Here’s how ice needles sculpt patterns into cold, rocky landscapes

Neat rings, stripes and swirls embellish many cold, rocky landscapes. Although these beautiful stone patterns look like humanmade artwork, they’re all natural. Scientists have long known that such rocky patterns result from freezing and thawing. But precisely how they develop has been a mystery — until now.

New experiments reveal that so-called “ice needles” can sort and organize rocks into many patterns, Anyuan Li of the University of Tsukuba in Japan and colleagues report the Oct. 5 Proceedings of the National Academy of Sciences.

“The beauty of [our] experiments is that you can actually see direct information on how the patterns form,” says Bernard Hallet of the University of Washington in Seattle, who has studied natural patterns in surface rocks around the world.
The researchers spread pebbles atop a pan holding moist, fine-grained soil, then froze and thawed this mini-landscape over and over. When the moist soil had not yet frozen but the air temperature dropped below freezing, tiny, needlelike columns of ice sprouted up from the soil. These ice needles, each up to a few centimeters high, lifted any stones atop them. When temperatures rose again, the ice collapsed and the stones tumbled off. Because the ice needles curved as they grew, the stones tended to fall off their icy pedestals to one side.

Over many freeze-thaw cycles, the ice needles cleared patches of exposed soil. Since needles could more easily form in spots where there were fewer rocks in the way, they more efficiently cleared out any remaining pebbles. Stones were gradually shuffled into clusters between stone-free areas to form larger patterns. The pattern that builds on a landscape “strongly depends on its [local stone] concentration,” says study coauthor Quan-Xing Liu, a theoretical ecologist at East China Normal University in Shanghai.
In lab experiments, the team “was able to able to get patterns after 30 freeze cycles,” says Hallet. That could equate to 30 cold nights — or 30 years, if each freeze lasted a whole winter. In the real world, Hallet says, some patterns might take “thousands, if not tens of thousands, of years to form.”

Using observations from their soil experiments, the researchers built a computer simulation of ice needle landscaping. This simulation could predict stone movement in the open environment under a range of conditions. The simulation confirmed that the rate of pattern formation depended on how dense the stone cover was, among other factors. The shapes and formation rates of patterns were also related to how moist the soil was, how the ground sloped and how tall the ice needles grew.

“We see identical patterns in different systems, such as fluids,” Hallet says of the rock formations. Materials with different characteristics or sizes often start all mixed together but don’t stay that way (SN: 4/22/21). Phase separation is the process that morphs these mixes into patterns. The new study is among the first to show how phase separation applies to landscapes.

The combination of experiments and computer modelling in this study provides a new way to connect how natural landscapes form and how their materials behave, says Rachel Glade, a geologist at the University of Rochester in New York who was not involved in the work. This approach “is vital for our understanding of complex materials,” she adds, and it could help us understand how landscapes may evolve differently in a changing climate.

A new particle accelerator aims to unlock secrets of bizarre atomic nuclei

Inscribed on an Italian family’s 15th century coat of arms and decorating an ancient Japanese shrine, the Borromean rings are symbolically potent. Remove one ring from the trio of linked circles and the other two fall apart. It’s only when all three are entwined that the structure holds. The rings have represented the concepts of unity, the Christian Holy Trinity and even certain exotic atomic nuclei.

A rare variety, or isotope, of lithium has a nucleus that is made of three conjoined parts. Lithium-11’s nucleus is separated into a main cluster of protons and neutrons flanked by two neutrons, which form a halo around the core. Remove any one piece and the trio disbands, much like the Borromean rings.

Not only that, lithium-11’s nucleus is enormous. With its wide halo, it is the same size as a lead nucleus, despite having nearly 200 fewer protons and neutrons. The discovery of lithium-11’s expansive halo in the mid-1980s shocked scientists (SN: 8/20/88, p. 124), as did its Borromean nature. “There wasn’t a prediction of this,” says nuclear theorist Filomena Nunes of Michigan State University in East Lansing. “This was one of those discoveries that was like, ‘What? What’s going on?’ ”
Lithium-11 is just one example of what happens when nuclei get weird. Such nuclei, Nunes says, “have properties that are mind-blowing.” They can become distorted into unusual shapes, such as a pear (SN: 6/15/13, p. 14). Or they can be sheathed in a skin of neutrons — like a peel on an inedible nuclear fruit (SN: 6/5/21, p. 5).

A new tool will soon help scientists pluck these peculiar fruits from the atomic vine. Researchers are queuing up to use a particle accelerator at Michigan State to study some of the rarest atomic nuclei. When it opens in early 2022, the Facility for Rare Isotope Beams, or FRIB (pronounced “eff-rib”), will strip electrons off of atoms to make ions, rev them up to high speeds and then send them crashing into a target to make the special nuclei that scientists want to study.

Experiments at FRIB will probe the limits of nuclei, examining how many neutrons can be crammed into a given nucleus, and studying what happens when nuclei stray far from the stable configurations found in everyday matter. With FRIB data, scientists aim to piece together a theory that explains the properties of all nuclei, even the oddballs. Another central target: pinning down the origin story for chemical elements birthed in the extreme environments of space.

And if scientists are lucky, new mind-blowing nuclear enigmas, perhaps even weirder than lithium-11, will emerge. “We’re going to have a new look into an unexplored territory,” says nuclear physicist Brad Sherrill, scientific director of FRIB. “We think we know what we’ll find, but it’s unlikely that things are going to be as we expect.”
Exploring instability
Atomic nuclei come in a dizzying number of varieties. Scientists have discovered 118 chemical elements, distinguished by the number of protons in their nuclei (SN: 1/19/19, p. 18). Each of those elements has a variety of isotopes, different versions of the element formed by switching up the number of neutrons inside the nucleus. Scientists have predicted the existence of about 8,000 isotopes of known elements, but only about 3,300 have made an appearance in detectors. Researchers expect FRIB will make a sizable dent in the missing isotopes. It may identify 80 percent of possible isotopes for all the elements up through uranium, including many never seen before.

The most familiar nuclei are those of the roughly 250 isotopes that are stable: They don’t decay to other types of atoms. The ranks of stable isotopes include the nitrogen-14 and oxygen-16 in the air we breathe and the carbon-12 found in all known living things. The number following the element’s name indicates the total number of protons and neutrons in the nucleus.

Stable nuclei have just the right combination of protons and neutrons. Too many or too few neutrons causes a nucleus to decay, sometimes slowly over billions of years, other times in mere fractions of a second (SN: 3/2/19, p. 32). To understand what goes on inside these unstable nuclei, scientists study them before they decay. In general, as the proton-neutron balance gets more and more off-kilter, a nucleus gets further from stability, and its properties tend to get stranger.

Such exotic specimens test the limits of scientists’ theories of the atomic nucleus. While a given theory might correctly explain nuclei that are near stability, it may fail for more unusual nuclei. But physicists want a theory that can explain the most unusual to the most banal.

“We would like to understand how the atomic nucleus is built, how it works,” says theoretical nuclear physicist Witold Nazarewicz, FRIB’s chief scientist.
A fast clip
Accelerating beams of ions in FRIB is like herding cats.

In the beginning, “it’s just a gaggle of cats,” says Thomas Glasmacher, FRIB’s laboratory director. The cats meander this way or that, but if you can nudge the unruly bunch in a particular direction — maybe you open a can of cat food — then the cats start moving together, despite their natural tendency to wander. “Pretty soon, it’s a stream of cats,” he says.

In FRIB’s case, the cats are ions — atoms with some or all of their electrons stripped off. And rather than cat food, electromagnetic forces get them moving en masse.

The journey starts in one of FRIB’s two ion sources, where elements are vaporized and ionized. After some initial acceleration to get the ions moving, the beam enters the linear accelerator, which is what sets the particles really cruising. The linear accelerator looks like a scaled-down freight train — a line of 46 boxes the color of pistachio ice cream, each about 2.5 meters tall, of varying lengths. But the accelerator sends the beam moving much faster than a cargo-filled train — up to about half the speed of light.

Within the green boxes, called cryomodules, superconducting cavities are cooled to just a few kelvins, a smidge above absolute zero. At those temperatures, the cavities can accelerate the ions using rapidly oscillating electromagnetic fields. The chain of pistachio modules wends around the facility in the shape of a paper clip, a contortion necessary so that the approximately 450-meter-long accelerator fits in the 150-meter-long tunnel that houses it.

When the beam is fully accelerated, it’s slammed into a graphite target. That hard hit knocks protons and neutrons off the nuclei of the incoming ions, forming new, rarer isotopes. Then, the specific one that a scientist wants to study is separated from the riffraff by magnets that redirect particles based on their mass and electric charge. The particles of interest are then sent to the experimental area, where scientists can use various detectors to study how the particles decay, measure their properties or determine what reactions they undergo.
The energy of FRIB’s beam is carefully selected for producing rare isotopes. Too much energy would blow the nuclei apart when they collide with the target. So FRIB is designed to reach less than a hundredth the energy of the Large Hadron Collider at CERN near Geneva, the world’s most energetic accelerator.

Instead, the new accelerator’s potential rests on its juiced-up intensity: Essentially, it has lots and lots of particles in its beam. For example, FRIB will be able to slam 50 trillion uranium ions per second into its target. As a result, it will produce more intense streams of rare isotopes than its predecessors could.

For isotopes that are relatively easily produced, FRIB will churn out about a trillion per second; plenty to study. That opens prospects for scrutinizing isotopes that are more difficult to make. Those isotopes might pop up once a week in FRIB, but that’s still much more often than in a weaker beam. It’s like a case of low water pressure in the bathroom: “You can’t have a shower if it’s just trickling,” says Nunes, who is one of the leaders of a coalition of theoretical physicists supporting research at FRIB. Now, “FRIB is going to come in with a fire hose.”
Dripping with neutrons
That fire hose will also come in handy for pinpointing a crucial boundary known as the neutron drip line.

Try to stuff too many neutrons in a nucleus, and it will decay almost immediately by spitting out a neutron. Imagine a greedy chipmunk with its cheeks so full of nuts that when it tries to shove in one more, another nut pops right back out. The threshold at which nuclei decay in this way marks the ultimate limits for bound nuclei. On a chart of the known elements and their isotopes, this boundary traces out a line, the neutron drip line. So far, scientists know the location of this crucial demarcation up through, at most, the 10th element on the periodic table, neon.

“FRIB is going to be the only way to go heavier and far enough out to define that drip line,” says nuclear physicist Heather Crawford of Lawrence Berkeley National Laboratory in California. FRIB is expected to determine the neutron drip line up to the 30th element, zinc, and maybe even farther.
Near that drip line, where neutrons greatly outnumber protons, is where nuclei get especially strange. Lithium-11, with its capacious halo, sits right next to the drip line. Crawford focuses on magnesium isotopes that are close to the drip line. The most common stable magnesium isotope has 12 protons and 12 neutrons. Crawford’s main target, magnesium-40, has 12 protons and more than double that number of neutrons — 28 — in its nucleus.

“That’s right out at the limits of existence,” Crawford says. Out there, theories that predict the properties of nuclei are no longer reliable. Theoretical physicists can’t always be sure what size and shape a given nucleus in this realm might be, or even whether it qualifies as a bound nucleus. A given theory might also fall short when predicting how much energy is needed to bump the nucleus into its various energized states. The spacing of these energy levels acts as a kind of fingerprint of an atomic nucleus, one that’s highly sensitive to the details of the nucleus’ shape and other properties.

Sure enough, magnesium-40 behaves unexpectedly, Crawford and colleagues reported in 2019 in Physical Review Letters. While theories predicted its energy levels would match those of magnesium isotopes with slightly fewer neutrons, magnesium-40’s energy levels were significantly lower than its neighbors’.

In August, Crawford learned that she will be one of the first scientists to use FRIB. Two experiments she and colleagues proposed were selected for the first round of about 30 experiments to take place over FRIB’s first two years. She’ll take a closer look at magnesium-40, which, like lithium-11, has a Borromean nucleus. Crawford now aims to determine if her chosen isotope also has a haloed nucleus. That’s one possible explanation for magnesium-40’s oddness. Despite the fact that nuclei with halos have been known for decades, theories still can’t reliably predict which nuclei will be festooned with them. Understanding magnesium-40 could help scientists firm up their accounting of nuclei’s neutron adornments.
Elemental origins
Physicists want to be able to poke around, like mechanics under the hood, to understand the cosmic nuclear reactions that make the universe go. “Nuclear physics is like the engine of a sports car. It’s what happens in the engine that determines how well the car performs,” says nuclear physicist Ani Aprahamian of the University of Notre Dame in Indiana.

The cosmos powered by that engine can be a violent place for nuclei, punctuated with dramatic stellar explosions and extreme conditions, including matter crammed into ultratight quarters by crushing gravity. These environments beget wonders of nuclear physics unlike those normally seen on Earth. FRIB will let scientists get a glimpse at some of those processes.

For example, physicists think that certain neutron-rich environments are the cauldron where many of the universe’s chemical elements are cooked. This cosmic connection allowed nuclear physicist Jolie Cizewski to make good on a childhood dream.

When Cizewski was a little girl, she caught the astronomy bug, she says. “I decided I was going to become an astronomer so I could go into space.” It might seem that she took a left turn from her childhood obsession. She never made it to orbit and she didn’t become an astronomer.

But echoes of that childhood dream now anchor her research. Instead of peering at the stars with a telescope, she’ll soon be using FRIB to reveal secrets of the cosmos.

Cizewski, of Rutgers University in New Brunswick, N.J., is working to unveil details of the cosmic nuclear reactions responsible for the nuclei that surround us. “I’m trying to understand how the elements, in particular those heavier than iron, have been synthesized,” she says.

Many of the elements around us — and in us — formed within stars. As large stars age, they fuse progressively larger atomic nuclei together in their cores, creating elements farther along the periodic table — oxygen, carbon, neon and others. But the process halts at iron. The rest of the elements must be born another way.

A process called the rapid neutron capture process, or r-process, is responsible for many of those other elements found in nature. In the r-process, atomic nuclei quickly soak up neutrons and bulk up to large masses. The neutronfest is interspersed with radioactive decays that form new elements. The sighting of two neutron stars merging in 2017 revealed that such collisions are one place where the r-process occurs (SN: 11/11/17, p. 6). But scientists suspect it might happen in other cosmic locales as well (SN: 6/8/19, p. 10).

Cizewski and colleagues are studying an abbreviated form of the r-process that might thrive in supernovas, which may not have enough oomph for the full r-process. The team has zeroed in on germanium-80, which plays a pivotal role in the weak r-process. Physicists want to know how likely this nucleus is to capture another neutron to become germanium-81. At FRIB, Cizewski will slam a beam of germanium-80 into deuterium, which has one proton and one neutron in its nucleus. Knowing how often germanium-80 captures the neutron will help scientists nail down the neutron-slurping chain of the weak r-process, wherever it might crop up.
A Borromean bent
Like the interlinked Borromean rings, different facets of nuclear physics are closely entwined, from mysteries of the cosmos to the inner workings of nuclei. The exotic nuclei that FRIB cooks up could also allow physicists to tap into the very bedrock of physics by testing certain fundamental laws of nature. And there’s a practical side to the facility as well. Scientists could collect some of the isotopes FRIB produces for use in medical procedures, for example.

Physicists are ready for surprises. “Every time we build such a facility, new discoveries come and breakthroughs in science come,” Nazarewicz says. Like the 1980s discovery of lithium-11’s Borromean nucleus, scientists may find something totally unexpected.

What the Perseverance rover’s quiet landing reveals about meteor strikes on Mars

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.
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.”