planetary science

The continuing search for life beyond Earth is driving many of the priorities for what’s next when it comes to U.S. planetary exploration. In a new report that could shape the next 10 years of planetary missions, Mars, Uranus and Saturn’s moon Enceladus have come out on top.

This report is the latest decadal survey for planetary science and astrobiology. Every 10 years, experts convened by the National Academies of Sciences, Engineering and Medicine compile a look at the state of the field and pull together a list of recommended priorities for the next decade of exploration. The new survey, which covers 2023 to 2032, will be used by NASA, the National Science Foundation and others to help guide which projects are pursued and funded.

The survey is meant in part “to identify the key scientific questions that are the most important” to pursue in the next decade and assess how best to answer them, astrophysicist Robin Canup said April 19 during a news conference after the report was released. Canup, of the Southwest Research Institute in Boulder, Colo., is a cochair of the steering committee for the decadal survey.

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At the top of the list, the report recommends continuing the Mars sample-return effort by developing a mission that will retrieve, as soon as possible, the rock and soil samples that NASA’s Perseverance rover is collecting and storing (SN: 9/10/21). This multipart sample-return mission was also the top priority of the previous decadal survey, released in 2011 (SN: 3/7/11). Those samples could hold hints of past signs of life on the Red Planet.

The report also suggests that the next Mars mission, after the sample-return one, should look for signs of life in the ice as well as gaseous biosignatures in the atmosphere. That one is farther down the priority list, though.

Next in the line after the Mars sample-return mission is a large, several-billion-dollar mission to send an orbiter and probe to Uranus to explore the planet, its ring system and its moons. Uranus and the solar system’s other ice giant, Neptune, were visited once, in the late 1980s, when Voyager 2 flew by each.

The time has come to go back, scientists say (SN: 2/10/16). “I’m really thrilled to see that they picked a mission to go back and follow up on those incredible discoveries and those wonderful images that Voyager took,” says planetary scientist Linda Spilker of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who was not involved in the decadal survey. Spilker began her career with Voyager.

What’s more, better understanding the ice giants in our solar system could help scientists decipher the mysteries of faraway worlds. In the hunt for planets outside our solar system, the most common type of known exoplanets are those like Neptune and Uranus.

A mission to Uranus “will be transformative,” says planetary scientist Amy Simon of NASA Goddard Space Flight Center in Greenbelt, Md., and a member of the decadal steering committee. “We’re sure there’s going to be fantastic discoveries.”  

This mission could launch in June 2031 or April 2032, the report suggests. After swinging by Jupiter to use the giant planet’s gravity to fling it faster, the spacecraft would arrive at Uranus 13 years after its launch. Once there, the orbiter would drop a probe in the atmosphere, sampling its composition as never before.

The next highest priority is sending an “orbilander” to Saturn’s moon Enceladus, a world known to have easily accessible liquid water (SN: 5/2/06). NASA’s now-defunct Cassini mission discovered in 2005 that this small moon spews geysers of water into space, and more recent research suggests that water coming from subsurface locales has salts, possibly indicating warm pockets of water interacting with rock — and brewing an environment that may host life (SN: 8/4/14).

Does Enceladus (shown) harbor life? A new planetary science report recommends planning a mission to the Saturnian moon to try to answer that question.JPL-Caltech/NASA, Space Science Institute

This proposed spacecraft would arrive at the moon in the early 2050s, where it would first spend 1.5 years orbiting Enceladus, flying through its watery plumes to sample the liquid. Then the spacecraft would land on the surface for a two-year mission.

“If you want to go and look for life, Enceladus is a very good place to do it,” says planetary scientist Francis Nimmo of the University of California, Santa Cruz, and a member of the decadal steering committee.

Life on other planets isn’t the only thing on planetary scientists’ minds. The report also recommends continuing work on a mission to find and characterize near-Earth objects, like asteroids and comets, in an effort to protect life on the only planet where it’s known to exist.

Two medium-sized missions should be funded in the next decade too, the report recommends. While the survey doesn’t specify targets for these missions, nine higher-priority locales are singled out, including Venus, Saturn’s moon Titan and Neptune’s moon Triton.

The decadal survey also considered the state of the fields of planetary science and astrobiology — namely decreasing funding opportunities and how to improve diversity, equity, inclusion and accessibility efforts. For the latter, the committee looked at whether the community has diverse representation through their members.

“The thing that became abundantly clear is that NASA has done a terrible job of collecting those kinds of statistics,” Nimmo says of demographics in planetary science. For now, the recommendation is to better survey the scientific community, he says.  “We’re not going to be able to solve anything until we actually have better statistics.” More

Europa’s frozen surface is covered with distinctive pairs of ridges that straddle troughs of ice. These double ridges are the most common features on the Jovian moon. But scientists don’t yet have a clear idea of how the oddities are created.

Now, an analysis of images of a similar set of ridges on Greenland’s ice sheet suggests that relatively shallow water within Europa’s thick icy shell may be behind their formation, scientists report April 19 in Nature Communications. If so, that could mean that Europa has much more shallow liquid water than scientists have thought.

Europa’s double ridge systems, which can stretch for hundreds of kilometers, include some of the oldest features on the moon, says Riley Culberg, a geophysicist at Stanford University. Some researchers have proposed that the flexing of the moon’s icy shell due to tides in an underlying liquid water ocean plays a role in the ridges’ formation (SN: 8/6/20). Yet others have suggested that water erupted from deep within the icy moon — a process known as cryovolcanism — to create the ridges. Without a closer look, though, it’s been hard to nail down a more solid explanation.

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But Culberg and his colleagues seem to have caught a break. Data gathered by NASA’s ICESat-2 satellite in March 2016 showed an 800-meter-long double ridge system in northwestern Greenland. So the team looked back at other images to see when the ridge system first appeared and to assess how it grew. The researchers found that the ridges appeared in images taken as early as July 2013 and are still there today.

When the ridges — which lie on either side of a trough, like those on Europa — reached full size, they averaged only 2.1 meters high. That’s a lot smaller than the ridges on Europa, which can rise 300 meters or more from the moon’s surface. But surface gravity is much lower on Europa, so ridges can grow much larger there, Culberg says. When he and his colleagues considered the difference between Earth’s gravity and Europa’s in their calculations, they found that the proportions of the two ridge systems are consistent.

Double ridge systems are common on Europa. The largest pair seen in this composite image from NASA’s Galileo spacecraft in the 1990s is about 2.6 kilometers wide and 300 meters tall.JPL-Caltech/NASA, ASU

Scientists will never get a perfect analog of Europa on Earth, but the ridges in Greenland “look just like the Europan ridges,” says Laurent Montési, a geophysicist at the University of Maryland in College Park who was not involved in the study.

Data from airplane-mounted radar gathered in March 2016 show that a water-filled layer of snow about 10 to 15 meters below the surface underlies the Greenland ridges, Culberg and his team say. That water comes from surface meltwater that sinks into and is then collected in the buried snow, which in turn sits atop an impermeable layer of ice.

Repeated freeze-thaw cycles of water in that layer of snow would squeeze water toward the surface, the researchers propose. In the first phase of refreezing, a solid plug of ice forms. Then, as more water freezes, it expands and is forced toward the surface on either side of that plug, pushing material upward and producing the double ridges at the surface.

On Europa, the process works the same way, the researchers suggest. But because there is no known meltwater or precipitation at the moon’s surface, near-surface water there probably would have to come from the ocean thought to be trapped beneath the moon’s icy shell (5/14/18). Once that water rose toward the surface through cracks, it could pool in thick layers of ice shattered by tidal flexing or the impacts of meteorites.

“There’s a general consensus that these ridges grow from cracks in the ice,” says William McKinnon, a planetary scientist at Washington University in Saint Louis who was not involved in the study. “But how do they do it is the question.”

The answer to that question may not be long in coming, McKinnon says. NASA’s Europa Clipper mission is scheduled to launch in late 2024. If all goes well, the orbiter will arrive at Jupiter in April 2030. “If there’s anything like what has happened in Greenland going on at Europa, we’ll be able to see it,” he says.

Researchers will also be interested to see if the mission can ascertain what sort of materials might have been brought to Europa’s surface from the ocean deep below, because the moon is considered to be one of the best places in the solar system to look for extraterrestrial life (SN: 4/8/20). More

One year ago, Ingenuity took its first flight on Mars. And its story since is that of a real-world little helicopter that could.

Ingenuity traveled to the Red Planet attached to the belly of NASA’s Perseverance rover, and both arrived in Jezero crater last February (SN: 2/17/21). About six weeks later, the helicopter began what was meant to be only a 30-day technology demonstration to see if flight is possible in the thin Martian atmosphere.

It proved it could fly — and then some (SN: 4/19/21). Over the next couple weeks, Ingenuity took four more flights, each time going a bit farther, a bit faster and a bit higher. After those first test flights, Ingenuity’s mission morphed from a technology demonstration to operations, helping Perseverance traverse the surface by scouting the terrain ahead (SN: 4/30/21; SN: 12/10/21).

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Before the helicopter arrived, scientists had two perspectives of Mars. “We have pictures taken from orbit around Mars, and then we have pictures taken by rovers driving on the ground,” says planetary scientist Kirsten Siebach of Rice University in Houston, who is not part of the Ingenuity team. “But now this has opened up an entirely new perspective on Mars.” 

Ingenuity has surpassed all expectations. It has shown not only that flight is possible but also what is possible with flight. Science News discussed the helicopter’s big moments, collaboration with the rover and upcoming flights with Håvard Fjær Grip. He’s Ingenuity’s chief pilot and an engineer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. His answers were edited for length and clarity.

SN: What does the “chief pilot” for a helicopter on another planet do?

Grip: The biggest part of the job is planning the flights. Ingenuity doesn’t know where it is or where it wants to go when it wakes up, so all of those decisions are made here [on Earth]. Every maneuver that the helicopter makes during the flight is planned here on the ground first, and then we uplink the instructions to Ingenuity. When it comes time to fly, it uses its onboard software to follow our instructions as precisely as possible.

On April 19, 2021, Ingenuity Chief Pilot Håvard Fjær Grip (pictured) logged the first flight on another world in an official logbook.JPL-Caltech/NASA

SN: Ingenuity has completed 25 flights. Can you talk about how it’s exceeded expectations?

Grip: It is pretty great. We came there expecting to perform at most five flights within the 30-day window. And all of that was going to happen within in a small area that we carefully selected. We spent weeks figuring out exactly where to place the helicopter, studying these tiny little rocks. Everything was mapped out. And then things went so well when we started flying that almost immediately people started thinking, “Wow, let’s try to make use of this beyond those five flights.”

We started this next phase where, to be useful at all, we had to fly away from this carefully selected area. I’m really proud of that. We’ve been able to take this technology that was designed for this very limited mission and extend it to go and land different places on Mars and to travel across terrain that, originally, we had never planned on traveling across.

It’s lasted now for over a year since we deployed it to the surface. I don’t think any of us had imagined that that would be possible.

SN: Have there been any specific flights that have stood out to you?

Grip: Obviously, the first flight. That was the most important flight; it still is. We had a more challenging [time on] flight six. It became exciting, because we had an anomaly during the flight. [A glitch led to navigation images being marked with the wrong time stamps, which caused Ingenuity to sway back and forth during its flight.] Ingenuity had to power through that and survive and get down on the ground in one piece.

We’ve had some flights that have been dedicated to scouting activities. We went to an area where the rover was going to spend several months, and we went ahead of the rover and scouted [it] out so the rover drivers could be more efficient in finding safe ways to drive. Those were flights 12 and 13. Then some of these longer flights have been exciting. Flight No. 9, until a few days ago, was the biggest thing we’d ever done, at [a distance of] 625 meters. And with flight 25, we just beat that and flew more than 700 meters.

SN: There was a flight recently that had to be postponed because of a dust storm, right?

Grip: That’s correct. That was flight No. 19. With flying, whether it’s on Mars or here on Earth, you’re worried about weather. We always look at the weather before flying. And every time we’ve done that [on Mars], it had been more or less the same. Then the afternoon before we were about to open flight 19, we were notified that we had a dust storm. That delayed us by quite a bit. When we woke up from that, we had dust on our navigation camera lens, and sand covered our legs partially. We had to fly out of that, and it was a new challenge for the helicopter, but again, it tackled that perfectly.

During Ingenuity’s 22nd flight, on March 19, 2022, the helicopter captured a picture of its own shadow on the ground below.JPL-Caltech/NASA

SN: Ingenuity has flown through two seasons on Mars. As seasons change, so does air pressure. Does that affect the helicopter?

Grip: Yeah, that’s a pretty big deal. We knew, for several years before launch, exactly when we were going to land and where we were going to land. Our design was geared towards the first few months after landing, and that coincided with a particular season [spring] in Jezero crater on Mars. We could [ahead of launch] predict reasonably well what the air density would be. And when we extended [the mission] beyond that, the air density started dropping. To be able to keep flying, we had to increase our rotor speed. In fact, we increased it above anything we tested on Earth. Now we’ve come out of summer, the density has started climbing again, and we’ve been able to go back to our original rotor speed and also extend our flight time.

SN: What comes next? Are there any big flights planned soon?

Grip: We’re going to make our way over to the river delta that Perseverance is headed toward. We’ve just completed the biggest obstacle to doing that, flight 25, which was getting across this region called Séítah, which has a lot of sand and varied terrain. And when we get to the river delta, there are a few different options on the table: to help the rover drivers, to scout out targets, or even potentially to do some scouting on behalf of the next Mars mission. Perseverance is the first part of a sample return campaign. It’s sampling right now. And those samples will be left on the surface and will be eventually picked up — that’s the plan anyway — and sent back to Earth.

SN: What does Ingenuity mean for future exploration?

Grip: This is a new era. Aviation in space is now a thing. We can’t think about Mars exploration without aerial assets as part of that. I think that’s the most exciting thing. More

The nucleus of a comet discovered in 2014 is the largest ever spotted.

The “dirty snowball” at the center of comet C/2014 UN271 is about 120 kilometers across, researchers report in the April 10 Astrophysical Journal Letters. That makes this comet — also known as Bernardinelli-Bernstein, after its discoverers — about twice as wide as Rhode Island, says David Jewitt, an astronomer at UCLA.

Though the comet is big — and vastly larger than Halley’s comet, which measures a little more than 11 kilometers across — it will never be visible to the naked eye from Earth because it’s too far away, Jewitt says (SN: 12/14/15). The object is now about 3 billion kilometers from Earth. At its closest approach in 2031, the comet will come no closer to the sun than 1.6 billion kilometers, about the same distance as Saturn.

Jewitt and colleagues sized up the comet with the help of new images from the Hubble Space Telescope, combined with images taken by another team at far-infrared wavelengths. The analysis also revealed that the comet’s nucleus reflects only about 3 percent of the light that strikes it. That makes the object “blacker than coal,” Jewitt says.

Comet Bernardinelli-Bernstein takes about 3 million years to circle the sun in a highly elliptical orbit. At its farthest, the comet may reach about half a light-year from the sun — about one-eighth of the distance to the next nearest star.

The comet is likely “just the tip of the iceberg” as far as undiscovered comets of this size go, Jewitt says. And for every comet this size, he suggests, there could be tens of thousands of smaller objects circling the sun undetected.

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Neptune’s atmospheric temperature is on an unexpected roller-coaster ride, and it could take decades for scientists to piece together what’s happening at the distant planet.

The ice giant’s global temperature dropped about 8 degrees Celsius between 2003 and 2012 at the start of Neptune’s summer, researchers report April 11 in Planetary Sciences Journal. Then from 2018 to 2020, thermal images show that the planet’s south pole brightened dramatically, indicating a spike of 11 degrees C (SN: 10/2/07).

Naomi Rowe-Gurney, a planetary scientist at NASA Goddard Space Flight Center in Greenbelt, Md., and colleagues looked at 17 years of mid-infrared data from ground-based telescopes and the no-longer-functioning Spitzer Space Telescope (SN: 7/18/18; SN: 1/28/20). The researchers used infrared light to pierce Neptune’s top cloud layer and peer at its stratosphere, where the planet’s atmospheric chemistry comes into view.

Each Neptune year lasts 165 Earth years, so the time period analyzed — from 2003 to 2020 — is essentially equivalent to five weeks on Earth. The wildest temperature shift occurred from 2018 to 2020, when the atmospheric temperature at Neptune’s south pole rose from –121° C to –110° C.

“We weren’t expecting any seasonal changes to happen in this short time period, because we’re not even seeing a full season,” says Rowe-Gurney. “It’s all very strange and interesting.”

The researchers don’t yet know what’s causing the temperature changes. The sun’s ultraviolet rays break up methane molecules in the stratosphere, so that chemistry or even the sun’s activity cycle could be a trigger. Nailing down specifics requires more observations. “We need to keep observing over the next 20 years to see a full season and see if something else changes,” says Rowe-Gurney.

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On Mars, the speed of sound depends on its pitch.

All sound travels slower through Mars’ air compared with Earth’s. But the higher-pitched clacks of a laser zapping rocks travels slightly faster in the thin Martian atmosphere than the lower-pitched hum of the Ingenuity helicopter, researchers report April 1 in Nature.

These sound speed measurements from NASA’s Perseverance rover are part of a broader effort to monitor minute-by-minute changes in atmospheric pressure and temperature, like during wind gusts, on the Red Planet.

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“The wind is the sound of science for us,” says astrophysicist Baptiste Chide of Los Alamos National Laboratory in New Mexico.

To listen to the wind, Perseverance carries two microphones. One was meant to record audio during the mission’s complex entry, descent and landing, and while it didn’t work as hoped, it is now turned on occasionally to listen to the rover’s vitals (SN: 2/22/21; SN: 2/17/21). The other microphone is part of the rover’s SuperCam instrument, a mast-mounted mishmash of cameras and other sensors used to understand the properties of materials on the planet’s surface.

But these microphones also pick up other sounds, such as those made by the rover itself as its wheels crunch the surface, and by Perseverance’s flying companion, the robotic helicopter Ingenuity. The SuperCam instrument, for example, has a laser, which Perseverance fires at interesting rocks for further analysis (SN: 7/28/20). The microphone on SuperCam captures sounds from those laser shots, which helps researchers learn about the hardness of the target material, says planetary scientist Naomi Murdoch of the Institut Supérieur de l’Aéronautique et de l’Espace in Toulouse, France.

Murdoch, Chide and their colleagues listened to the laser’s clack-clack when zapping rocks. (“It doesn’t do, really, ‘pew pew,’” Murdoch says). When the laser hits a target, that blast creates a sound wave. Because scientists know when the laser fires and how far away a target is, they can measure the speed at which that sound wave travels through the air toward the SuperCam microphone.

The speed of this sound is about 250 meters per second, the team reports. That’s slower than on Earth, where sound travels through the air at about 340 m/s.

The slower speed isn’t surprising. What we hear as sound is actually pressure waves traveling through a medium like air, and the speed of those waves depends on the medium’s density and composition (SN: 10/9/20). Our planet’s atmosphere is 160 times as dense as the Martian atmosphere, and Earth’s air is mostly nitrogen and oxygen, whereas the Martian air is predominately carbon dioxide. So sound on Mars travels slower in that different air.

The team also used the SuperCam microphone to listen to the lower-pitch whirl of Ingenuity’s helicopter blades (SN: 12/10/21). From this lower-pitched sound, the researchers learned that there is a second speed of sound at the Martian surface at frequencies below 240 hertz, or slightly deeper than middle C on a piano: 240 m/s.

In contrast, at Earth’s surface, sound moves through the air at only one speed, no matter the pitch. The two speeds on Mars, the researchers say, are because of its carbon dioxide–rich atmosphere. Carbon dioxide molecules behave differently with one another when sound waves with frequencies above 240 hertz move through the air compared with those below 240 hertz, affecting the waves’ speed.

“We’ve proved that we can do science with a microphone on Mars,” Chide says. “We can do good science.”

The SuperCam microphone captures thousands of sound snippets per second. Those sounds are affected by air pressures, so the researchers can use that acoustic data to track detailed changes in air pressures over short timescales, and, in doing so, learn more about the Martian climate. While other Mars rovers have had wind, temperature and pressure sensors, those could sense changes only over longer periods.

“Listening to sounds on another planet is another way that helps all of us place ourselves as if we were there,” says Melissa Trainer, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., who was not part of this work.

The team is focusing on next collecting acoustic data at different times of day and different seasons on Mars.

“The pressure changes a lot on Mars throughout the year with the seasons,” Trainer says. “I’m really excited to see how the data might change as it gets collected through proceeding seasons.” More

A treasure trove of diamonds may be sown into Mercury’s cratered crust.

Billions of years of meteorite impacts may have flash-baked much of Mercury’s surface into the glittery gemstones, planetary scientist Kevin Cannon reported March 10 at the Lunar and Planetary Science Conference in The Woodlands, Texas. His computer simulations predict that such impacts may have transformed about one-third of the little planet’s crust into a diamond stockpile many times that of Earth’s.

Diamonds are forged under immense pressures and temperatures. On Earth, the gemstones crystallize deep underground — at least 150 kilometers down — then ride to the surface during volcanic eruptions (SN: 9/14/20). But studies of meteorites suggest diamonds can also form during impact.

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“When those [impacts] happen, they create very high pressures and temperatures that can transform carbon into diamond,” says Cannon, of the Colorado School of Mines in Golden.

With impact-born diamonds on his mind, Cannon turned to the closest planet to the sun. Surveys of the planet’s surface and experiments with molten rock suggest that the planet’s crust may retain fragments of an old shell of graphite — a mineral made from carbon (SN: 3/7/16). “What we think happened is that when [Mercury] first formed, it had a magma ocean, and that graphite crystallized out of that magma,” Cannon says.

Then, the bombardment. Mercury’s surface today is heavily cratered, evidence of an impact-rich history. Much of the purported graphite crust would have been battered and transformed into diamond, Cannon hypothesized.

Curious how pervasive this diamond forging could have been, Cannon used computers to simulate 4.5 billion years of impacts on a graphite crust. The findings show that if Mercury had possessed a skin of graphite 300 meters thick, the battering would have generated 16 quadrillion tons of diamonds — about 16 times Earth’s estimated reserves.

“There’s no reason to doubt that diamonds could be produced in this way,” says Simone Marchi, a planetary scientist at the Southwest Research Institute in Boulder, Colo., who was not involved with the research. But how many might have survived, that’s another story, he says. Some of the gemstones would probably have been destroyed by later impacts.

Cannon agrees that subsequent impacts would probably obliterate some diamonds. But the losses would have been “very limited,” he says, as the ultimate melting point of diamond exceeds 4000° Celsius. Future simulations will incorporate remelting from impacts, he says, to refine the potential size of Mercury’s present day diamond reserves.

An opportunity to scout for diamonds on Mercury may come in 2025, when the BepiColombo mission reaches the planet. Diamonds reflect a distinct signature of infrared light, Cannon says. “And potentially, this could be detected.” More

A bevy of craters formed by material blasted from the carving of another, larger crater — a process dubbed secondary cratering — have finally been spotted on Earth. Several groupings of craters in southeastern Wyoming, including dozens of pockmarks in all, have the hallmarks of secondary cratering, researchers report February 11 in GSA Bulletin.

When an asteroid or another type of space rock smacks into a planet or moon, it blasts material from the surface and creates a crater (SN: 12/18/18). Large blocks of that material can be thrown far from the initial crater and blast out their own holes when they land, explains Thomas Kenkmann, a planetary scientist at the Albert Ludwig University of Freiburg in Germany. Astronomers have long observed secondary cratering on our moon, Mars and other orbs in the solar system, but never on Earth.

When Kenkmann and his colleagues first investigated a series of craters near Douglas, Wyo., in 2018, they thought the pockmarks were formed by fragments of a large meteorite that had broken up in the atmosphere. But Kenkmann and his team later discovered similar groups of craters of the same age, somewhere around 280 million years old, throughout the region.

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Altogether, the team found more than 30 impact craters that range between 10 and 70 meters in diameter at six different locales. Based on subtle but distinct differences in the alignment of elliptical craters in the groups, the researchers suggest that the impactors that blasted each set of craters struck the ground from slightly different directions.

The impactors that created these secondary craters probably ranged between 4 and 8 meters in diameter and struck the ground at speeds between 2,520 and 3,600 kilometers per hour, Kenkmann says. Extrapolating the paths of these impactors back to their presumed sources suggests the original crater from which they flew straddles the Wyoming–Nebraska border northeast of Cheyenne.

The team’s evidence “comes together very well to make a compelling story,” says Gareth Collins, a planetary scientist at Imperial College London who was not involved in the new study.

The original crater was probably between 50 and 65 kilometers across and was created by an impactor 4 to 5.4 kilometers wide, Kenkmann and the team estimate. The crater is also probably buried under more than 2 kilometers of sediment that accumulated in the 280 million years since it formed. An equivalent amount of sediment eroded away to expose the secondary craters when the Rocky Mountains rose in the meantime.

“What a fortuitous discovery that these folks have made,” says Beau Bierhaus, a planetary scientist at Lockheed Martin Space Systems in Littleton, Colo. He likens the short geological interval during which these craters could be discovered to the brief period between the time a fossil is first exposed to the elements and when it is eroded to dust.

Scouring measurements of gravitational and magnetic fields in the region for anomalies could help reveal the buried crater, the researchers note. The team may also look for heavily fractured rock and other evidence of the ancient crater in sediment cores that have been drilled during oil and gas exploration in the region, Kenkmann says. More