The bathymetry of a Titan sea

Mastrogiuseppe, Marco; Poggiali, Valerio; Hayes, Alexander G.; Lorenz, Ralph D.; Lunine, Jonathan I.; Picardi, G.; Seu, R.; Flamini, E.; Mitri, G.; Notarnicola, Claudia; Paillou, Philippe; Zebker, H. A.

doi:10.1002/2013gl058618

Cited by 133 publications

(152 citation statements)

References 20 publications

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“…These profiles, which are both above their respective noise floors, are correlated and the lesser T92 NRCS, relative to that of T91, is due to its greater incidence angle. This behaviour is consistent, for example, with the radar transmitting through the liquid and scattering off the seabed as claimed by other analyses 3,19,22,23 . In the region of the anomalies, however, the T92 profile exhibits a large spike in NRCS but no similar anomalous spike is observed in the T91 profile.…”

supporting

confidence: 91%

“…We used the recently measured index of refraction for Ligeia Mare to calculate the refracted incidence angles of the radar 19 and considered a perfectly flat surface for Ligeia Mare, consistent with recent measurements 8 , when calculating the Fresnel transmission coefficients.…”

Section: Modellingmentioning

confidence: 95%

“…2 and their parameters are given in the legend. We also considered models for submerged seamounts, using constraints for the surface roughness and dielectric constant of Ligeia Mare derived from recent analyses of the nadir (0 degrees incidence) signal in the T91 observation 8,19 and found that these models are also ruled out to 88% confidence (Methods). We note that it is the combination of the small likelihood that the NRCS at 3 degrees is larger than at 6 degrees with the low upper limits at higher incidence angles that inhibits the models from fitting the observations.…”

mentioning

confidence: 99%

“…Therefore, the evolution of the anomalous features seems to have included a reversion after the T92 pass and we do not consider further hypotheses that predict the formation of a new permanent structure, such as an island through cryovolcanism and restrict further discussion to hypotheses for ephemeral features. From recent analysis of the nadir signal in the T91 observation 19 , the absorption of the radar energy as it propagates through the sea is constrained to be small. Thus, variations in sea level should not strongly influence the measured NRCS and are unlikely to explain the transient anomalies.…”

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confidence: 99%

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Transient features in a Titan sea

et al. 2014

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Titan's surface-atmosphere system bears remarkable similarities to Earth's, the most striking being an active, global methane cycle akin to Earth's water cycle 1,2 . Like the hydrological cycle of Earth, Titan's seasonal methane cycle is driven by changes in the distribution of solar energy 2 . The Cassini spacecraft, which arrived at Saturn in 2004 in the midst of northern winter and southern summer, has observed surface changes, including shoreline recession, at Titan's south pole 3,4 and equator 5 . However, active surface processes have yet to be confirmed in the lakes and seas in Titan's north polar region 6-8 . As the 2017 northern summer solstice approaches, the onset of dynamic phenomena in this region is expected 6,7,9-12 . Here we present the discovery of bright features in recent Cassini RADAR data that appeared in Titan's northern sea, Ligeia Mare, in July 2013 and disappeared in subsequent observations. We suggest that these bright features are best explained by the occurrence of ephemeral phenomena such as surface waves, rising bubbles, and suspended or floating solids. We suggest that our observations are an initial glimpse of dynamic processes that are commencing in the northern lakes and seas as summer nears in the northern hemisphere.Anomalous, bright features were detected in Titan's north polar sea, Ligeia Mare, by the Cassini Titan Radar Mapper 13 (RADAR) during the T92 synthetic aperture radar (SAR) pass (Fig. 1). Three preceding SAR observations (T25, T29 and T64) and a subsequent low-resolution SAR observation (T95) did not detect the anomalous features. The faint, grey spots in the circle of the T95 image are consistent with the speckle noise in the surrounding sea region and thus are not anomalous. Radar backscatter above the noise floor, however, was also detected during preceding T91 radar scatterometry-mode observations 13 but we argue that this signal may not have originated from the anomalies. Subsequent Visual and Infrared Mapping Spectrometer (VIMS) and Imaging Science Subsystem observations (T93 and T94) also did not detect the anomalies. These eight passes, constituting all of the highresolution observations up to the present of the region of the anomalous features, are shown in Fig. 1. In radar images, brightness is determined by the normalized radar cross-section (NRCS), the ratio of the radar energy backscattered to the receiver compared with that from an isotropic scatterer 14 . Dynamic processes such as waves 7 , suspended particles 3 , or bubbles 15 increase the NRCS. Such phenomena have not been confirmed in Titan's northern lakes and seas, which have a dielectric constant that indicates a methane-ethane composition and surface height variations of less than 1 mm (ref. 8). The progressive seasonal increase in insolation that is occurring however has been predicted to power the onset of energetic processes 6,7,9-12 and we argue that these anomalous features are the observation of transient features in the seas. The regional extent of the anomalous signal, which does not s...

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supporting

confidence: 91%

Section: Modellingmentioning

confidence: 95%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Transient features in a Titan sea

et al. 2014

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“…Spectroscopic observations of Ontario Lacus suggest the presence of ethane (Brown et al, 2008), although the abundance is not constrained by these measurements. Recent observations of Ligeia Mare, conducted by the Cassini RADAR instrument, have demonstrated that it is primarily composed of methane (Mastrogiuseppe et al, 2014). Evaporation rates from a lake that is mostly methane will be much greater than from a lake that is mostly ethane (Mitri et al, 2007;Tokano, 2009).…”

Section: Introductionmentioning

confidence: 99%

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

Ádámkovics

Mitchell

Hayes

et al. 2016

Icarus

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The spatial distribution of the tropospheric methane on Titan was measured using near-infrared spectroscopy. Ground-based observations at 1.5 µm (H-band) were performed during the same night using instruments with adaptive optics at both the W. M. Keck Observatory and at the Paranal Observatory on 17 July 2014 UT. The integral field observations with SINFONI on the VLT covered the entire H-band at moderate resolving power, R = λ/∆λ ≈ 1, 500, while the Keck observations were performed with NIRSPAO near 1.5525 µm at higher resolution, R ≈ 25, 000. The moderate resolution observations are used for flux calibration and for the determination of model parameters that can be degenerate in the interpretation of high resolution spectra. Line-by-line calculations of CH 4 and CH 3 D correlated k distributions from the HITRAN 2012 database were used, which incorporate revised line assignments near 1.5 µm. We fit the surface albedo and aerosol distributions in the VLT SINFONI observations that cover the entire H-band window and used these quantities to constrain the models of the high-resolution Keck NIRSPAO spectra when retrieving the methane abundances. Cassini VIMS images of the polar regions, acquired on 20 July 2014 UT, are used to validate the assumption that the opacity of tropospheric aerosol is relatively uniform below 10 km. We retrieved methane abundances at latitudes between 42 • S and 80 • N. The tropospheric methane in the Southern mid-latitudes was enhanced by a factor of ∼10-40% over the nominal profile that was measured using the GCMS on Huygens. The Northern hemisphere had ∼90% of the nominal methane abundance up to polar latitudes (80 • N). These measurements suggest that a source of saturated polar air is equilibrating with dryer conditions at lower latitudes.

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

Zebker

Hayes

Janssen

et al. 2014

Geophysical Research Letters

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Cassini 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 +/À 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.

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The bathymetry of a Titan sea

Cited by 133 publications

References 20 publications

Transient features in a Titan sea

Transient features in a Titan sea

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

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

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