October 21st 05, 06:02 PM
Caltech News Release
For Immediate Release
October 20, 2005
Cracks or Cryovolcanoes? Surface Geology Creates Clouds on Titan
PASADENA, Calif.-Like the little engine that could, geologic activity
on the surface of Saturn's moon Titan-maybe outgassing cracks and
perhaps icy cryovolcanoes-is belching puffs of methane gas into the
atmosphere of the moon, creating clouds.
This is the conclusion of planetary astronomer Henry G. Roe, a
postdoctoral researcher, and Michael E. Brown, professor of planetary
astronomy at the California Institute of Technology. Roe, Brown, and
their colleagues at Caltech and the Gemini Observatory in Hawaii
based their analysis on new images of distinctive clouds that
sporadically appear in the middle latitudes of the moon's southern
hemisphere. The research will appear in the October 21 issue of the
journal Science.
The clouds provide the first explanation for a long-standing Titan
mystery: From where does the atmosphere's copious methane gas keep
coming? That methane is continuously destroyed by the sun's
ultraviolet rays, in a process called photolysis. This photolysis
forms the thick blanket of haze enveloping the moon, and should have
removed all of Titan's atmospheric methane billions of years ago.
Clearly, something is replenishing the gas-and that something, say
Roe and his colleagues, is geologic activity on the surface. "This is
the first strong evidence for currently active methane release from
the surface," Roe says.
Adds Brown: "For a long time we've wondered why there is methane in
the atmosphere of Titan at all, and the answer is that it spews out
of the surface. And what is tremendously exciting is that we can see
it, from Earth; we see these big clouds coming from above these
methane vents, or methane volcanoes. Everyone had thought that must
have been the answer, but until now, no one had found the spewing
gun."
Roe, Brown, and their colleagues made the discovery using images
obtained during the past two years by adaptive optics systems on the
10-meter telescope at the W. M. Keck Observatory on Mauna Kea in
Hawaii and the neighboring 8-meter telescope at the Gemini North
Observatory. Adaptive optics is a technique that removes the blurring
of atmospheric turbulence, creating images as sharp as would be
obtained from space-based telescopes.
"These results came about from a collaborative effort between two
very large telescopes with adaptive optics capability, Gemini and
Keck," says astronomer Chadwick A. Trujillo of the Gemini
Observatory, a co-author of the paper. "At both telescopes, the
science data were collected from only about a half an hour of images
taken over many nights. Only this unusual 'quick look' scheduling
could have produced these unique results. At most telescopes, the
whole night is given to a single observer, which could not have
produced this science."
The two telescopes observed Titan on 82 nights. On 15 nights, the
images revealed distinctive bright clouds-two dozen in all-at
midlatitudes in the southern hemisphere. The clouds usually popped up
quickly, and generally had disappeared by the next day. "We have
several observations where on one night, we don't see a cloud, the
next night we do, and the following night it is gone," Roe says.
Some of the clouds stretched as much as 2,000 km across the 5,150 km
diameter moon. "An equivalent cloud on Earth would cover from the
east coast to the west coast of the United States," Roe says.
Although the precise altitude of the clouds is not known, they fall
somewhere between 10 km and 35 km above the surface, within Titan's
troposphere (most cloud activity on the earth is also within its
troposphere).
Notably, all of the clouds were located within a relatively narrow
band at around 40 degrees south latitude, and most were clustered
tightly near 350 degrees west longitude. Both their sporadic
appearance and their specific geographic location led the researchers
to conclude that the clouds were not arising from the regular
convective overturn of the atmosphere due to its heating by the sun
(which produces the cloud cover across the moon's southern pole) but,
rather, that some process on the surface was creating the clouds.
"If these clouds were due only to the global wind pattern, what we
call general circulation, there's no reason the clouds should be
linked to a single longitude. They'd be found in a band around the
entire moon," Roe says.
Another possible explanation for the clouds' patchy formation is
variation in the albedo, or brightness, of the surface. Darker
surfaces absorb more sunlight than lighter ones. The air above those
warmer spots would be heated, then rise and form convective clouds,
much like thunderstorms on a summer's day on Earth. Roe and his
colleagues, however, found no differences in the brightness of the
surface at 40 degrees south latitude. Clouds can also form over
mountains when prevailing winds force air upward, but in that case
the clouds should always appear in the identical locations. "We see
the clouds regularly appear in the same geographic region, but not
always in the exact same location," says Roe.
The other way to make a cloud on Titan is to raise the humidity by
directly injecting methane into the atmosphere, and that, the
scientists say, is the most likely explanation here.
Exactly how the methane is being injected is still unknown. It may
seep out of transient cracks on the surface, or bubble out during the
eruption of icy cryovolcanoes.
Although no such features have yet been observed on the moon, Roe and
his colleagues believe they may be common. "We think there are
numerous sources all over the surface, of varying size, but most
below the size that we could see with our instruments," he says.
One large feature near 350 degrees west longitude is probably
creating the clump of clouds that forms in that region, while also
humidifying the band at 40 degrees latitude, Roe says, "so you end up
creating areas where the humidity is elevated by injected methane,
making it easier for another, smaller source to also generate clouds.
They are like weather fronts that move through. So we are seeing
weather, on another planet, with something other than water. With
methane. That's cool. It's better than science fiction."
Images are available upon request. For advance copies of the
embargoed paper, contact the AAAS Office of Public Programs, (202)
326-6440 or .
###
For Immediate Release
October 20, 2005
Cracks or Cryovolcanoes? Surface Geology Creates Clouds on Titan
PASADENA, Calif.-Like the little engine that could, geologic activity
on the surface of Saturn's moon Titan-maybe outgassing cracks and
perhaps icy cryovolcanoes-is belching puffs of methane gas into the
atmosphere of the moon, creating clouds.
This is the conclusion of planetary astronomer Henry G. Roe, a
postdoctoral researcher, and Michael E. Brown, professor of planetary
astronomy at the California Institute of Technology. Roe, Brown, and
their colleagues at Caltech and the Gemini Observatory in Hawaii
based their analysis on new images of distinctive clouds that
sporadically appear in the middle latitudes of the moon's southern
hemisphere. The research will appear in the October 21 issue of the
journal Science.
The clouds provide the first explanation for a long-standing Titan
mystery: From where does the atmosphere's copious methane gas keep
coming? That methane is continuously destroyed by the sun's
ultraviolet rays, in a process called photolysis. This photolysis
forms the thick blanket of haze enveloping the moon, and should have
removed all of Titan's atmospheric methane billions of years ago.
Clearly, something is replenishing the gas-and that something, say
Roe and his colleagues, is geologic activity on the surface. "This is
the first strong evidence for currently active methane release from
the surface," Roe says.
Adds Brown: "For a long time we've wondered why there is methane in
the atmosphere of Titan at all, and the answer is that it spews out
of the surface. And what is tremendously exciting is that we can see
it, from Earth; we see these big clouds coming from above these
methane vents, or methane volcanoes. Everyone had thought that must
have been the answer, but until now, no one had found the spewing
gun."
Roe, Brown, and their colleagues made the discovery using images
obtained during the past two years by adaptive optics systems on the
10-meter telescope at the W. M. Keck Observatory on Mauna Kea in
Hawaii and the neighboring 8-meter telescope at the Gemini North
Observatory. Adaptive optics is a technique that removes the blurring
of atmospheric turbulence, creating images as sharp as would be
obtained from space-based telescopes.
"These results came about from a collaborative effort between two
very large telescopes with adaptive optics capability, Gemini and
Keck," says astronomer Chadwick A. Trujillo of the Gemini
Observatory, a co-author of the paper. "At both telescopes, the
science data were collected from only about a half an hour of images
taken over many nights. Only this unusual 'quick look' scheduling
could have produced these unique results. At most telescopes, the
whole night is given to a single observer, which could not have
produced this science."
The two telescopes observed Titan on 82 nights. On 15 nights, the
images revealed distinctive bright clouds-two dozen in all-at
midlatitudes in the southern hemisphere. The clouds usually popped up
quickly, and generally had disappeared by the next day. "We have
several observations where on one night, we don't see a cloud, the
next night we do, and the following night it is gone," Roe says.
Some of the clouds stretched as much as 2,000 km across the 5,150 km
diameter moon. "An equivalent cloud on Earth would cover from the
east coast to the west coast of the United States," Roe says.
Although the precise altitude of the clouds is not known, they fall
somewhere between 10 km and 35 km above the surface, within Titan's
troposphere (most cloud activity on the earth is also within its
troposphere).
Notably, all of the clouds were located within a relatively narrow
band at around 40 degrees south latitude, and most were clustered
tightly near 350 degrees west longitude. Both their sporadic
appearance and their specific geographic location led the researchers
to conclude that the clouds were not arising from the regular
convective overturn of the atmosphere due to its heating by the sun
(which produces the cloud cover across the moon's southern pole) but,
rather, that some process on the surface was creating the clouds.
"If these clouds were due only to the global wind pattern, what we
call general circulation, there's no reason the clouds should be
linked to a single longitude. They'd be found in a band around the
entire moon," Roe says.
Another possible explanation for the clouds' patchy formation is
variation in the albedo, or brightness, of the surface. Darker
surfaces absorb more sunlight than lighter ones. The air above those
warmer spots would be heated, then rise and form convective clouds,
much like thunderstorms on a summer's day on Earth. Roe and his
colleagues, however, found no differences in the brightness of the
surface at 40 degrees south latitude. Clouds can also form over
mountains when prevailing winds force air upward, but in that case
the clouds should always appear in the identical locations. "We see
the clouds regularly appear in the same geographic region, but not
always in the exact same location," says Roe.
The other way to make a cloud on Titan is to raise the humidity by
directly injecting methane into the atmosphere, and that, the
scientists say, is the most likely explanation here.
Exactly how the methane is being injected is still unknown. It may
seep out of transient cracks on the surface, or bubble out during the
eruption of icy cryovolcanoes.
Although no such features have yet been observed on the moon, Roe and
his colleagues believe they may be common. "We think there are
numerous sources all over the surface, of varying size, but most
below the size that we could see with our instruments," he says.
One large feature near 350 degrees west longitude is probably
creating the clump of clouds that forms in that region, while also
humidifying the band at 40 degrees latitude, Roe says, "so you end up
creating areas where the humidity is elevated by injected methane,
making it easier for another, smaller source to also generate clouds.
They are like weather fronts that move through. So we are seeing
weather, on another planet, with something other than water. With
methane. That's cool. It's better than science fiction."
Images are available upon request. For advance copies of the
embargoed paper, contact the AAAS Office of Public Programs, (202)
326-6440 or .
###