Investigaion of X-ray emission from Martian exosphere at solar minimum with Suzaku

We observed Mars in the X-ray wavelength range in April 2008 with Japanese Suzaku observatory. As far as we know, this was the first X-ray observation of Mars at solar minimum. Mars has been known to emit X-rays via scattering of solar X-rays and also charge exchange between neutral atom in its exosphere and solar wind ions (Dennerl 2002, Dennerl et al. 2006). Past theoretical studies (e.g., Krasnopolsky and Gladstone 1996) suggest that the neutral density varies at most by a factor of 10 due to the change of solar X-ray activities. Aiming at possible X-ray variation of Martian exsosphere, we analyzed image and spectrum data taken with Suzaku. No clear X-ray source is detected at the position of Mars in the energy band of 0.2-5 keV. We placed a stringent upper limit of X-ray oxygen line flux (0.5-0.65 keV) from Mars of 8.6e-5 [ph/cm2/s]. A comparison with XMM-Newton's results at solar maximum indicates that the exospheric density at solar minimum is less tha 4.4 times one at solar maximum.