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Solar System
Saturn
Like Jupiter, Saturn has a strong magnetic field so it was expected that Saturn would also show a concentration of X-rays toward the poles. However, Chandra's observation revealed instead an increased X-ray brightness in the equatorial region. Furthermore, Saturn's X-ray spectrum, or the distribution of its X-rays according to energy, was found to be similar to that of X-rays from the Sun. This indicates that Saturn's X-radiation is due to the reflection of solar X-rays by Saturn's atmosphere, the same process that may be responsible for the weak equatorial X-radiation observed from Jupiter. Further observations should help clarify whether Saturn's magnetic polar regions ever flare up in X-rays, as do Jupiter's.
Titan
Astronomers have used the lack of X-rays from Saturn's largest moon, Titan, to draw some interesting conclusions. On January 5, 2003, Titan - the only moon in the solar system with a thick atmosphere - crossed in front of the Crab Nebula, a bright, extended X-ray source. Titan's transit enabled Chandra to image the one-arcsecond-diameter X-ray shadow cast on Chandra by the moon. This tiny shadow corresponds to the size of a dime as viewed from two and a half miles. The diameter of Titan's shadow was found to be larger than the known diameter of its solid surface. This difference in diameters yields a measurement of about 550 miles (880 kilometers) for the height of the X-ray absorbing region of Titan's atmosphere.
The extent of Titan's upper atmosphere is consistent with, or slightly (10-15%) larger, than that implied by Voyager I observations made at radio, infrared, and ultraviolet wavelengths in 1980. Saturn was about 5% closer to the Sun in 2003, so increased solar heating of Titan may have caused its atmosphere to expand.
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