Estimate of the erosion rate from H₂O mass-loss measurements from SWAN/SOHO in previous perihelions of comet 67P/Churyumov-Gerasimenko and connection with observed rotation rate variations

Jean-Loup Bertaux (1)

1 PLANETO - LATMOS

The SWAN Lyman α photometer onboard SOHO monitored the hydrogen cloud around comet 67P/Churyumov- Gerasimenko (67P) post perihelion at the three last perihelions in 1996, 2002, and 2009. The Rosetta instruments are collecting a wealth of new data during the satellite rendezvous with this comet. Aims. Combining the SWAN results with some new Rosetta data allows estimating the erosion rate of the comet at each orbit. Methods. By integrating the production rates measured with SWAN in time and adding some estimates for periods that are not covered by SWAN measurements, we estimate the total H₂O mass loss per orbit to be 2.7± 0.4x10⁹ kg. It is possible to measure the erosion rate more accurately with the precise knowledge of the area of the nucleus (47.4 km²), the outgassing rates, and dust-to- gas mass ratios (4±2) determined from Rosetta instruments, along with the mean density of the nucleus. The erosion rate is quantified by the thickness of a layer that is disposed of at each orbit, mainly around perihelion. We also tried to explain the observed change in the rotation rate during the 2009 orbit (a decrease in the period of 1285 s) and the change observed by Rosetta from June 2014 to February 2015 (increase in the period of 32 s and 98 s up to 17 May 2015) with three different mechanisms: sublimation-induced torque, thermal dilatation, and separation between the two lobes. With a dust-to gas mass ratio of 2, the estimated layer is between 0.5 to 0.6 m thick, while it reaches 1.0 to 1.4 m for a dust-to gas mass ratio of 6. This means that a layer of 1.0 ±0.5 m thickness is lost at each orbit. The outgassing-induced torque may explain the observed changes in the rotation rate around perihelion in 2009 and recent changes. We argue that dilatation and separation of lobes cannot explain the perihelion change in 2009 (increase) because these mechanisms would either let the rotation remain unchanged or would decrease the rotation rate. These mechanisms are quantified and cannot be totally excluded for the observed changes from June 2014 to February 2015. They would need to become three times larger to explain the 98 s change in period up to 17 May, which is somewhat unrealistic. Our preferred scheme is the sublimation-induced torque, which our computations show to be fully compatible with the known outgassing rates. The torque decelerated the rotation from August 2014 to 17 May 2015, at which time it changed sign and began to accelerate the rotation, consistent with the average behavior observed for the 2009 apparition. The thickness of lost material needs to be kept in mind when interpreting all surface features. It probably rules out the existence of a ubiquitous crust mantle that would survive many orbits. At 1 m ± 0.5 m, the erosion rate per orbit is high and supports the idea that the composition of the material that is measured in the coma (gas and solid) is indeed representative of the bulk material of the nucleus. We also argued that monitoring the rotation rate yields a very accurate and precious indicator of the global activity of the comet with which other activity measurements can be compared.

Mots clés: SOHO; comets; 67P/Churyumov-Gerasimenko; erosion; mass loss; rotation rate; Rosetta; SWAN

Domaines: Planétologie et astrophysique de la terre [astro-ph.EP]

Jean-Loup Bertaux. Estimate of the erosion rate from H₂O mass-loss measurements from SWAN/SOHO in previous perihelions of comet 67P/Churyumov-Gerasimenko and connection with observed rotation rate variations. Astronomy and Astrophysics - A&A, 2015, 583, A38 (10 p.). ⟨10.1051/0004-6361/201525992⟩. ⟨insu-01175722⟩