how porous is the Moon? Many millions of years ago, it was one-third as porous as a pumice stone. This was due to the incessant impacts that the Moon suffered. They shattered much of the crust. Scientists at MIT discovered this.
Impacts upon impacts
The study is in ‘Nature Geoscience’. The fierce bombardment occurred about 4.4 billion years ago. Massive asteroids and comets were hitting the moon and other infant terrestrial bodies. This period ended about 3.8 billion years ago. The Moon was left with a heavily cratered face and a cracked and porous crust.
Continued attack by smaller impactors compacted its surface. They served to squeeze and compact some of the moon’s existing cracks and faults. They estimate that the moon experienced twice as many impacts as can be seen on the surface.
“Previous estimates are much higher. It wasn’t.” Says it in a statement study co-author Jason Soderblom, a research scientist at MIT.
“What we see on the surface is no longer a record of all the impacts the Moon has had. At some point, the impacts were erasing the earlier impacts,” Soderblom notes.
They used the youngest craters with the highest current porosity as a starting point. The team studied the 77 craters in chronological order, based on their previously determined ages. For each crater, the team modeled the amount by which the underlying porosity was changing.
Porous as pumice
These simulations showed a clear trend. At the beginning of the heavy bombardment of the Moon, 4.3 billion years ago, the crust was very porous. About 20% (pumice porosity is between 60% and 80%). About 3.8 billion years ago, the crust became less porous. It remains at its present porosity of about 10 percent.
This change in porosity is from smaller impactors compacting a fractured crust. The incessant impacts suffered by the Moon were modifying its internal structure. Our satellite has not yet finished telling us all its secrets.