Investigation of dust and water ice in comet 9P/Tempel 1 fromSpitzerobservations of the Deep Impact event

Abstract

Context. The Spitzer spacecraft monitored the Deep Impact event on 2005 July 4 providing unique infrared spectrophotometric data that enabled exploration of comet 9P/Tempel 1's activity and coma properties prior to and after the collision of the impactor. Aims. The time series of spectra take with the Spitzer Infrared Spectrograph (IRS) show fluorescence emission of the H 2 O ν 2 band at 6.4 μm superimposed on the dust thermal continuum. These data provide constraints on the properties of the dust ejecta cloud (dust size distribution, velocity, and mass), as well as on the water component (origin and mass). Our goal is to determine the dust-to-ice ratio of the material ejected from the impact site. Methods. The temporal evolution of the continuum was analyzed using a dust thermal model which considers amorphous carbon and intimate silicate-carbon mixtures. The water emission was extracted from the spectra and the water columns within the Spitzer extraction aperture were inferred using a fluorescence excitation model. Time-dependent models simulating the development of the ejecta cloud and the sublimation of icy grains were developed to interpret the temporal evolution of both dust and water emissions within the field of view. Results. Both the color temperature of the grains in the ejecta cloud (375 ± 5 K) and their size distribution show that a large quantity of submicron grains were ejected by the impact. The velocity of the smallest grains is 230 m s −1 , with a power index for the size dependence of 0.3−0.5, in agreement with gas loading. The total mass of dust is (0.7−1.2) × 10 5 kg for grain sizes 0.1−1 μm and (0.5−2.1) × 10 6 kg for sizes 0.1−100 μm. A sustained production of water is observed, which can be explained by the sublimation of pure ice grains with sizes less than 1 μm and comprising a mass of ice of (0.8−1.8) × 10 7 kg. The contribution of dirty ice grains to the ice budget is negligible. Assuming that water was essentially produced by icy grains present in the ejecta cloud, our measurements suggest a very high ice-to-dust ratio of about 10 in the excavated material, which greatly exceeds the gas-to-dust production rate ratio of ∼0.5 measured for the background coma. Alternately, if a large amount of material felt back to the surface and sublimated, ice-to-dust ratios of 1−3 are not excluded. A better understanding of the cratering event on 9P/Tempel 1 is required to disentangle between the two hypotheses. Evidence for grain fragmentation in the ejecta cloud is found in the data. The pre-impact water production rate is measured to be (4.7 ± 0.7) × 10 27 mol s −1 .

The Composition of Comets

“…Observations of color changes after the Deep Impact encounter in the visible and near IR domains also suggested an increase in small grains and in ice relative to refractory dust in the coma (Knight et al 2007;Schleicher et al 2006). Indeed, remote observations from the Spitzer observatory suggested a very high ice-to-dust ratio of about 10 in the excavated material (which greatly exceeds the gas-to-dust production rate ratio of about 0.5 measured for the background coma), although a ratio in the 1 to 3 range cannot be excluded if a large amount of material fell back to the surface and sublimated (Gicquel et al 2012).…”

Orphaned exomoons: Tidal detachment and evaporation following an exoplanet–star collision

“…We can crudely calibrate our expectations by considering the dust grain distributions found in rapidly sublimating Solar System comets. For example, the Deep Impact space mission to the comet Temple 1 (A'Hearn et al 2005;Gicquel et al 2012) found maximum particle sizes of b max < 100µm, which safely satisfy this limit. The PR drag timescale we have assumed (Eq.…”

Energetic Particle Showers Over Mars from Comet C/2013 A1 Siding Spring

“…In terms of comet activity, and so, in terms of interaction region sizes, Siding Spring can be considered similar to other comets at the time they were visited by previous missions, such as comets 21P/Giacobini‐Zinner, 19P/Borrelly, 81P/Wild, 103P/Hartley2, and 67P/Churyumov‐Gerasimenko (when at its perihelion). On the other hand, Siding Spring was about an order of magnitude more active than comets 26P/Grigg‐Skjellerup, 9P/Tempel1, and comet 67P/Churyumov‐Gerasimenko (before and after perihelion), and an order of magnitude less active than comet P/Halley (e.g., A'Hearn et al, ; Gicquel et al, ; Hansen et al, ; Huddleston et al, ; Johnstone et al, ; Mäkinen et al, ; McFadden et al, ; Meech et al, ). In terms of solar wind and solar radiation factors, each comet is different as the comet interaction with the solar wind strongly depends on the in situ solar activity conditions.…”