Thanks to Rosetta, comet 67P / Churyumov-Gerasimenko continues to reveal its secrets

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Several teams of French researchers of the LATMOS1, LPC2E2, CRPG3, LAM4, IRAP5 involved in the analysis of the observations made by the instruments onboard Rosetta (ESA) reveal the lack of connection, for certain chemical elements, between our Earth and cometary atmospheres. Meanwhile, researchers of the Observatoire de la Côte d’Azur showed that the early activity of the comet is due to large temperature fluctuations caused by shading process on the topographic surface. These works are published in the journals Science and The Astrophysical Journal Letters, 810: L22

Comparison between the map of temperature variation (∆T/∆t)max at the surface of 67P during the period August-December 2014 and an image of 67P taken on 2 September 2014 (© ESA/Rosetta/Navcam/Bob King).

Cold and inactive far from the sun, the icy cometary nuclei vaporize as they approach the inner solar system, releasing a flow of gas and dust under the effect of solar radiation. The hair and tail of the comet thus formed, the coma, then differentiates them from the other small inactive bodies of the solar system: the asteroids.

Water, carbon, terrestrial nitrogen would not be of cometary origin…

The ROSINA instrument developed by an international team under the coordination of Kathrin Altwegg (University of Bern, Switzerland) and embarked on board the ROSETTA probe, analyses the gas composition of comet 67P/Churyumov-Gerasimenko by mass spectrometry. This instrument allows the elemental and isotopic analysis of these gases.

The results show that cometary ice is rich in deuterium, with a Deuterium/Hydrogen ratio three times higher than that of the Earth’s oceans, which precludes a direct relationship between this type of comet and the Earth’s water (6).

Moreover, for the first time a rare gas, argon was detected in a cometary coma, and in large quantities (7). Noble gases are important as tracers of the origin and evolution of the atmospheres of the inner planets (Venus the Earth and Mars). This argon measurement fully confirms that the major elements that make up the Earth’s atmosphere and oceans (water, carbon, and nitrogen) cannot have originated from comets of type 67P, and would have been brought by volatility-rich asteroids. On the other hand, they suggest that a significant fraction of rare gases are of cometary origin (Marty et al., submitted).

This instrument also measured continuously the composition of coma (H2O, CO2, CO, N2…) (8) and showed its chemical heterogeneity. These measurements provide a better understanding of the conditions of cometary ice formation, including its temperature (around 30-40 K) (9).

The activity of the comet betrayed by its shadow…

The NAVCAM imager unexpectedly revealed that the early activity of 67P, materialized by gas and dust jets and still poorly understood, occurred mainly in the concave neck area between the 2 main lobes (see Fig.). However, this region is the least exposed to the Sun and should be on average colder, and thus less conducive to ice sublimation than the other regions of the comet.

To understand this paradox, researchers at the Observatoire de la Côte d’Azur (10) have used a thermophysical model taking into account the thermal conductivity and the complex topography of the comet to calculate a temperature map of its surface during its rotations. This model enabled them to show that the neck region had the fastest temperature variations between August and December 2014 in response to shading by the surrounding terrain. A new cause and effect relationship is thus revealed between these surface temperature variations and the comet’s early activity.

Temperature variation (∆T/∆t)max at the surface of 67P during the period August-December 2014 © Marco Delbo and Victor Ali-Lagoa/Observatoire de la Côte d’Azur.

It has already been observed that rapid temperature changes can induce fracturing at the surface of small bodies in the solar system (Delbo et al. 2014). The authors propose in this paper that the rate of erosion of the comet surface, related to this thermal fracturing, is higher in the neck than elsewhere. This fracturing of the surface material allows solar radiation to penetrate deeper. This would explain why the neck region reveals more ice than the other regions and why it is the main source of the comet’s gas (see Fig.). More generally, these results suggest that thermal fracturing (regolith formation) must be much faster at the surface of atmosphericless bodies with large concavities (shadow formation) than currently available estimates.


French People and Laboratories contributing to the ROSINA experiment :

  • J.-J. Berthelier1, C. Briois2, B. Marty3, O. Mousis4, H. Rème5
  • 1-LATMOS/IPSL-CNRS-UPMC-UVSQ, 4 Avenue de Neptune, F-94100 Saint-Maur, France.
  • 2-Laboratoire de Physique et Chimie de l’Environnement et de l’Espace (LPC2E), UMR 6115 CNRS – Université d’Orléans, France.
  • 3-Centre de Recherches Pétrographiques et Géochimiques, CRPG-CNRS, Université de Lorraine, 15 rue Notre Dame des Pauvres, BP 20, 54501 Vandoeuvre lès Nancy, France.
  • 4-Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France.
  • 5-Université de Toulouse, CNRS–UPS-OMP–IRAP, 9 avenue du Colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France.

Further Resources

  • 6 – Altwegg, K et al. 2015. 67P/Churyumov-Gerasimenko, a Jupiter Family Comet with a High D/H Ratio. Science 347: 1261952–1.
  • 7 – Balsiger, H. et al. 2015. Detection of argon in the coma of comet 67P/Churyumov-Gerasimenko Science Advances 2015, 1500377 (online)
  • 8 – Hässig, M. et al. 2015. Time Variability and Heterogeneity in the Coma of 67P/Churyumov-Gerasimenko. Science 347: aaa0276–1.
  • 9 – Rubin, M. et al. 2015. Molecular Nitrogen in Comet 67P/Churyumov-Gerasimenko Indicates a Low Formation Temperature. Science : 1–4. aaa6100.
  • 10 – Alí-Lagoa V., Delbo M., Libourel G. (2015) Rapid temperature changes and the early activity on comet 67P/CHURYUMOV-GERASIMENKO. The Astrophysical Journal Letters, 810 :L22
  • Communiqué de presse de l’INSU :

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