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Testing Two Materials As Potential Cometary And Asteroid Analogs

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August 7, 2018
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The advent of newspace and commercial space (SpaceX, Blue Origin, etc.) in the early 2000s has brought about the resurgence of the decades-old concept of obtaining valuable resources from the space environment. Known in the space community as ISRU (in situ resource utilization) and more broadly as “space resources,” the idea involves extracting minerals, elements (carbon, oxygen, etc.), and other precious metals and volatiles from the surfaces of planetary bodies that mankind may encounter.

Recovering these materials in the space environment reduces the need to launch resources from earth, dramatically dropping launch costs, extending the life of interplanetary missions, and relieving the impact of resource extraction on earth.

In order to harvest these valuables, we must have knowledge of their physical properties, like strength and cohesion (similar to drilling for oil or digging into an ore body on earth). Extraction equipment design is based on the expectation of these properties, and limitations in available power and energy off-earth, as well as the high cost of launching mass from our planet, place constraints on these designs. Without proper knowledge of the strength of the surfaces we are likely to encounter, the risk of mission failure increases dramatically, as evidenced by the failure of the MUPUS-PEN device on the Philae lander to penetrate the surface of Comet 67P in 2014 (Biele et al., 2015).

As a step towards reducing the risk of future missions as well as providing insight into the surface processes of comets and asteroids in our solar system, MIT researchers Jared Atkinson, Sara Seager, and William Durham endeavored to characterize the cryogenic behavior of two terrestrial rocks – Bishop tuff and Indiana limestone – as potential cometary and asteroid analogs, respectively. Laboratory measurements of the failure strength (under compressional stress) of dried and fully water-saturated samples at temperatures ranging from 77 K to 295 K and at confining pressures (for most) of 5 MPa provide a surprising look at the behavior of ice-saturated materials that may be found on extraterrestrial surfaces and below.

Below the ice-point (0oC or 273 K), the strength of the ice-saturated analogs increases dramatically with decreasing temperature, to such an extent that the icy rock is stronger than either the dry rock alone or the ice filling its pores (Figure 1). Generally, combinations of materials have properties that are some average of the individual material properties, but here the dry rock is significantly strengthened by the pore ice through what is known as “grouting.” The ice forms a structural element within the pore, essentially replacing weak gas in the pore space (air, typically) with a hard solid. As the ice strengthens with decreasing temperature (Durham et al., 1992) due to the retardation of certain thermally-activated processes (e.g. dislocation creep) within ice’s crystal structure, the icy rock similarly strengthens.

Figure 1. Experimental results plotted as ultimate strength vs. temperature (decreasing towards the right). A decrease in temperature results in a large strength increase for saturated samples, and a relatively small increase for the air-dried and oven-dried samples. Note that the water-saturated limestone samples at 150 K and 77 K did not reach failure; they thus represent lower bounds (indicated by dotted arrows). The red circle with error bar at 150 K represents the spread of the three repeat measurements of saturated Bishop tuff, while the error bar added to the Indiana limestone measurement at 240 K is taken from Mellor (1971). The two overlapping red squares at 295 K have been shifted slightly in temperature and stress for plotting purposes. Republished with permission from Elsevier from: https://doi.org/10.1016/j.icarus.2018.06.016

The magnitude of the strengthening effect of ice on porous rock at cryogenic temperatures is striking (Fig 1). The saturated limestone experienced a four-fold increase in strength while the tuff strengthened almost ten times. Similar hardening results on granular cometary simulants were reported by Kochan et al., 1989 during the so-called “KOSI” experiments, while Mellor (1971) and Podnieks et al. (1968) observed just such a strengthening of similar limestone at unconfined pressure conditions.

Of particular interest to planetary scientists and ISRU researchers alike is the behavior of the saturated analogs at these cryogenic temperatures; specifically the transition from dominantly ductile-like behavior to dominantly brittle. Figure 2 shows a comparison of tuff behavior ranging from ductile-like at 240 K (indicated by the bulging center of the sample) to the brittle-like nature of the failure at 150 K (indicated by the large fracture and lack of bulging along its length). The transition between these states occurs between 150 K and 200 K and is very likely dominated by the transition of ice from brittle to ductile at temperatures greater than 180 K (Durham et al., 1983).

Figure 2. Deformation in saturated tuff samples at various temperatures, showing brittle-like behavior at 150 K (A), and more ductile-like behavior in samples at 200 K and above (B–D). Republished with permission from Elsevier from: https://doi.org/10.1016/j.icarus.2018.06.016

The MIT researchers suggest that the results of the study should be taken as an upper bound on the strength of ice-saturated material found on asteroids or comets. Both limestone and tuff are terrestrial rocks and have properties unlikely to be found on comet or asteroids; limestone, in particular, is a cemented carbonate rock yet has a density and porosity similar to carbonaceous chondrites (Britt et al., 1987). The surfaces of comets and asteroids are unlikely to be fully saturated, and surface pressures would generally be expected to be much less than 5 MPa.

However, the results imply that significant strengths exceeding at least several MPa should be expected by any mission that interacts with an icy surface at cryogenic temperatures. With multiple sample retrieval mission planned for the near future, and a strong push in the space community to make ISRU a reality, the increase in strength at cryogenic temperatures shown by MIT will be a critical consideration in ensuring the mission success.

These findings are described in the article entitled The strength of ice-saturated extraterrestrial rock analogs, recently published in the journal Icarus. This work was conducted by Jared Atkinson, William B. Durham, and Sara Seager from the Massachusetts Institute of Technology.

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