The lakes of Titan

Abstract

The surface of Saturn's haze-shrouded moon Titan has long been proposed to have oceans or lakes, on the basis of the stability of liquid methane at the surface. Initial visible and radar imaging failed to find any evidence of an ocean, although abundant evidence was found that flowing liquids have existed on the surface. Here we provide definitive evidence for the presence of lakes on the surface of Titan, obtained during the Cassini Radar flyby of Titan on 22 July 2006 (T16). The radar imaging polewards of 70 degrees north shows more than 75 circular to irregular radar-dark patches, in a region where liquid methane and ethane are expected to be abundant and stable on the surface. The radar-dark patches are interpreted as lakes on the basis of their very low radar reflectivity and morphological similarities to lakes, including associated channels and location in topographic depressions. Some of the lakes do not completely fill the depressions in which they lie, and apparently dry depressions are present. We interpret this to indicate that lakes are present in a number of states, including partly dry and liquid-filled. These northern-hemisphere lakes constitute the strongest evidence yet that a condensable-liquid hydrological cycle is active in Titan's surface and atmosphere, in which the lakes are filled through rainfall and/or intersection with the subsurface 'liquid methane' table.

Ground-based radar observations of Titan: 2000–2008

“…Lake features observed in Cassini SAR imagery (Stofan et al, 2007) have sizes ranging from $3 km to $100 km, comparable to the length of the track of the sub-radar point in these groundbased data. To test for any such short length variability in these narrow specular components, the daily runs were reprocessed in shorter segments, although still long enough so as to maintain a useful signal level.…”

“…Owen, 1982) because the surface temperature is near the condensation point of the methane occurring in the atmosphere at a few percent level, and that Titan may even have a methane cycle similar to Earth's water cycle (Atreya et al, 2006). While dynamics (Sears, 1995;Sagan and Dermott, 1982), the first infrared maps of the surface (Smith et al, 1996; Combes , 1997), and the first radar observations (Muhleman et al, 1995) effectively ruled out a global ocean, Cassini observations of drainage and flow features Burr et al, 2009) and even lake-like features themselves (Stofan et al, 2007) have confirmed that this picture is generally correct. As first discussed by Campbell et al (2003) using a subset of these ground-based radar data, a straightforward interpretation of the narrow echoes is that they are the expected to result from liquid bodies which can have arbitrarily flat surfaces given appropriately low wind excitation of waves.…”

“…Observations of the high abundance of methane in its nitrogen atmosphere led to suspicion that there might be a surface-atmosphere methane cycle similar to Earth's water cycle (Owen, 1982;Atreya et al, 2006). New evidence, particularly from Cassini, demonstrates this is truer than was expected with observations providing evidence for wind processes (Radebaugh et al, 2008;Lorenz et al, 2006), continuing erosion Barnes et al, 2007b;Elachi et al, 2006), clouds (Brown et al, 2002;Griffith et al, 1998), and even apparent liquid bodies (Stofan et al, 2007). Changes due to an active methane cycle (Turtle et al, 2009) and possible recent cryovolcanic activity (Lopes et al, 2007;Wall et al, 2009) further strengthens the Earth-like parallels.…”

“…Despite the quite convincing discovery with the Cassini RADAR of lakes only at high latitudes (Stofan et al, 2007) including a specular glint from Fig. 9 but using the fit parameters from the re-aligned spectra of Fig.…”

Meridional variation in tropospheric methane on Titan observed with AO spectroscopy at Keck and VLT

“…Seasonal variations in the general circulation (Mitchell et al, 2009) as well as predictions of the locations and frequency of clouds Preprint submitted to Icarus September 30, 2015 (Schneider et al, 2012) depend on the distribution of methane near the surface, both in the regolith and the lower atmosphere. Lakes and seas of liquid hydrocarbons (Stofan et al, 2007) provide both sinks and sources of methane on the surface. The north pole contains by far the greatest extent of open liquids on Titan Lorenz et al, 2008;Sotin et al, 2012;Lorenz et al, 2014).…”