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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The exoplanet perspective on future ice giant exploration
Exoplanets number in their thousands, and the number is ever increasing with the advent of new surveys and improved instrumentation. One of the most surprising things we have learnt from these discoveries is not that small-rocky planets in their stars habitable zones are likely to be common, but that the most typical size of exoplanets is that not seen in our solar system—radii between that of Neptune and the Earth dubbed mini-Neptunes and super-Earths. In fact, a transiting exoplanet is four times as likely to be in this size regime than that of any giant planet in our solar system. Investigations into the atmospheres of giant hydrogen/helium dominated exoplanets has pushed down to Neptune and mini-Neptune-sized worlds revealing molecular absorption from water, scattering and opacity from clouds, and measurements of atmospheric abundances. However, unlike measurements of Jupiter, or even Saturn sized worlds, the smaller giants lack a ground truth on what to expect or interpret from their measurements. How did these sized worlds form and evolve and was it different from their larger counterparts? What is their internal composition and how does that impact their atmosphere? What informs the energy budget of these distant worlds? In this we discuss what characteristics we can measure for exoplanets, and why a mission to the ice giants in our solar system is the logical next step for understanding exoplanets. This article is part of a discussion meeting issue ‘Future exploration of ice giant systems’.
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- Award ID(s):
- 1903811
- NSF-PAR ID:
- 10214801
- Date Published:
- Journal Name:
- Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
- Volume:
- 378
- Issue:
- 2187
- ISSN:
- 1364-503X
- Page Range / eLocation ID:
- 20200054
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1098/rsta.2020.0054
