Cryovolcanism on Charon and other Kuiper Belt Objects

1. Cryovolcanism on Charon and other Kuiper Belt Objects Steve Desch School of Earth and Space Exploration Arizona State University

2. Outline • Observational Signatures of Cryovolcanism • Correlation with KBO Size • Thermal Evolution Models • Astrobiological Implications

3. Observational Signatures: Imaging and Spectra • We will make the claim that the spectral signature of crystalline water ice on KBOs is diagnostic of cryovolcanism. • One (unnamed) researcher has scoffed at this, saying crystalline water ice is found on nearly all the icy satellites (e.g., Ariel), and we “know” those satellites are geologically dead.

4. Observational Signatures: Imaging • Europa: • Young surface (few craters) • Linear troughs

6. Enceladus Dione Mix of heavily cratered and relatively uncratered terrains

7. Tethys Rhea

8. Titania linear troughs Miranda

9. Linear troughs = extensional stresses Ariel Some terrains on Ariel < 100 million years old (Plescia 1989)

10. N2 frost CH4 frost geysers driven by solid-state greenhouse effect Triton geysers

11. more linear troughs from extensional stresses = “grabens”

12. “lobate flows” From NASA Photojournal. Original caption says: two depressions (impact basins?) extensively modified by flooding, melting, faulting and collapse, several episodes of filling and partial removal of material. Hardly any craters. 500 km

13. Observational Signatures: Imaging • Imaging of icy satellites shows that resurfacing is very, very common: they are not geologically dead. • One common mode: extensional stresses produce grabens, into which ice or liquid can flow (think: mid-ocean ridge). • Another observed mode: “lobate” flows of liquid on the surface, or cryovolcanism.

14. Observational Signatures: Crystalline Water Ice All water ice absorbs at 1.5 and 2.0 microns Crystalline water ice alone absorbs strongly at 1.65 microns

15. 0.12 = crystal-line 0.03 = amorph -ous

16. Why is crystalline water ice diagnostic of cryovolcanism? • Cosmic rays doses of 2 - 3 eV / molecule destroy the crystalline structure of water ice (Strazzulla et al. 1992; Mastrapa & Brown 2006); this takes 1.5 Myr in the Kuiper Belt (Cooper et al. 2003). • Heat can anneal the ice, but it takes > 5 Gyr unless T > 90 K (Kouchi et al. 1994; Schmitt et al. 1989). • Solar UV photons also amorphize ice if they deliver the same dose (Leto & Baratta 2003) .

17. Why is crystalline water ice diagnostic of cryovolcanism? Ice amorphized in < 105 years! Cook et al. (2007), ApJ

18. Crystalline water ice on KBOs indicates a surface that has been replenished or transientally heated (to > 100 K) in the last 105 years. • Possible mechanisms (besides cryovolcanism): • Impact gardening uncovering unirradiated crystalline ice • Frost created by micrometeorites vaporizing ice • Transient heating of ice by micrometeorite impacts • Solid-state greenhouse effect • Solid-state convection • All were reviewed by Cook et al. (2007) and found not to work for KBOs.

19. Only mechanism that comes close is transient heating by kinetic energy of impact by micrometeorites. At average impact speed of 1.8 km/s (Zahnle et al. 2004), each particle can anneal 10 times its mass of ice (Cook et al. 2007). Mass flux observed by Pioneer 10(Humes 1980) , scaled to 1.8 km/s, yields flux 2.4 x 10-17 g cm-2 s-1. Can anneal to depths probed by H and K on 1/e timescales of 3.1 Myr.

20. Observational Signatures: Ammonia Hydrates Ammonia hydrates have absorption feature at 2.21 microns. These should be decomposed by cosmic ray doses of 100 eV / molecule (Strazzulla & Palumbo 1998). This should take about 20 Myr in the Kuiper Belt (Cooper et al. 2003). Their presence requires a physical replenishment.

21. Observational Signatures • Ammonia hydrates on surface show surface material has been replenished in last 20 Myr. • Crystalline water ice on surface shows that surface has been transientally heated or replenished in the last 105 years. • Annealing by micrometeorite impacts might just barely be competitive with amorphization by cosmic rays, but it would then apply equally to KBOs of all sizes...

22. Correlation with KBO Size Strength of the absorption feature of crystalline water ice at 1.65 microns correlates with KBO size. The largest KBOs with water ice features have crystalline water ice. No smaller objects (KBOs, comets and Centaurs) have crystalline water ice.

25. Charon Radius 603.6 km (Sicardy et al. 2006) Surface has crystalline (band ratio 0.13) water ice and ammonia hydrates (Brown & Calvin 2000; Dumas et al. 2001; Cook et al. 2007)

26. Cook et al. (2007), ApJ, in press

28. Water ice clearly crystalline: Hapke models including amorphous water ice call for > 95% crystalline

29. Feature of ammonia hydrates at 2.21 microns present. Variations in hydration state?

30. Quaoar Radius 630 +/- 95 km (Brown & Trujillo 2004) Surface has crystalline water ice (we compute band ratio 0.12), and ammonia hydrates (Jewitt & Luu 2004)

31. Quaoar Jewitt & Luu (2004)

34. 2003 EL61 Radius 980 x 759 x 498 km (Rabinowitz et al. 2006) Triaxial ellipsoid because of rapid rotation; consistent with mean density 2.6 g cm-3 (!) 2003 EL61 is largest member of a collisional family (Brown et al. 2007); collision probably the cause of the rapid rotation.

35. 2003 EL61 Mean density consistent with mass fraction of rock ~ 0.90(!). Water and other volatiles probably lost during collision? Surface has water ice that is apparently weakly (band ratio 0.06) crystalline (Barkume et al. 2006)

36. Barkume et al. (2006)

37. Orcus Radius 600 +55/-90 km (Lykawka & Mukai 2005) Surface has water ice, apparently crystalline (de Bergh et al. 2005)

38. de Bergh et al. (2005)

39. 2002 TX300 Radius upper limit = 555 km (3 sigma), or 455 km (2 sigma) (Ortiz et al. 2004) Surface has water ice, apparently crystalline (Licandro et al. 2001; Cook et al. 2007, in prep.) If confirmed, 2002 TX300 would be the smallest KBO with crystalline water ice on its surface.

40. Licandro et al. (2006)

41. Cook et al. (2007), in prep.

42. 1996 TO66 Radius ~ 325 km (Hainaut et al. 2000) Surface has water ice, but probably amorphous (we find band ratio 0.04). Brown et al. (1999) say: “the weakness or absence of this [the 1.65 micron] band in our data is consistent with amorphous water ice rather than crystalline water ice...” 1996 TO66 is the largest KBO not to have crystalline water ice.

43. 1.65 micron feature very weak. Brown et al. (1999)

44. S/2005 (2003 EL61) 1 Radius about 160 km (Barkume et al. 2006) Surface has water ice, but not obviously crystalline: we estimate band ratio 0.03

45. Barkume et al. (2006)

46. Centaur 1997 CU26 (Chariklo) Radius 118 +/- 6 km (Groussin et al. 2004) Surface has water ice that is clearly amorphous (we estimate band ratio 0.03). Brown et al. (1998) say “within the precision of the full-resolution data... the weakness of this [1.65 micron] band is consistent with mostly amorphous rather than mostly crystalline water ice on 1997 CU26.”

47. Brown et al. (1998)

48. 1.65 micron feature very weak Brown et al. (1998)

49. C/1995 O1 Hale-Bopp Radius ~ 30 km (Fernandez 2000) Water ice detected while comet at 7 AU, but not crystalline (we estimate band ratio 0.03). Davies et al. (1997) say “The absence of the [1.65 micron] absorption feature is therefore suggestive of the presence of amorphous ice.”

50. models including crystalline water ice do not match at 1.65 microns 1.4 1.6 1.8 2.0 2.2 2.4 Davies et al. (1997)