At last, the missing storm clouds on Saturn's moon Titan may have been found.
In the last decade, researchers have monitored clouds at both of Titan's poles, where large lakes of methane have been spotted by Earth-based observers and by the Cassini spacecraft, which has been orbiting the moon for the last three years. But the moon's clouds seemed inexplicably confined to those areas.
"We've seen a lot of clouds at the poles. But we'd never seen a major storm at the equator," said Michael Brown, a professor of planetary astronomy at Caltech.
This result has had scientists scratching their heads. When the European probe Huygens parachuted to the surface of Saturn's large moon from NASA's Cassini spacecraft in January 2005, the probe's cameras showed what looked to geologists to be dry channels left by methane rivers cutting through dune fields in the equatorial zone.
If there were no storms at the equator, scientists wondered, what made the river channels? "Some people even suggested maybe these are not rain-carved," Brown said.
Now, Brown and one of his former graduate students, Emily Schaller, have found the first evidence of a storm over Titan's tropical latitudes. Their research, titled "Storms in the tropics of Titan," is published in this week's journal Nature.
"The exciting thing is, this maybe points to an answer" to what caused the channels at the equator, Brown said.
The next question, he added, is: Where did the equatorial clouds come from?
Long neglected, Titan has become a hotbed of scientific research in recent years, primarily because of the discoveries of the Cassini-Huygens mission. It's not only the second-largest moon in the solar system (behind Jupiter's Ganymede) and bigger even than the planet Mercury, it's the only moon with a thick atmosphere. As on Earth, Titan's smog-like atmosphere is mostly nitrogen, with some methane thrown in.
It's also the only object in the solar system other than Earth with stable bodies of liquid on the surface; unlike Earth's water-filled oceans and lakes, however, these are thought to be composed mainly of methane and ethane.
On Earth, methane is a gas used to heat homes. But on Titan, temperatures are so low -- at minus-289 degrees Fahrenheit -- that it flows like water.
Many scientists believe Titan represents a time capsule of what the early Earth may have been like, before life began. Titan could never host life as we know it because it is far too cold.
In April 2008, Schaller was using NASA's Infrared Telescope Facility on Mauna Kea, Hawaii, to track daily weather activity on Titan. One day, after weeks of frustration, she checked the data from the previous night and found that "Titan suddenly had the biggest clouds ever."
After that, Brown, Schaller and their colleagues began tracking the clouds with the giant Gemini telescope on Mauna Kea. Brown said the first cloud appeared in the tropics and rapidly spread around the moon.
The source of the clouds is still a mystery. On Earth, the hydrologic cycle is driven by the evaporation of water from the surface to high altitudes where it cools, condenses into clouds and falls back to Earth.
On Titan, methane plays that role. But it's far from certain, Brown said, that clouds of methane circulate around Titan as they do on Earth.
The methane that formed the clouds at Titan's equator may have come from some explosive event in the equatorial region of the moon itself, such as volcanic activity that sent a "methane burp" hurtling into Titan's orange sky.