New NASA Study Improves Search For Habitable Worlds

New NASA research is helping to refine our understanding of candidate planets beyond our solar system that might support life.

“Using a model that more realistically simulates atmospheric conditions, we discovered a new process that controls the habitability of exoplanets and will guide us in identifying candidates for further study,” said Yuka Fujii of NASA’s Goddard Institute for Space Studies (GISS), New York, New York and the Earth-Life Science Institute at the Tokyo Institute of Technology, Japan, lead author of a paper on the research published in the Astrophysical Journal Oct. 17.

Previous models simulated atmospheric conditions along one dimension, the vertical. Like some other recent habitability studies, the new research used a model that calculates conditions in all three dimensions, allowing the team to simulate the circulation of the atmosphere and the special features of that circulation, which one-dimensional models cannot do. The new work will help astronomers allocate scarce observing time to the most promising candidates for habitability.

Liquid water is necessary for life as we know it, so the surface of an alien world (e.g. an exoplanet) is considered potentially habitable if its temperature allows liquid water to be present for sufficient time (billions of years) to allow life to thrive. If the exoplanet is too far from its parent star, it will be too cold, and its oceans will freeze. If the exoplanet is too close, light from the star will be too intense, and its oceans will eventually evaporate and be lost to space. This happens when water vapor rises to a layer in the upper atmosphere called the stratosphere and gets broken into its elemental components (hydrogen and oxygen) by ultraviolet light from the star. The extremely light hydrogen atoms can then escape to space. Planets in the process of losing their oceans this way are said to have entered a “moist greenhouse” state because of their humid stratospheres.

In order for water vapor to rise to the stratosphere, previous models predicted that long-term surface temperatures had to be greater than anything experienced on Earth – over 150 degrees Fahrenheit (66 degrees Celsius). These temperatures would power intense convective storms; however, it turns out that these storms aren’t the reason water reaches the stratosphere for slowly rotating planets entering a moist greenhouse state.

“We found an important role for the type of radiation a star emits and the effect it has on the atmospheric circulation of an exoplanet in making the moist greenhouse state,” said Fujii. For exoplanets orbiting close to their parent stars, a star’s gravity will be strong enough to slow a planet’s rotation. This may cause it to become tidally locked, with one side always facing the star – giving it eternal day – and one side always facing away -giving it eternal night.

When this happens, thick clouds form on the dayside of the planet and act like a sun umbrella to shield the surface from much of the starlight. While this could keep the planet cool and prevent water vapor from rising, the team found that the amount of near-Infrared radiation (NIR) from a star could provide the heat needed to cause a planet to enter the moist greenhouse state. NIR is a type of light invisible to the human eye. Water as vapor in air and water droplets or ice crystals in clouds strongly absorbs NIR light, warming the air. As the air warms, it rises, carrying the water up into the stratosphere where it creates the moist greenhouse.

This process is especially relevant for planets around low-mass stars that are cooler and much dimmer than the Sun. To be habitable, planets must be much closer to these stars than our Earth is to the Sun. At such close range, these planets likely experience strong tides from their star, making them rotate slowly. Also, the cooler a star is, the more NIR it emits. The new model demonstrated that since these stars emit the bulk of their light at NIR wavelengths, a moist greenhouse state will result even in conditions comparable to or somewhat warmer than Earth’s tropics. For exoplanets closer to their stars, the team found that the NIR-driven process increased moisture in the stratosphere gradually. So, it’s possible, contrary to old model predictions, that an exoplanet closer to its parent star could remain habitable.

This is an important observation for astronomers searching for habitable worlds, since low-mass stars are the most common in the galaxy. Their sheer numbers increase the odds that a habitable world may be found among them, and their small size increases the chance to detect planetary signals.

The new work will help astronomers screen the most promising candidates in the search for planets that could support life. “As long as we know the temperature of the star, we can estimate whether planets close to their stars have the potential to be in the moist greenhouse state,” said Anthony Del Genio of GISS, a co-author of the paper. “Current technology will be pushed to the limit to detect small amounts of water vapor in an exoplanet’s atmosphere. If there is enough water to be detected, it probably means that planet is in the moist greenhouse state.”

In this study, researchers assumed a planet with an atmosphere like Earth, but entirely covered by oceans. These assumptions allowed the team to clearly see how changing the orbital distance and type of stellar radiation affected the amount of water vapor in the stratosphere. In the future, the team plans to vary planetary characteristics such as gravity, size, atmospheric composition, and surface pressure to see how they affect water vapor circulation and habitability.

Dawn Mission Extended At Ceres

NASA has authorized a second extension of the Dawn mission at Ceres, the largest object in the asteroid belt between Mars and Jupiter. During this extension, the spacecraft will descend to lower altitudes than ever before at the dwarf planet, which it has been orbiting since March 2015. The spacecraft will continue at Ceres for the remainder of its science investigation and will remain in a stable orbit indefinitely after its hydrazine fuel runs out.

The Dawn flight team is studying ways to maneuver Dawn into a new elliptical orbit, which may take the spacecraft to less than 120 miles (200 kilometers) from the surface of Ceres at closest approach. Previously, Dawn’s lowest altitude was 240 miles (385 kilometers).

A priority of the second Ceres mission extension is collecting data with Dawn’s gamma ray and neutron spectrometer, which measures the number and energy of gamma rays and neutrons. This information is important for understanding the composition of Ceres’ uppermost layer and how much ice it contains.

The spacecraft also will take visible-light images of Ceres’ surface geology with its camera, as well as measurements of Ceres’ mineralogy with its visible and infrared mapping spectrometer.

The extended mission at Ceres additionally allows Dawn to be in orbit while the dwarf planet goes through perihelion, its closest approach to the Sun, which will occur in April 2018. At closer proximity to the Sun, more ice on Ceres’ surface may turn to water vapor, which may in turn contribute to the weak transient atmosphere detected by the European Space Agency’s Herschel Space Observatory before Dawn’s arrival. Building on Dawn’s findings, the team has hypothesized that water vapor may be produced in part from energetic particles from the Sun interacting with ice in Ceres’ shallow surface.Scientists will combine data from ground-based observatories with Dawn’s observations to further study these phenomena as Ceres approaches perihelion.

The Dawn team is currently refining its plans for this next and final chapter of the mission. Because of its commitment to protect Ceres from Earthly contamination, Dawn will not land or crash into Ceres. Instead, it will carry out as much science as it can in its final planned orbit, where it will stay even after it can no longer communicate with Earth. Mission planners estimate the spacecraft can continue operating until the second half of 2018.

Dawn is the only mission ever to orbit two extraterrestrial targets. It orbited giant asteroid Vesta for 14 months from 2011 to 2012, then continued on to Ceres, where it has been in orbit since March 2015.

MAVEN Mission Finds Mars Has A Twisted Tail

Mars has an invisible magnetic “tail” that is twisted by interaction with the solar wind, according to new research using data from NASA’s MAVEN spacecraft.

NASA’s Mars Atmosphere and Volatile Evolution Mission (MAVEN) spacecraft is in orbit around Mars gathering data on how the Red Planet lost much of its atmosphere and water, transforming from a world that could have supported life billions of years ago into a cold and inhospitable place today. The process that creates the twisted tail could also allow some of Mars’ already thin atmosphere to escape to space, according to the research team.

“We found that Mars’ magnetic tail, or magnetotail, is unique in the solar system,” said Gina DiBraccio of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “It’s not like the magnetotail found at Venus, a planet with no magnetic field of its own, nor is it like Earth’s, which is surrounded by its own internally generated magnetic field. Instead, it is a hybrid between the two.” DiBraccio is project scientist for MAVEN and is presenting this research at a press briefing Thursday, Oct. 19 at 12:15pm MDT during the 49th annual meeting of the American Astronomical Society’s Division for Planetary Sciences in Provo, Utah.

The team found that a process called “magnetic reconnection” must have a big role in creating the Martian magnetotail because, if reconnection were occurring, it would put the twist in the tail.

“Our model predicted that magnetic reconnection will cause the Martian magnetotail to twist 45 degrees from what’s expected based on the direction of the magnetic field carried by the solar wind,” said DiBraccio. “When we compared those predictions to MAVEN data on the directions of the Martian and solar wind magnetic fields, they were in very good agreement.”

Mars lost its global magnetic field billions of years ago and now just has remnant “fossil” magnetic fields embedded in certain regions of its surface. According to the new work, Mars’ magnetotail is formed when magnetic fields carried by the solar wind join with the magnetic fields embedded in the Martian surface in a process called magnetic reconnection. The solar wind is a stream of electrically conducting gas continuously blowing from the Sun’s surface into space at about one million miles (1.6 million kilometers) per hour. It carries magnetic fields from the Sun with it. If the solar wind field happens to be oriented in the opposite direction to a field in the Martian surface, the two fields join together in magnetic reconnection.

The magnetic reconnection process also might propel some of Mars’ atmosphere into space. Mars’ upper atmosphere has electrically charged particles (ions). Ions respond to electric and magnetic forces and flow along magnetic field lines. Since the Martian magnetotail is formed by linking surface magnetic fields to solar wind fields, ions in the Martian upper atmosphere have a pathway to space if they flow down the magnetotail. Like a stretched rubber band suddenly snapping to a new shape, magnetic reconnection also releases energy, which could actively propel ions in the Martian atmosphere down the magnetotail into space.

Since Mars has a patchwork of surface magnetic fields, scientists had suspected that the Martian magnetotail would be a complex hybrid between that of a planet with no magnetic field at all and that found behind a planet with a global magnetic field. Extensive MAVEN data on the Martian magnetic field allowed the team to be the first to confirm this. MAVEN’s orbit continually changes its orientation with respect to the Sun, allowing measurements to be made covering all of the regions surrounding Mars and building up a map of the magnetotail and its interaction with the solar wind.

Magnetic fields are invisible but their direction and strength can be measured by the magnetometer instrument on MAVEN, which the team used to make the observations. They plan to examine data from other instruments on MAVEN to see if escaping particles map to the same regions where they see reconnected magnetic fields to confirm that reconnection is contributing to Martian atmospheric loss and determine how significant it is. They also will gather more magnetometer data over the next few years to see how the various surface magnetic fields affect the tail as Mars rotates. This rotation, coupled with an ever-changing solar wind magnetic field, creates an extremely dynamic Martian magnetotail. “Mars is really complicated but really interesting at the same time,” said DiBraccio.

Samples Brought Back From Asteroid Reveal ‘Rubble Pile’ Had A Violent Past

Curtin University planetary scientists have shed some light on the evolution of asteroids, which may help prevent future collisions of an incoming ‘rubble pile’ asteroid with Earth.

The scientists studied two incredibly small particles brought back to Earth from the asteroid Itokawa, after they were collected in 2005 from the surface of the 500 metre-wide asteroid, by the Japanese Hayabusa spacecraft.

The capsule and its precious cargo returned to Earth in 2010, landing near Woomera, Australia with only about 1500 asteroid dust particles on board – most of them much smaller than the width of a human hair.

The Geology-published research, “Collisional history of asteroid Itokawa,” used the Argon-Argon dating technique to investigate when impact crater events happened on Itokawa, offering a glimpse into the asteroid’s impact history.

Lead author of the study, Associate Professor Fred Jourdan from the Department of Applied Geology within the Curtin WA School of Mines, explained Itokawa was no ordinary asteroid, with fly-by pictures taken by Hayabusa prior to sampling in 2005 showing it had a peanut-like shape and resembled a rubble pile of boulders and dust more than solid rock.

“In fact, analyses by Japanese scientists revealed the asteroid had a violent past. Prior to being a rubble pile, Itokawa was part of a much larger asteroid that was destroyed by a collision with another asteroid. Our job was to try to find out when that collision happened,” Dr. Jourdan said.

Dr. Jourdan explained that the analyses were not without challenges, due to the extremely small size of the particles.

“Using our noble gas mass spectrometer at Curtin University, a revolutionary new machine that we customised for extra-terrestrial samples, we were able to measure tiny amounts of gas and analyse these fragments from Itokawa,” Dr. Jourdan said.

“The impact-shocked particle indicated a small-scale collision that occurred 2.1 billion years ago, whereas the other non-shocked particle preserves a very old age, similar to the formation age of the solar system itself.”
According to these results and a series of models, the scientists concluded that asteroids do not always break up due to a single cataclysmic impact. Instead, they can internally fragment due to the medium-sized collisions that constantly batter large asteroids until they shatter from impact.

“The final impact could be seen as ‘the straw that broke the camel’s back’,” Dr. Jourdan said.

“Our results tell us that Itokawa was already broken and re-assembled as a rubble pile about 2.1 billion years ago, showing that ‘rubble pile’ asteroids can survive a much longer time in this state than researchers previously thought.

“This is due to their cushion-like nature and the abundance of dust in between the boulders.”

He continued to explain these research results are not only important to understand how our solar system works, but can inform us on the best way to prevent any future collisions of an incoming ‘rubble pile’ asteroid with Earth.

Due to the success of the team’s study, they have been awarded four new particles from Itokawa, and will now look for more information to be unlocked from this asteroid.

Manager of the Curtin Argon-Argon Laboratory Ms Celia Mayers said the team plans to work on samples from the Hayabusa 2 mission, which is on its way to Asteroid Ryugu, and is anticipated to bring back samples in 2020.

“We also recently set up a collaboration with China that plans to bring back samples from the moon in a few years,” Ms Mayers said.

Dr. Jourdan and his colleagues at Curtin University conducted their research at the John de Laeter Centre.

Spinning Comet Observed To Rapidly Slow Down During Close Approach To Earth

Astronomers at Lowell Observatory observed comet 41P/Tuttle-Giacobini-Kresak last spring and noticed that the speed of its rotation was quickly slowing down. A research team led by David Schleicher studied the comet while it was closer to the Earth than it has ever been since its discovery. The comet rotational period became twice as long, going from 24 to more than 48 hours within six weeks, a far greater change than ever observed before in a comet. If it continues to slow down, it might stop completely and then begin rotating in the opposite direction.

Comet 41P/Tuttle-Giacobini-Kresak is a short period comet that now completes an orbit around the Sun every 5.4 years. First discovered by H. Tuttle in 1858, it was lost for years until is was rediscovered by M. Giacobini in 1907. Lost again and rediscovered for a third time in 1951 by K. Kresak, now the comet holds the names of its three independent discoverers.

Astronomers had a hard time studying this comet in detail until early 2017 when it passed within 13 million miles (21 million kilometers) from Earth, the closest since its discovery.With a relatively inactive nucleus estimated to be less than one mile in size (about 1.4 km), this comet was finally sufficiently bright for an extensive observing campaign.

During eight weeks between March and May of this year, the comet remained at a distance of less than 18 million miles (30 million kilometers) from Earth. In comparison, the distance between the Sun and the Earth is 93 million miles. These conditions allowed astronomers to study it in great detail.

Remnants from the formation of the Solar System, comets have changed very little during the past 4.5 billion years. As a comet gets closer to the Sun and the ice on its surface vaporizes, it develops gas and dust jets thousands of miles in length that ultimately create the coma or head, and the tail that distinguish comets from asteroids and other celestial bodies. One of the most common gases found in comets is the cyanogen radical, a molecule composed of one carbon atom and one nitrogen atom.

Schleicher and his collaborators used Lowell Observatory’s Discovery Channel Telescope, together with the Hall telescope and the Robotic telescope located on Anderson Mesa near Flagstaff, Arizona. They found and measured the motion of two cyanogen jets coming from comet 41P/Tuttle-Giacobini-Kresak. From these jets, they determined that the rotation period changed from 24 hours in March to 48 hour in late April, slowing down to less than half the rotation speed by the end of the observing campaign in May.

“While we expected to observe cyanogen jets and be able to determine the rotation period, we did not anticipate detecting a change in the rotation period in such a short time interval. It turned out to be the largest change in the rotational period ever measured, more than a factor of ten greater than found in any other comet,” said Schleicher, who lead the project.

This result also implies that the comet has a very elongated shape, a low density, and that the jets are located near the very end of its body, providing the torque needed to produce the observed change in rotation.
“If future observations can accurately measure the dimensions of the nucleus, then the observed rotation period change would set limits on the comet’s density and internal strength. Such detailed knowledge of a comet is usually only obtained by a dedicated spacecraft mission like the recently completed Rosetta mission to comet 67P/Churyumov-Gerasimenko,” said collaborator Matthew Knight.

Looking to the past on the other hand, brings another possible scenario. If the comet behaved similarly on previous orbits, it could have been rotating so fast that the nucleus might have broken, allowing more gas to escape and causing an increase in brightness for a short period of time. Such an outburst was observed in 2001.

The preliminary results were presented during the 49th Meeting of the American Astronomical Society Division for Planetary Sciences held in Provo, Utah. The full team consists of David Schleicher from Lowell Observatory, Nora Eisner from the University of Sheffield, Matthew Knight from the University of Maryland, and Audrey Thirouin also from Lowell Observatory.

Hubble Observes Source Of Gravitational Waves For The First Time

The NASA/ESA Hubble Space Telescope has observed for the first time the source of a gravitational wave, created by the merger of two neutron stars. This merger created a kilonova — an object predicted by theory decades ago — that ejects heavy elements such as gold and platinum into space. This event also provides the strongest evidence yet that short duration gamma-ray bursts are caused by mergers of neutron stars. This discovery is the first glimpse of multi-messenger astronomy, bringing together both gravitational waves and electromagnetic radiation.

On 17 August 2017 the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo Interferometer both alerted astronomical observers all over the globe about the detection of a gravitational wave event named GW170817 . About two seconds after the detection of the gravitational wave, ESA’s INTEGRAL telescope and NASA’s Fermi Gamma-ray Space Telescope observed a short gamma-ray burst in the same direction.

In the night following the initial discovery, a fleet of telescopes started their hunt to locate the source of the event. Astronomers found it in the lenticular galaxy NGC 4993, about 130 million light-years away. A point of light was shining where nothing was visible before and this set off one of the largest multi-telescope observing campaigns ever — among these telescopes was the NASA/ESA Hubble Space Telescope.

Several different teams of scientists used Hubble over the two weeks following the gravitational wave event alert to observe NGC 4993. Using Hubble’s high-resolution imaging capabilities they managed to get the first observational proof for a kilonova, the visible counterpart of the merging of two extremely dense objects — most likely two neutron stars. Such mergers were first suggested more than 30 years ago but this marks the first firm observation of such an event. The distance to the merger makes the source both the closest gravitational wave event detected so far and also one of the closest gamma-ray burst sources ever seen.

“Once I saw that there had been a trigger from LIGO and Virgo at the same time as a gamma-ray burst I was blown away,” recalls Andrew Levan of the University of Warwick, who led the Hubble team that obtained the first observations. “When I realised that it looked like neutron stars were involved, I was even more amazed. We’ve been waiting a long time for an opportunity like this!”

Hubble captured images of the galaxy in visible and infrared light, witnessing a new bright object within NGC 4993 that was brighter than a nova but fainter than a supernova. The images showed that the object faded noticeably over the six days of the Hubble observations. Using Hubble’s spectroscopic capabilities the teams also found indications of material being ejected by the kilonova as fast as one-fifth of the speed of light.

“It was surprising just how closely the behaviour of the kilonova matched the predictions,” said Nial Tanvir, professor at the University of Leicester and leader of another Hubble observing team. “It looked nothing like known supernovae, which this object could have been, and so confidence was soon very high that this was the real deal.”

Connecting kilonovae and short gamma-ray bursts to neutron star mergers has so far been difficult, but the multitude of detailed observations following the detection of the gravitational wave event GW170817 has now finally verified these connections.

“The spectrum of the kilonova looked exactly like how theoretical physicists had predicted the outcome of the merger of two neutron stars would appear,” says Levan. “It ties this object to the gravitational wave source beyond all reasonable doubt.”

The infrared spectra taken with Hubble also showed several broad bumps and wiggles that signal the formation of some of the heaviest elements in nature. These observations may help solve another long-standing question in astronomy: the origin of heavy chemical elements, like gold and platinum. In the merger of two neutron stars, the conditions appear just right for their production.

The implications of these observations are immense. As Tanvir explains: “This discovery has opened up a new approach to astronomical research, where we combine information from both electromagnetic light and from gravitational waves. We call this multi-messenger astronomy — but until now it has just been a dream!”

Levan concludes: “Now, astronomers won’t just look at the light from an object, as we’ve done for hundreds of years, but also listen to it. Gravitational waves provide us with complementary information from objects which are very hard to study using only electromagnetic waves. So pairing gravitational waves with electromagnetic radiation will help astronomers understand some of the most extreme events in the Universe.”

Solar Eruptions Could Electrify Martian Moons

Powerful solar eruptions could electrically charge areas of the Martian moon Phobos to hundreds of volts, presenting a complex electrical environment that could possibly affect sensitive electronics carried by future robotic explorers, according to a new NASA study. The study also considered electrical charges that could develop as astronauts transit the surface on potential human missions to Phobos.

Phobos has been considered as a possible initial base for human exploration of Mars because its weak gravity makes it easier to land spacecraft, astronauts and supplies. The idea would be to have the astronauts control robots on the Martian surface from the moons of Mars, without the considerable time delay faced by Earth-based operators. “We found that astronauts or rovers could accumulate significant electric charges when traversing the night side of Phobos — the side facing Mars during the Martian day,” said William Farrell of NASA’s Goddard Space Flight Center, Greenbelt, Maryland. “While we don’t expect these charges to be large enough to injure an astronaut, they are potentially large enough to affect sensitive equipment, so we would need to design spacesuits and equipment that minimizes any charging hazard.” Farrell is lead author of a paper on this research published online Oct. 3 in Advances in Space Research.

Mars has two small moons, Phobos and Deimos. Although this study focused on Phobos, similar conditions are expected at Deimos, since both moons have no atmosphere and are directly exposed to the solar wind — a stream of electrically conducting gas, called a plasma, that’s constantly blowing off the surface of the Sun into space at around a million miles per hour.

The solar wind is responsible for these charging effects. When the solar wind strikes the day side of Phobos, the plasma is absorbed by the surface. This creates a void on the night side of Phobos that the plasma flow is obstructed from directly entering. However, the composition of the wind — made of two types of electrically charged particles, namely ions and electrons — affects the flow. The electrons are over a thousand times lighter than the ions. “The electrons act like fighter jets — they are able to turn quickly around an obstacle — and the ions are like big, heavy bombers — they change direction slowly,” said Farrell. “This means the light electrons push in ahead of the heavy ions and the resulting electric field forces the ions into the plasma void behind Phobos, according to our models.”

The study shows that this plasma void behind Phobos may create a situation where astronauts and rovers build up significant electric charges. For example, if astronauts were to walk across the night-side surface, friction could transfer charge from the dust and rock on the surface to their spacesuits. This dust and rock is a very poor conductor of electricity, so the charge can’t flow back easily into the surface — and charge starts to build up on the spacesuits. On the day side, the electrically conducting solar wind and solar ultraviolet radiation can remove the excess charge on the suit. But, on the night side, the ion and electron densities in the trailing plasma void are so low they cannot compensate or ‘dissipate’ the charge build-up. The team’s calculations revealed that this static charge can reach ten thousand volts in some materials, like the Teflon suits used in the Apollo lunar missions. If the astronaut then touches something conductive, like a piece of equipment, this could release the charge, possibly similar to the discharge you get when you shuffle across a carpet and touch a metal door handle.

The team modeled the flow of the solar wind around Phobos and calculated the buildup of charge on the night side, as well as in obstructed regions in shadow, like Stickney crater, the largest crater on Phobos. “We found that excess charge builds up in these regions during all solar wind conditions, but the charging effect was especially severe in the wake of solar eruptions like coronal mass ejections, which are dense, fast gusts of solar wind,” said Farrell.

This study was a follow-up to earlier studies that revealed the charging effects of solar wind in shadowed craters on Earth’s Moon and near-Earth asteroids. Some conditions on Phobos are different than those in the earlier studies. For example, Phobos gets immersed in the plasma flowing behind Mars because it orbits Mars much closer than the Moon orbits Earth. The plasma flow behind Mars’ orbit was modeled as well.

The research was funded by Goddard’s Dynamic Response of the Environment at Asteroids, the Moon, and moons of Mars (DREAM2) center, as well as the Solar System Exploration Research Virtual Institute (SSERVI), based and managed at NASA’s Ames Research Center in Moffett Field, California.

SSERVI is a virtual institute that, together with international partnerships, brings science and exploration researchers together in a collaborative virtual setting. SSERVI is funded by the Science Mission Directorate and Human Exploration and Operations Mission Directorate at NASA Headquarters in Washington.