The history of the Moon is a tale told by geology, apparent in its rocks, craters, and other surface features. For centuries, astronomers have studied the Moon from afar and for the past few decades, it has been visited by countless robotic missions. Between 1969 and 1972, a total of twelve astronauts walked on its surface, conducted lunar science, and brought samples of lunar rock back to Earth for study.
These efforts have taught us a lot about the things that have shaped the lunar surface, be they one-off events like the massive impact that formed the Shakleton crater to things that happened regularly throughout its 4.51 billion-year history. For instance, scientists recently discovered something unusual about the Antoniadi crater: a large boulder was perched on the rim of a smaller crater within after rolling about 1000 meters (1093 yards) downhill.
The image was taken by the Lunar Reconnaissance Orbiter Camera (LROC), which is a system of three cameras mounted on the Lunar Reconnaissance Orbiter (LRO) that capture high-resolution black and white (and moderate resolution multi-spectral) images of the lunar surface. The image zooms in on the Antoniadi crater, which measures 138 km (~86 mi) in diameter and is located in the southern hemisphere on the far side of the Moon.
To the east of the crater (right side of the image), rock outcroppings are visible that are part of Antoniadi’s rim. To the west, a young impact crater is located on Antoniadi’s floor that measures about 30 m (~100 ft) across and is partially erased. Between them lies a slope that gradually runs downhill from northwest to southeast that is part of the crater’s inner rim.
It is because of its location at the bottom of this slop that the young crater has been partially erased. Over time, it has been filled by loose regolith that runs downhill, possibly as a result of moonquakes. In this case, however, the LRO caught sight of a boulder that came loose from the rock outcropping and rolled towards the young crater.
This is indicated by the tracks it left in the lunar regolith. And while the rock is partially obscured by shadow, the illuminated portion is estimated to be about 15 m (49 ft) in diameter, which means its probably as big as an 18-wheeler truck. Based on the tracks, it appears that the rock bounced a few times as it plowed downhill before coming to a stop on the young crater’s rim.
As with other tracks, craters and geological features on the Moon, this rock and the path it carved tells a story. And it is because of instruments and lunar explorers of increasing sophistication that we are privy to them.
Further Reading: LROC