At the final landing site, called Abydos, the MUPUS experiment on Philae was
able to deploy an arm with a penetration device. The device encountered very hard
material just below the surface. The resulting stress was measured by the device and
resulted in compressive strengths >2 MPa (Spohn et al. 2015; Boehnhardt et al.
2017) and therefore 2–3 orders of magnitude higher than that found from the
rebound. However, it needs to be noted that the orientation of the lander at this
time was far from nominal (Spohn et al. 2015).
These results might be reconciled if the sub-surface ice content and its spatial
variability are important and may be evidence of recondensation of volatiles emitted
from the surface and forced back into the nucleus surface layer by the gas pressure
over the surface. This effect was identified in the “KOSI” (“KOmeten-SImulation”)
experiments in the late 1980s (e.g. Thiel et al. 1989) and was modelled by, for
example, Prialnik (1991). In the experiments, the crust thickness reached 7 cm with a
mechanical strength exceeding 5 MPa while some degree of porosity was
maintained. Hence, this seems to be a way to produce a more dense and therefore
potentially harder ice layer immediately under a desiccated surface layer of relative
low thermal conductivity. Evidence from the SESAME-PP and CONSERT instruments also indicates that the porosity within the first metre of the surface layer is
lower than at depth (Brouet et al. 2016a). Hérique et al. (2019), analysing
CONSERT data, concluded that porosity variations could not exceed 10% and that
larger porosity variations needed to be restricted to at most metre-scales within the
interior. This is probably still consistent with the analysis of Brouet et al. (2016a).
The Deep Impact experiment on 9P/Tempel 1 also showed through infrared
spectroscopy that the uppermost layer of the surface was lacking in water ice but,
after the impact, water emissions (including evidence for long-lived ice particles)
were detected in the ejecta. Analysis of the crater produced by the impactor
suggested it was 50 m in diameter surrounded by a low rim about 180 m in diameter.
The impacting mass was 372 kg of which nearly 50% was copper in a spherical cap
at the front of the impactor. At an impact velocity of 10.3 km s
À1 , the energy
delivered was just under 20 GJ or 5 tons of TNT (A’Hearn 2008). Schultz et al.
(2007) compared laboratory simulations to observations of the Deep Impact event
and concluded that some of the features of ejecta pattern landing uprange of the
crater produced by the oblique impact were consistent with experiments using
layered targets. The observations, including subsequent observations of the final
impact crater from the Stardust-NExT mission to 9P/Tempel 1, were interpreted as
possibly arising from impact into a layered target with a loose particulate surface
about 1–2 m deep over a slightly more competent substrate (Schultz et al. 2013).
These diverse observations suggest that locally the sub-nuclei have significant
compressive strength just below the surface and that this material contains water ice
(Fig. 2.26). This is covered with a variably thick layer of a dusty component. Moving
further in, this hard layer is over a more porous and homogeneous interior. Our
knowledge of the interior beyond these aspects is limited to modelling work based
on numerous assumptions and specifically how energy is input to or lost from the
interior via the surface.
74
2 The Nucleus
able to deploy an arm with a penetration device. The device encountered very hard
material just below the surface. The resulting stress was measured by the device and
resulted in compressive strengths >2 MPa (Spohn et al. 2015; Boehnhardt et al.
2017) and therefore 2–3 orders of magnitude higher than that found from the
rebound. However, it needs to be noted that the orientation of the lander at this
time was far from nominal (Spohn et al. 2015).
These results might be reconciled if the sub-surface ice content and its spatial
variability are important and may be evidence of recondensation of volatiles emitted
from the surface and forced back into the nucleus surface layer by the gas pressure
over the surface. This effect was identified in the “KOSI” (“KOmeten-SImulation”)
experiments in the late 1980s (e.g. Thiel et al. 1989) and was modelled by, for
example, Prialnik (1991). In the experiments, the crust thickness reached 7 cm with a
mechanical strength exceeding 5 MPa while some degree of porosity was
maintained. Hence, this seems to be a way to produce a more dense and therefore
potentially harder ice layer immediately under a desiccated surface layer of relative
low thermal conductivity. Evidence from the SESAME-PP and CONSERT instruments also indicates that the porosity within the first metre of the surface layer is
lower than at depth (Brouet et al. 2016a). Hérique et al. (2019), analysing
CONSERT data, concluded that porosity variations could not exceed 10% and that
larger porosity variations needed to be restricted to at most metre-scales within the
interior. This is probably still consistent with the analysis of Brouet et al. (2016a).
The Deep Impact experiment on 9P/Tempel 1 also showed through infrared
spectroscopy that the uppermost layer of the surface was lacking in water ice but,
after the impact, water emissions (including evidence for long-lived ice particles)
were detected in the ejecta. Analysis of the crater produced by the impactor
suggested it was 50 m in diameter surrounded by a low rim about 180 m in diameter.
The impacting mass was 372 kg of which nearly 50% was copper in a spherical cap
at the front of the impactor. At an impact velocity of 10.3 km s
À1 , the energy
delivered was just under 20 GJ or 5 tons of TNT (A’Hearn 2008). Schultz et al.
(2007) compared laboratory simulations to observations of the Deep Impact event
and concluded that some of the features of ejecta pattern landing uprange of the
crater produced by the oblique impact were consistent with experiments using
layered targets. The observations, including subsequent observations of the final
impact crater from the Stardust-NExT mission to 9P/Tempel 1, were interpreted as
possibly arising from impact into a layered target with a loose particulate surface
about 1–2 m deep over a slightly more competent substrate (Schultz et al. 2013).
These diverse observations suggest that locally the sub-nuclei have significant
compressive strength just below the surface and that this material contains water ice
(Fig. 2.26). This is covered with a variably thick layer of a dusty component. Moving
further in, this hard layer is over a more porous and homogeneous interior. Our
knowledge of the interior beyond these aspects is limited to modelling work based
on numerous assumptions and specifically how energy is input to or lost from the
interior via the surface.
74
2 The Nucleus