CONSERT probing of 67P/C-G nucleus during the ROSETTA mission, operations and results
Résumé
The structure and composition of cometary nuclei is one of the
major unknowns in cometary science. These were the scientific
objectives of the Comet Nucleus Sounding Experiment by
Radiowave Transmission (CONSERT) aboard ESA spacecraft
Rosetta. The knowledge of the interior is important to understand
the formation and evolution of comets. CONSERT was a bi-static
radar composed of two parts, one on the lander Philae on the
comet’s surface, and another on Rosetta spacecraft (Kofman et
al., 2007). CONSERT operated during 9 hours after the landing
and made measurements through the small lobe (head) of 67P/
C-G. The analysis and interpretation have been done using
measurements of the propagation time of the signal through the
comet, the shape of the received signals and then 3D modeling
of the propagation through the comet. The modelling and the
propagation time inside the comet permit to derive the bulk
permittivity value (= 1.27) of the cometary interior (Kofman et
al, 2015). Permittivity values for porous mixture of ices and dusts
were compared with laboratory experimental values in order
to obtain constraints on the possible constituents of the comet
nucleus (ices, silicates and organics) and its porosity (70-85%).
The minimum required content of carbonaceous material is 75 %
in volume. This suggests that comets represent a massive carbon
reservoir (Kofman et al., 2015, Herique et al., 2016). The shape of
the signal, very close to the shape of the calibration one, shows
that the scattering by inhomogeneities in the medium is not
observable. This indicates that the interior is homogenous at the
scale of few wavelengths (Kofman et al., 2015). 3D simulations of
the signal propagation in non-homogeneous media have been
run to define the sensitivity of CONSERT to inhomogeneities and
to find constrains on the internal structures in terms of size and
composition at a scale commensurate with the wavelength. Given
the high bulk porosity of about 75% inside the sounded part of
the nucleus, a likely model would be obtained by a mixture, at this
3-m size scale, of voids (vacuum) and blobs with material made
of ices and dust with a porosity larger than 60%. The absence
of any pulse spreading due to scattering allows us to exclude
heterogeneity with higher contrast (0.25) and larger size (3m) (but
remaining on the few wavelengths scale, since larger scales can
be responsible for multipath propagation) (Ciarletti et al., 2017).
Properties of meterscale inhomogeneities inside the comet are
essential to understand cometary formation. The knowledge of
the precise position of Philae on the comet, since the September
2016, permitted to improve the determination of the propagation
paths inside the comet and therefore to describe better the
interior. The influence of the close environment on antennas
lobes and polarization was calculated using the Digital Terrain
Model of the landing site and used to study the received signal
power. We will describe shortly measurements that explored
the interior of the comet, discuss results, their interpretation in
terms of the internal structure and composition. References [1]
Ciarletti et al, CONSERT constrains the internal structure of 67P
at a fewmetre size scale, MNRAS, 469 (Suppl. 2), pp.S805-S817,
2017 [2] Herique et al, Cosmochemical implications of CONSERT
permittivity characterization of 67P/CG, MNRAS 462, S516-S532,
2016 [3] Kofman et al, The Comet Nucleus Sounding Experiment
by Radiowave Transmission (CONSERT). Space Science Reviews,
Volume 128, Issue 1-4, 413-432, 2007 [4] Kofman et al, Properties of
the 67P/Churyumov-Gerasimenko interior revealed by CONSERT
radar, Science, 349, 6247 aab0639, 2015.