  We observed Comet C/Siding Spring using the Hubble Space Telescope (HST)
during its close approach to Mars. The high spatial resolution images obtained
through the F689M, F775W, and F845M filters reveal the characteristics of the
dust coma. The dust production rate of C/Siding Spring, quantified by $Af\rho$,
is 590$\pm$30, 640$\pm$30, and 670$\pm$30 cm in a 420 km-radius aperture at
38$^\circ$ solar phase angle through the three filters, respectively,
consistent with other observations at similar time and geometry, and with model
predictions based on earlier measurements. The dust expansion velocity is
~150-250 m s$^{-1}$ for micron-sized dust grains, similar to the speeds found
for other comets. The coma has a color slope of (5.5$\pm$1.5)%/100 nm between
689 and 845 nm, similar to previous HST measurements at comparable aperture
sizes, consistent with the lack of color dependence on heliocentric distance
for almost all previously observed active comets. The rotational period of the
nucleus of C/Siding Spring is determined from the periodic brightness variation
in the coma to be 8.00$\pm$0.08 hours, with no excited rotational state
detected. The dust coma shows a broad and diffuse fan-shaped feature in the
sunward direction, with no temporal morphological variation observed. The
projected orientation of the dust feature, combined with the previous analysis
of the coma morphology and other characteristics, suggests secular activity
evolution of the comet in its inner solar system passage as one previously
observed active region turns off whereas new regions exposed to sunlight due to
seasonal illumination change.
