Tracking spatial and temporal variability of H₂O and CO₂ from the flyby of Comet Hartley 2

Cometary activity is driven by the sublimation of ices originating from the formation of the Solar System. Studying cometary comae allows one to make inferences about the composition and compositional history of the cometary nucleus. The presence of different species of volatile gas and their mixing ratios can provide insight as to the degree of heterogeneity within a cometary nucleus and ultimately where in the protosolar nebula a comet formed.

Comet 103P/Hartley (H2) is a Jupiter-family comet in a complex rotation state with an orbital period of 6.46 years and perihelion distance 1.061 AU. The Deep Impact spacecraft performed a flyby of H2 (4 Nov 2010) collecting data for 56 days during its perihelion passage (28 Oct 2010). During this time, the spacecraft scanned the comet with an infrared spectrometer (HRI-IR). 24 days of IR observations were systematically calibrated, 1321 spatial/spectral cubes in total, and a spectral continuum removal analysis was performed to measure the time-varying distribution of water vapor (H₂O, 2.59–2.77 μm) and carbon dioxide (CO₂, 4.17–4.31 μm) in the comet’s coma. Aperture analyses were performed to produce lightcurves of the two gas species. The lightcurves exhibit a peculiar 3-peaked pattern, whose periods are consistent with the rotation rates of the comet nucleus. Comparing the two species against one another shows CO₂ brightness is more tightly coupled to the rotation of the nucleus, consistent with the observation of a CO₂ jet originating from the small lobe of H2’s bilobed nucleus. The H₂O lightcurve is less coherent than the CO₂, suggesting H₂O distribution is more diffuse and consistent with the notion of multiple water vapor sources in the observations. More work is necessary to fully characterize the distribution of these volatile gases, its relation to the motion and geometry of the cometary nucleus, and ultimately determine the source locations of the volatiles in greater detail using the spatial maps of the volatile gases.