X-ray Observation of Mars with Suzaku at Solar Minimun
  Mars was observed in X-rays during April 3-5 2008 for 82 ksec with the Japanese Suzaku observatory. Mars has been known to emit X-rays via the scattering of solar X-rays and via the charge exchange between neutral atoms in the exosphere and solar wind ions. Past theoretical studies suggest that the exospheric neutral density may vary by a factor of up to 10 over the solar cycle. To investigate a potential change of the exospheric charge exchange emission, Mars was observed with Suzaku at solar minimum. Significant signals were not detected at the position of Mars in the energy band of 0.2-5 keV. A 2 sigma upper limit of the O VII line flux in 0.5-0.65 keV was 4.3$\times10^{-5}$ ph cm$^{-2}$ s$^{-1}$. Comparing this upper limit to the past Chandra and XMM-Newton observations conducted near solar maximum, it was found that the exospheric density at solar minimum does not exceed that near solar maximum by more than 6-70 times. 
Kumi Ishikawa, Yuichiro Ezoe, Takaya Ohashi, Naoki Terada, Yoshifumi   Futaana
astro-ph.EP astro-ph.HE