Free Astronomy Magazine September-October 2018

28 SEPTEMBER-OCTOBER 2018 PLANETOLOGY According to the researchers, it could be just that layer of ice gained by Uranus that in- hibits the propagation of internal heat to- wards the outside. Among the fifty scenarios developed by the Kegerreis team, those that in the final result over- lap better with reality re- quire a grazing impact, pos- sibly occurring in two distinct phases: a first crawled con- tact, lasting a few hours and enough to slow and curb the trajectory of the intruder, then a true impact occurring after about a day, with the total destruction and assimi- lation of the impactor within the next 24 hours. This “soft” dynamic would have allowed the young Uranus to keep about half of its origi- nal atmosphere in case of an impactor of 2 A summary of the single- phase collision suffered by Uranus, with the indication of the times. Everything is completed in about two days. As in the previous series, R indicates the radius of Uranus. The video on the side shows six different im- pact simulations, with a decreasing centrality. [ApJ, J. Kegerreis et al.]