FROST has been seen vaporising on Pluto for the first time, though the pictures of it were taken in the 1930s, soon after the dwarf planet was discovered.
A team led by Bradley Schaefer from Louisiana State University in Baton Rouge measured Pluto鈥檚 brightness on 32 photographic plates taken at US observatories in 1933 and 1934. Using modern techniques, the team was able to measure Pluto鈥檚 brightness far more accurately than at the time.
Comparing the results with observations from the early 1950s, the team showed that Pluto darkened by about 5 per cent in the intervening years. The results will be reported in the journal Icarus.
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Pluto鈥檚 brightness always appears to be changing because its colour is uneven and its rotation axis is highly tilted, so our view of it gradually changes from its north pole to its south. But in the early 1930s and early 1950s the same southern latitudes faced Earth, so geometry can鈥檛 explain the darkening.
The finding suggests Pluto鈥檚 thin atmosphere froze when it was farthest from the sun in the 1930s, then the bright methane and nitrogen frost vaporised as it moved closer in the 1950s, as had been expected. 鈥淏ut now we鈥檝e actually got the smoking gun,鈥� says Schaefer.
