First observations of H₂O and CO₂vapor in comet 67P/Churyumov-Gerasimenko made by VIRTIS onboard Rosetta

Abstract

Context. Outgassing from cometary nuclei involves complex surface and subsurface processes that need to be understood to investigate the composition of cometary ices from coma observations. Aims. We investigate the production of water, carbon dioxide, and carbon monoxide from the nucleus of comet 67P/ChuryumovGerasimenko (67P). These species have different volatility and are key species of cometary ices. Methods. Using the high spectral-resolution channel of the Visible InfraRed Thermal Imaging Spectrometer (VIRTIS-H), we observed the ν 3 vibrational bands of H 2 O and CO 2 at 2.67 and 4.27 µm, respectively, from 24 November 2014 to 24 January 2015, when comet 67P was between 2.91 and 2.47 AU from the Sun. Observations were undertaken in limb-viewing geometry at distances from the surface of 0 to 1.5 km and with various line-of-sight (LOS) orientations in the body-fixed frame. A geometry tool was used to characterize the position of the LOS with respect to geomorphologic regions and the illumination properties of these regions. Results. The water production of 67P did not increase much from 2.9 to 2.5 AU. High water column densities are observed for LOS above the neck regions, suggesting they are the most productive in water vapor. While water production is weak in regions with low solar illumination, CO 2 is outgassing from both illuminated and non-illuminated regions, which indicates that CO 2 sublimates at a depth that is below the diurnal skin depth. The CO 2 /H 2 O column density ratio varies from 2 to 60%. For regions that are in sunlight, mean values between 2 and 7% are measured. The lower bound value is likely representative of the CO 2 /H 2 O production rate ratio from the neck regions. For carbon monoxide, we derive column density ratios CO/H 2 O < 1.9% and CO/CO 2 < 80%. An illumination-driven model, with a uniformly active surface releasing water at a mean rate of 8 × 10 25 s −1 , provides an overall agreement to VIRTIS-H data, although some mismatches show local surface inhomogeneities in water production. Rotational temperatures of 90-100 K are derived from H 2 O and CO 2 averaged spectra.

Spatial and diurnal variation of water outgassing on comet 67P/Churyumov-Gerasimenko observed from Rosetta/MIRO in August 2014

“…The most active region coincides with the neck area of the nucleus, suggesting a larger deposit of ices in the neck than in other parts of the nucleus. The VIRTIS spectrometer and the OSIRIS science camera on Rosetta have observed similar trends Capaccioni et al 2015;Bockelée-Morvan et al 2015). In addition, MIRO brightness temperatures in the neck region are consistent with sublimation of ice in the neck region, as the measured temperatures are lower than thermal model predictions of a pure dust layer with thermal inertia of 10−30 JK −1 m −2 s −1/2 (Schloerb et al 2015).…”

Seasonal erosion and restoration of the dust cover on comet 67P/Churyumov-Gerasimenko as observed by OSIRIS onboard Rosetta

“…This scenario is in keeping with the observed increase in activities of outgassing and dust ejection with decreasing heliocentric distance (Bockelée-Morvan et al 2015;Della Corte et al 2015;Fougere et al 2016;Fulle et al 2016c). Since the thickness of the dust cover is small relative to the width of a single honeycomb feature, it suffices to quantify the surface erosion in thickness.…”

Comparison of 3D kinetic and hydrodynamic models to ROSINA-COPS measurements of the neutral coma of 67P/Churyumov-Gerasimenko

“…Despite the decrease in heliocentric distance we find that a constant production rate from August to 1 November fits the COPS data best. Bockelée-Morvan et al (2015) similarly report no significant increase in the total H 2 O production rate for heliocentric distances from 2.9 to 2.5 AU as measured with the Rosetta VIRTIS instrument. We can calculate the total production rate of 67P by integrating the fluxes of all the model facets over the total surface area.…”

“…We therefore assume that the COPS signal mostly shows the same characteristics as H 2 O. In their recent work, Bockelée-Morvan et al (2015) indeed find that water production rates are lower over nucleus areas with low illumination. Our boundary conditions are hence a reasonable choice for the description of neutral H 2 O.…”