X-rays from the Solar System
The research on X-ray emission from the Solar System is a very young field. Until 1996, the only planets which were known to emit X-rays were the Earth and Jupiter. Comets were not even generally considered as candidates for X-ray sources.
Thus, the discovery of X-rays from comet Hyakutake with ROSAT in March 1996 came as a big surprise to many scientists. The surprise increased even more, when additional comets were detected in archival X-ray data obtained during the ROSAT all-sky survey, establishing comets as a new class of X-ray sources.
The process which causes the X-ray emission of comets is now understood as the result of charge exchange interactions between heavy, highly ionized atoms in the solar wind with cometary gas. This process reproduces the observed X-ray luminosity and X-ray morphology very well, explains the temporal variability of the X-ray flux, and predicts characteristic signatures in the X-ray spectrum, which can be tested with the spectroscopic capabilities of Chandra and XMM-Newton. All the observed X-ray properties are consistent with the charge exchange process.
Comets can thus be used as natural spacecrafts to probe the heavy ion content of the solar wind at various heliographic latitudes and at different phases in the solar cycle, which is otherwise only accessible by in-situ measurements. Furthermore, X-ray observations of comets provide insights into the physics of the charge exchange process itself, complementing laboratory experiments and theoretical studies.
Venus was clearly detected as a half-lit crescent, with considerable brightening on the sunward limb. This morphology agrees well with that expected from fluorescent scattering of solar X-rays in the planetary atmosphere. The radiation is observed at discrete energies, mainly at 0.53 keV and at 0.28 keV, corresponding to K-alpha fluorescence on oxygen and carbon, the main constituents of the carbon dioxide atmosphere of Venus. In contrast to the optical radiation from Venus, which is sunlight reflected from clouds at 50-70 km height, most of the X-ray fluorescence takes place at heights of 120-140 km. Thus, X-ray observations of Venus can be used to monitor remotely the properties of the upper atmospheric layers of the Venus atmosphere, which are difficult to investigate otherwise, and their response to solar activity.
For this observation the Low Energy Transmission Grating (LETG), developed at MPE, played an essential role. It did not only allow us to obtain a high resolution X-ray spectrum, but provided also an efficient way of diffracting the extremely intense optical light to areas outside the X-ray CCDs, so that optical photons would not interfere with the X-ray observation. With this technique it was possible to detect unambiguously X-rays from Venus, despite the fact that the X-ray intensity did not exceed one ten-billionth of the optical intensity.
Also Mars is an X-ray source. In the first Chandra observation, in July 2001, Mars was cleary detected as an almost fully illuminated disk, with an indication of limb brightening at the sunward side, accompanied by some fading at the opposite side. Although the atmosphere of Mars is much more tenuous than that of Venus, it is still opaque to solar X-rays. As the chemical composition of the Martian atmosphere is very similar to that of Venus (predominantly carbon dioxide), the observed X-rays are also here the result of fluorescent scattering of solar X-rays, mainly on carbon and oxygen. Compared to Venus, the region where most of the X-ray scattering happens is more extended and somewhat closer to the surface, at heights of 100-150 km.
In addition to the fluorescent radiation, evidence for an additional source of X-ray emission was found, indicated by a faint X-ray halo which could be traced out to about three Mars radii, and by an additional component in the X-ray spectrum of Mars, which has a similar spectral shape as the halo. It is very likely that this halo is caused by the same process which is responsible for the X-ray emission of comets (see above).
Scattering of solar X-rays on very small dust particles was one of the early suggestions for explaing the X-ray emission from comets. Such tiny dust grains might be present in the upper Mars atmosphere, in particular during episodes of global dust storms. The Chandra observation happened to take place when a vigorous dust storm was raging on Mars and had covered roughly one hemisphere. This hemisphere was visible at the beginning of the observation and had mainly rotated away from our view by the end of the observation, which lasted for one third of a Mars rotation. The X-ray flux from Mars, however, did not show any modulation, so that it can be excluded that the dust storm had whirled up a sufficient amount of such particles in the upper atmosphere to have a detectable effect on the X-ray radiation.
Saturn was unambiguously detected in X-rays in 2003 with Chandra. In the Chandra image, Saturn is clearly resolved. Although the spatial information is limited by the small number of photons (about 100), it is evident that the X-ray photons came almost exclusively from the southern hemisphere, which was tilted towards us. No X-ray photons were detected from the regions which were covered by the rings. This is an indication that the rings of Saturn are optically thick to X-rays and have a low X-ray albedo.
Despite the low number of photons, it is not an easy task to find a simple spectral model which reproduces the measured energy distribution. The only acceptable single component model, a 0.18 keV blackbody, is physically not plausible. A 0.39 keV thermal spectrum, with an oxygen fluorescence emission line superimposed, however, provides an acceptable and physically motivated fit. The oxygen line accounts for one quarter of the energy emitted in the 0.3-2.0 keV band. This suggests that the observed X-rays from Saturn are maily solar X-rays, scattered in its upper atmosphere by a superposition of elastic scattering, mainly on hydrogen, and fluorescent scattering, mainly on oxygen. The intensity of the oxygen fluorescence line is comparable to that observed from Mars, if the different size of both planets and their different distance from the Sun and Earth is taken into account. The X-ray intensity of Saturn, however, exceeds that expected for scattering of solar X-rays, suggesting the presence of an additional mechanism. There are similarities in the X-ray emission of Saturn and the equatorial X-ray emission of Jupiter. However, while the X-ray intensity of Jupiter increases towards the magnetic poles, it was observed to decrease towards Saturn's south pole.