by David Powell
Special to SPACE.com
26 March 2007

from Space Website

 

The Cassini spacecraft's radar sweep of Saturn's largest moon Titan in January revealed a portion of what appears to be a 110 mile (180 kilometer) diameter impact crater.

If its impact origin is confirmed it would only be the fourth such crater discovered on Titan, a surprisingly small number.

Impact cratering is pervasive in our solar system, and the number of craters on the surface of a moon or planet can reveal its age in just the same way the accumulation of potholes on a highway reveals how long ago the asphalt was laid.

Earth's Moon remains heavily pockmarked because it has no significant weather or geological processes to wipe its face clean. Earth, similarly bombarded over the eons, shows many scars from relatively recent impacts that have not had time to weather away. Craters are common on several other satellites of Saturn.

"If Titan's surface had the same density of craters that other Saturnian moons have, there should be thousands of craters," said Ralph Lorenz of the Johns Hopkins University Applied Physics Laboratory. "With radar having examined about 10 percent of Titan's surface, we have only three definite craters, and perhaps a half-dozen probables."

 


Far too few

The trio of impact craters confirmed to date are named Menrva, Sinlap and Ksa and have diameters of 273 miles (440 kilometers), 50 miles (80 kilometers) and 17 miles (28 kilometers), respectively.

 

The huge impact crater Menrva was spotted by the Cassini radar instrument on 15 February 2005 on Titan and has an outer diameter of 440 kilometers. It resembles a large crater or part of a ringed basin, either of which could be formed when a comet or asteroid tens of kilometers in size crashed into Titan. Credit: NASA/JPL

 

 

The appearance of Sinlap crater and the extremely bright (hence rough) blanket of material surrounding it is indicative of an origin by impact, in which a hypervelocity comet or asteroid, in this case, roughly 5-10 kilometers (3-6 miles) in size, slammed into the surface of Titan. Credit: NASA/JPL

 

 

 

This image from Cassini's radar instrument shows impact crater Ksa, with a diameter of 30 kilometers (19 miles), on the surface of Saturn's moon Titan. The difference in overall appearance between this crater, which has a central peak, and those without, such as Sinlap, indicates variations in the conditions of impact, thickness of the crust, or properties of the meteorite that made the crater. The dark floor indicates smooth or highly absorbing materials. Credit: NASA/JPL

 

Titan's thick nitrogen atmosphere hinders the formation of impact craters less than about 12 miles (20 kilometers) in diameter, because smaller space rocks burn up before they reach the surface. This is one reason the shrouded moon's crater count is so modest.

As Cassini continues to map up to 30 percent of Titan's surface at radar wavelengths the crater tally is set to grow.

"We've seen three craters on 10 percent of the surface, so we will probably find another 10 to 30. Maybe there are 30-100 in total, although we will need a follow-on mission to Titan to find and document them all," Lorenz told SPACE.com.

Still, researchers consider the surface nearly pothole free when it should be on the road to ruin. Something yet to be determined must be keeping the crater count down.
 

 


Buried

Clues to Titan's smooth finish can be seen in the presence of vast tracts of sand dunes, river channels and evidence for cryo-volcanism visible in Cassini images.

It is likely that a combination of burial in sand, erosion by methane or obliteration by the cold hand of cryo-volcanism is responsible for paving over the craters. Cassini has already spotted vague circular features among Titan's sand dunes that may be evidence of craters undergoing burial.

Pinning down the rate of crater removal will be an important factor in dating the age of Titan's surface features, including those craters that have survived.

"We have no way of knowing how recent the known craters are, although Menrva is old enough that it has a fairly eroded rim, cut by river channels," Lorenz said.
 


The road toward life

Though craters are the most obvious legacy of impacts, these dramatic events can leave much more than a hole in the ground. Indeed, the shock of an impact can affect the atmosphere. In Titan's case, a big blow can add ethylene to the mix and aid the conversion of ammonia into molecular nitrogen.

Chemical reactions on Titan's surface might also be instigated by impacts and though the craters may be few and far between, fresher examples such as Ksa could be biologically interesting sites for future missions to veer toward.

It's possible that for a brief period, the heat of an impact could create liquid water on a moon that is otherwise icy cold.

"One of the most exciting aspects about impact craters on Titan, is that by melting a chunk of Titan's icy crust, it forms a local area of liquid water on the surface, which can then chemically react with all the photochemical organics drizzling down from the atmosphere," Lorenz explained.

Such organics when exposed to water in the laboratory have been seen to form the prebiotic molecules essential to the formation of proteins and DNA.

"So in the thousands of years it might take for this water to freeze, some very interesting prebiotic synthesis can occur. No-one knows how close to self-replicating chemical systems such environments might get before they freeze solid. Perhaps by visiting Titan in the future with a lander or balloon able to sample such material we can find out," Lorenz said.

 





 Saturn's Moon Titan a World of Rivers and Lakes
by Ker Than
03 January 2007
from Space Website
Images shot last summer by NASA's Cassini spacecraft provide the strongest evidence yet that Titan, a saturnian moon and one of the most Earth-like celestial bodies in the solar system (image right), is dotted with a multitude of liquid lakes.

"At the time we first announced it, we were like, 'Well, we think these are probably lakes,' but that was about our level of confidence," said study team member Ellen Stofan of University College London and Caltech. "I would say at this point, we've analyzed the data to the extent that we feel very confident that they are liquid-filled lakes."

Instead of water, however, the Titan lakes [below image] are likely filled with methane, and possibly even ethane, organic compounds that are gases on Earth but liquid on the frigid surface of Titan.

"It's going to behave like water," Stofan said about liquid methane. "It's transparent just the way water is. So if you were standing by the shoreline, you would be able to see down to whatever pebbles or gunk that was on the bottom."

A false-color image of Titan's surface snapped by NASA's Cassini spacecraft on July 22, 2006.

The lakes appear darker than the surrounding terrain beca

 

Titan is the only moon in the solar system to have a dense atmosphere with thin layers of methane and nitrogen clouds - a setup similar to that of early Earth. Atmospheric methane is destroyed by sunlight over time and must constantly be renewed. Scientists thus speculated that lakes or even oceans of methane might exist on, or just beneath, the moon's icy surface and that evaporation from these liquid bodies was replenishing the atmosphere.

 

The first confirmation of this thinking came last July (see below insert) when Cassini's radar spotted more than 75 large, dark patches around the surface of the moon's northern pole.

"The lakes are basically black in the [radar] data, which is how a liquid would behave," Stofan said.


Lakes Found on Saturn's Moon Titan
by Robert Roy Britt
Senior Science Writer
28 July 2006

from Space Website


One of the major goals of NASA's Cassini mission was to find lakes or seas on Saturn's moon Titan.

Now scientist say they've found lakes.

These are not bodies of water like those on Earth, but rather dark lakes of methane and possibly ethane. They are likely the source of the hydrocarbon smog in the moon's atmosphere that has long made it impossible to even see the surface.

Several dark patches, some with channels running out of them, were spotted near Titan's north pole during a July 22 Cassini flyby, NASA said in a statement yesterday.

"This is a big deal," said Steve Wall, deputy radar team leader at NASA's Jet Propulsion Laboratory. "We've now seen a place other than Earth where lakes are present."

This was Cassini's first look at the region. Its radar, which penetrates the smog, was used to find several dozen lakes ranging from less than a mile wide to one that is about 62 miles long.

"What we see is darker than anything we've ever seen elsewhere on Titan. It was almost as though someone laid a bull's-eye around the whole north pole of Titan, and Cassini sees these regions of lakes just like those we see on Earth," said Larry Soderblom, Cassini interdisciplinary scientist at the U.S. Geological Survey, Flagstaff, Ariz.

On radar, dark areas indicate smoother terrain. These apparent lakes are so dark that the scientists assume they must be liquid. Any water on Titan would be constantly frozen, however, so the assumption is these lakes are made of hydrocarbons, which can stay liquid at much colder temperatures.

The shapes of outflow channels strongly suggest liquid carved them, the researchers say.

"We've always believed Titan's methane had to be maintained by liquid lakes or extensive underground 'methanofers,' the methane equivalent of aquifers," said Jonathan Lunine, a Cassini scientist at the University of Arizona. "We can't see methanofers but we can now say we've seen lakes."

Lakes should change shape slightly with the seasons, and winds ought to roughen their surfaces, so future passes by Cassini will look for these effects.

Other Cassini observations have revealed apparent river channels elsewhere on the moon, as well as shorelines that might represent lakes or seas. Scientists say the moon likely experiences methane rains.

But most observations, until now, have not shown conclusively that the methane exists in large quantities in liquid form now.

Cassini has been observing Saturn and its moons and rings since it arrived there two years ago. It is a cooperative project between NASA, the European Space Agency and the Italian Space Agency.

 

Radar data alone wasn't enough, however. A very smooth deposit of fine soil would also appear black on radar, Stofan explained.

The clincher that the patches were liquid lakes came from looking at the surrounding terrain. Some of the patches appeared to be fed by sinuous channels, or "rivers," some more than 62 miles (100 kilometers) long. Others appeared to be contained within rimmed circular depressions, similar to crater lakes or volcanic calderas on Earth.

"The morphological evidence points completely away from it being a smooth deposit of soil or sediment. It's just not consistent," Stofan said. "Combining these two sets of data, it led us to feel very confident about the interpretation that they're actually liquid."

In a study published in the Jan. 4 issue of the journal Nature, the researchers also suggest that the rivers and lakes are being filled by rainfall from methane clouds or by seepage from beneath the moon's surface.

"You can think of all the exotic words you want to call it--a 'methanofer' because it's not an aquifer. It's a subsurface methane table and not a water table," Stofan told SPACE.com.

The researchers predict that as the seasons progress, lakes in the winter hemisphere should expand while those in the summer hemisphere should shrink or dry up entirely.

Cassini is slated to perform 22 more Titan flybys, the next of which is scheduled for later this month.

 

 

Cassini's Latest Discoveries

 

These three views of Titan from the Cassini spacecraft illustrate how different the same place can look in different wavelengths of light.

Cassini's cameras have numerous filters that reveal features above and beneath the shroud of Titan's atmosphere.

All of these images were taken with the Cassini spacecraft wide angle camera on April 16, 2005,

at distances ranging from approximately 173,000 to 168,200 kilometers (107,500 to 104,500 miles)

from Titan and from a Sun-Titan-spacecraft, or phase, angle of 56 degrees.