How Earth Life Could Come Back from a Sterilizing Asteroid Impact

Supervillains take note: Even the biggest and baddest asteroids may not be 100% effective as doomsday devices.

A cosmic impact powerful enough to wipe out all life on Earth's surface would loft large amounts of rock into orbit around the sun. And most of these bits and pieces would end up falling back onto our bruised and battered planet, potentially bringing life back with them, said Steinn Sigurðsson, a professor in the Department of Astronomy and Astrophysics at Penn State University.

"This is peculiarly reassuring," Sigurðsson said last month at the Breakthrough Discuss conference at the University of California, Berkeley.

"If you have a sterilizing impact — if you have a beyond dinosaur killer, something that’s going to flash fry the entire planet — there is a significant probability that some biota is ejected and returns to the planet, hopefully gently, fast enough to reseed the planet," he added.

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The existence of such "space refuges" is supported by computer simulations Sigurðsson and his colleagues recently performed, which tracked the trajectories of rock blasted off Earth and the other rocky planets into orbit around the sun. 

This is a relatively small subclass of ejected material, by the way; most of the liberated rock would not reach escape velocity and would therefore come back down in short order. Indeed, scientists think the biggest killer in the dino-offing impact 66 million years ago may have been a global firestorm that flared up when returning rock heated Earth's upper atmosphere to about 2,700 degrees Fahrenheit (1,482 degrees Celsius).

Sigurðsson and his team followed the simulated ejecta's orbital evolution for 10 million years. They chose this time span "because there's a meme in the literature that you might keep biota viable [inside a rock in space] for about 10 million years," Sigurðsson said. "Beyond that, you're pushing your luck."

The ejecta starts out in a solar orbit similar to that of its home planet, and most of the material ends up getting reabsorbed. But gravitational tugs from passing planets yank some of the rocky bits onto different paths.

For example, in the simulations, a few percent of orbiting ejecta made their way to one of the other rocky planets. We know this happens, of course; scientists have identified more than 100 Mars meteorites here on Earth. But the extent of the rock swapping in the inner solar system was unexpected, Sigurðsson said.

"That was actually surprisingly high," he said. "There really is a rain of rocks."

Less than 0.1% of the ejecta makes it to the outer solar system, the realm of the potentially habitable Jupiter moon Europa and the Saturn satellites Enceladus and Titan, both of which may also be capable of supporting life. 

That may not sound like much, but it amounts to tens of thousands of rocks over the course of the solar system's 4.5-billion-year history, according to the team's simulations. And these results represent a conservative estimate, Sigurðsson stressed.

"So, the solar system is vulnerable to cross-contamination, and we should be cognizant of that when we look for life in the outer system," he said.

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And we are going to look for life out there soon, if all goes according to plan. NASA plans to launch a mission to Europa in the early to mid-2020s. The Europa Clipper probe will characterize the moon's subsurface ocean over the course of dozens of flybys and also scout out places for a life-hunting lander to touch down. (The lander mission isn't officially on NASA's books yet, but Congress has instructed the space agency to develop it.)

NASA is also considering developing a Titan drone mission called Dragonfly, which would study the big moon's atmospheric chemistry in detail. Dragonfly could spot possible signs of life in Titan's air, in the form of gases in chemical disequilibrium. (Dragonfly is one of two finalists, along with a comet sample-return mission called CESAR, for a medium-class mission launch spot in the mid-2020s. The agency is expected to announce its choice by the end of the year.)

In addition, a few percent of ejected rocks escape our solar system entirely, raising the possibility that life from Earth (or Mars) may have seeded worlds circling other stars, Sigurðsson said. Such seeding may have happened in the other direction, too; some scientists think life may have come to Earth long ago aboard an interstellar object.

This is all speculation, of course; nobody actually knows where or how Earth life started, or how far afield it may have spread. But other research suggests that it's quite possible for life to make the impact-aided trip from world to world. 

For example, experiments have shown that some bacteria, and supertough little animals called tardigrades, can survive the harsh conditions of space. And the powerful impacts that send such beasties on an interplanetary or interstellar trek aren't nearly as deadly as you may think.

Benjamin Weiss, a professor of planetary sciences at the Massachusetts Institute of Technology, presented research to this effect at the Breakthrough Discuss conference. Work by Weiss and his colleagues suggests that at least some Mars meteorites experienced surprisingly low maximum temperatures when they were launched from their planets — meaning they were likely not sterilized. 

And life can probably survive the trip down from space as well, both Weiss and Sigurðsson said.

"I think atmospheric entry is basically a nonissue here; it’s the easiest part of the problem," Weiss said during a panel discussion at the conference.

So, life may commonly hop from planet to planet, especially in tightly packed solar systems such as TRAPPIST-1, in which multiple potentially habitable worlds reside cheek-to-jowl.

"You’d expect systems like that — if they develop life at all, if life is common — to completely cross-fertilize," Sigurðsson said.

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Mike Wall's book about the search for alien life, "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), is out now. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.