230316_image1-750.jpg
Artist’s illustration of Uranus and its five largest moons (innermost to outermost): Miranda, Ariel, Umbriel, Titania and Oberon. (Credit: NASA/Johns Hopkins APL/Mike Yakovlev)
How can Uranus be used to indirectly study its moons and identify if they possess subsurface oceans? This is what a recent study presented at the 56th Lunar and Planetary Science Conference hopes to address as a team of scientists investigated using passive radar sounding methods from Uranus to study its five largest moons: Miranda, Ariel, Umbriel, Titania, and Oberon. This study has the potential to help researchers better understand the formation and evolution of Uranus and its largest moons despite a spacecraft not currently visiting Uranus.
Here, Universe Today discusses this incredible research with Dr. Gregor Steinbrügge, who is a geodesist and geophysicist at NASA Jet Propulsion Laboratory and lead author of the study, regarding the motivation behind the study, significant takeaways, the type of mission that could carry out this research, next steps in developing their passive radar sounding method, and the importance of studying the five largest Uranian moons. Therefore, what was the motivation behind the study?
“There is significant scientific interest in the possibility that the Uranian icy moons host or once hosted subsurface oceans,” Dr. Steinbrügge tells Universe Today. “But detecting such oceans—likely buried beneath hundreds of kilometers of ice—has historically proven challenging. Any future mission to Uranus would therefore benefit from being equipped with a broad set of geophysical tools. Passive radar sounding offers a valuable, complementary approach to established techniques like gravity science and magnetic induction. At the same time, it leverages Uranus as a natural radio source, avoiding the need for a dedicated active transmitter—an advantage for a mission likely to face tight resource constraints.”
For the study, the researchers discussed how passive radar sounding could be used by a spacecraft visiting Uranus to use the planet’s radio emissions, known as Uranian Kilometric Radio (UKR) for studying its five largest moons with the goal of ascertaining if they possess subsurface oceans and analyzing them. This study builds off a 2021 concept study led by Dr. Steinbrügge that discussed using passive radar sounding to study Jupiter’s volcanic moon, Io, which is the most volcanically active planetary body in the solar system.
For the proposed spacecraft to perform the UKR, the researchers suggest using a modified version of the NASA Cassini spacecraft’s Radio and Plasma Wave Science instrument, which studied plasma waves emitted by Saturn to study the gas giant and its many moons. The researchers hypothesize that this method could use UKR reflective waves to study Miranda, Ariel, Umbriel, Titania, and Oberon while learning about their interior compositions, specifically subsurface oceans. Therefore, what were the most significant takeaways from this study?
“Actually, we were initially skeptical that kilometric radio emissions from Uranus could support a viable ocean detection experiment,” Dr. Steinbrügge tells Universe Today. “The broader idea of using natural radio sources for subsurface sounding was considered provocative, and we identified several potential showstoppers at the outset. However, as we investigated these challenges, it became increasingly clear that the concept holds up and even offers notable advantages. Since then, we have developed a detailed proof of concept, supported by field and laboratory experiments, which demonstrates the feasibility of passive radar sounding for this application.”
NASA’s Voyager 2 spacecraft continues to be the only spacecraft to visit Uranus and its many moons, which occurred during its flyby in early 1986. Since then, several missions have been proposed to return to this intriguing part of the solar system. This culminated with the Uranus Orbiter and Probe (UOP) mission being selected by the 2023-2032 Planetary Science Decadal Survey as a high-priority Flagship mission, which is the largest and most expensive type of NASA mission. For context, the Cassini mission to Saturn was a Flagship mission.
While Voyager 2 was a flyby mission and Cassini spent several years orbiting Saturn and its many moons, the question then becomes which type of mission this passive radar sounding instrument should fly on. In this study, the researchers briefly mention UOP but mention the word “flyby” several times. Therefore, what type of mission could this passive radar sounding instrument be best suited for?
Dr. Steinbrügge tells Universe Today, “The Uranus Orbiter and Probe mission concept involves a spacecraft orbiting Uranus with multiple targeted flybys of its major moons—analogous to how Cassini orbited Saturn while conducting repeated flybys of satellites like Titan and Enceladus. Our instrument is currently being developed for this type of mission architecture, where repeated flybys of the satellites, while in Uranus orbit, provide the necessary opportunities for subsurface sounding.”
The development of space mission instruments typically takes years to go from a concept to operational hardware and software, requiring multiple phases of tests, re-designs, and budget concerns. NASA uses their Technology Readiness Levels (TRL) system to monitor the various phases of development, whether it’s an instrument of full-blown mission, with TRL ratings from 1 to 9. As development progresses, the mission/instrument gets a rating promotion and is allowed to proceed in their development.
For example, the concept for the Voyager 1 & 2 spacecraft were conceived in the 1960s but they weren’t launched until 1977. During this time, systems and subsystems were being designed, tested, evaluated, re-designed, re-tested, and finally launched in 1977. Therefore, what are the next steps in developing this passive radar sounding instrument?
Dr. Steinbrügge tells Universe Today, “We are currently transitioning from the conceptual phase to hardware design, which would require a system optimized for the harsh environmental conditions of the Uranian system. This includes building and testing a prototype. At the same time, we are working to better characterize how Uranus’ radio emissions interact with and illuminate the icy moons, and how our experiment influences key mission parameters such as optimal flyby altitudes and velocities.”
Despite the only up-close data of Uranus and its moons coming from Voyager 2’s brief flyby in early 1986, scientists continue to pour over that data while gaining greater insight into its five largest moons and their unique geological features. This includes a recent study that analyzed potential cryovolcanism on Umbriel and Oberon based on similar geological features on the dwarf planet Ceres.
Additionally, the moon Miranda looks like it was blown to pieces and haphazardly pieced back together, raising questions regarding its own formation and evolution. The moon Ariel is has a predominantly dark surface with bright bands near the poles and most of the moon hasn’t been imaged. Finally, the moon Titania has a brighter surface with even brighter spots across its surface, with most of its surface also being unmapped. But why is it so important to learn more about Uranus’ largest moons like Miranda, Ariel, Umbriel, Titania, and Oberon and whether or not they possess subsurface oceans?
“The existence or non-existence of oceans within the Uranian satellites is a very interesting ongoing scientific debate which ultimately links to the fundamental questions of how ocean worlds form and evolve,” Dr. Steinbrügge tells Universe Today. “Some of these oceans might be old but constantly freezing, some might have formed surprisingly late in the geologic history of these moons. And understanding the formation and evolution of ocean worlds gives us fundamental insights into the working principles of our solar system.”
Dr. Steinbrügge continues, “But understanding these processes not only informs our knowledge of the Uranian system but also contributes to our understanding of how ocean worlds form and persist more generally. Given the prevalence of Uranus- and Neptune-sized exoplanets, many of which may have their own satellite systems, this research could have implications well beyond our solar system.”
How will passive radar sounding help scientists better understand Uranus and its largest moons in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!
Laurence Tognetti, MSc
Laurence Tognetti is a six-year USAF Veteran with extensive journalism, science communication, and planetary science research experience for various outlets. He specializes in space and astronomy and is the author of “Outer Solar System Moons: Your Personal 3D Journey”. Follow him on X (Twitter) and Instagram @ET_Exists.
You can email Laurence for article inquiries or if you're interested in showcasing your research to a global audience.