chapter 10 Flashcards
discovery
- Jupiter and Saturn are bright in our sky and were well known to the ancients. Uranus was the first planet discovered in recorded history. Wilhelm Herschel found it quite by accident in 1781. He thought he might have discovered a comet until he determined that its orbit was more rounded (less eccentric) like that of the other planets.
- The discovery of Neptune in 1846 was not accidental at all. By this time Uranus had completed 65 years of its 84-year orbit around the Sun, and it was becoming clear that its orbit showed deviations from the orbit predicted by interactions with the Sun and the known planets. It was suspected that these deviations were caused by the gravitational pull of an undiscovered planet further out. Theoreticians predicted where this unknown planet might be located – and the telescope observers found Neptune at once.
atmosphere
Jupiter’s Great Red Spot (see Image #3) has been visible all during the four centuries we have made telescopic observations of Jupiter, but its size and color have varied. The smaller white ovals come and go. These are all the tops of cyclonic storms similar (in type but not in size) with those we are familiar with on Earth.
There have been several special studies aimed at a better understanding of Jupiter’s atmosphere. In 1992 a comet, Shoemaker-Levy 9, came too close to Jupiter and was broken up by its powerful gravitational field. (See Image #4) In the summer of 1994 the comet fragments plunged into Jupiter’s atmosphere (Images #5 & #6), giving us some information about the material below the cloud tops. At the end of 1995, the Galileo spacecraft (Image #7), in orbit
around Jupiter, sent a probe into the atmosphere. It continued to transmit information to a depth of about 160 km below the cloud tops (Image #8), when it presumably burned up. There have also been a number of orbiters, most recently Juno which went into a polar orbit around Jupiter in 2016. The images on page 271 are from Juno.
We had one close look at the atmosphere of Saturn in 2017 when the Cassini spacecraft, in the Saturnian system since 2004, ran out of propellant and was crashed into the atmosphere
Planet Migration
Planet migration suggests that the large gas planets moved between orbits close to the Sun and orbits farther out during the early history of the solar system. To understand this, we need to review some of the ideas about orbits from Chapter Four. An object in gravitational orbit has both kinetic energy (energy of motion which can only be positive [or zero]) and potential energy (energy of position which can only be negative). For a bound orbit, like a planet around the Sun, the total energy must be negative. The more energy the body has, the less tightly bound it is. Example: the Moon is pulled forward by Earth’s tidal bulge and is gaining energy. The result is that it is becoming less bound - moving away from Earth into a higher, more energetic (but still negative energy) orbit. If you could give the object enough energy to make its total energy zero, it would break free of the Sun and no longer be bound in the solar system. It would have escape speed. Its orbit would be an open orbit: parabolic or hyperbolic.
prober
One of the many surprises found by the Galileo probe is this: the concentration of some of the noble gases (neon, argon, krypton, xenon) are much larger than the concentration in the Sun. These elements do not generally form chemical compounds, so the amounts of the elements themselves should remain constant. Since the whole of the original solar nebula contained the same concentration of elements, it has been suggested that the extra amounts of inert gas were carried to Jupiter aboard comets. The problem is, however, that these gases do not freeze at the temperatures which exist at Jupiter’s present location. So, apparently Jupiter has spent some time at colder locations farther out than its present orbit. This was one of the early motivations for the idea of planet migration
