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Titan: Frozen in Time or Constantly Evolving?
Color image of Titan and a small part of Saturn.
Titan approaches the limb of Saturn in this view captured by the Cassini spacecraft.

"Saturn's Titan: Surface Change, Ammonia, and Implications for Atmospheric and Tectonic Activity."

Before the Cassini-Huygens mission began bringing back crucial data about Saturn and its largest moon, Titan, scientists knew very little about the nature of Titan or its hazy atmosphere. It was thought of as a "pre-biotic Earth, frozen in time."

But now, Cassini instruments have provided tantalizing evidence that Titan is currently geologically active, recording what could be a large area where icy ammonia fog vents from the interior and falls to the local surface. The finding indicates that Titan continues to replenish its atmosphere today by outgassing ammonia, which serves as a source for the satellite's nitrogen-dominated atmosphere.

Scientists who recently authored the paper "Saturn's Titan: Surface Change, Ammonia, and Implications for Atmospheric and Tectonic Activity," conclude that distinct changes have occurred on or near the ground over a 73,000 square kilometer-area viewed by Cassini's Visual Infrared Mapping Spectrometer (VIMS) several times over a two-year period. The area in question - twice as large as Hawaii's Big Island -- was found to repeatedly grow and diminish in brightness from 2004-2006.

Series of black and white images showing epochs on Titan.
Fig. 1. Mosaics of the eight epochs. (a) Mosaics of the images (at 2.02 µm) of Titan at the eight epochs. They are (left to right, top) T0 (2-Jul-2004), T4 (31-Mar-2005), T5 (16-Apr-2005), T7 (6-Sep-2005), and (left to right, bottom) T8 (27-Oct-2005), T9 (26-Dec-2005), T10 (14-Jan-2006) and T12 (17-Mar-2006). Mosaics were created from the individual data cubes that comprise the observations of each epoch.

The scientists say the spectral signature of the reflective material is consistent with ammonia frost, most likely overlying water ice. They theorize that the brightened feature is due to ammonia frost that has vented from the interior and fallen to the surface. The later darkening of the bright frost could be explained by camouflaging from Titan's reddish hydrocarbon rain. Or the bright material could be darkened by weathering, chemical decomposition or evaporation.

Chart showing ranges of brightness and darkness of reflectance on Titan.
Change in reflectance at each of the atmospheric windows where VIMS is able to see Titan's surface. The vertical red lines (error bars facing right) show the range in I/F at each wavelength where the spot appeared bright at 2 µm (T4, T5, and T9). The vertical blue lines (error bars facing left) show the range in I/F where the spot was not bright (T0, T8 and T12).

The authors considered but ruled out some other possible causes for the alternating bright-dark region, including migration of a dark cloud or haze, a polar cloud or a thermal anomaly such as a hot spot.

The region in question is also comparable to the large volcano, Loki, on Jupiter's Io. "If the entire reflective region is part of the activity, its size is of Krakatoan proportions," says lead author Robert M. Nelson of NASA's Jet Propulsion Laboratory (JPL). "This may be the largest active surface region in the solar system."

Black and white image of bright spots on Titan's surface.
VIMS image of Titan from the T9 data set. Here, unlike Fig. 1, the image is shown without projection to the observational perspective of the T0 observations. The region of changing brightness is indicated with yellow fiducial marks; the tropospheric cloud is indicated by red fiducial marks.

Earlier studies provide evidence for ammonia mixed with water in a liquid layer below Titan's water-ice surface. Given such a layer, it is possible that ammonia, alone or mixed with water, may be vented through deep-seated volcanism, or via a diapir that taps the liquid layer, or by more localized geysering of ammonia-rich deposits in the crust. Ammonia decomposes into nitrogen. Ongoing episodes of ammonia venting from the interior could be feeding nitrogen into Titan's atmosphere now and in the future. Titan now joins Earth, Io, Triton, Enceladus, probably Venus, and possibly Europa as the small community of planetary objects in the solar system that exhibit active volcanic processes. Most significantly, these findings could shed light on how Earth's nitrogen-rich atmosphere evolved. In summary, the results also cause the scientists to ask 'Are the chemical processes happening on Titan today the best approximation among the planets to the processes under which life evolved on Earth?'

Reference: "Saturn's Titan: Surface Change, Ammonia, and Implications for Atmospheric and Tectonic Activity." Icarus, August 4, 2008

Scientists: R..M. Nelson, L.W. Kamp, D.L.Matson (JPL); PG.J. Irwin (Clarendon Lab, U.K.); K.H. Baines (JPL); M.D. Boryta Mt. San Antonio Coll.); F.E. Leader (JPL); R. Jaumann (Inst. for Planetary Exploration, Germany); W.D. Smythe (JPL) C. Sotin (U. Nantes); R. N. Clark (USGS Denver); D.P. Cruikshank (NASA Ames);P. Drossart (Obs. de Paris-Meudon); J.C. Pearl (NASA Goddard); B.W. Hapke (U of Pittsburgh); J. Lunine (U of AZ); M. Combes (Obs. de Paris); G. Bellucci (Inst. di Astrofisica Spaziale, Rome); J.-P. Bibring (Univ. de Paris Sud-Orsay); f. Capaccioni, P. Cerroni, A. Coradini, V. Formisano, G. Filacchione (Instit. Di Astrofisica Spaziale); R. Y. Langevin (Univ. de Paris Sud-Orsay); T. B McCord (U of WA); V. Mennella (Oservatorio Astronomico di Capodimonte); P.D. Nicholson (Cornell U.); B. Sicardy (Obs. de Paris-Meudon)

Last Updated: 9 February 2011

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Last Updated: 9 Feb 2011