First observation of Mars with XMM-Newton

Abstract

In the first observation of Mars with XMM-Newton, on 20-21 November 2003, this planet is clearly detected as an X-ray source. High resolution X-ray spectroscopy with the Reflection Grating Spectrometer (RGS) confirms that the X-ray radiation from Mars is composed of two different components: one due to fluorescent scattering of solar X-rays in its upper atmosphere and the other one due to solar wind charge exchange in its exosphere. Close to Mars, the RGS spectrum is dominated by two pronounced CO 2 fluorescence lines at 23.5 A and 23.7 A. Fluorescence from N 2 at 31.5 A is also observed. With increasing distance from Mars, these lines fade, while numerous (∼12) emission lines become prominent at the positions expected for de-excitation of highly ionized C, N, O, and Ne atoms, strongly resembling a cometary X-ray spectrum. The He-like O 6+ multiplet is resolved and is dominated by the spin-forbidden magnetic dipole transition 2 3 S 1 → 1 1 S 0 , confirming charge exchange as the origin of the emission, while the resonance line 2 1 P 1 → 1 1 S 0 increases in intensity closer to Mars, where the density of the exosphere is higher. The high spectral dispersion and throughput of XMM-Newton/RGS make it possible to produce X-ray images of the Martian exosphere in individual emission lines, free from fluorescent radiation. They show extended emission out to ∼8 Mars radii, with morphological differences between individual ions and ionization states. This is the first definite detection of charge exchange induced X-ray emission from the exosphere of another planet, providing a direct link to cometary X-ray emission.