planetary science

Saturn’s icy moon Enceladus sprays water vapor into space. Scientists have thought that the plumes come from a deep subsurface ocean — but that might not be the case, new simulations suggest.

Instead, the water could come from pockets of watery mush in the moon’s icy shell, scientists report December 15 at the American Geophysical Union’s fall meeting.

“Maybe we didn’t get the straw all the way through the ice shell to the ocean. Maybe we’re just getting this weird pocket,” says planetary scientist Jacob Buffo of Dartmouth College.

The finding is “a cautionary tale,” Buffo says. The hidden ocean makes Enceladus one of the best places to search for life in the solar system (SN: 4/8/20). Concepts for future missions to Enceladus rely on the idea that taking samples of the plumes would directly test the contents of the ocean, without needing to drill or melt through the ice. “That could be true,” Buffo says. But the simulations suggest “you could be sampling this slushy region in the middle of the shell, and that might not be the same chemistry as is down in the ocean.”

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Enceladus has beguiled planetary scientists since NASA’s Cassini spacecraft revealed the moon’s dramatic plumes in 2005 (SN: 8/23/05). At the time, researchers wondered if the spray originated on Enceladus’ icy surface, where friction from quakes could melt ice and let it escape as pure water vapor into space. But later evidence collected by Cassini convinced most scientists that the geysers are from fractures in the shell that reach all the way to a salty, subsurface sea (SN: 8/4/14).

One of the most convincing pieces of evidence was the fact that the plumes contain salts, said physicist Colin Meyer of Dartmouth in a talk at the meeting, which was held virtually and in New Orleans. Early versions of the quake idea couldn’t account for those salts, and instead suggested that any salts in the melted ice would be left on the surface as the water escaped into space, like the sheen of salt left on your skin after you sweat, he says.

But Meyer, who has studied the physics of sea ice on Earth, realized that pockets of meltwater in the ice shell could concentrate salts and other compounds. He, Buffo and colleagues applied computer simulations developed for sea ice on Earth to the observed icy conditions on Enceladus. The team found that Enceladus could easily generate pockets of mush within its shell and vent the contents of that mush out into space, salts and all.

That does not mean Enceladus doesn’t have an ocean, Meyer says — it almost certainly does. And it does not mean the ocean isn’t habitable, Buffo adds.

The implications of the results “are huge,” especially for proposed life-finding missions to Enceladus, says planetary scientist Emily Martin of the Smithsonian National Air and Space Museum in Washington, D.C., who was not involved in the work.

“If those plumes aren’t tapping into the ocean, it will really shift our perspective on what that plume is telling us about the interior of Enceladus,” Martin says. “And that’s a big deal.” More

A newly discovered exoplanet is really making astronomers prove their mettle. Planet GJ 367b is smaller than Earth, denser than iron and hot enough to melt, researchers report in the Dec. 3 Science.

“We think the surface of this exoplanet could be molten,” says astronomer Kristine Wei Fun Lam of the Institute of Planetary Research at the German Aerospace Center in Berlin.

Signals of the planet were first spotted in data from NASA’s TESS telescope in 2019. The small world swung around its host star every 7.7 hours.

Using data from TESS and the ground-based HARPS spectrograph at the European Southern Observatory in Chile, Lam and her colleagues measured the planet’s radius and mass. GJ 367b clocked in at about 0.72 times Earth’s radius and 0.55 times its mass. That makes it the first ultrashort-period planet — a class of worlds with years shorter than one Earth day and with mysterious origins — known to be smaller and lighter than Earth.

Using those measurements, the team then calculated the planet’s density: about 8.1 grams per cubic centimeter, or slightly denser than iron. A computer analysis of the planet’s interior structure suggests that 86 percent of it could comprise an iron core, with only a sliver of rock left on top.

Mercury has a similarly large core, Lam notes (SN: 4/22/19). Scientists think that’s a result of a giant impact with another planet that stripped away most of its outer layers. GJ 367b could have formed after a similar collision. It could also have once been a gaseous planet whose atmosphere was blasted off by radiation from its star (SN: 7/1/20).

Whatever its origins, GJ 367b is so close to its star that it’s almost certainly covered in melted metallic lava now. “At 1400° Celsius, I don’t think it would be very nice to stand on it,” Lam says. More

Pluto’s dark side has come into dim view, thanks to the light of the dwarf planet’s moon.

When NASA’s New Horizons spacecraft flew past Pluto in 2015, almost all the images of the dwarf planet’s unexpectedly complex surface were of the side illuminated by the sun (SN: 7/15/15). Darkness shrouded the dwarf planet’s other hemisphere. Some of it, like the area near the south pole, hadn’t seen the sun for decades.

Now, mission scientists have finally released a grainy view of the dwarf planet’s dark side. The researchers describe the process to take the photo and what it tells them about how Pluto’s nitrogen cycle affects its atmosphere October 20 in the Planetary Science Journal.

Before New Horizons passed by Pluto, the team suspected the dwarf planet’s largest moon, Charon, might reflect enough light to illuminate the distant world’s surface. So the researchers had the spacecraft turn back toward the sun to take a parting peek at Pluto.

New Horizons captured this view of the backlit dark side of Pluto as the spacecraft receded from the dwarf planet in 2015. Some light and dark splotches were illuminated by the dim light of Pluto’s moon Charon.NOIRLab, SwRI, JHUAPL, NASA

At first, the images just showed a ring of sunlight filtering through Pluto’s hazy atmosphere (SN: 7/24/15). “It’s very hard to see anything in that glare,” says planetary scientist John Spencer of the Southwest Research Institute in Boulder, Colo. “It’s like trying to read a street sign when you’re driving toward the setting sun and you have a dirty windshield.”

Spencer and colleagues took a few steps to make it possible to pull details of Pluto’s dark side out of the glare. First, the team had the spacecraft take 360 short snapshots of the backlit dwarf planet. Each was about 0.4 seconds long, to avoid overexposing the images. The team also took snapshots of the sun without Pluto in the frame so that the sun could be subtracted out after the fact.

Tod Lauer of the National Optical Astronomy Observatory in Tucson, Ariz., tried to process the images when he got the data in 2016. At the time, the rest of the data from New Horizons was still fresh and took up most of his attention, so he didn’t have the time to tackle such a tricky project.

But “it was something that just sat there and ate away at me,” Lauer says. He tried again in 2019. Because the spacecraft was moving as it took the images, each image was a little bit smeared or blurred. Lauer wrote a computer code to remove that blur from each individual frame. Then he added the reflected Charon light in each of those hundreds of images together to produce a single image.

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“When Tod did that painstaking analysis, we finally saw something emerging in the dark there … giving us a little bit of a glimpse of what the dark pole of Pluto looks like,” Spencer says.

That the team got anything at all is impressive, says planetary scientist Carly Howett, also of the Southwest Research Institute and who is on the New Horizons team but was not involved in this work. “This dataset is really, really hard to work with,” she says. “Kudos to this team. I wouldn’t have wanted to do this.”

The image, Howett says, can help scientists understand how Pluto’s frigid nitrogen atmosphere varies with its decades-long seasons. Pluto’s atmosphere is controlled by how much nitrogen is in a gas phase in the air and how much is frozen on the surface. The more nitrogen ice that evaporates, the thicker the atmosphere becomes. If too much nitrogen freezes to the ground, the atmosphere could collapse altogether.

In this image of Pluto’s sunlit side from NASA’s New Horizons spacecraft, different colors represent different kinds of ices. A faint glimpse of Pluto’s dark hemisphere in newly released mission images reveals some new details about how those ices behave.SwRI, JHUAPL, NASA

When New Horizons was there, Pluto’s south pole looked darker than the north pole. That suggests there was not a lot of fresh nitrogen frost freezing out of the atmosphere there, even though it was nearing winter. “The previous summer ended decades ago, but Pluto cools off pretty slowly,” Spencer says. “Maybe it’s still so warm [that] the frost can’t condense there, and that keeps the atmosphere from collapsing.”

There was a bright spot in the middle of the image, which could be a fresh ice deposit. That’s also not surprising, Howett says. The ices may still be moving from the north pole to the south pole as Pluto moves deeper into its wintertime.

“We’ve thought this for a long time. It makes sense,” she says. “But it’s nice to see it happening.” More

The lander was listening. On February 18, NASA’s InSight lander on Mars turned its attention to the landing site for another mission, Perseverance, hoping to detect its arrival on the planet.

But InSight heard nothing.

Tungsten blocks ejected by Perseverance during entry landed hard enough to create craters on the Martian surface. Collisions like these — whether from space missions or meteor strikes — send shock waves through the ground. Yet in the first experiment of its kind on another world, InSight failed to pick up any seismic waves from the blocks’ impacts, researchers report October 28 in Nature Communications.

As a result, researchers think that less than 3 percent of the energy from the impacts made its way into the Martian surface. The intensity of impact-generated rumblings varies from planet to planet and is “really important for understanding how the ground will change from a big impact event,” says Ben Fernando, a geophysicist at the University of Oxford.

Perseverance left behind several craters (one indicated with the arrow) after pieces of the mission disengaged as planned during entry, creating a rare opportunity to see how Mars absorbs energy from impacts. Univ. of Arizona, JPL-Caltech/NASA

But getting these measurements is tricky. Scientists need sensitive instruments placed relatively near an impact site. Knowing when and where a meteor will strike is nearly impossible, especially on another world.

Enter Perseverance: a hurtling space object set to hit Mars at an exact time and place (SN: 2/17/21). To help with its entry, Perseverance dropped about 78 kilograms of tungsten as the rover landed about 3,450 kilometers from InSight. The timing and weight of the drop provided a “once-in-a-mission opportunity” to study the immediate seismic effects of an impact from space, Fernando says.

The team had no idea whether InSight would be able to detect the blocks’ impacts or not, but the quiet arrival speaks volumes. “It lets us put an upper limit on how much energy from the tungsten blocks turned into seismic energy,” Fernando says. “We’ve never been able to get that number for Mars before.”

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The Perseverance rover has captured its first two slices of Mars.

NASA’s latest Mars rover drilled into a flat rock nicknamed Rochette on September 1 and filled a roughly finger-sized tube with stone. The sample is the first ever destined to be sent back to Earth for further study. On September 7, the rover snagged a second sample from the same rock. Both are now stored in airtight tubes inside the rover’s body.

Getting pairs of samples from every rock it drills is “a little bit of an insurance policy,” says deputy project scientist Katie Stack Morgan of NASA’s Jet Propulsion Lab in Pasadena, Calif. It means the rover can drop identical stores of samples in two different places, boosting chances that a future mission will be able to pick up at least one set.

The successful drilling is a comeback story for Perseverance. The rover’s first attempt to take a bit of Mars ended with the sample crumbling to dust, leaving an empty tube (SN: 8/19/21). Scientists think that rock was too soft to hold up to the drill.

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Nevertheless, the rover persevered.

“Even though some of its rocks are not, Mars is hard,” said Lori Glaze, director of NASA’s  planetary science division, in a September 10 news briefing.

Rochette is a hard rock that appears to have been less severely eroded by millennia of Martian weather (SN: 7/14/20). (Fun fact: All the rocks Perseverance drills into will get names related to national parks; the region on Mars the rover is now exploring is called Mercantour, so the name Rochette — or “Little Rock”  — comes from a village in France near Mercantour National Park.)

Rover measurements of the rock’s texture and chemistry suggests that it’s made of basalt and may have been part of an ancient lava flow. That’s useful because volcanic rocks preserve their ages well, Stack Morgan says. When scientists on Earth get their hands on the sample, they’ll be able to use the concentrations of certain elements and isotopes to figure out exactly how old the rock is — something that’s never been done for a pristine Martian rock.

Rochette also contains salt minerals that probably formed when the rock interacted with water over long time periods. That could suggest groundwater moving through the Martian subsurface, maybe creating habitable environments within the rocks, Stack Morgan says.

“It really feels like this rich treasure trove of information for when we get this sample back,” Stack Morgan says.

Once a future mission brings the rocks back to Earth, scientists can search inside those salts for tiny fluid bubbles that might be trapped there. “That would give us a glimpse of Jezero crater at the time when it was wet and was able to sustain ancient Martian life,” said planetary scientist Yulia Goreva of JPL at the news briefing.

Scientists will have to be patient, though — the earliest any samples will make it back to Earth is 2031. But it’s still a historic milestone, says planetary scientist Meenakshi Wadhwa of Arizona State University in Tempe.

“These represent the beginning of Mars sample return,” said Wadhwa said at the news briefing. “I’ve dreamed of having samples back from Mars to analyze in my lab since I was a graduate student. We’ve talked about Mars sample return for decades. Now it’s starting to actually feel real.” More

The shrinking mass of Pluto — Science News, August 28, 1971

Pluto was the last of the planets to be discovered (in 1930). If astronomers continue to make it lighter, it may be the first to disappear.… [The latest measurement] brings Pluto down to 0.11 of Earth’s mass, less than an eighth of its former self.… The wide discrepancies among the figures presented for the mass of Pluto illustrate the particular difficulties of measuring its mass.… If a planet has satellites, its mass can be determined from studying their motions.… But Pluto has no known satellites.

Update

The discovery of Pluto’s moon Charon in 1978 (SN: 7/15/78, p. 36) finally allowed astronomers to accurately calculate the planet’s mass: about 0.2 percent of Earth’s mass. Decades after scientists resolved Pluto’s heft, the planet received arguably the greatest demotion of all — a downgrade to dwarf planet (SN: 9/2/06, p. 149). Some astronomers have since proposed alternate definitions for the term “planet” that, if widely adopted, would restore Pluto to its former rank. More

In February, NASA’s Perseverance rover touched down on Mars and went to work. The rover has seen the first flight of a Martian robot, gotten its drill bit dirty and begun traversing the floor of Jezero crater, thought to be the remains of an ancient lake (SN: 4/30/21).

And what Perseverance is finding isn’t exactly what scientists expected. “The crater floor is super interesting,” says planetary scientist Briony Horgan of Purdue University in West Lafayette, Ind., one of the mission’s long-term science planners. “We didn’t really know what we were getting into from orbit.”

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Perseverance is getting views of enormous boulders that may have been transported by ancient floods, fine rock layers that look like they settled in calm waters, and rocks with large crystals that look volcanic. The rover’s landing site may include a volcanic lava flow from long ago, or signs of an earlier episode of water — or something else.

“It’s not as obvious as we thought,” Horgan says. “Whatever it is, it’s cool.”

Here are some of the image highlights from the rover so far.

Taking the long view

Before the rover landed, the Perseverance team knew that Jezero crater looked like the dry basin of an ancient lake, with a river delta flowing into it. The prospect of finding preserved lake floor sediments made the site good for searching for past life, one of the mission’s primary goals.

This picture, taken March 17, is a mosaic of five images taken with Perseverance’s Remote Microscopic Imager camera. The tilted layers of sedimentary rock (arrows) and other textures in this escarpment were probably formed by interaction between an ancient river and a lake.JPL-Caltech/NASA, LANL, CNES, CNRS, ASU, MSSS

Perseverance took this snapshot March 17 of a steep slope in a part of Jezero’s delta, from more than two kilometers away. The rover probably won’t reach that spot until sometime next year. But already, the rover’s Remote Microscopic Imager camera is uncovering details that could reveal new insight into the crater’s watery past.

For example, the tilted layers of sedimentary rock and cementlike mixtures of coarse sand and pebbles in this rock feature, nicknamed “Delta Scarp,” confirm the delta’s wet history. There are also individual large boulders cemented into the front of the scarp, suggesting that the region saw high floods, says Perseverance deputy project scientist Katie Stack Morgan of NASA’s Jet Propulsion Lab in Pasadena, Calif.

Closer to home

Even eroded outcrops close to Perseverance’s landing site look like they had a watery history. This image of a remnant of part of the delta rising out of the crater floor was taken with Perseverance’s Mastcam-Z camera February 22.

Perseverance’s Mastcam-Z camera took this image (shown in false color) February 22 of a relatively nearby escarpment, which probably preserves ancient lake sediments. Click to enlargeJim Bell/ASU, Mastcam-Z

“Many of us expected these outcrops to be quite uninteresting, based on orbital data,” Stack Morgan says. But images from the ground showed beautiful layers, just like what you would find in a deep-lake deposit.

“We weren’t expecting to find them here, but maybe they’re right next door to our landing site,” she says. These outcrops could be remnants of the edge of the lake that used to fill Jezero crater or could represent an even older lake that was replaced.

Even closer

Perseverance is taking close-ups of the rocks around it too. This closeup image of a rock nicknamed “Foux” was taken July 11 using the WATSON camera on the end of the rover’s robotic arm. The area in the image is only about 4 centimeters by 3 centimeters.

This close-up image of a larger rock was taken with Perseverance’s WATSON camera, part of the SHERLOC instrument on the rover’s robotic arm. It shows textured rocks with an interesting coating that might indicate interaction with water. JPL-Caltech/NASA, MSSS

The textures in this image are fascinating, as are the “crazy red coatings” that are more purple than typical Mars dust, Horgan says. “What rocks are these?” The coatings probably imply alteration by water, and the purple color suggests that they contain some iron, she adds.

Volcanic grains?

Perseverance has also found evidence of igneous, or volcanic, rocks on Jezero’s crater floor. That wasn’t surprising — observations from orbit suggested that volcanic rocks should be there, and scientists hoped to pick up some to help researchers back on Earth figure out the rocks’ absolute ages. Right now, the timing of past events on Mars is based on the sizes of craters and the ages of rocks from the moon, and it’s not extremely precise.

This image, taken August 2, shows mysterious holes and light and dark patches that are potential crystals. The Perseverance rover abraded the rock to prepare for drilling into it. JPL-Caltech/NASA

Igneous rocks on Mars tend to be old and preserve a record of their ages well. “If you want to figure out when things happened on Mars, you want an igneous rock,” Stack Morgan says.

On the ground, though, things are a little more complicated. This rock was the first that Perseverance cleared dust from in preparation for taking a sample. The image shows mysterious holes, which could have been formed by erosion or by air bubbles trapped in lava as it cooled. And the surface is divided into light and dark patches that could be individual crystals, or cemented grains.

If they’re crystals, that suggests volcanic activity, Stack Morgan says — but these crystals are bigger than expected for lava that would have cooled at the planet’s surface. Similar crystals form deep in the subsurface of Earth, where magma solidifies slowly. When lava cools at Earth’s surface, the crystals “don’t have time to grow big,” Stack Morgan says. The next step, she says, is “thinking through how rocks like this could have formed here, if they are indeed igneous or volcanic rocks. How would we get a rock that looks like this?” Maybe this rock formed underground and was transported to the surface, but it’s not clear how.

First sample attempt

That same rock carried more surprises when the rover team tried to drill into it August 6. The drill worked perfectly, to the team’s elation. “One of the most complex robotic systems ever designed and executed worked perfectly with no faults the first time,” Stack Morgan says. “We were like ‘Oh my god, this is amazing.’”

But when they looked inside the tube that was meant to capture the rock sample, it was empty.

“It’s been a bit of an emotional roller coaster,” Stack Morgan says.

Perseverance’s shadow (left) looms over the borehole that the rover made on its first attempt to drill into the Red Planet. The rover’s WATSON imager took a close-up of that hole (composite image at right). These images were taken August 6.JPL-Caltech/NASA, MSSS

The team thinks that the rock was more crumbly than expected, and essentially turned to dust. “The rock was not able to keep its act together,” Stack Morgan says. The drill is designed to sweep the small grains produced in the drilling process, called cuttings, up and out of the sample tube. Stack Morgan thinks the entire sample was treated as cuttings and ended up in a pile of dust on the ground.

There is a silver lining: Now the rover has a sealed sample of Martian atmosphere. And the rover will attempt to take another sample of a hardier rock sometime soon, Stack Morgan says.

In the wind

Mars may have had lakes and rivers in its past, but today the dry, dusty landscape is shaped mostly by wind (SN: 7/14/20). Perseverance has seen a number of dust devils and windstorms sweep through Jezero crater as a beautiful reminder of how environments are always changing, even on a dried-up planet like Mars.

A dust devil swirls across the Martian landscape. This image was captured with the Perseverance rover’s left Mastcam-Z camera June 15.JPL-Caltech/NASA, ASU

“We often think of Mars as this barren wasteland where not much happens today,” Stack Morgan says. “But when you see these dust devils move across the images, you’re kind of reminded that Mars, even though not Earthlike, is its own very active planet still.” More

Mars has had its first CT scan, thanks to analyses of seismic waves picked up by NASA’s InSight lander. Diagnosis: The Red Planet’s core is at least partially liquid, as some previous studies had suggested, and is somewhat larger than expected.

InSight reached Mars in late 2018 and soon afterward detected the first known marsquake (SN: 11/26/18; SN: 4/23/19). Since then, the lander’s instruments have picked up more than a thousand temblors, most of them minor rumbles. Many of those quakes originated at a seismically active region more than 1,000 kilometers away from the lander. A small fraction of the quakes had magnitudes ranging from 3.0 to 4.0, and the resulting vibrations have enabled scientists to probe Mars and reveal new clues about its inner structure.

Simon Stähler, a seismologist at ETH Zurich, and colleagues analyzed seismic waves from 11 marsquakes, looking for two types of waves: pressure and shear. Unlike pressure waves, shear waves can’t pass through a liquid, and they move more slowly, traveling side to side through solid materials, rather than in a push-and-pull motion in the same direction a wave is traveling like pressure waves do.

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Of those 11 events, six sets of vibrations included shear waves strong enough to stand out from background noise. The strength of those shear waves suggests that they reflected off of the outer surface of a liquid core, rather than entering a solid core and being partially absorbed, Stähler says. And the difference in arrival times at InSight for the pressure waves and shear waves for each quake suggest that Mars’ core is about 3,660 kilometers in diameter, he and colleagues report in the July 23 Science.

That’s a little more than half of the diameter of the entire planet, larger than most previous estimates. The Red Planet’s core is so big, in fact, that it blocks InSight from receiving certain types of seismic waves from a large part of the planet. That, in turn, suggests that Mars may be more seismically active than the lander’s sensors can detect. Indeed, one of the regions in the lander’s seismic blind spot is the Tharsis region, home to some of Mars’ largest volcanoes. Volcanic activity there, as well as the motion of molten rock within the crust in that region, could trigger quakes or seismic waves.

Seismic waves (red lines in this illustration) traveling through Mars from a quake’s source (example, red dot) to the InSight lander (white dot) reveal the Red Planet’s internal structure, including a massive core (yellow-white) more than half the diameter of the planet.Chris Bickel/Science

While the newly analyzed data confirm the planet’s outer core is liquid, it’s not clear yet whether Mars has a solid inner core like Earth, says study coauthor Amir Khan, a geophysicist also at ETH Zurich. “The signal should be there in the seismic data,” he says. “We just need to locate it.”

In a separate analysis also published in Science, Khan and colleagues suggest that InSight’s seismic blind spot may also stem, in part, from the way that seismic waves slow down and bend as they travel deep within the planet. Changes in seismic wave speed and direction can result from gradual variations in rock temperature or density, for example.

Mars’ seismic waves also hint at the thickness of the planet’s crust. As they bounce back and forth within the planet, the waves bounce off interfaces between different layers and types of rocks, says Brigitte Knapmeyer-Endrun, a seismologist at the University of Cologne in Bergisch Gladbach, Germany. In a separate study in Science, she and her team analyzed seismic signals that reflected off several such interfaces near Mars’ surface, making it difficult to determine the depth at which the planet’s crust ends and the underlying mantle begins, she says. The researchers concluded, however, that the average thickness of the crust likely lies between 24 and 72 kilometers. For comparison, Earth’s oceanic crust is about 6 to 7 kilometers thick, while the planet’s continental crust averages from 35 to 40 kilometers thick.

Together, these seismic analyses are the first to investigate the innards of a rocky planet other than Earth, Stähler says. As such, they provide “ground truth” for measurements made by spacecraft orbiting Mars, and could help scientists better interpret data gathered from orbit around other planets, such as Mercury and Venus.

The findings could also provide insights that would help planetary scientists better understand how Mars formed and evolved over the life of the solar system, and how the Red Planet ended up so unalike Earth, says Sanne Cottaar, a geophysicist at the University of Cambridge. Cottaar wrote a commentary, also published in Science, on the new research. “Mars was put together with similar building blocks” as Earth, she says, “but had a different result.” More