A paper describing the theory appears in the November 29th advance online edition of Nature Geoscience.
As revealed by Synthetic Aperture Radar (SAR) imaging data taken by NASA's Cassini spacecraft, which has been surveying Saturn and its moons since 2004, liquid hydrocarbon–filled lakes in Titan's northern high latitudes cover 20 times more area than lakes in the southern high latitudes. There are also significantly more partially filled and now-empty lakes in the north. (In the SAR data, smooth features—like the surfaces of lakes—appear as dark areas, while rougher features—such as the bottom of an empty lake—appear bright.)
Assuming that the asymmetry is not a statistical fluke (which is unlikely because of the large amount of data collected by Cassini), scientists initially considered the idea that "there is something inherently different about the northern polar region versus the south in terms of topography, such that liquid rains, drains, or infiltrates the ground more in one hemisphere," says Oded Aharonson, associate professor of planetary science at Caltech and lead author of the Nature Geoscience paper. However, he notes, there are no substantial known differences between the north and south to support this possibility.
Alternatively, the mechanism may be seasonal. One year on Titan lasts 29.5 Earth years. Every 15 Earth years the seasons reverse, so that it becomes summer in one hemisphere and winter in the other. (Currently, summer has just begun in the northern hemisphere, and winter in the south.) According to the seasonal hypothesis, methane rainfall and evaporation vary in different seasons—recently filling lakes in the north while drying lakes in the south.
The problem with this idea, Aharonson says, is that it explains decreases of about one meter per year in the depths of lakes in the summer hemisphere. But Titan's lakes are a few hundred meters deep on average, and wouldn't drain (or fill) in just 15 years.
In addition, seasonal variation can't account for the disparity between the hemispheres in the number of empty lakes; the northern pole has roughly three times as many dried-up lake basins as the south (and seven times as many partially filled ones).
"How do you move the hole in the ground?" Aharonson asks. "The seasonal mechanism may be responsible for part of the global transport of liquid methane, but it's not the whole story."
A more plausible explanation, say Aharonson and his colleagues, is related to the eccentricity of the orbit of Saturn—and hence of Titan, its satellite—around the sun.
Like Earth and the other planets, Saturn's orbit is not perfectly circular, but is instead somewhat elliptical—or eccentric—and oblique. Because of this, during its southern summer, Titan is about 12 percent closer to the sun than it is during the northern summer. As a result, northern summers are long and subdued while southern summers are short and intense.
Aharonson and his colleagues think these differences in the characteristics of the seasons could somehow affect the relative amounts of precipitation and evaporation of methane in the hemispheres' respective summers.
"We propose that, in this orbital configuration, the difference between evaporation and precipitation is not equal in opposite seasons, which means there is a net transport of methane from south to north," he says. This imbalance would lead to an accumulation of methane—and hence the formation of many more lakes—in the northern hemisphere.
This situation is only true right now, however. Over very long time scales of tens of thousands of years, Saturn's orbital parameters vary, at times causing Titan to be closer to the sun during its northern summer and farther away in southern summers, and producing a reverse in the net transport of methane. This should lead to a buildup of the hydrocarbon—and an abundance of lakes—in the southern hemisphere.
"Like Earth, Titan has tens-of-thousands-of-year variations in climate driven by orbital motions," Aharonson says. On Earth, these variations, known as Milankovitch cycles, are linked to the global redistribution of water in the form of glaciers, and are responsible for ice-age cycles. "On Titan, there are long-term climate cycles in the global movement of methane that make lakes and carve lake basins. In both cases we find a record of the process embedded in the geology," he adds.
"We may have found an example of present-day climate change, analogous to Milankovitch climate cycles on Earth, on another object in the solar system," he says.
The paper, "Titan's Asymmetric Lake Distribution and its Potential Astronomical Evolution," was coauthored by Caltech graduate student Alexander G. Hayes; Jonathan I. Lunine of the Lunar and Planetary Laboratory; Ralph D. Lorenz of the Applied Physics Laboratory at the Johns Hopkins University; Michael D. Allison of the NASA Goddard Institute for Space Studies; and Charles Elachi, director of the Jet Propulsion Laboratory. The work was partially funded by the Cassini Project.
UCI and NASA document accelerated glacier melting in West Antarctica
26.10.2016 | University of California - Irvine
Ice shelf vibrations cause unusual waves in Antarctic atmosphere
25.10.2016 | American Geophysical Union
Physicists from the University of Würzburg have designed a light source that emits photon pairs. Two-photon sources are particularly well suited for tap-proof data encryption. The experiment's key ingredients: a semiconductor crystal and some sticky tape.
So-called monolayers are at the heart of the research activities. These "super materials" (as the prestigious science magazine "Nature" puts it) have been...
Ultrafast lasers have introduced new possibilities in engraving ultrafine structures, and scientists are now also investigating how to use them to etch microstructures into thin glass. There are possible applications in analytics (lab on a chip) and especially in electronics and the consumer sector, where great interest has been shown.
This new method was born of a surprising phenomenon: irradiating glass in a particular way with an ultrafast laser has the effect of making the glass up to a...
Terahertz excitation of selected crystal vibrations leads to an effective magnetic field that drives coherent spin motion
Controlling functional properties by light is one of the grand goals in modern condensed matter physics and materials science. A new study now demonstrates how...
Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.
"The quantum socket is a wiring method that uses three-dimensional wires based on spring-loaded pins to address individual qubits," said Jeremy Béjanin, a PhD...
In a paper in Scientific Reports, a research team at Worcester Polytechnic Institute describes a novel light-activated phenomenon that could become the basis for applications as diverse as microscopic robotic grippers and more efficient solar cells.
A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used...
14.10.2016 | Event News
14.10.2016 | Event News
12.10.2016 | Event News
28.10.2016 | Power and Electrical Engineering
28.10.2016 | Life Sciences
28.10.2016 | Life Sciences