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Poor Pluto, already thrown out of the club of the "normal" planets of our solar system, could also risk loosing the title of dwarf planet, in favour of that of comet. This unexpected (and perhaps exaggerated) conclusion can be reached from results obtained by J.S. Greaves, Ch. Helling and P. Friberg (University of St Andrews, UK, and University Park, USA) with data from the 15 metre, sub-mm, James Clerk Maxwell Telescope in Hawaii. They studied Pluto's tenuous atmosphere.
Pluto has an elliptical orbit which brings it within that of Neptune for a period of 20 years. The last such period was between 1979 and 1999, with perihelion in 1989. The increased heating around this time, and the arrival of a warmer season in regions that had been immersed in freezing darkness for
more than a century, must have provoked an exchange of volatile material between the surface and the atmosphere.
In fact, images taken during recent decades show that the planet's albedo (reflectivity) changes with time.
In 2000, and in subsequent years, an increase in the levels of CO (carbon monoxide) was measured in Pluto's atmosphere, due to the sublimation of ices at the illuminated pole. As the planet should now be cooling down as it moves further from the Sun, it was expected that the volatile materials (including nitrogen, methane and ethane) would rapidly condense onto the other pole, thus being removed from the atmosphere.
But instead, Greaves and colleagues have found the the levels of CO have continued to increase, rather than decrease, and CO has also risen to an altitude of 4500 km from the centre of the planet (4 planetary radii). A possible explanation for this unexpected observation is that the ices of the south pole, after coming out of a night that lasted for 120 years, take less time to sublime than those at the north pole do to condense.
The most interesting discovery, however, regards the slight redshift detected in the CO spectrum, indicating a motion of the gas away from our point of observation, and implying the presence of a comet-like tail. This is caused by the weak solar wind that reaches even these remote regions of the solar system.
What is less clear is whether the material is actually lost, and
subsequently replenished from the surface.
Pluto undoubtedly shows certain similarities to comets, and what with its elliptical and very large orbital radius it's orbit is also similar. NASA's New Horizons probe, due to arrive in 4 years, should answer many of the doubts about this strange body... |
