Our nearest celestial compatriot continues to surprise us. A team of researchers from the Southwest Research Institute (SwRI) has just spotted two new lunar craters, one of which – at an age of 16 million years – is decidedly young, geologically speaking. Both are less than 2 kilometers (1.2 miles) across.

The other has an amusingly large margin of error attached to it: It could be anywhere between 75 million and 420 million years old, which means that it could have formed near the end-Cretaceous mass extinction event that wiped out the non-avian dinosaurs, or at a similar time that the first land plants on Earth appeared.

“These 'young' impact craters are a really exciting discovery,” SwRI Senior Research Scientist Dr. Kathleen Mandt, lead author of the Icarus paper, said in a statement. “Finding geologically young craters and honing in on their age helps us understand the collision history in the solar system.”

Using the Lunar Reconnaissance Orbiter (LRO) and its onboard instrumentation – specifically, an instrument called the Lyman-Alpha Mapping Project (LAMP) – the regions of the lunar surface permanently covered in shadows could be imaged. Instead of using sunlight to see things, the LRO in this instance used LAMP to pick up ultraviolet radiation.

This part of the electromagnetic spectrum allowed the team to measure minuscule changes in surface reflectivity, something known as albedo. Impacts radically change the topography of an object when they occur, and the resulting surface geology is redistributed, creating signature differences in how radiation reflects off the surface.

^{The south pole Aitkin Basin, the largest impact crater on the Moon, contains many smaller, younger impact craters, including these two newly found ones. Red represents higher elevation; purple depicts extremely low elevation. The purple and gray rings trace the inner and outer walls of the basin, respectively. JAXA/Ittiz/Wikimedia Commons; CC BY-SA 3.0}

Backed up by radar data, the team used LAMP to spot two previously hidden deep craters near the south pole. The areas around the two craters were brighter and rougher than the surrounding landscape, indicating that several sheets of debris, known as ejecta blankets, had covered the surface.

The Moon has an incredibly tenuous atmosphere, meaning that weathering and the redistribution of surficial dust does occur on very long timescales. Therefore, the more altered and weathered the ejecta blankets of impact craters are, the older the impacts are likely to be.

One blanket was so fresh and unaltered that it was estimated to be around 16 million years old.

The other blanket had significantly faded in comparison, meaning that it could be no younger than 75 million years old. However, it hasn’t been completely weathered away or covered in lightweight lunar dust, meaning that it cannot be older than 420 million years. Without any additional information, a more precise cratering date cannot be ascertained.

Either way, discovering new craters on the Moon is always a joy for astronomers. Impact craters are the mausoleums of the creators of the Solar System, whose origin and early history was full of sound and fury as huge rocky and metallic objects collided into each other. The Moon, with its extremely low erosional rates, is an ancient catalog full of young and old impacts; the more we find, the more we can understand about how the Solar System formed.

Despite being so close and so thoroughly studied – and visited, of course – it’s clear that the Moon still hides plenty of secrets waiting to be uncovered by intrepid scientists. One recent study revealed how our watchful protector may be helping to generate Earth’s magnetic field, without which complex life on Earth would likely have never evolved.