Two craters have been spotted on the Moon’s perpetually dark nether regions, and they’re fresh – in a geological sense. These lunar babies are 16 million and 75-420 million years old.
A US team led by Kathleen Mandt from the Southwest Research Institute in San Antonio, Texas used the Lunar Reconnaissance Orbiter to measure starlight bouncing from the permanently shaded base of the Moon which revealed the craters. They were published in the journal Icarus.
There are some parts of the Moon we simply can’t see, such as around the south pole, purely because sunlight doesn’t reach. Those regions are interesting to scientists, though, as without being bombarded by light, they may trap and hang onto volatile compounds.
Identifying new craters and comparing them with older counterparts will help map how space weathering, such as particles sputtering from the solar wind or impacts from micrometeorites, happens in those areas too.
Of course, optical telescopes are all but useless when it comes to exploring shaded parts, but this is where the Lyman Alpha Mapping Project (LAMP) instrument aboard the Lunar Reconnaissance Orbiter shines.
LAMP doesn’t pick up reflected photons from the Sun. It uses reflected far ultraviolet light, such as that emitted by stars that shine in ultraviolet, which is, as it sounds, extremely dim by the time it gets to the Moon.
But LAMP is sensitive enough to identify tiny differences in albedo, or reflectivity, of ultraviolet light.
New craters are relatively bright, as the smattering of dust and rocks around the impact point hasn’t been affected by space weathering. And for the first time it picked up evidence of two bright young craters near the south pole – one 1.4 kilometres at its widest point and another 800 metres in diameter.
The new craters are in larger craters of Faustini and Slater. They happen to be ancient – matter of the floor of Faustini, for instance, is estimated to be around 3.5 billion years old.
Figuring out the age of the new craters was a matter of measuring their brightness. The first crater – the largest – was clearly surrounded by a bright, rough “ejecta blanket” made of rocks thrown up by the impact.
But the second, older crater’s blanket was dimmer. Given this, calculations showed it must be at least 75 million years old.
The upper limit – 420 million years – was the amount of time it would take for the entire blanket to be covered in fine, fluffy dust.
“Discovering these two craters and a new way to detect young craters in the most mysterious regions of the Moon is particularly exciting,” Mandt says.
“This method will be useful not only on the Moon, but also on other interesting bodies, including Mercury, the dwarf planet Ceres and the asteroid Vesta.”