Sunday 23 March 2014

The origin of Ceres.

Ceres is the largest body in the Main Asteroid Belt, comprising roughly 1/3 of all the mass of the belt. It has the designation (1) Ceres, indicating that it was the first asteroid discovered  (by Giuseppe Piazzi in 1801), but has recently been declared to be a Dwarf Planet, due to its large size, a designation that places it in the same class of bodies as the Trans-Neptunian Objects Pluto, Haumea, Eris and Makemake. As such it is the subject of considerable interest to planetary scientists, and was one of two bodies chosen to be visited by NASA’s Dawn Mission, along with (4) Vesta, the second largest body in the Main Asteroid Belt.

The results of the Dawn Mission have revealed striking differences between the two bodies, with Vesta having a subspherical shape and a cratered, volcanic surface (much as was expected from an asteroid), but Ceres has an (unexpected) smooth, icy surface and a more-or-less spherical shape. Moreover Ceres is considerably less dense than Vesta (at 2.077 g cm¯³ compared to 3.456 g cm¯³ for Vesta), suggesting that the ice forms a significant proportion of its mass, rather than simply being a thin surface layer, and several points of cryovolcanic activity, where water vapour is being released from the surface at a rate of about 6 kg s¯¹ have been discovered.

The surface of Ceres. NASA/JPL/Dawn Mission.

The surface of Vesta. NASA/JPL/Dawn Mission.

In a paper published on the arXiv online database at Cornell University Library on 20 March 2014, Yury Rogozin of the VEDA LLC in Moscow speculates that Ceres may have begun it existence not as a Main Asteroid Belt object, but as the moon of a now destroyed planet beyond the snowline of the early Solar System (the snowline being the point beyond which it was cool enough for water-ice to form, not possible within the inner Solar System due to the heat from the early Sun), and that it may have reached its current position by interaction with the gravity of the giant planet Jupiter.

Rogozin cites as evidence of this the theory that the planets Mercury and Mars may also have started out as the satellites of larger bodies (a theory which is not currently widely supported among planetary scientists). That theory goes something like this: Mercury and Mars are significantly smaller than the other two rocky planets, Earth and Venus, but are of comparable size to the larger moons of the Solar System, such as Earth’s Moon, the four Galilean moons of Jupiter, Titan etc. Furthermore Mercury and Mars have greater orbital eccentricities than any other planets in the Solar System (i.e. their distance from the Sun varies more than that of other planets). This theory speculates that Mercury is an escaped moon of Venus, and that Mars was formerly a moon of the planet Phaeton, which existed within what is now the Main Asteroid Belt, but which was destroyed by the gravitational influence of Jupiter early in the history of the Solar System.

Rogozin reasons that Mercury is in a 5:2 orbital resonance with Venus (i.e. it completes five orbits four every two orbits of Venus), and Mars is in a 5:2 resonance with the (hypothetical) orbit of the former planet Phaeton. Therefore Ceres could be in a 5:2 orbital resonance with another now destroyed planet, which Rogozin names Yurus, which would therefore have had a semi major axis (average orbital distance from the Sun) of 5.0951 AU (i.e. 5.0951 times the distance at which the Earth orbits the Sun), and an orbital period of 11.5 years.

The orbit of Ceres. JPL Small Body Database Browser.

Rogozin further suggests that the destruction of a large icy planet in such an orbit might account for the large volumes of water present on Earth and now believed to formerly have been present on Mars, both planets which are thought to have formed within the snow line, and which might therefore be expected to be largely waterless.

While the idea that Ceres may have formed beyond the early Solar System’s snow line has some merit, the existence of the planet Yurus seems highly speculative. The separation of Mercury and Mars from the other rocky planets as moon-like objects is not widely supported among planetary scientists. While Mercury is of similar size to several moons, Mars is in fact of intermediate size between these bodies and the larger rocky planets, and studies of other stellar systems have revealed a variety of rocky planets of intermediate sizes. Therefore most planetary scientists now either regard the four rocky planets as a discreet group, or use a grouping of ‘rocky worlds’ which includes the four planets, plus the fifteen largest moons in the Solar System.

The destruction of a large icy planet at a distance of 5.0951 AU from the Sun would be easy to explain, due to the closeness of such a planet to the orbit of Jupiter, a body which will excerpt considerable tidal stress on any nearby body, which has a semi major axis of 5.204267 AU, and which at its perihelion (the closest point in its orbit to the Sun) is only 4.950429 AU from the Sun; however the formation of a planet in such a position would require considerable explanation for the same reason, and explanation that Rogozin does not provide. The presence of water on Earth and Mars is more usually explained by hypothesizing a large number of comet impacts during the early history of the Solar System (the Early Bombardment Theory); comets that are thought to have formed in the outer parts of the Solar System, safely beyond the snow line.

Furthermore the speed at which a body orbits the Sun, and its distance from the Sun, are usually thought to be connected, with bodies that accelerate or slow in their orbits correspondingly moving towards or away from the Sun. Orbital resonances are usually explained by the exchange of inertia between bodies. A faster body approaching a slower body in a similar orbit will impart some of its inertia to it via tidal exchange, causing the slower body to accelerate and the faster body to slow down. The bodies will continue to exchange energy each time they pass, with one body accelerating and the other slowing each time they pass, until they reach a stable resonance. 

Several bodies within the Solar System (and in other known planetary systems) are in such resonances, most notably the three inner Galilean moons of Jupiter, which have a 4:2:1 orbital resonance. Where two bodies are in similar orbits but cannot reach a stable resonance, it is predicted that one of them will be expelled into a quite different orbit. Thus an origin of Ceres as a fifth large moon of Jupiter, unable to form a stable resonance with the other four Galilean moons and therefore expelled from the Jovian system by tidal forces, would present an alternative theory for the origin of Ceres (and an equally hypothetical one). The presence of an icy body in the Jovian system requires no explanation, as Jupiter is beyond the snow line, and has several icy moons.

The icy surface of the Jovian moon Europa. NASA/Galileo.

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