The moon had a magnetic field for at least 2 billion years, or maybe longer.
Analysis of a relatively young rock collected by Apollo astronauts reveals the moon had a weak magnetic field until 1 billion to 2.5 billion years ago, at least a billion years later than previous data showed. Extending this lifetime offers insights into how small bodies generate magnetic fields, researchers report August 9 in Science Advances. The result may also suggest how life could survive on tiny planets or moons. “A magnetic field protects the atmosphere of a planet or moon, and the atmosphere protects the surface,” says study coauthor Sonia Tikoo, a planetary scientist at Rutgers University in New Brunswick, N.J. Together, the two protect the potential habitability of the planet or moon, possibly those far beyond our solar system.
The moon does not currently have a global magnetic field. Whether one ever existed was a question debated for decades (SN: 12/17/11, p. 17). On Earth, molten rock sloshes around the outer core of the planet over time, causing electrically conductive fluid moving inside to form a magnetic field. This setup is called a dynamo. At 1 percent of Earth’s mass, the moon would have cooled too quickly to generate a long-lived roiling interior. Magnetized rocks brought back by Apollo astronauts, however, revealed that the moon must have had some magnetizing force. The rocks suggested that the magnetic field was strong at least 4.25 billion years ago, early on in the moon’s history, but then dwindled and maybe even got cut off about 3.1 billion years ago. Tikoo and colleagues analyzed fragments of a lunar rock collected along the southern rim of the moon’s Dune Crater during the Apollo 15 mission in 1971. The team determined the rock was 1 billion to 2.5 billion years old and found it was magnetized. The finding suggests the moon had a magnetic field, albeit a weak one, when the rock formed, the researchers conclude. A drop in the magnetic field strength suggests the dynamo driving it was generated in two distinct ways, Tikoo says. Early on, Earth and the moon would have sat much closer together, allowing Earth’s gravity to tug on and spin the rocky exterior of the moon. That outer layer would have dragged against the liquid interior, generating friction and a very strong magnetic field (SN Online: 12/4/14).
Then slowly, starting about 3.5 billion years ago, the moon moved away from Earth, weakening the dynamo. But by that point, the moon would have started to cool, causing less dense, hotter material in the core to rise and denser, cooler material to sink, as in Earth’s core. This roiling of material would have sustained a weak field that lasted for at least a billion years, until the moon’s interior cooled, causing the dynamo to die completely, the team suggests.
The two-pronged explanation for the moon’s dynamo is “an entirely plausible idea,” says planetary scientist Ian Garrick-Bethell of the University of California, Santa Cruz. But researchers are just starting to create computer simulations of the strength of magnetic fields to understand how such weaker fields might arise. So it is hard to say exactly what generated the lunar dynamo, he says.
If the idea is correct, it may mean other small planets and moons could have similarly weak, long-lived magnetic fields. Having such an enduring shield could protect those bodies from harmful radiation, boosting the chances for life to survive.
August’s total solar eclipse won’t be the last time the moon cloaks the sun’s light. From now to 2040, for example, skywatchers around the globe can witness 15 such events.
Their predicted paths aren’t random scribbles. Solar eclipses occur in what’s called a Saros cycle — a period that lasts about 18 years, 11 days and eight hours, and is governed by the moon’s orbit. (Lunar eclipses follow a Saros cycle, too, which the Chaldeans first noticed probably around 500 B.C.)
Two total solar eclipses separated by that 18-years-and-change period are almost twins — compare this year’s eclipse with the Sept. 2, 2035 eclipse, for example. They take place at roughly the same time of year, at roughly the same latitude and with the moon at about the same distance from Earth. But those extra eight hours, during which the Earth has rotated an additional third of the way on its axis, shift the eclipse path to a different part of the planet. This cycle repeats over time, creating a family of eclipses called a Saros series. A series lasts 12 to 15 centuries and includes about 70 or more eclipses. The solar eclipses of 2019 and 2037 belong to a different Saros series, so their paths too are shifted mimics. Their tracks differ in shape from 2017’s, because the moon is at a different place in its orbit when it passes between the Earth and the sun. Paths are wider at the poles because the moon’s shadow is hitting the Earth’s surface at a steep angle.
Predicting and mapping past and future eclipses allows scientists “to examine the patterns of eclipse cycles, the most prominent of which is the Saros,” says astrophysicist Fred Espenak, who is retired from NASA’s Goddard Spaceflight Center in Greenbelt, Md.
He would know. Espenak and his colleague Jean Meeus, a retired Belgian astronomer, have mapped solar eclipse paths from 2000 B.C. to A.D. 3000. For archaeologists and historians peering backward, the maps help match up accounts of long-ago eclipses with actual paths. For eclipse chasers peering forward, the data are an itinerary.
“I got interested in figuring out how to calculate eclipse paths for my own use, for planning … expeditions,” says Espenak, who was 18 when he witnessed his first total solar eclipse. It was in 1970, and he secured permission to drive the family car from southern New York to North Carolina to see it. Since then, Espenak, nicknamed “Mr. Eclipse,” has been to every continent, including Antarctica, for a total eclipse of the sun.
“It’s such a dramatic, spectacular, beautiful event,” he says. “You only get a few brief minutes, typically, of totality before it ends. After it’s over, you’re craving to see it again.”
Speculation is running rampant about potential new discoveries of gravitational waves, just as the latest search wound down August 25.
Publicly available logs from astronomical observatories indicate that several telescopes have been zeroing in on one particular region of the sky, potentially in response to a detection of ripples in spacetime by the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO. These records have raised hopes that, for the first time, scientists may have glimpsed electromagnetic radiation — light — produced in tandem with gravitational waves. That light would allow scientists to glean more information about the waves’ source. Several tweets from astronomers reporting rumors of a new LIGO detection have fanned the flames of anticipation and amplified hopes that the source may be a cosmic convulsion unlike any LIGO has seen before. “There is a lot of excitement,” says astrophysicist Rosalba Perna of Stony Brook University in New York, who is not involved with the LIGO collaboration. “We are all very anxious to actually see the announcement.”
An Aug. 25 post on the LIGO collaboration’s website announced the end of the current round of data taking, which began November 30, 2016. Virgo, a gravitational wave detector in Italy, had joined forces with LIGO’s two on August 1 (SN Online: 8/1/17). The three detectors will now undergo upgrades to improve their sensitivity. The update noted that “some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis, and we have shared what we currently know with astronomical observing partners.”
When LIGO detects gravitational waves, the collaboration alerts astronomers to the approximate location the waves seemed to originate from. The hope is that a telescope could pick up light from the aftermath of the cosmic catastrophe that created the gravitational waves — although no light has been found in previous detections.
LIGO previously detected three sets of gravitational waves from merging black holes (SN: 6/24/17, p. 6). Black hole coalescences aren’t expected to generate light that could be spotted by telescopes, but another prime candidate could: a smashup between two remnants of stars known as neutron stars. Scientists have been eagerly awaiting LIGO’s first detections of such mergers, which are suspected to be the sites where the universe’s heaviest elements are formed. An observation of a neutron star crash also could provide information about the ultradense material that makes up neutron stars. Since mid-August, seemingly in response to a LIGO alert, several telescopes have observed a section of sky around the galaxy NGC 4993, located 134 million light-years away in the constellation Hydra. The Hubble Space Telescope has made at least three sets of observations in that vicinity, including one on August 22 seeking “observations of the first electromagnetic counterparts to gravitational wave sources.”
Likewise, the Chandra X-ray Observatory targeted the same region of sky on August 19. And records from the Gemini Observatory’s telescope in Chile indicate several potentially related observations, including one referencing “an exceptional LIGO/Virgo event.”
“I think it’s very, very likely that LIGO has seen something,” says astrophysicist David Radice of Princeton University, who is not affiliated with LIGO. But, he says, he doesn’t know whether its source has been confirmed as merging neutron stars.
LIGO scientists haven’t commented directly on the veracity of the rumor. “We have some substantial work to do before we will be able to share with confidence any quantitative results. We are working as fast as we can,” LIGO spokesperson David Shoemaker of MIT wrote in an e-mail.
Alien megastructures are out. The unusual fading of an oddball star is more likely caused by either clouds of dust or an abnormal cycle of brightening and dimming, two new papers suggest.
Huan Meng of the University of Arizona in Tucson and his colleagues suggest that KIC 8462852, known as Tabby’s star, is dimming thanks to an orbiting cloud of fine dust particles. The team observed the star with the infrared Spitzer and ultraviolet Swift space telescopes from October 2015 to December 2016 — the first observations in multiple wavelengths of light. They found that the star is dimming faster in short blue wavelengths than longer infrared ones, suggesting smaller particles. “That almost absolutely ruled out the alien megastructure scenario, unless it’s an alien microstructure,” Meng says.
Tabby’s star is most famous for suddenly dropping in brightness by up to 22 percent over the course of a few days (SN Online: 2/2/16). Later observations suggested the star is also fading by about 4 percent per year (SN: 9/17/16, p. 12), which Meng’s team confirmed in a paper posted online August 24 at arXiv.org.
Joshua Simon of the Observatories of the Carnegie Institution for Science in Pasadena, Calif., found a similar dimming in data on Tabby’s star from the All Sky Automated Survey going back to 2006. Simon and colleagues also found for the first time that the star grew brighter in 2014, and possibly in 2006, they reported in a paper August 25 at arXiv.org.
“That’s fascinating,” says astrophysicist Tabetha Boyajian of Louisiana State University in Baton Rouge. She first reported the star’s flickers in 2015 (the star is nicknamed for her) and is a coauthor on Meng’s paper. “We always speculated that it would brighten sometime. It can’t just get fainter all the time — otherwise it would disappear. This shows that it does brighten.”
The brightening could be due to a magnetic cycle like the sun’s, Simon suggests. But no known cycle makes a star brighten and dim by quite so much, so the star would still be odd. Brian Metzger of Columbia University previously suggested that a ripped-up planet falling in pieces into the star could explain both the long-term and short-term dimming. He thinks that model still works, although it needs some tweaks.
“This adds some intrigue to what’s going on, but I don’t think it really changes the landscape,” says Metzger, who was not involved in the new studies. And newer observations could complicate things further: The star went through another bout of dimming between May and July. “I’m waiting to see the papers analyzing this recent event,” Metzger says.
West German power companies have decided to go ahead with two nuclear power station projects…. Compared with the U.S. and Britain, Germany has been relatively backward in the application of nuclear energy…. The slow German start is only partly the result of restrictions placed upon German nuclear research after the war. — Science News, September 16, 1967
Update Both East and West Germany embraced nuclear power until antinuclear protests in the 1970s gathered steam. In 1998, the unified German government began a nuclear phaseout, which Chancellor Angela Merkel halted in 2009. The 2011 Fukushima nuclear disaster in Japan caused a rapid reversal. Germany closed eight of its nuclear plants immediately, and announced that all nuclear power in the country would go dark by 2022 (SN Online: 6/1/11). A pivot to renewable energy — wind, solar, hydropower and biomass — produced 188 billion kilowatt-hours of electricity in 2016, nearly 32 percent of German electricity usage.
After 20 years in space and 13 years orbiting Saturn, the veteran spacecraft spent its last 90 seconds or so firing its thrusters as hard as it could to keep sending Saturnian secrets back to Earth for as long as possible.
The spacecraft entered Saturn’s atmosphere at about 3:31 a.m. PDT on September 15 and immediately began running through all of its stabilizing procedures to try to keep itself upright. The signal that Cassini had reached its destination arrived at Earth at 4:54 a.m., and cut out about a minute later as the spacecraft lost its battle with Saturn’s atmosphere. “The signal from the spacecraft is gone, and within the next 45 seconds, so will be the spacecraft,” Cassini project manager Earl Maize announced from the mission control center at NASA’s Jet Propulsion Lab. “I hope you’re all as deeply proud of this amazing accomplishment. Congratulations to you all. This has been an incredible mission, an incredible spacecraft, and you’re all an incredible team. I’m going to call this the end of mission. Project manager, off the net.”
With that, the mission control team erupted in applause, hugs and some tears. It’s the end of an era. But the spacecraft’s last moments at Saturn will answer questions that couldn’t have been addressed any other way. Going out in a blaze of glory seems fitting. Since its launch in 1997, the probe traveled a total of 7.9 billion kilometers. It gathered more than 635 gigabytes of science data and took more than 450,000 images. It completed 294 orbits of Saturn, discovered six named moons and made 162 close, deliberate flybys of the ringed planet’s largest and most interesting moons. The last flyby sealed Cassini’s fate. On September 11, at 12:04 p.m., Cassini passed by Saturn’s largest moon Titan one last time ( SN Online: 9/11/17 ). The moon’s gravity nudged Cassini on an irretrievable trajectory into the giant planet’s atmosphere. Also blame the moons — particularly lake-dappled Titan and watery Enceladus — for why Cassini went out in such dramatic fashion. The mission team decided to sacrifice the spacecraft when it ran out of fuel, rather than risk a collision with one of those potentially habitable moons and contaminating it with any still-lingering earthly microbes.
“Because of planetary protection and our desire to go back to Enceladus, go back to Titan, go back to the Saturn system, we must protect those bodies for future exploration,” Jim Green, director of NASA’s planetary science division, said at a news conference on September 13.
Even in its months-long death spiral, Cassini collected unprecedented observations. Starting in April, the spacecraft made 22 dives through the unexplored region between Saturn and its rings. Measurements of the gravity and composition in that zone will help solve outstanding mysteries. How long is Saturn’s day? How much material is in the rings? When and how did the rings form?
To answer that last question in particular, “you have to fly between the planet and the rings,” says planetary scientist Matthew Hedman of the University of Idaho in Moscow, who uses Cassini data to study the rings. “That’s risky. We had to wait until the end of the mission to take that risk.” On September 13 and 14, Cassini took a last look around the Saturn system’s greatest hits, taking a color mosaic image of Saturn and the rings, a movie sequence of Enceladus setting behind Saturn, Titan and tiny moonlets in the rings that pull the icy ring particles around themselves to form features called propellers.
Inside the mission control center on the afternoon of September 14, a hushed operations team waited for Cassini to come online for the last time to start sending the last pictures back (SNOnline: 9/15/17). Then flight engineer Michael Staab at JPL suddenly broke the silence. “Yeah!” he yelled, pumping both arms in the air. Cassini’s last signal had just come in.
“That tells us that the spacecraft is nice and healthy, she’s doing just fine. She’s doing exactly what she’s supposed to do, like she’s done for 13 years,” Staab said. “We’re just gonna track her now, all the way in.” In the wee hours of September 15, the spacecraft reconfigured itself to shift from a recording device to a transmitting probe. As of that moment, its last and only job was to stream everything it could sense directly back to Earth in real time. Turning so that its ion neutral mass spectrometer was facing directly towards Saturn, Cassini could taste the atmosphere for the first time and investigate a phenomenon called “ring rain,” in which water and ice from the rings splash into the atmosphere. This idea was introduced in the early 1980s, but Cassini has already shown that it’s more complicated than previously thought.
“We’re trying to find out exactly what is coming from the rings and what is due to the atmosphere,” Hunter Waite, Cassini team lead for the mass spectrometer instrument and an atmospheric scientist at the Southwest Research Institute in San Antonio, said at the Sept. 13 news conference. “That final plunge will allow us to do that.” That plunge happened at about 3:31 a.m., when Cassini entered the atmosphere about 10 degrees north of the equator, falling at around 34 kilometers per second. It took data constantly, directly measuring the temperature, magnetic field, plasma density and composition of the upper layers of Saturn’s atmosphere for the first time ever. When it hit the atmosphere, Cassini started firing its thrusters to keep its antenna pointed at Earth despite the forces of the atmosphere trying to knock it askew. But about a minute later, the atmosphere won, when Cassini was about 1,400 kilometers above the cloud tops.
What happened next, scientists can only imagine. Models suggest this fiery demise: The spacecraft attempted to stabilize itself, but to no avail. It started to tumble faster and faster. Atmospheric friction broke the spacecraft apart, bit by bit — first its thermal blankets burned off, then aluminum components started to melt. The spacecraft probably fell another 1,000 kilometers as it disintegrated like a meteor, Maize said.
Saturn’s atmosphere crushed and melted the bits and pieces, until they completely dissociated and became part of the very planet the spacecraft had been sent to observe.
When all was said and done, the spacecraft lasted about 30 seconds longer than expected. That may help ensure the team got enough data to figure out Saturn’s rotation period, science team member Michele Dougherty of Imperial College London said at a post-mission news conference September 15. “I’m hoping we can do it, I’m not going to promise. Ask me in three months’ time.”
There are no planned future missions to Saturn, although some Cassini alumni are already working on proposals. Outer solar system astronomers are now setting their sights on Jupiter and its icy, possibly life-friendly moons. The European Space Agency’s Jupiter Icy Moons Explorer (JUICE) and NASA’s Europa Clipper both hope to launch around 2022. Those missions may pave the way for a lander on Europa (SN Online: 2/18/17), which could directly look for life in that moon’s subsurface seas.
Planetary scientist Kevin Hand at JPL, who is leading the science definition team for the proposed Europa lander, feels a debt to Cassini.
“When you’re at the earliest frontiers of exploration, it’s hard to feel sad,” he said. “The wake we’re experiencing right now for Cassini, it’s not so much an end but the early steps that pave the way for the next stage of exploration.”
Even Maize is more proud than mourning.
“This is exactly the way we always planned it. It’s sad that we’re losing this incredible discovery machine,” he said in the moments leading up to Cassini’s disintegration. “But the real sense here is just, all right, we got it!”
As far as last meals go, squid isn’t a bad choice. Cephalopod remains appear to dominate the stomach contents of a newly analyzed ichthyosaur fossil from nearly 200 million years ago.
The ancient marine reptiles once roamed Jurassic seas and commonly pop up in England’s fossil-rich coast near Lyme Regis. But a lot of ichthyosaur museum specimens lack records of where they came from, making their age difficult to place.
Dean Lomax of the University of Manchester and his colleagues reexamined one such fossil. Based on its skull, they identified the creature as a newborn Ichthyosaurus communis. Microfossils of shrimp and amoeba species around the ichthyosaur put the specimen at 199 million to 196 million years old, the researchers estimate.
Tiny hook structures stand out in the newborn’s ribs — most likely the remnants of prehistoric black squid arms. Another baby ichthyosaur fossil that lived more recently had a stomach full of fish scales. So the new find suggests a shift in the menu for young ichthyosaurs at some point in their evolutionary history, the researchers write October 3 in Historical Biology.
The only lemurs so dependent on bamboo that they gnaw on hardened, nutrient-poor stems during the dry season might dwindle away as those seasons grow longer.
Reconstructing the history of the greater bamboo lemur (Prolemur simus) in Madagascar suggests that drier areas over thousands of years already have lost their populations. As the region dries further due to climate change and the bad-bamboo months in the last holdouts lengthen, remaining populations of these critically endangered lemurs might go hungry and fade away too, an international research team warns online October 26 in Current Biology.
Other animals, even another lemur species, will eat lots of bamboo shoots and leaves. But the greater bamboo lemur is the only mammal besides the giant panda that sticks with bamboo during the dry season. That’s when the plants stop sprouting and offer only culm, the tough, old, yellowing stems poor in nutrients. Culm hasn’t reached the hard stage of bamboo that’s used as a building material. “Nobody wants to eat that,” says study coauthor Alistair Evans, an evolutionary morphologist at Monash University in Melbourne, Australia.
This lemur species with its extreme diet had already been feared extinct once, around the middle of the last century, but relic populations turned up. Current survivors remain more toward the eastern part of the island, where dry seasons are apparently survivable, at least for now.
In a big step toward catching up with the rest of the world, the United States cleared the way for using mosquitoes as a commercial pest control for the first time.
The U.S. Environmental Protection Agency has approved using a strain of male Asian tiger mosquitoes (Aedes albopictus) as a biopesticide in the District of Columbia and 20 states, including California and New York. Kentucky-based MosquitoMate was granted the right to sell these mosquitoes, called ZAP Males, for the next five years, the agency announced November 7. These male mosquitoes are not genetically modified. Instead they carry a strain of Wolbachia bacteria that turns them into saboteur dads. When they mate with wild females not carrying the strain, the offspring will die and the population should dwindle. Males don’t bite, so releasing them should not add extra vexation.
Releases of Wolbachia-bearing mosquitoes for pest control already go on in other countries, such as Brazil, although with a different bacterial strain and a different strategy.
This same company has also been testing the effectiveness of a different mosquito species, Aedes aegypti, also carrying bad-dad Wolbachia, near Key West, Fla. (These mosquitoes are not commercially available.) The tests “ended a bit early due to [Hurricane] Irma,” says Stephen Dobson of MosquitoMate, “but we think that we have some good data despite this complication.”
The human brain is teeming with diversity. By plucking out delicate, live tissue during neurosurgery and then studying the resident cells, researchers have revealed a partial cast of neural characters that give rise to our thoughts, dreams and memories.
So far, researchers with the Allen Institute for Brain Science in Seattle have described the intricate shapes and electrical properties of about 100 nerve cells, or neurons, taken from the brains of 36 patients as they underwent surgery for conditions such as brain tumors or epilepsy. To reach the right spot, surgeons had to remove a small hunk of brain tissue, which is usually discarded as medical waste. In this case, the brain tissue was promptly packed up and sent — alive — to the researchers. Once there, the human tissue was kept on life support for several days as researchers analyzed the cells’ shape and function. Some neurons underwent detailed microscopy, which revealed intricate branching structures and a wide array of shapes. The cells also underwent tiny zaps of electricity, which allowed researchers to see how the neurons might have communicated with other nerve cells in the brain. The Allen Institute released the first publicly available database of these neurons on October 25.
A neuron called a pyramidal cell, for instance, has a bushy branch of dendrites (orange in 3-D computer reconstruction, above) reaching up from its cell body (white circle). Those dendrites collect signals from other neural neighbors. Other dendrites (red) branch out below. The cell’s axon (blue) sends signals to other cells that spur them to action. Like the chandelier cell, a Martinotti cell (below) quiets other cells with messages coming from its tangled, tall axon, which spans several layers of the brain’s cortex — the wrinkly, outer layer that’s involved in higher-level thought. And in a basket cell (above), axon branches, which allow the nerve cell to send messages to other neurons, cluster densely around the cell body. Because the neurons play different roles in the brain, the new collection could help researchers figure out the details of those diverse jobs. Similar data exist for cells taken from the brains of other animals, such as mice, but until now, data on live cells from people have been scarce.
“These neurons are amazingly beautiful,” says Ed Lein, a neuroscientist at the Allen Institute who works on the project. “They look like trees. They’re much more complex than similar cells in a mouse.”
