Data from the comet 67P/Churyumov–Gerasimenko suggest that some of the xenon in Earth's atmosphere may have originated from comets. Credit: ESA/Rosetta/NAVCAM (CC BY-SA IGO 3.0)

Asteroids, comets and Kuiper belt

Xenon came from comets

Comets may have delivered more than one-fifth of Earth’s atmospheric xenon.

The planet’s mixture of xenon isotopes is unique within the Solar System, and the origins of a proportion of the heavy noble gas have long been a mystery. Bernard Marty at the University of Lorraine in Vandœuvre-lès-Nancy, France, and his colleagues analysed the xenon emanating from comet 67P/Churyumov–Gerasimenko, which was measured by the ROSINA spectrometer aboard the European Space Agency’s Rosetta spacecraft. 

Data from close flybys of the comet in May 2016 revealed a xenon isotopic signature that, when combined with xenon signatures from other known sources within the early Solar System, can account for Earth’s xenon mix. Comets that struck the planet early in its history probably brought enough of the element to account for 22% of its total today, say the authors. 

More Research Highlights...


Stable gene pool on the Silk Road

Human migration left few genetic marks on the female population of the Caucasus.

The female gene pool in the southern Caucasus seems little changed since Neolithic times, even though this geographical bridge ­— part of the old Silk Road trade route between Europe and Asia — has been a crossroads for human migration.

Ashot Margaryan and Morten Allentoft from the Natural History Museum of Denmark in Copenhagen and their colleagues extracted mitochondrial DNA, which is passed from mother to child, from 52 ancient skeletons found in Armenia and a neighbouring region. The samples spanned 7,800 years.

The researchers analysed the DNA sequences and compared them with 206 mitochondrial genomes from modern Armenians. They found that the maternal gene pool remained stable.

Archaeological records show major cultural upheavals in Armenia over the past eight millennia. The results imply that these may have occurred through exchanges of ideas rather than through population shifts, or that it was mainly men who migrated into the region.

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Climate sciences

Sunnier summers propel Greenland melting

Accelerated melting of island’s vast ice sheet is due mainly to clearer skies.

The accelerated surface melting observed on Greenland in recent years is due primarily to the island receiving more summer sunshine.

The Greenland ice sheet has been losing mass since the mid-1990s, both because more ice is being discharged into the sea and as a result of increased melting across its vast surface. Earlier work suggested that higher atmospheric temperatures were the main cause of the rise in surface melting.

But a team led by Stefan Hofer at the University of Bristol, UK, analysed satellite data on cloud cover. Between 1995 and 2009, summer cloud cover over Greenland decreased by nearly 1% per year. Each percentage point represents enough sunshine to melt an extra 27 gigatonnes of ice per year, a computer model suggests. Overall, Greenland lost about 4,000 gigatonnes of ice in the 20 years from 1995, contributing to sea-level rise.

Greenland’s clouds seem to be changing in direct response to atmospheric-circulation changes, which, in turn, are thought to be driven by shrinking Arctic summer sea ice.

NASA's Mars Curiosity rover landed on Mars in 2012

Planetary science

Curiosity rover gets a boost from artificial intelligence

Autonomy-enhancing software has sped up exploration of Mars geochemistry.

The use of artificial intelligence (AI) in space exploration — instead of human commands sent all the way from Earth — saves precious mission time and has boosted the speed at which the Mars Curiosity rover gathers data.

The AEGIS software is designed to allow the robot to autonomously select appropriate rock and soil targets for analysis, and has been in routine use since NASA scientists installed it in May 2016. Raymond Francis at the Jet Propulsion Laboratory in Pasadena, California, and his colleagues now report that the software’s performance in selecting desired target material for the rover’s Chemistry and Camera (ChemCam) instrument exceeds 93% in unknown territory. Having Curiosity pick targets at random would yield a success rate of only about 24%, they estimate.


Sleep is a neuronal-network matter

Scientists decode the way worms switch between wakefulness and sleep.

How the brain switches between sleep and wakefulness is an open question, but in the worm Caenorhabditis elegans it seems the process occurs passively.

Manuel Zimmer and his colleagues at the Research Institute of Molecular Pathology in Vienna used a new calcium-imaging technology to simultaneously view the activity of most of the worm’s brain cells during a particular stage of larval development. During this stage the creatures are prone to falling asleep — providing oxygen levels remain as low as those in their normal soil environment.

When the scientists lowered oxygen levels, around three-quarters of the neurons became silent. Those not silenced included neurons responsible for monitoring alarming environmental signals such as high oxygen levels, and these caused the animals to awaken when the scientists raised oxygen levels.

This indicates that sleep is an emergent property of neuronal circuits, rather than an activity strictly enforced by specific brain areas, as others have suggested.


Plug pulled on ocean carbon sink

Organic particle behaviour may predict less carbon stored in the sea bed.

The ocean’s biological carbon pump may be shuttling less carbon from the surface to the depths than is assumed in most climate models.

Chelsey Baker of the National Oceanography Centre in Southampton, UK, and her colleagues analysed the amount of particulate organic carbon — microscopic detritus from marine life — in samples taken from 144 locations during eight Atlantic Ocean cruises from 2009 to 2013. The results suggest that the bulk of the carbon in the deep ocean is in the form of small particles, partly because larger ones become fragmented in midwater. But smaller particles sink more slowly than large ones and so are more likely to be chemically broken down before they reach the ocean floor, where they can be locked into sediment.

The researchers suggest that biogeochemical models, which assume a larger proportion of big, fast-sinking particles at great depths, might overestimate long-term carbon storage in the ocean.


Noise pollution kills zooplankton

Air-gun pulses wipe out key animals in marine ecosystems.

The widespread use of air guns in underwater seismic surveys could be dramatically reducing populations of the microscopic animals at the base of the marine food chain.

Plankton are vital for the health of ocean ecosystems, but their sensitivity to human-made noise is not well understood. Robert McCauley at Curtin University in Perth, Australia, and his team decided to investigate the effects of air-gun signals, similar to those used for the detection of oil reserves, in a bay in Tasmania. They towed nets through the water before and after firing an air gun, and found that the abundance of collected zooplankton fell by more than 60% within an hour of the noise impulse, and that the number of dead animals more than doubled.

Sonar measurements suggest that zooplankton abundance dropped more than a kilometre away from the source of the gun shot.

Heat-plagued people crowd China’s largest swimming pool in Suining

Climate sciences

Deadly heat is on the rise

More than two billion people face at least 20 days of potentially lethal temperatures each year.

Heatwaves have killed thousands of people in 164 cities around the world since 1980 — and the risk of such events looks set to increase.

To identify climatic conditions associated with unusually high human mortality rates, Camilo Mora at the University of Hawaii at Manoa, Honolulu, and his colleagues analysed more than 900 papers published between 1980 and 2014. From this, they determined a threshold beyond which air temperatures, humidity and other factors can be lethal, and found that about 30% of the global population is currently exposed to potentially deadly heat for at least 20 days per year.

Climate projections suggest that by 2100, that percentage will be at least 48%, even if greenhouse-gas emissions are aggressively reduced. Heat-related health risks and mortality are likely to rise disproportionately in cities and humid regions in the tropics, the authors say.


Tumour DNA highlights targets for therapy

Blood test offers a way to identify those most likely to benefit from treatment for prostate cancer.

DNA floating in the blood provides a window into how people with prostate cancer will fare on drugs known as PARP inhibitors.

PARP inhibitors are used to treat tumours with mutations that disrupt particular DNA-repair pathways. Johann de Bono of the Institute of Cancer Research in London and his colleagues analysed blood samples from 46 people with prostate cancer in a clinical trial of the PARP inhibitor olaparib.

The team found that the amount of tumour DNA in the blood tended to fall more markedly in participants whose tumours shrank following therapy. In some patients who initially responded to treatment but then relapsed, the researchers detected additional mutations that corrected the original DNA-repair defect, allowing tumours to escape the effects of olaparib.

Scans of a mouse cornea get sharper by using improved optical coherence tomography


Innovative approach promises speckle-free scans

Image stacking can slash noise and boost resolution of medical technology.

An imaging technique used in cancer research and for diagnosing eye and heart disease has received a boost in resolution.

Optical coherence tomography allows scientists to map fine structure in living tissue, but it is limited by an image artefact called speckle. This is caused by interference that occurs when the laser beam strikes rough tissue surfaces.

Current speckle-reduction methods degrade image resolution. A team led by Adam de la Zerda at Stanford University in California improved on these by modulating the phase of the beam to vary the speckle pattern, and by capturing up to 100 different images that were stacked and averaged to almost eliminate speckle noise.

The improved technique can resolve structures as small as sweat ducts on a human fingertip and the inner structure of a live mouse’s cornea.

Grass snakes common in Great Britain are at risk of a fungal disease


Snake disease turns up in Europe

A deadly fungus that causes skin lesions in snakes has been detected in wild European species for the first time.

Snake fungal disease, caused by Ophidiomyces ophiodiicola, infects at least 30 wild species in North America and can be fatal, but until now had not been reported in wild snakes on other continents.

Becki Lawson at the Institute of Zoology in London and her colleagues screened more than 300 moulted snake skins and around 30 carcasses, from 3 species in the United Kingdom and 1 in the Czech Republic. They found skin lesions on nearly one-quarter of carcasses and moulted skins. DNA analysis confirmed O. ophiodiicola infection in 31% of these samples, including grass snakes (Natrix natrix) and a dice snake (Natrix tessellata).

The European strains of the fungal pathogen were distinct from the North American ones. They were slower-growing and may have existed at low levels in Europe for years, the authors say.

Wildfires as those currently raging in Portugal emit more harmful soot than thought

Atmospheric science

Wildfire pollution grossly underestimated

US survey records unprecedented levels of soot.

Wildfires pollute the air more than previously thought — especially when it comes to tiny particles that can lodge in the lungs and cause health problems.

Greg Huey at the Georgia Institute of Technology in Atlanta and his colleagues analysed data gathered by research aeroplanes from the smoke plumes of three major wildfires in the western United States in 2013. The burns emitted more than three times the amount of fine particulate matter, or soot, accounted for in the US National Emissions Inventory.

Earlier estimates of the amount of particulate matter produced by such fires came from controlled burns, but it seems wildfires release much more pollution into the air. The team also measured a range of chemicals coming from the wildfires, including some nitrate compounds spotted for the first time in plumes from burning biomass. Setting controlled burns that improve forest health and reduce the risk of devastating wildfires may be one way to prevent future air-quality problems, the authors say.


Ground-to-air quantum link achieved?

Researchers have transmitted photons to a speeding aircraft, demonstrating technology for space-based quantum communication.

Quantum encryption is secure because the laws of quantum physics guarantee that any eavesdropping would be detectable. But photons, which are used encode a shared quantum key, can travel only relatively short distances through fibres or air before being absorbed, making transmission through space more appealing for long-distance communication.

Thomas Jennewein at the University of Waterloo, Canada, and his collaborators sent photons to a receiver mounted on an aircraft passing overhead at the same apparent rate as a satellite and over distances of up to 10 kilometres. The team successfully transmitted photons in 7 of 14 attempts, generating quantum keys more than 800,000 bits long.

The authors say that this ‘uplink’ scheme, in which photons are generated at ground stations, could be simpler than a set-up used by rivals, which requires complex devices to be put into orbit.


How cats came to dominate the world

Domestic felines followed trade routes to spread into Europe.

Feline remains at ancient burial sites suggest that domestic cats existed in early societies in the Near East and Egypt. But scientists know little about how the animals advanced across the world.

Eva-Maria Geigl at the Institute Jacques Monod in Paris and her colleagues analysed mitochondrial DNA from 209 cats that lived between about 8000 BC and the twentieth century.

They found two distinct populations that contribute to modern domestic cats, one of which appeared in the Middle East and spread to Europe as early as 4400 BC. A separate lineage, initially common only to ancient Egyptian cats, spread to Europe and the Middle East from the fifth century AD onwards.

This move mirrors ancient trade routes, suggesting that a role as ship’s rat catcher might have helped cats to spread to Europe and beyond.


Hunting spiders lose web skills

Adaptations to create sticky traps leaves ground spiders without a safety harness.

Ground spiders (Gnaphosidae) specialize in catching prey that fights back fiercely, including ants and other spiders. They snare their prey with an unusually glue-like silk, which evolved from a thread normally used to anchor a web’s structural silks to surfaces.

Jonas Wolff at Macquarie University in Sydney, Australia, and his colleagues studied 11 species in slow-motion video and examined the biomechanics of their silk and silk-producing structures.

The researchers found that as the gluey silk evolved into a hunting tool, the glands that produce it became bigger, while those that make the main thread shrank. As a result, these ground spiders can no longer spin abseiling ‘draglines’ and are barely able to attach webs to the environment.

Drug discovery

New class of antibiotic found

Natural compound kills bacteria that are resistant to multiple antibiotics.

Richard Ebright at Rutgers University in Piscataway, New Jersey, Stefano Donadio at the biotechnology firm Naicons in Milan, Italy, and their team extracted the antibiotic — called pseudouridimycin — from microbes in soil. They found that it cured bacterial infections in mice and killed several strains of antibiotic-resistant bacteria in culture.

The drug works by stopping bacteria from making RNA — it inhibits the binding of a nucleoside triphosphate, a building block of nucleic acids. Other antibiotics also stop RNA synthesis, but not by the same mechanism. This means that few bacteria will be prepared for pseudouridimycin’s mode of attack, say the authors.The team showed that resistance to pseudouridimycin develops ten times more slowly than it does to rifampin, an existing drug that also inhibits RNA synthesis.

Antiviral drugs that block nucleoside triphosphates have transformed the treatment of hepatitis C and HIV, add the authors, who hope that the new drug will be similarly successful against bacteria.

Computer science

Robot can single out humans

A robot can learn to detect human motion by the tell-tale smoothness with which people move.

To work with humans, robots must discriminate between movements of living and non-living things — a challenging task. Alessia Vignolo at the Italian Institute of Technology in Genoa and her team developed a machine-learning algorithm that exploits the characteristic smoothness with which humans change speed and trajectory on a split-second timescale.

By observing how sharply motion changed, the algorithm learned to differentiate between human actions, such as rolling dough, and the movements of inanimate objects, such as toy trains. The program does not interpret objects on the basis of their appearance, so it was successful even when viewing unfamiliar actions and when a scene was partly obscured.

Installed in a humanoid robot, the algorithm was able to automatically direct the machine’s gaze at humans, a useful skill for social interaction, say the authors.

Astronomy and astrophysics

How galaxies grew

Milky-Way-like galaxies formed through gentle mergers according to mega-simulations.

Large disk-shaped galaxies such as the Milky Way grow bigger when external gas and dark matter merge into them with just the right speed and trajectory.

A team led by Robert Grand at the Heidelberg Institute for Theoretical Studies in Germany created one of the largest cosmological simulations ever run, and used it to show how features such as the Milky Way’s spiral arms came to be. The results suggest that when material flows gently into a growing galaxy, it bulks up the central disk and allows structures such as bars and spiral arms to form. By contrast, violent collisions tend to shred the galaxy and limit its growth, say the authors.

The model ran on two supercomputers over several months and included, for the first time, the effects of magnetic fields that pervade interstellar space. Astronomers can compare the results of the simulations with their observations to better understand galaxy formation, says the team.


How termite mounds begin

Collective digging, rather than depositing of soil, triggers mound building by termites.

No one knows for sure how termites collaborate to build complex structures, although researchers have long thought that a chemical, secreted by the creatures when they deposit soil, prompts others to start dropping soil in the same place.

Ben Green at Harvard University in Cambridge, Massachusetts, and his colleagues observed the behaviour of workers from two species of termite (Macrotermes) in soil-lined Petri dishes. Tracking the insects’ motion and behaviour, the researchers first observed random digging, after which termites clustered at the busiest excavation sites. Soil deposited at these sites became the basis for new mounds.

The observations fit with a simulation by the team in which mound building is initially driven by termites joining in at the most popular excavation sites; they fit less well, the authors say, with a model in which the insects deposit soil in response to a chemical signal.

Cell biology

A UV-free tan that protects skin

Drug that stimulates melanin production could shield skin from damage caused by the Sun.

A drug that can be applied to the skin can boost production of the pigment melanin and potentially protect against cancer.

A type of melanin known as eumelanin darkens skin and helps to shield cells from damage caused by ultraviolet (UV) light, but drugs known to promote its production in mice struggle to penetrate the tougher barrier of human skin.

David Fisher of Massachusetts General Hospital in Boston and his colleagues therefore looked for alternatives. They focused on compounds that inhibit SIK, a protein involved in the suppression of pigment synthesis. One such compound was found to stimulate eumelanin production in mice that produce low levels of the pigment owing to a genetic mutation.

To increase the chances of penetrating human skin, the researchers developed SIK inhibitors that were smaller and more soluble in lipids than the original compound. Two of these increased eumelanin production in human skin cells. The researchers hope that the compounds might be useful in combination with sunscreens.


Sea-ice ‘treadmill’ speeds up for polar bears

Faster-drifting sea ice forces bears to travel farther for food.

Shrinking Arctic sea ice has already reduced the roaming range of polar bears (Ursus maritimus), but the impact of changes in ice drift has not been explored. George Durner at the US Geological Survey in Anchorage, Alaska, and his team analysed data on sea-ice drift across the Beaufort and Chukchi seas between 1987 and 2013, along with the movements of a total of 273 radio-tagged polar bears during that period. They found an increased rate of westward ice drift since 1987, which coincided with bears walking eastward faster or for longer to reach their seal-hunting grounds.

Bears would need to catch up to three more seals each year to fuel this extra activity, say the authors, who add that faster-drifting ice could exacerbate the physiological stress that bears already face in the warming Arctic.