Seasonal Evolution on the Nucleus of Comet C/2013 A1 (Siding Spring)

Abstract

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.

Accepted by ApJ Lett

References in corpus (8)

• Color Systematics of Comets and Related Bodies
• An Early Look of Comet C/2013 A1 (Siding Spring): Breathtaker or Nightmare?
• Delivery of dust grains from comet C/2013 A1 (Siding Spring) to Mars
• A Study of Dust and Gas at Mars from Comet C/2013 A1 (Siding Spring)
• Constraining the Dust Coma Properties of Comet C/Siding Spring (2013 A1) at Large Heliocentric Distances
• The pre-perihelion activity of dynamically new comet C/2013 A1 (Siding Spring) and its close encounter with Mars
• High precision comet trajectory estimates: the Mars flyby of C/2013 A1 (Siding Spring)
• NEOWISE observations of comet C/2013 A1 (Siding Spring) as it approaches Mars

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