Giant Impacts on Earth-Like Worlds

Earth has experienced a large number of impacts, from the cratering events that may have caused mass extinctions to the enormous impact believed to have formed the Moon. A new study examines whether our planet’s impact history is typical for Earth-like worlds.

N-Body Challenges

The final stages of terrestrial planet formation are thought to be dominated by giant impacts of bodies in the protoplanetary disk. During this stage, protoplanets smash into one another and accrete, greatly influencing the growth, composition, and habitability of the final planets.

There are two major challenges when simulating this N-body planet formation. The first is fragmentation: since computational time scales as N^2, simulating lots of bodies that split into many more bodies is very computationally intensive. For this reason, fragmentation is usually ignored; simulations instead assume perfect accretion during collisions.

The second challenge is that many-body systems are chaotic, which means it’s necessary to do a large number of simulations to make statistical statements about outcomes.

Adding Fragmentation

A team of scientists led by Elisa Quintana (NASA NPP Senior Fellow at the Ames Research Center) has recently pushed at these challenges by modeling inner-planet formation using a code that does include fragmentation. The team ran 140 simulations with and 140 without the effects of fragmentation — using similar initial conditions — to understand how including fragmentation affects the outcome.

Quintana and collaborators then used the fragmentation-inclusive simulations to examine the collisional histories of Earth-like planets that form. Their goal is to understand if our solar system’s formation and evolution is typical or unique.

How Common Are Giant Impacts?

The authors find that including fragmentation does not affect the final number of planets that are formed in the simulation (an average of 3–4 in each system, consistent with our solar system’s terrestrial planet count). But when fragmentation is included, fewer collisions end in merger — which results in typical accretion timescales roughly doubling. So the effects of fragmentation influence the collisional history of the system and the length of time needed for the final system to form.

Examining the 164 Earth-analogs produced in the fragmentation-inclusive simulations, Quintana and collaborators find that impacts large enough to completely strip a planet’s atmosphere are rare; fewer than 1% of the Earth-like worlds experienced this.

But giant impacts that are able to strip ~50% of an Earth-analog’s atmosphere — roughly the energy of the giant impact thought to have formed our Moon — are more common. Almost all of the authors’ Earth-analogs experienced at least 1 giant impact of this size in the 2-Gyr simulation, and the average Earth-like world experienced ~3 such impacts.

These results suggest that our planet’s impact history — with the Moon-forming impact likely being the last giant impact Earth experienced — is fairly typical for Earth-like worlds. The outcomes also indicate that smaller impacts that are still potentially life-threatening are much more common than bulk atmospheric removal. Higher-resolution simulations could be used to examine such smaller impacts.

Citation

Elisa V. Quintana et al 2016 ApJ 821 126. doi:10.3847/0004-637X/821/2/126