Hit-and-Run Heist of Water by Terrestrial Planets in the Early Solar System

A study simulating the final stages of terrestrial planet formation shows that ‘hit-and-run’ encounters play a significant role in the acquisition of water by large protoplanets, like those that grew into Mars and Earth.
The results will be presented by Christoph Burger at the European Planetary Science Congress (EPSC) 2018 in Berlin.

Four and a half billion years ago, the inner solar system was a chaotic place with around 50-100 protoplanets ranging in size from the Moon to Mars that were prone to giant collisions. Bodies that formed within what is now the orbit of Mars contained no water as the conditions were too hot for volatile material, like water or methane, to condense. For water to make its way onto the developing terrestrial planets, water needed to be delivered from outside this region via a sequence of collisions.

Burger and colleagues from the University of Vienna and Tübingen have used high-resolution simulations to track the fate of water and other materials through a series of different impact scenarios. Outcomes of collisions could include bodies sticking together, material being lost, or being redistributed between the two objects. The results depend on various factors like the speed and angle of impact, the difference in mass between the bodies and their total mass.

“We found that ‘hit-and-run’ collisions, where the impact is off-centre and the bodies have enough speed to separate again after the encounter, are very common. In these scenarios, tens of percent of water can be transferred between the colliding bodies or ejected and lost entirely,” said Burger.

The smaller of the colliding pair is often modified down to the core and effectively stripped of water, while the more-massive body remains more-or-less unaltered. The team is now focusing on how long chains of successive collisions affect the evolution of a disk of planetesimals and protoplanets.

“Recent research shows that comets can only account for a small fraction of the terrestrial planets’ water. These giant collisions early in the solar system’s history must also be a major source. Our results strongly suggest we need to track the water in both survivors following hit-and-run encounters. This will help us predict the properties of planets that form as the end-product of a long sequence of successive collisions,” said Burger.

http://www.europlanet-eu.org/wp-content/uploads/2018/09/Burger_hitandrun_still.jpg
Snapshots from the simulations illustrating water transfer and loss in a typical hit-and-run
encounter. The blue and white colours represent water on the initial bodies, while red is rocky material from their interiors. Credit: Burger et al.

http://www.europlanet-eu.org/wp-content/uploads/2018/09/Protoplanet_impact.jpg
Artist’s concept of a celestial body about the size of the moon colliding with a body the size of Mercury. Credit: NASA/JPL-Caltech

The European Planetary Science Congress (EPSC) 2018 is taking place at the Technische Universität (TU) Berlin, from Sunday 16 to Friday 21 September 2018. EPSC is the major European annual meeting on planetary science. Around 1000 scientists from Europe and around the world will attend EPSC 2018 and will give around 1,250 oral and poster presentations about the latest results on our own solar system and planets orbiting other stars.
Details of the Congress and a full schedule of EPSC 2018 scientific sessions and events can be found at the official website: http://www.epsc2018.eu/

Europlanet provides Europe’s planetary science community with a platform to exchange ideas and personnel, share research tools, data and facilities, define key science goals for the future, and engage stakeholders, policy makers and European citizens with planetary science. Europlanet is the parent organisation of the European Planetary Science Congress (EPSC).

The Europlanet 2020 Research Infrastructure (RI) has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 654208 to provide access to state-of-the-art research facilities across the European Research Area and a mechanism to coordinate Europe’s planetary science community. The project builds on a €2 million Framework 6 Coordination Action and €6 million Framework 7 Research Infrastructure funded by the European Commission.