One paper in MNRAS (Monthly Notices of the Royal Astronomical Society) for July:
Volume 540, Issue 3
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Volume 540, Issue 4
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Volume 541, Issue 1
Smyth-Moore, A. Kaufmann, E. Jia, Q. et al. Exploring planetary regolith: deriving geotechnically meaningful properties from penetrometry Page 251 staf982
Regolith is the layer of rubble- dust to gravel, possibly boulders- acting as “soil” on Solar System bodies. All asteroids visited so far have a regolith; we will see if the near-Earth object 1998 KY26 is too small for regolith when the Hayabusa2 probe gets there in a few years. Bodies that are too small simply lack the gravity to retain fine particles; Itokawa, for one, had gravel but not dust. This is new territory: humanity’s prior experience included only large masses, with their large gravities.
A simple regolith instrument is a penetrometer- a gauge driven into the “soil”, all the while measuring the resistance of the material. Some early space landers had flown penetrometers; others replicated them somewhat. Mars rovers, for example, study “soil” mechanics by dragging one wheel; this has been proposed for asteroid rovers, and the IDEFIX rover bound for Phobos. Smyth-Moore et al. propose including penetrometers on more missions- it just doesn’t cost very much to do this. We’ll see just how readily we can do this…