We already know a decent amount about how planets form, but moon formation is another process entirely, and one we’re not as familiar with. Scientists think they understand how the most important Moon in our solar system (our own) formed, but its violent birth is not the norm, and can’t explain larger moon systems like the Galilean moons around Jupiter. A new book chapter (which was also released as a pre-print paper) from Yuhito Shibaike and Yann Alibert from the University of Bern discusses the differing ideas surrounding the formation of large moon systems, especially the Galileans, and how we might someday be able to differentiate them.

The Galilean moons form what is known as the circum-Jovian disc (CJD), and analogue of the circum-stellar disc (CSD) that surrounds the Sun, but instead has Jupiter at its center. The other 93+ non-Galilean moons around Jupiter also define the CJD, but their creation might be different due to the size differentials.

According to the paper, there are three main differences between the formation of planets and the formation of moons. Moon formation happens on a much faster time scale - around 10-100 times faster than planet formation. The system itself is also always gaining additional material from the CSD and losing it to whatever is at the center of the disk, which in the CJD’s case is Jupiter. And finally, there aren’t nearly as many examples of systems with multiple large moons as there are planetary systems, at least since the discovery of exoplanets 30 years ago. Jupiter and Saturn remain our only examples of large moon systems, and it will be awhile before any multi-exo-moon system will be found.

Fraser discusses the formation of our own Moon, which was dramatically different than that of the Galileans.

So what we can tell about the formation of these moon systems from the two we know about. The paper breaks the process down into a three-step process. First is the formation of the CJD, which includes gas and dust as well as moons. This was originally supported by a “minimum mass model” developed in the 1980s that assumed the disc was static and contained approximately the overall mass of the Galilean moons. In 2002, a new theory was developed that modeled the CJD as a “gas-starved disc” where the original CJD was relatively material poor but had plenty of additional material added to it by gravitational capture from the CSD.

That gravitational capture is believed to have played a key role in the formation of the Galilean moons and marks the second phase of their creation. However, Jupiter is a planet, and one of the requirements of a planet is that it clears its orbital path. Since Jupiter is the largest planet, it does so very effectively, which includes what astronomers consider “pebbles” (but on Earth could be considered a decent-sized boulder a few meters across).

One way for moons to accrete given this paucity of small material is by using even smaller material - small dust particles can make their way into the CJD without being disrupted by Jupiter, though there’s some debate about how effective this process is. Another method would be "planetesimal capture” where Jupiter’s gravity well catches the core of what would have ended up being a planet, but then ends up simply being one of the giant planet’s moons. They could have been gravitationally disturbed by Saturn, and then slowed in their orbit by running through the gas cloud surrounding early Jupiter that made up the CJD.

There are some differences in the Galilean moons themselves that can be used to prove or disprove these different formation theories. For example, Callisto isn’t in resonance with Jupiter at all, unlike the rest of its Galilean brethren. One potential theory for that is that Jupiter’s fourth moon was formed under different conditions, or maybe was hit by its own impactor that knocked it from its natural course. Callisto is again an outlier as it’s only partially “differentiated” (meaning it has a separate core, mantle, and outer shell), unlike its three compatriots. Some pebble accretion models think that Callisto is still early on in its formation journey and will eventually begin to look more like its peers.

But ultimately those questions, and many more about the formation of large moon systems, will be hard to answer without more data. The Jupiter Icy Moon Explorer (JUICE) mission will help shed some light on those questions, but even then it's still only one, or at the most two, data sets that we have available. Until exoplanet hunting telescopes become powerful enough to start finding exomoons as often as they currently find planets, many of these formation theories will remain untested. That data will eventually come along someday, and when it does it will help us understand some important parts of our own solar system better.

Learn More:

• Y. Shibaike & Y. Alibert - Origin of Ganymede and the Galilean Moons
• UT - What are the Galilean Moons?
• UT - Jupiter's Moon Io has Dunes. Dunes!?
• UT - All of Jupiter's Large Moons Have Auroras

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Our Moon is a seismically active world with a long history of quakes stretching back to its early history. It turns out those quakes can and will affect the safety of permanent base structures for anybody planning to explore and inhabit the Moon. That's one conclusion from a study of quakes along the Lee-Lincoln fault in the Taurus-Littrow valley where the Apollo 17 astronauts landed in 1972. “The global distribution of young thrust faults like the Lee-Lincoln fault, their potential to be still active and the potential to form new thrust faults from ongoing contraction should be considered when planning the location and assessing stability of permanent outposts on the Moon,” said Smithsonian senior scientist emeritus Thomas R. Watters, lead author of the paper.

They base their work on evidence of moonquakes in the region over the past 90 million years, largely in material gathered by the Apollo astronauts. Chunks of rocks and landslides are mute proof of the power of magnitude 3.0 quakes to shift the surface materials around. Along with other active faults on the Moon, the Taurus-Littrow rocks and landslides show that our lunar companion is likely still geologically active.

Why Lunar Seismicity?

Here on Earth, we get earthquakes all the time. By some estimates, our planet shakes about 55 times a day, although many of these tremors are so weak we don't feel them. They happen largely due to plate tectonics and volcanic activity. Plates slip past each other very gradually, which releases energy that gets dissipated as an earthquake. We all know about the really famous spots on Earth for that kind of action - the San Andreas Fault line, the Ring of Fire in the Pacific, and parts of southeast Asia, for example. Volcanic activity also spurs earthquakes when underground magma causes "shudders" as it moves. Recent events such as the ongoing Kilauea eruptions in Hawai'i and those near Grindavik, Iceland, cause swarms of earthquakes as a result of that magma movement.

However, that's not how it works on the Moon. The two most likely causes for lunar quakes are tidal pulling and the continual cooling and shrinking of the Moon. The tidal quakes happen because Earth's gravity pulls on the Moon, which results in deep quakes up to hundreds of miles inside. Weaker quakes originate closer to the surface and those are generally thought to be due to lunar shrinkage. Since the Moon formed billions of years ago, it has lost about 150 feet of its diameter due to the gradual cooling after its birth. There are also very minor temblors that happen when a meteoroid slams into the surface, or when surface rocks react to heating and cooling from the Sun. All this activity describes a world that is constantly shaking and shuddering.

This artist’s concept shows the Moon’s hot interior and volcanism about 2 to 3 billion years ago. It is thought that volcanic activity on the lunar near side (the side facing Earth) helped create a landscape dominated by vast plains called mare, which are formed by molten rock that cooled and solidified. As the Moon has continued to cool, it has shrunk and its surface contracted. That causes scarps and fault lines to form.

NASA/JPL-Caltech

Quakes and Risks

To understand the risk of quakes to future bases, Watters and research partner Nicholas Schmerr of the University of Maryland, studied materials from the Apollo 17 landing site. These rock samples, along with other details about rock falls and landslides on the Moon, told them that there are thousands of young thrust faults on the Moon. They point to a continual evolution of surface units, many caused by earthquake activities that create lunar thrust faults. That happens when rocks are compressed and one block is pushed up over another, generally as a result of the ongoing contraction of the Moon.

According to Watters and Schmerr, mission planners are going to have to consider those fault lines and the ongoing related lunar quakes when planning bases on the Moon. Short-term missions, like the Apollo landing, which had astronauts on the Moon for nearly 2 weeks, didn't face much danger from a quake or two. However, permanent bases face significant chances of damage during a quake, simply due to numbers. “If astronauts are there for a day, they’d just have very bad luck if there was a damaging event,” Schmerr pointed out. “But if you have a habitat or crewed mission up on the Moon for a whole decade, that’s 3,650 days times 1 in 20 million, or the risk of a hazardous moonquake becoming about 1 in 5,500. It’s similar to going from the extremely low odds of winning a lottery to much higher odds of being dealt a four of a kind poker hand.”

Taurus-Littrow valley taken by NASA’s Lunar Reconnaissance Orbiter spacecraft. The valley was explored in 1972 by the Apollo 17 mission astronauts Eugene Cernan and Harrison Schmitt. They had to zig-zag their lunar rover up and over the cliff face of the Lee-Lincoln fault scarp that cuts across this valley.

Credits: NASA/GSFC/Arizona State University

Planning for Quakes

It's not just habitats and science missions that could be damaged by lunar quakes. Russia, China, and the U.S. are planning to put nuclear power plants on the Moon. Such facilities could supply all the power anyone needs for bases and exploration, but they come with a safety price and could be quite susceptible to quake damage. That's why any these and other places need to be built with tough safety margins, and not located near any active fault lines. That's going to be a tall order, considering the extent of quakes and the numbers of fault lines that thread through the Moon.

This is why the scientists' study of lunar paleoseismology is so important. Gathering evidence of past quakes (going back many millennia), as well as more recent ones, is going to help chart the safest places to build bases, habitats, and power plants. “If astronauts are there for a day, they’d just have very bad luck if there was a damaging event,” Schmerr added. “But if you have a habitat or crewed mission up on the Moon for a whole decade, that’s 3,650 days times 1 in 20 million, or the risk of a hazardous moonquake becoming about 1 in 5,500. It’s similar to going from the extremely low odds of winning a lottery to much higher odds of being dealt a four of a kind poker hand.”

For More Information

• University of Maryland Geophysicist Helps Identify Moonquake Dangers that Could Threaten Future Missions
• Paleoseismic Activity in the Moon's Taurus-Littrow Valley Inferred from Boulder Falls and Landslides

Our Moon is a seismically active world with a long history of quakes stretching back to its early history. It turns out those quakes can and will affect the safety of permanent base structures for anybody planning to explore and inhabit the Moon. That's one conclusion from a study of quakes along the Lee-Lincoln fault in the Taurus-Littrow valley where the Apollo 17 astronauts landed in 1972. “The global distribution of young thrust faults like the Lee-Lincoln fault, their potential to be still active and the potential to form new thrust faults from ongoing contraction should be considered when planning the location and assessing stability of permanent outposts on the Moon,” said Smithsonian senior scientist emeritus Thomas R. Watters, lead author of the paper.

They base their work on evidence of moonquakes in the region over the past 90 million years, largely in material gathered by the Apollo astronauts. Chunks of rocks and landslides are mute proof of the power of magnitude 3.0 quakes to shift the surface materials around. Along with other active faults on the Moon, the Taurus-Littrow rocks and landslides show that our lunar companion is likely still geologically active.

Why Lunar Seismicity?

Here on Earth, we get earthquakes all the time. By some estimates, our planet shakes about 55 times a day, although many of these tremors are so weak we don't feel them. They happen largely due to plate tectonics and volcanic activity. Plates slip past each other very gradually, which releases energy that gets dissipated as an earthquake. We all know about the really famous spots on Earth for that kind of action - the San Andreas Fault line, the Ring of Fire in the Pacific, and parts of southeast Asia, for example. Volcanic activity also spurs earthquakes when underground magma causes "shudders" as it moves. Recent events such as the ongoing Kilauea eruptions in Hawai'i and those near Grindavik, Iceland, cause swarms of earthquakes as a result of that magma movement.

However, that's not how it works on the Moon. The two most likely causes for lunar quakes are tidal pulling and the continual cooling and shrinking of the Moon. The tidal quakes happen because Earth's gravity pulls on the Moon, which results in deep quakes up to hundreds of miles inside. Weaker quakes originate closer to the surface and those are generally thought to be due to lunar shrinkage. Since the Moon formed billions of years ago, it has lost about 150 feet of its diameter due to the gradual cooling after its birth. There are also very minor temblors that happen when a meteoroid slams into the surface, or when surface rocks react to heating and cooling from the Sun. All this activity describes a world that is constantly shaking and shuddering.

This artist’s concept shows the Moon’s hot interior and volcanism about 2 to 3 billion years ago. It is thought that volcanic activity on the lunar near side (the side facing Earth) helped create a landscape dominated by vast plains called mare, which are formed by molten rock that cooled and solidified. As the Moon has continued to cool, it has shrunk and its surface contracted. That causes scarps and fault lines to form.

NASA/JPL-Caltech

Quakes and Risks

To understand the risk of quakes to future bases, Watters and research partner Nicholas Schmerr of the University of Maryland, studied materials from the Apollo 17 landing site. These rock samples, along with other details about rock falls and landslides on the Moon, told them that there are thousands of young thrust faults on the Moon. They point to a continual evolution of surface units, many caused by earthquake activities that create lunar thrust faults. That happens when rocks are compressed and one block is pushed up over another, generally as a result of the ongoing contraction of the Moon.

According to Watters and Schmerr, mission planners are going to have to consider those fault lines and the ongoing related lunar quakes when planning bases on the Moon. Short-term missions, like the Apollo landing, which had astronauts on the Moon for nearly 2 weeks, didn't face much danger from a quake or two. However, permanent bases face significant chances of damage during a quake, simply due to numbers. “If astronauts are there for a day, they’d just have very bad luck if there was a damaging event,” Schmerr pointed out. “But if you have a habitat or crewed mission up on the Moon for a whole decade, that’s 3,650 days times 1 in 20 million, or the risk of a hazardous moonquake becoming about 1 in 5,500. It’s similar to going from the extremely low odds of winning a lottery to much higher odds of being dealt a four of a kind poker hand.”

Taurus-Littrow valley taken by NASA’s Lunar Reconnaissance Orbiter spacecraft. The valley was explored in 1972 by the Apollo 17 mission astronauts Eugene Cernan and Harrison Schmitt. They had to zig-zag their lunar rover up and over the cliff face of the Lee-Lincoln fault scarp that cuts across this valley.

Credits: NASA/GSFC/Arizona State University

Planning for Quakes

It's not just habitats and science missions that could be damaged by lunar quakes. Russia, China, and the U.S. are planning to put nuclear power plants on the Moon. Such facilities could supply all the power anyone needs for bases and exploration, but they come with a safety price and could be quite susceptible to quake damage. That's why any these and other places need to be built with tough safety margins, and not located near any active fault lines. That's going to be a tall order, considering the extent of quakes and the numbers of fault lines that thread through the Moon.

This is why the scientists' study of lunar paleoseismology is so important. Gathering evidence of past quakes (going back many millennia), as well as more recent ones, is going to help chart the safest places to build bases, habitats, and power plants. “If astronauts are there for a day, they’d just have very bad luck if there was a damaging event,” Schmerr added. “But if you have a habitat or crewed mission up on the Moon for a whole decade, that’s 3,650 days times 1 in 20 million, or the risk of a hazardous moonquake becoming about 1 in 5,500. It’s similar to going from the extremely low odds of winning a lottery to much higher odds of being dealt a four of a kind poker hand.”

For More Information

• University of Maryland Geophysicist Helps Identify Moonquake Dangers that Could Threaten Future Missions
• Paleoseismic Activity in the Moon's Taurus-Littrow Valley Inferred from Boulder Falls and Landslides

When global events set our minds to wondering if humanity has what it takes to persist, it's natural to wonder about other worlds, other life, other intelligent species, and if those others might be better suited to survive whatever Great Filters they face. Those are fanciful thoughts, but there's an underpinning of nuts-and-bolts thinking to them. It starts with identifying which planets in habitable zones around other stars might actually be habitable.

That begins with liquid water and a life-friendly atmosphere that can contain it.

The discovery of the TRAPPIST-1 system generated a lot of excitement a few years ago. It contains seven roughly Earth-like worlds, and three or perhaps four of them are in the red dwarf's compact habitable zone. One of them, TRAPPIST-1 d, could host water on its surface, or at least on parts of its surface, according to some research. But without a suitable atmosphere, a planet can't retain surface water, and new observations from the JWST show that TRAPPIST-1 d does not have an Earth-like atmosphere.

The TRAPPIST-1 system compared to our Solar System. TRAPPIST-1 d is on the inner edge of the star's habitable zone.

Image Credit: NASA/JPL-Caltech

The JWST observed two consecutive transits of TRAPPIST-1 d with its NIRSpec/PRISM instrument in November, 2022. Researchers from Canada, the UK, France, and the USA analyzed the data from those transits and concluded that the promising exoplanet does not have an Earth-like atmosphere. Their results are in a paper in The Astrophysical Journal titled "Strict Limits on Potential Secondary Atmospheres on the Temperate Rocky Exo-Earth TRAPPIST-1 d." The lead author is Caroline Piaulet-Ghorayeb of the University of Chicago and Trottier Institute for Research on Exoplanets (IREx) at Université de Montréal.

"While TRAPPIST-1 d may prove a barren rock illuminated by a cruel red star, the outer planets TRAPPIST-1e, f, g, and h, may yet possess thick atmospheres." - co-author Ryan MacDonald, University of St. Andrews.

"The nearby TRAPPIST-1 system, with its seven small rocky planets orbiting a late-type M8 star, offers an unprecedented opportunity to search for secondary atmospheres on temperate terrestrial worlds," the authors write in their research. "Here we present the first 0.6–5.2 μm NIRSpec/PRISM transmission spectrum of TRAPPIST-1 d from two transits with JWST."

TRAPPIST-1 d is right at the inner edge of TRAPPIST-1's habitable zone. It's a great target for transmission spectroscopy, and these JWST observations provide the first detailed transmission spectrum of the planet's atmosphere. Unfortunately, the spectrum is flat, meaning there are no detectable atmospheric features.

“Ultimately, we want to know if something like the environment we enjoy on Earth can exist elsewhere, and under what conditions. While the James Webb Space Telescope is giving us the ability to explore this question in Earth-sized planets for the first time, at this point we can rule out TRAPPIST-1 d from a list of potential Earth twins or cousins,” said lead author Piaulet-Ghorayeb in a press release.

The JWST failed to detect the types of molecules present in Earth's atmosphere like methane, carbon dioxide, and water. However, that doesn't completely rule out an atmosphere; there are a couple of other possibilities.

“There are a few potential reasons why we don’t detect an atmosphere around TRAPPIST-1 d. It could have an extremely thin atmosphere that is difficult to detect, somewhat like Mars. Alternatively, it could have very thick, high-altitude clouds that are blocking our detection of specific atmospheric signatures — something more like Venus. Or, it could be a barren rock, with no atmosphere at all,” Piaulet-Ghorayeb said.

Studying TRAPPIST-1 d and its atmosphere is about more than just ruling out its habitability. There's a greater scientific endeavor involved.

A visual comparison of Solar System orbits, TRAPPIST-1 orbits, and Galilean moon orbits.

Image Credit: By ESO/O. Furtak - http://www.eso.org/public/images/eso1706b/, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=56526718

Red dwarfs, or M dwarfs, like TRAPPIST-1 are common, and are likely the most plentiful type of star in the Milky Way. They're known to host their share of rocky worlds where we can reasonably wonder if life persists. But red dwarfs are also known for their violent flaring, and TRAPPIST-1 is no exception. It flares every couple of days, and each year it emits between four and six superflares. This powerful flaring activity could shred any planetary atmospheres, rendering the TRAPPIST-1 planets inhabitable.

However, there's considerable uncertainty around red dwarf flaring and habitability. Some research shows that the planets couldn't retain atmospheres in the face of the coronal mass ejections coming from the star. But it's at least possible that some of these planets could retain their atmospheres. For example, powerful planetary magnetic fields could provide a protective barrier from the star's flaring. The JWST opens a path to understanding red dwarf flaring effects on atmospheres.

“Webb’s sensitive infrared instruments are allowing us to delve into the atmospheres of these smaller, colder planets for the first time,” said Björn Benneke of IREx at Université de Montréal, a co-author of the study. “We’re really just getting started using Webb to look for atmospheres on Earth-sized planets, and to define the line between planets that can hold onto an atmosphere, and those that cannot.”

The only features in the JWST's spectra are attributed to stellar contamination rather than atmospheric absorption. "Our precise transmission spectrum can be fully explained by stellar contamination alone, and therefore enables us to rule out cloud-free or thick atmosphere scenarios across a wide range of potential atmospheric metallicities," the authors write.

This artist's illustration of TRAPPIST-1 d is from several years ago when scientists wondered about its nature and if it could support liquid water.

Image Credit: By NASA/JPL-Caltech - Cropped from: PIA22093: TRAPPIST-1Planet Lineup -

Updated Feb. 2018, Public Domain, https://commons.wikimedia.org/w/index.php?curid=76364484

A low molecular weight atmosphere is harder for a planet to retain, and these observations ruled out those types of hydrogen-dominated atmospheres. The observations also ruled out thicker atmospheres like Venus' or Titan's. The only things left are extremely thin atmospheres unlikely to bolster habitability, or atmospheres dominated by high clouds that mask molecular absorption features from the JWST. But the research effectively rules them out.

"Therefore, we conclude that

1. thick cloud-free hydrogen-rich atmospheres are ruled out by our transmission spectrum;
2. thin H2-rich alternatives are strongly disfavored when considering TRAPPIST-1 d in the context of its formation and evolution under stellar irradiation; and
3. high-altitude clouds or hazes are not expected to form on TRAPPIST-1 d if it has a low-metallicity atmosphere," the researchers explain.

This work almost certainly eliminates TRAPPIST-1 d from a list of potentially habitable, water-supporting exoplanets. This is Nature, so TRAPPIST-1 d's elimination isn't absolutely certain. "Our observations cannot yet completely exclude other potential atmosphere scenarios for TRAPPIST-1 d which were predicted in the literature," the authors explain, noting that other research involving climate models hints at the possibility that the tidally-locked planet could form high-altitude water clouds at its terminator, blocking atmospheric absorption signals from view.

But what about the other planets in the system?

“All hope is not lost for atmospheres around the TRAPPIST-1 planets,” Piaulet-Ghorayeb said. “While we didn’t find a big, bold atmospheric signature at planet d, there is still potential for the outer planets to be holding onto a lot of water and other atmospheric components.”

However, the outer planets aren't the juicy scientific targets that planet d is. They're further from the star, and colder. Even the JWST's powerful instruments struggle in those conditions. While detailed spectra aren't available for those worlds, the researchers still reached some conclusion.

"We find that even complete atmosphere loss for TRAPPIST-1 d would not preclude atmosphere presence for the outer HZ planets TRAPPIST-1 e, f, and g," the authors write in their conclusion. Contrary to the inner planets, it's possible that these outer planets held onto their water "even if they initially accreted only a few Earth oceans of volatiles."

“Our detective work is just beginning. While TRAPPIST-1 d may prove a barren rock illuminated by a cruel red star, the outer planets TRAPPIST-1e, f, g, and h, may yet possess thick atmospheres," added Ryan MacDonald, a co-author of the paper, now at the University of St Andrews in the United Kingdom, and previously at the University of Michigan. “Thanks to Webb we now know that TRAPPIST-1 d is a far cry from a hospitable world. We're learning that the Earth is even more special in the cosmos."

Being a human being means bearing witness to humanity's greatest, most triumphant moments of accomplishment and unity, but also to the depraved actions we take against one another. The minds of thinking people are bound to wonder if there are other worlds out there that host life. Each potentially habitable world is a glimmer of hope that humans, with all their struggles, are not the only intelligent species out there.

If we look to the heavens, and to exoplanets, for some kind of reprieve from humanity's troubles, TRAPPIST-1 d won't provide it. If this research is correct, its stricken from the list of hope-inspiring exoplanets.

On to the next one.

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