A proposed form of ice acts like a cross between a solid and a liquid. Now, a new study strengthens the case that the weird state of matter really exists.

Hints of the special phase, called superionic ice, appeared in water ice exposed to high pressures and temperatures, researchers report February 5 in Nature Physics. Although such unusual ice isn’t found naturally on Earth, it might lurk deep inside frozen worlds like Uranus and Neptune (SN Online: 3/5/12).
Normal ice is composed of water molecules, each made of an oxygen atom bonded to two hydrogen atoms. As water freezes, those molecules link up to form a solid. But superionic ice is made up of ions, which are atoms with a positive or negative electric charge. Within the material, hydrogen ions flow freely through a solid crystal of oxygen ions.

“That’s really strange behavior for water,” says study coauthor Marius Millot, a physicist at Lawrence Livermore National Laboratory in California. Although the superionic state was first predicted 30 years ago, “up until now we didn’t really know whether this was something that was real.”

At extremely high pressures, familiar substances like water can behave in unusual ways (SN: 1/14/12, p. 26). Working with a sample of ice that was crushed between two diamonds, Millot and colleagues used a laser to create a shock wave that plowed through the ice, boosting the pressure even more. At first, the density and temperature of the ice ramped up smoothly as the pressure increased. But at around 1.9 million times atmospheric pressure and 4,800 kelvins (about 4,500° Celsius), the scientists observed a jump in density and temperature. That jump, the researchers say, is evidence that superionic ice melted at that point. Although we normally think of ice as being cold, at high pressures, superionic ice can form even when heated. The melting occurred at just the conditions that theoretical calculations predict such ice would melt. The physicists didn’t measure the pressure at which the superionic phase first formed.

The electrical conductivity of the material provided another hint of superionic ice: The level of conductivity was consistent with expectations for that phase of matter. Whereas metals conduct electricity via the motion of electrons, in superionic ice, the flowing hydrogen ions transmit electricity.
The researchers “provide quite good evidence” of the new phase, says Alexander Goncharov, a physicist at the Carnegie Institution for Science in Washington, D.C., who was not involved with the study.

Others are more cautious about the significance of the work. “It’s definitely providing more insight into water at these conditions,” says physicist Marcus Knudson of Washington State University in Pullman. But, he says, “I don’t see strong evidence that there’s a melting transition in their data.”

So more work remains before this weird kind of ice is fully understood. For now, the superionic state of water seems likelier, but still on thin ice.

Lab-grade flight tracking has gone wild, creating a broad new way of studying some of the flashiest of natural acrobats, wild hummingbirds.

One of the findings: Bigger hummingbird species don’t seem handicapped by their size when it comes to agility. A battleship may not be as maneuverable as a kayak, but in a study of 25 species, larger hummingbirds outdid smaller species at revving or braking while turning. Measurements revealed these species have more muscle capacity and their wings tended to be proportionately larger for their body size than smaller species. Those boosts could help explain how these species could be so agile despite their size, researchers report in the Feb. 9 Science.
Adapting a high-speed camera array and real-time tracking software to perform in field conditions let the researchers analyze more than 200 wild birds swerving and pivoting naturally. With over 330,000 bird maneuvers recorded, the researchers could compare the agility of the different species. It’s the first comparative study of natural flight moves in wild birds, says coauthor Roslyn Dakin, who is based in Ottawa with the Smithsonian Conservation Biology Institute.

“What makes this research a clear advance is the methods they used,” says Christopher J. Clark of the University of California, Riverside. His hummingbird studies have revealed how the birds’ feathers squeal during flight (SN: 4/4/15, p. 5), but he was not involved in the new research.

In the experimental setup, four cameras film a temporary flight chamber in the field. Customized computer software allows the team to track birds in 3-D as they explore the space in any way they choose.
The project began almost a decade ago when coauthor Paolo Segre adapted and then hauled the flight-tracking system to Ecuador, Costa Rica and Peru — great places to find hummingbirds with different wing shapes and body sizes, but hard on equipment. At the time, he needed five computers, sometimes running just on solar panels and a generator in the Amazon. “We were in a thatched hut,” accessible only by several hours’ boat ride, he says. “Monkeys poked their heads in.” Since then, computers have improved, and one machine is enough to run the software.

The new paper uses some of this hard-won data to focus on two kinds of turning maneuvers — a simple flying arc and a pitch-and-roll move that Dakin calls “turning on a dime.” That involves the bird slanting its body and then pivoting in place.

Birds’ agility did not appear to be affected in field sites at higher elevations. In theory, less oxygen and lower air pressure should make athletic flying tougher. An earlier study by the same researchers found that Anna’s hummingbirds (Calypte anna) accelerated more slowly and had other performance falloffs at altitudes higher than the birds’ home range. Yet birds that call those higher elevations home had adapted to the conditions.

Studying how hummingbirds, or even birds in general, maneuver has been hard to do. Previous approaches to understanding bird motion were limited to capturing data on animals performing set tasks. “A ballerina has a number of different moves,” Clark says, but “so far we’ve just studied individual moves.” Now researchers are starting “to put together the entire dance.”

Fossilized footprints from an iguana-like reptile provide what could be the earliest evidence of a lizard running on two legs.

The 29 exceptionally well-preserved lizard tracks, found in a slab of rock from an abandoned quarry in Hadong County, South Korea, include back feet with curved digits and front feet with a slightly longer third digit. The back footprints outnumber the front ones, and digit impressions are more pronounced than those of the balls of the feet. The lizard’s stride length also increases across the slab.
That’s what you’d expect to see in a transition from moseying along on four legs to scampering on two, says Yuong-Nam Lee, a paleontologist at Seoul National University who first came across the slab back in 2004. A closer examination two years ago revealed the telltale tracks.

Lee and his colleagues attribute the tracks to a previously unknown lizard ichnospecies, that is a species defined solely by trace evidence of its existence, rather than bones or tissue. Lee and his colleagues have dubbed the possible perpetrator Sauripes hadongensis and linked it to an order that includes today’s iguanas and chameleons in the Feb. 15 Scientific Reports.
Bipedal running certainly would have come in handy when escaping predatory pterosaurs some 110 million to 128 million years ago, the age of the rock slab. Lizard tracks are pretty rare in the fossil record, due to the reptiles’ lightweight bodies and penchant for habitats that don’t make great fossils. Though tracks appear in older fossils from the Triassic Epoch, 200 million to 250 million years ago, those prints belong to more primitive lizardlike reptiles. The new find edges out another set from the same region as the oldest true lizard tracks in the world by a few million years, the researchers say.
Plenty of modern lizards use two legs to scurry around. Some studies have linked similarities in ancient lizard bone structure to bipedal locomotion, but it is unclear exactly when lizards developed bipedalism. Lee’s team argues that these tracks represent the earliest and only direct evidence of bipedal running in an ancient lizard.

Martin Lockley, a paleontologist at the University of Colorado Denver who studies ancient animal tracks, points to alternative explanations. S. hadongensis might have trampled over front prints with its back feet, obscuring them and giving the appearance of two-legged running. Preservation can vary between back and front footprints. And the stride lengths aren’t quite as long as what Lockley says he’d expect to see in running. “Running or ‘leaping’ lizards make for a good story, but I am skeptical based on the evidence,” he adds.

So it may take the discovery of more fossilized lizard prints to determine whether S. hadongensis’ tracks truly represent running on two legs rather than simply scurrying on four.