A&A 583, A35 (2015)
Fig. 14.
Mosaic of the Imhotep region with smooth terrains, boulders,
and downslope directions drawn from the gravitational heights with the
ArcGis software.
too big to be lifted (
Groussin & Lamy 2003
;
Kelley et al. 2013
;
Pajola et al. 2015
;
Thomas et al. 2015a
). Since many boulders
larger than one meter are visible on Imhotep, including some on
smooth terrains, air-fall deposits cannot account for all of them.
Nevertheless, transport across the comet nucleus remains a likely
process for small boulders (<1 m), pebbles, and finer material.
With a dust and gas production rate of 255 kg
/s at perihelion
and assuming that 50% of the dragged material falls back on the
surface, a dust layer of 5 cm will cover the entire surface of the
nucleus in two months at perihelion. This deposit is most likely
distributed inhomogeneously on the surface, but it is certainly
not negligible.
Seismic shaking – in Itokawa and Eros, seismic shaking re-
mains the main mechanism to explain the presence of smooth
terrains in gravitational lows (
Richardson et al. 2004
,
2005
;
Miyamoto et al. 2007
). Seismic shaking tends to fluidize the
material and to sort grains by size, with the finest grains mi-
grating, by gravity, toward gravitational lows. However, consid-
ering the very low density of 470 kg
/m
3
of 67P and its high
porosity of 60–70% (
Sierks et al. 2015
), the seismic energy is
substantially attenuated compared to Itokawa and Eros (
Love
et al. 1993
), whose densities are estimated at 1950 kg
/m
3
(
Abe
et al. 2006
) and 2640 kg
/m
3
, respectively (
Thomas et al. 2002
).
Moreover, seismic shaking is usually triggered by a violent ener-
getic event such as an impact, but no large impact crater has been
unambiguously identified on the surface of 67P so far (
Thomas
et al. 2015b
). This makes this process unlikely.
Fluidization of material – for comet 9P
/Tempel 1,
Belton
& Melosh
(
2009
) invoked a fluidization process to explain the
observed smooth terrains. The source of the flow would be a
depression formed after the collapse of a cavity filled with gas.
The escape of the gas causes the cometary material to fluidize; it
is then transported by gravity to the gravitational lows. There
are interesting similarities between the two comets: just like
9P
/Tempel 1, the smooth terrains in Imhotep are restricted to the
gravitational low. They are also adjacent to depressions (basins),
and terraces and linear features may be interpreted as successive
flows. Figure
14
shows the downslope directions on Imhotep cal-
culated from the gravitational heights. It is clear that any material
transported by gravity will end in smooth terrains. However, un-
like smooth terrains on 9P
/Tempel 1, there is no evidence of flow
features in this area. This is a strong limitation that makes the flu-
idization of material and flows not fully satisfying for Imhotep.
Transport by gravity – The only hint of a displacement of the
fine material is located on the northern border of smooth terrains,
at the interface between the smooth and rocky terrains, where
the boulders and the fine material seem to interact (Fig.
15
).
The general direction appears to be downslope, as expected for a
gravity process. Segregation in size seems visible, the finest ma-
terial being located downhill. Following these observations, dust
originates in the north and moves downslope toward the south
until it is stopped by boulders and accumulates.
Based on this discussion, we propose the following scenario
for the formation and evolution of smooth terrains on Imhotep:
the fine material comes from the cli
ffs on the border of the basins
where mass wasting occurs. It is then transported by gravity
downslope to a flat surface where it stays. The wideness of the
smooth area can be explained by the progressive retreat of the
cli
ffs over a long time, probably some tens to hundreds of peri-
helion passages. The more distant the fine material from the cli
ff,
the older the deposit is. We suggest that the linear features visi-
ble in smooth terrains reveal the topography of the rocky terrain
underneath. Some might also be scars of the previous location
of the cli
ffs. This scenario is more suitable with an erosion of
cli
ff runs by events limited in time, such as the passage at peri-
helion that strongly increases the activity of the comet. Air-fall
deposits are also not excluded, but probably only account for a
small fraction of the smooth terrains.
4.2. Rocky terrains
While smooth terrains can be considered as the erosion prod-
uct of rocky terrains, the latter are then the exposed part of
the nucleus bedrock. Currently, we know little about this ma-
terial. The bulk density of 67P’s nucleus is very low and has
been evaluated to be 470
± 45 kg/m
3
, which can be explained
by a high porosity of 60–70% (
Sierks et al. 2015
). The upper
layer of rocky terrains is highly fractured and almost entirely de-
pleted in volatiles, as suggested by the VIRTIS infrared spectro-
scopic observations that did not reveal water ice on the surface
(
Capaccioni et al. 2015
). This is also confirmed by OSIRIS color
images that showed a spectrally red and very homogeneous ter-
rain across the Imhotep region (
Fornasier et al. 2015
).
Rocky terrains experience erosion, as emphasized by mass
wasting that consists of boulders and finer material. Di
fferent
processes can drive this erosion: a) fracturing; b) sublimation;
c) outburst; and d) gravity processes.
Fracturing – as illustrated in Fig.
6
, fractures on exposed
rocks have di
fferent orientations that cannot be related to clearly
identified structures. Moreover, since fractures are visible ev-
erywhere on the nucleus, from the hundred meter scale to the
decimeter scale (
Thomas et al. 2015b
), it seems unlikely that
they have been created by a single, catastrophic event such as
an impact. The fractures on Imhotep at the meter scale might
result from thermal fatigue and thermal shock, which are well
known to a
ffect rocks on airless bodies (
Tauber & Kuhrt 1987
;
Dombard & Freed 2002
;
Dombard et al. 2010
;
Molaro & Byrne
2012
). As
Pochat et al.
(
2009
) phrased it, “this near-surface pro-
cess may help to speed up other processes like mass wasting and
sublimation degradation”.
Sublimation – the second possible erosion process is the
sublimation of ices, known to be the main driver of cometary
activity and responsible for the nucleus erosion. Imhotep, lo-
cated close to the equator, is illuminated every day over the
entire comet revolution around the Sun. However, sublimation
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