A.-T. Auger et al.: Geomorphology of the Imhotep region on comet 67P
/Churyumov-Gerasimenko from OSIRIS observations
Fig. 15.
A) Mass wasting on the northern border of Imhotep showing evidence of transport of the fine material.
B) Zoom that shows textured
boulders of tens of meters that stop the fine material from moving downslope. This image was acquired with the NAC camera on 5 Oct. 2014 from
a distance of 18 km. The spatial resolution is 34 cm
/pix (NAC_2014-10-05T04.09.24.)
is stronger at perihelion when insulation, and therefore ero-
sion, is highest. It is therefore likely that the mass wasting ob-
served at 3 AU on Imhotep is just the beginning of an ero-
sion phenomenon that will increase up to perihelion at 1.2 AU.
Quantitatively, we expect surface erosion to be ten times stronger
at 1.2 AU than at 3 AU, if it is driven by water ice sublimation.
Outbursts – sporadic events such as outbursts can also erode
the nucleus and even break it in the most catastrophic scenario
(
Boehnhardt 2004
). Several small and short (a few hours) out-
bursts were observed by the Deep Impact spacecraft on comet
9P
/Tempel 1 during the weeks preceding the flyby (
A’Hearn
et al. 2005
). On comet 67P, an outburst was observed by OSIRIS
on 30 April 2014, at 4.1 AU from the Sun, which lasted ten days
(
Tubiana et al. 2015
). This outburst released 10
3
kg to 10
5
kg of
material, which corresponds to an excavated volume equivalent
to a sphere smaller than 15 m, assuming a density of 470 kg
/m
3
(
Sierks et al. 2015
). Such events, if they become more frequent
as 67P approaches the Sun, could thus play an important role in
eroding the nucleus surface and Imhotep in particular.
Gravity processes – when a rocky terrain has been eroded by
sublimation and
/or fragmented by fractures, gravity can modify
it by mass wasting or collapse that is due to subsurface internal
voids. The creation of overhangs by sublimation of a lower layer
is a simple mechanism for mass wasting, and these overhangs
have been observed in several places on the nucleus (
Thomas
et al. 2015b
;
Pommerol et al. 2015
). Mass wasting on the north-
western part of Imhotep, clearly associated with the surrounding
scarps, is a good candidate for an overhang (Fig.
16
).
To summarize, the most likely scenario for the erosion of
rocky terrains is that it is triggered by the sublimation of ices,
controlled by gravity, and probably exacerbated by fractures.
An interesting question is whether rocky terrains, which are
eroding, are also forming? Does an Earth-like rock cycle exist?
We already know that if there is indeed a cycle, it can only be
an open cycle since 67P loses some rocky material (i.e., dust) at
each perihelion passage. Dust falling back onto the nucleus sur-
face may re-form a rock, as it is the case for sediments on Earth.
This requires a compaction mechanism or a cement
/matrix be-
tween the grains to obtain a coherent material. The accumulation
of material in a basin may lead to the compaction of the deepest
layers. In this case, the new rocky terrain would no longer be
made of primordial material, but of remobilized material from
an older eroded rocky terrain. Such a rocky cycle is currently
purely speculative.
Fig. 16.
Mass wasting and overhangs in the northwest of Imhotep.
(NAC_2014-09-05T06.45.55.)
4.3. Accumulation basins
Accumulation basins are areas where the products of the erosion
of rocky terrains will accumulate if they have not escaped the
comet gravitational attraction for the finest grains. The boulders
at the feet of the cli
ffs, on the borders of the basins, show that
cli
ffs indeed erode and then retreat, as proposed before. The al-
cove shape of basins could result from their initial circular shape
and
/or from an erosion process. There have been several proposi-
tions to explain circular features, including pits, on other comets:
a) impact craters; b) locally enhanced cometary activity; c) or
collapse of a subsurface cavity.
Brownlee et al.
(
2004
) and
Vincent et al.
(
2014
) were able
to retrieve the shape of flat-floor pits and pit-halo features on
81P
/Wild 2 from experimental and numerical simulations of im-
pact craters.
Thomas et al.
(
2007
) considered that circular rim
remnants and isolated rimless depressions on 9P
/Tempel 1 are
consistent with impact craters, although they were unable to
prove their origin. An impact origin for the formation of some
basins cannot be completely ruled out.
On the other hand,
Belton et al.
(
2013
) and
Thomas et al.
(
2013a
) ascribed the majority of pits and depressions observed
A35, page 9 of
13
A&A 583, A35 (2015)
Fig. 17.
Formation and evolution of the accumulation basins on Imhotep. A) Primordial void into the nucleus, resulting from its formation process.
B) Progressive erosion of the surface and thinning down of the layer of material above the void.
C) and
D) collapse of the thin upper layer.
E) Enlargement of the basin and infill with eroded material of the edges. Possible formation of overhangs.
Fig. 18.
Basin F and its fractures (white lines). The colored area represents the density of intersection points between prolongated fractures: the
density increases from yellow to red. (NAC_2014-09-05T06.31.16.)
on 9P
/Tempel 1 to endogenic processes such as outbursts.
Belton
& Melosh
(
2009
) also proposed that the origin of some large
round features might be the collapse of a subsurface cavity, af-
ter the voiding of a gaseous material. However, this hypothe-
sis relies on the transition from amorphous to crystalline water
ice, which is exothermic and leads to the sublimation of the sur-
rounding CO and CO
2
ices. This process remains speculative
because transferring solar energy from the surface to tens of me-
ters into the nucleus is di
fficult to reconcile with the low thermal
inertia of the nucleus of 10–50 J
/m
2
/K/s
1/2
(
Gulkis et al. 2015
).
Indeed, the seasonal heat wave penetrates less than 1 m per revo-
lution, and at 10 m depth the material is thermally isolated from
the surface.
Our best interpretation for the formation of basins A to E
remains the presence of large primordial voids into the nucleus,
resulting from its formation process. After the surface above the
voids is made fragile by erosion and fracturing, it can eventually
collapse (Fig.
17
). Primordial internal voids must have a volume
equivalent to the formed basin. The resulting basin can then ex-
tend radially, with erosion. This scenario implies that the size
of the current basins may be significantly larger than that of the
initial basins formed just after collapse.
Basin F appears to be di
fferent from the other basins since
it shows many fractures that point radially toward its interior
(Fig.
18
). Radial fracturing results from mechanical stress that
cannot be triggered by a gravitational process such as a collapse,
since it will only a
ffect the collapsed material and not its sur-
roundings. Two processes previously mentioned may thus be at
the origin of these fractures and of basin F: 1) impact cratering
or 2) elevation by the rising up of a gas bubble from the interior
of the nucleus triggererd by cometary activity. It is currently not
possible to favor one scenario over the other.
4.4. Terraces
The terraces on Imhotep and on the nucleus in general (
Thomas
et al. 2015b
) strongly suggest layering (
Massironi et al. 2015
).
Layers can be material of di
fferent compositions or compaction
and
/or successive deposits. There is no variation in color or
albedo between the layers, but an important observation is that
they have a relatively constant thickness of a few meters, which
implies a repetitive process. Layers may be primordial, resulting
from the formation process of the nucleus (
Belton et al. 2007
), or
formed later by evolutionary processes (
Belton & Melosh 2009
).
The fractures of basin F cross several terraces around it
(Figs.
8
and
18
), indicating that fractures are posterior to the for-
mation of the layers. Chronologically, layers were formed before
basin F. However, we cannot prove whether or not they are pri-
mordial.
4.5. Roundish features
In the center of Imhotep, the region of roundish features presents
clear characteristics. In addition to the fact that it is the only
place so far on the nucleus where this type of features has been
detected, this region is
– located in the lowest part of the Imhotep region in terms of
gravitational heights (basin E),
– depleted in boulders,
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