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MercuryThe remaining inner planet, Mercury, has not been directly sampled for evidence of volcanic activity. Spectral studies have shown there is a feldspar-rich crust and basaltic lava’s similar to the lunar surface. The regolith has a low albedo, with low abundances of iron and titanium with plagioclase feldspar. The only indication of large-scale volcanic features is a radar bright spot on the unphotographed side of Mercury. This spot may be a large shield volcano 500km across with a small dark centre 70km across. The smooth plains of Mercury formed around 3.8Ga, with two possible origins: volcanic deposits or sheets of impact melt. Colour edges seen on the smooth plains indicate volcanic flows. This is supported by apparent sagging of the volcanic surface layers near the Caloris basin, due to the excess weight of volcanic deposits. Thrust faults and inter-crater plains are evidence for planetary contraction and contemporaneous volcanism. Mantle cooling has shrunk Mercury’s radius by 6-10km. The asthenosphere likely cooled to a solid shroud around the cooling core approximately 1.5Ga.
CryovolcanismCryovolcanism is the dominant form of volcanism in the Outer Solar System. Cryovolcanism involves eruption and surface deposition of low temperature volatiles e.g. ice and ammonia on satellites of the outer planets. The study of this recently discovered form of volcanism is severely limited, but processes involve aqueous as well as silicate materials. Studies of the chemical and physical make-up of the Outer Solar System satellites has shown that ammonia hydrates may comprise 10% of their total mass. If ammonia is present in water ice the melting point of the mixture (~35% NH3) to 174K (~-100oC). This temperature is easily reached by radioactive heating within satellites with a 500-1000km radius, therefore low temperature volatiles can be erupted. Jovian Moons: Europa
Europa is predominantly rocky in overall composition, but its distance from Jupiter compared to Io means it has proportionally more water. The surface of this Jovian moon is covered by water, indicating past differentiation processes have occurred. Europa may have 100km deep-water oceans covered by a few kilometres of ice. The surface of Europa is virtually flat, crossed by an extensive network of graben, indicating extensional tectonic forces. These fractures act as conduits for the forceful injection of water-rich materials. The fractures are a result of lesser tidal interactions, as seen on Io in an extreme form. Tidal heating may be presently causing the upwelling of ice-slush, producing the bright strips of ‘clean’ ice seen in the bottom of graben. The injection of ice-slush is further supported by the implication of a young surface (100 million years), as determined by crater counts. Resurfacing would occur via geysers and volcanoes of liquid water disgorging onto the surface. Jovian Moons: Ganymede
A complex history of dark mantle volcanism ended 3.8Ga, followed by a different style of volcanism producing lighter grooved terrain. Several theories have been put forward to explain the different processes, the most likely one developed by Murchie and Head in 1988 (in: Planetary Volcanism, by Peter Cattermole): - Old, low albedo material erupted as a cool, silicate-bearing water-ammonia liquid mixture. The younger, higher albedo material erupted as a higher temperature, silicate-bearing water-rich liquid. - Older materials darken due to the sublimation rate of low-pressure ammonia hydrate, leaving the silicate material behind. Neptunian Moon: TritonVoyager 2 reached the moon of Neptune Triton in 1989, finding the 2700km body covered by nitrogen, carbon monoxide and methane clathrates. The small number and mainly degraded craters on Triton indicates cryovolcanic processes operate. Many small vents and smooth plains of clathrate ice were imaged, along with four large ice calderas with pits and flows concentrated in the centre. Stereoscopic views of the moon’s surface led to the discovery of two horizontal plumes stretching west of 8km dark eruption columns. Another six plumes were promptly discovered, but all located in the polar regions 40-60ºS, presently facing the distant Sun. Cryovolcanism on Triton therefore must be in some way linked to solar energy. This would not be enough to alone drive geysers, therefore other heating mechanisms must be involved, e.g. a frozen nitrogen covering on the moon acts as a transmitter for sunlight, which is then converted to infrared light by carbon impurities in the ice. Infrared light wavelengths become trapped, thereby raising the temperature below the surface and causing high pressure blow-outs of nitrogen. Since 1989, surface pressure at 50km has risen from 14 microbars to 20-45 microbars in 1997, due to the effects of a temperature increase of 1-2K. The temperature increase may be a result of global warming, geyser eruption, frost pattern change, or the movement of subsolar latitude from 45-50ºS. Triton is clearly a dynamic world, needing much more attention along with many of the satellites of the Outer planets.
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30/09/2003
This website has been researched and created by Beverley Coldwell. If you have any questions, comments or problems email beverleycoldwell@yahoo.co.uk |
Ganymede
displays a mix of old, dark, cratered, heavily fractured terrain and younger,
lighter, grooved terrain, composing 60% of the surface.
