PH104 Lecture 7

Observations and the Solar System : Lecture 7

The Gas Planets

Moving out in the Solar system we encounter 4 large planets - these are

Jupiter, Saturn, Uranus, Neptune

These four planets are very different from the four planets we have seen previously. The four terrestrial planets are rocky with a density of around 5000 kg/m³ ( see the planet data sheet ) These four planets are much less dense with densities between 600 and 1300 kg/m³ (recall that water is 1000 kg/m³). These four planets are actually composed of very dense gases. The gases include large amounts of hydrogen and helium. The gases are at very high pressure also (it take a lot of pressure to make gases as dense as water!!). The four planets are often called the Jovian planets. The gas giants were explored by Voyager I+II which passed by these planets and gained much information.

Jupiter

Jupiter is, quite simply, enormous. It has 317 times the mass of earth and around 71% of the mass in the solar system outside the sun lies in Jupiter. It does not have a surface as such but instead we speak of "clouds-top" where the atmosphere begins. If one were to pass through the clouds we would progressively be subjected to higher and higher pressures until we reached the centre. Jupiters atmosphere is very different from the terrestrial planets. It is 86% hydrogen and 14% Helium. Its atmosphere is very turbulant-stormy- and some of the features are very long-lived. The most prominent -the "red spot" - is 3 times larger than the earth and has persisted since the 1600s.

Jupiter is spinning very quickly - despite its enormouse size it rotates much faster than the earth. Its rotation period at the equator is 9hrs 50 minutes. Unlike the terrestrial planets which are rigid, since jupiter is a gas object there are several effects. Firsty it does not all rotate at the same speed. The poles rotate slightly slower with a rotation period of 9hrs 55 minutes. This is called Differential Rotation . Secondly becaus of the rotation is is not a perfect sphere but is rather squashed. This is known as being oblate . Jupiters diameter at the equator is 11.2 earth diameters but the distance "pole-to-pole" is only 10.4 earth diameters. Jupiter has an oblateness of about 6.5% - it can be noticed by the sharp-eyed observer.

some pictures of Jupiter

The four moons of Jupiter

Jupiter has four large moons which are easily seen with a telescope. These were first observed by Galileo and are known as the Galilean Moons or Satelites. They are large are rather diverse. In order, closest first they are

Io, Europa, Ganymede, Callisto

The Jupiter system is much like a mini solar system. The speculations that life might exist on these other worlds led to the execution of the former priest Giordano Bruno in 1600.

We looked at some characteristics. Europa in particular is peculular. It has a smooth surface and covered with crack-like features (but no craters). It is thought that this is an ice covering. Underneath the surface there could be water and perhaps even life..... Io the nearest moon is covered in very active volcanoes. It is thought that the tidal effects of Jupiters gravity squeeze Io periodically thus heating it up - like a squash ball.

Io

Close ups of Io's surface reveal an extremely volcanically active surface. We also have a couple of nice images ( one and two ) of the total surface and a close up of a volcano and a good side view of a active volcano

Europa

Close ups of Europa's surface reveal a ``cracked'' surface which could well be the frozen surface of liquid water. This can also be seen in this (false colour) image We also have a couple of nice double images ( one and two ) of the total surface.

Ganymede

image

Calisto