The Favorite Solar System Moons of Planetary Geologists; An In-Depth Discussion

The moons of our Solar System have garnered quite a lot of attention in the last few years, especially pertaining to astrobiology and the search for life beyond Earth. From the Galilean moons of Jupiter to the geysers of Enceladus to the methane lakes on Titan, these small worlds continue to humble us with both their awe and mystery. But do the very same scientists who study these mysterious and intriguing worlds have their own favorite moons? As it turns out, seven such planetary geologists were kind enough to share their favorite Solar System moons with Universe Today!

“My favorite moon is Enceladus, for two reasons,” said Dr. Francis Nimmo, who is a professor in the Earth & Planetary Sciences Department at UC Santa Cruz. “First, it is geologically active, which was very surprising given how tiny it is – it is spewing jets of ice and water vapor into space. Second, because it is kind enough to be giving us free samples of its interior, it makes a very attractive target for future spacecraft missions – you can analyze the composition of the ocean (and even look for life) without having to drill through the ice.”

Image mosaic of Enceladus taken by NASA’s Cassini spacecraft in October 2008 from approximately 25 kilometers (15.6 miles) of the moon’s surface. (Credit: NASA/JPL/Space Science Institute)

Saturn’s sixth-largest moon, Enceladus, was discovered in 1789 by William Herschel, and whose diameter is approximately the size of the State of Arizona. As noted by Dr. Nimmo, Enceladus possesses geysers that discharge ice and water vapor from a series of fissures known as “tiger stripes”. These geysers were first observed by NASA’s Cassini spacecraft during its mission in the 2000s, and Cassini even flew through them to test their composition, finding water vapor, a variety of salts, methane, and carbon dioxide.

“My favorite moon of the Solar System is Io, the innermost of the Galilean satellites of Jupiter,” said Dr. David Williams, who is a research professor in the School of Earth and Space Exploration at Arizona State University. “Discovered by Galileo Galilei in January of 1610, Io is the most geologically active of all of the moons of our Solar System. A Laplace orbital resonance with Jupiter’s other moons Europa and Ganymede results in tidal flexing and heating of Io’s interior, producing an enormous amount of energy that powers over 400 volcanoes on Io’s surface. Io’s volcanic activity, which manifests as both lava flows and lava lakes in caldera-like craters, and in explosive eruption plumes that shoot silicate ash, dust, and sulfur-bearing gases hundreds of kilometers above the surface, results in a world without any large impact craters. This indicates that Io has the geologically youngest surface in the Solar System. Thus, Io serves as an example of potentially active, volcanic lava planets discovered around other stars in our Galaxy.”

Image of Io taken by NASA’s Galileo spacecraft in July 1999. (Credit NASA/JPL/University of Arizona)

Jupiter’s first Galilean moon, Io, was first visited by NASA’s Pioneer 10 and 11 in December 1973 and December 1974, respectively, but only one image was taken by Pioneer 11 during the brief flyby. It wasn’t until Voyager 1 and 2 flew through the Jupiter system in 1979 that scientists got their first real look at this mysterious moon, revealing a crater-less surface and was the first planetary object other than Earth to be observed exhibiting volcanic activity, which is due to the tidal heating between Io and the much more massive Jupiter, along with Europa orbiting just beyond Io.

“The truth is, when it comes to moons, I could never pick one,” said Dr. Alyssa Rhoden, who is a principal scientist at the Southwest Research Institute. “They are all intriguing in their own ways, and each one teaches us something different. Although I don’t have a favorite, I will take the opportunity to highlight one particular moon that doesn’t get much attention: Proteus, a small moon of Neptune. Compared to Neptune’s large active moon, Triton, it seems reasonable to neglect battered little Proteus. But here’s the thing…Proteus is in the same size range as Mimas and Enceladus (around Saturn), and Miranda (around Uranus), which are much more round and brighter than Proteus,” Dr. Rhoden continues. “Enceladus is geologically active with very high heat flows and plumes at its South Pole, showing that even small moons can be quite interesting. And yet, Proteus is heavily cratered, with so many large craters that it doesn’t even look spherical anymore.”

Image gallery of Proteus with other moons. (Credit: Dr. Alyssa Rhoden)

Proteus is Neptune’s second-largest moon, and was discovered by Voyager 2 in 1989 when the spacecraft flew through the Neptune system. Despite its non-spherical shape, Proteus shows no signs of current geologic activity, unlike Neptune’s much larger moon, Triton, and is one of the darkest objects in the Solar System.

“Of course, my favorite moon is Triton!!” Dr. Candice Hansen-Koharchek, who is a planetary scientist and was a Voyager Imaging Team Assistant Experiment Representative during the Voyager missions, exclaimed. “There is so much that we still don’t know…very fundamental questions like whether or not it has an internal ocean, whether or not the bizarre features on the surface are cryovolcanic and whether or not the surface and the sub-surface ocean interact. What is the composition of the bright south polar region? How are different ices distributed across the surface? Sooooo many interesting questions…”

Global color mosaic of Triton taken by NASA’s Voyager 2 in 1989. (Credit: NASA/NASA-JPL/USGS)

Triton was discovered by William Lassell in 1846. It is the largest of Neptune’s 13 moons, and possibly the most intriguing, with its cantaloupe terrain and dark streaks from geysers across its surface, which Voyager scientists determined to be geysers when Voyager 2 flew past in 1989, Triton could possibly contain an interior liquid ocean. Despite the very brief flyby, scientists learned a great deal about this small moon, whose diameter is approximately one-half the width of the United States at 2,700 kilometers (1,680 miles). No spacecraft are currently exploring Triton or are scheduled to travel out there, so Voyager 2 remains the only human-made object to visit this mysterious and intriguing moon way out in the depths of the Solar System.

“Europa, the sixth-largest moon in the solar system, is without a doubt my favorite moon,” said Dr. Antonio Paris, who is the Chief Research Scientist with The Center for Planetary Science. “Recent research of Europa has uncovered inferred evidence of an ocean of water below the moon’s icy surface. Europa, therefore, may have the necessary ingredients for life: water, energy, and complex molecules known as organics. The current data, however, is still speculation at best. Therefore, planetary scientists like myself hope to find the answers with the Europa Clipper mission!”

True color image of Europa taken by NASA’s Juno spacecraft in September 2022. (Credit: NASA/JPL-Caltech/Southwest Research Institute/Malin Space Science Systems/Kevin M. Gill)

Like Io, Jupiter’s second Galilean moon, Europa, was discovered by Galileo Galilei in 1610, and also exhibits a crater-less surface due to tidal heating, as well. But instead of extreme volcanism, Europa harbors an interior ocean that is estimated to contain more than twice the volume of all of Earth’s oceans combined despite Europa being smaller than Earth’s Moon. Europa was first explored up close by Voyager 1 and 2 in 1979, which presented strong evidence of an interior ocean beneath the Europa’s ice shell. Dr. Paris mentions NASA’s Europa Clipper mission, which is a NASA Flagship mission designed to explore Europa for potential signs of habitability within the small moon’s deep ocean.

“My favorite moon in the Solar System is Saturn’s giant moon, Titan,” said Dr. Jason Barnes, who is a professor in the Department of Physics at the University of Idaho. “Titan is particularly awesome because it is a member of so many different planetary clubs. Titan’s subsurface liquid water mantle makes it an Ocean World, like Europa, Ganymede, and Enceladus. But at the same time Titan is one of just four places that we know of in the entire universe that sport both a solid surface and a thick atmosphere, along with Venus, Earth, and Mars. Only Earth and Titan have lakes and seas of surface liquid, and it’s just Earth and Titan again that have extensive water in the vicinity of complex organic molecules. All of these make Titan a logical choice for future exploration, and that’s why we’re sending the Dragonfly relocatable lander to Titan to investigate possibly prebiotic chemistry, to ascertain its habitability, and to search for chemical signatures of potential life there. Dragonfly launches in 2027 June and will arrive at Titan after a 6.5-year space cruise, after which it will fly in Titan’s air to more than 20 different landing sites as a nearly one-ton octocopter. We look forward to sharing Dragonfly’s adventure with you all once it arrives by 2034!”

False color image of Titan taken by NASA’s Cassini spacecraft taken in October 2004. (Credit: NASA/JPL/Space Science Institute)

Saturn’s largest moon, Titan, which is also the second largest moon in the Solar System, was discovered by Christiaan Huygens in March 1655, and is the only moon to possess a dense atmosphere comprised of a thick haze that cameras in the visible spectrum cannot penetrate. Titan was first explored by NASA’s Pioneer 11 and later by Voyager 1 and 2, but none of the spacecraft possessed the equipment to penetrate the thick atmosphere and see the surface. It wasn’t until NASA’s Cassini mission with its radar and infrared instruments that scientists were able to see the surface for the first time, revealing countless lakes of liquid methane and ethane, making Titan the only known planetary body other than Earth to have bodies of liquid on its surface. During the mission, Cassini deployed the Huygens probe from the European Space Agency that landed on Titan’s surface, becoming the first spacecraft to land on a planetary body in the outer Solar System. As Dr. Barnes stated, NASA’s Dragonfly mission will be sent to Titan to explore the moon’s potential habitability, and will cover hundreds of kilometers of Titan’s surface during two-year mission.

“My favorite moon in the Solar System is Ganymede, simply because it’s a planet by any other name,” said Dr. Paul Byrne, who is an associate professor in the Department of Earth and Planetary Sciences at Washington University in St. Louis. While Dr. Byrne believes that Ganymede would be called a planet if it wasn’t a moon around Jupiter, he’s quick to point out that Ganymede wouldn’t have stayed a planet if it didn’t form around Jupiter in the first place.

Image of Ganymede taken by NASA’s Juno spacecraft in 2021. (Credit: NASA/JPL-Caltech/Southwest Research Institute/Malin Space Science Systems/Kevin M. Gill)

“But Ganymede is magnificent,” Dr. Byrne continues. “It’s got a highly geologically complex outer shell of water ice, showing both ancient and relatively recent regions. Beneath that shell is an ocean of liquid water up to 900 kilometers deep. More likely, instead of a single water ocean, there’s a layer of high-pressure ice at the base of a somewhat thinner ocean. In fact, it’s even possible that there are interleaved layers of ocean and ice, forming an onion-like interior beneath the icy exterior. And then, under all the ice and water is a rocky planetary body about the same size of the Moon. And that rocky body must surely be differentiated, just like the Moon, and Earth, Venus, Mars, and Mercury—because the rocky interior of Ganymede has at its center a liquid iron core, the movement of which generates a modern magnetic field. That field makes Ganymede one of only three rocky bodies in the Solar System to generate a modern magnetic field, the other two being Earth and Mercury. There are lots of other cool things about Ganymede, but it’s its size, interior structure, and modern magnetic field that together fascinate me, and make it my favorite Solar System moon.”

Much like Io and Europa, Jupiter’s third Galilean moon, Ganymede, was also discovered by Galileo Galilei in 1610, and is the largest moon in the Solar System, even bigger than the planet Mercury and the dwarf planet Pluto. Ganymede was first visited by NASA’s Pioneer 10 and then Pioneer 11, but received its first up close study from Voyager 1 and 2 in 1979, with Voyager 1 imaging a surface that had a combination of craters and smooth terrain, which contrasts both Io and Europa’s respective surfaces. NASA’s Galileo spacecraft became the first spacecraft to orbit Jupiter and was able to provide the most in-depth analyses of Ganymede, including the identification of a magnetosphere that Dr. Byrne mentions, along with up close images revealing a very diverse surface. NASA’s Hubble Space Telescope, which is in Earth orbit, later provided evidence that Ganymede harbors an interior ocean much like Europa.

What are your favorite moons of the Solar System and which do you think will be the first to confirm the existence of life beyond Earth?

As always, keep doing science & keep looking up!

Laurence Tognetti

Laurence Tognetti is a six-year USAF Veteran who earned both a BSc and MSc from the School of Earth and Space Exploration at Arizona State University. Laurence is extremely passionate about outer space and science communication, and is the author of “Outer Solar System Moons: Your Personal 3D Journey”.

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