THE CORES OF THE PLANETS

Jupiter, the largest planet in the Solar System, is known to have a diameter 11,19 times that of the Earth, a mass of 317,8 times and a volume of 1318 times that of the Earth. Truly a giant! But how does Jupiter's core which must consist of elements heavier than hydrogen and helium, the so-called metals, compare with the cores of the Earth and the other planets?

Jupiter's atmosphere of hydrogen and helium extends many thousands of kilometres above the cloud decks which are fairly near to Jupiter's surface. The surface layer of hydrogen, with an admixture of helium, is calculated to be 25 000 km thick. Because of the great pressure exerted by the atmosphere, the surface layer is in the liquid state and the hydrogen and helium are in the molecular form. Below this there is a layer of metallic hydrogen, calculated to be 31 000 km thick. Right in the centre is the core which is calculated to have a radius of 14 400 km. These figures give Jupiter an average density of 1,33 grams per cubic centimetre. Saturn, which is very similar to Jupiter, has an average density of only 0,704 gm per cubic cm, so that it would be able to float on water!

The radius of Jupiter's core is only 2,26 times that of the Earth's radius of 6738 km and 2,38 times the radius of Venus, 6057 km. The Earth and the other terrestrial planets consist entirely of heavy elements - its atmosphere and surface water being insignificant compared to the thousands of kilometres of heavy elements. Saturn's core is calculated to have a radius of 10 000 km and those of Uranus and Neptune 8 000 km each. Yet the latter two planets have radii of almost 4 times that of the Earth and volumes of 64 and 60 times respectively.

Because the amounts of heavy elements in the cores of the planets are so very similar, the distribution of heavy elements in the primeval nebula from which the planets condensed, must have been fairly uniform throughout. This process of condensation is known as accretion and the greater masses of the giant planets are due, largely to the masses of the overlying lighter elements. The volumes of space in the primeval nebula from which the giant planets accreted their cores, are considerably larger than those from which the terrestrial planets accreted.

At the start of the process of accretion, the finely divided material must have coagulated into many thousands of small clumps. These clumps must then have attracted each other gravitationally, thus becoming larger and larger. These clumps are called planetesimals. G W Wetherill ( Scientific American, October 1969 ) made a computer study starting with 100 planetesimals and allowed them to grow by accretion. He found that after only 30,2 million years these would have accumulated to only 22 separate bodies; after 79 million years to 11 bodies and after 100 million years to only 4 separate bodies. The four terrestrial planets, Mercury, Venus, the Earth and Mars could thus have formed by accretion in a period of only one hundred million years which comprises only 2% of the age of the Solar System.

Only four bodies in the Solar System, Jupiter, Saturn, Uranus and Neptune, have radii of heavy elements, more than 8 000 km; two, Earth and Venus have radii of 6000 km and Mars 3 400 km. Mercury and Jupiter's largest satellite, Ganymede, have radii of 2 000 km. There are 7 bodies, Io, the Moon, Titan, Europa, Callisto, Pluto and Triton, which have cores with radii between 1 800 and 1 00 km. Another eight, Titania, Rhea, Oberon, lapetos, Umbriel, Ariel, Tethys and Ceres, have radii between 800 and 500 km. The average radius of the 24 bodies listed above, is only 3 160 km - half that of the Earth and Venus. Ceres is the largest of the Minor Planets and there are thousands with radii less than 500 km.

The gravities of bodies such as Venus and the Earth were not great enough to hold on to lighter gases such as helium and hydrogen, while bodies such as Uranus and Neptune, with cores only one-third those of Venus and Earth, did hold on to extensive atmospheres of hydrogen. Thus, somewhere between radii of 6 000 and 8 000 km, is the dividing line between cores able to become giant planets, shrouded in hydrogen and small planets which are able to hold on to atmospheres of the heavier gases, such as carbon dioxide, nitrogen and oxygen. Since both Venus and Mars have atmospheres consisting of more than 95% of carbon dioxide, it seems safe to assume that the Earth also started off with an atmosphere consisting largely of carbon dioxide. This atmosphere must have been consumed by the first forms of life, the algae, which ruled the Earth for three thousand million years and during that time exuded the poisonous gas oxygen which today comprises 20% of our atmosphere.

Eben van Zyl