Ices of the Outer Solar System: Triton, Pluto/Charon, Centaurs and Kuiper Belt Objects

D. P. Cruikshank

Our knowledge of the compositions of the bodies in the outer Solar System is derived from remote sensing observations, primarily by the methods of near-infrared spectroscopy with ground-based telescopes, but also with the help of ultraviolet and photo-visual spectral region data from ground-based and orbiting telescopes. Ices of many compositions have active and diagnostic vibrational spectra between 1 and 5 micrometers, the most productive spectral region for conventional astronomical observations (1,2)

Triton

Near-IR spectroscopy shows that Triton's high-albedo surface is covered largely with N2 ice, with <1% CH4 and a small amount of CO dissolved in the N2. The central frequencies of the CH4 bands are shifted because of the matrix isolation of the methane molecules. Solid CO2 and H2O are also seen in the spectrum. The very low vapor pressures of CO2 and H2O suggest that these two ices are spatially isolated from the more volatile species on Triton's surface (3,4). Voyager pictures show that Triton's surface is variegated, and that low-albedo surface deposits occur on small spatial scales. The sources and compositions of the colored regions and the dark material are not known. Triton's surface and its spectrum are variable, having had a significant change circa 1980-1982 (5). The three most volatile components are probably exchanged between the surface and the atmosphere during the satellite's seasonal (and perhaps diurnal) cycle.

Pluto

Pluto's surface shows spectral evidence for N2, CH4, CO, and H2O, but no CO2. The CH4 appears in two states; one component is dissolved in the N2, as on Triton, and a second component appears as relatively pure, free CH4 (4,6). All components except H2O are probably exchanged between the surface and the atmosphere during the seasonal cycle of the planet. Low- albedo regions on Pluto's surface appear to contain material that has not yet been identified; no clear spectral signature of the dark material has been recognized.

Charon

Pluto's satellite Charon has a surface covered with H2O ice, as deduced from a spectrum of low resolution and moderate S/N; other ices and refractory materials may exist there, but their spectral signatures have not been recognized (7).

Centaurs

Spectroscopy of two Centaurs and limited data on a few others show them to be a diverse group. 2030 Chiron shows no spectral signatures of ices, and it exhibits episodic cometary behavior. 5145 Pholus has a very red spectrum that is modelled with organic refractory solids (Titan tholin), and shows absorption bands of H2O ice and a light hydrocarbon, possibly CH3OH ice or (CH2)6N4 (8,9).

Kuiper Belt Objects

The near-IR spectrum of only one KBO (1993 SC) has been described (10); it resembles the spectrum of Triton in that it shows a band coincident with solid CH4. Colorimetry of the KBOs suggests considerable compositional diversity among this population (11).

List of References

  1. Cruikshank, D. P. and R. H. Brown, "Remote sensing of ices and ice-mineral mixtures in the outer solar system", in Remote Geochemical Analysis: Elemental and Mineralogical Composition, C. M Pieters and P. A. J. Englert, eds. Cambridge Univ. Press, p. 455, 1993.
  2. Brown, R. H. and D. P. Cruikshank, Ann. Rev. Earth Planet Sci., 1997.
  3. Cruikshank, D. P., et al. Science 261, p. 742, 1993.
  4. Quirico, E. , Thesis, Universite Joseph Fourier-Grenoble I, 1995.
  5. Brown, R. H., et al., in Neptune and Triton, D. P. Cruikshank, ed., Univ. Arizona Press, p. 991, 1995.
  6. Owen, T. C., et al., Science 261, p. 745, 1993.
  7. Roush, T. L., Icarus 108, 243 (1994); T. L. Roush et al. Icarus vol. 119, p. 214, 1995.
  8. Cruikshank, D. P., et al., Icarus (submitted).
  9. Luu, J., et al. Icarus vol. 109, p. 133, 1994.
  10. Brown, R. H., et al. (submitted).
  11. Luu, J. and D. Jewitt, A.J. vol. 112, p. 2310, 1996.

Contact Information:

Dale P. Cruikshank
NASA Ames Research Center
Moffett Field, CA 94035
dale@ssa1.arc.nasa.gov