Earth & Sky

Is there such a thing as a shower of comets?

I’ve seen meteor showers, but is there such a thing as a ‘comet shower?’

Comet showers could exist – but you would have to live a long time to see one. As it is now, several comets per year whisk through the inner solar system before heading back out into deep space. Most pass too far from Earth to be seen without telescopes. But astronomers believe a “rain” of comets could be triggered by the gravitational tug from a passing star on the Oort Cloud – a hypothetical cloud of comets surrounding our solar system. A passing star might kick a huge number of comets from the Oort cloud into elongated, sun approaching orbits. And then many more comets each year might be visible from Earth.

But, events in outer space – and events on Earth – don’t always take place over the same time scale. A comet shower might last a long time – perhaps several hundred thousand years. During that time, any people on Earth might see a lot of comets. But these comets might come in one by one with years or even centuries in between. So could you see a lot of comets all at once in the sky – a comet shower visible over the scale of a human lifetime?

The answer is . . . possibly . . . but, considering the vast scale of space and time, it’s probably not very likely.

A “comet shower” might last several hundred thousands years. During that period earth might be hit frequently with comets. Each impact could unleash the equivalent energy of all the nuclear weapons on earth exploding at once. This would pump enough dusty debris into the atmosphere to block out the sun for years. Such a gloom would spell doomsday for most species.

Recently, scientists found evidence from a rock quarry in northern Italy that suggests a shower of comets hit Earth about 36 million years ago. The quarry was on the ocean floor 36 million years ago and accumulated sediments that contain an unusual abundance of a form of helium (an isotope known as helium-3). Helium-3 was formed in the big bang. But most of the Earth’s primordial helium escaped into space long ago. So, this isotope is rare on Earth but common in comets and meteorites.

Dust falling from space contains 200 times more helium-3 than the Earth’s atmosphere. In deep ocean sediments, this dust does not get mixed with large quantities of other debris, making the helium-3 easier to detect.

The helium-3 that was deposited on earth 36 million years ago suggests there was a lot of dust in the inner solar system back then, and that it rained onto earth. The most likely source of the dust would be active comets spewing tons of dust into space.

This means there were a large number of Earth-crossing comets and much dust from their tails for a period of about 2.5 million years. Some fraction of these comets must have collided with earth. Microscopic debris from space and from comet impacts also accumulated on the ocean bed. The debris was preserved because it was covered with dying organisms over the eons.

Supporting the idea of a bombarded earth, are a giant ancient impact craters in Siberia and at Chesapeake Bay in Maryland, which date back to this period.

More resources on “space”…