Cassini Data Solves Mystery of Arecibo Radar Signals on Titan

Image credit: NASA

For nearly two decades, exceptionally bright radar reflections from Saturn’s moon, Titan, have puzzled scientists. A new study, published in Nature Communications, has finally resolved the mystery.

“Using new evidence from NASA’s Cassini mission, we were able to reopen the Cold Case,” said lead author Dr. Jason Hofgartner of the Jet Propulsion Laboratory. “Our investigation showed that the anomalously bright reflections are likely from dried ancient lakes, known as paleolakes.”

The exceptionally bright radar reflections were initially thought to be a sign of current hydrocarbon lakes or seas. However, when Cassini flew past the moon, it only saw lakes at the poles, not near the equator where the Arecibo radar signals were observed.

“We looked at all of the available data from Cassini: the high-resolution radar images, the near-infrared spectral data, and - most importantly for this work - the radar brightness in altimetry data that primarily measures the topography of Titan,” explained Dr. Hofgartner, who was an associate team member of the Cassini radar instrument.

After comparing those data with Arecibo radar observations, Dr. Hofgartner and his team noted that the unique radar signals were reflecting from two specific geological regions on Titan: the Hotei and Tui Regiones.

“At that time, it was unknown whether these regions were created by cryovolcanism (icy volcanoes) or if they were remnants of ancient lakes or seas.” Dr. Hofgartner continued, “This new analysis indicates that the paleolakes were the culprit.”

“The Cassini observations helped Arecibo because we didn’t fully understand those anomalously bright observations until Cassini could take a closer look,” Dr. Hofgartner reported. “The reverse is true, too, because Arecibo’s data gave us the ‘shining beacon’ for areas of high interest that Cassini needed to look at!”

“The reverse is true, too, because Arecibo’s data gave us the ‘shining beacon’ for areas of high interest that Cassini needed to look at!” - Dr. Jason Hofgartner, Jet Propulsion Laboratory

Dr. Hofgartner shared these new results with the staff at Arecibo in a virtual seminar on August 5, 2020.
“This work was enabled - well, would have been impossible - without the Arecibo Observatory. It is an extremely valuable, one-of-a-kind resource,” Dr. Hofgartner conveyed.

“This work was enabled - well, would have been impossible - without the Arecibo Observatory. It is an extremely valuable, one-of-a-kind resource.” -Dr. Jason Hofgartner, Jet Propulsion Laboratory

“It is the only facility that can make these types of observations with the necessary resolution.” He added, “When Titan is visible again in 2028, we want to be ready!”

The detection of paleolakes is a clear reminder of how science is advanced through synergies between ground-based and spacecraft observations. Dr. Hofgartner asserted, “The combination of Arecibo radar data with spacecraft observations has helped unlock the history of Titan, and could be used to better understand other ocean worlds in our Solar System.”

Future Arecibo radar observations could support the investigation of NASA’s upcoming Dragonfly mission, which will use a rotorcraft to fly to and analyze multiple locations on Titan’s surface in 2034.

“Titan is a very exciting, Earth-like world. It is the only other place we know that has stable liquids on the surface.” Dr. Hofgartner proclaimed, “There are a lot more discoveries to make!”

You can read more about Titan’s paleolakes and their unusual radar properties in the Nature Research Astronomy Community Blog written by Dr. Hofgartner.

About Arecibo
The Arecibo Planetary Radar Program is funded by NASA’s Near-Earth Object Observations Program. The Arecibo Observatory is operated by the University of Central Florida (UCF) in partnership with Universidad Ana G. Méndez - Universidad Metropolitana and Yang Enterprises Inc., under a cooperative agreement with the National Science Foundation (NSF).