Jupiter: King of the planets

by Christopher Seddon

A concise history of Jupiter from the earliest pre-telescopic observations, to Galileo's discovery of four large moons, to the robotic probes of the Space Age. This 12,000 word Kindle Short Read describes how our knowledge of Jupiter and its four planet-sized moons Io, Europa, Ganymede, and Callisto has been transformed by the space probes that have visited this giant world and how the moons could possibly host life in subsurface oceans holding more water than all of Earth's oceans put together.Did you know?*Notwithstanding Jupiter’s enormous size, larger and more massive planets have been found orbiting other stars. The most massive of these are on the borderline between planets and ‘brown dwarf’ stars, where core temperatures and pressures are sufficient to sustain nuclear fusion. The International Astronomical Union classifies anything of more than thirteen Jupiter masses as a ‘brown dwarf’. Objects of more than one Jupiter mass in distant orbits tend to be denser, and hence no larger than Jupiter. On the other hand, so-called ‘hot Jupiters’ in very close orbits tend to less dense and larger than Jupiter, although not necessarily more massive.*Although Jupiter is well below the mass needed for it to become a dwarf star, it still radiates more heat than it receives from the Sun. Jupiter is slowly contracting under its gravity, releasing gravitational energy, and heating the interior.*Jupiter’s rotation period of 9 hours 55 minutes 30 seconds is shorter than that of any other planet in the Solar System. It is closely followed by Saturn, with a day of 10 hours 33 minutes 38 seconds. Both are far shorter than third-placed Neptune, with a day of 16 hours 6 minutes 36 seconds. Due to the high speed of its rotation, Jupiter is visibly oblate, i.e., its equatorial diameter is visibly greater than its polar diameter. However, Jupiter is not a solid body and does not rotate in the same way as a solid body. The region from the southern edge of the North Equatorial Belt and the northern edge of the South Equatorial Belt has a rotation period of 9 hours 55 minutes 30 seconds, but regions outside this zone have a rotation period of 9 hours 55 minutes 41 seconds. Furthermore, individual features have their own rotation periods; for example, that of the Great Red Spot varies between 9 hours 55 minutes 36 seconds and 9 hours 55 minutes 42 seconds.*Jupiter’s axial tilt is just over 3 degrees, less than that of any other planet. Consequently, the planet experiences no noticeable seasonality throughout its 11.86-year passage around the Sun.*The first observed impact of a comet on a planet occurred in July 1994, when Jupiter was bombarded with fragments of the comet Shoemaker-Levy 9. The comet was detected on 26 March 1993 on photographs taken at Mount Palomar. It was found to have been extended orbit around Jupiter since around 1970 and to have been tidally disrupted by a close pass in 1992, which had taken it to within 22,000 km (13,670 miles) of the planet. The fragments were on a collision course for Jupiter and impacted between 16 and 22 July 1994. Unfortunately, none of the impacts were visible from Earth, although the impact sites were soon brought into view by the planet’s swift rotation, and large dark spots were seen. The largest fragment, Fragment G, was 3 to 4 km (1.8 to 2.5 miles) across. It produced a fireball 3,000 km (1,860 miles) high and clouds of ammonia, sulphur, and hydrogen sulphide that remained visible from Earth for several months. Had the fragment hit Earth, it has been estimated that it would have left a crater 60 km (37 miles) across.*Jupiter’s magnetosphere is the largest permanent structure in the Solar System, apart from the Sun’s own magnetosphere.

69 pages, Kindle Edition

Published May 8, 2021