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We used the UltraViolet-Optical Telescope on board Swift to systematicallyfollow the dynamically new comet C/2013 A1 (Siding Spring) on its approach tothe Sun. The comet was observed from a heliocentric distance of 4.5 AUpre-perihelion to its perihelion at 1.4 AU. From our observations, we estimatethat the water production rate during closest approach to Mars was 1.5 +/- 0.3x 1E28 molecules/s, that peak gas delivery rates were between 4.5-8.8 kg/s, andthat in total between 3.1-5.4 x 1E4 kg cometary gas was delivered to theplanet. Seasonal and evolutionary effects on the nucleus govern thepre-perihelion activity of comet Siding Spring. The sudden increase of itswater production between 2.46-2.06 AU suggests the onset of the sublimation oficy grains in the coma, likely driven by CO2. As the comet got closer to theSun, the relative contribution of the nucleus' water production increased,while CO2 production rates decreased. The changes in the comet's activity canbe explained by a depletion of CO2, but the comet's high mass loss ratesuggests they may also reflect primordial heterogeneities in the nucleus.

The pre-perihelion activity of dynamically new comet C/2013 A1 (Siding Spring) and its close encounter with Mars