Bright patches seen decades ago on Saturn’s moon Titan could be dry lake beds, say scientists
The suggestion, published June 16 in Nature Communications, may solve a 20-year-old mystery
Scientists had discovered strange bright patches on Saturn's largest moon Titan between 2000 to 2008. Researchers now suggest that these mysterious spots on Titan's surface are likely the beds of ancient dry lakes or seas. The findings not only shed light on a nearly 20-year-old mystery but it could also have implications for potentially habitable environments on other planets.
The Arecibo Observatory in Puerto Rico and Green Bank Telescope in West Virginia observed several anomalously bright regions between 2000-2008 from the southern tropical region of Titan, and each was more than 3,000 miles wide. They were referred to as “anomalously specular radar reflections” and were interpreted as evidence for liquid surfaces. Meanwhile, NASA’s Cassini Saturn orbiter had imaged Titan’s surface at “infrared and microwave wavelengths” from 2004-2017 and discovered more than 500 lakes or seas. The lakes did not have water, but the liquid was ethane and methane. However, Cassini did not observe liquid surfaces in the regions that are anomalously specular, says the research team. Accordingly, the researchers decided to reassess the nature of the bright spots.
The team reviewed available data on the bright regions, which includes information from the Arecibo Observatory, the Green Bank Observatory, and the Cassini spacecraft. It had experts from US institutes such as Jet Propulsion Laboratory at California Institute of Technology and Cornell University among others and France such as UVSQ Université Paris-Saclay.
After analyzing the information, they determined that the bright regions could only be either dry lake beds or settled rainfall or dunes. The scientists eliminated rainfall as a possibility as rain events are not frequent enough to explain the patches. The Cassini has observed just two rainfall events on Titan, one in 2004 that was far from the location of the observed regions and another in 2010, which postdated all the observations from Arecibo Observatory, and Green Bank Telescope. Weather on the moon enables the formation of dunes in other locations, and hence dunes were also excluded. The team found only lake beds explain the timing and locations of the observed patches.
“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 (ancient lakes/ancient seas),” say researchers in their findings published in Nature Communications.
The current analysis throws more light on how conditions may have evolved and changed on Titan over the years. What is currently known is that among our solar system’s more than 150 known moons, Titan is the only one with a substantial atmosphere. Of all the places in the solar system, Titan is the only place besides Earth known to have liquids in the form of rivers, lakes and seas on its surface. According to scientists, Titan’s atmosphere is made mostly of nitrogen, like Earth’s, but with a surface pressure 50% higher than Earth’s. Titan has clouds, rain, rivers, lakes and seas of liquid hydrocarbons like methane and ethane and the largest seas are hundreds of feet deep and hundreds of miles wide.
The suggestion in the current study that the bright patches are ancient lake beds or seas that have dried up raises questions as to what happened to the liquid that once formed the lakes over them. According to scientists, one possibility is that it moved to other parts of the moon. It is also possible that the liquid may have evaporated and was destroyed by solar radiation. Based on their findings, the experts suggest that future studies on habitable planets should be careful when searching for evidence of liquid as it may dry up. “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,” says the study.