- Seasonal Methane Rain Discovered on Titan
- Japan Quake Epicenter Was in Unexpected Location
- Crop Tops: Strange Agricultural Landscapes Seen From Space
Posted: 17 Mar 2011 12:12 PM PDT
Spring may bring methane showers to the deserts of Titan, Saturn's largest moon. NASA's Cassini spacecraft recently saw a large, dark puddle appear in the wake of a storm cloud at the moon's dune-filled equator.
"It's the only easy way to explain the observations," said planetary scientist Elizabeth Turtle of Johns Hopkins University Applied Physics Laboratory, lead author of a study March 18 in Science. "We're pretty confident that it has just rained on Titan."
Aside from Earth, Titan is the only world known to have liquid lakes, clouds and a weather cycle to move moisture between them. But on chilly Titan, where temperatures plunge to -297 degrees Fahrenheit, the frigid lakes are filled with liquid methane and ethane, not water.
Titan's lakes are also exclusively confined to the poles. The moon's dry central regions are covered in rippling dunes and arid deserts.
But the dunes are crisscrossed by a network of dry channels, suggesting a wetter past. In 2006, Cassini observations showed hints of drizzle at the equator, but not enough rain to explain the riverbeds.
"So the question was, 'When was the last rainfall near the equator of Titan?'" said planetary scientist Tetsuya Tokano of the University of Cologne in Germany, who was not involved in the new work. Some researchers suggested that the rivers were a relic of a bygone era, or carved by things other than rain.
"This observation by Turtle et al. showed for the first time that there is rainfall on present Titan, not merely millions of years ago but at the present Titan," Tokano said. "This is extraordinary."
In the new study, Turtle's team describes a large cloud system moving eastward across Titan's equator on Sept. 27, 2010. By October, observations show, a dune field called Belet that lies east of the clouds suddenly darkened. The dark patch extended for more than 190,000 square miles, and started fading fast. Some spots that were dark on Oct. 14 were bright again by Oct. 29, and even more bright spots were visible on Jan. 15.
Turtle thinks the shadow is wet ground after rainfall, like a sidewalk darkened by a shower. Titan's winds aren't strong enough to wreak such sudden or vast changes, she says, and it's doubtful that the kind of explosive volcanic activity that could explain the dark patch is possible on Titan.
It's not clear how much rain fell, she adds. Some areas could have flooded or sustained small puddles, but it may just be that the surface got wet.
The showers were probably prompted by Titan's changing seasons. Cassini has been orbiting Saturn since 2004, but since a full year on Saturn — and therefore all its moons — lasts 29 Earth years, the spacecraft has only observed one 7-year season on Titan. Astronomers saw storms and rain at Titan's south pole during the summer, and then the clouds cleared after the spring equinox in August 2009.
"It's kind of the equivalent on Titan right now of early April, just into northern spring," Turtle said. "What we think triggered this huge storm is that the weather patterns are seasonal." Major cloud patterns move north as the southern summer ends, similar to the way they do on Earth, she says. The only difference is, Earth's tropics sustain rain clouds year round. On Titan, the equator may see rain only a few times a year.
The difference comes, at least in part, from Titan's leisurely rotation rate, Tokano said. Titan takes 16 Earth days to rotate once, meaning its atmospheric circulation patterns are somewhat more simple. Titan's clouds shift quickly from north to south, filling the polar lakes with rain but mostly leaving the equator out to dry.
As for whether the spring showers are good news for the possibility life on Titan, Turtle and Tokano are agnostic.
"There's no liquid water involved in any of the processes we're describing here, so life as we know it can't exist," Turtle said. "But there's clearly so much scope for prebiotic chemistry on Titan…. Understanding Titan better in general helps us to understand what the possibilities are."
Images: 1) NASA/JPL/Space Science Institute. 2) P. Huey/Science AAAS
Posted: 17 Mar 2011 09:43 AM PDT
Japan has been expecting and preparing for the "big one" for more than 30 years. But the magnitude-9.0 temblor that struck March 11 — the world's fourth biggest quake since 1900 — wasn't the catastrophe the island nation had in mind. The epicenter of the quake was about 80 miles east of the city of Sendai, in a strip of ocean crust previously thought unlikely to be capable of unleashing such energy.
"This area has a long history of earthquakes, but [the Sendai earthquake] doesn't fit the pattern," says Harold Tobin, a marine geophysicist at the University of Wisconsin-Madison. "The expectation was high for a 7.5, but that's a hundred times smaller than a 9.0."
Understanding where big earthquakes will emerge is extraordinarily difficult, and nowhere more so than Japan. The northern part of the island nation sits at the intersection of four moving pieces of the Earth's crust. Where one tectonic plate slides beneath another, forming a subduction zone, sudden slippages can unleash tremendous amounts of energy.
The Sendai earthquake occurred at the Japan Trench, the junction of the westward-moving Pacific Plate and the plate beneath northern Japan. Historical records, one of seismologists' best tools for identifying areas at risk, suggest that this segmented fault has produced several earthquakes bigger than 7.0 in the 20th century alone — but none bigger than 8.0.
That's why the Japanese government has long focused on the nation's southern coast and the northward-moving Philippine Plate, which has a proven ability to generate large quakes. Quakes larger than 8.0 tend to strike the Tokai region in central Japan every 150 years or so, with the last big one appearing in 1854.
In 1976 researcher Katsuhiko Ishibashi of Kobe University warned that Suruga trough, a subduction zone just off the coast of Tokai, was due for a big one. In the years since, the Japanese government and research community have braced for this predicted Tokai earthquake — deploying GPS systems to monitor the movements of islands on the Philippine Plate and even generating computer simulations of how crowds in train stations might behave during such an event.
Current thinking about the mechanisms that govern megaquakes also favored the Philippine Plate as the site of greatest risk. About 80 percent of all earthquakes above magnitude 8.5 occur at the edges of such geologically young, warm tectonic plates. Kilometer-thick sediment layers carried by these plates are thought to grind smooth patches that allow long stretches of fault to rupture at once. The Pacific Plate, some of the oldest ocean crust on the planet, doesn't fit this description.
But preliminary computer simulations at Harvard that crunched early data from the Sendai quake suggest that a long stretch of the Japan Trench ruptured during the event — about 390 kilometers [240 miles]. Multiple segments that usually behave independently broke over the course of two to three minutes.
"It looks like three of the segments all slipped together," says Miaki Ishii, a seismologist at Harvard. "There is some evidence that a fourth may have been involved as well." She doesn't know why these particular segments ruptured together, or why other similar segments nearby didn't join them.
What does seem to be clear is that the slip happened in a relatively shallow region of the subduction zone. According to computer simulations run by geophysicist Chen Ji at the University of California, Santa Barbara, the quake originated 8 to 20 kilometers [5 to 12 miles] below the ocean floor. The shallower an earthquake, the more easily it flexes the Earth's crust, raising a mountain of water that can turn into a tsunami. The Sendai quake lifted the seafloor several meters and generated a tsunami up to 7 meters [23 feet] high.
"We're learning that we can't discount any of these big subduction zones," says Tobin. "They're all capable of producing large earthquakes." The magnitude-9.1 earthquake that struck Sumatra in 2004 also broke the rules: It, too, happened on the edge of an old piece of crust, hurling a tsunami across the Indian Ocean that was more deadly than any in recorded history.
In the United States, seismologists are now eyeing the Cascadia fault zone that flanks Oregon and Washington, which last gave way in 1700 to produce the largest known earthquake in North American history.
"Perhaps the earthquake in Japan shouldn't have been as surprising as it was," says Stanford seismologist Greg Beroza.
Beroza explains that deposits of sand found kilometers from shore have revealed a large tsunami that struck the Sendai area during the Jogan earthquake of 869. Ever since this magnitude-8.0+ quake, the Pacific Plate has been moving more than 8 centimeters [3 inches] per year — a tectonic sprint — pushing against its neighbor plate and perhaps building a tremendous amount of strain.
Seismologists hope that the detailed Sendai earthquake data collected by Japan's advanced monitoring technologies — hundreds of sensors spaced an average of 20 to 30 kilometers [12 to 18 miles] apart across the Japanese islands — will lead to a better understanding of subduction zone quakes. Researchers will also analyze the emerging pattern of aftershocks, which now includes at least three bigger than 7.0 and dozens bigger than 6.0.
But being able to spot signs far in advance of a big earthquake — currently far beyond the reach of modern science — may require digging deeper. Tobin and his Japanese colleagues have for the first time embedded strain sensors directly inside a subduction zone, the Nankai trough located southwest of Tokai.
Large earthquakes have struck this region every 100 to 120 years, from 686 to 1946. The researchers hope to catch the next big one in the act and find a warning sign that could provide more than a minute's notice that a monster quake is on its way.
Images: The March 11 Sendai earthquake (epicenter shown as star) occurred when the westward-moving Pacific Plate took a sudden dive beneath northern Japan's plate, the identity of which is disputed among scientists. (USGS)
Posted: 17 Mar 2011 04:00 AM PDT
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Agriculture is one of the oldest and most pervasive human impacts on the planet. Estimates of the land surface affected worldwide range up to 50 percent. But while driving through the seemingly endless monotony of wheat fields in Kansas may give you some insight into the magnitude of the change to the landscape, it doesn't compare to the view from above.
When seen from space, those same boring wheat fields are transformed into a strange and even beautiful pattern. Some of the most arresting agricultural landscapes occur in the Midwestern United States in areas that rely on center-pivot irrigation (shown at right). The area pictured above near Garden City, Kansas, is being farmed to the point of resembling abstract art or a Magic Eye illusion. Groundwater from the Ogallala Aquifer is used to grow corn, wheat and sorghum in the region.
The image above, taken by the USGS' Landsat 7 satellite on Sept. 25, 2000, is a false-color composite made using data from near infrared, red and green wavelengths and sharpened with a panchromatic sensor. The red areas actually represent the greenest vegetation. Bare soil or dead vegetation ranges from white to green or brown.
The image below is a simulated true-color shot from the same county in Kansas taken June 24, 2001 by NASA's Terra satellite. Bright greens are healthy, leafy crops such as corn; sorghum would be less mature at this time of year and probably a bit paler; wheat is ready for harvest and appears a bright gold; brown fields have been recently harvested. The circles are perfectly round and measure a mile or a half mile in diameter.
In this gallery, we've collected some of the most interesting views of crops from space, including rice paddies in Thailand, cotton fields in Kazakhstan and alfalfa growing in the middle of the Libyan desert.
Images: 1) USGS/NASA. 2) USGS. 3) NASA.
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