Ceres and the minor planets

3.5.6 Ceres and the minor planets

Mass

20 kg

Radius (equatorial)

487 km

Radius (polar)

455 km

Rotation period

0.378 days

Semi-major axis of orbit

414 703 838 km

2.77 AU

Orbital period

Orbital inclination

In 1768, Johann Elert Bode suggested that there might be a planet in orbit between Mars and Jupiter. He based this on the so-called Titius–Bode Law (see box), an empirical law proposed by Johann Daniel Titius in 1766 which (roughly) gave the relative orbital distances of the planets from the Sun. The Law ‘predicted’ that

a planet should lie at a distance from the Sun of 2.8 AU and, with the discovery some years earlier of Uranus which fi tted the Law’s predictions quite well, a group of 24 astronomers, who became known as the ‘Celestial Police’, combined their efforts to make a methodical search for the possible planet. Though they did not discover Ceres that did fi nd several minor planets – also known as asteroids—in what is now called ‘the main asteroid belt’.

The body which became known as Ceres was discovered on New Year’s Day 1801 by Guiseppe Piazzi who fi rst thought it to be a comet. Ceres has a diam- eter of ∼950 km, and has suffi cient mass to be in hydrostatic equilibrium (mak- ing it round) and so has become the smallest of the dwarf planets. It contains almost one-third of the mass of the main belt asteroids of which over 170 000 now have computed orbits. In 2006 the IAU classed these as Small Solar Sys- tem Bodies.

It is though that Ceres is suffi ciently large to have become differentiated, that is, the heavier rocky elements have concentrated in the centre giving Ceres a core over which lies an ice mantle (a mixture of water ice and minerals such as carbon- ates and clay).

The Titius–Bode law

If one takes the number 0.3, continuously doubles it and adds 0.4 to each value one gets a sequence of values which closely agree with the distance of the planets in AU. (Zero is used as an initial value.)

Our Solar System 2 – The Planets

0.4 0.7 1.0 1.6 2.8 5.2 10 19.6 The planets then known fell at distances: 0.39 0.72 1.0 1.52 ?

5.2 9.5 19.2 This suggested that a planet might exist at a distance of ∼2.8 AU.

Near-Earth Objects

Through gravitational interaction within the main belt of asteroids, occasion- ally an asteroid can acquire an orbit which will bring it into the inner Solar System. Should they come within the orbit of the Earth, they have the potential to impact the Earth’s surface. Such asteroids, along with comets and meteoroids (⬍50 m across), which come close to the Earth are termed Near-Earth Objects (NEOs). They are usually only spotted as they pass close to the Earth. NASA has

a mandate to fi nd all NEOs that have a diameter greater than 1 km, as these have the potential to cause catastrophic local damage and even global effects. By the end of 2007, approximately 800 above this size had been detected out of an expected 1000. Nearly 5000 NEOs have been detected including over 60 near- Earth comets. Of these, around 800 have been classifi ed as potentially hazard- ous. The hope is that should a NEO be discovered that might impact the Earth at some time in the future it would be possible to alter its orbit suffi ciently to miss the Earth.

In June 1908, a giant explosion, 1000 times greater than the Hiroshima atomic bomb, occurred close to the Tungusta river in northern Siberia. It is though that a comet or asteroid of about 50 m diameter exploded above the

ground – felling over 800 million trees in an area of over 2000 km 2 . If the impact had occurred 4 h and 47 min later, the city of St Petersburg (Leningrad) would have been destroyed. It is estimated that there are over 200 000 NEOs of comparable size and the probability of an impact of any one with Earth is of the order of one in 100 million per year. One such impact might thus be expected every 500 years so that there is a 1 in 500 chance of an impact in any one year. It is quite likely that such an impact would be into the oceans causing a tsunami which would have repercussions over a signifi cant part of the globe.

As of 2008, the most potentially serious threat to Earth is from Asteroid (29075) 1950 DA. It has a diameter of ∼1 km and a 1 in 300 chance of hitting the Earth on March 16, 2880.

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317.8 times that of Earth

Radius (equatorial)

71.492 km

11.21 times that of Earth

Radius (polar)

66 854 km

10.52 times that of Earth

Rotation period

9.925 h

Axial tilt

Semi-major axis of orbit 778 547 199 km

5.2 AU

Orbital period

4334.5 days

Average orbital speed

13.07 km s

Eccentricity

Orbital inclination

With Saturn, Uranus and Neptune, Jupiter is one of the gas giants of the Solar System and its mass exceeds that of all the other planets combined by two and a half times. Its interior mass is primarily made up of hydrogen (∼71%) and helium (24%) with ∼5% of heavier elements. Its composition thus closely follows that of the solar nebula from which it was formed. Interestingly, if Jupiter were to acquire more mass, its diameter would actually decrease, so it is about as large as a planet of its composition could be.

Jupiter is thought to consist of a dense core surrounded by a layer of liquid metallic hydrogen lying under an outer layer, about 1000 km thick, composed very largely of molecular hydrogen. Jupiter is perpetually covered with a cloud layer about 50 km thick. The clouds are composed of ammonia crystals arranged into bands of different latitudes made up of light coloured zones between darker belts. The orange and brown colours in the Jovian clouds are caused by compounds containing phosphorus and sulphur exposed to ultraviolet light from the Sun. At differing latitudes, the darker clouds so formed deeper within the atmosphere are masked out by higher clouds of crystallizing ammonia producing the pale zones seen between the belts (Figure 3.16).

The Great Red Spot

Wind speeds of up to 100 m s are common in the atmosphere and opposing cir- culation patterns caused, in part, by Jupiter’s rapid rotation rate cause storms and turbulence in the atmosphere. The belts and zones are seen to vary in colour and form from year to year, but the general pattern remains stable. The best known fea- ture in the atmosphere is undoubtedly the Great Red Spot. It is a persistent anticy- clonic storm, more than twice the diameter of the Earth, which has been observed

Our Solar System 2 – The Planets

Figure 3.16 Jupiter as imaged by the Hubble Space Telescope showing the Equatorial Bands and the Great Red Spot below and to the left of which is a new feature nicknamed ‘Red Spot Junior’. Image: NASA, ESA, A. Simon-Miller (Goddard Space Flight Centre), I. de Pater, M. Wong (UC Berkeley).

since at least 1831. It rotates in an anticlockwise direction with a rotation period of about 6 days and is thought to be stable and so has become a permanent, or at least a very long term feature of the Jovian atmosphere. It is not, however, fi xed in position, and though staying at latitude 22° south has moved around the planet several times since it was fi rst observed. Similar, but smaller, features are common, with white ovals of cool clouds in the upper atmosphere and warmer brown ovals lower down. These smaller storms can sometimes merge to form larger features, as happened in 2000 when three white ovals, fi rst observed in 1938, combined into one. In the following years its colour has reddened and it has been nicknamed ‘Red Spot Junior’.

The rings of Jupiter

Jupiter has a very faint planetary ring system composed of three main segments: an inner halo, a brighter main ring, and an outer ‘gossamer’ ring having two dis- tinct components. They appear to be made of dust with the main ring probably made of material ejected from the satellites Adrastea and Metis as a result of

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meteorite impact. Jupiter’s strong gravitational pull prevents the material falling back onto their surfaces and they gradually move towards Jupiter. It is thought that the two components of the gossamer ring are produced in similar fashion by the moons Thebe and Amalthea.

The Comet Shoemaker-Levy 9 impact

Perhaps the most exciting event in recent Jovian history was when the fragments of Comet Shoemaker-Levy 9 impacted on its surface in July 1994 (Figure 3.17). It is thought that Jupiter had captured the comet in the late 1960s or early 1970s so it had become a temporary satellite of Jupiter orbiting it once every 2 years or so. It appears that the comet had passed within 40 000 km of Jupiter’s surface on July 7, 1992. This distance is within what is called the Roche limit of the planet within which the gravitational forces acting on the comet were able to break it up into a number of fragments. If a body has a particular size, the force of gravity acting on those parts of the body furthest from the centre of mass of a nearby planet will be less than that on the nearer parts. A differential ‘tidal’ force thus acts across the body. If this force is greater than the gravitational forces keeping the body whole, the body will break up into smaller parts. As Jupiter is very mas- sive, its tidal forces are very great and also have a signifi cant effect on Jupiter’s innermost Galilean moon, Io.

The comet was discovered on the night of March 24, 1993 by Carolyn and Eugene Shoemaker and David Levy in a photograph taken with the 0.4 m Schmidt

Figure 3.17 The impact sites where fragments of the Comet Shoemaker-Levy 9 impacted on

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Telescope at the Mount Palomar Observatory in California. (They were conducting

a programme of observations designed to uncover NEOs.) High resolution images, such as that made by the Hubble Space Telescope, showed a number of fragments (labelled A to W) which ranged in size from a few hundred metres up to 2 km. It is thought that the initial diameter of the comet would have been about 5 km. As the orbits of the fragments were refi ned, it became apparent that they were very likely to impact on the surface of Jupiter the following year with the individual impacts to be spread out over just under

a week.

Further observations showed that the impact sites would lie just over Jupiter’s limb, so would not be directly visible from Earth. However, in the same way that a nuclear explosion creates a giant mushroom cloud that rises up into the

atmosphere, the Hubble Space Telescope was trained on the limb in the hope of observing a fi reball above the impact site. The spacecraft Galileo, en route to Jupiter, was in a position to image the impact site and on July 16, 1994 detected a fi reball which had a peak temperature of 24 000 K. Observations from Earth soon detected the plume from the fi reball that reached over 3000 km above the Jovian atmosphere.

The material thrown up into space soon fell back to the surface and formed dark spots, similar in size to the Earth, on the surface. They were easily visible with a small telescope and the author will never forget his fi rst sight of the pair of giant spots produced by the F and G fragments. The latter produced a spot some

12 000 km across and is thought to have released the equivalent of 6 000 000 t of TNT! Over 6 days, 21 discrete impacts were observed (the last on July 22 when fragment W struck the planet). The visible effects of these impacts highlighted the effects such events would have should the Earth, not Jupiter, be the target and spurred on the programmes that were already in place to detect those asteroids that might be a threat to the Earth.

Jupiter’s Galilean moons

Even a very small telescope can detect the four major moons of Jupiter as they weave their way around it. In order of distance from Jupiter, they are called Io, Europa, Ganymeade and Callisto and are comparable in size with our Moon

(Figure 3.18). Discovered by Galileo in 1610, they showed him that Solar System objects did not all have to orbit the Sun, giving further evidence for the Copernican model of the Solar System.

Observations in 1676 made by the Danish astronomer Christensen Roemar of the times of their eclipses as they passed behind Jupiter led to the fi rst determination of the speed of light. An eclipse of Io occurs every 42.5 h – the period of its orbit – and it thus provides a form of cosmic clock. However, Roemar

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Figure 3.18 Jupiter’s moon Io showing volcanoes and sulphur dioxide frost. Image: Galileo Project, JPL, NASA.

observed that the 40 orbits of Io during the time that the Earth was moving towards Jupiter took a total of 22 min less than when the Earth was moving away from Jupiter ∼6 months later. The change in apparent period is due to the Dop- pler effect and this enabled him to calculate the ratio of the velocity of light to the orbital speed of the Earth around the Sun. He derived a value for this ratio of ∼9300. As the orbital speed of the Earth is ∼30 km s this gave a value (actually calculated by Christiaan Huygens from Roemar’s observations) for the speed of light of ∼279 000 km s .

The two innermost moons, Io and Europa, are of great interest. Io is the fourth largest moon in the Solar System with a diameter of 3642 km. When high resolution images of Io were received on Earth from the Voyager spacecraft in 1979, astronomers were amazed to fi nd that Io was pockmarked with over 400 volcanoes. It was soon realised that giant tidal forces due to the close proximity of Jupiter would pummel the interior, generating heat and so give Io a molten interior. As a result, in contrast with most of the other moons in the outer Solar System which have an icy surface, Io has a rocky silicate crust overlying a molten iron or iron sulphide core. A large part of Io’s surface is formed of planes covered by red and orange sulphur compounds and brilliant white sulphur dioxide frost. Above the planes, are seen over 100 mountains, some higher than Mt Everest – a strange world indeed.

Our Solar System 2 – The Planets

In contrast, Europa, the sixth largest moon in the Solar System with a diameter of just over 3000 km, has an icy crust above an interior of silicate rock overlying

a probable iron core. The icy surface is one of the smoothest in the Solar System. Close up images show breaks in the ice as though parts of the surface are break- ing apart and then being fi lled with fresh ice. This implies that the crust is fl oating above a liquid ocean, warmed by the tidal heating from its proximity with Jupiter. This could thus conceivably be an abode for life and some ambitious proposals have been made for a spacecraft to land and burrow beneath the ice to investigate whether any life forms are present!