Huge Space Lake Confirmed on Saturn's Moon Titan
Anne Minard
for National Geographic News
July 31, 2008
Scientists have confirmed the presence of a suspected big, black, glassy lake on Saturn's moon Titan, adding to the growing body of evidence that the giant satellite is oddly Earthlike.
Titan is thought to host mountains, desert-like flats, rivers, lakes and—possibly—an underground ocean.
Whereas water dominates climate cycles on Earth, weather on Titan—where surface temperatures can be -300 degrees Fahrenheit (about -200 degrees Celsius)—revolves around thick, dark hydrocarbons.
Methane and ethane fill the air with a dirty haze that rains down as ash, and rivers run with a gasoline-like liquid.
And now scientists know the hydrocarbons collect in black lakes with surfaces as smooth as glass—perhaps as many as a hundred lakes and seas in the north pole region alone.
An instrument aboard the Cassini orbiter, called the visual and infrared mapping spectrometer, or VIMS, confirmed the presence of the lake called Ontario Lacus when the craft detected ethane.
"Detection of liquid ethane in Ontario Lacus confirms a long-held idea that lakes and seas filled with methane and ethane exist on Titan," said Larry Soderblom, a study co-author from the U.S. Geological Survey in Flagstaff, Arizona.
The study, led by Robert Brown at the University of Arizona in Tucson, appears in today's issue of the journal Nature.
Murkier Than Mud
Cassini—along with its Huygens probe, which detached and landed on Titan's surface in 2005—is part of a cooperative mission of NASA, the European Space Agency, and the Italian Space Agency.
Previous observations of the lake with the Cassini radar imager, as far back as 2005, were unable to fully distinguish the lake's liquid hydrocarbons from the atmosphere's thick hydrocarbon haze. (See "First Space Lakes Found on Saturn Moon" [July 27, 2006].)
But Cassini's VIMS camera uses spectroscopy—a technique that identifies the chemical composition of objects by the way they reflect light.
Ethane is a simple hydrocarbon produced when ultraviolet light from the sun breaks up ethane's parent molecule. The ethane in the lake is mixed with methane, nitrogen, and other hydrocarbons, which are also dominant in the atmosphere.
The lake is about 150 miles (235 kilometers) long—just a bit larger than Lake Ontario.
And it looks weird, even for researchers who knew what to expect.
"It was hard for us to accept the fact that the feature was so black when we first saw it," study leader Brown said. "More than 99.9 percent of the light that reaches the lake never gets out again."
No Ocean
Jonathan Lunine, a University of Arizona planetary scientist, proposed back in 1983 that Titan would be awash in global oceans of ethane and other light hydrocarbons.
That was the only way to account for all the methane and ethane that would have been produced over the 4.5 billion years of solar system history, he suggested.
But 40 close flybys of Titan by the Cassini spacecraft have shown that no such oceans exist.
"The beautiful work described by Brown et al actually strengthens the basic premise that methane is being converted to ethane, which in turn is deposited on Titan's surface," said Lunine, who was not involved in the new study.
"But the lakes … do not hold enough ethane to account for what can be produced over the age of the solar system. So we still have a mystery here."
Study co-author Larry Soderblom said indications have pointed to an underground ocean, though that has not been confirmed.
"Everybody can be right at once," he said.
Endless Summer
Retired University of Arizona scientist Martin Tomasko dedicated much of his career to the Huygens probe.
He called the new results "quite spectacular," adding that they "provide crucial information about the methane-ethane 'hydrological' cycle on Titan."
Study co-author Larry Soderblom says even more will be revealed in the coming years. Titan tilts like the Earth, and it has seasons like Earth. But a year there equals 16 Earth years, he said.
"Over the next several years, those lakes will move into the sunlight," Soderblom said, adding that the Cassini mission was originally designed to end this year but now has a good chance of an extension.
"We're now looking at the possibility of Cassini operations going well into the next decade," he said. "That would allow us to study these lakes in a time-lapse sort of way."
Soderblom expects the lakes region could become even weirder—revealing seasonal changes in composition, storms, and even seasonal climate cycles that transfer liquids among different regions on Titan—or even between the surface and subsurface.
Already, the VIMS observations suggest Ontario Lacus is evaporating, and ringed with a dark beach.
"There may be a whole new chapter," he said.
Anne Minard
for National Geographic News
July 31, 2008
Scientists have confirmed the presence of a suspected big, black, glassy lake on Saturn's moon Titan, adding to the growing body of evidence that the giant satellite is oddly Earthlike.
Titan is thought to host mountains, desert-like flats, rivers, lakes and—possibly—an underground ocean.
Whereas water dominates climate cycles on Earth, weather on Titan—where surface temperatures can be -300 degrees Fahrenheit (about -200 degrees Celsius)—revolves around thick, dark hydrocarbons.
Methane and ethane fill the air with a dirty haze that rains down as ash, and rivers run with a gasoline-like liquid.
And now scientists know the hydrocarbons collect in black lakes with surfaces as smooth as glass—perhaps as many as a hundred lakes and seas in the north pole region alone.
An instrument aboard the Cassini orbiter, called the visual and infrared mapping spectrometer, or VIMS, confirmed the presence of the lake called Ontario Lacus when the craft detected ethane.
"Detection of liquid ethane in Ontario Lacus confirms a long-held idea that lakes and seas filled with methane and ethane exist on Titan," said Larry Soderblom, a study co-author from the U.S. Geological Survey in Flagstaff, Arizona.
The study, led by Robert Brown at the University of Arizona in Tucson, appears in today's issue of the journal Nature.
Murkier Than Mud
Cassini—along with its Huygens probe, which detached and landed on Titan's surface in 2005—is part of a cooperative mission of NASA, the European Space Agency, and the Italian Space Agency.
Previous observations of the lake with the Cassini radar imager, as far back as 2005, were unable to fully distinguish the lake's liquid hydrocarbons from the atmosphere's thick hydrocarbon haze. (See "First Space Lakes Found on Saturn Moon" [July 27, 2006].)
But Cassini's VIMS camera uses spectroscopy—a technique that identifies the chemical composition of objects by the way they reflect light.
Ethane is a simple hydrocarbon produced when ultraviolet light from the sun breaks up ethane's parent molecule. The ethane in the lake is mixed with methane, nitrogen, and other hydrocarbons, which are also dominant in the atmosphere.
The lake is about 150 miles (235 kilometers) long—just a bit larger than Lake Ontario.
And it looks weird, even for researchers who knew what to expect.
"It was hard for us to accept the fact that the feature was so black when we first saw it," study leader Brown said. "More than 99.9 percent of the light that reaches the lake never gets out again."
No Ocean
Jonathan Lunine, a University of Arizona planetary scientist, proposed back in 1983 that Titan would be awash in global oceans of ethane and other light hydrocarbons.
That was the only way to account for all the methane and ethane that would have been produced over the 4.5 billion years of solar system history, he suggested.
But 40 close flybys of Titan by the Cassini spacecraft have shown that no such oceans exist.
"The beautiful work described by Brown et al actually strengthens the basic premise that methane is being converted to ethane, which in turn is deposited on Titan's surface," said Lunine, who was not involved in the new study.
"But the lakes … do not hold enough ethane to account for what can be produced over the age of the solar system. So we still have a mystery here."
Study co-author Larry Soderblom said indications have pointed to an underground ocean, though that has not been confirmed.
"Everybody can be right at once," he said.
Endless Summer
Retired University of Arizona scientist Martin Tomasko dedicated much of his career to the Huygens probe.
He called the new results "quite spectacular," adding that they "provide crucial information about the methane-ethane 'hydrological' cycle on Titan."
Study co-author Larry Soderblom says even more will be revealed in the coming years. Titan tilts like the Earth, and it has seasons like Earth. But a year there equals 16 Earth years, he said.
"Over the next several years, those lakes will move into the sunlight," Soderblom said, adding that the Cassini mission was originally designed to end this year but now has a good chance of an extension.
"We're now looking at the possibility of Cassini operations going well into the next decade," he said. "That would allow us to study these lakes in a time-lapse sort of way."
Soderblom expects the lakes region could become even weirder—revealing seasonal changes in composition, storms, and even seasonal climate cycles that transfer liquids among different regions on Titan—or even between the surface and subsurface.
Already, the VIMS observations suggest Ontario Lacus is evaporating, and ringed with a dark beach.
"There may be a whole new chapter," he said.
