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Precise measurements of Cassini’s final trajectory have now allowed scientists to make the first accurate estimate of the amount of material in the planet’s rings, weighing them based on the strength of their gravitational pull.

That estimate — about 40 percent of the mass of Saturn’s moon Mimas, which itself is 2,000 times smaller than Earth’s moon — tells them that the rings are relatively recent, having originated less than 100 million years ago and perhaps as recently as 10 million years ago.

Their young age puts to rest a long-running argument among planetary scientists. Some thought that the rings formed along with the planet 4.5 billion years ago from icy debris remaining in orbit after the formation of the solar system. Others thought the rings were very young and that Saturn had, at some point, captured an object from the Kuiper belt or a comet and gradually reduced it to orbiting rubble.

The new mass estimate is based on a measurement of how much the flight path of Cassini was deflected by the gravity of the rings when the spacecraft flew between the planet and the rings on its final set of orbits in September 2017. Initially, however, the deflection did not match predictions based on models of the planet and rings. Only when the team accounted for very deep flowing winds in atmosphere on Saturn — something impossible to observe from space — did the measurements make sense, allowing them to calculate the mass of the rings.

“The first time I looked at the data I didn’t believe it, because I trusted our models and it took a while to sink in that there was some effect that changed the gravity field that we had not considered,” said Burkhard Militzer, a professor of earth and planetary science at the University of California, Berkeley, who models planetary interiors. “That turned out to be massive flows in the atmosphere at least 9,000 kilometers deep around the equatorial region. We thought preliminarily that these clouds were like clouds on Earth, which are confined to a thin layer and contain almost no mass. But on Saturn they are really massive.”

They also calculated that the surface clouds at Saturn’s equator rotate 4 percent faster than the layer 9,000 kilometers (about 6,000 miles) deep. That deeper layer takes 9 minutes longer to rotate than do the cloud tops at the equator, which go around the planet once every 10 hours, 33 minutes.

“The discovery of deeply rotating layers is a surprising revelation about the internal structure of the planet,” said Cassini project scientist Linda Spilker of NASA’s Jet Propulsion Laboratory in Pasadena, California. “The question is what causes the more rapidly rotating part of the atmosphere to go so deep and what does that tell us about Saturn’s interior.”

Militzer also was able to calculate that the rocky core of the planet must be between 15 and 18 times the mass of Earth, which is similar to earlier estimates.

The team, led by Luciano Iess at the Sapienza University of Rome, Italy, reported their results today in the journal Science.

Did rings come from icy comet?

Earlier estimates of the mass of Saturn’s rings — between one-half and one-third the mass of Mimas — came from studying the density waves that travel around the rocky, icy rings. These waves are caused by the planet’s 62 satellites, including Mimas, which creates the so-called Cassini division between the two largest rings, A and B. Mimas is smooth and round, 246 kilometers in diameter. It has a big impact crater that makes it resemble the Death Star from the Star Wars movies.

“People didn’t trust the wave measurements because there might be particles in the rings that are massive but are not participating in the waves,” Militzer said. “We always suspected there was some hidden mass that we could not see in the waves.”

Luckily, as Cassini approached the end of its life, NASA programmed it to perform 22 dives between the planet and the rings to probe Saturn’s gravity field. Earth-based radio telescopes measured the spacecraft’s velocity to within a fraction of a millimeter per second.

The new ring mass value is in the range of earlier estimates and allows the researchers to determine their age.

These age calculations, led by Philip Nicholson of Cornell University and Iess, built on a connection that scientists had previously made between the mass of the rings and their age. Lower mass points to a younger age, because the rings are initially made of ice and are bright but over time become contaminated and darkened by interplanetary debris.

“These measurements were only possible because Cassini flew so close to the surface in its final hours,” Militzer said. “It was a classic, spectacular way to end the mission.”

Two scientists, Ruixuan Gao and Shoh Asano, wanted to use his team’s microscope on brain samples expanded to four times their usual size — blown up like balloons. The duo, part of Howard Hughes Medical Institute (HHMI) Investigator Ed Boyden’s lab at the Massachusetts Institute of Technology (MIT), uses a chemical technique to make small specimens bigger so scientists can more easily see molecular details.

Their technique, called expansion microscopy, worked well on single cells or thin tissue sections imaged in conventional light microscopes, but Boyden’s team wanted to image vastly larger chunks of tissue. They wanted to see complete neural circuits spanning millimeters or more. The scientists needed a microscope that was high-speed, high resolution, and relatively gentle — something that didn’t destroy a sample before they could finish imaging it.

So, they turned to Betzig. His team at HHMI’s Janelia Research Campus had used their lattice light-sheet microscope to image the rapid subcellular dynamics of sensitive living cells in 3-D. Combining the two microscopy techniques could potentially offer rapid, detailed images of wide swaths of brain tissue.

“I thought they were full of it,” Betzig remembers. “The idea does sound a bit crude,” Gao says. “We’re stretching tissues apart.” But Betzig invited Gao and Asano to try the lattice scope out.

“I was going to show them,” Betzig laughs. Instead, he was blown away. “I couldn’t believe the quality of the data I was seeing. You could have knocked me over with a feather.”

Now, he and his Janelia colleagues have teamed up with Boyden’s group and imaged the entire fruit fly brain and sections of mouse brain the thickness of the cortex. Their combined method offers high resolution with the ability to visualize any desired protein — and it’s fast, too. Imaging the fly brain in multiple colors took just 62.5 hours, compared to the years it would take using an electron microscope, Boyden, Betzig, and their colleagues report January 17, 2018, in the journal, Science.

“I can see us getting to the point of imaging at least 10 fly brains per day,” says Betzig, now an HHMI investigator at the University of California, Berkeley. Such speed and resolution will let scientists ask new questions, he says, like how brains differ between males and females, or how brain circuits vary between flies of the same type.

Boyden’s group dreams of making a map of the brain so detailed you can simulate it in a computer. “We’ve crossed a threshold in imaging performance,” he says. “That’s why we’re so excited. We’re not just scanning incrementally more brain tissue, we’re scanning entire brains.”

Expanding the brain

Making detailed maps of the brain requires charting its activity and wiring — in humans, the thousands of connections made by each of more than 80 billion neurons. Such maps could help scientists spot where brain disease begins, build better artificial intelligence, or even explain behavior. “That’s like the holy grail for neuroscience,” Boyden says.

Years ago, his group had an idea to figure out how everything was organized: What if they could actually make the brain bigger — big enough to look inside? By infusing samples with swellable gels — like the stuff in baby diapers — the team invented a way to expand tissues, making the molecules inside less crowded and easier to see under a microscope. Molecules lock into a gel scaffold, keeping the same relative positions even after expansion.

But it wasn’t easy to image large tissue volumes. The thicker a specimen gets, the harder it is to illuminate only the parts you want to see. Shining too much light on samples can photobleach them, burning out the fluorescent “bulbs” scientists use to light up cells.

Expanding a sample just four-fold increases its volume 64-fold, so imaging speed also becomes paramount, Gao says. “We needed something that was fast and didn’t have much photobleaching, and we knew there was a fantastic microscope at Janelia.”

The lattice light-sheet microscope sweeps an ultrathin sheet of light through a specimen, illuminating only that part in the microscope’s plane of focus. That helps out-of-focus areas stay dark, keeping a specimen’s fluorescence from being extinguished.

When Gao and Asano first tested their expanded mouse tissues on the lattice scope, they saw a thicket of glowing nubs protruding from neurons’ branches. These nubs, called dendritic spines, often look like mushrooms, with bulbous heads on skinny necks that can be hard to measure. But the scientists were able to see even “the smallest necks possible,” Asano says, while simultaneously imaging synaptic proteins nearby.

“It was incredibly impressive,” says Betzig. The team was convinced that they should explore the combined technique further. “And that’s what we’ve been doing ever since,” he says.

The brain and beyond

Over the last two years, Gao and Asano have spent months at Janelia, teaming up with biologists, microscopists, physicists, and computer scientists across the campus to capture and analyze images. “This is like an Avengers-level collaboration,” Gao says, referring to the crew of comic book superheroes.

Yoshinori Aso and the FlyLight team provided high-quality fly brain specimens, which Gao and Asano expanded and used to collect some 50,000 cubes of data across each brain — forming a kind of 3-D jigsaw puzzle. Those images required complicated computational stitching to put the pieces back together, work led by Stephan Saalfeld and Igor Pisarev. “Stephen and Igor saved our bacon,” Betzig says. “They dealt with all the horrible little details of image processing and made it work on each multi-terabyte data set.”

Next, Srigokul Upadhyayula from Harvard Medical School, a co-first author of the report, analyzed the combined 200 terabytes of data and created the stunning movies that showcase the brain’s intricacies in vivid color. He and his coauthors investigated more than 1,500 dendritic spines, imaged fatty sheaths that insulate mouse nerve cells, highlighted all of the dopaminergic neurons, and counted all the synapses across the entire fly brain.

The nuances of Boyden’s team expansion technique make it well suited for the lattice scope; the technique produces nearly transparent samples. For the microscope, it’s almost like looking through water, rather than a turbid sea of molecular gunk. “The result is that we get crystal clear images at blazingly fast speeds over very large volumes compared to earlier microscopy techniques,” Boyden says.

Still, challenges remain. As with any kind of super resolution fluorescence microscopy, Betzig says, it can be hard to decorate proteins with enough fluorescent bulbs to see them clearly at high resolution. And since expansion microscopy requires many processing steps, there’s still the potential for artifacts to be introduced. Because of this, he says, “we worked very hard to validate what we’ve done, and others would be well advised to do the same.”

Now, Gao and the Janelia team are building a new lattice light-sheet microscope, which they plan to move to Boyden’s lab at MIT. “Our hope is to rapidly make maps of entire nervous systems,” Boyden says.

In a study published today in Science, the team shows the number of asteroid impacts on the Moon and Earth increased by two to three times starting around 290 million years ago.

“Our research provides evidence for a dramatic change in the rate of asteroid impacts on both Earth and the Moon that occurred around the end of the Paleozoic era,” said lead author Sara Mazrouei, who recently earned her PhD in the Department of Earth Sciences in the Faculty of Arts & Science at the University of Toronto (U of T). “The implication is that since that time we have been in a period of relatively high rate of asteroid impacts that is 2.6 times higher than it was prior to 290 million years ago.”

It had been previously assumed that most of Earth’s older craters produced by asteroid impacts have been erased by erosion and other geologic processes. But the new research shows otherwise.

“The relative rarity of large craters on Earth older than 290 million years and younger than 650 million years is not because we lost the craters, but because the impact rate during that time was lower than it is now,” said Rebecca Ghent, an associate professor in U of T’s Department of Earth Sciences and one of the paper’s co-authors. “We expect this to be of interest to anyone interested in the impact history of both Earth and the Moon, and the role that it might have played in the history of life on Earth.”

Scientists have for decades tried to understand the rate that asteroids hit Earth by using radiometric dating of the rocks around them to determine their ages. But because it was believed erosion caused some craters to disappear, it was difficult to find an accurate impact rate and determine whether it had changed over time.

A way to sidestep this problem is to examine the Moon, which is hit by asteroids in the same proportions over time as Earth. But there was no way to determine the ages of lunar craters until NASA’s Lunar Reconnaissance Orbiter (LRO) started circling the Moon a decade ago and studying its surface.

“The LRO’s instruments have allowed scientists to peer back in time at the forces that shaped the Moon,” said Noah Petro, an LRO project scientist based at NASA Goddard Space Flight Center.

Using LRO data, the team was able to assemble a list of ages of all lunar craters younger than about a billion years. They did this by using data from LRO’s Diviner instrument, a radiometer that measures the heat radiating from the Moon’s surface, to monitor the rate of degradation of young craters.

During the lunar night, rocks radiate much more heat than fine-grained soil called regolith. This allows scientists to distinguish rocks from fine particles in thermal images. Ghent had previously used this information to calculate the rate at which large rocks around the Moon’s young craters — ejected onto the surface during asteroid impact — break down into soil as a result of a constant rain of tiny meteorites over tens of millions of years. By applying this idea, the team was able to calculate ages for previously un-dated lunar craters.

When compared to a similar timeline of Earth’s craters, they found the two bodies had recorded the same history of asteroid bombardment.

“It became clear that the reason why Earth has fewer older craters on its most stable regions is because the impact rate was lower up until about 290 million years ago,” said William Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colorado and another of the paper’s coauthors. “The answer to Earth’s impact rate was staring everyone right in the face.”

The reason for the jump in the impact rate is unknown, though the researchers speculate it might be related to large collisions taking place more than 300 million years ago in the main asteroid belt between the orbits of Mars and Jupiter. Such events can create debris that can reach the inner solar system.

Ghent and her colleagues found strong supporting evidence for their findings through a collaboration with Thomas Gernon, an Earth scientist based at the University of Southampton in England who works on a terrestrial feature called kimberlite pipes. These underground pipes are long-extinct volcanoes that stretch, in a carrot shape, a couple of kilometers below the surface, and are found on some of the least eroded regions of Earth in the same places preserved impact craters are found.

“The Canadian shield hosts some of the best-preserved and best-studied of this terrain — and also some of the best-studied large impact craters,” said Mazrouei.

Gernon showed that kimberlite pipes formed since about 650 million years ago had not experienced much erosion, indicating that the large impact craters younger than this on stable terrains must also be intact.

“This is how we know those craters represent a near-complete record,” Ghent said.

While the researchers weren’t the first to propose that the rate of asteroid strikes to Earth has fluctuated over the past billion years, they are the first to show it statistically and to quantify the rate.

“The findings may also have implications for the history of life on Earth, which is punctuated by extinction events and rapid evolution of new species,” said Ghent. “Though the forces driving these events are complicated and may include other geologic causes, such as large volcanic eruptions, combined with biological factors, asteroid impacts have surely played a role in this ongoing saga.

“The question is whether the predicted change in asteroid impacts can be directly linked to events that occurred long ago on Earth.”

The findings are described in the study “Earth and Moon impact flux increased at the end of the Paleozoic,” published in Science. Support for the research was provided by the National Science and Engineering Research Council of Canada, NASA’s Solar System Exploration Research Virtual Institute, and the Natural Environment Research Council of the United Kingdom.