Astronomy and Astrophysics – Astrophysics
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21810204d&link_type=abstract
American Astronomical Society, AAS Meeting #218, #102.04; Bulletin of the American Astronomical Society, Vol. 43, 2011
Astronomy and Astrophysics
Astrophysics
Scientific paper
X-ray observations have opened up a completely new possibility of remote global imaging of planetary exospheres and their spatial and temporal variability. The talk will focus on the planets Venus and Mars, where the absence of a global magnetic field enables a straightforward study of how a planetary atmosphere responds to the incident solar photon and ion flux. Chandra was the first satellite to detect X-rays from Venus and Mars and to reveal that they are the result of two different processes: scattering of solar X-rays and charge exchange reactions between highly charged heavy ions in the solar wind with atmospheric neutrals. As a consequence of the characteristic photoabsorption cross sections, scattering of solar X-rays is most efficient at atmospheric heights of 100-140 km, i.e., well above the cloud layers. Here, X-rays provide direct observational access to regions which are difficult to study by in-situ measurements or by remote observations at other wavelengths. X-ray observations of charge exchange reactions in planetary exospheres have a particulary high scientific potential, because this process is considered as an important nonthermal escape mechanism, which may be responsible for a significant loss of the atmosphere. Although this process is mainly due to charge exchange with solar wind protons, which are about 1000 times more abundant than heavy ions and which do not produce X-rays, the high cross section makes charge exchange an efficient tracer of planetary outgassing, thus linking X-ray astrophysics to astrobiology.
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