The crater-scarred landscape of the moon only shows about half the number of impacts it has received in its 4.5-billion-year lifetime, reveals a new study. Researchers have found a more accurate way of measuring the moon’s impact history by studying the density of rock on and just below the surface – its porosity.
Around the time of the Earth’s and moon’s formation, asteroids, comets and other space debris flew around the solar system smashing into the young planet and its satellite, leaving the moon with the heavily cratered face we see today. This tumultuous time ended about 3.8 billion years ago and impacts have been smaller and much less frequent since.
These early, massive bombardments smashed the surface rock and created a fragmented, porous crust with large gaps that stretch well below the surface. Now, scientists from the Massachusetts Institute of Technology (MIT) have been studying that porosity to learn more about the moon’s impact history.
“We know the moon was so bombarded that what we see on the surface is no longer a record of every impact the moon has ever had, because at some point, impacts were erasing previous impacts,” says study co-author Dr. Jason Soderblom, MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS), in a statement. “What we’re finding is that the way impacts created porosity in the crust is not destroyed and that can give us a better constraint on the total number of impacts that the moon was subject to.
“Previous estimates put that number much higher, as many as 10 times the impacts as we see on the surface, and we’re predicting there were fewer impacts,” he continues. “That matters because that limits the total material that impactors like asteroids and comets brought to the moon and terrestrial bodies and gives constraints on the formation and evolution of planets throughout the solar system.”
Scientists had previously assumed that the onslaught of massive impacts would have compressed the surface. But the MIT team found this was from subsequent, smaller impacts and that the earlier, larger ones actually caused more fragmentation.
The study used measurements of the moon’s surface gravity taken by NASA to create detailed maps. They reveal how the areas surrounding the youngest craters are the most porous while older craters are surrounded by more compressed, dense rock.
Researchers examined 77 craters, spanning all ages from 4.3 billion to 3.8 billion years and modeled how the porosity changed in relation to the craters’ age. The team reasoned that older craters would have been exposed to more impacts over time that would have compacted the surrounding rock while younger craters would have experienced far less, if any impacts. Thus, the underlying porosity of these younger craters would be more representative of the moon’s initial conditions.
“We use the youngest basin that we have on the moon, that hasn’t been subject to too many impacts, and use that as a way to start as initial conditions,” says EAPS postdoc Ya Huei Huang. “We then use an equation to tune the number of impacts needed to get from that initial porosity to the more compacted, present-day porosity of the oldest basins.”
These simulations show a clear trend, that at the start of the lunar bombardment – 4.3 billion years ago – the crust was highly porous, about 20 percent (by comparison, the porosity of pumice is about 60-80 percent). Closer to 3.8 billion years ago, the crust became less porous and has remained about 10 percent since then.
This shift is likely the result of smaller impacts compacting the fractured crust and the researchers estimate that the moon has experienced about double the number of small impacts as can be seen on the surface today.
“This puts an upper limit on the impact rates across the solar system. We also now have a new appreciation for how impacts govern porosity of terrestrial bodies,” adds Soderblom.
The research is published in the journal Nature Geoscience.
Report by South West News Service writer Danny Halpin