Impact craters leave quite an impression on the surface of planets and moons — just think of Earth’s moon, which gets its distinctive appearance from millions of encounters of asteroids over the centuries. But Earth is a different story altogether, with only 128 impact craters recorded in the most recent count. That can’t be right, can it?
Wrong, writes Science’s Eric Hand. He reports that a new study shows that the low number found by past scientists isn’t “just the result of lazy searching”: it’s the surprising truth about a planet that’s astonishingly crater-free.
The study looked at the ways Earth erosion affects existing craters and concluded that the current count of 70 craters larger than 6 km (3.7 miles) in diameter should be just about right. That’s a rare instance of a complete geologic record, writes Hand — and one that may discourage people on the hunt for new craters.
But don’t put away your crater-catching gear just yet. The study’s authors note that just because we’ve already found all of the likely large impact craters on Earth doesn’t mean there aren’t more to discover. The real opportunity, they write, lies in smaller craters: they estimate that more than 90 craters between .6 miles and 3.7 miles in diameter should still be undiscovered and more than 250 between 0.1 miles and .6 miles.
So why are there so few craters on Earth? NASA notes that Earth is equipped with three processes that eat up craters relatively quickly: erosion, tectonics, and volcanism. These forces leave only the largest scars from meteorites or asteroids — unlike, say, the moon, which can’t gobble up craters. Hand writes that the parameters of the study also play a part in the low number — it looks at just surface craters, not those that lie beneath sediment. And the study also didn’t look at volcanic craters, which formed some of Earth’s most distinctive basins and lakes.