Ligeia Mare Research Articles

First depth measurements taken for extraterrestrial lake

Since 2004, NASA's Cassini mission has been orbiting Saturn and taking measurements of the planet itself as well as its largest moon, Titan, which has an atmosphere that can serve as an analog for a young Earth. During Cassini's flyby of Titan in May 2013, it collected data from the moon's second‐largest sea, Ligeia Mare.

A radar map of Titan Seas: Tidal dissipation and ocean mixing through the throat of Kraken

We present a radar map of the Titan’s seas, with bathymetry estimated as proportional to distance from the nearest shore. This naïve analytic bathymetry, scaled to a recent radar sounding of Ligeia Mare, suggests a total liquid volume of ∼32,000km3, at the low end of estimates made in 2008 when mapping coverage was incomplete. We note that Kraken Mare has two principal basins, separated by a narrow (∼17km wide, ∼40km long) strait we refer to as the ‘throat’. Tidal currents in this strait may be dramatic (∼0.5m/s), generating observable effects such as dynamic topography, whirlpools, and acoustic noise, much like tidal races on Earth such as the Corryvreckan off Scotland. If tidal flow through this strait is the dominant mixing process, the two basins take ∼20 Earth years to exchange their liquid inventory. Thus compositional differences over seasonal timescales may exist, but the composition of solutes (and thus evaporites) over Croll–Milankovich timescales should be homogenized.

Cassini sheds light on Titan's second largest lake, Ligeia Mare

Saturn's largest moon, Titan, is known for its dense, planet‐like atmosphere and large lakes most likely made of methane and ethane. It has been suggested that Titan's atmosphere and surface are a model of early Earth. Since the early 2000s, NASA's Cassini space probe has been unlocking secrets of the distant moon.

The bathymetry of a Titan sea

AbstractWe construct the depth profile—the bathymetry—of Titan's large sea Ligeia Mare from Cassini RADAR data collected during the 23 May 2013 (T91) nadir‐looking altimetry flyby. We find the greatest depth to be about 160 m and a seabed slope that is gentler toward the northern shore, consistent with previously imaged shoreline morphologies. Low radio signal attenuation through the sea demonstrates that the liquid, for which we determine a loss tangent of 3 ± 1·10−5, is remarkably transparent, requiring a nearly pure methane‐ethane composition, and further that microwave absorbing hydrocarbons, nitriles, and suspended particles be limited to less than the order of 0.1% of the liquid volume. Presence of nitrogen in the ethane‐methane sea, expected based on its solubility and dominance in the atmosphere, is consistent with the low attenuation, but that of substantial dissolved polar species or suspended scatterers is not.

Surface of Ligeia Mare, Titan, from Cassini altimeter and radiometer analysis

AbstractCassini radar observations of the surface of Ligeia Mare collected during the 23 May 2013 (T91) Cassini flyby show that it is extremely smooth, likely to be mostly methane in composition, and exhibits no surface wave activity. The radar parameters were tuned for nadir‐looking geometry of liquid surfaces, using experience from Cassini's only comparable observation, of Ontario Lacus on 21 December 2008 (T49), and also include coincident radiometric observations. Radar echoes from both passes show very strong specular radar reflections and limit surface height variations to 1 mm rms. The surface physical temperature at 80°N is 92 +/− 0.5 K if the sea is liquid hydrocarbon and the land is solid hydrocarbon, essentially the same as Cassini CIRS measurements. Furthermore, radiometry measurements over the surrounding terrain suggest dielectric constants from 2.2 to 2.4, arguing against significant surface water ice unless it is extremely porous.

Plumbing the depths of Ligeia: considerations for depth sounding in Titan's hydrocarbon seas.

Saturn's moon Titan is the only satellite in this solar system with a dense atmosphere and hydrocarbon seas. The Titan Mare Explorer (TiME) mission would splashdown a capsule to float for 3 months on Ligeia Mare, a several-hundred-kilometer wide sea near Titan's north pole. Among TiME's scientific goals is the determination of the depth of Ligeia, to be achieved with an acoustic depth sounder. Since Titan's surface temperature is known to vary around 92 K, all instruments must be ruggedized to operate at cryogenic temperatures. This paper's contributions include an approach to infer key acoustic properties of this remote environment and the extraterrestrial environment's influence on the development of a cryogenic depth sounder. Additionally, an approach is formulated to infer the transducer's response, sensitivity, and performance when in situ calibration is impossible or when replicating key environmental conditions is too costly.

Estimates of fluvial erosion on Titan from sinuosity of lake shorelines

Titan has few impact craters, suggesting that its surface is geologically young. Titan's surface also has abundant landforms interpreted to be fluvial networks. Here we evaluate whether fluvial erosion has caused significant resurfacing by estimating the cumulative erosion around the margins of polar lakes. A scarcity of detailed topographic data makes it difficult to measure fluvial incision on Titan, but images of drowned fluvial features around the lake margins, where elevated levels of hydrocarbon liquids appear to have partly flooded fluvial valleys, offer a constraint on the topography. We mapped the shorelines of several lakes to obtain topographic contours that trace the fluvially dissected topography. We then used a numerical landscape evolution model to calibrate a relationship between contour sinuosity, which reflects the extent of fluvial valley incision, and cumulative erosion. We confirmed this relationship by analyzing a partially dissected surface adjacent to the Minnesota River, USA. Comparison of the mapped Titan contours with the sinuosity‐erosion relationship suggests that cumulative fluvial erosion around the margins of Titan's polar lakes, including Ligeia Mare, Kraken Mare, and Punga Mare in the north and Ontario Lacus in the south, ranges from 4% to 31% of the initial relief. Additional model simulations show that this amount of fluvial erosion does not render craters invisible at the resolution of currently available imagery, suggesting that fluvial erosion is not the only major resurfacing mechanism operating in Titan's polar regions.

A geological characterization of Ligeia Mare in the northern polar region of Titan

Ligeia Mare, the second largest sea on Titan, resides in an endorheic basin dominated by seas in the northeastern polar region. Ligeia's shoreline morphology resembles terrestrial man-made reservoirs where water is dammed and valleys are flooded. Here we describe the mare and surrounding geologically diverse terrain including rugged highlands and lowlands, incised valleys, and smooth plains. Observations include active processes evidenced by drainage flow directions, areal extent of rivers and river valleys, sediment volume estimates, and varied drainage patterns. Headward erosion has carved valley and ridge systems in the flanks of the highlands, while the more distal highland plateau is mostly uncut by rivers or extensive erosion and contains smaller lakes, lakebeds, and mottled terrain.

A global topographic map of Titan

Cassini RADAR SARtopo and altimetry data are used to construct a global gridded 1×1° elevation map, for use in Global Circulation Models, hydrological models and correlative studies. The data are sparse, and so most of the map domain (∼90%) is populated with interpolated values using a spline algorithm. The highest (∼+520m) gridded point observed is at 48°S, 12°W. The lowest point observed (∼1700m below a 2575km sphere) is at 59°S, 317°W: this may be a basin where liquids presently in the north could have resided in the past. If the deepest point were once a sea with the areal extent of present-day Ligeia Mare, it would be ∼1000m deep. We find four prominent topographic rises, each ∼200km wide, radar-bright and heavily dissected, distributed over a ∼3000kmarc in the southeastern quadrant of Titan (∼40–60°S, 15–150°W).

Are tropical cyclones possible over Titan’s polar seas?

While extratropical cyclones cannot be expected in Titan’s barotropic troposphere, tropical cyclones which gain their energy from the latent heat of sea evaporation cannot be entirely dismissed over Titan’s polar hydrocarbon seas. The most essential condition for the genesis of tropical cyclones on Titan is a methane-rich composition of the polar seas. The most likely season for Titan’s hypothetical tropical cyclones is around the northern summer solstice when the sea surface gets warmer and the relative vorticity of the near-surface air increases by seasonal convergence and equatorial wave activity. A tropical cyclone would manifest itself as an anti-clockwise swirling vortex right over one of the northern seas (Kraken Mare, Ligeia Mare, Punga Mare) and increase the surface wind over the seas by an order of magnitude. On the other hand, tropical cyclones are unlikely to emerge over Titan’s few tropical lakes for dynamic reasons such as negligible Coriolis parameter and large vertical wind shear.

Observations of Titan’s Northern lakes at 5 μm: Implications for the organic cycle and geology

Since Titan entered Northern spring in August 2009, the North Pole has been illuminated allowing observations at optical wavelengths. On June 5, 2010 the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft observed the Northern Pole area with a pixel size from 3 to 7km. Since, as we demonstrate, little of the solar flux at 5μm is scattered by the atmosphere, these observations were obtained at relatively large incidence angles and allowed us to build a mosaic covering an area of more than 500,000km2 that overlaps and complements observations made by the Synthetic Aperture Radar (SAR) in 2007. We find that there is an excellent correlation between the shape of the radar dark area, known as Ligeia Mare and the VIMS 5-μm dark unit. Matching most of the radar shoreline, the 2010 VIMS observations suggest that the 125,000-km2 surface area of Ligeia Mare measured by RADAR in 2007 has not significantly changed. The VIMS observations complement the radar observations to the west of Ligeia Mare and suggest that Ligeia Mare is connected to Kraken Mare by either a diffuse network similar to a swamp area, or by well-defined, sub-pixel rivers. Considering the results of recent evaporation models of methane, our preferred interpretation of the relative constancy in surface area of Ligeia is that it is principally composed of ethane although we cannot rule out the possibility that methane evaporation is balanced with replenishment by either precipitation or underground seepage. There is also strong correlation between the location of the small radar lakes and the small VIMS 5-μm dark patches. The geographic location of the small lakes are within a VIMS pixel of the SAR location, suggesting that the non-synchronous component of Titan’s spin rate, if it exists, was less than 2.3×10−4deg/day between 2007 and 2010 in agreement with the recent T64 radar observations. These observations question the existence of non-synchronous rotation. Two radar-bright features appear dark at 5-μm. The simplest interpretation is that these are very shallow lakes, less than one meter deep. Three new small lakes, named Freeman, Cardiel, and Towada by the IAU, are found outside of the area mapped with the SAR. A single-scattering model describing reflection of sunlight at 5-μm suggests that the lake surface is mirror-like and that the albedo of the solid surfaces surrounding the lakes is about 8%. These observations together with information of the haze aerosols allow us to show that Titan’s lakes, atmospheric ethane and aerosol haze are smaller carbon reservoirs than Titan’s sand dunes and atmospheric methane. A simple model involving an outburst of methane a few hundreds of Myr ago followed by the dissociation of methane in the atmosphere leading to the formation of the haze particles that constitute the dune fields would be consistent with both the present observations and recent measurements of isotopic ratios in atmospheric methane (Mandt, K.E. et al. [2012]. Astrophys. J. 749(160), 14).

Plumbing the depths of Ligeia: Considerations for acoustic depth sounding in Titan's hydrocarbon seas

Saturn’s moon Titan is the only satellite in our solar system with a dense atmosphere and hydrocarbon seas. The proposed Titan Mare Explorer (TiME) mission would splashdown a capsule to float for 3 months on Ligeia Mare, a several-hundred-kilometer wide sea near Titan’s north pole. Among TiME’s scientific goals is the determination of the depth of Ligeia, to be achieved with an acoustic depth-sounder. Since Titan’s surface temperature is known to vary around 92 K, all instruments must be ruggedized to operate at cryogenic temperatures. This paper’s contributions include an approach to infer key acoustic properties of this remote environment, their influence on the development of a cryogenic depth sounder, and on an approach to infer the transducer’s response, sensitivity and performance when unable to perform in-situ calibration measurements or to replicate key environmental conditions. This effort was conducted under the auspices of the Civilian Space Independent Research and Development program from the Johns Hopkins University Applied Physics Laboratory.

The growth of wind-waves in Titan’s hydrocarbon seas

Ocean wave growth on Titan is considered. The classic Sverdrup–Munk theory for terrestrial wave growth is applied to Titan, and is compared with a simple energy balance model that exposes the effect of Titan’s environmental parameters (air density, gravity, and fluid density). These approaches are compared with the only previously-published (semi-empirical) model (Ghafoor, N.A.-L., Zarnecki, J.C., Challenor, P., Srokosz, M.A. [2000] J. Geophys. Res. 105, 12,077–12,091, hereafter G2k), and allow the impact of various parameters such as atmospheric density to be transparently explored.Our model, like G2k, suggests fully-developed significant wave heights on Titan Hs=0.2U2, where U is the windspeed (SI units): in dimensionless terms this is rather close to Hs=0.2U2/g, a rule of thumb previously noted for terrestrial waves (we find various datasets where the prefactor varies by ∼2). It is noted that liquid and air densities affect the growth rate of waves, but not their fully-developed height: for 1m/s winds wave amplitude reaches 0.15m (75% of fully-developed) with a fetch of only 1km, rather faster than predicted by G2k. Liquid viscosity has no major effect on gravity wave growth, but does influence the threshold windspeed at which gravity–capillary waves form in the first place.The model is used to develop predicted ranges for wave height to guide the design of the Titan Mare Explorer (TiME), a proposed Discovery-class mission to float a capsule on Ligeia Mare in 2023. For the expected maximum 1m/s winds, a significant wave height of 0.2m and wavelength of ∼4m can be expected. Assuming that wave heights follow Rayleigh statistics as they do on Earth, then given the wave period of ∼4s, individual waves of ∼0.6m might be encountered over a 3month period.For predicted Titan winds at Kraken Mare, significant wave heights may reach ∼0.6m in the peak of summer but do not exceed the tidal amplitude at its northern end, consistent with the area around Mayda Insula being a tidal flat, while elsewhere on Kraken and Ligeia and at Ontario Lacus, shorelines may be wave- or tidally-dominated, depending on the specific location.

Winds and tides of Ligeia Mare, with application to the drift of the proposed time TiME (Titan Mare Explorer) capsule

We use two independent General Circulation Models (GCMs) to estimate surface winds at Titan’s Ligeia Mare (78°N, 250°W), motivated by a proposed mission to land a floating capsule in this ∼500km hydrocarbon sea. The models agree on the overall magnitude (∼0.5–1m/s) and seasonal variation (strongest in summer) of windspeeds, but details of seasonal and diurnal variation of windspeed and direction differ somewhat, with the role of surface exchanges being more significant than that of gravitational tides in the atmosphere. We also investigate the tidal dynamics in the sea using a numerical ocean dynamics model: assuming a rigid lithosphere, the tidal amplitude is up to ∼0.8m. Tidal currents are overall proportional to the reciprocal of depth—with an assumed central depth of 300m, the characteristic tidal currents are ∼1cm/s, with notable motions being a slosh between Ligeia’s eastern and western lobes, and a clockwise flow pattern.We find that a capsule will drift at approximately one tenth of the windspeed, unless measures are adopted to augment the drag areas above or below the waterline. Thus motion of a floating capsule is dominated by the wind, and is likely to be several km per Earth day, a rate that will be readily measured from Earth by radio navigation methods. In some instances, the wind vector rotates diurnally such that the drift trajectory is epicyclic.

FHP motor for machine tool applications

Titan’s lakes and seas are targets of particular interest for future exploration. We review candidate splashdown areas in Ligeia and Kraken Mare, and Ontario Lacus. Titan’s thick and dense atmosphere means that landing dispersions of spacecraft are dominated by uncertainties in wind drift, and thus the feasibility of landing safely in the sea with a simple Huygens-like descent system (i.e. without guidance or propulsion) is dependent upon these uncertainties being small enough that the landing point dispersion lies within the sea. Because Titan’s winds vary with season, notably through the formation of a high-speed stratospheric jet in the winter hemisphere, landing point dispersions are seasonally-dependent as well as latitude-dependent. Ligeia Mare, at 78oN, sees relatively small dispersions but offers viable Direct-to-Earth (DTE) communication only until 2024. A wide part of Kraken Mare (450 × 90 km) at 70oN can be comfortably reached at all times, and is viable for assured Direct-to-Earth communication until 2026, or with a relay spacecraft thereafter. The seasonal geometry permits DTE from the northern seas again after 2040. Wind dispersions are always too large for Ontario, unless a steerable parachute or similar system is used to tighten the landing ellipse.