"Most of the Saturnian satellites – Titan's siblings – have thousands and thousands of craters on their surface. So far on Titan, of the 50 percent of the surface that we've seen in high resolution, we've only found about 60 craters," said Catherine Neish, a Cassini radar team associate based at NASA's Goddard Space Flight Center, Greenbelt, Md.
This image taken with the Cassini radar instrument shows two craters on Titan: the crater Sinlap (left), which is a relatively 'fresh' crater, with a depth-to-diameter ratio close to what we see on Ganymede, and Soi (right), an extremely degraded crater, with a very small depth compared to similar craters on Ganymede. These craters are both about 80 km (almost 50 miles) in diameter. The Sinlap image was taken on Feb. 15, 2005. The Soi image is a mosaic of two images from May 21, 2009 and July 22, 2006. Credit: Catherine Neish/NASA/JPL-Caltech/ASI/GSFC
"It's possible that there are many more craters on Titan, but they are not visible from space because they are so eroded. We typically estimate the age of a planet's surface by counting the number of craters on it (more craters means an older surface). But if processes like stream erosion or drifting sand dunes are filling them in, it's possible that the surface is much older that it appears."
"This research is the first quantitative estimate of how much the weather on Titan has modified its surface," adds Neish.
Titan is the only moon in the solar system with a thick atmosphere, and the only world besides Earth known to have lakes and seas on its surface. However, with a frigid surface temperature of around minus 290 degrees Fahrenheit (94 kelvins), the rain that falls from Titan's skies is not water but instead liquid methane and ethane, compounds that are normally gases on Earth.
Neish and her team made the discovery by comparing craters on Titan to craters on Jupiter's moon Ganymede. Ganymede is a giant moon with a water ice crust, similar to Titan, so craters on the two moons should have similar shapes. However, Ganymede has almost no atmosphere and thus no wind or rain to erode its surface.
"We found that craters on Titan were on average hundreds of yards (meters) shallower than similarly sized craters on Ganymede, suggesting that some process on Titan is filling its craters," says Neish, who is lead author of a paper about this research published online in the journal Icarus Dec. 3, 2012.
The team used the average depth-versus-diameter trend for craters on Ganymede derived from stereo images from NASA's Galileo spacecraft. The same trend for craters on Titan was calculated using estimates of the crater depth from data derived from images made by Cassini's radar instrument.
Titan's atmosphere is mostly nitrogen with a trace of methane and other, more complex molecules made of hydrogen and carbon (hydrocarbons). The source of Titan's methane remains a mystery because methane in the atmosphere is broken down over relatively short timescales by sunlight. Fragments of methane molecules then recombine into more complex hydrocarbons in the upper atmosphere, forming a thick, orange smog that hides the surface from view. Some of the larger particles eventually rain out on to the surface, where they appear to get bound together to form the sand.
"Since the sand appears to be produced from the atmospheric methane, Titan must have had methane in its atmosphere for at least several hundred million years in order to fill craters to the levels we are seeing," says Neish. However, researchers estimate Titan's current supply of methane should be broken down by sunlight within tens of millions of years, so Titan either had a lot more methane in the past, or it is being replenished somehow.
Team members say it's possible that other processes could be filling the craters on Titan: erosion from the flow of liquid methane and ethane for example. However, this type of weathering tends to fill a crater quickly at first, then more slowly as the crater rim gets worn down and less steep. If liquid erosion were primarily responsible for the infill, then the team would expect to see a lot of partially filled craters on Titan. "However, this is not the case," says Neish. "Instead we see craters at all stages; some just beginning to be filled in, some halfway, and some that are almost completely full. This suggests a process like windblown sand, which fills craters and other features at a steady rate."
All solid materials under stress flow very slowly over time. This is called viscous flow, and it is like what happens when someone takes a scoop out of a fresh tub of whipped cream -- the material slowly flows in to fill the hole and flatten the surface. Craters on icy satellites tend to get shallower over time as the ice flows viscously, so it's possible that some of the shallow craters on Titan are simply much older or experienced a higher heat flow than the similarly sized, fresh craters on Ganymede studied in this work.
However, Titan's crust is mostly water ice, and at the extremely low temperatures on Titan, ice shouldn't flow enough to account for such a large difference in depth compared to the Ganymede craters, according to the team. Also, just like stream erosion, deformation from viscous flow tends to happen rapidly at first, then more slowly as the material adjusts, so one would expect to see a lot of partially filled craters on Titan if its surface was deforming easily through viscous flow.
As Cassini flies past Titan on its multi-year tour of Saturn and its moons, the radar instrument gradually builds up a map of the surface. To date, the instrument has provided data in strips covering approximately 50 percent of Titan's surface. The craters measured by the team are all within about 30 degrees of the equator, a relatively dry region on Titan.
"However, the presence of liquids on the surface and in the near subsurface can also cause extensive modification to crater shape, as is observed on Earth," says Neish. "In the case of Titan, liquids consist of hydrocarbons, either as wet sediments (such as those observed at the Huygens landing site) or shallow marine environments (such as the lakes observed at the north and south poles). Craters formed in similar environments on Earth lack any significant surface topography, including the absence of a raised rim, as wet sediments slump into the crater. It is possible that the lack of topography associated with marine-target impacts may help to explain the relative scarcity of impact craters observed near the poles of Titan. If Titan's polar regions are saturated by liquid hydrocarbons, craters formed in those regions may lack any recognizable topographic expression."
The team thinks these considerations are good areas for more research, but based on the data so far, the difference in depth between craters on Titan and Ganymede is best explained by filling from windblown sand, although erosion from liquids and viscous flow might contribute to the modification of Titan's craters.
NASA's Cassini mission, managed by NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and the NASA Postdoctoral Program, administered by Oak Ridge Associated Universities, funded the research.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and ASI, the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter was designed, developed and assembled at JPL. The radar instrument was built by JPL and the Italian Space Agency, working with team members from the US and several European countries.Nancy Neal-Jones / Bill Steigerwald
Bill Steigerwald | EurekAlert!
Igniting a solar flare in the corona with lower-atmosphere kindling
29.03.2017 | New Jersey Institute of Technology
NASA spacecraft investigate clues in radiation belts
28.03.2017 | NASA/Goddard Space Flight Center
The Institute of Semiconductor Technology and the Institute of Physical and Theoretical Chemistry, both members of the Laboratory for Emerging Nanometrology (LENA), at Technische Universität Braunschweig are partners in a new European research project entitled ChipScope, which aims to develop a completely new and extremely small optical microscope capable of observing the interior of living cells in real time. A consortium of 7 partners from 5 countries will tackle this issue with very ambitious objectives during a four-year research program.
To demonstrate the usefulness of this new scientific tool, at the end of the project the developed chip-sized microscope will be used to observe in real-time...
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
29.03.2017 | Materials Sciences
29.03.2017 | Physics and Astronomy
29.03.2017 | Earth Sciences