Time variability and heterogeneity in the coma of 67P/Churyumov-Gerasimenko

Abstract

Abstract:Comets contain the best-preserved material from the beginning of our planetary system. Their nuclei and comae composition reveal clues about physical and chemical conditions during the early Solar system when comets formed. ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) onboard the Rosetta spacecraft has measured the coma composition of comet 67P/Churyumov-Gerasimenko with well sampled time resolution per rotation. Measurements were made over many comet rotation periods and a wide range of latitudes. These measurements show large fluctuations in composition in a heterogeneous coma that has diurnal and possibly seasonal variations in the major outgassing species: H 2 O, CO, and CO 2 . These results indicate a complex coma-nucleus relationship where seasonal variations may be driven by temperature differences just below the comet surface.One Sentence Summary: ROSINA/DFMS shows that 67P/Churyumov-Gerasimenko has a highly heterogeneous coma with large diurnal and possibly seasonal variations. Main Text:Initially, comets were classified depending on the location where they formed in the protoplanetary disc (1,2). This classification assumed a similar composition of the nucleus within a given formation region. No cometary nucleus composition has been sampled in situ. Rather, it is implicitly assumed that measurements of the outgassing of comets reveal the composition of the volatile components of the nucleus. However, compositional homogeneity of at least one comet was confirmed by studying outgassing from the fragments of the broken up comet Schwassmann-Wachmann 3 (3). Detailed observations of other cometary comae indicated that there is evidence of heterogeneity. Missions to comet Halley detected release of volatiles in multiple jet-like features that were dominantly seen on the sunlit side of the nucleus (4, 5). The Deep Impact mission detected asymmetries in composition in the coma of Tempel 1 (6). In particular, these remote sensing observations at Tempel 1 indicated an absence of correlation between H 2 O and CO 2 in the coma.Detailed, close up cometary images have also showed visible differences between different areas of cometary nuclei. These images suggested that heterogeneity in the coma of a comet may be related to heterogeneity of the nucleus. Observations by EPOXI at Hartley 2 in 2010 near perihelion indicated that the nucleus is complex, with two different sized lobes separated by a middle waist region that is smoother and lighter in color (7). Outgassing from sunlit surfaces of the nucleus revealed that the waist and one of the lobes were very active. A CO 2 source was detected at the small lobe of the comet, while the waist was more active in H 2 O and had a significantly lower CO 2 content. Based on these coma observations, it has been tentatively suggested that the heterogeneity in the comet's nucleus was primordial (7). Seasonal effects could not be ruled out because the observations also showed a complex rotational state for the comet (7). The smaller of the two lobes ...

Statistical analysis of suprathermal electron drivers at 67P/Churyumov–Gerasimenko

“…We have highlighted two intervals of interest near 07:00 (Event 1) and 10:00 (Event 2) on November 6. The 6h periodic enhancements in neutral density are the same as those described by Hässig et al (2015), and are more pronounced on this day relative to our previously discussed observations. We note four spikes in neutral density, which are caused by spacecraft slews and are similar to the spikes shown in Fig.…”

“…Given the relatively small motion of Rosetta relative to the comet, as shown in Fig. 1, the gradual changes in density are likely the result of enhanced neutral density regions that vary with the rotation of 67P (Hässig et al 2015). The gradual variations in density are reasonably correlated with the warm electron temperature (panel b).…”

“…On 2014 November 1, the local neutral density varied between ∼0.7 × 10 7 and ∼1.5 × 10 7 cm −3 because of the local heterogeneity in 67P's coma (Hässig et al 2015). The cross-section for 20 eV electrons to transfer momentum with water is ∼5 × 10 −16 cm 2 , and is smaller for electrons at higher energies (Itikawa & Mason 2005).…”

“…They infer the bulk electron density with Rosetta's Langmuir Probe (LAP) and Mutual Impedance Probe (MIP) (Eriksson et al 2006;Trotignon et al 2006). They found that the bulk electron density is inversely proportional to the distance from the comet, and that the bulk electron density is proportional to the local neutral density enhancements (Hässig et al 2015). Clark et al (2015) performed the first study of cometary suprathermal electrons at 67P by fitting the electrons below a few hundred eV with a flattop distribution (Thomsen et al 1986).…”

Decay of COSAC and Ptolemy mass spectra at comet 67P/Churyumov-Gerasimenko

“…Owing to the zenith directed position of Ptolemy's vent tubes and a siphon-shaped inlet (Figure 1, right panel), also preventing grains from entering the system, its mass spectrometer could not have received organic-abundant grains from the impact-excavated subsurface directly. During the landing event, the coma composition would have been modified by gas vaporising from newly excavated dust exposed to solar illumination and contact with the lander (including the entrance to the Ptolemy vent tubes), thus explaining the high concentration of organics measured compared to orbiter measurements (Hässig et al 2015). Wright et al (2015) therefore confirm measuring gaseous CO 2 present in the nebulous coma atmosphere over the illuminated nucleus surface.…”