A mysterious dark spot has appeared in the clouds of Uranus 鈥� something that has never before been seen clearly on the icy planet. 杏吧原创 do not know what makes it dark and why it is appearing now.
Its more distant neighbour, Neptune, has displayed prominent dark spots from time to time. They also display a puzzling range of behaviours, sometimes moving back and forth in latitude, and sometimes changing shape with time.
They are thought to be related to eddies in the atmosphere, but exactly how the eddies cause dark patches is unclear.
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Now, the Hubble Space Telescope, which has frequently scrutinised the planet since 1994, has seen the first such spot on Uranus. Lawrence Sromovsky of the University of Wisconsin, in Madison, US, led the team that found the spot, which appeared in an image taken on 24 August.
Like the dark spots on Neptune, Uranus鈥檚 spot is probably related to an eddy in the atmosphere, but why it is dark is unclear, Sromovsky says.
鈥淚t could be the spot is a region where there鈥檚 less cloudiness, where we鈥檙e seeing deeper into the atmosphere,鈥� he told New 杏吧原创. 鈥淥r it could be a region of darker materials.鈥�
Dredged up
If it is dark material, it might have been dredged up from deeper in the atmosphere. Alternatively, it might be converging at the spot from higher up in the atmosphere, he says.
The timing of the spot鈥檚 appearance may have to do with the approach of spring in the northern hemisphere, where the spot was found, says team member Heidi Hammel of the Space Science Institute in Boulder, Colorado, US.
Uranus, unlike the solar system鈥檚 other planets, is tilted almost completely on its side, with its rotational axis lying nearly in its orbital plane. So its south pole has been facing the Sun for a couple dozen years during the northern hemisphere鈥檚 summer.
As more of its surface is exposed to the Sun during the lead-up to spring, which will occur in December 2007, its atmosphere is expected to become more active, like that of Neptune.
鈥淲e have hypothesised that Uranus might become more Neptune-like as it approached its equinox,鈥� she says. 鈥淭he sudden appearance of this unusual dark feature suggests we might be right.鈥�
Different wavelengths
The changing circulation patterns may be to blame for the spot, says Sromovsky: 鈥淲hat we might be seeing is an uplifting of some of the eddies that we normally don鈥檛 see because they鈥檙e normally deeper in the atmosphere,鈥� he says.
The team plans to look at Uranus again in October using the Keck telescope on Mauna Kea in Hawaii, US. Sromovsky is not sure whether they will see the spot in those observations, because they will not cover exactly the same wavelengths as Hubble did. 鈥淲e鈥檙e going to certainly look for it though,鈥� he says.
The Hubble images were taken with the High Resolution Channel of the Advanced Camera for Surveys (ACS), which shut down unexpectedly on 23 September (see Hubble鈥檚 key camera shuts down again).
The use of the High Resolution Channel and the rest of the ACS have been suspended while engineers investigate the problem.
