Tchouri under the eye of Rosetta

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Surprisingly shaped into two lobes and of high porosity, the nucleus of the comet 67P / Churyumov-Gerasimenko (nicknamed Tchouri) reveals a wide range of features thanks to the MIRO, OSIRIS and VIRTIS instruments of the ESA Rosetta mission, to which contribute researchers of the CNRS, of the Observatoire de Paris and of several universities 1, with the support of the CNES. Seven in number, their studies, published on January 23, 2015 in Science, also show that the comet is rich in organic materials and that the geological structures observed result primarily from surface erosion phenomena. The RPC-ICA instrument has meanwhile traced the evolution of the magnetosphere of the comet while the ROSINA instrument seeks the witnesses for the birth of the Solar System.

The nucleus of 67P/Churyumov-Gerasimenko

The images of the comet 67P taken by the OSIRIS camera show an unusual overall shape composed of two lobes separated by a “neck” whose origin remains unexplained. Its overall homogeneous composition surface hosts a wide variety of geological structures resulting from erosion phenomena, collapse and re-deposition. The activity of the comet, surprising at a so large distance from the sun, is currently focused in the area of the “neck”. All the images has led to a three-dimensional model of the comet and to the detailed topography of the original landing site of Philae. Combined with the mass measurement, this model gave the first direct determination of the density of a comet nucleus which implies a very high porosity. This model also provides the “cartographic” context for the interpretation of the results of other experiments.

Example of a circular hole observed on the nucleus of comet 67P. The increase in contrast reveals the presence of activity. Image taken by the OSIRIS-NAC camera on August 28, 2014 from a distance of 60 km, with a spatial resolution of 1 m/pixel. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The surface properties of 67P/Churyumov-Gerasimenko

The MIRO instrument allowed the researchers to map the temperature of the near subsurface of 67P. This shows seasonal and diurnal temperature variations which suggest that the surface of 67P is weakly conductive on a thermal level because of a porous and less dense structure. The researchers also made measurements of the rate of water production of the comet. It varies throughout the rotation of the nucleus, the water released by the comet being located in the region of its “neck”.

Temperature map of the near subsurface temperature of the core (in iso-contour) measured by the MIRO instrument. The illumination of the core surface is shown in the background. The lowest temperatures (-250 °C, in blue) are on the non-sunny side (left in the figure). Credits: Gulkis et al.

A comet rich in organic materials

VIRTIS provided the first detection of organic materials on a cometary nucleus. Its spectroscopy measurements indicate the presence of various materials containing carbon-hydrogen bonds and/or oxygen-hydrogen bonds, the nitrogen-hydrogen bond being not detected at present. These species are associated with opaque and dark minerals such as iron sulfides (pyrrhotite or troilite). Moreover, these measurements indicate that no rich area of ice larger than twenty meters is observed in regions illuminated by the Sun, which suggest a strong dehydration of the first centimeters of the surface.

The comet’s surface composition is very homogeneous with a small difference in the neck region which may contain water ice. Credits: F. Capaccioni et al.

The birth of the magnetosphere of a comet

Using the RPC-ICA (Ion Composition Analyser) instrument, researchers have traced the evolution of aqueous ions, from the first detections until the cometary atmosphere began to stop the solar wind (around 3,3 UA2). They have registered the spatial configuration of the early interaction between the solar wind and the cometary fine atmosphere, causing the formation of the magnetosphere of “Tchouri”.

67P/Churyumov-Gerasimenko, witness of the birth of the Solar System

Formed about 4.5 billion years ago and remained frozen since that time, comets preserve traces of the primitive material of the solar system. The composition of their nucleus and coma thus gives clues on the physico-chemical conditions of the early solar system. The ROSINA instrument of the Rosetta probe measured the composition of the coma of 67P (the coma or hair, is a kind of fairly dense atmosphere surrounding the core, it is composed of a mixture of dust and gas molecules) by following the rotation of the comet. These results indicate large fluctuations in the composition of the heterogeneous coma and a coma-nucleus complex relationship where seasonal variations could be caused by differences in temperature just below the surface of the comet.

CO2/H2O ratio measured by ROSINA on the comet during the period from 17 August to 22 September 2014. Credits: ESA/Rosetta/ROSINA/UBern, IBAR, LATMOS, LMM, IRAP, MPS, SwRI, TUB, Umich (short version: ESA/Rosetta/ROSINA)

Dust from the comet 67P/Churyumov-Gerasimenko

The GIADA dust sensor has already collected a wealth of data (size, speed, direction, composition) on dust whose dimensions vary from 0.1 to several millimeters and which are issued directly from the nucleus. Additionally, the OSIRIS images have led to detect larger dust orbiting the nucleus, probably released during the previous passage of the comet.

Notes

1 Aix-Marseille Université, UPMC, Université Paris Diderot, UVSQ, Université Toulouse III – Paul Sabatier, Université d’Orléans, Université Joseph Fourier, Université de Cergy-Pontoise, Université Paris-Sud, Université de Lorraine, Ecole Normale Supérieure.

2 The astronomical unit (AU) is the average Earth-Sun distance. The value of 150 million kilometers is commonly accepted to 1 AU.

Laboratories involved :

The French laboratories involved in this study are :

  • Laboratoire d’astrophysique de Marseille (CNRS/ Aix-Marseille Université)
  • Laboratoire d’études spatiales et d’instrumentation en astrophysique (CNRS/Observatoire de Paris/UPMC/Université Paris Diderot)
  • Laboratoire atmosphères, milieux, observations spatiales (CNRS/UPMC/UVSQ)
  • Institut de recherche en astrophysique et planétologie (CNRS/Université Toulouse III – Paul Sabatier)
  • Laboratoire de physique et de chimie de l’environnement et de l’espace (CNRS/Université d’Orléans)
  • Institut de planétologie et astrophysique de Grenoble (CNRS/Université Joseph Fourier)
  • Laboratoire d’étude du rayonnement et de la matière en astrophysique et atmosphères (CNRS/Observatoire de Paris/UPMC/ENS/Université de Cergy-Pontoise)
  • Institut d’astrophysique spatiale (CNRS/Université Paris-Sud)
  • Centre de recherches pétrographiques et géochimiques (CNRS/Université de Lorraine)

Bibliography

  • Subsurface properties and early activity of comet 67P/Churyumov-Gerasimenko., S.Gulkis et al.,Science, 23 janvier 2015.
  • 67P/Churyumov-Gerasimenko: The Organic-rich surface of a Kuiper Belt comet as seen by VIRTIS/Rosetta., F. Capaccioni et al.,Science, 23 janvier 2015.
  • On the nucleus structure and activity of comet 67P/Churyumov-Gerasimenko., H. Sierks et al.,Science, 23 janvier 2015.
  • The Morphological Diversity of Comet 67P/Churyumov-Gerasimenko., N.Thomas et al.,Science, 23 janvier 2015.
  • Dust Measurements in the Coma of Comet 67P/Churyumov-Gerasimenko Inbound to the Sun Between 3.7 and 3.4 AU., A. Rotundi et al.,Science, 23 janvier 2015.
  • Birth of a comet magnetosphere: A spring of water ions., H. Nilsson et al.,Science, 23 janvier 2015.
  • Time variability and heterogeneity in the coma of 67P/Churyumov-Gerasimenko., M. Hässig et al.,Science, 23 janvier 2015.

IRAP Contacts:

  • Henri Rème l T 05 61 55 66 65 / 06 18 58 50 49 l Henri.Reme@irap.omp.eu
  • Jean-André Sauvaud l T 05 61 55 66 76 / 06 77 72 70 76 l jean-andre.sauvaud@irap.omp.eu

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