First observation of Mars with XMM-Newton

High resolution X-ray spectroscopy with RGS

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany e-mail: kod@mpe.mpg.de
² Johns Hopkins University APL, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
³ NASA Marshall Space Flight Center, NSSTC/XD12, 320 Sparkman Drive, Huntsville, AL 35805, USA On leave from: Space Physics Laboratory, Vikram Sarabhai Space Center, Trivandrum 695022, India
⁴ Swedish Institute of Space Physics, Box 812, 981 28 Kiruna, Sweden
⁵ European Space Agency, ESTEC Postbus 299, Noordwijk 2200AG, The Netherlands
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁷ XMM-Newton SOC, Apartado 50727 Villafranca, Madrid 28080, Spain

Received: 24 September 2005
Accepted: 30 December 2005

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 CO₂ fluorescence lines at 23.5 Å and 23.7 Å. Fluorescence from N₂ at 31.5 Å 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 O⁶⁺ multiplet is resolved and is dominated by the spin-forbidden magnetic dipole transition 2  1 , confirming charge exchange as the origin of the emission, while the resonance line 2  1  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.