The Voyager 2 space probe was launched from Cape Canaveral in August 1977. It flew by Uranus and sent back over 1,600 close-up images in January 1986. Because computing power in 2014 far exceeds that which was available 28 years ago, an astronomer has been able to go back and re-examine the images. He found that a strange rotational pattern in the atmosphere likely denotes unusual characteristics within the planet. The research was performed by Erich Karkoschka of the University of Arizona, who presented his work on Wednesday at the meeting of the Division for Planetary Science of the American Astronomical Association in Tucson.
When the images of the seventh planet were first transmitted back to Earth, scientists were only able to identify eight features from the southern hemisphere of the powder blue gas giant. Subsequent observations with the Hubble Space Telescope didn’t yield much additional information either. Using the Voyager images, Karkoschka found that the southern hemisphere of Uranus rotates in a way that is unlike anything that has been seen in other gas giants of our solar system.
"Some of these features probably are convective clouds caused by updraft and condensation," Karkoschka said in a press release. "Some of the brighter features look like clouds that extend over hundreds of kilometers.”
Uranus, as seen by Voyager 2 in 1986. Credit: NASA/JPL
Astronomers have been analyzing the rotation of gas giants through features of their atmosphere for 350 years. By tracking the movement of clouds and storms on these planets, scientists can deduce wind speed and calculate the rotation.
"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 explained. "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’s work fills in gaps that had existed about Uranus’ rotation, though they were highly unusual. No existing theories or models could explain why the southern hemisphere would be rotating so differently. He attributed the difference to anomalies within the planet’s interior that are hidden from view, but can be discerned from the atmosphere. Though what those odd features could be isn’t known yet, the data from the planet’s rotation can help eliminate some models and narrow down the list of possibilities until more testing can be done.
Going back and using the old Voyager images was done mostly out of necessity. Due to the planet’s tilt causing incredibly long seasons, the southern hemisphere has been out of Earth’s view since 2007 and won’t be seen again until 2050. Since he couldn’t make new observations himself, Karkoschka used the data that was available. Because advances in technology allow scientists to look at old data with a new approach, this could be an economical solution to answering many questions about the outer planets.
"The computer memory necessary to process 1,600 images was not available at the time Voyager took these images," Karkoschka concluded. "As computers and calibration methods get better, we can now do this kind of work, at a tiny fraction of the cost necessary to send a spacecraft to a planet.”