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

Abstract

Context. The SWAN Lyman α photometer onboard SOHO monitored the hydrogen cloud around comet 67P/Churyumov-Gerasimenko (67P) postperihelion at the three last perihelions in 1996, 2002, and 2009. Aims. Combining the SWAN results with some new Rosetta data allows estimating the erosion rate of the comet, quantified by the thickness of a layer that is disposed of 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, we estimate the total H 2 O mass loss per orbit to be 2.7 ± 0.4 × 10 9 kg. We also tried to explain the observed change in the rotation rate during the 2009 orbit (period decrease of 1285 s) and the change observed by Rosetta from June 2014 to February 2015 (period increase 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. Results. The total ejected mass depends on dust-to-gas mass ratios (4 ± 2) determined from Rosetta. 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. 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. Conclusions. The thickness of lost material needs to be kept in mind when interpreting all surface features. 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.

Gas outflow and dust transport of comet 67P/Churyumov–Gerasimenko

“…Based on the studies of the surface mass erosion and transport before and in the early phase of the Rosetta mission (Bertaux 2015;Keller et al 2015;Thomas et al 2015), a first-generation numerical model of the regolith growth on comet 67P is produced by integrating several key elements. These include the time profile of the water production rates derived from ground-based, space-born and Rosetta observations (de Almeida et al 2009;Bertaux 2015;Biver et al private communication).…”

“…These include the time profile of the water production rates derived from ground-based, space-born and Rosetta observations (de Almeida et al 2009;Bertaux 2015;Biver et al private communication). The peak value of the water production rate is taken to be 10 28 molecules s −1 which is subject to change upon more detailed analyses (Biver et al private communication).…”

“…From a compilation of the estimated Q values within 2 au to perihelion from different returns, de Almeida et al (2009) produced a radial dependence of Q ∼ Q dA (r/q) −7 preperihelion and Q ∼ Q dA (r/q) −3.3 post-perihelion with q = 1.25 au and Q dA = 3 × 10 28 H 2 O molecules s −1 . The SWAN Lyman α photometer onboard SOHO measured a peak value of Q = 1.5 × 10 28 H 2 O s −1 near perihelion for the 2002 return but a smaller peak value of about 9 × 10 27 H 2 O molecules s −1 at 20 d after perihelion for the 2009 return (Bertaux 2015). Preliminary results from the MIRO observations on Rosetta indicate that the pattern of outgassing process and gas production rate of 67P during the 2015 return could be similar to the SWAN values in 2009 (Biver et al private communication) (see Fig.…”

“…From the estimate of total mass-loss of 2.7 ± 0.4 × 10 9 kg, Bertaux (2015) suggested that the surface layer of comet 67P could be eroded by as much as 1.0 ± 0.5 m per orbit. This value is compatible with average mass-loss rate estimated for periodic comets (Sekanina 2008).…”

“…The airfall process Figure 1. A schematic description of global gas production rates Q for different periods of time before and after perihelion according to de Almeida et al (2009), Bertaux (2015 and the preliminary results from the MIRO microwave spectrometer experiment on Rosetta (Biver et al private communication). associated with erosion sublimation and dust mantle buildup as comet 67P moves around the sun is addressed in Section 4.…”