More Youth on Titan
Hopes that Saturn’s giant moon Titan might have volcanoes just dropped. A new paper in Icarus1 concludes Titan gets its geology from the outside, not the inside. If confirmed, it implies all the surface features were created by wind, impacts and weather – not by active geology. The hopeful cryovolcano announced last year (Sotra Facula, see 12/24/2010, bullet 12) was disputed by Moore and Pappalardo, authors of the new paper. Titan may be a geologically dead world.
Titan’s atmosphere, however, remains a subject of intense interest. Scientists were eager to visit Titan via Cassini because of its thick atmosphere of nitrogen and methane. Because precipitation of methane and its byproducts was considered inevitable, astrobiologists were eager to find liquid as possible abodes for life. Some proposed a global ocean several kilometers deep. When the Huygens Probe landed in January 2005 with a thud on a moist but mostly dry lake bed, those hopes evaporated.
Planetary scientists have also had an age conundrum with Titan. They know that the methane in the atmosphere is destroyed and converted to other compounds in a one-way process. This puts strong upper limits on the age of the atmosphere – far less than the 4.5-billion-year age assumed for the solar system. They had hoped that a reservoir of methane under the surface would be found to erupt in cryovolcanos to replenish the atmosphere. The new paper casts doubt on that solution; see the Cassini press release for a summary of the findings, and also PhysOrg, Science Daily.
Instead of volcanoes, another possible large crater has been found. The “ghost crater” reported by New Scientist is disputed by others. The surprising dearth of volcanoes leads many planetary scientists to say they are quickly erased by erosion. If it weren’t for the atmosphere, scientists expect Titan would look like Callisto, a dead moon orbiting Jupiter.
Another paper in press in Icarus analyzed Titan’s equatorial sand dunes.2 The longitudinal dunes, covering about 12.5% of the surface, were a surprise when discovered, because scientists were expecting large lakes or even a global ocean. They had also doubted that the winds were strong enough at the surface to move particles around. Dunes also exist on Mars, Venus, and of course, Earth, but on Titan, the average 300-foot-high dunes are nearly 3 km apart, getting farther apart at higher latitudes. Unlike the silica sands on Earth, the particles in Titan’s dunes are thought to be composed of hydrocarbon dust and ice precipitated out of the atmosphere. All together, they constitute the largest known reservoir of organics on Titan, because the combined area of dunes is about as large as the United States (Titan’s diameter is also about that size).
The dunes also impinge on theories of Titan’s age. For one, they are among Titan’s most youthful features; for another, they indicate a lack of persistent liquid on Titan’s equator, even though liquid ethane should have been raining onto the surface throughout Titan’s history. The presence of dunes implies that much of Titan is arid. If spread out evenly over the globe, the particles in this largest reservoir of organics (larger than all the observed lakes combined) would doubtless fail to cover Titan with the predicted accumulation of hydrocarbons that must have been produced in the assumed 4.5-billion-year age of the moon. “The dune distribution places constraints on Titan’s meteorology and geology,” the authors said.
1. Jeffrey M. Moore and Robert T. Pappalardo, “Titan: An Exogenic World?”, Icarus April 2011, doi:10.1016/j.icarus.2011.01.019.
2. LeGall, Janssen et al., “Cassini SAR, radiometry, scatterometry and altimetry observations of Titan’s dune fields,” Icarus (article in press), doi:10.1016/j.icarus.2011.03.026.
We are still discovering facts about Titan, so definitive conclusions are premature; however, enough is known to falsify many assumptions and predictions made by those who refuse to budge from their A.S.S. (age of the solar system, 4.5 billion years; see 02/19/2011). They were wrong about a global ocean; they were wrong about huge lakes of liquid ethane; they were dumbfounded to find sand dunes; and now it appears they were wrong about active geology.
The upper limits on age appear to be growing stronger with time. The puzzlement on their faces, and the silence about defending the consensus age, are tell-tale signs that their fascination with discovery is tempered by panic over looming destruction of favored beliefs about the age of the solar system (02/15/2008). Titan may be the old-agers’ Titanic.