Left: Ozone molecules in a planet's atmosphere could indicate biological activity, but ozone, carbon dioxide and carbon monoxide -- without methane, is likely a false positive. Right: Ozone, oxygen, carbon dioxide and methane -- without carbon monoxide, indicate a possible true positive.

NASA research gives guideline for future alien life search


Astronomers searching the atmospheres of alien worlds for gases that might be produced by life can't rely on the detection of just one type, such as oxygen, ozone, or methane, because in some cases these gases can be produced non-biologically, according to extensive simulations by researchers in the NASA Astrobiology Institute's Virtual Planetary Laboratory.

The researchers carefully simulated the atmospheric chemistry of alien worlds devoid of life thousands of times over a period of more than four years, varying the atmospheric compositions and star types. "When we ran these calculations, we found that in some cases, there was a significant amount of ozone that built up in the atmosphere, despite there not being any oxygen flowing into the atmosphere," said Shawn Domagal-Goldman of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "This has important implications for our future plans to look for life beyond Earth."

Methane is a carbon atom bound to four hydrogen atoms. On Earth, much of it is produced biologically (flatulent cows are a classic example), but it can also be made inorganically; for example, volcanoes at the bottom of the ocean can release the gas after it is produced by reactions of rocks with seawater.

Ozone and oxygen were previously thought to be stronger biosignatures on their own. Ozone is three atoms of oxygen bound together. On Earth, it is produced when molecular oxygen (two oxygen atoms) and atomic oxygen (a single oxygen atom) combine, after the atomic oxygen is created by other reactions powered by sunlight or lightning. Life is the dominant source of the molecular oxygen on our planet, as the gas is produced by photosynthesis in plants and microscopic, single-cell organisms. Because life dominates the production of oxygen, and oxygen is needed for ozone, both gases were thought to be relatively strong biosignatures. But this study demonstrated that both molecular oxygen and ozone can be made without life when ultraviolet light breaks apart carbon dioxide (a carbon atom bound to two oxygen atoms). Their research suggests this non-biological process could create enough ozone for it to be detectable across space, so the detection of ozone by itself would not be a definitive sign of life.

"However, our research strengthens the argument that methane and oxygen together, or methane and ozone together, are still strong signatures of life," said Domagal-Goldman. "We tried really, really hard to make false-positive signals for life, and we did find some, but only for oxygen, ozone, or methane by themselves." Domagal-Goldman and Antígona Segura from the Universidad Nacional Autónoma de México in Mexico City are lead authors of a paper about this research, along with astronomer Victoria Meadows, geologist Mark Claire, and Tyler Robison, an expert on what Earth would look like as an extrasolar planet. The paper appeared in the Astrophysical Journal Sept. 10, and is available online.

Methane and oxygen molecules together are a reliable sign of biological activity because methane doesn't last long in an atmosphere containing oxygen-bearing molecules. "It's like college students and pizza," says Domagal-Goldman. "If you see pizza in a room, and there are also college students in that room, chances are the pizza was freshly delivered, because the students will quickly eat the pizza. The same goes for methane and oxygen. If both are seen together in an atmosphere, the methane was freshly delivered because the oxygen will be part of a network of reactions that will consume the methane. You know the methane is being replenished. The best way to replenish methane in the presence of oxygen is with life. The opposite is true, as well. In order to keep the oxygen around in an atmosphere that has a lot of methane, you have to replenish the oxygen, and the best way to do that is with life."

Scientists have used computer models to simulate the atmospheric chemistry on planets beyond our solar system (exoplanets) before, and the team used a similar model in its research. However, the researchers also developed a program to automatically compute the calculations thousands of times, so they could see the results with a wider range of atmospheric compositions and star types.

In doing these simulations, the team made sure they balanced the reactions that could put oxygen molecules in the atmosphere with the reactions that might remove them from the atmosphere. For example, oxygen can react with iron on the surface of a planet to make iron oxides; this is what gives most red rocks their color. A similar process has colored the dust on Mars, giving the Red Planet its distinctive hue. Calculating the appearance of a balanced atmosphere is important because this balance would allow the atmosphere to persist for geological time scales. Given that planetary lifetimes are measured in billions of years, it's unlikely astronomers will happen by chance to be observing a planet during a temporary surge of oxygen or methane lasting just thousands or even millions of years.

It was important to make the calculations for a wide variety of cases, because the non-biological production of oxygen is subject to both the atmospheric and stellar environment of the planet. If there are a lot of gases that consume oxygen, such as methane or hydrogen, then any oxygen or ozone produced will be destroyed in the atmosphere. However, if the amount of oxygen-consuming gases is vanishingly small, the oxygen and the ozone might stick around for a while. Likewise, the production and destruction of oxygen, ozone, and methane is driven by chemical reactions powered by light, making the type of star important to consider as well. Different types of stars produce the majority of their light at specific colors. For example, massive, hot stars or stars with frequent explosive activity produce more ultraviolet light. "If there is more ultraviolet light hitting the atmosphere, it will drive these photochemical reactions more efficiently," said Domagal-Goldman. "More specifically, different colors (or wavelengths) of ultraviolet light can affect oxygen and ozone production and destruction in different ways."

Astronomers detect molecules in exoplanet atmospheres by measuring the colors of light from the star the exoplanet is orbiting. As this light passes through the exoplanet's atmosphere, some of it is absorbed by atmospheric molecules. Different molecules absorb different colors of light, so astronomers use these absorption features as unique "signatures" of the type and quantity of molecules present.

"One of the main challenges in identifying life signatures is to distinguish between the products of life and those compounds generated by geological processes or chemical reactions in the atmosphere. For that we need to understand not only how life may change a planet but how planets work and the characteristics of the stars that host such worlds", said Segura.

The team plans to use this research to make recommendations about the requirements for future space telescopes designed to search exoplanet atmospheres for signs of alien life. "Context is key – we can't just look for oxygen, ozone, or methane alone," says Domagal-Goldman. "To confirm life is making oxygen or ozone, you need to expand your wavelength range to include methane absorption features. Ideally, you'd also measure other gases like carbon dioxide and carbon monoxide [a molecule with one carbon atom and one oxygen atom]. So we're thinking very carefully about the issues that could trip us up and give a false-positive signal, and the good news is by identifying them, we can create a good path to avoid the issues false positives could cause. We now know which measurements we need to make. The next step is figuring out what we need to build and how to build it."


NASA has selected four ideas from the public for innovative uses of climate projections and Earth-observing satellite data. The agency also has announced a follow-on challenge with awards of $50,000 to build climate applications based on OpenNEX data on the Amazon cloud computing platform.

Both challenges use the Open NASA Earth Exchange, or OpenNEX, a data, cloud computing, and knowledge platform where users can share modeling and analysis codes, scientific results, information and expertise to solve big data challenges in the Earth sciences. OpenNEX provides users a large collection of climate and Earth science satellite data sets, including global land surface images, vegetation conditions, climate observations and climate projections.

The four winners of the "ideation" stage of the OpenNEX challenge, which ran from July 1 through Aug. 1, will share a $10,000 award for their ideas on novel uses of the datasets. Abdal Elhassani of Indiana University, Bloomington, proposed an app to predict how plant hardiness zones will change in the future with a changing climate. Edward Aboufadel of Grand Valley State University, Allendale, Michigan, suggested using the data to compare a local community's future predicted climate with the historical record of another community.

A team led by Raymond Milowski of San Francisco proposed converting the storehouse of OpenNEX climate model data to formats compatible with the Open Web Platform to facilitate wider use by web developers. Reuben Cummings from Peoria, Illinois, suggested a web application to map potential and actual climate-related environmental hazards such as wildfires, flood, and drought across the United States.

"The ideas generated by this OpenNEX challenge demonstrate the value of these NASA data assets when put in the hands of citizen scientists," said Ramakrishna Nemani, principal scientist for the NEX project at NASA's Ames Research Center in Moffett Field, California. "Our second challenge seeks to rapidly turn these ideas into practical applications."

The second "builder" challenge that opens Friday offers awards for the development of an application or algorithm that communicates climate change impacts to the general public using the OpenNEX data. Submissions based on the winning proposals in the "ideation" challenge are encouraged, in addition to new ideas that focus on climate change impacts.

Applications should communicate through concise summaries of impacts over time that can be easily related to familiar climate-related events and processes. The summaries may rely on key climatic events or observable events dependent on climate, such as changes in the timing of snow melt and runoff, plant flowering and the start of the allergy season, and the annual migration of birds. Developers are not limited to these examples, and are encouraged to consider solutions that incorporate other scientifically-based climate summaries and analogs.

"NASA is committed to engaging and enabling individuals and groups to make use of these high-quality scientific data and innovative technologies to better communicate climate change impacts to the general public," said Tsengdar Lee, program manager in the Earth Science Division of the Science Mission Directorate at NASA Headquarters in Washington.

Entries are due by Oct. 21 and NASA plans to announce the winners on Dec. 15.

NASA's OpenNEX challenge ties in to a number of White House initiatives, including Open Data, Big Data and Climate Data. These initiatives advance national goals to address climate change impacts on economic growth, health and livelihood, and include the use of competitions and challenges to foster regional innovation.

The challenges are managed by NASA's Center of Excellence for Collaborative Innovation. The center was established in coordination with the White House Office of Science and Technology Policy to advance NASA's open innovation efforts and extend that expertise to other federal agencies. The challenges are released on the NASA Innovation Pavilion, one of the center's platforms available to NASA team members, through its contract with InnoCentive, Inc.

To educate citizen scientists about how the data on OpenNEX can be used, NASA has created a series of online video lectures and hands-on lab modules. To view this material, and for information on registering for the challenges, visit: https://nex.nasa.gov/OpenNEX

OpenNEX is hosted on the Amazon Web Services (AWS) cloud and available to the public through a Space Act Agreement. Challenge developers are eligible for credits on the AWS platform to build their applications.

NASA monitors Earth's vital signs from land, air and space with a fleet of satellites and ambitious airborne and ground-based observation campaigns. NASA develops new ways to observe and study Earth's interconnected natural systems with long-term data records and computer analysis tools to better see how our planet is changing. The agency shares this unique knowledge with the global community and works with institutions in the United States and around the world that contribute to understanding and protecting our home planet.

For more information about NASA's Earth science activities in 2014, visit: http://www.nasa.gov/earthrightnow

Astronomers using NASA's Hubble Space Telescope have applied a new image processing technique to obtain near-infrared scattered light photos of five disks observed around young stars in the Mikulski Archive for Space Telescopes database. These disks are telltale evidence for newly formed planets.

If astronomers initially miss something in their review of data, they can make new discoveries by revisiting earlier data with new image processing techniques, thanks to the wealth of information stored in the Hubble data archive. This is what Rémi Soummer, of the Space Telescope Science Institute (STScI) in Baltimore, Md., and his team recently did while on a hunt for hidden Hubble treasures.

The stars in question initially were targeted with Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) based on unusual heat signatures obtained from NASA's Spitzer Space Telescope and the Infrared Astronomical Satellite that flew in 1983. The previous data provided interesting clues that dusty disks could exist around these stars. Small dust particles in the disks might scatter light and therefore make the disks visible. But when Hubble first viewed the stars between 1999 and 2006, no disks were detected in the NICMOS pictures.

Recently, with improvements in image processing, including algorithms used for face-recognition software, Soummer and his team reanalyzed the archived images. This time, they could unequivocally see the debris disks and even determine their shapes.

The NICMOS instrument, which began collecting data in 1997, has been so cutting-edge that ground-based technology only now is beginning to match its power. Because Hubble has been in operation for almost 24 years, it provides a long baseline of high-quality archival observations.

"Now, with such new technologies in image processing, we can go back to the archive and conduct research more precisely than previously possible with NICMOS data," said Dean Hines of STScI.

"These findings increase the number of debris disks seen in scattered light from 18 to 23. By significantly adding to the known population and by showing the variety of shapes in these new disks, Hubble can help astronomers learn more about how planetary systems form and evolve," said Soummer.

The dust in the disks is hypothesized to be produced by collisions between small planetary bodies such as asteroids. The debris disks are composed of dust particles formed from these grinding collisions. The tiniest particles are constantly blown outward by radiation pressure from the star. This means they must be replenished continuously though more collisions. This game of bumper cars was common in the solar system 4.5 billion years ago. Earth's moon and the satellite system around Pluto are considered to be collisional byproducts.

"One star that is particularly interesting is HD 141943," said Christine Chen, debris disk expert and team member. "It is an exact twin of our sun during the epoch of terrestrial planet formation in our own solar system."

Hubble found the star exhibits an asymmetrical, edge-on disk. This asymmetry could be evidence the disk is being gravitationally sculpted by the tug of one or more unseen planets.

"Being able to see these disks now also has let us plan further observations to study them in even more detail using other Hubble instruments and large telescopes on the ground," added Marshall Perrin of STScI.

"We also are working to implement the same techniques as a standard processing method for NASA's upcoming James Webb Space Telescope," said STScI teammate Laurent Pueyo. "These disks will also be prime targets for the Webb telescope."

Soummer's team has just begun its work. They next will search for structures in the disks that suggest the presence of planets.

Planets in eccentric orbits can experience powerful tidal forces. A planet covered by a very thick ice shell (left) is springy enough to flex a great deal, generating a lot of internal friction and heat. Some terrestrial planets (right) also will flex, especially with partially molten inner layers.  Credit: NASA's Goddard Space Flight Center

As anybody who has started a campfire by rubbing sticks knows, friction generates heat. Now, supercomputer modeling by NASA scientists shows that friction could be the key to survival for some distant Earth-sized planets traveling in dangerous orbits.

The findings are consistent with observations that Earth-sized planets appear to be very common in other star systems. Although heat can be a destructive force for some planets, the right amount of friction, and therefore heat, can be helpful and perhaps create conditions for habitability.

"We found some unexpected good news for planets in vulnerable orbits," said Wade Henning, a University of Maryland scientist working at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the new study. "It turns out these planets will often experience just enough friction to move them out of harm's way and into safer, more-circular orbits more quickly than previously predicted."

Simulations of young planetary systems indicate that giant planets often upset the orbits of smaller inner worlds. Even if those interactions aren't immediately catastrophic, they can leave a planet in a treacherous eccentric orbit – a very elliptical course that raises the odds of crossing paths with another body, being absorbed by the host star, or getting ejected from the system.

Another potential peril of a highly eccentric orbit is the amount of tidal stress a planet may undergo as it draws very close to its star and then retreats away. Near the star, the gravitational force is powerful enough to deform the planet, while in more distant reaches of the orbit, the planet can ease back into shape. This flexing action produces friction, which generates heat. In extreme cases, tidal stress can produce enough heat to liquefy the planet.

In this new study, available online in the July 1, 2014, issue of the Astrophysical Journal, Henning and his colleague Terry Hurford, a planetary scientist at Goddard, explored the effects of tidal stresses on planets that have multiple layers, such as rocky crust, mantle or iron core.

One conclusion of the study is that some planets could move into a safer orbit about 10 to 100 times faster than previously expected – in as a little as a few hundred thousand years, instead of the more typical rate of several million years. Such planets would be driven close to the point of melting or, at least, would have a nearly melted layer, similar to the one right below Earth's crust. Their interior temperatures could range from moderately warmer than our planet is today up to the point of having modest-sized magma oceans.

The transition to a circular orbit would be speedy because an almost-melted layer would flex easily, generating a lot of friction-induced heat. As the planet threw off that heat, it would lose energy at a fast rate and relax quickly into a circular orbit. (Later, tidal heating would turn off, and the planet's surface could become safe to walk on.)

In contrast, a world that had completely melted would be so fluid that it would produce little friction. Before this study, that is what researchers expected to happen to planets undergoing strong tidal stresses.

Cold, stiff planets tend to resist the tidal stress and release energy very slowly. In fact, Henning and Hurford found that many of them actually generate less friction than previously thought. This may be especially true for planets farther from their stars. If these worlds are not crowded by other bodies, they may be stable in their eccentric orbits for a long time.

"In this case, the longer, non-circular orbits could increase the 'habitable zone,' because the tidal stress will remain an energy source for longer periods of time," said Hurford. "This is great for dim stars or ice worlds with subsurface oceans."

Surprisingly, another way for a terrestrial planet to achieve high amounts of heating is to be covered in a very thick ice shell, similar to an extreme "snowball Earth." Although a sheet of ice is a slippery, low-friction surface, an ice layer thousands of miles thick would be very springy. A shell like this would have just the right properties to respond strongly to tidal stress, generating a lot of heat. (The high pressures inside these planets could prevent all but the topmost layers from turning into liquid water.)

The researchers found that the very responsive layers of ice or almost-melted material could be relatively thin, just a few hundred miles deep in some cases, yet still dominate the global behavior.

The team modeled planets that are the size of Earth and up to two-and-a-half times larger. Henning added that superEarths – planets at the high end of this size range – likely would experience stronger tidal stresses and potentially could benefit more from the resulting friction and heating.

Now that the researchers have shown the importance of the contributions of different layers of a planet, the next step is to investigate how layers of melted material flow and change over time.

Outdoor enthusiasts in Colorado's Front Range are occasionally rewarded with remarkable visibility brought about by dry, clear air and wind. But it's what people in the mountainous U.S. West can't see in conditions like this – ozone plunging down to the ground from high in the stratosphere, the second layer of the atmosphere – that has attracted the interest of NASA scientists, university scientists and air quality managers.

Ozone in the stratosphere, located on average 10 to 48 kilometers (6 to 30 miles) above the ground, typically stays in the stratosphere. Not on days like April 6, 2012.

On that day, a fast-moving area of low pressure moved northeast across states in the Western U.S., clipping western and northern Colorado. Ozone-rich stratospheric air descended, folding into tropospheric air near the ground. Winds took hold of the air mass and pushed it in all directions, bringing stratospheric ozone to the ground in Colorado and along the Northern Front Range. The event, called a stratospheric ozone intrusion, raised ground-level ozone concentrations in some areas to potentially unhealthy levels. Watch the intrusion unfold in a new NASA simulation of the event.

Ozone high in the atmosphere, in the stratosphere, forms naturally when sunlight mingles with oxygen molecules to form the well-known "layer" that protects life on Earth from the sun's harmful ultraviolet rays. That's in contrast to ozone near the ground, in the troposphere, which forms from complex reactions involving chemicals emitted from industrial processes, vehicle exhaust, and other byproducts of fossil fuel combustion. Ozone at ground level can damage lung tissue and pose an immediate threat to sensitive groups such as people with asthma.

For this reason, the Clean Air Act requires the U.S. Environmental Protection Agency to set a threshold for ground-level ozone, as outlined in the National Ambient Air Quality Standards. States that exceed this threshold can be fined, although the EPA can grant exceptions for natural events or those proven to be beyond reasonable control.

That's why ozone intrusions are on the minds of air quality managers like Patrick Reddy, lead forecast meteorologist at Colorado's Department of Public Health and Environment in Denver, Colo. Reddy co-leads the EPA Stratospheric Intrusion Work Group, tasked to identify ozone intrusion events and collect input for improved analysis.

The state of Colorado flagged the concentrations associated with the April 6 event as possibly exceeding the EPA's allowable threshold. Now it's up to Reddy and colleagues to determine if the intrusion on April 6 is a viable candidate for the preparation of documentation to be classified as an exceptional event.

"We need to use the best science that we can to demonstrate conclusively that 'but for' this intrusion there would not have been an exceedence," Reddy said.

Resolution Requirements

Reddy says it's fairly obvious when a stratospheric ozone intrusion has occurred, based on signatures in satellite data, air quality monitoring stations and meteorological data. For example, low water vapor, wind and high ozone at remote locations are often characteristic of stratospheric air.

Evidence of the intrusions, however, doesn't show up in the models currently used by air quality managers. Many of those models assume ozone moves from the stratosphere to the troposphere at a constant, average rate. This fails to capture the episodic intrusion events.

Meiyun Lin, an atmospheric scientist at Princeton University and NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, New Jersey, set out to better quantify the impact of stratospheric ozone intrusions. Lin and colleagues used satellite and meteorological observations alongside a global chemistry-climate model to simulate intrusions in high-resolution.

Like the pixels in a photograph, the resolution of a model refers to the size of three-dimensional boxes of atmosphere. Models simulate the chemistry and atmospheric processes inside each box. For perspective, a model with 200-kilometer (124-mile) resolution is typical of today's high-end climate models, and 25-kilometer (16-mile) resolution is typical of high-end weather forecasts.

"We absolutely need to use a model with a grid size at least as small as, or smaller than, 50-by-50 kilometers (31-by-31 miles) to look at where and when the stratospheric air reaches the surface," Lin said.

Lin's analysis, based on a GFDL model with 50-kilometer (31-mile) resolution, suggests that the impact on ground level ozone in the U.S. West from springtime intrusion events is two to three times greater than previously estimated. The study was published October 2012 in Journal of Geophysical Research.

Steven Pawson and Eric Nielsen, atmospheric scientists at NASA's Goddard Space Flight Center in Greenbelt, Md., are also in pursuit of improved model simulations of stratospheric ozone intrusions. The team set out to see if the Goddard Earth Observing System Model, Version 5 (GEOS-5) Chemistry-Climate Model could replicate stratospheric intrusions at 25-kilometer (16-mile) resolution.

They show that indeed, the model could replicate small-scale features, including finger-like filaments, within the apron of ozone-rich stratospheric air that descended over Colorado on April 6, 2012.

"High-resolution modeling is giving us the capability to examine these events comprehensively for the first time," Nielsen said.

High-resolution models are possible due to computing power now capable of simulating the chemistry and movement of gasses and pollutants around the atmosphere and calculating their interactions. The addition of chemistry to these models, however, is not without a computational cost. For example, a weather forecast that takes about one hour of computational time would take five hours to run at the same resolution with the chemistry included.

"For a long time people thought excluding stratospheric chemistry was a reasonable approximation to make," said Lesley Ott, an atmospheric scientist at NASA Goddard. "But recent work has shown that you really need to consider what the stratosphere is doing. It's not just something you can totally ignore, despite the fact that it's more computationally intensive."

Atmospheric measurements from the ground and from aircraft suggest the higher resolution models are on track. In June and July 2011, NASA aircraft flew at low altitude over the Baltimore-Washington area as part of DISCOVER-AQ, a NASA airborne campaign to study urban air quality. Comparing data from the aircraft with the model output, Ott says the models performed well.

Tying it Together

Scientists already know that intrusions reaching surface air are more frequent in spring and early summer, when chemistry and weather conditions are more favorable for such events. Also, intrusions are more likely to affect mountainous regions in the U.S. West simply because land at elevation is closer to the stratosphere.

The next step is to find out how the frequency of intrusions changes from year to year and what controls its variability. "This is really the first time that our models are giving us the chance to try to answer those questions," Ott said.

Reddy, too, looks forward to seeing if the models can streamline reporting and forecasting efforts. "The nice thing about the new model products is that they could help us potentially do a better job forecasting these events and documenting what happened for those events that we want to submit to the EPA," he said.

The models could also help Reddy as his agency works to refine and expand its services. Models that could more accurately focus the timing and scale of intrusion effects would enhance the state's ability to issue advisories that better target affected populations.

Does that mean that spring skiers will have an additional forecast to consider before heading to the slopes?

"In the West, don't be surprised if on a clean-looking, windy day in spring there's an ozone health advisory," Reddy said.

As for Bryan Duncan, an atmospheric scientist at NASA Goddard, "It wouldn't stop me from enjoying the powder conditions."

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