Hubble captures rare, fleeting shadow on Uranus

With the help of NASA’s Hubble Space Telescope, astronomers have captured a rare sight in the solar system — a glimpse of Uranus as one of its moons, Ariel, and its shadow traverse the disk of the planet.

The sight has never been observed before because Uranus travels a large, 84-year orbit around the sun and the orientation of the planet on its polar axis is sideways. The combination of sideways orientation and large orbit make the planet’s seasons last decades and keep unexposed regions of Uranus in frigid darkness for as long as 42 years.

“Uranus has the most extreme seasonal variation of sunlight you could imagine,” says University of Wisconsin–Madison space scientist Lawrence Sromovsky, the leader of the team that made the new observations. “If you lived at the pole of Uranus, you wouldn’t see the sun for 42 years, and then you wouldn’t see darkness for another 42 years.”

The northern hemisphere of Uranus is now nearly fully exposed to the sun — and to detailed imaging for the first time.

In addition to the view of Ariel and its shadow, the new images of the planet show that the southern hemisphere is brighter than the northern hemisphere. This asymmetry in cloud structure, Sromovsky explains, probably arises from the sluggish response of Uranus to solar heating due to its extremely low temperatures.

On Earth, seasonal changes are more modest because the Earth’s rotational axis is tilted much less (only 23.5 degrees compared to Uranus’ 97 degrees). The planet’s southern and northern hemispheres are alternately tipped toward and away from the sun as the Earth moves in its solar orbit. Summer in the United States occurs when the northern hemisphere is tilted toward the sun.

Uranus is now approaching its 2007 equinox when the sun hovers directly over the Uranian equator. It is only near the planet’s equinox when scientists can observe the shadows of the moons in its orbit as they pass between the planet and the sun.

The last time Uranian equinox occurred, according to team member Heidi B. Hammel of the Space Science Institute in Boulder, Colo., was in 1965, when Lyndon B. Johnson was president, a first class postage stamp cost a nickel, the Sound of Music was a cinematic smash — and the Hubble Space Telescope was but a distant vision for astronomers.

The availability of the orbiting space telescope, says Hammel, is, of course, the other reason scientists can now take in the view of Uranus as its numerous satellite moons begin to eclipse the sun and cast shadows on the dense Uranian atmosphere of hydrogen, helium and methane.

And just as the planet is flipped on its side, so are its ring and moon systems, a condition that will provide astronomers with a “ring-plane crossing,” which is when the ring system can be viewed edge-on from Earth.

“The time of equinox and ring-plane crossing,” explains Hammel, “is excellent for observing the faintest rings and smallest satellites of Uranus. The edge-on viewing geometry permits direct observations of the main rings’ thickness and the timing of satellite eclipses.”

“As equinox draws closer, more shadow crossings may be captured in images by Hubble and the Keck Telescope,” a large ground-based telescope on the dormant Hawaiian volcano Mauna Kea, says Kathy Rages, another member of the team based at the SETI Institute in Mountain View, Calif.

Ariel was the first to be seen because it is the nearest to Uranus of the planet’s four large satellites.

Although Ariel, named for a mischievous airy spirit in Shakespeare’s “The Tempest,” is only one-third the size of Earth’s moon, it casts a much larger and sharper shadow than our moon does during a solar eclipse on Earth, notes Sromovsky. “This is a result of Uranus being 20 times the Earth’s distance from the sun, which makes the sun appear 10 times smaller than Ariel, while our moon appears to be about the same angular size as the sun.