Free Astronomy Magazine September-October 2018

29 SEPTEMBER-OCTOBER 2018 PLANETOLOGY Earth masses. If the impactor was 3 Earth masses, only one third of the early at- mosphere would have remained within the Roche radius (about 6 Uranus radii), the limit beyond which the expelled ma- terial (rocks, ice, and gas) would end up either contributing to form moons and rings or would end up being ejected far- ther into space. It is interesting to note that the simula- tions of the Kegerreis team, in addition to providing valid answers to the questions of the magnetic field and the severe tem- perature of the upper atmosphere, pro- duce a final planet with an inclination of the rotation axis fully compatible with the current one of Uranus. There is still one aspect that remains un- solved that not even the simulations can account for presently. It concerns the real position of the geographical poles: as strange as it may seem, even today there is no agreed upon answer among the plan- etologists on which is the north pole of Uranus and which is the south pole. In fact, the rotation axis could be inclined either slightly less than 98° or slightly more than 82°. In the first case, the north pole would be below the orbital plane, and the planet would have a direct rotation, such as that of the Earth. In the second case, the north pole would be above the orbital plane, and the rotation would be retrograde, such as that of Venus (whose retrograde motion can almost certainly be attributed to grav- itational braking operated by the Sun on the dense atmosphere of the planet). It seems near-impossible to understand how the ice giant’s geographic poles are arranged. Perhaps the answer could come from future simulations with even higher numerical resolution, reconstructing with great precision the physical properties of the planet by reducing all of the possible variables down to a single set. This high- est-quality model would hopefully provide great insights into the mysterious world that hit Uranus 4 billion years ago. ! T his view of Uranus was recorded by Voyager 2 on Jan- uary 25, 1986, as the spacecraft left the plan- et behind and set forth on its cruise to Neptune. Voyager was 1 million kilometers (about 600,000 miles) from Uranus when it acquired this wide-angle view. [NASA/JPL]