- Moon’s craters came from bombardment by impactors
- Now, first comprehensive map of craters has been made
- Shows the bombardment changed dramatically at a certain stage
Take a cursory look at the Moon, and it can resemble a pockmarked golf ball. The dimples and divots on its surface are testament that our satellite has withstood a barrage of impacts from comets, asteroids and other space matter throughout much of its history.
Because the geological record of that pummelling remains largely intact, scientists have used the Moon to help reconstruct the history of the chaotic early days of the inner Solar System.
Now a team led by Brown University planetary geologists has produced the first uniform, comprehensive catalogue of large lunar craters…which could help shed light on the full-scale, planetary bombardment suffered by the planets of the inner Solar System more than 4 billion years ago.
In a paper appearing in the journal Science, the team used data from the Lunar Orbiter Laser Altimeter (LOLA), one of a suite of instruments aboard NASA’s Lunar Reconnaissance Orbiter, to identify and map 5,185 craters that are 20 kilometres in diameter or larger.
From the crater count and analysis, the team (which includes scientists from the Massachusetts Institute of Technology and the NASA Goddard Space Flight Centre) determined the Moon’s oldest regions are the southern near side and the north-central far side.
The group also confirmed that the South Pole–Aitken Basin is the oldest basin, meaning that any samples from there could be invaluable to further understanding the Moon and other bodies of the inner Solar System.
Rethinking the Moon’s bombardment
A major finding deals with the stream of projectiles pinballing throughout the inner Solar System in its earliest days.
For years, the prevailing wisdom was that the Moon was buffeted by a volley of space matter that held a steady ratio between larger and smaller objects, which planetary scientists refer to as “size-frequency distribution.”
The bombardment activity has never been questioned. But in 2005, the size-frequency distribution part of it was challenged. In a paper in Science, a group led by University of Arizona geologist Robert Strom hypothesised that the ratio of larger and smaller objects striking the Moon had differed during its first billion years of existence.
The Brown-led team’s crater analysis lends added credence to that hypothesis.
The researchers studied impact craters formed early in the Moon’s history (when major basins were created by large projectiles striking the surface) and compared them with those they knew were formed later (when objects struck lava flows that had covered these basins).
They found that the oldest surfaces (located in the lunar highlands) bore crater markings indicating a greater ratio of larger impactors. The group looked in particular at Orientale Basin, formed by a massive impactor about 3.8 billion years ago, and determined that this is approximately when the era of larger projectiles versus smaller projectiles ended.
Much more to do
The finding opens a set of intriguing questions for what was going on in the inner Solar System leading up to roughly the time that Orientale Basin was formed.
“We know the asteroid belt has been spinning off projectiles at a relatively constant rate for three and a half billion years,” said Caleb Fassett, a postdoctoral researcher at Brown. “But now we go back earlier in the Solar System’s history, and suddenly things are completely different.”
The scientists think the change may have been caused by the gravitational pull on the asteroid belt exerted by larger planets such as Jupiter and Saturn as they settled into their orbits…or it could have been a temporary abundance of comets, an unexplained change in the size of impactors emanating from the asteroid belt, or something else.
The Lunar Orbiter Laser Altimeter measures the Moon’s surface topography with a vertical precision of 10 centimetres using laser pulses bounced off the lunar surface just 25 metres apart.
In all, the findings “are telling us something about the infancy of the Solar System,” said James W. Head III, a planetary geologist at Brown and the paper’s lead author. “It is clear we can find out and learn so much more from future missions, robotic or otherwise. There is so much to do.”
Adapted from information issued by NASA / LRO / LOLA / GSFC / MIT / Brown.
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