In 1998 when Voyager 2 flew by the distant planet in 1998, what we saw was one of the most boringly featureless blue planets we could have imagined. Recently, though, it has been surrendering some of its secrets, including a fair amount of information about its atmospheric phenomena, and some strange things going on at its south pole that suggest at an unusual feature in the interior of the planet.
Nobody has sent out a fresh probe to take another look, so where did the new information come from? As it happens, we already had it. By re-analyzing images that NASA’s Voyager-2 spacecraft took 28 years ago, University of Arizona astronomer Erich Karkoschka has teased out hidden features in Uranus’ atmosphere. Mostly this consisted of increasing the contrast on the images Voyager sent back until these features emerged. What Karkoschka found suggests new ideas about the interior structure of gas giants.
The animation at the upper right, provided by Karkoschka, shows Uranus as Voyager-2 saw it during its fly-by in 1986, superimposed with the new look of the giant gas planet as a result of this study. As you can see, Uranus’ southern hemisphere is doing some pretty strange stuff, unlike anything we have discovered so far. Karkoschka presented his findings at the meeting of the Division for Planetary Science of the American Astronomical Association in Tucson.
“Some of these features probably are convective clouds caused by updraft and condensation,” said Karkoschka, a senior staff scientist at the UA’s Lunar and Planetary Laboratory. “Some of the brighter features look like clouds that extend over hundreds of kilometers.”
“What we’re really looking at when we observe the giant planets are their thick atmospheres,” Karkoschka explained. “Cloud features tracking winds move mostly east or west at a speed depending on the latitude. Once we know the wind speed or rotational period at each latitude, we know the circulation of the planet’s atmosphere.”
In 1665, Giovanni Cassini performed the first rotational measurement of a giant planet when he tracked the Great Red Spot on Jupiter. Over the last three and a half centuries, astronomers have pinned down essentially the complete circulation of Jupiter and Saturn and about 75 percent for Uranus and Neptune. Karkoschka’s new work fills in the remaining 25 percent for Uranus.
“All previous observations of the giant planets indicated that these planets rotate in a regular way, meaning the rotational rates in their respective southern and northern latitudes are about the same,” Karkoschka said. “My analysis suggests rotational rates in the high latitudes of Uranus are highly asymmetrical, with some southern latitudes possibly rotating as much as 15 percent faster than their northern counterparts.”
Karkoschka found several sharp kinks in the rotational profile, defying all previous observations and theoretical considerations.
“The unusual rotation of high southern latitudes of Uranus is probably due to an unusual feature in the interior of Uranus,” he said. “While the nature of the feature and its interaction with the atmosphere are not yet known, the fact that I found this unusual rotation offers new possibilities to learn about the interior of a giant planet.”
Astronomers have tried to find clues about the interior of the giant planets, but there hasn’t been a whole lot of information to work with. Gas giants generate radio fields which give us clues about the rotation of their magnetic fields, which is probably indicative of the rotation of the deep interior core. This doesn’t tell us much about the interior structure, though we can gleen a bit more from the gravitational fields. Karkoschka’s detailed rotational measurements of Uranus may help determine the interior structure of Uranus fairly accurately by eliminating some of the proposed models of the planet’s interior.
“Most of the more than a thousand planets discovered around other stars are similar in size to Uranus,” Karkoschka said. “They are too far for us to be able to measure their rotational profiles for the foreseeable future, but with an improved knowledge about Uranus, we might be better able to draw conclusions about their interior structure.”
Uranus is an oddball in the solar system. Its rotational axis is tilted by almost 90 degrees, like a spinning top lying on its side. It orbits the sun about once every 85 years. Uranus’ spring equinox in 2007 marked the beginning of a 43-year long period of darkness for the south pole and its surroundings, hidden from Earth’s view, so we aren’t going to be doing much study of it for the next few decades. Karkoschka dealt with the problem by reexamining the data we already had. The contrast in the images was so low that he had to develop special image processing techniques to find and enhance the features he was looking for. By enhancing the contrast of the images by a factor of 300, he was able to identify enough features on Uranus’ southern hemisphere to get a detailed understanding of the circulation pattern.
Karkoschka’s work shows that even old scientific data can yield new revelations, thanks to the increase in computing power of modern platforms. The 1600 photographic images from Voyager 2 were well beyond the capability of computers at the time they were taken. He hused a similar technique on 13-year-old Voyager images of the space around Uranus’ and discovered a new satellite called Perdita.