The landscape of Titan as witness to its climate evolution

Abstract

We investigated the range of Titan climate evolution hypotheses regulated by the role, sources, and availability of methane. We analyzed all available image data (principally synthetic aperture radar (SAR)) of Titan's landscape through the T-86 encounter, starting with focused examinations of terrains that carry the markers of climate evolution. Traditional geologic and geomorphic landscape analysis was used to perform morphometric characterization, establish time-stratigraphic relationships, and interpret local and regional geologic process-oriented evolutionary histories. We then assayed the distribution of terrains we identified with respect to both their latitudinal and altimetric occurrence. Our analysis of the terrain types and distributions was used to evaluate and rank the various climate evolution scenarios. We favor progressive hypotheses, which include a relatively brief period in which precipitation was able to affect geomorphic change in low latitudes at scales perceivable in SAR data, with subsequent gradual decline of precipitation intensity coupled with an increasing poleward restriction.

Formation of gravel pavements during fluvial erosion as an explanation for persistence of ancient cratered terrain on Titan and Mars

“…This banding is interspersed with fluvial channels, and we interpret this banding to be ridgelines and slopes that comprise a densely fluvially-dissected terrain (Moore et al, 2014). This conclusion is supported by imaging during descent by the Huygens probe which revealed dendritic channel networks and bordering hillslopes that are too fine-textured to be resolvable by the Cassini RADAR imaging (Tomasko et al, 2005;Soderblom et al, 2007).…”

“…There are no physical or chemical weathering mechanisms observed operating in situ to break down on Titan massive water ice (or the alkanes) into particles (gravel and smaller) capable of being transported fluvially, despite abundant evidence for fluvial sculpting of its landscape (Collins, 2005;Perron et al, 2006;Burr et al, 2013). Dissected plains in the mid-latitudes of Titan (Moore et al, 2014) as well as extensive dune fields (e.g., Le Gall et al, 2011Gall et al, , 2012 suggest large amounts of transportable sediment. This sediment may have been derived from the physico-chemical breakdown of originally massive water ice, reworked impact-generated megaregolith (a scenario investigated here), or formation of hydrocarbon particle aggregates from photodissociation of methane (Lorenz et al, 2006;Clark et al, 2010).…”

“…The ice crust at that time would have been reworked into a regolith of loose sediment. A number of definitive craters have been identified on Titan, and there is evidence that at least some of the equatorial highlands may retain a heavily cratered surface, albeit fluvially modified (e.g., Moore and Pappalardo, 2011;Moore et al, 2014). Using the Lunar highland megaregolith as an analog, Titan's original surface might have been composed of poorly graded cohesionless granular sediment possibly dominated by sand and finer sediment but containing a small fraction of gravel and boulder-sized material.…”

“…However, the amount of fluvial reworking has been modest by terrestrial standards, as evidenced by the preservation of craters. A number of scenarios might explain the limited degree of dissection (Moore and Pappalardo, 2011;Moore et al, 2014), including a brief interval of favorable conditions at the equator for precipitation during Titans' history, erosion processes being very inefficient (e.g., infrequent precipitation, low precipitation intensity), or the scenario that we investigate here; that fluvial erosion, driven by methane precipitation, may have eroded Titan's megaregolith until a sufficient armoring of grains too coarse to be readily moved accumulated on channel beds.…”

“…If some coarse gravel is mixed with mostly finer sediment erosion may progress rapidly at first but become increasingly slower as relief diminishes and a coarse lag covers the landscape. In the context of Titan and Mars the issue is whether the landscape may have initially eroded rapidly but erosion subsequently slowed to the point that little further modification of the landscape occurred due to progressive armoring with coarse debris, despite the landscape having a dissected appearance (e.g., the equatorial martian highlands and Titan's ''crenulated terrain'' (Moore et al, 2014) (Fig. 1).…”