planetary science

armada-to-apophis—scientists-recycle-old-ideas-for-rare-asteroid-encounter

Armada to Apophis—scientists recycle old ideas for rare asteroid encounter

Apophis, asteroid, european space agency, NASA, osiris rex, planetary science, Ramses, Science, solar system, Space / /

Tick-tock —

“It will miss the Earth. It will miss the Earth. It will miss the Earth.”

–

This artist's concept shows the possible appearance of ESA's RAMSES spacecraft, which will release two small CubeSats for additional observations at Apophis.

Enlarge / This artist’s concept shows the possible appearance of ESA’s RAMSES spacecraft, which will release two small CubeSats for additional observations at Apophis.

For nearly 20 years, scientists have known an asteroid named Apophis will pass unusually close to Earth on Friday, April 13, 2029. But most officials at the world’s space agencies stopped paying much attention when updated measurements ruled out the chance Apophis will impact Earth anytime soon.

Now, Apophis is again on the agenda, but this time as a science opportunity, not as a threat. The problem is there’s not much time to design, build and launch a spacecraft to get into position near Apophis in less than five years. The good news is there are designs, and in some cases, existing spacecraft, that governments can repurpose for missions to Apophis, a rocky asteroid about the size of three football fields.

Scientists discovered Apophis in 2004, and the first measurements of its orbit indicated there was a small chance it could strike Earth in 2029 or in 2036. Using more detailed radar observations of Apophis, scientists in 2021 ruled out any danger to Earth for at least the next 100 years.

“The three most important things about Apophis are: It will miss the Earth. It will miss the Earth. It will miss the Earth,” said Richard Binzel, a professor of planetary science at MIT. Binzel has co-chaired several conferences since 2020 aimed at drumming up support for space missions to take advantage of the Apophis opportunity in 2029.

“An asteroid this large comes this close only once per 1,000 years, or less frequently,” Binzel told Ars. “This is an experiment that nature is doing for us, bringing a large asteroid this close, such that Earth’s gravitational forces and tidal forces are going to tug and possibly shake this asteroid. The asteroid’s response is insightful to its interior.”

It’s important, Binzel argues, to get a glimpse of Apophis before and after its closest approach in 2029, when it will pass less than 20,000 miles (32,000 kilometers) from Earth’s surface, closer than the orbits of geostationary satellites.

“This is a natural experiment that will reveal how hazardous asteroids are put together, and there is no other way to get this information without vastly complicated spacecraft experiments,” Binzel said. “So this is a once-per-many-thousands-of-years experiment that nature is doing for us. We have to figure out how to watch.”

This week, the European Space Agency announced preliminary approval for a mission named RAMSES, which would launch in April 2028, a year ahead of the Apophis flyby, to rendezvous with the asteroid in early 2029. If ESA member states grant full approval for development next year, the RAMSES spacecraft will accompany Apophis throughout its flyby with Earth, collecting imagery and other scientific measurements before, during, and after closest approach.

The challenge of building and launching RAMSES in less than four years will serve as good practice for a potential future real-world scenario. If astronomers find an asteroid that’s really on a collision course with Earth, it might be necessary to respond quickly. Given enough time, space agencies could mount a reconnaissance mission, and if necessary, a mission to deflect or redirect the asteroid, likely using a technique similar to the one demonstrated by NASA’s DART mission in 2022.

“RAMSES will demonstrate that humankind can deploy a reconnaissance mission to rendezvous with an incoming asteroid in just a few years,” said Richard Moissl, head of ESA’s planetary defense office. “This type of mission is a cornerstone of humankind’s response to a hazardous asteroid. A reconnaissance mission would be launched first to analyze the incoming asteroid’s orbit and structure. The results would be used to determine how best to redirect the asteroid or to rule out non-impacts before an expensive deflector mission is developed.”

Shaking off the cobwebs

In order to make a 2028 launch feasible for RAMSES, ESA will reuse the design of a roughly half-ton spacecraft named Hera, which is scheduled for launch in October on a mission to survey the binary asteroid system targeted by the DART impact experiment in 2022. Copying the design of Hera will reduce the time needed to get RAMSES to the launch pad, ESA officials said.

“Hera demonstrated how ESA and European industry can meet strict deadlines and RAMSES will follow its example,” said Paolo Martino, who leads ESA’s development of Ramses, which stands for the Rapid Apophis Mission for Space Safety.

ESA’s space safety board recently authorized preparatory work on the RAMSES mission using funds already in the agency’s budget. OHB, the German spacecraft manufacturer that is building Hera, will also lead the industrial team working on RAMSES. The cost of RAMSES will be “significantly lower” than the 300-million-euro ($380 million) cost of the Hera mission, Martino wrote in an email to Ars.

“There is still so much we have yet to learn about asteroids but, until now, we have had to travel deep into the Solar System to study them and perform experiments ourselves to interact with their surface,” said Patrick Michel, a planetary scientist at the French National Center for Scientific Research, and principal investigator on the Hera mission.

“For the first time ever, nature is bringing one to us and conducting the experiment itself,” Michel said in a press release. “All we need to do is watch as Apophis is stretched and squeezed by strong tidal forces that may trigger landslides and other disturbances and reveal new material from beneath the surface.”

Assuming it gets the final go-ahead next year, RAMSES will join NASA’s OSIRIS-APEX mission in exploring Apophis. NASA is steering the spacecraft, already in space after its use on the OSIRIS-REx asteroid sample return mission, toward a rendezvous with Apophis in 2029, but it won’t arrive at its new target until a few weeks after its close flyby of Earth. The intricacies of orbital mechanics prevent a rendezvous with Apophis any earlier.

Observations from OSIRIS-APEX, a larger spacecraft than RAMSES with a sophisticated suite of instruments, “will deliver a detailed look of what Apophis is like after the Earth encounter,” Binzel said. “But until we establish the state of Apophis before the Earth encounter, we have only one side of the picture.”

At its closest approach, asteroid Apophis will closer to Earth than the ring of geostationary satellites over the equator.

Enlarge / At its closest approach, asteroid Apophis will closer to Earth than the ring of geostationary satellites over the equator.

Scientists are also urging NASA to consider launching a pair of mothballed science probes on a trajectory to fly by Apophis some time before its April 2029 encounter with Earth. These two spacecraft were built for NASA’s Janus mission, which the agency canceled last year after the mission fell victim to launch delays with NASA’s larger Psyche asteroid explorer. The Janus probes were supposed to launch on the same rocket as Psyche, but problems with the Psyche mission forced a delay in the launch of more than one year.

Despite the delay, Psyche could still reach its destination in the asteroid belt, but the new launch trajectory meant Janus would be unable to visit the two binary asteroids scientists originally wanted to explore with the probes. After spending nearly $50 million on the mission, NASA put the twin Janus spacecraft, each about the size of a suitcase, into long-term storage.

At the most recent workshop on Apophis missions in April, scientists heard presentations on more than 20 concepts for spacecraft and instrument measurements at Apophis.

They included an idea from Blue Origin, Jeff Bezos’s space company, to use its Blue Ring space tug as a host platform for multiple instruments and landers that could descend to the surface of Apophis, assuming research institutions have enough time and money to develop their payloads. A startup named Exploration Laboratories has proposed partnering with NASA’s Jet Propulsion Laboratory on a small spacecraft mission to Apophis.

“At the conclusion of the workshop, it was my job to try to bring forward some consensus, because if we don’t have some consensus on our top priority, we may end up with nothing,” Binzel said. “The consensus recommendation for ESA was to more forward with RAMSES.”

Workshop participants also gently nudged NASA to use the Janus probes for a mission to Apophis. “Apophis is a mission in search of a spacecraft, and Janus is a spacecraft in search of a mission,” Binzel said. “As a matter of efficiency and basic logic, Janus to Apophis is the highest priority.”

A matter of money

But NASA’s science budget, and especially funding for its planetary science vision, is under stress. Earlier this week, NASA canceled an already-built lunar rover named VIPER after spending $450 million on the mission. The mission had exceeded its original development cost by greater than 30 percent, prompting an automatic cancellation review.

The funding level for NASA’s science mission directorate this year is nearly $500 million less than last year’s budget, and $900 million below the White House’s budget request for fiscal year 2024. Because of the tight budget, NASA officials have said, for now, they are not starting development of any new planetary science missions as they focus on finishing projects already in the pipeline, like the Europa Clipper mission, the Dragonfly quadcopter to visit Saturn’s moon Titan, and the Near-Earth Object (NEO) Surveyor telescope to search for potentially hazardous asteroids.

These grainy radar views of asteroid Apophis were captured using radars at NASA's Goldstone Deep Space Communications Complex in California and Green Bank Telescope in West Virginia.

Enlarge / These grainy radar views of asteroid Apophis were captured using radars at NASA’s Goldstone Deep Space Communications Complex in California and Green Bank Telescope in West Virginia.

NASA has asked the Janus team to look at the feasibility of launching on the same rocket as NEO Surveyor in 2027, according to Dan Scheeres, the Janus principal investigator at the University of Colorado. With such a launch in 2027, Janus could capture the first up-close images of Apophis before RAMSES and OSIRIS-APEX get there.

“This is something that we’re currently presenting in some discussions with NASA, just to make sure that they understand what the possibilities are there,” Scheeres said in a meeting last week of the Small Bodies Advisory Group, which represents the asteroid science community.

“These spacecraft are capable of performing future scientific flyby missions to near-Earth asteroids,” Scheeres said. “Each spacecraft has a high-quality Malin visible imager and a thermal infrared imager. Each spacecraft has the ability to track and image an asteroid system through a close, fast flyby.”

“The scientific return from an Apophis flyby by Janus could be one of the best opportunities out there,” said Daniella DellaGiustina, lead scientist on the OSIRIS-APEX mission from the University of Arizona.

Binzel, who has led the charge for Apophis missions, said there is also some symbolic value to having a spacecraft escort the asteroid by Earth. Apophis will be visible in the skies over Europe and Africa when it is closest to our planet.

“When 2 billion people are watching this, they are going to ask, ‘What are our space agencies doing?’ And if the answer is, ‘Oh, we’ll be there. We’re getting there,’ which is OSIRIS-APEX, I don’t think that’s a very satisfying answer,” Binzel said.

“As the international space community, we want to demonstrate on April 13, 2029, that we are there and we are watching, and we are watching because we want to gain the most knowledge and the most understanding about these objects that is possible, because someday it could matter,” Binzel said. “Someday, our detailed knowledge of hazardous asteroids would be among the most important knowledge bases for the future of humanity.”

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Seismic data shows Mars is often pummeled by planet-shaking meteorites

astronomy, Impacts, Mars, meteorites, planetary science, Science, seismology / /

Brace for impact —

Seismic information now allows us to make a planet-wide estimate of impact rates.

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One of the craters identified seismically, then confirmed through orbital images.

Enlarge / One of the craters identified seismically, then confirmed through orbital images.

Mars trembles with marsquakes, but not all of them are driven by phenomena that occur beneath the surface—many are the aftermath of meteorite strikes.

Meteorites crash down to Mars every day. After analyzing data from NASA’s InSight lander, an international team of researchers noticed that its seismometer, SEIS, detected six nearby seismic events. These were linked to the same acoustic atmospheric signal that meteorites generate when whizzing through the atmosphere of Mars. Further investigation identified all six as part of an entirely new class of quakes known as VF (very high frequency) events.

The collisions that generate VF marsquakes occur in fractions of a second, much less time than the few seconds it takes tectonic processes to cause quakes similar in size. This is some of the key seismological data that has helped us understand the occurrence of earthquakes caused by meteoric impacts on Mars. This is also the first time seismic data was used to determine how frequently impact craters are formed.

“Although a non-impact origin cannot be definitively excluded for each VF event, we show that the VF class as a whole is plausibly caused by meteorite impacts,” the researchers said in a study recently published in Nature.

Seismic shift

Scientists had typically determined the approximate meteorite impact rate on Mars by comparing the frequency of craters on its surface to the expected rate of impacts calculated using counts of lunar craters that were left behind by meteorites. Models of the lunar cratering rate were then adjusted to fit Martian conditions.

Looking to the Moon as a basis for comparison was not ideal, as Mars is especially prone to being hit by meteorites. The red planet is not only a more massive body that has greater gravitational pull, but it is located near the asteroid belt.

Another issue is that lunar craters are often better preserved than Martian craters because there is no place in the Solar System dustier than Mars. Craters in orbital images are often partly obscured by dust, which makes them difficult to identify. Sandstorms can complicate matters by covering craters in more dust and debris (something that cannot occur on the Moon due to the absence of wind).

InSight deployed its SEIS instrument after it landed in the Elysium Planitia region of Mars. In addition to detecting tectonic activity, the seismometer can potentially determine the impact rate through seismic data. When meteorites strike Mars, they produce seismic waves just like tectonic marsquakes do, and the waves can be detected by seismometers when they travel through the mantle and crust. An immense quake picked up by SEIS was linked to a crater 150 meters (492 feet) wide. SEIS would later detect five more marsquakes that were all associated with an acoustic signal (detected by a different sensor on InSight) that is a telltale sign of a falling meteorite.

A huge impact

Something else stood out about the six impact-driven marsquakes detected with seismic data. Because of the velocity of meteorites (over 3,000 meters or 9,842 feet per second), these events happened faster than any other type of marsquake, even faster than quakes in the high frequency (HF) class. That’s how they earned their own classification: very high frequency, or VF, quakes. When the InSight team used the Mars Reconnaissance Orbiter’s (MRO) Context Camera (CTX) to image the locations of the events picked up by SEIS, there were new craters present in the images.

There are additional seismic events that haven’t been assigned to craters yet. They are thought to be small craters formed by meteorites about the size of basketballs, which are extremely difficult to see in orbital images from MRO.

The researchers were able to use SEIS data to estimate the diameters of craters based on distance from InSight (according to how long it took seismic waves to reach the spacecraft) and the magnitude of the VF marsquakes associated with them. They were also able to derive the frequency of quakes picked up by SEIS. Once a frequency estimate based on the data was applied to the entire surface area of Mars, they estimated that around 280 to 360 VF quakes occur each year.

“The case is strong that the unique VF marsquake class is consistent with impacts,” they said in the same study. “It is, therefore, worthwhile considering the implications of attributing all VF events to meteoroid impacts.”

Their detection has added to the estimated number of impact craters on Mars since many could not be seen from space before. What can VF impacts tell us? The impact rate on a planet or moon is important for determining the age of that object’s surface. Using impacts has helped us determine that the surface of Venus is constantly being renewed by volcanic activity, while most of the surface of Mars has not been covered in lava for billions of years.

Figuring out the rate of meteorite impacts can also help protect spacecraft and, someday, maybe Martian astronauts, from potential hazards. The study suggests that there are periods where impacts are more or less frequent, so it might be possible to predict when the sky is a bit more likely to be clear of falling space rocks—and when it isn’t. Meteorites are not much of a danger to Earth since most of them burn up in the atmosphere. Mars has a much thinner atmosphere that more can make it through, and there is no umbrella for a meteor shower.

Nature Astronomy, 2024. DOI: 10.1038/s41550-024-02301-z

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Saturn’s moon Titan has shorelines that appear to be shaped by waves

astronomy, Cassini, geology, planetary science, Saturn, Science, Titan / /

Surf the moon —

The liquid hydrocarbon waves would likely reach a height of a meter.

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Ligeia Mare, the second-largest body of liquid hydrocarbons on Titan.

Enlarge / Ligeia Mare, the second-largest body of liquid hydrocarbons on Titan.

During its T85 Titan flyby on July 24, 2012, the Cassini spacecraft registered an unexpectedly bright reflection on the surface of the lake Kivu Lacus. Its Visual and Infrared Mapping Spectrometer (VIMS) data was interpreted as a roughness on the methane-ethane lake, which could have been a sign of mudflats, surfacing bubbles, or waves.

“Our landscape evolution models show that the shorelines on Titan are most consistent with Earth lakes that have been eroded by waves,” says Rose Palermo, a coastal geomorphologist at St. Petersburg Coastal and Marine Science Center, who led the study investigating signatures of wave erosion on Titan. The evidence of waves is still inconclusive, but future crewed missions to Titan should probably pack some surfboards just in case.

Troubled seas

While waves have been considered the most plausible explanation for reflections visible in Cassini’s VIMS imagery for quite some time, other studies aimed to confirm their presence found no wave activity at all. “Other observations show that the liquid surfaces have been very still in the past, very flat,” Palermo says. “A possible explanation for this is at the time we were observing Titan, the winds were pretty low, so there weren’t many waves at that time. To confirm waves, we would need to have better resolution data,” she adds.

The problem is that this higher-resolution data isn’t coming our way anytime soon. Dragonfly, the next mission to Titan, isn’t supposed to arrive until 2034, even if everything goes as planned.

To get a better idea about possible waves on Titan a bit sooner, Palermo’s team went for inferring their presence from indirect cues. The researchers assumed shorelines on Titan could have been shaped by one of three candidate scenarios. They first assumed there was no erosion at all; the second modeled uniform erosion caused by the dissolution of the bedrock by the ethane-methane liquid; and the third assumed erosion by wave activity. “We took a random topography with rivers, filled up the basin-flooding river valleys all around the lake. Then, we then used landscape evolution computer model to erode the coast to 50 percent of its original size,” Palermo explains.

Sizing the waves

Palermo’s simulations showed that wave erosion resulted in coastline shapes closely matching those actually observed on Titan.

The team validated its model using data from closer to home. “We compared using the same statistical analysis to lakes on Earth, where we know what the erosion processes are. With certainty greater than 77.5 percent, we were able to predict those known processes with our modeling,” Palermo says.

But even the study that claimed there were waves visible in the Cassini’s VIMS imagery concluded they were roughly 2 centimeters high at best. So even if there are waves on Titan, the question is how high and strong are they?

According to Palermo, wave-generation mechanisms on Titan should work just like they do on Earth, with some notable differences. “There is a difference in viscosity between water on Earth and methane-ethane liquid on Titan compared to the atmosphere,” says Palermo. The gravity is also a lot weaker, standing at only one-seventh of the gravity on Earth. “The gravity, along with the differences in material properties, contributes to the waves being taller and steeper than those on Earth for the same wind speed,” says Palermo.

But even with those boosts to size and strength, could waves on Titan actually be any good for surfing?

Surf’s up

“There are definitely a lot of open questions our work leads to. What is the direction of the dominant waves? Knowing that can tell us about the winds and, therefore, about the climate on Titan. How large do the waves get? In the future, maybe we could tell that with modeling how much erosion occurs in one part of the lake versus another in estimated timescales. There is a lot more we could learn,” Palermo says. As far as surfing is concerned, she said that, assuming a minimum height for a surfable wave of around 15 centimeters, surfing on Titan should most likely be doable.

The key limit on the size and strength of any waves on Titan is that most of its seas are roughly the size of the Great Lakes in the US. The largest of them, the Kraken Mare, is roughly as large as the Caspian Sea on Earth. There is no such thing as a global ocean on Titan, and this means the fetch, the distance over which the wind can blow and grow the waves, is limited to tens of kilometers instead of over 1,500 kilometers on Earth. “Still, some models show that the waves on Titan be as high as one meter. I’d say that’s a surfable wave,” Palermo concluded.

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How the perils of space have affected asteroid Ryugu

asteroid, astronomy, magneticsm, planetary science, Ryugu, Science / /

Magnets: how do they stop working? —

Ryugu’s parent body appears to have had a fair amount of water present, too.

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Grey image of a complicated surface composed of many small rocks bound together by dust.

Enlarge / The surface of Ryugu. Image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, Aizu University, AIST

An asteroid that has been wandering through space for billions of years is going to have been bombarded by everything from rocks to radiation. Billions of years traveling through interplanetary space increase the odds of colliding with something in the vast emptiness, and at least one of those impacts had enough force to leave the asteroid Ryugu forever changed.

When the Japanese Space Agency’s Hayabusa2 spacecraft touched down on Ryugu, it collected samples from the surface that revealed that particles of magnetite (which is usually magnetic) in the asteroid’s regolith are devoid of magnetism. A team of researchers from Hokkaido University and several other institutions in Japan are now offering an explanation for how this material lost most of its magnetic properties. Their analysis showed that it was caused by at least one high-velocity micrometeoroid collision that broke the magnetite’s chemical structure down so that it was no longer magnetic.

“We surmised that pseudo-magnetite was created [as] the result of space weathering by micrometeoroid impact,” the researchers, led by Hokkaido University professor Yuki Kimura, said in a study recently published in Nature Communications.

What remains…

Ryugu is a relatively small object with no atmosphere, which makes it more susceptible to space weathering—alteration by micrometeoroids and the solar wind. Understanding space weathering can actually help us understand the evolution of asteroids and the Solar System. The problem is that most of our information about asteroids comes from meteorites that fall to Earth, and the majority of those meteorites are chunks of rock from the inside of an asteroid, so they were not exposed to the brutal environment of interplanetary space. They can also be altered as they plummet through the atmosphere or by physical processes on the surface. The longer it takes to find a meteorite, the more information can potentially be lost.

Once part of a much larger body, Ryugu is a C-type, or carbonaceous, asteroid, meaning it is made of mostly clay and silicate rocks. These minerals normally need water to form, but their presence is explained by Ryugu’s history. It is thought that the asteroid itself was born from debris after its parent body was smashed to pieces in a collision. The parent body was also covered in water ice, which explains the magnetite, carbonates, and silicates found on Ryugu—these need water to form.

Magnetite is a ferromagnetic (iron-containing and magnetic) mineral. It is found in all C-type asteroids and can be used to determine their remanent, or remaining, magnetization. The remanent magnetization of an asteroid can reveal how intense the magnetic field was at the time and place of the magnetite’s formation.

Kimura and his team were able to measure remanent magnetization in two magnetite fragments (known as framboids because of their particular shape) from the Ryugu sample. It is proof of a magnetic field in the nebula our Solar System formed in, and shows the strength of that magnetic field at the time that the magnetite formed.

However, three other magnetite fragments analyzed were not magnetized at all. This is where space weathering comes in.

…and what was lost

Using electron holography, which is done with a transmission electron microscope that sends high-energy electron waves through a specimen, the researchers found that the three framboids in question did not have magnetic chemical structures. This made them drastically different from magnetite.

Further analysis with scanning transmission electron microscopy showed that the magnetite particles were mostly made of iron oxides, but there was less oxygen in those particles that had lost their magnetism, indicating that the material had experienced a chemical reduction, where electrons were donated to the system. This loss of oxygen (and oxidized iron) explained the loss of magnetism, which depends on the organization of the electrons in the magnetite. This is why Kimura refers to it as “pseudo-magnetite.”

But what triggered the reduction that demagnetized the magnetite in the first place? Kimura and his team found that there were more than a hundred metallic iron particles in the part of the specimen that the demagnetized framboids had come from. If a micrometeorite of a certain size had hit that region of Ryugu then it would have produced approximately that many particles of iron from the magnetite framboids. The researchers think this mystery object was rather small, or it would have had to have been moving incredibly fast.

“With increasing impact velocity, the estimated projectile size decreases,” they said in the same study.

Pseudo-magnetite might sound like an imposter, but it will actually help upcoming investigations that seek to find out more about what the early Solar System was like. Its presence indicates the former presence of water on an asteroid, as well as space weathering, such as micrometeoroid bombardment, that affected the asteroid’s composition. How much magnetism was lost also affects the overall remanence of the asteroid. Remanence is important in determining an object’s magnetism and the intensity of the magnetic field around it when it formed. What we know of the Solar System’s early magnetic field has been reconstructed from remanence records, many of which come from magnetite.

Some magnetic properties of those particles might have been lost eons ago, but so much more could be gained in the future from what remains.

Nature Communications, 2024.  DOI: 10.1038/s41467-024-47798-0

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How the Moon got a makeover

moon, planetary science, Science, volcanism / /

Putting on a new face —

The Moon’s former surface sank to the depths, until volcanism brought it back.

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Image of the moon.

Our Moon may appear to shine peacefully in the night sky, but billions of years ago, it was given a facial by volcanic turmoil.

One question that has gone unanswered for decades is why there are more titanium-rich volcanic rocks, such as ilmenite, on the near side as opposed to the far side. Now a team of researchers at Arizona Lunar and Planetary Laboratory are proposing a possible explanation for that.

The lunar surface was once flooded by a bubbling magma ocean, and after the magma ocean had hardened, there was an enormous impact on the far side. Heat from this impact spread to the near side and made the crust unstable, causing sheets of heavier and denser minerals on the surface to gradually sink deep into the mantle. These melted again and were belched out by volcanoes. Lava from these eruptions (more of which happened on the near side) ended up in what are now titanium-rich flows of volcanic rock. In other words, the Moon’s old face vanished, only to resurface.

What lies beneath

The region of the Moon in question is known as the Procellarum KREEP Terrane (PKT). KREEP signifies high concentrations of potassium (K), rare earth elements (REE), and phosphorus (P). This is also where ilmenite-rich basalts are found. Both KREEP and the basalts are thought to have first formed when the Moon was cooling from its magma ocean phase. But the region stayed hot, as KREEP also contains high levels of radioactive uranium and thorium.

“The PKT region… represents the most volcanically active region on the Moon as a natural result of the high abundances of heat-producing elements,” the researchers said in a study recently published in Nature Geoscience.

Why is this region located on the near side, while the far side is lacking in KREEP and ilmenite-rich basalts? There was one existing hypothesis that caught the researchers’ attention: it proposed that after the magma ocean hardened on the near side, sheets of these KREEP minerals were too heavy to stay on the surface. They began to sink into the mantle and down to the border between the mantle and core. As they sank, these mineral sheets were thought to have left behind trace amounts of material throughout the mantle.

If the hypothesis was accurate, this would mean there should be traces of minerals from the hardened KREEP magma crust in sheet-like configurations beneath the lunar surface, which could reach all the way down to the edge of the core-mantle boundary.

How could that be tested? Gravity data from the GRAIL (Gravity Recovery and Interior Laboratory) mission to the Moon possibly had the answer. It would allow them to detect gravitational anomalies caused by the higher density of the KREEP rock compared to surrounding materials.

Coming to the surface

GRAIL data had previously revealed that there was a pattern of subsurface gravitational anomalies in the PKT region. This appeared similar to the pattern that the sheets of volcanic rock were predicted to have made as they sank, which is why the research team decided to run a computer simulation of sinking KREEP to see how well the hypothesis matched up with the GRAIL findings.

Sure enough, the simulation ended up forming just about the same pattern as the anomalies GRAIL found. The polygonal pattern seen in both the simulations and GRAIL data most likely means that traces of heavier KREEP and ilmenite-rich basalt layers were left behind beneath the surface as those layers sank due to their density, and GRAIL detected their residue due to their greater gravitational pull. GRAIL also suggested there were many lesser anomalies in the PKT region, which makes sense considering that a large part of the crust is made of volcanic rocks thought to have sunk and left behind residue before they melted and surfaced again through eruptions.

We now also have an idea of when this phenomenon occurred. Because there are impact basins that dated to around 4.22 billion years ago (not to be confused with the earlier far-side impact), but the magma ocean is thought to have hardened before that, the researchers think that the crust also began to sink before that time.

“The PKT border anomalies provide the most direct physical evidence for the nature of the post-magma ocean… mantle overturn and sinking of ilmenite into the deep interior,” the team said in the same study.

This is just one more bit of information regarding how the Moon evolved and why it is so uneven. The near side once raged with lava that is now volcanic rock, much of which exists in flows called mare (which translates to “sea” in Latin). Most of this volcanic rock, especially in the PKT region, contains rare earth elements.

We can only confirm that there really are traces of ancient crust inside the Moon by the collection of actual lunar material far beneath the surface. When Artemis astronauts are finally able to gather samples of volcanic material from the Moon in situ, who knows what will come to the surface?

Nature Geoscience, 2024.  DOI: 10.1038/s41561-024-01408-2

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Io: New image of a lake of fire, signs of permanent volcanism

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Io: New image of a lake of fire, signs of permanent volcanism

Ever since the Voyager mission sent home images of Jupiter’s moon Io spewing material into space, we’ve gradually built up a clearer picture of Io’s volcanic activity. It slowly became clear that Io, which is a bit smaller than Mercury, is the most volcanically active body in the Solar System, with all that activity driven by the gravitational strain caused by Jupiter and its three other giant moons. There is so much volcanism that its surface has been completely remodeled, with no signs of impact craters.

A few more details about its violence came to light this week, with new images being released of the moon’s features, including an island in a lake of lava, taken by the Juno orbiter. At the same time, imaging done using an Earth-based telescope has provided some indications that this volcanism has been reshaping Io from almost the moment it formed.

Fiery, glassy lakes

The Juno orbiter’s mission is primarily focused on studying Jupiter, including the dynamics of its storms and its internal composition. But many of its orbital passes have taken it right past Io, and this week, the Jet Propulsion Laboratory released some of the best images from these flybys. They include a shot of Loki Patera, a lake of lava that has an island within it. Also featured: the impossibly sheer slopes of Io’s Steeple Mountain.

Looking more closely at the lake, the Juno team found that some of the areas within it were incredibly smooth, raising the possibility that obsidian glass had formed on the surface where it had cooled enough to solidify. Given the level of volcanism on Io, this may be more widespread than the Loki Patera.

Volcanic ash would also create a relatively smooth surface, and is likely to be even more common, but it would have significantly different reflective properties.

How long has this been going on?

But we don’t have to send hardware to Jupiter to learn something about Io. A US-based team got time on the Atacama Large Millimeter Array (ALMA) and used it to record emissions from atoms in Io’s sparse atmosphere. By combining the imaging power of lots of smaller telescopes scattered across a plateau, ALMA is able to spot regional differences in the presence of specific elements in Io’s atmosphere, as well as identify different isotopes of those elements.

What can isotopes tell us? Any atoms that reach Io’s upper atmosphere are at risk of being lost to space. And, because of their relative atomic weights, lighter isotopes have a higher probability of being lost. So, it’s possible to compare the present ratio of elements in the atmosphere with the expected ratio, and we can make inferences about the history of loss of lighter isotopes. And, since the material is put into the atmosphere by volcanoes in the first place, that tells us something about the history of volcanism.

The research team focused on two particular elements: sulfur and chlorine. Sulfur has two common non-radioactive isotopes, 32S and 34S, and chlorine, its neighbor on the periodic table, has 35Cl and 37Cl. There are differences in the ratio of these isotopes throughout the bodies of the Solar System, but those differences are generally small. And, because we think we know what sort of material contributed to the formation of Io, we can focus on the ratios found in bodies that have a similar origin.

Chlorine enters the atmosphere from volcanoes primarily in the form of sodium and potassium salts. These have a very short half-life before they’re split up by exposure to light and radiation. The ALMA data indicated both these chemicals were present in localized regions, likely corresponding to active volcanic plumes. The data from the chlorine isotopes were a bit noisy, so were largely used as a sanity check for the ones obtained from sulfur isotopes.

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Mars may not have had liquid water long enough for life to form

astronomy, carbon dioxide, Mars, planetary science, Science, water / /

Subliminal —

Lab experiments suggest gullies on Mars might form when carbon dioxide heats up.

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Image of a grey-colored slope with channels cut into it.

Mars has a history of liquid water on its surface, including lakes like the one that used to occupy Jezero Crater, which have long since dried up. Ancient water that carried debris—and melted water ice that presently does the same—were also thought to be the only thing driving the formation of gullies spread throughout the Martian landscape. That view may now change thanks to new results that suggest dry ice can also shape the landscape.

It’s sublime

Previously, scientists were convinced that only liquid water shaped gullies on Mars because that’s what happens on Earth. What was not taken into account was sublimation, or the direct transition of a substance from a solid to a gaseous state. Sublimation is how CO2 ice disappears (sometimes water ice experiences this, too).

Frozen carbon dioxide is everywhere on Mars, including in its gullies. When CO2 ice sublimates on one of these gullies, the resulting gas can push debris further down the slope and continue to shape it.

Led by planetary researcher Lonneke Roelofs of Utrecht University in the Netherlands, a team of scientists has found that the sublimation of CO2 ice could have shaped Martian gullies, which might mean the most recent occurrence of liquid water on Mars may have been further back in time than previously thought. That could also mean the window during which life could have emerged and thrived on Mars was possibly smaller.

“Sublimation of CO2 ice, under Martian atmospheric conditions, can fluidize sediment and creates morphologies similar to those observed on Mars,” Roelofs and her colleagues said in a study recently published in Communications Earth & Environment.

Into thin air

Earth and Martian gullies have basically the same morphology. The difference is that we’re certain that liquid water is behind their formation and continuous shaping and re-shaping on Earth. Such activity includes new channels being carved out and more debris being taken to the bottom.

While ancient Mars may have had enough stable liquid water to pull this off, there is not enough on the present surface of Mars to sustain that kind of activity. This is where sublimation comes in. CO2 ice has been observed on the surface of Mars at the same time that material starts flowing.

After examining observations like these, the researchers hypothesized these flows are pushed downward by gas as the frozen carbon dioxide sublimates. Because of the low pressure on Mars, sublimation creates a relatively greater gas flux than it would on Earth—enough power to make fluid motion of material possible.

There are two ways sublimation can be triggered to get these flows moving. When part of a more exposed area of a gully collapses, especially on a steep slope, sediment and other debris that have been warmed by the Sun can fall on CO2 ice in a shadier and cooler area. Heat from the falling material could supply enough energy for the frost to sublimate. Another possibility is that CO2 ice and sediment can break from the gully and fall onto warmer material, which will also trigger sublimation.

Mars in a lab

There is just one problem with these ideas: since humans have not landed on Mars (yet), there are no in situ observations of these phenomena, only images and data beamed back from spacecraft. So, everything is hypothetical. The research team would have to model Martian gullies to watch the action in real time.

To re-create a part of the red planet’s landscape in a lab, Roelofs built a flume in a special environmental chamber that simulated the atmospheric pressure of Mars. It was steep enough for material to move downward and cold enough for CO2 ice to remain stable. But the team also added warmer adjacent slopes to provide heat for sublimation, which would drive movement of debris. They experimented with both scenarios that might happen on Mars: heat coming from beneath the CO2 ice and warm material being poured on top of it. Both produced the kinds of flows that had been hypothesized.

For further evidence that flows driven by sublimation would happen under certain conditions, two further experiments were conducted, one under Earth-like pressures and one without CO2 ice. No flows were produced by either.

“For the first time, these experiments provide direct evidence that CO2 sublimation can fluidize, and sustain, granular flows under Martian atmospheric conditions,” the researchers said in the study.

Because this experiment showed that gullies and systems like them can be shaped by sublimation and not just liquid water, it raises questions about how long Mars had a sufficient supply of liquid water on the surface for any organisms (if they existed at all) to survive. Its period of habitability might have been shorter than it was once thought to be. Does this mean nothing ever lived on Mars? Not necessarily, but Roelofs’ findings could influence how we see planetary habitability in the future.

Communications Earth & Environment, 2024. DOI: 10.1038/s43247-024-01298-7

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The best robot to search for life could look like a snake

astrobiology, astronomy, Biology, Enceladus, planetary science, Robots, Science / /
Image of two humans sitting behind a control console dressed in heavy clothing, while a long tube sits on the ice in front of them.

Enlarge / Trying out the robot on a glacier.

Icy ocean worlds like Europa or Enceladus are some of the most promising locations for finding extra-terrestrial life in the Solar System because they host liquid water. But to determine if there is something lurking in their alien oceans, we need to get past ice cover that can be dozens of kilometers thick. Any robots we send through the ice would have to do most of the job on their own because communication with these moons takes as much as 155 minutes.

Researchers working on NASA Jet Propulsion Laboratory’s technology development project called Exobiology Extant Life Surveyor (EELS) might have a solution to both those problems. It involves using an AI-guided space snake robot. And they actually built one.

Geysers on Enceladus

The most popular idea to get through the ice sheet on Enceladus or Europa so far has been thermal drilling, a technique used for researching glaciers on Earth. It involves a hot drill that simply melts its way through the ice. “Lots of people work on different thermal drilling approaches, but they all have a challenge of sediment accumulation, which impacts the amount of energy needed to make significant progress through the ice sheet,” says Matthew Glinder, the hardware lead of the EELS project.

So, instead of drilling new holes in ice, the EELS team focuses on using ones that are already there. The Cassini mission discovered geyser-like jets shooting water into space from vents in the ice cover near Enceladus’ south pole. “The concept was you’d have a lander to land near a vent and the robot would move on the surface and down into the vent, search the vent, and through the vent go further down into the ocean”, says Matthew Robinson, the EELS project manager.

The problem was that the best Cassini images of the area where that lander would need to touch down have a resolution of roughly 6 meters per pixel, meaning major obstacles to landing could be undetected. To make things worse, those close-up images were monocular, which meant we could not properly figure out the topography. “Look at Mars. First we sent an orbiter. Then we sent a lander. Then we sent a small robot. And then we sent a big robot. This paradigm of exploration allowed us to get very detailed information about the terrain,” says Rohan Thakker, the EELS autonomy lead. “But it takes between seven to 11 years to get to Enceladus. If we followed the same paradigm, it would take a century,” he adds.

All-terrain snakes

To deal with unknown terrain, the EELS team built a robot that could go through almost anything—a versatile, bio-inspired, snake-like design about 4.4 meters long and 35 centimeters in diameter. It weighs about 100 kilograms (on Earth, at least). It’s made of 10 mostly identical segments. “Each of those segments share a combination of shape actuation and screw actuation that rotates the screws fitted on the exterior of the segments to propel the robot through its environment,” explains Glinder. By using those two types of actuators, the robot can move using what the team calls “skin propulsion,” which relies on the rotation of screws, or using one of various shape-based movements that rely on shape actuators. “Sidewinding is one of those gaits where you are just pressing the robot against the environment,” Glinder says.

The basic design also works on surfaces other than ice.

Enlarge / The basic design also works on surfaces other than ice.

The standard sensor suite is fitted on the head and includes a set of stereo cameras providing a 360-degree viewing angle. There are also inertial measuring units (IMUs) that use gyroscopes to estimate the robot’s position, and lidar sensors. But it also has a sense of touch. “We are going to have torque force sensors in each segment. This way we will have direct torque plus direct force sensing at each joint,” explains Robinson. All this is supposed to let the EELS robot safely climb up and down Enceladus’ vents, hold in place in case of eruptions by pressing itself against the walls, and even navigate by touch alone if cameras and lidar don’t work.

But perhaps the most challenging part of building the EELS robot was its brain.

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Webb telescope spots hints that Eris, Makemake are geologically active

astronomy, dwarf planet, Eris, geology, Kuiper belt, Makemake, planetary science, Science / /
Image of two small planets, one more reddish, the second very white.

Enlarge / Artist’s conceptions of what the surfaces of two dwarf planets might look like.

Active geology—and the large-scale chemistry it can drive—requires significant amounts of heat. Dwarf planets near the far edges of the Solar System, like Pluto and other Kuiper Belt objects, formed from frigid, icy materials and have generally never transited close enough to the Sun to warm up considerably. Any heat left over from their formation was likely long since lost to space.

Yet Pluto turned out to be a world rich in geological features, some of which implied ongoing resurfacing of the dwarf planet’s surface. Last week, researchers reported that the same might be true for other dwarf planets in the Kuiper Belt. Indications come thanks to the capabilities of the Webb telescope, which was able to resolve differences in the hydrogen isotopes found on the chemicals that populate the surface of Eris and Makemake.

Cold and distant

Kuiper Belt objects are natives of the distant Solar System, forming far enough from the warmth of the Sun that many materials that are gasses in the inner planets—things like nitrogen, methane, and carbon dioxide—are solid ices. Many of these bodies formed far enough from the gravitational influence of the eight major planets that they have never made a trip into the warmer inner Solar System. In addition, because there was much less material that far from the Sun, most of the bodies are quite small.

While they would have started off hot due to the process by which they formed, their small size means a large surface-to-volume ratio, allowing internal heat to radiate out to space relatively quickly. Since then, any heat has come from rare collision events or the decay of radioactive isotopes.

Yet New Horizons’ visit to Pluto made it clear that it doesn’t take much heat to drive active geology, although seasonal changes in sunlight are likely to account for some of its features. Sunlight is less likely to be an influence for worlds like Makemake, which orbits at a distance one and a half times Pluto’s closest approach to the Sun. Eris, which is nearly as large as Pluto, orbits at over twice Pluto’s closest approach.

Sending a mission to either of these planets would take decades, and none are in development at the moment, so we can’t know what their surfaces look like. But that doesn’t mean we know nothing about them. And the James Webb Space Telescope has added to what we know considerably.

The Webb was used to image sunlight reflected off these objects, obtaining its infrared spectrum—the amount of light reflected at different wavelengths. The spectrum is influenced by the chemical composition of the dwarf planets’ surfaces. Certain chemicals can absorb specific wavelengths of infrared light, ensuring they don’t get reflected. By noting where the spectrum dips, it’s possible to figure out which chemicals are present.

Some of that work has already been done. But Webb is able to image parts of the spectrum that were inaccessible earlier, and its instruments are even able to identify different isotopes of the atoms composing each chemical. For example, some molecules of methane (CH4) will, at random, have one of their hydrogen atoms swapped out for its heavier isotope, deuterium, forming CH3D. These isotopes can potentially act as tracers, telling us things about where the chemicals originally came from.

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What would the late heavy bombardment have done to the Earth’s surface?

asteroids, craters, Earth science, geology, Impacts, planetary science, Science, tsunamis / /

Under fire —

Early in Earth’s history, bombardment by enormous asteroids was common.

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Image of a projection of the globe, with multi-colored splotches covering its surface.

Enlarge / Each panel shows the modeled effects of early Earth’s bombardment. Circles show the regions affected by each impact, with diameters corresponding to the final size of craters for impactors smaller than 100 kilometers in diameter. For larger impactors, the circle size corresponds to size of the region buried by impact-generated melt. Color coding indicates the timing of the impacts. The smallest impactors considered in this model have a diameter of 15 kilometers.

Simone Marchi, Southwest Research Institute

When it comes to space rocks slamming into Earth, two stand out. There’s the one that killed the dinosaurs 65 million years ago (goodbye T-rex, hello mammals!) and the one that formed Earth’s Moon. The asteroid that hurtled into the Yucatan peninsula and decimated the dinosaurs was a mere 10 kilometers in diameter. The impactor that formed the Moon, on the other hand, may have been about the size of Mars. But between the gigantic lunar-forming impact and the comparatively diminutive harbinger of dinosaurian death, Earth was certainly battered by other bodies.

At the 2023 Fall Meeting of the American Geophysical Union, scientists discussed what they’ve found when it comes to just how our planet has been shaped by asteroids that impacted the early Earth, causing everything from voluminous melts that covered swaths of the surface to ancient tsunamis that tore across the globe.

Modeling melt

When the Moon-forming impactor smashed into Earth, much of the world became a sea of melted rock called a magma ocean (if it wasn’t already melted). After this point, Earth had no more major additions of mass, said Simone Marchi, a planetary scientist at the Southwest Research Institute who creates computer models of the early Solar System and its planetary bodies, including Earth. “But you still have this debris flying about,” he said. This later phase of accretion may have lacked another lunar-scale impact, but likely featured large incoming asteroids. Predictions of the size and frequency distributions of this space flotsam indicate “that there has to be a substantial number of objects larger than, say, 1,000 kilometers in diameter,” Marchi said.

Unfortunately, there’s little obvious evidence in the rock record of these impacts before about 3.5 billion years ago. So scientists like Marchi can look to the Moon to estimate the number of objects that must have collided with Earth.

Armed with the size and number of impactors, Marchi and colleagues built a model that describes, as a function of time, the volume of melt this battering must have produced at the Earth’s surface. Magma oceans were in the past, but impactors greater than 100 kilometers in diameter still melted a lot of rock and must have drastically altered the early Earth.

Unlike smaller impacts, the volume of melt generated by objects of this size isn’t localized within a crater, according to models. Any crater exists only momentarily, as the rock is too fluid to maintain any sort of structure. Marchi compares this to tossing a stone into water. “There is a moment in time in which you have a cavity in the water, but then everything collapses and fills up because it’s a fluid.”

The melt volume is much larger than the amount of excavated rock, so Marchi can calculate just how much melt might have spilled out and coated parts of the Earth’s surface with each impact. The result is an astonishing map of melt volume. During the first billion years or so of Earth’s history, nearly the entire surface would have featured a veneer of impact melt at some point. Much of that history is gone because our active planet’s atmospheric, surface, and tectonic processes constantly modify much of the rock record.

Balls of glass

Even between 3.5 and 2.5 billion years ago, the rock record is sparse. But two places, Australia and South Africa, preserve evidence of impacts in the form of spherules. These tiny glass balls form immediately after an impact that sends vaporized rock skyward. As the plume returns to Earth, small droplets begin to condense and rain down.

Spherule bed from impact S3 in drill core. Here, S3’s spherule beds were deposited in deep enough water to not be diluted by other detritus.” data-height=”2560″ data-width=”1735″ href=”https://cdn.arstechnica.net/wp-content/uploads/2024/01/Image2-IMG_1552-scaled.jpg”><span class=Spherule bed from impact S3 in drill core. Here, S3’s spherule beds were deposited in deep enough water to not be diluted by other detritus.” height=”1446″ src=”https://cdn.arstechnica.net/wp-content/uploads/2024/01/Image2-IMG_1552-980×1446.jpg” width=”980″>

Enlarge / Spherule bed from impact S3 in drill core. Here, S3’s spherule beds were deposited in deep enough water to not be diluted by other detritus.

Nadja Drabon, Harvard

“It’s remarkable that we can find these impact-generated spherule layers all the way back to 3.5 billion years ago,” said Marchi.

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