Life as we know it is based on water and organic compounds. Studying how water and organics form, evolve and move within our Solar System helps us to understand where and how life can form. The first place to look, of course, is Earth. But for a laboratory free of human interference, Mars is a perfect candidate. We know that Mars was hospitable to life in the past and that there is water on Mars today. But what about organics? One of the main questions of Mars research is whether there are any organics on Mars today or if they were present in the past.
Organics were first detected on Mars in 2012 by the Sample Analysis at Mars instrument suite aboard NASA’s Curiosity rover in several different samples collected in a region of Mars known as Gale Crater. However, organic molecules are unstable on the surface of Mars. They are destroyed by both UV radiation and cosmic-ray bombardment. Even at the depth at which they were dug up by Curiosity, organics cannot have sat longer than thousands of years, very short compared to geological timescales. How is that possible? One possibility is delivery from space, e.g., through impacts of C-type asteroids (a type of asteroids rich in organics and water), comets, and interplanetary dust particles.
Asteroids and comets are the leftovers of the Solar System formation in orbit around the Sun. Asteroids are mostly rocky, while comets are a mix of dust and ice. Interplanetary dust particles are the source of ”shooting stars”; while small, they collide with planets on a regular basis.
Previously, scientists assumed that dust was the main source of organics on Mars and did not consider asteroids and comets. Frantseva and her colleagues modeled the Solar System to investigate the importance of asteroids and comets. In their simulations, they have used millions of asteroids and comets, which takes weeks even on a supercomputer.
Astronomers estimate that comet impacts deliver ∼ 13 tonnes per year of carbon to Mars and that asteroid impacts deliver ∼ 50 tonnes per year. This is much more than previously assumed. They find comets and asteroids to deliver ∼ 5% and ∼ 20%, respectively, of the total organic material delivered to Mars.
Comet and asteroid-borne organics are mostly deposited locally, around impact locations. In those areas, up to 150 km from the crater center, comet/asteroid-borne organics will dominate over dust-borne organics. This means that if organics will be found near a crater, there is a high chance that the organics were delivered by an asteroid or a comet. Moreover, in search of asteroid- and comet-borne organics, one should look next to craters.
Asteroids and comets have been found around other stars than the Sun. These findings imply that exo-asteroids and exo-comets can pollute the surfaces of exo-planets, potentially enabling life by delivering the required ingredients: water and organic molecules.
These findings are described in the article entitled Delivery of organics to Mars through asteroid and comet impacts, recently published in the journal Icarus.
This work was conducted by Kateryna Frantseva, Michael Mueller, and Floris F.S. van der Tak from the SRON Netherlands Institute for Space Research and University of Groningen, Inge Loes ten Kate from Utrecht University, and Sarah Greenstreet from Las Cumbres Observatory and the University of California at Santa Barbara.
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