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We present the results of the two most recent (2005) XMM-Newton observationsof Saturn together with the re-analysis of an earlier (2002) observation fromthe XMM-Newton archive and of three Chandra observations in 2003 and 2004.While the XMM-Newton telescope resolution does not enable us to resolvespatially the contributions of the planet's disk and rings to the X-ray flux,we can estimate their strengths and their evolution over the years fromspectral analysis, and compare them with those observed with Chandra. Thespectrum of the X-ray emission is well fitted by an optically thin coronalmodel with an average temperature of 0.5 keV. The addition of a fluorescentoxygen emission line at ~0.53 keV improves the fits significantly. Inaccordance with earlier reports, we interpret the coronal component as emissionfrom the planetary disk, produced by the scattering of solar X-rays in Saturn'supper atmosphere, and the line as originating from the Saturnian rings. Thestrength of the disk X-ray emission is seen to decrease over the period 2002 -2005, following the decay of solar activity towards the current minimum in thesolar cycle. By comparing the relative fluxes of the disk X-ray emission andthe oxygen line, we suggest that the line strength does not vary over the yearsin the same fashion as the disk flux. We consider possible alternatives for theorigin of the line. The connection between solar activity and the strength ofSaturn's disk X-ray emission is investigated and compared with that of Jupiter.We also discuss the apparent lack of X-ray aurorae on Saturn and conclude thatthey are likely to lie below the sensitivity threshold of current Earth-boundobservatories. A similar comparison for Uranus and Neptune leads to the samedisappointing conclusion.

X-rays from Saturn: A study with XMM-Newton and Chandra over the years 2002-05