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Mercury’s Weather-Beaten Surface: Understanding Mercury in the Context of Lunar and Asteroidal Space Weathering Studies
Authors:Deborah L Domingue  Clark R Chapman  Rosemary M Killen  Thomas H Zurbuchen  Jason A Gilbert  Menelaos Sarantos  Mehdi Benna  James A Slavin  David Schriver  Pavel M Trávní?ek  Thomas M Orlando  Ann L Sprague  David T Blewett  Jeffrey J Gillis-Davis  William C Feldman  David J Lawrence  George C Ho  Denton S Ebel  Larry R Nittler  Faith Vilas  Carle M Pieters  Sean C Solomon  Catherine L Johnson  Reka M Winslow  Jörn Helbert  Patrick N Peplowski  Shoshana Z Weider  Nelly Mouawad  Noam R Izenberg  William E McClintock
Institution:1. Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, Tucson, AZ, 85719-2395, USA
2. Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, CO, 80302, USA
3. Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD, 20771, USA
4. Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI, 48109, USA
5. Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, 20771, USA
6. Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA, 90024, USA
7. Space Sciences Laboratory, University of California, Berkeley, CA, 90704, USA
8. School of Chemistry and Biochemistry and School of Physics, Georgia Institute of Technology, Atlanta, GA, 30332-0400, USA
9. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, 85721-0092, USA
10. The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, 20723, USA
11. Hawai’i Institute of Geophysics and Planetology, University of Hawai’i, Honolulu, HI, 96822, USA
12. Department of Earth and Planetary Sciences, Division of Physical Sciences, American Museum of Natural History, New York, NY, 10023-5192, USA
13. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC, 20015, USA
14. Department of Geological Sciences, Brown University, Providence, RI, 02912, USA
15. Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, 10964, USA
16. Department of Earth, Ocean, and Atmospheric Sciences, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada
17. Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489, Berlin, Germany
18. Department of Natural Sciences, Lebanese American University, Beirut, Lebanon
19. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, 80303, USA
Abstract:Mercury’s regolith, derived from the crustal bedrock, has been altered by a set of space weathering processes. Before we can interpret crustal composition, it is necessary to understand the nature of these surface alterations. The processes that space weather the surface are the same as those that form Mercury’s exosphere (micrometeoroid flux and solar wind interactions) and are moderated by the local space environment and the presence of a global magnetic field. To comprehend how space weathering acts on Mercury’s regolith, an understanding is needed of how contributing processes act as an interactive system. As no direct information (e.g., from returned samples) is available about how the system of space weathering affects Mercury’s regolith, we use as a basis for comparison the current understanding of these same processes on lunar and asteroidal regoliths as well as laboratory simulations. These comparisons suggest that Mercury’s regolith is overturned more frequently (though the characteristic surface time for a grain is unknown even relative to the lunar case), more than an order of magnitude more melt and vapor per unit time and unit area is produced by impact processes than on the Moon (creating a higher glass content via grain coatings and agglutinates), the degree of surface irradiation is comparable to or greater than that on the Moon, and photon irradiation is up to an order of magnitude greater (creating amorphous grain rims, chemically reducing the upper layers of grains to produce nanometer-scale particles of metallic iron, and depleting surface grains in volatile elements and alkali metals). The processes that chemically reduce the surface and produce nanometer-scale particles on Mercury are suggested to be more effective than similar processes on the Moon. Estimated abundances of nanometer-scale particles can account for Mercury’s dark surface relative to that of the Moon without requiring macroscopic grains of opaque minerals. The presence of nanometer-scale particles may also account for Mercury’s relatively featureless visible–near-infrared reflectance spectra. Characteristics of material returned from asteroid 25143 Itokawa demonstrate that this nanometer-scale material need not be pure iron, raising the possibility that the nanometer-scale material on Mercury may have a composition different from iron metal such as (Fe,Mg)S]. The expected depletion of volatiles and particularly alkali metals from solar-wind interaction processes are inconsistent with the detection of sodium, potassium, and sulfur within the regolith. One plausible explanation invokes a larger fine fraction (grain size <45 μm) and more radiation-damaged grains than in the lunar surface material to create a regolith that is a more efficient reservoir for these volatiles. By this view the volatile elements detected are present not only within the grain structures, but also as adsorbates within the regolith and deposits on the surfaces of the regolith grains. The comparisons with findings from the Moon and asteroids provide a basis for predicting how compositional modifications induced by space weathering have affected Mercury’s surface composition.
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