Pluto may wear its heart on its sleeve, but the Moon wears its dramatic history on its face, in the chaotic array of craters blanketing the lunar surface. The Moon has no atmosphere to drive erosion and no modern volcanic or tectonic activity, making impact cratering the primary agent of change.
Partly because of this activity, the Moon’s crust is highly porous. This porosity extends deep into the crust, between 10 and 25 kilometers, and possibly into the lunar mantle. Porosity affects many properties of the surface area, including heat conduction and liquid permeability. Studying crustal porosity—and how it develops in a terrestrial planetary body—offers scientists valuable insight into the thermal and chemical processes that drive the evolution of planets.
Soderblom et al. looked at data collected by the two Gravity Recovery and Interior Laboratory spacecraft, which measure the Moon’s gravitational field. The team studied gravitational anomalies in order to quantify the relationship between crater size and porosity generated by impact. They found evidence that the amount of porosity that is created by an impact is related to the crater size for craters up to 100 kilometers in diameter. (The porosity associated with larger craters is likely truncated at depth.) The amount of pore space after impact was also determined by impact variables like size, velocity, and angle.
The researchers further suggest that, in cases where the crustal porosity is very high, impacts are actually capable of reducing porosity. Thus, averaged over large areas, impact cratering leads to porosity equilibrium. In other words, after enough impacts occur in an area, the density of the Moon’s surface will not change, on average, with new impacts.
The constant bombardment also results in cratering equilibrium, where craters are created and destroyed at roughly the same rate. Although the Moon’s surface may be a kind of geologic Etch A Sketch, the gravity signature of the lunar subsurface still contains an intact record of past cratering that reveals its history.
These insights contribute to a scientific understanding of planetary bodies and how they evolve. The unique physical conditions of the lunar surface offer scientists a helpful analogue for crustal conditions on a young, Archean Earth and similar developmental stages on other planets, like pre-Noachian Mars. The study of surface features and the history they reveal is fundamental to our growing knowledge of the galaxy. (Geophysical Research Letters, doi:10.1002/2015GL065022, 2015)
—Lily Strelich, Freelance Writer
Citation: Strelich, L. (2015), Subsurface craters expose the Moon’s dramatic past, Eos, 96, doi:10.1029/2015EO037841. 22 October 2015.