Scars on the Moon Hint at Protoplanetary Impacts

Non-radial Imbrium Sculpture on the Moon – our Violent Past

Between 4.1 and 3.8 billion years ago, our young planet (and much of the inner solar system for that matter), was subjected to an intense bombardment of space debris left over from the formation of the planets.  This period in Earth’s history is known as the Late Heavy Bombardment (LHB) and if you look at the moon with a pair of binoculars or study pictures of the surface of Mercury or Venus you will see that much of the moon and these rocky planets is pot-marked with huge numbers of craters.  These craters are a testament to our solar system’s violent, destructive past.  Writing in the journal “Nature” astronomers from Brown University (Rhode Island) and Sandia National Laboratories (New Mexico), have conducted a detailed analysis of one such impact crater on the moon.  Their research suggests the type of objects that caused many of the craters were substantially bigger than previously thought.  The rocky bodies of the early solar system were subjected to bombardment from many protoplanets.

Analysing the Geology of the Imbrium Crater on the Moon

Imbrium basin ejector.

The trajectory of debris is plotted (green lines), whilst most ejecta radiates from the centre outwards other debris paths show an entirely different route.

Picture Credit: Professor Schultz

Looking up into the Man in the Moon’s Right Eye

The scientists concentrated their study on the Mare Imbrium, part of an extensive basin (Imbrium basin), located in the northern hemisphere of our satellite.  The 750 miles-wide Imbrium basin, which can be seen with the naked eye, is better known as the man in the moon’s right eye.  The researchers have calculated that this feature came about due to the impact of a huge object, a protoplanet bigger than Wales.  This object sent ejecta flying in all directions helping to fuel the Late Heavy Bombardment of Earth.  Previous models had suggested that the Imbrium basin was formed after an impact from a chunk of space rock some fifty miles in diameter, in this new paper, the scientists have calculated that the body that caused this particular feature on the moon was at least three times as big.

Rimmed grooves, lineations and elongate craters around Mare Imbrium shape much of the nearside Moon.  This pattern was coined the Imbrium Sculpture by astronomers, most of these features emanate radially outward from the impact crater’s centre, what you would expect to see from a collision event.  These features were created by rocks blasted from the crater when it was formed, and they are concentrated on the basin’s south-eastern side, suggesting the impact came from a north-westerly direction.  However, there are other scars on the moon’s landscape in the Imbrium basin that do not conform to this model, they don’t converge on the crater’s centre.

A Map of the Nearside of the Moon Showing the Location of the Imbrium Basin

The Imbrium basin on the moon.

A map of the nearside of the moon showing the Imbrium basin outlined in red.

Picture Credit: NASA

Blasting Objects from a Gun Designed by NASA

In order to understand how these strange features were formed, the researchers commissioned a large gun created by NASA to help them understand the nature of the surface of the moon.  The team then shot projectiles out of this gun at supersonic speeds to recreate the conditions of the LHB.

The corresponding author of this new paper, Professor Peter Schultz (Planetary Geosciences Group at Brown University), explained about the use of the NASA facility.

“It’s a three-storey tall gun at NASA’s Ames Research Centre [California], that was created during the Apollo programme to understand better the nature of the lunar surface.  It fires small projectiles at six to seven kilometres per second.  What this does is reveal dynamics you can’t recreate with a slingshot, because the objects are going so fast that they let off strong shockwaves.”

A sophisticated high speed camera was used to record the simulated impact events.  These images revealed that pieces of material break off up-range of the main crater and these items continue to travel at very high speeds scouring the impact crater’s surface in a non-radial fashion, reminiscent to the features seen on the moon in the vicinity of the Imbrium basin.

This research explains the non-radial scars observed in the area of the Mare Imbrium.  It also permitted the research team to estimate the impact object’s size based on the signature impact pattern that was created.  According to the team’s calculations the object that struck the northern hemisphere of the moon was many times larger than previously thought, it was a protoplanet.  A protoplanet is a large body in orbit around a sun that is becoming rounded by its own gravity and is massive enough to show signs of internal melting to produce a differentiated interior.

The research team then conducted a similar analysis on other moon features including the Mare Moscoviense and the Mare Orientale which are found on the far side of our satellite (does not face Earth).  The scientists concluded that these features too, were probably as a result of protoplanet impacts.

Experiments and Computer Models Assess Impact Events

Experiments to assess how moon craters were formed.

A comparison of scouring marks produced in the NASA experiments (top) and those produced in a computer modelling exercise assessing the impact of a 60-mile-wide object on the moon.

Picture Credit: Professor Schultz

Professor Schultz stated:

“We are not claiming the entire Late Heavy Bombardment was from protoplanets – other asteroids were going bump in the night as well, but this paper does suggest there were a lot of large protoplanets roaming the inner solar system.”

By modelling what happened after the Imbrium basin was created, Professor Schultz and his co-authors have shown that protoplanet impact events could have been responsible for many of the other smaller features on the moon.  Ejecta from the impact would have “rained down” onto the moon creating the pot-marked surface we see today.

The professor explained:

“It’s like shrapnel ripping off and coming back to hit us again and again.”

The research team are hoping to apply their modelling methodology to allow them to study impact craters on other rocky planets such as those found on Mercury.  Their research could also help to improve our understanding of the geography of Mars, the fourth rocky planet in our solar system, one that like Earth, was affected by the LHB and just like our own planet, subsequent erosion has masked the impact craters.

The scientific paper: “Origin and Implications of Non-radial Imbrium Sculpture on the Moon.”

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