The individual components of split comets can provide additional information on
homogeneity. In the case of 73P/Schwassmann-Wachmann 3, for example, Schleicher and Bair (2011) using visible emissions concluded that, to within the uncertainties, the largest components had the same composition and that this composition
was consistent with that measured in the pre-fragmented nucleus. A study by Harker
et al. (2011) in the mid-infrared noted the similarity in mineralogy and grain
properties between the two major fragments again implying homogeneity in composition and structure between sub-nuclei. Some differences in grain size in the
comae between fragments have been noted but varying grain sizes in a cometary
coma can be the result of many processes and is certainly not a clear indicator of
inhomogeneity.
2.8.2 The Surface Layer and the Strength of Cometary
Material
The disruption of D/Shoemaker-Levy 9 (Fig. 1.8) provided a clear indication that
comets are weakly bound objects. Asphaug and Benz (1994, 1996), for example,
showed that the break-up of D/Shoemaker-Levy 9 could be modelled as the separation of a strengthless rubble pile of smaller cometesimals. The resulting behaviour
was dependent upon the density of the material but the preferred value of about
0.6 g cm
À3 was remarkably close to that found for 67P. The impact of metre-sized
cometary material into the Earth’s atmosphere producing “fireballs” also led
Ceplecha (1994) to conclude that material of cometary origin is weak. It is nonetheless important to distinguish between large-scale tensile strength, small scale tensile
strength, and compressive strength. This is illustrated by the fact that consolidated
material generally shows decreasing strength at larger scales, following a power law
proportional to d
Àq , where d is a length scale and the exponent, q, $0.6 for water ice
(Petrovic 2003; Attree et al. 2018). Here, Rosetta has played a major role in
increasing our knowledge.
Attree et al. (2018) have studied overhangs to estimate the tensile strength of the
material comprising 67P. The rough irregular surface provides several examples of
overhangs. One is shown in Fig. 2.25. The tensile strength needed to support these
overhangs is remarkably small because of the low gravitational acceleration and
values for the stress of around 20 Pa were derived (where the stress is computed from
the maximum force per unit area). On the other hand, the compressive strength may
be significantly higher.
There is no straightforward relationship between tensile and compressive
strength. An everyday example is reinforced concrete. Concrete has a high compressive strength making it good to stand on. But its tensile strength is low.
Consequently, reinforcing the concrete with steel bars that have low compressive
strength but high tensile strength leads to very strong structures.
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2 The Nucleus
homogeneity. In the case of 73P/Schwassmann-Wachmann 3, for example, Schleicher and Bair (2011) using visible emissions concluded that, to within the uncertainties, the largest components had the same composition and that this composition
was consistent with that measured in the pre-fragmented nucleus. A study by Harker
et al. (2011) in the mid-infrared noted the similarity in mineralogy and grain
properties between the two major fragments again implying homogeneity in composition and structure between sub-nuclei. Some differences in grain size in the
comae between fragments have been noted but varying grain sizes in a cometary
coma can be the result of many processes and is certainly not a clear indicator of
inhomogeneity.
2.8.2 The Surface Layer and the Strength of Cometary
Material
The disruption of D/Shoemaker-Levy 9 (Fig. 1.8) provided a clear indication that
comets are weakly bound objects. Asphaug and Benz (1994, 1996), for example,
showed that the break-up of D/Shoemaker-Levy 9 could be modelled as the separation of a strengthless rubble pile of smaller cometesimals. The resulting behaviour
was dependent upon the density of the material but the preferred value of about
0.6 g cm
À3 was remarkably close to that found for 67P. The impact of metre-sized
cometary material into the Earth’s atmosphere producing “fireballs” also led
Ceplecha (1994) to conclude that material of cometary origin is weak. It is nonetheless important to distinguish between large-scale tensile strength, small scale tensile
strength, and compressive strength. This is illustrated by the fact that consolidated
material generally shows decreasing strength at larger scales, following a power law
proportional to d
Àq , where d is a length scale and the exponent, q, $0.6 for water ice
(Petrovic 2003; Attree et al. 2018). Here, Rosetta has played a major role in
increasing our knowledge.
Attree et al. (2018) have studied overhangs to estimate the tensile strength of the
material comprising 67P. The rough irregular surface provides several examples of
overhangs. One is shown in Fig. 2.25. The tensile strength needed to support these
overhangs is remarkably small because of the low gravitational acceleration and
values for the stress of around 20 Pa were derived (where the stress is computed from
the maximum force per unit area). On the other hand, the compressive strength may
be significantly higher.
There is no straightforward relationship between tensile and compressive
strength. An everyday example is reinforced concrete. Concrete has a high compressive strength making it good to stand on. But its tensile strength is low.
Consequently, reinforcing the concrete with steel bars that have low compressive
strength but high tensile strength leads to very strong structures.
72
2 The Nucleus