Free Astronomy Magazine September-October 2018

24 SEPTEMBER-OCTOBER 2018 PLANETOLOGY Earth. However, in some cases, what can- not be deduced by means the direct ob- servation can be understood through com- puter simulations, which offer the not- negligible advantage of being able to rep- resent a phenomenon with a decidedly long time duration if necessary. The first convincing attempt to simulate the hypothetical impact suffered by Uranus shortly after the formation of the planetary system dates back to ‘92 and was run by a team of researchers led by W. L. Slattery. Because of the limited computing power available at the time, less than 10,000 particles virtually endowed with mass were included in the simulations. This meant the original simulation was of low resolution and not able to effectively (or at all) represent the less massive compo- nents of the two colliding bodies – and ig- nored the young Uranian atmosphere. Nonetheless, Slattery and colleagues suc- ceeded in representing the collision event with sufficient detail to establish that the ideal mass of the impacting body had to be between 1 and 3 terrestrial masses. It was, in fact, by attributing these values to the virtual projectile that the dynamics of the impact provided scenarios similar to the real one. means that the fluid mass inside Uranus must be distrib- uted very inhomogeneously. Can all these oddities of Uranus be brought back to a single cause? Possibly. Both the thermal and magnetic anomalies are compat- ible with a planetary collision hypothesis: an event of that type would have con- ferred enormous quantities of material and energy to an existing planetary body already formed and differentiated, thus deeply altering its rotational structure and properties. Testing Safronov’s hypothesis has been a priority for planetologists for decades, but in spite of this, not much has been accom- plished, even when considering the objec- tive difficulty of studying an impenetrable planet placed almost 3 billion km from I n the images on this page, we see the disk of Uranus (taken from the Voyager 2 probe in 1986) surrounded by its ring system (pho- tographed by the Gemini Observatory in 2011, above, and by the Hubble Space Tele- scope, right). The white spots on the planet's disk are polar aurorae, imaged in the ultravi- olet by Hubble. The strong phase displace- ment between the rotation axis and the magnetic axis is apparent. [NASA, ESA, and L. Lamy (Observatory of Paris, CNRS, CNES)]