Category: Science &Technology

{New global images of Mars from the MAVEN mission show the ultraviolet glow from the Martian atmosphere in unprecedented detail, revealing dynamic, previously invisible behavior. They include the first images of “nightglow” that can be used to show how winds circulate at high altitudes. Additionally, dayside ultraviolet imagery from the spacecraft shows how ozone amounts change over the seasons and how afternoon clouds form over giant Martian volcanoes. The images were taken by the Imaging UltraViolet Spectrograph (IUVS) on the Mars Atmosphere and Volatile Evolution mission (MAVEN).}

“MAVEN obtained hundreds of such images in recent months, giving some of the best high-resolution ultraviolet coverage of Mars ever obtained,” said Nick Schneider of the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder. Schneider is presenting these results Oct. 19 at the American Astronomical Society Division for Planetary Sciences meeting in Pasadena, California, which is being held jointly with the European Planetary Science Congress.

Nightside images show ultraviolet (UV) “nightglow” emission from nitric oxide (abbreviated NO). Nightglow is a common planetary phenomenon in which the sky faintly glows even in the complete absence of external light. Mars’ nightside atmosphere emits light in the ultraviolet due to chemical reactions that start on Mars’ dayside. Ultraviolet light from the sun breaks down molecules of carbon dioxide and nitrogen, and the resulting atoms are carried around the planet by high-altitude wind patterns that encircle the planet. On the nightside, these winds bring the atoms down to lower altitudes where nitrogen and oxygen atoms collide to form nitric oxide molecules. The recombination releases extra energy, which comes out as ultraviolet light.

Scientists predicted NO nightglow at Mars, and prior missions detected its presence, but MAVEN has returned the first images of this phenomenon in the Martian atmosphere. Splotches and streaks appearing in these images occur where NO recombination is enhanced by winds. Such concentrations are clear evidence of strong irregularities in Mars’ high altitude winds and circulation patterns. These winds control how Mars’ atmosphere responds to its very strong seasonal cycles. These first images will lead to an improved determination of the circulation patterns that control the behavior of the atmosphere from approximately 37 to 62 miles (about 60 to 100 kilometers) high.

Dayside images show the atmosphere and surface near Mars’ south pole in unprecedented ultraviolet detail. They were obtained as spring comes to the southern hemisphere. Ozone is destroyed when water vapor is present, so ozone accumulates in the winter polar region where the water vapor has frozen out of the atmosphere. The images show ozone lasting into spring, indicating that global winds are inhibiting the spread of water vapor from the rest of the planet into winter polar regions. Wave patterns in the images, revealed by UV absorption from ozone concentrations, are critical to understanding the wind patterns, giving scientists an additional means to study the chemistry and global circulation of the atmosphere.

MAVEN observations also show afternoon cloud formation over the four giant volcanoes on Mars, much as clouds form over mountain ranges on Earth. IUVS images of cloud formation are among the best ever taken showing the development of clouds throughout the day. Clouds are a key to understanding a planet’s energy balance and water vapor inventory, so these observations will be valuable in understanding the daily and seasonal behavior of the atmosphere.

“MAVEN’s elliptical orbit is just right,” said Justin Deighan of the University of Colorado, Boulder, who led the observations. “It rises high enough to take a global picture, but still orbits fast enough to get multiple views as Mars rotates over the course of a day.”

MAVEN’s principal investigator is based at the University of Colorado’s Laboratory for Atmospheric and Space Physics, Boulder. The university provided two science instruments and leads science operations, as well as education and public outreach, for the mission. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the MAVEN project and provided two science instruments for the mission. The University of California at Berkeley’s Space Sciences Laboratory also provided four science instruments for the mission. Lockheed Martin built the spacecraft and is responsible for mission operations. NASA’s Jet Propulsion Laboratory in Pasadena, California, provides navigation and Deep Space Network support, as well as the Electra telecommunications relay hardware and operations.

{Tomorrow, two spacecraft will reach Mars after nearly seven months of traveling together through space — and they’ll both attempt to pull off two separate and incredible feats. One will put itself into orbit around the Red Planet, while the other will land on the surface, hopefully in one piece.}

It’s perhaps the biggest moment so far in the first phase of the ExoMars mission, a joint venture between the European Space Agency and the Russian Federal Space Agency, or Roscosmos. On Sunday, the two vehicles — the Trace Gas Orbiter (TGO) and the Schiaparelli lander — separated from one another in preparation for Wednesday’s events.

The main goal of ExoMars is to figure out if there is, or ever was, life on Mars. The TGO will try to answer this question from orbit, by sniffing out the gases in Mars’ atmosphere. It’ll be looking for specific compounds like methane, which on Earth is often produced when biological matter breaks down. Its presence around Mars could indicate life on the planet’s surface, as well. Researchers have long debated whether or not the gas exists around the Red Planet, and the ExoMars mission hopes to finally quell that dispute.

As for Schiaparelli, its role is just to land intact. Its only mission is to demonstrate that ESA and Roscosmos have the necessary technology to safely land scientific hardware on Mars. That’s key for the next phase of the ExoMars mission, which involves landing a 680-pound rover on the Red Planet in 2021 to explore the surface and look for signs of biological life. The same technologies used to land Schiaparelli will be used to land the ExoMars rover as well.

Tomorrow, things get started a little after 9AM ET, when the TGO ignites its main engine to help slow the spacecraft down. The orbiter will be moving at more than 8,700 miles per hour, so it needs to put on the brakes just enough to get captured by Mars’ gravitational pull. To do this, the vehicle’s engine will burn for two and a half hours, putting the TGO into a highly elliptical orbit around Mars that takes four Martian days to complete. If for some reason the engine burn doesn’t work correctly, the orbiter will fly straight past Mars and its mission will be blown.

While the TGO’s engine continues to burn, Schiaparelli will move closer to Mars and start its descent around 10:42AM ET. The lander then has just six intense minutes to slow itself down enough so that it doesn’t crash into the planet’s surface. And that poses a big challenge, since Schiaparelli will enter the planet’s atmosphere at an altitude of about 75 miles, traveling at nearly 13,000 miles per hour.

But Schiaparelli is equipped to handle the descent. First, it will free fall for three and a half minutes, as the Martian atmosphere slows the spacecraft’s speed down to around 10,500 miles per hour. The lander has a heat shield to keep the vehicle from burning up while entering the atmosphere — a process that creates a lot of intense heating. At about six miles above the surface, Schiaparelli will deploy its parachute, which will help slow the vehicle down to about 155 miles per hour.

At less than a mile, Schiaparelli will ignite a series of onboard thrusters for about 30 seconds to slow down even more. This extra step is necessary because the air around Mars is about 1/100th the density of Earth’s atmosphere and the parachute alone won’t be enough. These tiny engines embedded in the hull of the spacecraft will push against the Martian surface, slowing Schiaparelli to 2.5 miles per hour. Eventually, they’ll turn off and the lander will fall the final few feet to the surface. Schiaparelli is equipped with a “crushable structure” designed to absorb the force of that final impact.

Throughout the entire landing, Schiaparelli will be sending back data to the TGO. ESA’s Mars Express probe, which is already in orbit around Mars, will also record the entire event, and an array of 30 radio telescopes in India will be picking signals of the landing from Earth. Once the entire thing is over, other orbiters like NASA’s Mars Reconnaissance Orbiter will pick up additional data and photos taken by Schiaparelli. All this data will tell researchers how lander’s fall to Mars went.

Once the landing is over, that’s about it for Schiaparelli, since it doesn’t have a way of generating its own power. Just a few days after landing, Schiaparelli will likely use up all of its onboard batteries to send data to the orbiters around Mars before it dies.

But the Trace Gas Orbiter’s mission is just getting started. The orbiter will spend the majority of next year slowing down and adjusting its orbit to get closer to Mars. It’s a process known as aerobraking, and it will bring TGO into a much more circular orbit about 250 miles above Mars’ surface. Then in March 2018, the orbiter will start its science observations — a phase that will last for two years.

ESA plans to live stream all of the big events from mission control tomorrow starting at 9AM ET. Tune back in to see if the ExoMars spacecraft pull everything off.

{Europe’s Schiaparelli spacecraft is on course to land on Mars.}

The 577kg probe separated successfully from its mothership on Sunday at 14:42 GMT (15:42 BST; 16:42 CEST).

It is now on a direct path to intercept the top of the Red Planet’s atmosphere on Wednesday.

The module will then have just under six minutes to reduce its 21,000km/h entry speed to zero in order to make a relatively soft flop-down on to Mars’ dusty surface.
Schiaparelli is a technology demonstrator. It is intended to showcase the European Space Agency’s (Esa) ability to land on Earth’s near neighbour.

The organisation’s only previous attempt was a very short-lived effort – the UK-led Beagle-2 robot in 2003.

This craft managed to make an intact touch-down but then failed to deploy its solar panels properly, blocking any contact with home.

Schiaparelli will hope to fare better, albeit with a planned surface operation of only a few days that will be sustained by batteries.

Esa controllers in Darmstadt, Germany, were able to confirm the separation of the module from its carrier satellite – the ExoMars Trace Gas Orbiter – thanks to a radio transmission beamed across more than 170 million km of space.

Schiaparelli is now on its own; there is nothing anyone can do to change its trajectory or to give it new commands.

Its landing sequence on Wednesday is fully automated. The probe will rub off most of its entry speed thanks to a heatshield that will push up against the Martian air. The combination of a big parachute and a cluster of rockets will then bring it to a near standstill just above the surface. Schiaparelli’s final two metres will see it dump down on to its belly.

The Esa probe will emit UHF tones during its entry, descent and landing phases, which an Indian radio telescope will endeavour to capture and relay to Darmstadt. If the Indian facility can still hear Schiaparelli at 15:00 GMT (16:00 BST; 17:00 CEST) on Wednesday, it will mean the Italian-built module is safely on the surface.

While the landing attempt will no doubt occupy the media’s and the public’s attention in the coming days, Esa also has the very important task of “parking” the Trace Gas Orbiter at Mars.

Twelve hours after ejecting Schiaparelli, the satellite was due to change course to avoid following on behind the module and its collision path with the planet.

This manoeuvre will be followed by a “big burn” on the TGO’s main engine on Wednesday, to put it on a large ellipse around Mars.

The orbiter will spend the coming years studying the behaviour of atmospheric components such as methane, water vapour and nitrogen dioxide. Although present in only small amounts, these gases – methane in particular – hold clues about the planet’s current state of activity. They may even hint at the existence of life.

{The Moon’s surface is being “gardened” — churned by small impacts — more than 100 times faster than scientists previously thought. This means that surface features believed to be young are perhaps even younger than assumed. It also means that any structures placed on the Moon as part of human expeditions will need better protection.}

This new discovery comes from more than seven years of high-resolution lunar images studied by a team of scientists from Arizona State University and Cornell University. The team is led by ASU’s Emerson Speyerer, who is also the lead author of the scientific paper published October 13 in Nature.

“Before the Lunar Reconnaissance Orbiter was launched in 2009, we thought that it took hundreds of thousands to millions of years to change the lunar surface layer significantly,” Speyerer said. “But we’ve discovered that the Moon’s uppermost surface materials are completely turned over in something like 80,000 years.”

The images used in the discovery come from the Lunar Reconnaissance Orbiter Camera (LROC) on NASA’s Lunar Reconnaissance Orbiter spacecraft. LROC is run from the Science Operations Center on ASU’s Tempe campus; the instrument’s principal investigator is Mark Robinson, a professor in ASU’s School of Earth and Space Exploration (SESE). Robinson is a co-author on the paper along with Reinhold Povilaitis and Robert Wagner, both SESE research specialists, and Peter Thomas of Cornell.

Before and after

“We used before and after images taken by LROC’s Narrow Angle Camera,” Speyerer said. During the seven years the mission has run so far, he said the team identified 222 new impact craters that formed during the mission. “These range in size from several meters wide up to 43 meters (140 feet) wide.”

The number of new craters found by Speyerer and colleagues is greater than anticipated by standard impact-modeling rates used by lunar scientists. The discovery has the effect of giving lunar surface features younger ages.

Theory says that a lunar geologic unit should accumulate a certain number of craters of a given size in a million years, for example. But if it turns out that impacts are making craters more quickly, then it takes less time to reach the benchmark number, and the geologic unit is in reality younger than theory predicts.

“A higher rate of impacts on geologic units with ages assumed to be already young turn out in fact to be even younger than we previously thought,” Speyerer said. However, to be certain, he said, LROC needs to continue taking images to verify the discovery and firm up the actual impact rate.

“Measuring the recent impact rate was one of the important tasks that led NASA to fly the Lunar Reconnaissance Orbiter,” Robinson said. Besides the value to scientists in pinning down surface ages, he noted there are also practical aspects.

Any future human exploration of the Moon will involve supply structures, rockets, and other equipment being parked on the surface for long periods of time, even if living quarters are underground. Knowing the present-day rate of impacts will be important in planning to protect equipment left on the surface.

Zones of disturbance

When the team examined the new craters found by the LROC survey, they noticed that the craters were surrounded by starburst patterns that obviously formed during the impact.

While the pattern details are complex, the researchers found that an impact throws out several kinds of debris. Some of it lands nearby. But impacts also throw small amounts of debris in hyper-velocity jets at speeds of 16 kilometers (10 miles) per second. This material — vaporized and molten rock — shoots over the surface, disturbing the upper layer of lunar soil and changing its brightness.

“In addition to the new impact craters and starburst debris patterns, we observed a surprising number of small surface changes which we call splotches,” Speyerer said.

While splotches lack the detectable rims that craters show, the team thinks the splotches are most likely caused by small impacts of material thrown from larger impacts.

“We see dense clusters of splotches around new impact sites,” he said. “This suggests that many splotches may be secondary effects caused by material thrown out from the primary impact event.”

From 14,000 pairs of before-and-after LROC images, the scientists identified more than 47,000 splotches.

“We estimated their accumulation over time and measured their sizes,” Speyerer said. “From this, we inferred how deeply each splotch dug up the surface. That gave us an estimate of how long it takes to effectively churn or ‘garden’ the upper few inches of lunar soil.”

The gardening time amounted to a geological eyeblink: not millions of years, and not even hundreds of thousands of years. As Speyerer explains, “We found that 99 percent of the surface would be overturned by forming splotches after about 81,000 years.”

Why the big difference in turnover time?

“Earlier estimates considered only direct hits from micrometeorites, and ignored entirely the role of small secondary impacts,” Robinson said.

Two additional findings come from revising the impact rate. First, remote sensing observations of the surface need to factor in a much higher turnover rate when looking at data from mineral-detecting X-ray and Gamma-ray spectrometers, which probe this upper surface layer.

Second, the churning rate will be important information for future planners of Moon bases. Surface assets will have to be designed to withstand the impacts of small particles traveling up to 500 meters per second, or 1,100 miles per hour.

Looking to the future, NASA has recently approved the Lunar Reconnaissance Orbiter for a two-year extended mission, and LROC will continue to acquire valuable cratering observations.

“As the mission goes on,” Robinson said, “the odds increase for us to find the larger impacts that occur less frequently on the Moon. Such discoveries will let us better pin down the lunar impact rate and also better characterize the most common process that shapes planetary bodies across the Solar System.”

{Mars’ largest moon Phobos has captured public imagination and been shrouded in mystery for decades. But numerical simulations recently conducted at Lawrence Livermore National Laboratory (LLNL) have shed some light on the enigmatic satellite.}

The dominant feature on the surface of Phobos (22-kilometers across) is Stickney crater (9-km across), a mega crater that spans nearly half the moon. The crater lends Phobos a physical resemblance to the planet-destroying Death Star in the film “Star Wars.” But over the decades, understanding the formation of such a massive crater has proven elusive for researchers.

For the first time, physicists at LLNL have demonstrated how an asteroid or comet impact could have created Stickney crater without destroying Phobos completely. The research, which also debunks a theory regarding the moon’s mysterious grooved terrain, was published in Geophysical Review Letters.

“We’ve demonstrated that you can create this crater without destroying the moon if you use the proper porosity and resolution in a 3D simulation,” said Megan Bruck Syal, an author on the paper and member of the LLNL planetary defense team. “There aren’t many places with the computational resources to accomplish the resolution study we conducted.”

The study showed that there is a range of possible solutions for the size and speed of the impactor, but Syal says one possible scenario is an impact object 250 meters across traveling close to 6 kps.

Previous studies used 2D simulations at lower resolutions, and they were ultimately unable to replicate Stickney crater successfully. Additionally, prior studies failed to account for the porosity of the Phobos’ crust in their calculations, critical given that Phobos is less dense than the Martian surface.

While the simulations show how a massive impact could have created Stickney crater, they also appear to disprove a related theory. Some have theorized that the hundreds of parallel grooves that appear to radiate from the crater were caused by the impact. However, the simulations in this study show that fracture patterns in the crust of Phobos would be nothing like the straight, long, parallel grooves. On the other hand, the simulations do support the possibility of slow-rolling boulders mobilized by the impact causing the grooves. But more study would be required to fully test that theory.

The research served as a benchmarking exercise for the LLNL planetary defense team in their use of an open source code developed at LLNL called Spheral. The team uses codes like Spheral to simulate various methods of deflecting potentially hazardous Earth-bound asteroids.

“Something as big and fast as what caused the Stickney crater would have a devastating effect on Earth,” Syal said. “If NASA sees a potentially hazardous asteroid coming our way, it will be essential to make sure we’re able to deflect it. We’ll only have one shot at it, and the consequences couldn’t be higher. We do this type of benchmarking research to make sure our codes are right when they will be needed most.”

The foundation this research is built upon is decades of investment in LLNL computational capabilities used to ensure the safety, security and effectiveness of the U.S. nuclear deterrent in the absence of nuclear testing — commonly known as stockpile stewardship. This research was also funded in part by the Laboratory Directed Research and Development Program at LLNL.

The study was spearheaded by Jared Rovny, a summer student visiting from Yale University. Other coauthors include LLNL computational physicist Mike Owen, who supported the research by mentoring Rovny and aligning the study to benchmark the Spheral code, and Paul Miller, who leads the planetary-defense team at LLNL. Syal conducted follow-up modeling to confirm the findings and wrote the published paper. She will be giving a talk on the paper in Pasadena this month during the annual meeting of the American Astronomical Society’s Division of Planetary Science.

{A star with a ring of planets orbiting around it — that is the picture we know from our own solar system and from many of the thousands of exoplanets observed in recent years. But now researchers from the Niels Bohr Institute have discovered a system consisting of two stars with three rotating planet-forming accretion discs around them. It is a binary star where each star has its own planet-forming disc and in addition, there is one large shared disc. All three planet-forming discs are misaligned in relation to one another. The spectacular results are published in the scientific journal, Astrophysical Journal Letters.}

A solar system is formed by a large cloud of gas and dust. The cloud of gas and dust condenses and eventually becomes so compact that it collapses into a ball of gas in the centre. Here the pressure heats up the matter and creates a glowing ball of gas, a star. The remainder of the gas and dust cloud rotates as a disc around the newly formed star. In this rotating disc of gas and dust, the material begins to accumulate and form larger and larger clumps, which finally become planets.

Often it is not just one, but two stars that are formed in the dense cloud of gas and dust. This is called a binary star and they are held together by their mutual gravity and orbit in a path around each other. About half of all stars are binary stars and they can each have a rotating disc of gas and dust.

Never before seen

But now the researchers have observed something highly unusual: a binary star with not just two, but three rotating gas discs.

“The two newly formed stars are both the size of our sun and they each have a rotating disc of gas and dust similar to the size of our solar system. In addition, they have a shared disc that is much larger and crosses over the other two discs. All three discs are staggered and this breaks with everything we have seen so far,” says Christian Brinch, assistant professor in the research group Astrophysics and Planetary Science and the Niels Bohr International Academy at the Niels Bohr Institute, University of Copenhagen.

The stars were observed with the large international telescope, the Atacama Large Millimeter Array (ALMA) in northern Chile by an international team of researchers from Denmark, England and the Netherlands. The stars are about 400 light years away from the Earth. The stars are about 100-200,000 years old and planet formation may already have started. They cannot see this. But when they can see the accretion discs, it is because they are still mostly made up of gases.

Tumble around

“What we can observe is the gas itself, because the molecules are excited by the heat from the stars and therefore emit light in the infrared and microwave range. By studying the wavelength of the light you can see whether the light source is moving farther away or is getting closer. If the light shifts towards red wavelengths it is moving farther away, while blue shift light is moving closer and thus we can see that the three planet-forming discs are almost ‘tumbling around’ and are skewed relative to each other,” explains Christian Brinch.

The researchers do not know why it is not a ‘nice’ system where the rotating discs of gas lie flat in relation to each other. Perhaps the formation occurred in a particularly turbulent manner.

“We will use computer simulations to try to understand the physics of the formation process. Perhaps it is a dynamic process of formation, which happens often and then it corrects itself later on. We will try to clarify this. We will also apply for more observation time on the ALMA telescope to study the planet-forming discs in even higher resolution to get more detailed information about their chemical composition,” says Jes Jørgensen, associate professor in the research group Astrophysics and Planetary Science at the Niels Bohr Institute and Centre for Star and Planet Formation, University of Copenhagen.

{Will astronauts traveling to Mars remember much of it? That’s the question concerning University of California, Irvine scientists probing a phenomenon called “space brain.”}

UCI’s Charles Limoli and colleagues found that exposure to highly energetic charged particles — much like those found in the galactic cosmic rays that will bombard astronauts during extended spaceflights — causes significant long-term brain damage in test rodents, resulting in cognitive impairments and dementia.

Their study appears in Nature’s Scientific Reports. It follows one last year showing somewhat shorter-term brain effects of galactic cosmic rays. The current findings, Limoli said, raise much greater alarm.

“This is not positive news for astronauts deployed on a two-to-three-year round trip to Mars,” said the professor of radiation oncology in UCI’s School of Medicine. “The space environment poses unique hazards to astronauts. Exposure to these particles can lead to a range of potential central nervous system complications that can occur during and persist long after actual space travel — such as various performance decrements, memory deficits, anxiety, depression and impaired decision-making. Many of these adverse consequences to cognition may continue and progress throughout life.”

For the study, rodents were subjected to charged particle irradiation (fully ionized oxygen and titanium) at the NASA Space Radiation Laboratory at New York’s Brookhaven National Laboratory and then sent to Limoli’s UCI lab.

Six months after exposure, the researchers still found significant levels of brain inflammation and damage to neurons. Imaging revealed that the brain’s neural network was impaired through the reduction of dendrites and spines on these neurons, which disrupts the transmission of signals among brain cells. These deficiencies were parallel to poor performance on behavioral tasks designed to test learning and memory.

In addition, the Limoli team discovered that the radiation affected “fear extinction,” an active process in which the brain suppresses prior unpleasant and stressful associations, as when someone who nearly drowned learns to enjoy water again.

“Deficits in fear extinction could make you prone to anxiety,” Limoli said, “which could become problematic over the course of a three-year trip to and from Mars.”

Most notably, he said, these six-month results mirror the six-week post-irradiation findings of a 2015 study he conducted that appeared in the May issue of Science Advances.

Similar types of more severe cognitive dysfunction are common in brain cancer patients who have received high-dose, photon-based radiation treatments. In other research, Limoli examines the impact of chemotherapy and cranial irradiation on cognition.

While dementia-like deficits in astronauts would take months to manifest, he said, the time required for a mission to Mars is sufficient for such impairments to develop. People working for extended periods on the International Space Station, however, do not face the same level of bombardment with galactic cosmic rays because they are still within the Earth’s protective magnetosphere.

Limoli’s work is part of NASA’s Human Research Program. Investigating how space radiation affects astronauts and learning ways to mitigate those effects are critical to further human exploration of space, and NASA needs to consider these risks as it plans for missions to Mars and beyond.

Partial solutions are being explored, Limoli noted. Spacecraft could be designed to include areas of increased shielding, such as those used for rest and sleep. However, these highly energetic charged particles will traverse the ship nonetheless, he added, “and there is really no escaping them.”

Preventive treatments offer some hope. Limoli’s group is working on pharmacological strategies involving compounds that scavenge free radicals and protect neurotransmission.

Vipan Kumar Parihar, Barrett Allen, Chongshan Caressi, Katherine Tran, Esther Chu, Stephanie Kwok, Nicole Chmielewski, Janet Baulch, Erich Giedzinski and Munjal Acharya of UCI and Richard Britten of Eastern Virginia Medical School contributed to the study, which NASA supported through grants NNX13AK70G, NNX14AE73G, NNX13AD59G, NNX10AD59G, UARC NAS2-03144 and NNX15AI22G.

{For at least a billion years of the distant past, planet Earth should have been frozen over but wasn’t. Scientists thought they knew why, but a new modeling study from the Alternative Earths team of the NASA Astrobiology Institute has fired the lead actor in that long-accepted scenario.}

Humans worry about greenhouse gases, but between 1.8 billion and 800 million years ago, microscopic ocean dwellers really needed them. The sun was 10 to 15 percent dimmer than it is today — too weak to warm the planet on its own. Earth required a potent mix of heat-trapping gases to keep the oceans liquid and livable.

For decades, atmospheric scientists cast methane in the leading role. The thinking was that methane, with 34 times the heat-trapping capacity of carbon dioxide, could have reigned supreme for most of the first 3.5 billion years of Earth history, when oxygen was absent initially and little more than a whiff later on. (Nowadays oxygen is one-fifth of the air we breathe, and it destroys methane in a matter of years.)

“A proper accounting of biogeochemical cycles in the oceans reveals that methane has a much more powerful foe than oxygen,” said Stephanie Olson, a graduate student at the University of California, Riverside, a member of the Alternative Earths team and lead author of the new study published September 26 in the Proceedings of the National Academy of Sciences. “You can’t get significant methane out of the ocean once there is sulfate.”

Sulfate wasn’t a factor until oxygen appeared in the atmosphere and triggered oxidative weathering of rocks on land. The breakdown of minerals such as pyrite produces sulfate, which then flows down rivers to the oceans. Less oxygen means less sulfate, but even 1 percent of the modern abundance is sufficient to kill methane, Olson said.

Olson and her Alternative Earths coauthors, Chris Reinhard, an assistant professor of earth and atmospheric sciences at Georgia Tech University, and Timothy Lyons, a distinguished professor of biogeochemistry at UC Riverside, assert that during the billion years they assessed, sulfate in the ocean limited atmospheric methane to only 1 to 10 parts per million — a tiny fraction of the copious 300 parts per million touted by some previous models.

The fatal flaw of those past climate models and their predictions for atmospheric composition, Olson said, is that they ignore what happens in the oceans, where most methane originates as specialized bacteria decompose organic matter.

Seawater sulfate is a problem for methane in two ways: Sulfate destroys methane directly, which limits how much of the gas can escape the oceans and accumulate in the atmosphere. Sulfate also limits the production of methane. Life can extract more energy by reducing sulfate than it can by making methane, so sulfate consumption dominates over methane production in nearly all marine environments.

The numerical model used in this study calculated sulfate reduction, methane production, and a broad array of other biogeochemical cycles in the ocean for the billion years between 1.8 billion and 800 million years ago. This model, which divides the ocean into nearly 15,000 three-dimensional regions and calculates the cycles for each region, is by far the highest resolution model ever applied to the ancient Earth. By comparison, other biogeochemical models divide the entire ocean into a two-dimensional grid of no more than five regions.

“There really aren’t any comparable models,” says Reinhard, who was lead author on a related paper in Proceedings of the National Academy of Sciences that described the fate of oxygen during the same model runs that revealed sulfate’s deadly relationship with methane.

Reinhard notes that oxygen dealt methane an additional blow, based on independent evidence published recently by the Alternative Earths team in the journals Science and Geology. These papers describe geochemical signatures in the rock record that track extremely low oxygen levels in the atmosphere, perhaps much less than 1 percent of modern values, up until about 800 million years ago, when they spiked dramatically.

Less oxygen seems like a good thing for methane, since they are incompatible gases, but with oxygen at such extremely low levels, another problem arises.

“Free oxygen [O2] in the atmosphere is required to form a protective layer of ozone [O3], which can shield methane from photochemical destruction,” Reinhard said. When the researchers ran their model with the lower oxygen estimates, the ozone shield never formed, leaving the modest puffs of methane that escaped the oceans at the mercy of destructive photochemistry.

With methane demoted, scientists face a serious new challenge to determine the greenhouse cocktail that explains our planet’s climate and life story, including a billion years devoid of glaciers, Lyons said. Knowing the right combination other warming agents, such as water vapor, nitrous oxide, and carbon dioxide, will also help us assess habitability of the hundreds of billions of other Earth-like planets estimated to reside in our galaxy.

“If we detect methane on an exoplanet, it is one of our best candidates as a biosignature, and methane dominates many conversations in the search for life on Mars,” Lyons said. “Yet methane almost certainly would not have been detected by an alien civilization looking at our planet a billion years ago — despite the likelihood of its biological production over most of Earth history.”

{When the maps appeared at the end of March, experts were electrified. The images revealed an orange-red disk pitted with circular gaps that looked like the grooves in an old-fashioned long-playing record. But this was no throwback to the psychedelic Sixties. It was a detailed portrait of a so-called protoplanetary disk, made up of gas and dust grains, associated with a young star — the kind of structure out of which planets could be expected to form. Not only that, the maps showed that the disk around the star known as TW Hydrae exhibits several clearly defined gaps. Astronomers speculated that these gaps might indicate the presence of protoplanets, which had pushed away the material along their orbital paths. And to make the story even more seductive, one prominent gap is located at approximately the same distance from TW Hydrae as Earth is from the Sun — raising the possibility that this putative exoplanet could be an Earth-like one.}

Now an international team led by Professor Barbara Ercolano at LMU’s Astronomical Observatory has compared the new observations with theoretical models of planet formation. The study indicates that the prominent gap in the TW Hydrae system is unlikely to be due to the action of an actively accreting protoplanet. Instead, the team attributes the feature to a process known as photoevaporation. Photoevaporation occurs when the intense radiation emitted by the parent star heats the gas, allowing it to fly away from the disk. But although hopes of a new exo-Earth orbiting in the inner gap of TW Hydrae may themselves have evaporated, the system nevertheless provides the opportunity to observe the dissipation of a circumstellar disk in unprecedented detail. The new findings appear in the journal Monthly Notices of the Royal Astronomical Society (MNRAS).

Only 175 light-years from Earth

The dusty disk that girdles TW Hydrae has long been a favored object of observation. The star lies only 175 light-years from Earth, and is it relatively young (around 106 years old). Moreover, the disk is oriented almost perpendicular to our line of sight, affording a well-nigh ideal view of its structure. The spectacular images released in March were made with the Atacama Large Millimeter/submillimeter Array (ALMA), an array of detectors in the desert of Northern Chile. Together, they form a radiotelescope with unparalleled resolving power that can detect the radiation from dust grains in the millimeter size range.

Photoevaporation is one of the major forces that shape the fate of circumstellar disks. Not only can it destroy such disks — which typically have a life expectancy of around 10 million years — it can also stop young planets being drawn by gravity and by the interaction with the surrounding disc gas into their parent star. The gaps caused by the action of photoevaporation on the disk, park the planets at their location by removing the gas, allowing the small dusty clumps to grow into fully fledged planets and steering them into stable orbits. However, in the case of the TW Hydrae system, Barbara Ercolano believes that the inner gap revealed by the ALMA maps is not caused by a planet, but represents an early stage in the dissipation of the disk. This view is based on the fact that many characteristic features of the disk around TW Hydrae, such as the distance between the gap and the star, the overall mass accretion rate, and the size and density distributions of the particles, are in very good agreement with the predictions of her photoevaporation model.

{A subsurface ocean lies deep within Saturn’s moon Dione, according to new data from the Cassini mission to Saturn. Two other moons of Saturn, Titan and Enceladus, are already known to hide global oceans beneath their icy crusts, but a new study suggests an ocean exists on Dione as well.}

In this study, researchers of the Royal Observatory of Belgium show gravity data from recent Cassini flybys can be explained if Dione’s crust floats on an ocean located 100 kilometers below the surface. The ocean is several tens of kilometers deep and surrounds a large rocky core. Seen from within, Dione is very similar to its smaller but more famous neighbor Enceladus, whose south polar region spurts huge jets of water vapor into space. Dione seems to be quiet now, but its broken surface bears witness of a more tumultuous past. The study is published online this week in Geophysical Research Letters.

The authors modeled the icy shells of Enceladus and Dione as global icebergs immersed in water, where each surface ice peak is supported by a large underwater keel. Scientists have used this approach in the past but previous results have predicted a very thick crust for Enceladus and no ocean at all for Dione. “As an additional principle, we assumed that the icy crust can stand only the minimum amount of tension or compression necessary to maintain surface landforms,” said Mikael Beuthe, lead author of the new study. “More stress would break the crust down to pieces.”

According to the new study, Enceladus’ ocean is much closer to the surface, especially near the south pole where geysers erupt through a few kilometers of crust. These findings agree well with the discovery last year by Cassini that Enceladus undergoes large back-and-forth oscillations, called libration, during its orbit. Enceladus’ libration would be much smaller if its crust was thicker. As for Dione, the new study finds it harbors a deep ocean between its crust and core. “Like Enceladus, Dione librates but below the detection level of Cassini,” said Antony Trinh, co-author of the new study. “A future orbiter hopping around Saturn’s moons could test this prediction.”

Dione’s ocean has probably survived for the whole history of the moon, and thus offers a long-lived habitable zone for microbial life. “The contact between the ocean and the rocky core is crucial,” said Attilio Rivoldini, co-author of the study. “Rock-water interactions provide key nutrients and a source of energy, both being essential ingredients for life.” The ocean of Dione seems to be too deep for easy access, but Enceladus as well as Jupiter’s moon Europa are generous enough to eject water samples in space, ready to be picked up by a passing spacecraft.

The club of “ocean worlds” — icy moons or planets with subsurface oceans in common parlance — gains new members with each new mission to the outer solar system. Three ocean worlds orbit Jupiter, three orbit Saturn and Pluto could also belong to the club, according to recent observations of the New Horizons spacecraft. The approach to modeling planetary bodies used in this study is a promising tool to study these worlds if we can measure their shape and gravity field, according to Mikael Beuthe. “Future missions will visit Jupiter’s moons, but we should also explore Uranus’ and Neptune’s systems,” he said.