Fluvial erosion and post-erosional processes on Titan

Abstract

The surface of Titan has been revealed by Cassini observations in the infrared and radar wavelength ranges as well as locally by the Huygens lander instruments. Sand seas, recently discovered lakes, distinct landscapes and dendritic erosion patterns indicate dynamic surface processes. This study focus on erosional and depositional features that can be used to constrain the amount of liquids involved in the erosional process as well as on the compositional characteristics of depositional areas. Fluvial erosion channels on Titan as identified at the Huygens landing site and in RADAR and VIMS observations have been compared to analogous channel widths on Earth yielding average discharges of up to 1600 m 3 /s for short recurrence intervals that are sufficient to move centimetre-sized sediment and significantly higher ones for long intervals. With respect to the associated drainage areas, this roughly translates to 1 -150 cm/day runoff production rates with 10 years recurrence intervals and by assuming precipitation this implies 0.6 -60 mm/hr rainfall rates. Thus the observed surface erosion fits with the methane convective storm models as well as with the rates needed to transport sediment. During Cassini's T20 fly-by, the Visible and Infrared Mapping Spectrometer (VIMS) observed an extremely eroded area at 30°W, 7°S with resolutions of up to 500 m/pixel that extends over thousands of square kilometres. The spectral characteristics of this area change systematically, reflecting continuous compositional and/or particle size variations indicative of transported sediment settling out while flow capacities cease. To account for the estimated runoff production and widespread alluvial deposits of finegrained material, release of area-dependent large fluid volumes are required. Only frequent storms with heavy rainfall or cryovolcanic induced melting can explain these erosional features.

Precipitation-induced surface brightenings seen on Titan by Cassini VIMS and ISS

“…Subsequent Cassini observations showed that similar channels are seen globally on all types of Titan terrain [2][3][4][5][6][7]. While some channels terminate in broad alluvial fans, like those at the spot of Huygens' touchdown, others lead to polar seas [8,9] or dry lakebeds [10,11].…”

AVIATR—Aerial Vehicle for In-situ and Airborne Titan Reconnaissance

“…The resulting theory has application to dunes on Mars, Earth, and Venus. The large fluid discharge rates inferred from some of Titan's channel systems and outflow plains [57] imply catastrophic flooding that resembles that seen on Mars and in the Channeled Scablands on Earth. Titan lakes share striking morphological similarities with Earth lakes in semi-arid regions [18,84].…”

“…Ephemeral streams carve channels [155]. At the equator the streams carry sediments into low-lying plains like the Huygens landing site [57]. At the poles, runoff deposits sediment into lakes and seas [146].…”

“…But with widths of individual links of ∼1 km [8], Cassini cannot resolve structure within the features themselves or even distinguish river channels from river valleys. Cassini has observed other water-ice-rich fluvial depositional plains like the landing site [121] and analog areas (like Chosuk Planitia; [57]), and also noted water-ice-rich ejecta-derived plumes from impact craters Sinlap [65] and Selk [138]. That the water-ice-rich unit covers only a few percent of Titan's surface area, however, is surprising considering that Titan's low bulk density (1.88 g/cm 3 ) indicates that water-ice makes up a substantial fraction of its total mass.…”

Numerical modelling of sedimentary structures in rivers on Earth and Titan

“…Later, in the form of the rain, it reaches the moon's sur face (Jaumann et al, 2008). Next, the liq uid, accord ing to grav ity, flows from an up per to a lower region, even tu ally form ing streams, rivers, and lakes (Lunine and Lorenz, 2009).…”