Starship Asterisk*

http://www.planetary.org/blog/article/00002267/ wrote:
Cassini VIMS sees the long-awaited glint off a Titan lake
Dec. 17, 2009 | By Emily Lakdawalla

Specular reflection off of a Titan lake


<<On July 8, 2009, Cassini finally saw the telltale glint of sunlight specularly reflecting off of the mirror-like surface of a lake on northern Titan. This image is from the VIMS instrument employing infrared light at a wavelength of 5 microns, and has been colorized to match visible-light pictures of Titan. Credit: NASA / JPL / University of Arizona / DLR>>

geckzilla wrote:I think out of all the planets, moons, planetoids, stars, etc, Titan is my favorite. Well, other than Earth. Kind of hard to beat your home planet. It probably wouldn't be so intriguing if it weren't for that thick, fuzzy atmosphere it has.

http://www.planetary.org/blog/article/00002267/ wrote:
Cassini VIMS sees the long-awaited glint off a Titan lake
Dec. 17, 2009 | By Emily Lakdawalla

<<The first possible lake-like feature to be found on Titan was reported from a single ISS image
captured very close to the south pole on June 6, 2005, now named Ontario Lacus.

The footprint-like feature in the upper left corner of this image is the unusual-looking feature that Cassini imaging scientists think may be a hydrocarbon lake. It is roughly 234 kilometers long by 73 kilometers wide (145 miles by 45 miles), about the size of Lake Ontario. The red cross below center identifies the location of Titan's south pole.

Finally, in July 2006, the RADAR instrument got data over Titan's north pole, and the images were just screaming "lakes"! They looked exactly like what you'd expect lakes to look like, and furthermore were evidently fed by branching networks, river-like features. The discovery was written up in this Nature paper published on January 4, 2007, by Ellen Stofan and coauthors. It became clear that Titan did have lakes, but only near its poles, because local climatic conditions make the rest of Titan too arid.

Probable lakes near Titan's north pole

Click to enlarge
Cassini's "T16" flyby on July 22, 2006 took it up to high latitudes near the north pole. RADAR images across the region contain numerous very dark splotches with sharp-edged boundaries, which may be the long-sought methane or ethane lakes on the surface of Titan. This image is centered near 78 degrees north, 18 degrees west measures about 475 kilometers by 150 kilometers (295 miles by 93 miles). Credit: NASA / JPL

Interestingly, while RADAR also saw lakes in the south, there were not nearly as many there (at the summer pole) as in the north (the winter pole). Oded Aharonsen and coauthors presented an explanation at the American Geophysical Union meeting a year ago, having to do with "superseasons" on Titan; that work was just published a couple of weeks ago in Nature Geoscience.

As compelling as the RADAR images of the lakes were, scientists are happier when there is more than one line of evidence supporting a conclusion, especially when the first line of evidence is morphological -- there are lots of examples of ways that nature can make similar-looking features using dissimilar processes. So the search continued for further evidence for liquid properties and methane-ethane composition in those lakes. Last year the VIMS team looked at spectra from Ontario Lacus and identified the presence of ethane (that was Bob Brown and coauthors). Long-term study of Titan's poles has revealed changes in the appearance of lakes, interpreted to mean that they've dried up under the southern summer sun.

Lakes drying up near Titan's south pole, Oct. 2007 to Dec. 2008

A comparison of the same area as seen on two different Cassini RADAR flybys shows that some lakes near Titan's south pole dried up during the intervening 14 months. Credit: NASA / JPL / blink gif by Emily Lakdawalla

In the end, we don't really need the VIMS specular reflection to be sure there are lakes near Titan's north pole. But I have to say that I am awfully relieved to finally see it, and I wonder if Titan researchers feel the same way. A specular reflection should have been the "smoking gun" for Titan lakes; its absence was deeply puzzling. Now we know that the reason nobody saw it was because the lakes are mostly only found near the north pole. To see a specular reflection from a flat surface near the north pole of a world, you first need to have light shining on it -- and it's been winter in Titan's north since Cassini arrived. Only recently did sunlight start shining on Titan's lake district.>>

Researchers at the California Institute of Technology (Caltech) suggest that the eccentricity of Saturn's orbit around the sun may be responsible for the unusually uneven distribution of lakes over the northern and southern polar regions of the planet’s largest moon, Titan. A paper describing the theory appears in the November 29th advance online edition of Nature Geoscience.

Saturn's oblong orbit around the sun exposes different parts of Titan to different amounts of sunlight, which affect the cycles of precipitation and evaporation in those areas. Similar variations in Earth's orbit also drive long-term ice-age cycles on our planet. ...

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, Aharonson notes that there are no substantial known differences between the north and south regions to support this possibility.

Alternatively, the mechanism responsible for this regional dichotomy may be seasonal. One year on Titan lasts 29.5 Earth years. Every 15 Earth years, the seasons of Titan reverse, so that it becomes summer in one hemisphere and winter in the other. According to this seasonal variation hypothesis, methane rainfall and evaporation vary in different seasons—recently filling lakes in the north while drying lakes in the south. ...

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.

"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," says Aharonson. This imbalance would lead to an accumulation of methane—and hence the formation of many more lakes—in the northern hemisphere. ...

Saturn's largest moon, Titan, looks to be the only place in the solar system—aside from our home planet, Earth—with copious quantities of liquid (largely, liquid methane and ethane) sitting on its surface. According to planetary astronomer Mike Brown of the California Institute of Technology (Caltech), Earth and Titan share yet another feature, which is inextricably linked with that surface liquid: common fog.

The presence of fog provides the first direct evidence for the exchange of material between the surface and the atmosphere, and thus of an active hydrological cycle, which previously had only been known to exist on Earth. ...