A shattered lunar landscape

Moon's gravity map

Astronomers using gravity maps from NASA’s twin GRAIL (Gravity Recovery And Interior Laboratory) satellites have discovered that the Moon’s upper crust – the megaregolith –  is highly porous.

The GRAIL satellites, Ebb and Flow, determined the Moon’s gravity field by measuring tiny movements of the spacecraft due to the push and pull of gravity upon them. Gravity is stronger over denser regions and weaker over loosely packed ones, so by studying the gravity maps from the GRAIL observations, it’s possible to determine whether the lunar crust is either more or less porous, or sponge-like.

NASA's GRAIL satellites, Ebb and Flow, in orbit over the Moon
NASA’s GRAIL satellites, Ebb and Flow, in orbit over the Moon. Credit: NASA/JPL-Caltech/MIT

A new study of the GRAIL data, looking at about 1200 craters in the lunar highlands on the far side of the Moon, has revealed major fracturing of its crustal layers. This fragmentation is due to pulverisation about four billion years ago during the Late Heavy Bombardment (sometimes called the lunar cataclysm). The effect of countless asteroids smashing into the Moon is evident by looking at the multitude of craters, cracks, and seams. GRAIL can ‘see’ features below the surface which are not visible in optical images.

The authors of the new study suggest that the megaregolith is currently in a state of equilibrium – or steady state. Because it’s taken so many impacts, it can’t get any more porous than it already is. In fact, more recent craters are thought to have a solidifying effect on the rubble-like structure near the surface because of heating and compaction from incoming projectiles.

Because of the porosity equilibrium in the upper crust, scientists find it hard to investigate other scientific subjects (like chemistry) there, so have to look deeper into the lithosphere (the rigid outer layer which includes the crust and upper mantle) where the gravity signature of larger craters is preserved. To find the gravitational fields further down, and therefore the porosity, researchers use a special technique, called the Bouguer Correction, to subtract surface features (like mountains and valleys) from the overall gravity observations to find the porosity of the lower crust.

the far side of the Moon
The far side of the Moon, captured by Apollo 16 astronauts. Credit: NASA/Apollo 16 Crew

The far side of the Moon has a more ancient, cratered terrain than the near side. This shattered environment is a great place for scientists to study porosity both at the surface and deeper down. Rocky fractures are perfect places for water or ice to get in, and for crystals to form. There are some scientists who believe that chemical life may have started in similar environments on Earth. But unlike on Earth, where active geology has concealed any evidence of this happening, these lunar cracks and fissures could hold the key to primitive evolution on the young Earth and on other terrestrial Solar System bodies.

© Melanie Davies 2015

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