X-ray studies of planets in our solar system

Prof. Graziella Branduardi-Raymont

Over the last decade the Chandra and XMM-Newton observatories have revealed the beauty and multiplicity of X-ray emissions from the planets in our solar system. This research field encompasses planetary physics, solar science and the response of solar system objects under the effects of the Sun’s activity.

Jupiter's polar regions show bright soft X-ray aurorae, with a line-rich spectrum arising from the charge exchange interactions of atmospheric neutrals with local and/or solar wind high charge-state heavy ions, accelerated in the planet’s powerful magnetic environment. At energies above ~3 keV the X-ray spectrum of the Jovian aurora becomes featureless, pointing to an origin from electron bremsstrahlung. Jupiter’s atmosphere also scatters solar X-rays, so that at low latitudes the planet's disk displays an X-ray spectrum that closely resembles that of solar flares.

Saturn has not revealed X-ray aurorae (yet), but its disk X-ray brightness, like Jupiter’s, strictly correlates with the Sun's X-ray output, pointing again to scattering of solar X-rays. Remarkably, we see X-rays from Saturn's rings as well, generally from bright spots localised on their East ansa.

Mars and Venus lack a strong magnetic field, yet they both show X-ray emissions from their disks and exospheres: here solar X-ray scattering and charge exchange (by solar wind ions with the exosphere neutrals) are thought to be at work, respectively; the two spectral components have been clearly separated at high spectral and spatial resolution.

Finally, the Earth's X-ray aurorae, like those in visible light, show a high degree of variability in intensity and morphology, both at soft and hard X-ray energies. And solar wind charge exchange in the Earth’s exosphere has been directly revealed by XMM-Newton along lines of sight crossing the terrestrial magnetosphere.

In summary, we have come to realise that planetary X-ray emissions are powerful probes of the conditions of the solar wind and of the planetary response to solar activity, and that their study provides novel insights about the close relationships between planets and their parent star.

The PhD project is envisaged to be based on the investigation of Jupiter, through the analysis of Chandra and XMM-Newton data. 

X-ray spectral map of Jupiter (0.2-1 keV) X-ray spectral map of Jupiter (1-3 keV)
X-ray spectral map of Jupiter (3-5 keV) X-ray spectral map of Jupiter (5-10 keV)

X-ray spectral maps (energy increasing from left to right and from top to bottom) of Jupiter obtained from XMM-Newton observations. The morphology of the planet is clearly different at different energies: the aurorae are bright at soft (charge exchange) and hard energies (bremsstrahlung) while the disk emission (scattered solar X-rays) disappears above ~3 keV.

References

Branduardi-Raymont et al. A study of Jupiter's aurorae with XMM-Newton,
Astronomy and Astrophysics, Volume 463, 761-774 (2007)
Branduardi-Raymont et al. Spectral morphology of the X-ray emission from
Jupiter's aurorae, Journal of Geophysical Research, Volume 113, issue A2, A02202 (2008)