"Long before it's in the papers"
January 27, 2015


“Weirdly Earth-like” rivers adorn Saturn moon

July 23, 2012
Courtesy of MIT
and World Science staff

Riv­ers of liq­uid meth­ane creep­ing across the sur­face of a moon of Sat­urn are eerily Earth-like, but they car­ry some mys­ter­ies, sci­en­tists say.

For many years, the thick, ha­zy at­mos­phere of the moon, Ti­tan, kept as­tro­no­mers from see­ing what lies be­neath. Sat­urn’s larg­est moon ap­peared through tele­scopes as a ha­zy or­ange orb, in con­trast to oth­er heavily crat­ered moons in the so­lar sys­tem.

Images from the Cassini mission show river networks draining into lakes in Titan's north polar region.

In 2004, NASA’s Cas­si­ni-Huy­gens space­craft — a probe that flies by Ti­tan as it or­bits Sat­urn — pen­e­trated the haze, of­fer­ing sci­en­tists their first de­tailed im­ages of the sur­face. Ra­dar im­ages re­vealed an icy ter­rain carved out over mil­lions of years by riv­ers of liq­uid meth­ane, si­m­i­lar to how riv­ers of wa­ter have etched in­to Earth’s rocky con­ti­nents.

While im­ages of Ti­tan have re­vealed its pre­s­ent land­scape, lit­tle is known of its ge­o­log­ic past. Now re­search­ers at the Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy and the Uni­vers­ity of Ten­nes­see at Knox­ville have an­a­lyzed im­ages of Ti­tan’s riv­er net­works and de­ter­mined that in some re­gions, riv­ers have cre­at­ed sur­pris­ingly lit­tle ero­sion. The re­search­ers say there are two pos­si­ble ex­plana­t­ions: ei­ther the ero­sion is ex­tremely slow, or the moon’s sur­faces are rap­idly chang­ing, wip­ing out old­er riv­erbeds and land­forms.

“It’s a sur­face that should have erod­ed much more than what we’re see­ing, if the riv­er net­works have been ac­tive for a long time,” said Tay­lor Per­ron, a ge­ol­o­gist at MIT. “It raises some very in­ter­est­ing ques­tions about what has been hap­pen­ing on Ti­tan in the last bil­lion years.”

A pa­per de­tail­ing the group’s find­ings will ap­pear in the Jour­nal of Geo­phys­i­cal Research-Planets.

Com­pared to most moons in our so­lar sys­tem, Ti­tan is fairly smooth and crat­er-free. Per­ron said the rea­son may be si­m­i­lar to what hap­pens on Earth. “We don’t have many im­pact crat­ers on Earth,” Per­ron said. “Peo­ple flock to them be­cause they’re so few, and one ex­plana­t­ion is that Earth’s con­ti­nents are al­ways erod­ing or be­ing co­vered with sed­i­ment. That may be the case on Ti­tan, too.”

For ex­am­ple, plate tec­ton­ics, erupt­ing vol­ca­noes, ad­vanc­ing glaciers and riv­er net­works have all re­shaped Earth’s sur­face over bil­lions of years. On Ti­tan, si­m­i­lar pro­cesses — tec­ton­ic up­heav­al, icy la­va erup­tions, ero­sion and sed­i­menta­t­ion by riv­ers — may be at work, he said.

But iden­ti­fy­ing which of these phe­nom­e­na may have mod­i­fied Ti­tan’s sur­face is hard. Im­ages gen­er­at­ed by the Cas­si­ni space­craft, si­m­i­lar to aer­i­al pho­tos but with much coars­er res­o­lu­tion, are flat, de­pict­ing ter­rain from a bird’s-eye per­spec­tive, with no in­forma­t­ion about a land­for­m’s eleva­t­ion or depth. “It’s an in­ter­est­ing chal­lenge,” Per­ron said—it’s al­most like maps on Earth would be if “we were thrown back a few cen­turies.”

Per­ron and a grad­u­ate stu­dent, Ben­ja­min Black, set out to de­ter­mine the ex­tent to which riv­er net­works may have re­newed Ti­tan’s sur­face. The team an­a­lyzed im­ages tak­en from Cas­si­ni-Huygens, and mapped 52 prom­i­nent riv­er net­works from four re­gions. The re­search­ers com­pared the im­ages with a mod­el of riv­er net­work ev­o­lu­tion de­vel­oped by Per­ron.

Black com­pared his mea­sure­ments of Ti­tan’s riv­er net­works with the mod­el, and found the moon’s riv­ers most re­sem­bled the early stages of a typ­i­cal ter­res­tri­al riv­er’s ev­o­lu­tion. The ob­serva­t­ions in­di­cate that riv­ers in some re­gions have caused very lit­tle ero­sion, and hence very lit­tle modifica­t­ion of Ti­tan’s sur­face, he said.

Black al­so com­pared Ti­tan’s im­ages with re­cently re­newed land­scapes on Earth, in­clud­ing vol­can­ic ter­rain on the is­land of Ka­u­ai and re­cently gla­ci­at­ed land­scapes in North Amer­i­ca. The riv­er net­works in those loca­t­ions are si­m­i­lar in form to those on Ti­tan, sug­gest­ing that ge­o­log­ic pro­cesses may have re­shaped the moon’s icy sur­face in the re­cent past, he added.

“It’s a weirdly Earth-like place, even with this ex­ot­ic com­bina­t­ion of ma­te­ri­als and tem­per­a­tures,” Per­ron said. “And so you can still say some­thing de­fin­i­tive about the ero­sion. It’s the same physics.”

* * *

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Rivers of liquid methane creeping across the surface of a moon of Saturn are eerily Earth-like, but they carry some mysteries, scientists are reporting. For many years, the thick, hazy atmosphere of the moon, Titan, kept astronomers from seeing what lies beneath. Saturn’s largest moon appeared through telescopes as a hazy orange orb, in contrast to other heavily cratered moons in the solar system. In 2004, the Cassini-Huygens spacecraft — a probe that flies by Titan as it orbits Saturn — penetrated the haze, offering scientists their first detailed images of the surface. Radar images revealed an icy terrain carved out over millions of years by rivers of liquid methane, similar to how rivers of water have etched into Earth’s rocky continents. While images of Titan have revealed its present landscape, little is known of its geologic past. Now researchers at the Massachusetts Institute of Technology and the University of Tennessee at Knoxville have analyzed images of Titan’s river networks and determined that in some regions, rivers have created surprisingly little erosion. The researchers say there are two possible explanations: either the erosion is extremely slow, or the moon’s surfaces are rapidly changing, wiping out older riverbeds and landforms. “It’s a surface that should have eroded much more than what we’re seeing, if the river networks have been active for a long time,” said Taylor Perron, a geologist at MIT. “It raises some very interesting questions about what has been happening on Titan in the last billion years.” A paper detailing the group’s findings will appear in the Journal of Geophysical Research-Planets. What accounts for a low crater count? Compared to most moons in our solar system, Titan is fairly smooth and crater-free. Perron said the reason may be similar to what happens on Earth. “We don’t have many impact craters on Earth,” Perron said. “People flock to them because they’re so few, and one explanation is that Earth’s continents are always eroding or being covered with sediment. That may be the case on Titan, too.” For example, plate tectonics, erupting volcanoes, advancing glaciers and river networks have all reshaped Earth’s surface over billions of years. On Titan, similar processes — tectonic upheaval, icy lava eruptions, erosion and sedimentation by rivers — may be at work, he said. But identifying which of these phenomena may have modified Titan’s surface is hard. Images generated by the Cassini spacecraft, similar to aerial photos but with much coarser resolution, are flat, depicting terrain from a bird’s-eye perspective, with no information about a landform’s elevation or depth. “It’s an interesting challenge,” Perron said. “It’s almost like we were thrown back a few centuries, before there were many topographic maps, and we only had maps showing where the rivers are.” Perron and a graduate student, Benjamin Black, set out to determine the extent to which river networks may have renewed Titan’s surface. The team analyzed images taken from Cassini-Huygens, and mapped 52 prominent river networks from four regions. The researchers compared the images with a model of river network evolution developed by Perron. Black compared his measurements of Titan’s river networks with the model, and found the moon’s rivers most resembled the early stages of a typical terrestrial river’s evolution. The observations indicate that rivers in some regions have caused very little erosion, and hence very little modification of Titan’s surface, he said. Black also compared Titan’s images with recently renewed landscapes on Earth, including volcanic terrain on the island of Kauai and recently glaciated landscapes in North America. The river networks in those locations are similar in form to those on Titan, suggesting that geologic processes may have reshaped the moon’s icy surface in the recent past, he added. “It’s a weirdly Earth-like place, even with this exotic combination of materials and temperatures,” Perron said. “And so you can still say something definitive about the erosion. It’s the same physics.”