Dynamics of non-spherical dust in the coma of 67P/Churyumov– Gerasimenko constrained by GIADA and ROSINA data

Abstract

Among the comet 67P/Churyumov-Gerasimenko (67P/C-G) in situ measurements, the closest that have ever been performed at a comet nucleus, are also those of speed, mass, and crosssection of cometary grains performed by the Grain Impact Analyser and Dust Accumulator (GIADA) instrument. To interpret GIADA data, we performed dust dynamical numerical simulations with both spherical and non-spherical (spheroids) shapes. This allowed us to analyse how the grain non-sphericity affects the data interpretation. We find that some measured dust speeds are unlikely reproducible when a spherical shape is considered. We considered two GIADA observational periods, 2015 February 19-27 and 2015 March 13-28. Gas parameters calibrated with the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) measurements have been used to retrieve the gas conditions to set up the dust particle motion. The dust grains are assumed to be out of the near nucleus coma, i.e. where the gas velocity is radial and constant, therefore they are either aligned or have random but constant orientation with respect to the gas drag. We reproduced the GIADA dust speeds, using spheres and two different spheroidal shapes. We find that the particle shapes that reproduce best the GIADA dust speeds are consistent with the particle shape constrained by the GIADA data. We obtain different terminal velocities for spherical and non-spherical particles of the same mass. The shape, which reproduces the GIADA data, is oblate rather than prolate spheroid. We obtain rotational frequencies of the spheroidal particles that best fit the GIADA measurements in these periods.

Airfall on Comet 67P/Churyumov–Gerasimenko

“…We here consider chunks shaped as oblate ellipsoids with axis ratio 4, exposing their largest cross section to the gas flow at =0 h ( 2.5 times that of an equal-mass sphere), that turn with constant radial velocity to expose their smallest cross section ( 0.63 times that of an equal-mass sphere) during the first 500 m of flight. The chunk shape is close to the axis ratio of 5 needed in order to match the particle speeds measured by Rosetta/GIADA according to Ivanovski et al (2017a). The time needed for the chunks to reach the height of 500 m is similar to that needed by the aerodynamic torque to set non-spherical chunks in rotation according to simulations by Ivanovski et al (2017b).…”

Summer outbursts in the coma of comet 67P/Churyumov–Gerasimenko as observed by Rosetta–VIRTIS

“…The first set includes the physical properties of the simulated particles. We considered homogeneous, isothermal spheres, oblate and prolate spheroids with aspect ratio a/b = 0.2 and 5.0, elongation values able to reproduce GIADA data (Ivanovski et al 2017b). We considered the dust physical properties retrieved by the GIADA measurements of 2015 August outbursts, i.e.…”

Cometary Comae-Surface Links

“… 2015 ) that have oblate or prolate shapes will also affect the dynamics as studied by Ivanovski et al. ( 2017a , b ). These latter authors have shown that not only may non-spherical particles begin to rotate in the gas flow but they may also accelerate to higher speeds than spherical ones.…”