Titanic jigsaw challenge: Piecing together a global color map of Saturn’s largest moon

ScienceDaily (Oct. 5, 2011) — An international team led by the University of Nantes has pieced together images gathered over six years by the Cassini mission to create a global mosaic of the surface of Titan.

The global maps and animations of Saturn's largest moon are being presented by Stéphane Le Mouélic at the EPSC-DPS Joint Meeting 2011 in Nantes, France.

The team has compiled all the infrared images acquired by the Visual and Infrared Mapping Spectrometer (VIMS) during Cassini's first seventy flybys of Titan. Fitting the pieces of the puzzle together is a painstaking task. The images must be corrected for differences in the illuminating conditions and each image is filtered on a pixel-by-pixel basis to screen out atmospheric distortions. Titan is veiled by a thick, opaque atmosphere composed mainly of nitrogen. It has clouds of methane and ethane and there is increasing evidence for methane rain. Only a few specific infrared wavelengths can penetrate the cloud and haze to provide a window down to Titan's surface. An exotic frozen world with many Earth-like geological features has progressively emerged from darkness.

Stéphane Le Mouélic explains: "As Cassini is orbiting Saturn and not Titan, we can observe Titan only once a month on average. The surface of Titan is therefore revealed year after year, as pieces of the puzzle are progressively put together. Deriving a final map with no seams is challenging due to the effects of the atmosphere -- clouds, mist etc. -- and due to the changing geometries of observation between each flyby."

Cassini has made 78 flybys of Titan since it arrived in orbit around Saturn in July 2004. A further 48 flybys are planned up to 2017. On flybys to date, VIMS has only had a few opportunities to observe Titan with a high spatial resolution. This means that the global map currently shows some regions of Titan in more detail than others.

"We have created the maps using low resolution images as a background with the high resolution data on top. In the few opportunities where we have VIMS imagery from the closest approach, we can show details as low as 500 metres per pixel. An example of this is from the 47th flyby, which allowed the observation of the site where the Huygens descent module landed. This observation is a key one as it might help us to bridge the gap between the ground truth provided by Huygens, and ongoing global mapping from orbit, which will continue up to 2017."

In addition to improving the spatial coverage, future mapping will allow the observation of seasonal changes in both the atmosphere and the surface. As spring comes to the northern hemispheres of Saturn and its moons, some areas are only now coming into view.

"Lakes in Titan's northern hemisphere were first discovered by the RADAR instrument in 2006, appearing as completely smooth areas. However, we had to wait up to June 2010 to obtain the first infrared images of the northern lakes, emerging progressively from the northern winter darkness," says Le Mouélic. "The infrared observations provide the additional opportunity to investigate the composition of the liquids within the lakes area. Liquid ethane has already been identified by this means."

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Europlanet Media Centre, via AlphaGalileo.

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Numbers flap has minor implications for global extinctions

Statistical technique inflates predictions of species disappearances, two ecologists chargeWeb edition : Thursday, May 19th, 2011 LOST Carolina parakeets are one of several bird species that have gone extinct in the eastern United States in a complicated relationship of extinction numbers and habitat shrinkage that ecologists are still debating.John James Audubon

Two ecologists have poked a hornet’s nest by contending that a long-overlooked flaw in one form of analysis has inflated estimates of looming species extinctions.

Correcting the flaw wouldn’t change the basic projection that extinctions will be massive, says one of the ecologists, Stephen Hubbell of UCLA. Even so, he and a colleague write in the May 19 Nature, the problem requires revisions in a technique that ecologists use to estimate species extinctions due to habitat destruction.

Outspoken ecologists have already dismissed the work as technical quibbling that threatens to distract attention from the most cataclysmic loss of biodiversity in at least 65 million years. A long-time thinker about extinctions, Stuart Pimm of Duke University, objects to the paper as “truly disastrous.” He says it does advance understanding on a technical point but ignores bigger issues and practices among extinction predictors. “This is going to be the biggest bun-fight in my profession in a very long time,” Pimm says.

But Hubbell insists that he is raising a scientifically substantive issue: “Come hell or high water, we have to be self-correcting. Or why are we doing science?”

Behind the flying baked goods is a long-standing observation, formalized in a standard species-area relationship equation, that bigger areas support a greater number of species than smaller areas do. Hubbell, Pimm and other ecologists agree that humanity’s accelerating destruction of other species’ habitats has become a major force squeezing species into extinction. What they disagree on is how to quantify that process.

The equation that ecologists use to estimate how many species could disappear if a given area is, say, turned into city sprawl has roots in surveys of how many kinds of organisms crowd into an area. A biologist counting all the species in an area has to tramp just far enough to find the first of its kind before adding that name to the list. A species-area relationship based on such data predicts that even a relatively modest land loss dooms a whole species.

Another approach is to find the outer limits of an organism’s range, but that requires traversing much more area than it would take to find one example of a species. Either way, species living only within a surveyed area, considered endemic to that area, are those at immediate risk of going extinct. The new paper proposes focusing on endemics for a more realistic view of what habitat loss might do to the number of species in an area.  

“It’s embarrassingly simple,” says Hubbell, who discovered the flaw in collaboration with Fangliang He of Sun Yat-sen University in Guangzhou, China, and the University of Alberta in Edmonton, Canada.

To test this possible flaw in count-the-first-one surveys, Hubbell and He turned to detailed tree data from eight forest patches and two sets of bird data with substantial information on the birds’ ranges. Indeed, the traditional count-the-first surveys didn’t do a good job of reflecting how much area endemic species covered. The discrepancy could cause the standard species-area relationship to add an extra 160 percent to estimates of species loss. To use the better, endemic-based method to project losses of an area’s species from amoeba to zebra, though, “we need a lot more data,” Hubbell says.

What the new paper will mean, if anything much, for the practice of estimating extinctions is the question that launches a thousand, often disputative, buns.

Such a high-profile attack about a technical issue “is going to set us back a long, long time,” says Michael Rosenzweig of the University of Arizona. Estimates of extinction need to account not just for endemic species in an area that get their whole world trashed when the habitat goes but species that may have lost thriving populations that provided sources of immigrants for iffier habitats.

One prominent extinction estimate that Hubbell and He call into question comes from the 2005 Millennium Assessment, which predicted extinction rates in coming years as much as 1,000 to 10,000 times higher than the ho-hum background rates seen during times of relatively mild, natural change in life on Earth.

“It is true that in the 1980s and early 1990s, essentially all estimates of extinction rates were indeed based on species-area relations,” says Robert May of the University of Oxford in England. But more recent estimates, including the work behind the Millennium Assessment, draw on multiple lines of evidence, he says.

“All ways that we measure extinction, not just species-area relationships, point to [extinction] rates that are elevated today,” says paleobiologist Anthony Barnosky of the University of California, Berkeley. In March he and his colleagues published an analysis — not based on species-area relationships — suggesting that ongoing extinctions could within the next few centuries grow to proportions qualifying as the sixth of Earth’s historical mass extinctions. Even using Hubbell and He’s adjustments to dial back a recent high-profile prediction of 18 percent to 35 percent land species losses from climate change could still means losing 10 percent in the not-too-distant future. That’s “an unacceptably high loss by anyone's standards,” he says.

Broadly speaking, the exact scale of estimated extinctions may not matter much, says plant scientist Vernon Heywood of the University of Reading in England. “Today no one doubts that species extinction is a major issue, and the real debate is about how we are to prevent a further substantial loss of biodiversity,” he says. “That is more about politics and economics than precise modeling.”

Found in: Earth, Environment and Life

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