Abstract
Far away from the Sun, at around 3 AU, the activity of comet 67P/Churyumov-Gerasimenko is low and changes with local time (solar insolation), with location (chemical heterogeneity of the surface), and with season. When the activity is very low because the total cross section of the comet against the Sun is small, the solar wind has access to the surface of the comet and causes ion-induced sputtering of surface material, which we wish to observe. Methods. We used the Double Focussing Mass Spectrometer (DFMS) of the ROSINA experiment on ESA’s Rosetta mission to search for mass spectrometric evidence of sputtered refractory species. In high-resolution mode, DFMS can separate some of the mass peaks of refractory elements from the many volatile species present in the coma. Results. At present, the locations of solar wind surface access are in the southern hemisphere of the comet (the local winter). Of particular interest is sputtering of dust grains on the surface. We observe global averages over the winter hemisphere of the refractory elements Na, K, Si, and Ca, presumably sputtered from grains residing on the surface. Compared to carbonaceous chondrites, the comet has the same Na abundance, is depleted in Ca, and has an excess of K. In addition, for Si the signal strength is strong enough to compile a coarse compositional map of the southern hemisphere. Most, perhaps all, of the observed variation can be explained by the solar wind being affected by the atmosphere of the comet.
Highlights
While the Rosetta spacecraft of the European Space Agency (ESA) was orbiting close to comet 67P/ChuryumovGerasimenko (67P), we performed continuous measurements of the chemical inventory of its coma with the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument suite
ROSINA consists of two mass spectrometers, the Double Focusing Mass Spectrometer (DFMS) and the Reflectron-type Time-Of-Flight (RTOF), as well as the ROSINA COmet Pressure Sensor (COPS; Balsiger et al 2007)
The number densities in the coma of the volatile species show temporary variations that are controlled by solar illumination with the rotation period of the comet of 12.4 h and half of that, with seasonal variation, and compositional heterogeneity of the volatile inventory on the cometary surface
Summary
While the Rosetta spacecraft of the European Space Agency (ESA) was orbiting close to comet 67P/ChuryumovGerasimenko (67P), we performed continuous measurements of the chemical inventory of its coma with the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument suite. Most of the observed species in the coma are volatile material that are released from the comet’s surface by sublimation, for example, H2O, CO, CO2 (Hässig et al 2015), and many others (Le Roy et al 2015). The number densities in the coma of the volatile species show temporary variations that are controlled by solar illumination (diurnal cycle) with the rotation period of the comet of 12.4 h and half of that, with seasonal variation (summer and winter hemispheres), and compositional heterogeneity of the volatile inventory on the cometary surface. We normalize the measurements of the observed species onto a spherical surface of 10 km from the center of the nucleus to create coarse maps of their probable origin on the surface. Wind sputtering of refractory material, that is, of dust located on the cometary surface. For the periods of high density during current conditions, the distance of this cavity from the cometary surface is estimated to be in the range of 10−20 km (calculated with Eq (10.30) in Gombosi 2015)
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