Massive asteroids and comets, and later, smaller pebbles and galactic junk, pounded the moon and other infant terrestrial bodies some 4.4 billion years ago, making the early solar system look like a game of space rock dodgeball.
This time span came to an end 3.8 billion years ago. This turbulent period left the moon with a highly cratered face and a fractured, porous crust.
Now, scientists at MIT have discovered that the porosity of the moon’s crust, which extends far below the surface, can give significant information about the moon’s bombardment history.
The team used simulations to show that early on in the bombardment period, the moon was very porous, almost a third as porous as pumice. The early, powerful impacts that fractured much of the crust are probably to blame for this high porosity.
Surprisingly, the scientists discovered that almost all of the moon’s porosity developed quickly with these huge impacts and that the surface of the moon was actually compacted by the subsequent onslaught of smaller impactors.
These subsequent, smaller impacts instead served to compress and squeeze existing cracks and faults on the moon.
The scientists’ calculations also suggested that the moon had twice as many impacts as are visible on its surface. This estimate is lower than previous estimates.
The goal of the team’s most recent research was to track the moon’s changing porosity and calculate the number of impacts that have occurred on its surface using changes below the moon’s surface.
Researchers have created comprehensive maps of the density of the moon’s underlying crust using the mission’s gravity measurements. These density maps also show the lunar crust’s current porosity.
These maps reveal that areas next to the youngest craters are quite porous, whereas areas close to older craters are less porous.