No one ever said that Uranus was anything but a strange world. Nineteen times farther from the Sun than the Earth, the planet’s equator is tilted 98 degrees from its orbital plane. The tilt is so profound that if you work out the averages, the Uranian poles get more sunlight than the equator. That could lead to interesting weather patterns on a world with an 84-year orbit where seasons last twenty-one years. Such seasonal subjects have been the subject of recent study using imagery from the Keck II instrument in Hawaii, the results presented at the Division for Planetary Sciences meeting this week in Ithaca, NY.
Uranus reached equinox in 2007 when the Sun attained a position directly over the planet’s equator. Having equal amounts of sunlight over northern and southern hemispheres is obviously not a routine occurrence for this planet, but it’s a good chance to look at what’s happening on the meteorological front. Lawrence Sromovsky (University of Wisconsin) notes that seasonal weather changes are driven by the variance in solar energy caused by the tilt of a planet on its axis (the term for this is ‘seasonal forcing’). But the data gathered by Keck shows that Uranus features a unique lag in responding to that input. Says Sromovsky:
“Although both hemispheres were symmetrically heated by sunlight at equinox, the atmosphere itself was not symmetric, implying that it was responding to past sunlight instead of current sunlight, a result of Uranus’s cold atmosphere and long response time.”
Cold indeed. We’re talking about atmospheric temperatures at the cloud tops of minus 215 degrees Celsius. We’re also talking about winds that can reach speeds of 900 kilometers per hour. The image below displays recent atmospheric features and their changes:
Image: Near-infrared images from the Keck II telescope show the planet Uranus in 2005 (left), with the rings at an angle of 8 degrees, and at equinox in 2007 (right pair), with the planet’s ring system edge-on. In all images, the south pole is at the left and the equator is directly below the rings. Credit: Imke de Pater, University of California, Berkeley; Heidi Hammel, Space Science Institute; Lawrence Sromovsky and Patrick Fry, University of Wisconsin-Madison. Obtained at the Keck Observatory, Kamuela, Hawaii.
Notice the cloud structure in the planet’s southern hemisphere (at left of the image and near the bright band at bottom), which may be dissipating as it drifts north, a motion that is probably the result of seasonal change. The vortex has evidently been in existence for years, if not decades, at between 32 degrees and 36 degrees south latitude. At the same time, the bright feature in the northern hemisphere in the 2005 image seems to correspond to the smaller bright area in the rightmost 2007 image. Note that the southern band is not as bright as it was in 2005, while a new northern band is now brightening. The expectation is that the bands will completely reverse by the time of the next equinox.
…and then consider that there isn’t any reason why extrasolar terrestrial planets could not have obliquities similar to that of Uranus, which would certainly cause some interesting climate/habitability issues.
It often seems to me that Uranus has been rather unfairly labeled as a bland, quiescent and uninteresting place (with the possible exception of Miranda). I guess we can blame Voyager arriving at the wrong time for that. Images like this continue to help dispel this notion.
I still dont holdout much hope of any spacecraft returning in our lifetime though, Neptune has Triton, and I imagine that is always going to be a deciding factor :(
P
Thursday, July 9, 2009
“The Age of Wonder”…new book
The Age of Wonder: How the Romantic Generation Discovered the Beauty and Terror of Science
by Richard Holmes
ISBN-10: 0007149522
ISBN-13: 978-0007149520
“When Poets Were Scientists and Nature Their Mysterious Muse”
by Janet Maslin
July 9, 2009
The New York Times
William Herschel, the German-born, star-gazing musician who effectively doubled the size of the solar system with a single discovery in 1781, was not regarded as a scientist. That word had not been coined during most of the era that will now be known, thanks to Richard Holmes’s amazingly ambitious, buoyant new fusion of history, art, science, philosophy and biography, as “The Age of Wonder.” And Mr. Holmes’s excitement at fusing long-familiar events and personages into something startlingly new is not unlike the exuberance of the age that animates his groundbreaking book.
In Herschel’s day (and that of his sister, Caroline, who functioned as his doting assistant to the point of feeding him like a baby bird), science was deductively methodical. And astronomy was no amateur’s game. But Herschel charted the skies as if making musical notations. And when he lacked instruments with enough precision, he painstakingly invented a telescope with startling new powers of magnification.
Looking through it, he noted a starlike object, twice as far from the Sun as Saturn, that appeared to be moving yet did not have a comet’s tail. He identified this as the planet Georgium Sidus, first named for George III of Britain but later known as Uranus. (Mr. Holmes is much too spirited a writer to resist making a bon mot about the English pronunciation of that name.)
Beyond enlivening the story of Herschel’s discovery into a gripping narrative, this book speculates fascinatingly about the ramifications of such a breakthrough. Thanks to Herschel the idea of a fixed universe was challenged, replaced by a cosmos in flux. Was that cause for wonder or terror? What were its theological implications? How would it influence a future generation of poets? (The thrill of this breakthrough would later figure in one of Keats’s most famous sonnets, “On First Looking Into Chapman’s Homer.”) Where would it figure in the relay race of scientific discoveries?
Full review here:
http://philosophyofscienceportal.blogspot.com/2009/07/age-of-wondernew-book.html
http://www.technologyreview.com/blog/arxiv/24477/
Thursday, December 03, 2009
Collision-Free Theory Explains Why Uranus Is Lying on Its Side
Astronomers have always assumed that Uranus must have been knocked onto its side by a collision. Now a new idea suggests that the planet’s remarkable tilt could have another explanation.
One of the great mysteries of our Solar System is why Uranus is tilted on its side. Surely, if the solar system formed from the same rotating cloud of dust and gas, then all the bodies within it should rotate in the same way. And yet Uranus’ axis of rotation lies at 97 degrees to the plane of the solar system.
The standard explanation is that Uranus must have been involved in some kind of interplanetary collision with and earth-sized protoplanet in the early days of the solar system. That’s a tempting idea but it has some shortcomings. For example, it doesn’t explain why the orbits of the moons of Uranus are similarly tilted, not that of its rings.
Today, Gwenael Boue and Jacques Laskar at the Observatoire de Paris in France put forward another idea. They say that Uranus may have become tilted during the period soon after formation when the planets were migrating to the orbits we see now. They point out that the presence of satellites around a planet can increase its rate of precession, if it has a high initial inclination of more than say 17 degrees.
This increase can be by as much as a factor of 1000 if the mass of the moon and the radius of its orbit have certain values. For Uranus, this is for a moon of 0.01 Uranian mass and at 50 Uranian radii.
The problem, of course, is that Uranus does not have such a moon. Its most distant companion is Oberon with a mass of just 10^-5 Uranian masses and an orbit of 23 Uranian radii.
Boue and Laskar’s idea is that Uranus once had a moon of the required size and orbit, which caused the planet to tilt during the planetary migration, but that this moon was ejected during a close encounter towards the end of the migration.
To study whether this idea is feasible, they simulated the process of giant planet migration in the early solar system some 10,000 times. They then discarded all scenarios in which the planets collided or did not end up in the correct final order. They then selected only those outcomes in which Uranus had an inclination of more than 17 degrees and also rejected any simulation in which Uranus came within 50 Uranian radii of another planet, since that would be likely to eject Oberon as well as the additional hypothesised moon. That left 17 simulations.
Boue and Laskar then added the additional moon to see how it would effect the tilt of Uranus and repeated each of these 17 scenarios a further 100 times. In 37 cases, the new moon helped Uranus onto its side and then ended up being ejected after a close encounter with another gas giant.
That’s an interesting result and not just because of the tilt: some models of planet formation predict that Uranus ought to have had another moon (albeit somewhat smaller than the one Boue and Laskar introduce). Consequently, this idea has the elegant property of explaining two mysteries for the price of one, never a bad thing in science.
Ref: http://arxiv.org/abs/0912.0181: A Collissionless Scenario For Uranus Tilting