The root of anomalously specular reflections from solid surfaces on Saturn\u2019s moon Titan

Randolph D Kirk,Jonathan I Lunine,Stephen D Wall,Shannon M Mackenzie,Jason D Hofgartner,Gregory J Black,Ralph D Lorenz,Charles Elachi,Alice Le Gall,Samuel P D Birch,Alexander G Hayes,Donald B Campbell

doi:10.1038/s41467-020-16663-1

Abstract

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.

Highlights

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
Specular radar reflections from the southern tropical region of Saturn’s moon Titan were observed with the Arecibo Observatory and Green Bank Telescope from 2000-2008 and interpreted as evidence for liquid surfaces[1,2]
We show that the Arecibo Observatory (AO)/Green Bank Telescope (GBT) Anomalously specular radar reflections (ASRR) are correlated to one terrain unit and that this terrain unit has both smoother surfaces and a greater dielectric constant than its surroundings, both of which contribute to the anomalous reflections

Summary

Introduction

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. 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. 1234567890():,; Anomalously 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]. 3) imaged Titan’s surface at infrared and microwave wavelengths from 2004-2017 and discovered >500 lakes/seas[4] It did not, observe liquid surfaces in the regions that are anomalously specular to AO/GBT2. Our conclusion that Titan’s ASRR originate from solid surfaces, suggests that to identify liquids on exoearths by specular reflections, a stringent definition of specular should be used. We recommend a definition based on the coherence of the reflected electromagnetic waves

Methods

Results

Conclusion