From the article and in Nature (Editor's Summary)...
In the April 29, 2010 issue of the journal Nature, two independent groups of astronomers announced the discovery of water ice and organic molecules on the surface of the 123 mile diameter asteroid, 24 Themis.
Andrew S. Rivkin and Joshua P. Emery observed 24 Themis seven different times between 2002 and 2008 with the NASA Infrared Telescope Facility 3 meter telescope. An independent group of nine astronomers led by Humberto Campins observed 24 Themis over 84% of the asteroid's 8.37 hour rotational period on the night of January 23, 2008 using the same NASA telescope. Observations over most of the rotational period show that the water ice covers most of the asteroid's surface.
Both groups observed infrared spectra of the asteroid to determine the composition of its surface. There is a spectral feature at an infrared wavelength of 3.1 micrometers (A micrometer is a millionth of a meter.) that indicates the presence of water ice. Both groups of astronomers found strong absorption at this wavelength and concluded that 24 Themis has water ice covering the surface. The fact that two independent groups found the same results lends credence to the results.
After subtracting the effects of water ice in the spectrum, the astronomers found the residual spectral signature of organic molecules including hydrocarbons and aromatics. These organic molecules do not imply that there is life on the asteroid, however they do add to the considerable evidence that organic molecules, which are necessary for life, form easily.
24 Themis is about 479 million kilometers from the Sun in the main asteroid belt between Mars and Jupiter. At this distance from the Sun, water ice on the surface of the asteroid would sublimate into space. This fact suggests to astronomers that 24 Themis has a good supply of water ice beneath its surface to resupply the surface ice.
Many small bodies beyond the orbit of Jupiter have large amounts of ice in their composition, but this discovery is the first time that a rocky asteroid in the main asteroid belt has been found to contain ice. What are the implications of this discovery?
One theory for the origin of Earth's oceans is that impacts from asteroids or comets containing ice supplied the water. A competing theory is that the water originated in Earth's interior and was outgassed by volcanoes. If more asteroids have water ice and if a mission to asteroids shows that the water on the asteroids has the same ratio of hydrogen isotopes as Earth's water, then the theory that Earth's water originated from asteroid collisions would gain credence.
Link: Article
Letter
Nature 464, 1320-1321 (29 April 2010) | doi:10.1038/nature09029; Received 21 September 2009; Accepted 24 February 2010
Water ice and organics on the surface of the asteroid 24 Themis
Humberto Campins1, Kelsey Hargrove1, Noemi Pinilla-Alonso2, Ellen S. Howell3, Michael S. Kelley4, Javier Licandro5,6, T. Mothé-Diniz7, Y. Fernández1 & Julie Ziffer8
1. University of Central Florida, PO Box 162385, Orlando, Florida 32816-2385, USA
2. NASA-Ames Research Center, Moffett Field, California 94035, USA
3. NAIC-Arecibo Observatory, Arecibo, Puerto Rico 00612
4. University of Maryland, College Park, Maryland 20742, USA
5. Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, E-38200 La Laguna, Spain
6. Department of Astrophysics, University of La Laguna, E-38205 La Laguna, Spain
7. Universidade Federal Do Rio De Janeiro, RJ 20080-090, Brazil
8. University of Southern Maine, Department of Physics, Portland, Maine 04104, USA
Correspondence to: Humberto Campins1 Email: campins@physics.ucf.edu.
It has been suggested1, 2, 3 that Earth’s current supply of water was delivered by asteroids, some time after the collision that produced the Moon (which would have vaporized any of the pre-existing water). So far, no measurements of water ice on asteroids4, 5 have been made, but its presence has been inferred from the comet-like activity of several small asteroids, including two members of the Themis dynamical family6. Here we report infrared spectra of the asteroid 24 Themis which show that ice and organic compounds are not only present on its surface but also prevalent. Infrared spectral differences between it and other asteroids make 24 Themis unique so far, and our identification of ice and organics agrees with independent results7 that rule out other compounds as possible sources of the observed spectral structure. The widespread presence of surface ice on 24 Themis is somewhat unexpected because of the relatively short lifetime of exposed ice at this distance (~3.2 au) from the Sun. Nevertheless, there are several plausible sources, such as a subsurface reservoir that brings water to the surface through ‘impact gardening’ and/or sublimation.
Link: Nature Article
Link: Supplemental Material (PDF)
Letter
Nature 464, 1322-1323 (29 April 2010) | doi:10.1038/nature09028; Received 22 September 2009; Accepted 24 February 2010
Detection of ice and organics on an asteroidal surface
Andrew S. Rivkin1 & Joshua P. Emery2
1. Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723, USA
2. Earth and Planetary Science Department, University of Tennessee, Knoxville, Tennessee 37996, USA
Correspondence to: Andrew S. Rivkin1 Email: andy.rivkin@jhuapl.edu.
Recent observations, including the discovery1 in typical asteroidal orbits of objects with cometary characteristics (main-belt comets, or MBCs), have blurred the line between comets and asteroids, although so far neither ice nor organic material has been detected on the surface of an asteroid or directly proven to be an asteroidal constituent. Here we report the spectroscopic detection of water ice and organic material on the asteroid 24 Themis, a detection that has been independently confirmed2. 24 Themis belongs to the same dynamical family as three of the five known MBCs, and the presence of ice on 24 Themis is strong evidence that it also is present in the MBCs. We conclude that water ice is more common on asteroids than was previously thought and may be widespread in asteroidal interiors at much smaller heliocentric distances than was previously expected.
Link: Letter in Nature
Link: Supplemental Material (PDF)
Link: Nature Article - Editor's Summary
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