
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.


1 5 too little light
1.5 Too Little Light?
  • Intensity was 1/10,000 expected.
  • Obscured by dust? 180 deg?


1 6 ir saw carbonates clay
1.6 IR Saw Carbonates, Clay??
  • Lisse reports a best fit of IR to clay and carbonates in the coma.
  • They need liquid water!


1 7 ir temperature map 273k
1.7 IR Temperature map 273K?
  • Most of comet hovers just above freezing point



1 8 uv saw coma ice grains
1.8 UV saw coma ice grains
  • Expected, but consistent with HRS not seeing any ice? >273K surface?
  • Where did it come from? Deeper?


1 9 density determination
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.


wet comet model explanations
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
evacuation of 9p tempel 1
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.






2 1 stardust 10micron dust
2.1 Stardust ~10micron dust

Crystal only forms at >1400K. Not Oort Cloud?

Awaiting isotope analysis…

Forsterite olivine


biosphere heliocentric radii
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.
  • 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.