Free Astronomy Magazine September-October 2018

23 SEPTEMBER-OCTOBER 2018 PLANETOLOGY C ompositional differences among the giant planets and their relative sizes. Earth is shown for comparison. Jupiter and Saturn are primarily made of hydrogen and helium, the terres- trial planets are al- most pure rock, while Uranus and Neptune are thought to be mainly watery. [JPL/Caltech, based on material from the Lunar and Planetary Institute] Below, on the left, Uranus imaged by the Voyager 2 spacecraft in 1986; on the right, Nep- tune recently im- aged by the VLT in Chile. [NASA/JPL- Caltech, ESO/P. Weilbacher (AIP)] tion, which means that the heat flowing from below is negligible, a strange thing if we consider that the core temperature is es- timated to be 5000°C. It is as if there was a layer of insulating material that prevents in- ternal heat from radiating into the upper at- mosphere and to outer space. Almost at the same time as the discovery of this temperature anomaly, another equally enigmatic one was discovered: the magnetic field axis is highly asymmetrical, inclined more than 60° compared to the rotation axis and, moreover, not passing through the center of the planet. The origin of a planetary magnetic field is the rotation of a fluid mass – to have this axis not pass through the planet’s center