Dust mass distribution around comet 67P/Churyumov–Gerasimenko determined via parallax measurements using Rosetta's OSIRIS cameras

Abstract

The OSIRIS (optical, spectroscopic and infrared remote imaging system) instrument on board the ESA Rosetta spacecraft collected data of 67P/Churyumov-Gerasimenko for over 2 yr. OSIRIS consists of two cameras, a Narrow Angle Camera and a Wide Angle Camera. For specific imaging sequences related to the observation of dust aggregates in 67P's coma, the two cameras were operating simultaneously. The two cameras are mounted 0.7 m apart from each other, as a result this baseline yields a parallax shift of the apparent particle trails on the analysed images directly proportional to their distance. Thanks to such shifts, the distance between observed dust aggregates and the spacecraft was determined. This method works for particles closer than 6000 m to the spacecraft and requires very few assumptions. We found over 250 particles in a suitable distance range with sizes of some centimetres, masses in the range of 10 −6-10 2 kg and a mean velocity of about 2.4 m s −1 relative to the nucleus. Furthermore, the spectral slope was analysed showing a decrease in the median spectral slope of the particles with time. The further a particle is from the spacecraft the fainter is its signal. For this reason, this was counterbalanced by a debiasing. Moreover, the dust mass-loss rate of the nucleus could be computed as well as the Af ρ of the comet around perihelion. The summed-up dust mass-loss rate for the mass bins 10 −4-10 2 kg is almost 8300 kg s −1 .

Local Manifestations of Cometary Activity

“…For such large grains that in terms of cometary structures will present hierarchically built clusters of grains -gravity is the limiting factor. In situ observations Ott et al, 2017) and laboratory measurements (Gundlach et al, 2015) have confirmed that mm to dm grains that can be easily removed against cohesion and gravity and are common on the surface of comet 67P.…”

Experiments on cometary activity: ejection of dust aggregates from a sublimating water-ice surface

“…However, this might be a difficult task, because it is still unclear at which dominant size most of the dust mass is emitted. For instance, Blum et al (2017) showed that for comet 67P/Churyumov-Gerassimenko this size falls in the vast range of ∼ 1 mm to ∼ 10 m and Fulle et al (2016b) and Ott et al (2017) derived mass-frequency distribution functions with peak masses of ∼ 1 kg and, thus, typical dimensions of 10 cm. Future work might narrow this range down and allow a stricter assessment of the depth of the dust-ice boundary.…”

Towards New Comet Missions

“…The true significance of these observations was revealed by Rosetta. Large particles, moving at velocities close to the escape velocity, were indeed detected (Thomas et al , 2015bLin et al 2015;Ott et al 2017) but the vast number of individual slow particles seen had not been predicted. While their presence cannot be disputed, the means by which these particles are lifted from the surface remains a subject of significant debate.…”