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

Auroral displays are breathtaking light shows that can be seen across high latitude skies, created by the interaction between the solar wind and a planet's magnetic field. High-energy particles from the sun—mostly electrons and protons—hurtle through space until captured by magnetic field lines, which funnel them toward the poles. There, these charged particles collide dramatically with atmospheric molecules, transferring energy that excites atoms and molecules to higher states. As these excited particles return to their ground state, they release their excess energy as the shimmering curtains of coloured light we know as aurora.

Stunning northern lights display

It’s not just Earth that enjoys auroral displays though, in particular, Jupiter’s auroras dwarf Earth's aurora creating vast light shows that could swallow our entire planet. Powered by the gas giant's colossal magnetic field—14 times stronger than Earth's—these polar displays glow with an intensity  never seen on Earth and never fully disappear. Unlike Earth's auroras, which depend primarily on solar wind, Jupiter generates much of its auroral energy internally through its rapid 10-hour rotation and interactions with its volcanic moon Io, which pumps tons of sulphur and oxygen into Jupiter's magnetosphere daily.

Auroral displays on Jupiter captured by the Hubble Space Telescope in 2016

(Credit : NASA)

Recent observations by the James Webb Space Telescope's (JWST) Near-InfraRed Camera on Christmas Day 2023, led by Jonathan Nichols from the University of Leicester, have leveraged the telescope's exceptional sensitivity to capture the rapidly changing Jovian auroral features with unprecedented detail, revealing new insights into these massive electromagnetic storms.

“What a Christmas present it was – it just blew me away! We wanted to see how quickly the auroras change, expecting it to fade in and out ponderously, perhaps over a quarter of an hour or so. Instead we observed the whole auroral region fizzing and popping with light, sometimes varying by the second.” - Jonathan Nichols, University of Leicester.

Further observations were completed using the Hubble Space Telescope and together, the observations of Jupiter's auroras revealed that emissions from the trihydrogen ion (H3+) fluctuate much more dramatically than previously thought, offering new insights into the heating and cooling mechanisms of Jupiter's upper atmosphere.

However, the team encountered a mystery: the brightest infrared emissions captured by JWST had no corresponding features in Hubble's ultraviolet imagery. This suggests an apparently impossible phenomenon—what Nichols describes as "a tempest of drizzle," where large quantities of very low-energy particles somehow create intense auroral brightness visible only to JWST, leaving researchers baffled about the underlying mechanisms that could produce such contradictory observations.

Artist impression of the James Webb Space Telescope

The team acknowledge more work is required to investigate the discrepancy between the observations. They now hope to use additional JWST sessions to compare with NASA's Juno spacecraft data, hoping to solve the mystery.. These findings could prove valuable for the European Space Agency's Juice mission—which is currently traveling to Jupiter—to examine the gas giant's auroras using seven scientific instruments, including two imaging systems. They hope the study will improve our understanding of the interactions between Jupiter's magnetic field, atmosphere, and the charged particles from its moons, particularly Io.

Source : 

• Webb reveals new details and mysteries in Jupiter’s aurora

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How can artificial intelligence (AI) be used to advance mapping and imaging methods on other planets? This is what a recent study presented at the 56^th Lunar and Planetary Science Conference hopes to address as a lone researcher investigated using machine learning models to enhance mapping and imaging capabilities from orbital images obtained from the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX), which is currently orbiting Mars. This study has the potential to help scientists, engineers, and the public better understand the benefits of AI in conducting more advanced science, specifically regarding global images around Earth and other worlds.

Here, Universe Today discusses this incredible research with Dr. Andrew Annex, who is a Senior Science Systems Engineer at the SETI Institute, regarding the motivation behind the study, the next steps in developing these machine learning models, and the importance of using machine learning models to improve upon existing methods. Therefore, what was the motivation behind the study?

“The primary motivation behind my work was the desire to accelerate scientific discovery and inquiry and enhance the scientific return from existing Mars datasets,” Dr. Annex tells Universe Today. “Many studies of Mars start by the simple identification of features on the surface and figuring out where they are. This is typically achieved by a scientist literally looking at images, many hundreds to thousands of images, manually. This process, however, can be very slow and tedious when looking at the surface at moderate to high resolution, as there is simply a lot of ground to cover.”

For the study, Dr. Annex evaluated how current image analysis methods could potentially be improved using machine learning models and tools, including content-based image retrieval (CBIR), OpenAI CLIP (Contrastive Language-Image Pre-training), and cloud computing architecture. The purpose of CBIR is to take a starting image and scan a database for similar images by scanning the image content.

OpenAI has become a leading research company with the goal of enhancing AI to benefit humanity in our everyday lives, with ChatGPT arguably being its most used and well-known model. OpenAI CLIP is a machine learning model designed to learn how to compare images and text while dealing with large datasets. Cloud computing uses a network of remote servers to manage large amounts of data, including mobile technologies, databases, storage, applications, and more.

In the end, Dr. Annex successfully used machine learning models to analyze global CTX mosaic images on Mars, including search and identification of specific image similarities across the Red Planet. While noting this research could open doors for improvements, including specific search queries, Dr. Annex emphasized machine learning models could be used on planetary surfaces across the solar system.

“What I built, in the end, is a basic visual search engine, that makes it possible to search the surface of Mars at the pixel resolution of CTX,” Dr. Annex tells Universe Today. “The work isn’t a single model answering a specific question that is typically seen in other ML [machine learning] research in planetary science. It’s an application of software (and machine learning) to make it possible to search the data quickly for many different things rapidly.”

The first image from a Mars orbiter occurred on July 15, 1965, by NASA’s Mariner 4 spacecraft, which returned strips of code that scientists and engineers colored based on the code number, along with returning the first black & white orbital image on July 16. That historic mission revealed that Mars was not the watery and tropical landscape that scientists dreamed about since Percival Lowell declared Mars to have living inhabitants in the early 20^th century.

Since then, Mars orbiters from several nations have sent back incredible images of the Red Planet, revealing a world that once held oceans and rivers of liquid water possibly billions of years ago. Due to the tireless work of these robotic explorers, the entire surface of Mars has been imaged, and some in incredible detail, by NASA’s Context Camera and High Resolution Imaging Science Experiment (HiRISE) camera, respectively. Therefore, what is the importance of using machine learning models to improve upon existing image analysis methods for Mars?

Dr. Annex tells Universe Today, “I think the importance is that over the past 25+ years, while computing power has increased, so has the amount of data we have to look at to answer our scientific questions, but also our speed at using this data has not accelerated. Existing techniques have not kept pace because these techniques aren’t necessarily computational, but conventional and critical by-eye image analysis and geologic interpretation. Many revolutionary scientific discoveries for Mars occurred when seeing the surface at a higher resolution than was available before. But now different, important questions about Mars can now be asked with a complete picture of the surface that the CTX global mosaic provides.”

Dr. Annex continues, “But seeing the whole surface at 5 meters per pixel isn’t achievable for a single individual, again it’s simply a lot of area to look at and hold in your mind. Machine Learning is important not just for speed, but perhaps more critically for flexibility in the task you are automating in ways that conventional computational image analysis isn’t practical for or are simply too slow for given the amount of data you need to examine and the time available. I don’t see machine learning replacing all image analysis, but I see it as another tool in the toolbelt. One that can be used to complement and enhance existing methods and analysis.”

How will machine learning help improve Mars image analysis in the coming years and decades? Only time will tell, and this is why we science!

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

Binary asteroid systems, in which a large asteroid is orbited by a smaller satellite, are a growing field of interest. In recent years, this has included the asteroid Didymos, a 765-meter-diameter (2,510 ft) Near-Earth Asteroid (NEA) with an orbiting companion (Dimorphos). This moonlet was targeted by the Double Asteroid Redirect Test (DART), a kinetic impactor designed to test a promising technique for planetary defense. Similar binaries have been found across a wide array of small body populations in the Solar System and are being characterized by observatories and spacecraft.

To date, 13 asteroids measuring more than 100 km (62 mi) in diameter have been detected that have confirmed satellites. Interestingly, asteroid satellites are generally found around those with rapid rotations and an elongated shape. Previous models suggest that these satellites could be generated by impacts, but much remains unknown about how these systems came to be. By combining impact simulations, the team was able to track how spin and shape are related to collisional circumstances.

The research was led by Kevin J. Walsh, a Senior Research Scientist at the Southwest Research Institute (SwRI) in Boulder, Colorado. He was joined by researchers from the Johns Hopkins University Applied Physics Laboratory (JHUAPL), the Observatoire de la Côte d'Azur, Charles University, the Space Research and Planetary Sciences at the University of Bern, and the University of Tokyo. The paper describing his team's findings recently appeared online and is being reviewed for publication in the Astrophysical Journal Letters.

Asteroids are essentially material left over from the formation of the Solar System roughly 4.6 billion years ago. Therefore, the study of binary asteroid systems can provide important information regarding the collisional history of different asteroid populations. Changes in the orbital properties of the satellites can also provide insight into the internal properties of the primary. Similarly, differences in binary properties between asteroid classes may provide insight into the internal properties of different types of large asteroids and their formation process.

It is generally accepted that impacts are responsible for binary asteroid systems, since collisions are inevitable for large asteroids. Furthermore, investigations of Main Belt Asteroids like Ceres and Vesta (performed by the Dawn spacecraft between 2011 to 2018) have noted impact craters and basins. Previous research that modeled asteroid impacts, disruption, and reaccumulation has shown that satellite formation is a possible consequence. As Walsh told Universe Today via email:

"By 'impacts,' I simply mean that a large asteroid gets hit by some smaller asteroid. It builds a nice, big crater, and some of the ejected debris gets caught in orbit. The concept is sound, as we know, big asteroids get hit all the time. We also know that some get hit so hard that they literally break up – we see that in large families of asteroids that have very similar orbits and identical physical properties. However, the models never really explained why the ejecta doesn't just come back and hit the large asteroid. That is what simple physics suggests should happen if the target is a big, spherically symmetric object and the crater is relatively small."

Research has shown that the properties of asteroid satellites are highly dependent on the primary's size. For example, asteroids larger than 100 km (62 mi) in diameter typically have small satellites (<0.1 times the size of the primary). In addition, five of the 13 known systems have multiple satellites, with 130 Elektra having three. Meanwhile, fewer satellites have been observed around asteroids that are ~10 and 100 km (6.2 to 62 mi) in diameter, while those that are more than ~300 km (186.5 mi) appear to have none.

"What really caught our attention was a few things: first, the asteroids with satellites were all elongated and NOT spherical, and second, they were all rotating pretty fast," said Walsh. "We also noticed which asteroids didn’t have satellites, which is most of the ones with really big families of asteroids that would have been liberated in huge impacts. This data combined suggested that 'impacts' wasn’t enough to explain things, but that we needed to understand what types of impacts and the key mechanism that led some ejecta to end up as satellites and some to escape and become part of a larger asteroid family."

For their study, the team conducted simulations of asteroid impacts, which resulted in a wide range of post-impact shapes. These relied on a hydrodynamics code, which models the big shockwave and initial breaking of the target object. This was followed by an N-body granular dynamics code that simulated the gravity of the new fragments, how they interacted with each other, and the resulting shape and spin of the post-impact primaries. Lastly, they performed a long-term simulation to see how the smaller fragments orbited their primaries over time.

Ultimately, their results showed a correlation between satellite formation and the shape and spin of the largest remaining remnant. As Walsh explained:

"They revealed that it wasn’t about how big/energetic the impacts are, but rather about the angular momentum (from pre-impact spin of the target, or applied rotation from an oblique impact) that can produce the odd-shaped primary and a bunch of satellites on stable orbits. We could also dig deeper and figure out where the material was likely to originate from inside the parent body, and found that it was typically all located 10-20 km (6.2 to 12.4 mi) deep."

These last findings are particularly significant since they predict how the observation and study of asteroid satellites will provide valuable insight into material deep within their primaries. Since an asteroid's interior is naturally shielded from solar and cosmic radiation and the vacuum of space, scientists would be studying material as it was when the Solar System was still forming. In addition, their study clarifies many unanswered questions about this less-understood population of asteroids.

"It tells us why we don't see satellites around every big asteroid (they do all get impacted a lot!), and why we don't see them around parent asteroids with huge asteroid families," said Walsh. "It also helps us understand the genetic relationship between the primary and the satellite. Finally, it could help us search deeper for more satellites that have not yet been observed since we now know what the key properties of the primary body should be."

Several space missions are planned for the near future that will rendezvous with binary asteroids to learn more about the origin and evolution of the Solar System. These missions will also provide more information on the orbital mechanics of asteroid populations, which will help inform planetary defense. In addition, facilities like the Vera C. Rubin Observatory are expected to dramatically increase the list of candidate systems in the coming years.

Further Reading: 

• arXiv

More than sixty years ago, the Search for Extraterrestrial Intelligence (SETI) officially began with Project Ozma at the Greenbank Observatory in West Bank, Virginia. Led by famed astronomer Frank Drake (who coined the Drake Equation), this survey used the observatory's 25-meter (82-foot) dish to monitor Epsilon Eridani and Tau Ceti - two nearby Sun-like stars - between April and July of 1960. Since then, multiple surveys have been conducted at different wavelengths to search for indications of technological activity (aka. "technosignatures") around other stars.

While no conclusive evidence has been found that indicates the presence of an advanced civilization, there have been many cases where scientists could not rule out the possibility. In a recent paper, veteran NASA scientist Richard H. Stanton describes the results of his multi-year survey of more than 1300 Sun-like stars for optical SETI signals. As he indicates, this survey revealed two fast identical pulses from a Sun-like star about 100 light-years from Earth, that match similar pulses from a different star observed four years ago.

Dr. Stanton is a veteran of NASA's Jet Propulsion Laboratory (JPL), whose work includes participating in the Voyager missions and serving as the Engineering Manager of the Gravity Recovery And Climate Experiment (GRACE) mission. Since retiring, he has dedicated himself to the Search for Extraterrestrial Intelligence (SETI) using the 76.2-cm (30-inch) telescope at the Shay Meadow Observatory in Big Bear, California, and a multi-channel photometer he designed. The paper describing his survey's findings appeared in the journal Acta Astronautica.

For years, Stanton has used these instruments to observe more than 1,300 Sun-like stars for optical SETI signals. Unlike traditional SETI surveys that have used radio antennas to search for evidence of potential extraterrestrial transmissions, optical SETI looks for pulses of light that could result from laser communications or directed-energy arrays. This latter example has been considered in recent years thanks to Project Starshot, NASA's Directed Energy Propulsion for Interstellar Exploration (DEEP-IN) concept, and similar interstellar mission concepts.

As Stanton indicated, the field of optical SETI traces its roots to a 1961 study by Schwartz and Townes. They reasoned that the best way an extraterrestrial intelligence (ETI) could send an optical signal that outshone their star would be with intense nanosecond laser pulses. These pulses are searched for using special equipment in infrared wavelengths, high-resolution spectra, or visible light. As Stanton related to Universe Today via email, his SETI search differs from conventional optical surveys:

"My approach is to stare at a single star for roughly 1 hour using photon counting to sample the star’s light at what is considered a very high time-resolution for astronomy (100 microsecond samples). The resulting time series are then searched for pulses and optical tones. The instrument uses readily available off-the-shelf components that can be assembled into a PC-based system. I’m not sure if anyone else is doing this with a significant time commitment. I am not aware of any discovery of similar pulses."

After years of searching, Stanton reported an unexpected "signal" when observing HD 89389, an F-type star slightly brighter and more massive than our Sun, located in the constellation Ursa Major. According to Stanton's paper, this signal consisted of two fast, identical pulses 4.4 seconds apart that were not revealed in previous searches. He then ran comparisons against signals produced by airplanes, satellites, meteors, lightning, atmospheric scintillation, system noise, etc.

As he explained, several things about the pulses detected around HD89389 made them unique from anything seen previously:

"a. The star gets brighter-fainter-brighter and then returns to its ambient level, all in about 0.2s. This variation is much too strong to be caused by random noise or atmospheric turbulence. How do you make a star, over a million kilometers across, partially disappear in a tenth of a second? The source of this variation can't be as far away as the star itself.    

b. In all three events, two essentially identical pulses are seen, separated by between 1.2 and 4.4 seconds (the third event,  found in an observation on January 18^th of this year, was not included in the paper). In over 1500 hours of searching, no single pulse resembling these has ever been detected.

c. The fine structure in the star's light between the peaks of the first pulse repeats almost exactly in the second pulse 4.4s later. No one knows how to explain this behavior.

d. Nothing was detected moving near the star in simultaneous photography or in the background sensor that easily detects distant satellites moving close to a target star. Common signals from airplanes, satellites, meteors, birds, etc., are completely different from these pulses."

A re-examination of historical data for similar signals revealed another pair of pulses detected around HD 217014 (51 Pegasi) in 2021. This main-sequence G-type star is located about 50.6 light-years from Earth and is similar in size, mass, and age to our Sun. In 1995, astronomers at the Observatoire de Haute-Provence detected an exoplanet orbiting this star, a hot gas giant that has since been named Dimidium. This was one of the first exoplanets ever detected, and the first time an exoplanet was discovered around a main-sequence star.

At the time, said Stanton, the signal was dismissed as a false positive caused by birds. However, a detailed analysis ruled out this possibility for all the pulses observed. Other possibilities that Stanton explores include diffraction caused by the Earth's atmosphere, possibly due to a shock wave. However, this is unlikely since shockwaves would have had to occur with perfect timing to coincide with both optical pulses. Other possibilities include starlight diffraction by a distant body in the Solar System, partial eclipses caused by Earth satellites or distant asteroids, and "edge diffraction" by a straight edge (as described by the Sommerfeld Effect).

There's also the possibility that a gravity wave could have generated these pulses, which requires additional consideration. Another interesting possibility is that it could be the result of ETI. As Stanton indicated, whatever modulated these stars' light must be relatively close to Earth, implying that any ETI activity must be within our Solar System. However, Stanton stresses that more data is needed.

"None of these explanations are really satisfying at this point," he said. "We don't know what kind of object could produce these pulses or how far away it is. We don't know if the two-pulse signal is produced by something passing between us and the star or if it is generated by something that modulates the star's light without moving across the field. Until we learn more, we can't even say whether or not extraterrestrials are involved!"

There are several examples of Optical SETI (OSETI) or LaserSETI, including the collaborative effort launched by Breakthrough Listen and the Very Energetic Radiation Imaging Telescope Array System (VERITAS) Collaboration. However, Stanton's method presents many opportunities for future SETI surveys, which could search for similar examples of optical pulses. To that end, he suggests two approaches that could reveal more about this phenomenon and help astronomers place tighter constraints on their possible causes:

"Look for events using arrays of synchronized optical telescopes. If the object is moving between the star and us, this approach should tell us how fast it is moving normal to the line of sight, and potentially its size and distance. [Also,] it would be very interesting if the star's light is modulated without an object moving across the field. Observing events with telescopes separated by a few hundred kilometers might show that any separation in the time each pulse arrives is due only to differences in the light time from the star to each telescope. Then, unless the variation could somehow be attributed to the star itself, we would have even more to explain!"

Further Reading:

• Acta Astronautica