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