Solar System Studies
Venus, Earth and Moon, and Mars
Most of the X-ray flux observed from Venus, Earth, Moon, and Mars is from fluorescent scattering of solar X-rays, either in the upper atmospheres of these planets or on the lunar surface. Following Chandra detection of Venusian and Martian X-rays in 2001 (Dennerl et al. 2002) simulations of Venusian X-ray emission predict pronounced brightening on the sunward limb (as was detected), and this limb brightening is sensitive to properties of the Venusian atmosphere above 110 km, thus presenting a way to monitor properties of the Venusian atmosphere. In contrast, the presence of Earth's pronounced magnetosphere provides an additional source of X-ray emission: When electrons, accelerated in the magnetosphere, strike the upper atmosphere, they may emit X-rays by bremsstrahlung. The X-ray intensity of this emission, which occurs at higher energies than fluorescence, is directly related to the precipitated electron flux. The Martian X-ray properties observed were comparable to Venus due to their similar atmospheres and lack of strong magnetic fields.
Another source of X-ray emission observed from Mars exhibits properties similar to those seen in comets and the Moon's dark side. Chandra observations showed that this flux had spectral signatures resembling cometary X-ray spectra (Wargelin et al. 2004), suggesting X-ray emission from the geocorona, an extended cloud of hydrogen around the Earth. Chandra and XMM-Newton (Dennerl 2002) observations of Mars have revealed a faint, extended X-ray halo, displaying emission lines seen in cometary X-ray spectra. Understanding Mars' exosphere may aid in characterizing the X-ray properties of Earth's geocorona, subsequently revealing how this foreground emission could affect all X-ray observations.
References
Dennerl, K., Burwitz, V., Englhauser, J., Lisse, C. M., and Wolk, S.,2002, A&A, 386, 319Dennerl, K., 2002, A&A, 394, 1119
Wargelin, B.J., et al., 2004, ApJ, 607, 596
