How Life-Bearing Rocks from the Chicxulub Asteroid Impact must have Spread through the Solar System

Earth rocks capable of carrying and protecting life have probably to travelled to Europa, Titan and beyond, say astrobiologists

Some 65 million years ago, an asteroid the size of a small city hit the Yucatan Peninsula in what is now Mexico. The impact devastated Earth, generating huge tsunamis, massive wildfires and rapidly heating the atmosphere. It triggered earthquakes and volcanic activity and then cooled the world as smoke and dust blanketed the planet.

This impact changed the future of the Earth, triggering the sequence of events that led to the extinction of the dinosaurs, the rise of mammals and ultimately the domination of humans.

Now Christopher House and pals at Penn State Astrobiology Research Center in University Park, say this event may have had an equally profound influence on other parts of the Solar System.

These guys have calculated the number of rocks ejected into space in this collision that would have been capable of carrying life and protecting it in space. They say that significant numbers of these rocks would have made the journey to Mars and Venus but also to Jupiter and Saturn. In particular, they say these rocks would probably have travelled to moons such as Europa, Callisto and Titan, where astrobiologists believe that conditions are ripe for life.

The idea that life may have been seeded from space by comets, asteroids and dust clouds is by no means new. But only in recent years have astrobiologists begun to study the likelihood that rocks ejected in asteroid impacts could travel the vast distances between one planet and another.

The key factors here are the temperature that the rocks reach during the impact and the protection they then offer to organisms in space. The general consensus is that life as we know it cannot survive temperatures in excess of 100 degrees Centigrade and that the rocks must be at least 3 metres in diametre to protect any organisms from the damaging effects of radiation in space over timescales ranging from thousands to millions of years.

So how many of these potentially life-bearing rocks did the Chicxulub impact throw into space? House and co say the impact would have ejected 70 billion kilograms of rock into space. They go on to simulate how this stuff would have spread through the Solar System.

As an example, they calculate how much of this stuff would have ended up on Europa—a staggering 20,000 kilograms. “The probability of a fragment > 3 m reaching Europa is 0.6, and over 3.5 billion years we can expect one or two rocks to have made the journey,” say House and co.

What’s more, they say similar amounts of potentially life-bearing rock would have travelled to other Jovian and Saturnian moons such as Io, Europa, Ganymede, Callisto, Enceladus, and Titan.

Astronomers know that beneath the icy surfaces of Europa, Callisto and Titan lie salty oceans of liquid water and that this ice is, or has been, thin enough to be penetrated relatively easily.

The implication is that it’s by no means impossible for life from the time of the dinosaur extinction to be flourishing now in the warm, dark oceans on these moons. “Any planned missions to search for life on Titan or the moons of Jupiter will have to consider whether any biological material found represents an independent origin, rather than another branch in the family tree populated by Earth life,” conclude House and co.

Of course, this kind of seeding may have worked in the opposite direction too. Astrobiologists have long discussed the possibility that life on Earth might have originated on Venus or Mars when conditions on these planets were friendlier. Indeed, in 1996 NASA claimed to have found evidence of ancient life in a meteorite that had travelled to Earth from Mars; although it’s fair to say that many scientists remain unconvinced.

But House and co take this debate further by showing how life-bearing ejecta may play an important role in maintaining life on any given planet. They point out that most of the ejecta from Chicxulub, for example, would have eventually fallen back to Earth.

So while the impact itself had a sterilising effect on the world by destroying vast swathes of life, any life-bearing ejecta could have helped to reseed the planet later, when conditions had become less harsh.

That might have been a particularly important process during the period known as the late heavy bombardment some 4 billion years ago when Earth and the other terrestrial planets experienced huge numbers of impacts as they mopped up the debris left over during planet formation.

The bottom line is that astrobiologists may have to revise the view that the emergence of life on Earth is somehow lucky to have survived the various impacts that could have wiped it out.. Instead, it looks as if asteroid impacts could play a crucial role in preserving life and transporting it, both in time and space, from regions that are inhospitable to places that are much friendlier.

Indeed, we probably owe our very existence as a species to these seemingly cataclysmic events.

Ref:arxiv.org/abs/1311.2558: Seeding Life on the Moons of the Outer Planets via Lithopanspermia