Several lines of evidence indicate that most of the smaller asteroids (< 1 km) consist of granular material loosely bound together primarily by self-gravity; these are commonly called rubble piles. While the strength of these rubble piles is valuable information on their origin and fate, it is still debated in the literature. We report on a laboratory measurement campaign on fine-coarse mixtures of simulated asteroid regolith. In a series of table-top measurements, we have determined sample compression and shear strengths for various fine-fractions within coarse-grained samples. We used confined setups (less than 10cm in length) to measure the strength of the material in constricted environments such as an asteroid’s core and unconfined setups (greater than 10cm in length) to simulate open environments such as the surface of an asteroid.
Using CI Orgeuil high fidelity asteroid soil simulant, we performed three measurement types to determine the strength of our samples: shear yield, which in turn provided values for the Angle of Internal Friction (AIF), bulk cohesion, and tensile strength of the samples; compression strength, which allowed to calculate the Young’s Modulus (YM); and the Angle Of Repose (AOR). From the AOR, we determined the coefficient of friction of each sample.
Samples of regolith were created by measuring percentage by volume amounts of both coarse and fine grains into the measurement container. We prepared coarse grains in two size distributions, mm-sized and cm-sized. The fine fraction was composed of grains sieved between 100 and 250 μm.
For compression and AOR measurements, we find that the strength of the coarse grain samples increases with the addition of a fine fraction. However, we find that the increase of the fine fraction in a sample of coarse grains does not consistently increase the sample shear strength. With increasing fine fractions, the AIF and bulk cohesion of the mixed samples decrease (until a point of saturation). This could be indicative of the fine grains acting as a lubricant as the larger grains move across each other, aiding rolling and reducing interlocking strength.
Our findings suggest that in the case of the surface of an asteroid, the presence of fine grains does indeed increase the strength of coarse regolith material. However, fine grains in the regolith sublayers or the asteroid interior will reduce material strength due to grain interlocking and ease disruption. Therefore, rubble piles that are depleted in fine grains will have higher internal strength compared to those composed of grain size distributions that include sub-mm sized particles.
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FROM NEOS TO TNOS: HOW REGOLITH IS SHAPING THE SMALL WORLDS OF OUR SOLAR SYSTEM
Introduction: With the capture of the first high-
resolution, in-situ images of Near-Earth Objects
(NEOs) a couple of decades ago [1–4], the ubiquity of
regolith and the granular nature of small objects in the
Solar System became apparent. Benefiting from an
increased access to high computing power, new
numerical studies emerged, modeling granular
structures forming and evolving as small bodies in the
Solar System [5–7]. Now adding laboratory studies on
granular material strength for asteroid and other small
body applications [8,9], we are steadily progressing in
our understanding of how regolith is shaping the
interiors and surfaces of these worlds. In addition, our
ever-more powerful observation capabilities are
uncovering interesting dust-related phenomena in the
outer skirts of our Solar System, in the form of activity
at large heliocentric distances and rings [10–12]. We
find that our recent progress in understanding the
behavior of granular material in small body
environments also has applications to the more distant
worlds of Centaurs and Trans-Neptunian Objects
(TNOs).
Internal Strength: We currently deduce internal
friction of rubble piles from the observation of large
numbers of small asteroids and their rotation rates,
combined with the associated numerical simulations
[13,14]. In the laboratory, we study internal friction of
simulant materials using shear strength measurements
[8]. Combining observations, modeling, and laboratory
work, the picture emerges of rubble pile interiors being
composed of coarse grains in the mm to cm range. The
irregular shapes of the grains lead to mechanical
interlocking, thus generating the internal friction
required to match observations of the asteroid
population [8,9]. We find that the presence of a fine
fraction in the confined interior of a rubble pile
actually leads weaker internal strength [9].
Surface Strength: Deducing surface regolith
strength for NEOs is usually performed via average
slope measurements [15–17] or, most notably,
observing the outcome of an impact of known energy
[18]. In the laboratory, we measure the angle of repose
of simulant material via pouring tests, as well as its
bulk cohesion using shear strength measurements [8].
In some cases, this allows us to infer grain size ranges
for various regions of the surface and subsurface of
pictured NEOs, beyond the resolution of their in-situ
images.
Surface Activity: The Rosetta mission revealed
that a number of activity events on comet
67P/Churyumov–Gerasimenko were linked to active
surface geology, most notably avalanches and cliff
collapses [19]. In addition, the role of regolith strength
in asteroid disruption patterns has been inferred from
numerical simulations of rotating rubble piles [20]. By
studying strength differences in simulant samples, it
becomes apparent that a difference in cohesion
between a surface and its subsurface layer can lead to
activity events with surface mass shedding, without the
presence of volatiles sublimating as a driver [8]. We
show that such differences in surface strength can be
brought upon by a depletion in fine grains or a change
in composition (e.g. depletion in water ice) and could
account for regular activity patterns on small bodies,
independently of their distance to the Sun. This is of
particular interest to the study of Centaur activity and a
potential mechanism for feeding ring systems.
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- Award ID(s):
- 1830609
- NSF-PAR ID:
- 10475060
- Publisher / Repository:
- Lunar and Planetary Institute Contributions
- Date Published:
- Journal Name:
- Asteroids, Comets, Meteors Conference 2023
- Page Range / eLocation ID:
- 2851
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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