Ground-based radar observations of Titan: 2000–2008

Black, G. J.; Campbell, D. B.; Carter, Lynn M.

doi:10.1016/j.icarus.2010.10.025

Cited by 35 publications

(20 citation statements)

References 55 publications

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“…Titan's surface is not resolved in AO/ GBT radar observations, however, the Doppler shift from Titan's rotation results in an echo spectrum that is equivalent to the scan of a slit across Titan's disk 1,2 . Figure 1a-e show example AO/GBT echo spectra of Titan (from the 83 observations, acquired over eight oppositions, in Black et al 2 ). The central, 1-Hz wide, Doppler bin corresponds to a ≈14-km wide slit that includes the entire ≈5150 km (Titan's diameter) height of the disk.…”

Section: Resultsmentioning

confidence: 99%

“…The NRCS of a surface generally decreases with increasing incidence angle (e.g., refs. 2,7,8 ). As a result, AO/GBT spectra generally decrease away from the central Doppler bin (slit over the center of the disk), because the central Doppler bin includes the subradar point, where the incidence angle is zero degrees.…”

Section: Resultsmentioning

confidence: 99%

“…The transmitted signal in the AO/GBT observations was circularly polarized and the echo power was measured in both the opposite and same circular senses as that transmitted 1,2 . A specular reflection would originate from the subradar point and increase the NRCS of the central Doppler bin of the opposite circular polarization spectrum.…”

Section: Resultsmentioning

confidence: 99%

“…A nomalously specular radar reflections from the southern tropical region of Saturn's moon Titan (equator to ≈27°S, Saturn and Titan have a solar obliquity of ≈27°) were observed with the Arecibo Observatory and Green Bank Telescope from 2000-2008 and interpreted as evidence for liquid surfaces 1,2 . The Cassini Saturn orbiter (e.g., ref.…”

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confidence: 99%

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The root of anomalously specular reflections from solid surfaces on Saturn’s moon Titan

et al. 2020

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Saturn's moon Titan has a methane cycle with clouds, rain, rivers, lakes, and seas; it is the only world known to presently have a volatile cycle akin to Earth's tropospheric water cycle. Anomalously specular radar reflections (ASRR) from Titan's tropical region were observed with the Arecibo Observatory (AO) and Green Bank Telescope (GBT) and interpreted as evidence for liquid surfaces. The Cassini spacecraft discovered lakes/seas on Titan, however, it did not observe lakes/seas at the AO/GBT anomalously specular locations. A satisfactory explanation for the ASRR has been elusive for more than a decade. Here we show that the ASRR originate from one terrain unit, likely paleolakes/paleoseas. Titan observations provide ground-truth in the search for oceans on exoearths and an important lesson is that identifying liquid surfaces by specular reflections requires a stringent definition of specular; we propose a definition for this purpose.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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confidence: 99%

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The root of anomalously specular reflections from solid surfaces on Saturn’s moon Titan

et al. 2020

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“…The CPR, defined as the ratio between power reflected in the same sense of circular polarization (SC) as that transmitted and the echo in the opposite sense (OC) of circular polarization, is strongly modulated by roughness‐induced changes in scattering on or beneath a target surface. We have observations of μ c in a wide range of terrestrial [ Campbell et al , 1993; Rignot , 1995; Plaut et al , 2004; Campbell , 2009] and planetary settings: Mars [ Harmon et al , 1999; Harmon and Nolan , 2007], the Galilean satellites [ Campbell et al , 1978; Ostro et al , 1992], Venus [ Campbell et al , 1999; Carter et al , 2004], Mercury [ Harmon et al , 1994, 2001, 2011], Titan [ Black et al , 2011], asteroids [ Shepard et al , 2008; Benner et al , 2008], and the Moon [ Campbell et al , 2006, 2009; Spudis et al , 2010].…”

Section: Introductionmentioning

confidence: 99%

High circular polarization ratios in radar scattering from geologic targets

Campbell¹

2012

J. Geophys. Res.

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[1] We examine and model the occurrence of circular polarization ratio (CPR or m c ) values greater than unity in terrestrial and planetary radar observations as a guide to the range of associated surface morphology. Lunar crater deposits exhibit maximum m c values at 12.6 and 70-cm wavelength of 2 to 3. CPR values for Maxwell Montes on Venus range up to about 1.5 at 12.6-cm wavelength. Echoes from SP Flow in Arizona exhibit m c up to 2 at 24-cm wavelength. Scattering from rock edges and cracks (dipole-like) produces m c of unity for single scattering and up to about 2 for multiple reflections. Scattering from natural corner reflectors (dihedrals) formed by pairs of rock facets can yield an average m c of 3-4, but likely requires non-random or scale-limited surface roughness properties in order to dominate the observed echo. The dihedral mechanism is required to satisfy the highest lunar m c observations, while echoes from SP Flow and Maxwell Montes could be explained by just the dipole mechanism. The dihedral scenario requires rocky facets on scales many times the radar wavelength, which the blocks at SP Flow (and the unknown surface texture of Maxwell) may not provide. The random dipole model is less demanding in terms of structures and scales, and likely increases the CPR of lunar or asteroid regoliths through scattering from and between rocks.

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