Organic Matter in the Outer Solar System
Dale P. Cruikshank, Ted L. Roush,
Yvonne J. Pendleton, Cristina Dalle Ore,
Tobias C. Owen, Thomas R. Geballe,
Catherine de Bergh, Bishun N. Khare |
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Many solid bodies in the outer solar system are covered with ices of various compositions, including water, carbon dioxide, methane, nitrogen, and other molecules that are solid at the low temperatures that prevail there. These ices have all been detected by remote sensing observations made with telescopes on Earth, or more recently, spacecraft in orbit (notably Galileo at Jupiter). The data also reveal other solid materials that could be minerals or complex carbon-bearing organic molecules. A study in progress using large ground-based telescopes to acquire infrared spectroscopic data, and laboratory results on the optical properties of complex organic matter, seeks to identify the nonicy materials on several satellites of Saturn, Uranus, and Neptune. The work on the satellites of Saturn is in part preparatory to the Cassini spacecraft investigation of the Saturn system, which will begin in 2004 and extend for four years.
One of the Saturn satellites, Iapetus, exhibits a unique exposure of nonice surface material that has very low reflectivity, causing the surface to appear entirely black at certain positions in its orbit around the planet. The infrared spectrum of this black surface of Iapetus has been extended into new wavelength regions in the current study, exploring a part of the spectrum that has not heretofore been seen.
In addition to other characteristics of the spectrum of Iapetus, a very strong absorption band at
3 micrometers wavelength has been revealed clearly for the first time in the new study. Models of the spectrum using organic solid materials produced in realistic simulations in the laboratory strongly indicate that the black matter on half of the surface of Iapetus is indeed organic in nature. The origin and mechanism for emplacement of the black material on the surface of this moon are unknown. The Cassini mission to Saturn will provide data that have a high probability of resolving these issues and further clarifying the apparently unique history of Iapetus.
Other moons of Saturn show similar, though less dramatic, evidence for the presence of macromolecular organic matter mixed with their surface ices, but the chemistry of the organic material appears to be different. The moons of Uranus, the Neptune moon Triton, and the planet Pluto all have black materials on their surfaces that are presumed to be organic in nature. The origins of this material are likely to be different from that on Iapetus, as well, underscoring the extraordinary variety of compositions and histories that the small bodies of the outer solar system have undergone since their formation. The study in progress at Ames, with colleagues from many other institutions, seeks to explore the nature and origin of organic matter throughout the solar system, and to explicate any astrobiological connections that emerge.
Point of Contact: D. Cruikshank
(650) 604-4244
dale@ssa1.arc.nasa.gov
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