How to tell a greater crater tale

GAZE at the moon with a pair of binoculars or a small telescope when it is within two or three days of the first or last quarter, and you can see a wealth of detail.

A telescope, in particular, will show you a huge number of craters of many different sizes. Their number and size on the moon and other celestial bodies has long been a subject of much study, and recently two important results have been announced in connection with both the inner and the outer solar system.

Publishing in the journal Science recently, Sara Mazrouei of the Department of Earth Sciences at the University of Toronto in Canada, together with four other authors, give the results of their analysis of the number and age of large, relatively young craters on the moon.

The team studied the craters using NASA’s Lunar Reconnaissance Orbiter spacecraft, currently in orbit around the moon. They concluded that there was a large increase — a factor of 2.6 — in the cratering rate about 290 million years ago.

That’s not very long ago, considering the age of the solar system, which is about 4.6 billion years. To bring these figures back to more meaningful levels, if we were

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to allow that period of time to be represented by a full day from midnight to midnight, the increase appeared about 10.30pm.

Basically, craters in the solar system are impact features, where objects have collided with larger ones. However, what could have caused this dramatic increase in objects in our region of the solar system?

A likely possibility is the break-up of a large asteroid, possibly as a result of a catastrophic collision, even though this may have taken place a long way from Earth. The debris from such an event could have rained down on both the Earth and the moon, and as Mazrouei has commented, this may have continued for a very long time.

Indeed, as the Earth and moon are more or less in the same place in the solar system, the cratering rate on the moon would have been an indication of the rate on Earth, too. There is a possibility that this increase could have been the cause of an extinction event.

Another finding from the study is that the crater size distributions on Earth and the moon are about the same, so even though many craters on Earth have suffered considerable erosion, the erosion has affected all crater sizes about equally.

From a surplus of large craters on the moon — at least, over a particular period — to the outer reaches of the solar system, studies of Pluto and its largest moon, Charon, have shown a curious shortage of small craters.

Much of what we know about the Pluto system comes from the fly-by by the New Horizons spacecraft past the dwarf planet in 2015. The craft sent back many spectacular images, and even at that time scientists were commenting on the fact that fewer craters were seen than were expected.

Now, a detailed study of the Pluto and Charon images by Kelsi Singer and a large team of authors, also published in Science, has shown that there is indeed a major deficiency in the number of craters less than about 13km in diameter on the two bodies. This distribution of crater sizes seems to be telling a story.

We knew from the New Horizon observations that the surface of Pluto is unexpectedly young, with activity on its surface involving movement of materials, likely to be causing erosion. However, this cannot explain why smaller craters would have been eroded while larger ones are intact.

Once again, the craters are impact features, so the conclusion is that there is a lower than expected number of smaller objects in the socalled Kuiper Belt, of which the Pluto system is part.

On so many objects, craters are a source of much information about the history of our solar system.

Space

Martin George is manager of the Launceston Planetarium.