The threat of Centaurs for the Earth

The astrophysicists Mattia Galiazzo and Rudolf Dvorak from the University of Vienna, in collaboration with Elizabeth A. Silber (Brown University, USA) investigated the long-term path development of Centaurs (solar system minor bodies which originally have orbits between Jupiter and Neptune). These researchers have estimated the number of close encounters and impacts with the terrestrial planets after the so-called Late Heavy Bombardment (about 3.8 billion years ago) as well as the possible sizes of craters that can occur after a collision with the Earth (and the other terrestrial planets). The publication was recently published in Monthly Notices of the Royal Astronomical Society.

Centaurs are the solar system objects whose orbits are found between those of the giant planets. They originate mainly from the Trans-Neptunian objects, and are among the sources of Near-Earth Objects. Thus, it is crucial to understand their orbital evolution which in some cases might end in collision with terrestrial planets and produce catastrophic events. The researchers studied the orbital evolution of the Centaurs toward the inner solar system, and estimate the number of close encounters and impacts with the terrestrial planets after the Late Heavy Bombardment (from about 3.8 billion years ago, until now) assuming a steady state population of Centaurs. "We also estimate the possible crater sizes. Centaurs can become also active comets, because of the presence of water on a  good number of them, thus we also compute the approximate amount of water released to the Earth, which is comparable to the amount of water present on the Adriatic sea, now. We also found subregions of the Centaurs where the possible impactors originate from", explains Mattia Galiazzo.

While crater sizes could extend up to hundreds of kilometers in diameter given the presently known population of Centaurs the majority of the craters would be less than ~10 km. For all the planets and an average impactor size of ~12 km in diameter, we have on average 2 impacts since the Late Heavy Bombardment  for the Earth and between 1 and 2 for Venus. Luckily this "bombardment" is less intense (of at least one tenth) the impacts done by asteroids between Mars and Jupiter. However Centaurs come on average much faster and have larger bodies in general. For smaller Centaurs (e.g. with a diameter, D> 1 km), the impact frequency is way larger, about one every 14 Myr for the Earth, 13 Myr for Venus and, 46 for Mars, in the recent solar system. The researchers find that about half of the Centaurs can enter into the terrestrial planet region and ~7% of them can interact with terrestrial planets. In the case of an impact, Centaurs could be the cause of catastrophic, such as events extinction of life as we presently know it.
 
These results provide an important contribution in the analysis of catastrophic events of extraterrestrial origin, which might happen not only on our planet, but also on planets like Mars and Venus. "Our work also provides the framework for better understanding past events, and how they might had altered life on Earth and other terrestrial planets", tells Galiazzo. For example, Dvorak says: "Such events could have a direct impact of life by either destroying it (e.g. Earth) or creating conducive conditions (e.g. hydrotermal activity) for new life to form. In addition, our results give more answers on the evolution of the present solar system".

Another interesting fact, Silber asserts: "Centaurs can bring water to Mars after a collision" and recent missions confirm the presence of water on Mars.

Publication in "Monthly Notices of the Royal Astronomical Society"
M A Galiazzo, E A Silber, R Dvorak: The threat of Centaurs for terrestrial planets and their orbital evolution as impactors. Monthly Notices of the Royal Astronomical Society
https://academic.oup.com/mnras/advance-article-abstract/doi/10.1093/mnras/sty2614/5106371