Titan, the largest moon of Saturn, looks more Earth-like on its surface than any other place in the Solar System. With its thick atmosphere and liquid methane rain, it has lakes, rivers, sand dunes and seas. But appearances can be deceiving and in other ways, Titan is in fact a very alien world. One baffling difference, recently discovered, is that Titan's rivers do not seem to form deltas when they reach the sea.

Titan is the largest moon of Saturn. It was discovered in 1655 by Christiaan Huygens, and was the 6th moon to be discovered after our own, and the 4 Galilean moons around Jupiter. It is the second largest moon in the Solar System, and orbits Saturn at an average distance of roughly 1.2 million kilometers. Although it is larger than Mercury, it has less than half as much mass. It also has a thick, cloudy atmosphere, and until a few decades ago, that was almost all we knew.

A breathtaking view of Titan's mysterious hydrocarbon seas, where rivers end in deep pits, challenging our understanding of planetary geology.

Similarities with Earth

When the Cassini mission arrived, we learned that Titan was surprisingly similar to Earth. It has an atmosphere thicker than our own and is the only place in the entire universe, outside of Earth, where we have observed the presence of free-running liquids on the surface. Despite the extreme cold, it has weather systems complete with rain falling from the clouds.

This rain, when it lands, rolls downhill to form streams and rivers, which eventually empty out into lakes and seas. Like on Earth, these rivers carve channels unto the ground, forming river beds, and they carry sediment.

But despite these similarities, they are still very different worlds. Titan is a place of extreme cold. Being so far from the Sun, it doesn't receive a lot of warming sunlight, and it doesn't have a massive molten iron core. At these temperatures, water is frozen so hard that it is just another kind of rock. The liquids raining from the sky and flowing on the surface? Super-chilled ethane and methane.

Hydrology

On Earth, water circulates around the planet in a cycle. Liquid water gives up some of its molecules to the atmosphere, driven out by their internal heat energy, in a process we call evaporation. The water vapour in the atmosphere circulates around the globe until it finds a region where the pressure is high enough, the temperature low enough, that it condenses into tiny droplets around nucleation sites: specks of dust or airborne bacteria. Sometimes these droplets stay liquid, and combine to form larger and larger droplets, sometimes they freeze into ice crystals, but either way we can see them from the ground as clouds. If the droplets get big enough, they start to fall, and we get precipitation (rain, snow, hail, depending on conditions).

If the rain falls from low enough that it doesn't simply evaporate again, it reaches land and wets the ground. Some soaks into the soil, the rest trickles down to form small streams, which in turn combine to form rivers, and eventually flow into the sea (or not! Some rivers in arid areas simply fade away, either soaking into the parched earth or evaporating away entirely). As rivers flow, they erode the ground beneath them, carving river beds, and transporting silt. This silt can be deposited wherever the flow is slow, and eventually builds up enough to change the course of the river. When this happens at a river mouth, the mouth begins to block up with silt and the river eventually breaks a new path around the blockage. Over enough time, this happens often enough that you are left with the classic triangular river delta formation.

But for some reason, this doesn't seem to happen on Titan!

Cassini

Titan's thick soupy atmosphere makes it hard to observe any of these features. None of this would be known without Cassini's synthetic aperture radar (SAR). Unfortunately liquid methane and ethane are completely transparent to the SAR instrument, so many of the details are inferred. We don't observe rivers or seas directly, but instead we see what they've done to the ground beneath: river beds cutting across the landscape, emptying to large basins that make up lake and sea beds.

Given that Deltas are formed from silt accumulated over very long times, blocking up river mouths and forcing rivers to find new paths, you might expect these formations to be easy to spot. But researchers studying Cassini mission data have not found them.

"It's kind of disappointing as a geomorphologist because deltas should preserve so much of Titan’s history," said Sam Birch, an assistant professor in Brown University’s Department of Earth, Environmental and Planetary Sciences. "We take it for granted that if you have rivers and sediments, you get deltas. But Titan is weird. It’s a playground for studying processes we thought we understood."

The hunt continues

To test his assumptions, Birch developed a numerical model to process similar data from a more familiar world: Earth. The model simulated what Earth's underwater features might look like to the same SAR instruments, if they were under liquid methane and ethane instead of water, and confirmed that river deltas should have been easily visible.

"If there are deltas the size of the one at the mouth of the Mississippi River, we should be able to see it," Birch said. "If there are large barrier islands and similar coastal landscapes like those we see all along the U.S. Gulf Coast, we should be able to see those."

But when Birch and his colleagues returned to the Cassini data they did not find the missing features: Only two rivers, near the South Pole of Titan, showed possible delta formations. By their count based on the Cassini data, only 1.3% of large rivers on Titan terminate in deltas, compared to almost all comparable rivers on Earth.

We're unlikely to know for certain what's going on until another mission can be sent to Saturn to study its moons more closely. But Birch and his team do have some ideas: Perhaps the sea level rises and falls fast enough that the sediments are regularly submerged, washing the silt away before it has time to form a proper delta. Or possibly strong winds and coastal currents are doing the same thing. After all, radar imaging has also revealed deep river channels cut into the sea beds themselves, another mystery that hasn't yet been solved.

As usual, it will take more data, and a lot more hard work from planetary scientists to find answers.

"This is really not what we expected," Birch said. "But Titan does this to us a lot. I think that’s what makes it such an engaging place to study."

What steps can be taken to enhance in-situ resource utilization (ISRU) for future astronauts on Mars? This is what a recent study presented at the 56^th Lunar and Planetary Science Conference hopes to address as an international team of researchers investigated the reasons, benefits, and challenges of conducting ISRU on Mars. This study has the potential to help astronauts, scientists, engineers, and mission planners develop new methods for enhancing the survivability of future Mars astronauts while also maximizing mission success.

Here, Universe Today discusses this incredible research with Dr. Christoph Gross, who is a postdoctoral researcher at Freie Universität Berlin (Free University of Berlin) and lead author of the study, regarding the motivation behind the study, specific locations on Mars for ISRU purposes, and the importance of ISRU in future crewed Mars missions. Therefore, what was the motivation behind the study?

Dr. Gross tells Universe Today, “The main motivation is the prospect that one day humans will set foot on Mars and will need resources to survive there. It may be feasible for short duration stays to bring everything to Mars (comparable to the lunar Apollo missions), but for long duration missions at least propellant and water/oxygen resources are needed to sustain the landed crews.”

Based on a 2024 study by the same researchers, the team discussed the benefits of growing food on Mars for future crewed missions. Based on the EDEN ISS project in Antarctica that operated from 2018 to 2022 and managed by the German Aerospace Center, the team estimated amount of area required to produce the necessary amount of food for one crewmember over one year was between 40 m² to 65 m² (430 ft² to 700 ft²). Additionally, the team noted how growing plants on Mars could contribute to producing oxygen and removing carbon dioxide from the atmosphere.

The team also discussed various locations on Mars where resources could be exploited, including Juventae Chasma and Meridiani Planum, which the team notes possess hydrated minerals and uniform deposits, respectively. Juventae Chasma is a box canyon measuring 250 kilometers by 100 kilometers (155 miles by 62 miles) and located near the Martian equator just north of Valles Marineris, the latter of which is the largest canyon in the solar system. Meridiani Planum is a giant plain whose diameter stretches approximately 1,060 kilometers (659 miles) also located near the Martian equator and resides on top of hydrated sediments. But what other locations on Mars could be investigated for ISRU purposes?

“Our first study was in Juventae Chasma and more limited in Mawrth Vallis,” Dr. Gross tells Universe Today. “However, many places appear to be good candidates. Our investigations use remote sensing data from orbiting instruments. In Utopia Planitia, subsurface ice and salt deposits are suspected. However, remote sensing data is pretty sparse from this location, because the basin is so deep and the atmosphere thicker there, this makes the identification of specific minerals difficult.”

Dr. Gross continues, “Also, we try to find places which are also good candidates as landing sites, e.g. scientific interest, resources present, good location for transmissions to earth, good environmental conditions (not too extreme) etc. It also depends what kind of resources you are looking for. For example, larger impact craters could harbor important ore deposits too, depending on where they impacted (water-rich or water-poor substrate).”

ISRU involves using available resources to maximize mission success while also reducing the number of resources that are shipped from home. In the context of space exploration, this means astronauts on Mars would use available water from buried water ice for drinking, bathing, and producing oxygen from electrolysis. Since the atmosphere of Mars is incapable of having liquid water on its surface, buried water ice has become a target for future crewed mission plans.

Additionally, converting carbon dioxide, which is the dominant Martian atmospheric component, to oxygen using existing tools could reduce the amount of oxygen that is shipped from Earth. Finally, due to the harsh radiation that rains down on the Martian surface daily, Martian regolith could be used to cover habitats as a shield. Therefore, what is the importance of ISRU in future crewed Mars missions, and could it potentially lead to a self-sustaining settlement, someday?

“ISRU will make settlements self-sustaining one day,” Dr. Gross tells Universe Today. “There is no question about it. I think the fact that NASA demonstrated oxygen production with the MOXI experiment on the Perseverance rover shows in which direction the research is going. It will for sure not happen at once, but it will happen.”

Dr. Gross concludes, “I think it is important to note that many more exploration missions are needed since there are still so many question marks since we have only limited data from landed missions. This could be done with small and ‘cheap(er)’ scout missions that have specific tasks to discover and specify resource deposits.”

How will ISRU help enhance future crewed Mars missions in the coming years and decades? Only time will tell, and this is why we science!

• As always, keep doing science & keep looking up!

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If we could peel back the Moon's cratered crust and examine its mantle, we might find answers to some foundational questions about the Solar System. We lack the technological capability to excavate the Moon's mantle, but Nature has a way. A massive, ancient impact excavated material from deep beneath the Moon's crust and left it on the surface for us to find. It could help confirm the Moon's origins.

The Giant Impact Hypothesis (GIH) is the widely accepted explanation for the origin of the Moon. It proposes that a massive protoplanet about the size of Mars, named Theia, slammed into Earth about 4.5 billion years ago. The impact melted Theia and some of Earth, sending the material into orbit around Earth. Eventually, some of it coalesced into the Moon. The GIH was first proposed in 1946 but didn't attract much interest until decades later, when the Apollo lunar samples generated renewed interest.

This artist's illustration shows the protoplanet Theia impacting Earth more than 4 billion years ago.

Image Credit: By NASA/JPL-Caltech, Public Domain

The GIH says that the Moon formed primarily from the mantles of Earth and Theia. The lunar samples supported this idea because their isotopic ratios are similar to Earth's. However, surface rock has been exposed to space weathering and impacts for billions of years, altering its composition. What we need is a sample of the untouched mantle.

Ancient, massive impacts like the one that created the Imbrium Basin had the power to excavate material from the mantle and spread it around the crust near the impact site. China's Chang'e-5 mission returned its samples to Earth in 2020, and they contained glass beads. These beads are common near energetic impact sites, where the intense heat blasts rock and melts it into little pieces that land back on the ground near the site.

Normally, impact beads are made of crustal material. In new research, scientists from Curtin University, Nanjing University, and the Australian National University examined a large lunar bead from the Chang'e mission and found that it contains an unusually high level of magnesium oxide (MgO). This indicates that its parent rock is from the Moon's upper mantle.

The research, titled "A potential mantle origin for precursor rocks of high-Mg impact glass beads in Chang’e-5 soil," appears in Science Advances. The lead author is Chen-Long Ding from the School of Earth Sciences and Engineering at Nanjing University in China.

Evidence shows that all lunar rock contains glass beads. These beads are from lava eruptions and impacts and provide a collective record of lunar history. Samples from different sites on the Moon confirm this. However, the Chang'e 5 samples are different.

"The chemical compositions of most lunar impact glass beads reflect mixing of crustal components, including mare basalts, highlands rocks, and KREEP [from high concentrations of K, REE (rare earth element), and P]," the authors write in their research article. "However, a few glass beads in the soil from the Chang’e-5 mission have unusually high MgO contents that require distinct target compositions."

The young age of the glass beads indicates that they come from the impact melting of ultramafic rock, which generally contains higher amounts of MgO. "Of particular interest here is a group of glasses with MgO contents exceeding 18 wt% %," the authors write. "The high MgO concentrations clearly differentiate them from the local basalt and regolith at the Chang'e-5 landing site, which have MgO contents ~6.5 wt% %."

This figure from the research illustrates the high concentration of Magnesium Oxide in the Chang'e 5 glass beads in this study.

Image Credit: Ding et al. 2025, Science Advances.

Though these rocks could be from surface material, they don't appear similar to any of the Moon's known lithologies. "Alternatively, these high-Mg beads might be sampling the upper mantle brought to the surface by the Imbrium basin–forming event," the researchers write.

Professor Alexander Nemchin from the School of Earth and Planetary Sciences at Curtin University in Perth, Australia, is one of the study's co-authors. In a press release, Nemchin said, "These high-magnesium glass beads may have formed when an asteroid smashed into rocks that originated from the mantle deep within the Moon. This is exciting because we've never sampled the mantle directly before: the tiny glass beads offer us a glimpse of the Moon's hidden interior."

Professor Tim Johnson, also from Curtin's School of Earth and Planetary Sciences, is one of the paper's co-authors. Since the rocks' chemistry is so different from that of other lunar samples, they could've been excavated by a massive impact.

"One such event could be the formation of the Imbrium Basin, which is a huge crater formed more than 3 billion years ago," Professor Johnson said. "Remote sensing has shown the area around the basin's edge contains the kind of minerals that match the glass bead chemistry."

"This is a big step forward in understanding how the Moon evolved internally; if these samples really are pieces of the mantle, it tells us that impacts can excavate otherwise inaccessible mantle material to the surface," Johnson said.

While volcanism can produce similar types of glass beads, the authors explain why it's not likely that these beads are volcanic. The mission's sample includes other glass beads of various ages. For all of them to be volcanic, there must have been multiple volcanic eruptions in the region very early in the Moon's history. However, while impacts can spread their glass beads over a wide area, volcanoes don't have the same reach, and their glass beads tend to accumulate near the center of the eruption. There's no evidence of that accumulation. "Therefore, while the possibility of very young volcanism on the Moon is provocative, there is no geological evidence for this, and we interpret the high-MgO beads in the Chang'e-5 regolith to have an impact origin," they write.

These results can't confirm the Giant Impact Hypothesis. But they do support the idea that the Moon experienced a magma ocean phase during its formation, which the GIH predicts. This opens a window into the Moon's deeper interior that wasn't there before. Scientists will work with these results and see what they tell them about the Moon and the Solar System. The results may help them constrain lunar magma ocean crystallization models and determine whether the mantle is rich in olivine and pyroxene, as predicted.

"Understanding how the Moon’s interior is made helps us compare it to Earth and other planets," said co-author Professor Xiaolei Wang from Nanjing University. "It could even guide future missions, whether robotic or human, that aim to explore the Moon’s deep geology."

Press Release: 

• Glass beads offer a window into the Moon's hidden depths

Research: 

• A potential mantle origin for precursor rocks of high-Mg impact glass beads in Chang'e-5 soil