vendredi 17 novembre 2017

Taking a Spin on Plasma Space Tornadoes with NASA Observations












NASA - Magnetospheric Multiscale Mission (MMS) patch.

Nov. 17, 2017

Interplanetary space is hardly tranquil. High-energy charged particles from the Sun, as well as from beyond our solar system, constantly whizz by. These can damage satellites and endanger astronaut health — though, luckily for life on Earth, the planet is blanketed by a protective magnetic bubble created by its magnetic field. This bubble, called the magnetosphere, deflects most of the harmful high-energy particles.

Nevertheless, some sneak through — and at the forefront of figuring out just how this happens is NASA’s Magnetospheric Multiscale mission, or MMS. New results show that tornado-like swirls of space plasma create a boundary tumultuous enough to let particles slip into near Earth space.


Animation above: This simulation of the boundary shows how areas of low density plasma, shown by blue, mix with areas of higher density plasma, red, forming turbulent tornadoes of plasma. Animation Credits: NASA/Takuma Nakamura.

MMS, launched in 2015, uses four identical spacecraft flying in a pyramid formation to take a three-dimensional look at the magnetic environment around Earth. The mission studies how particles transfer into the magnetosphere by focusing on the causes and effects of magnetic reconnection — an explosive event where magnetic field lines cross, launching electrons and ions from the solar wind into the magnetosphere.

By combining observations from MMS with new 3-D computer simulations, scientists have been able to investigate the small-scale physics of what’s happening at our magnetosphere’s borders for the first time. The results, recently published in a paper in Nature Communications, are key for understanding how the solar wind sometimes enters Earth’s magnetosphere, where it can interfere with satellites and GPS communications.

Inside the magnetosphere, the density of the space plasma — charged particles, like electrons and ions — is much lower than the plasma outside, where the solar wind prevails. The boundary, called the

magnetopause, becomes unstable when the two different density regions move at different rates. Giant swirls, called Kelvin Helmholtz waves, form along the edge like crashing ocean waves. The once-smooth boundary becomes tangled and squeezed, forming plasma tornadoes, which act as portholes for the transportation of charged particles from the solar wind into the magnetosphere.


Image above: Kelvin-Helmholtz waves, with their classic surfer's wave shape, are found in nature wherever two fluids meet, such as in these clouds. Image Credits: Danny Ratcliffe.

Kelvin Helmholtz waves are found across the universe wherever two materials of different density move past one another. They can be seen in cloud formations around Earth and have even been observed in other planetary atmospheres in our solar system.

Using large-scale computer simulations of this mixing, performed at the Oak Ridge National Laboratory in Oak Ridge, Tennessee, on the Titan supercomputer, and comparing them to observations MMS took while passing through such a region in space, scientists were able to show that the tornadoes were extremely efficient at transporting charged particles — much more so than previously thought. The comparisons between the simulations and observations allowed the scientists to measure the exact dimensions of the tornadoes. They found these tornadoes to be both large and small — ones reaching 9,300 miles spawned smaller tornadoes 60 to 90 miles wide and over 125 miles long.

MMS recently moved into a new orbit, flying on the far side of Earth, away from the Sun. Here too, it will continue to study magnetic reconnection, but focus instead on how energy and particles interact within Earth’s magnetosphere, in the long trailing magnetotail. Understanding such fundamental processes in Earth’s neighborhood helps improve our situational awareness of the space that surrounds us — crucial information as it becomes ever more filled with satellites and communications systems we depend on.

Related Links:

Learn more about the Magnetospheric Multiscale Mission: https://www.nasa.gov/mission_pages/mms/index.html

Learn more about NASA’s research on the Sun-Earth environment: https://www.nasa.gov/mission_pages/sunearth/index.html

Animation (mentioned9, Image (mentioned), Text, Credits: NASA/Rob Garner.

Greetings, Orbiter.ch

Jovian Tempest












NASA - JUNO Mission logo.

Nov. 17, 2017


This color-enhanced image of a massive, raging storm in Jupiter’s northern hemisphere was captured by NASA’s Juno spacecraft during its ninth close flyby of the gas giant planet.

The image was taken on Oct. 24, 2017 at 10:32 a.m. PDT (1:32 p.m. EDT). At the time the image was taken, the spacecraft was about 6,281 miles (10,108 kilometers) from the tops of the clouds of Jupiter at a latitude of 41.84 degrees. The spatial scale in this image is 4.2 miles/pixel (6.7 kilometers/pixel).

The storm is rotating counter-clockwise with a wide range of cloud altitudes. The darker clouds are expected to be deeper in the atmosphere than the brightest clouds. Within some of the bright “arms” of this storm, smaller clouds and banks of clouds can be seen, some of which are casting shadows to the right side of this picture (sunlight is coming from the left). The bright clouds and their shadows range from approximately 4 to 8 miles (7 to 12 kilometers) in both widths and lengths. These appear similar to the small clouds in other bright regions Juno has detected and are expected to be updrafts of ammonia ice crystals possibly mixed with water ice.

Juno spacecraft orbiting Jupiter

Citizen scientists Gerald Eichstädt and Seán Doran processed this image using data from the JunoCam imager.

JunoCam's raw images are available for the public to peruse and process into image products at: http://www.missionjuno.swri.edu/junocam   

More information about Juno is at:

https://www.nasa.gov/juno and http://missionjuno.swri.edu

Image, Animation, Text, Credits: NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstädt/Seán Doran.

Greetings, Orbiter.ch

Hubble’s Cosmic Search for a Missing Arm












NASA - Hubble Space Telescope patch.

Nov. 17, 2017


This new picture of the week, taken by the NASA/ESA Hubble Space Telescope, shows the dwarf galaxy NGC 4625, located about 30 million light-years away in the constellation of Canes Venatici (The Hunting Dogs). The image, acquired with the Advanced Camera for Surveys (ACS), reveals the single major spiral arm of the galaxy, which gives it an asymmetric appearance. But why is there only one such spiral arm, when spiral galaxies normally have at least two?

Astronomers looked at NGC 4625 in different wavelengths in the hope of solving this cosmic mystery. Observations in the ultraviolet provided the first hint: in ultraviolet light the disk of the galaxy appears four times larger than on the image depicted here. An indication that there are a large number of very young and hot — hence mainly visible in the ultraviolet — stars forming in the outer regions of the galaxy. These young stars are only around one billion years old, about 10 times younger than the stars seen in the optical center. At first astronomers assumed that this high star formation rate was being triggered by the interaction with another, nearby dwarf galaxy called NGC 4618.

They speculated that NGC 4618 may be the culprit “harassing” NGC 4625, causing it to lose all but one spiral arm. In 2004 astronomers found proof for this claim. The gas in the outermost regions of the dwarf galaxy NGC 4618 has been strongly affected by NGC 4625.

Hubble Space Telescope (HST)

For images and more information about Hubble, visit:

http://hubblesite.org/
http://www.nasa.gov/hubble
http://www.spacetelescope.org/

Image, Animation, Text,  Credits: ESA/Hubble & NASA/Text Credits: European Space Agency/NASA/Karl Hille.

Best regards, Orbiter.ch

jeudi 16 novembre 2017

Lava or Not, Exoplanet 55 Cancri e Likely to have Atmosphere












NASA - Spitzer Space Telescope patch.

November 16, 2017


Image above: The super-Earth exoplanet 55 Cancri e, depicted with its star in this artist's concept, likely has an atmosphere thicker than Earth's but with ingredients that could be similar to those of Earth's atmosphere. Image Credits: NASA/JPL.

Twice as big as Earth, the super-Earth 55 Cancri e was thought to have lava flows on its surface. The planet is so close to its star, the same side of the planet always faces the star, such that the planet has permanent day and night sides. Based on a 2016 study using data from NASA's Spitzer Space Telescope, scientists speculated that lava would flow freely in lakes on the starlit side and become hardened on the face of perpetual darkness. The lava on the dayside would reflect radiation from the star, contributing to the overall observed temperature of the planet.

Now, a deeper analysis of the same Spitzer data finds this planet likely has an atmosphere whose ingredients could be similar to those of Earth's atmosphere, but thicker. Lava lakes directly exposed to space without an atmosphere would create local hot spots of high temperatures, so they are not the best explanation for the Spitzer observations, scientists said.

"If there is lava on this planet, it would need to cover the entire surface," said Renyu Hu, astronomer at NASA's Jet Propulsion Laboratory, Pasadena, California, and co-author of a study published in The Astronomical Journal. "But the lava would be hidden from our view by the thick atmosphere."

Using an improved model of how energy would flow throughout the planet and radiate back into space, researchers find that the night side of the planet is not as cool as previously thought. The "cold" side is still quite toasty by Earthly standards, with an average of 2,400 to 2,600 degrees Fahrenheit (1,300 to 1,400 Celsius), and the hot side averages 4,200 degrees Fahrenheit (2,300 Celsius). The difference between the hot and cold sides would need to be more extreme if there were no atmosphere.

"Scientists have been debating whether this planet has an atmosphere like Earth and Venus, or just a rocky core and no atmosphere, like Mercury. The case for an atmosphere is now stronger than ever," Hu said.

Researchers say the atmosphere of this mysterious planet could contain nitrogen, water and even oxygen -- molecules found in our atmosphere, too -- but with much higher temperatures throughout. The density of the planet is also similar to Earth, suggesting that it, too, is rocky. The intense heat from the host star would be far too great to support life, however, and could not maintain liquid water.

Spitzer Space Telescope. Image Credits: NASA/JPL

Hu developed a method of studying exoplanet atmospheres and surfaces, and had previously only applied it to sizzling, giant gaseous planets called hot Jupiters. Isabel Angelo, first author of the study and a senior at the University of California, Berkeley, worked on the study as part of her internship at JPL and adapted Hu's model to 55 Cancri e.

In a seminar, she heard about 55 Cancri e as a potentially carbon-rich planet, so high in temperature and pressure that its interior could contain a large amount of diamond.

"It's an exoplanet whose nature is pretty contested, which I thought was exciting," Angelo said.

Spitzer observed 55 Cancri e between June 15 and July 15, 2013, using a camera specially designed for viewing infrared light, which is invisible to human eyes. Infrared light is an indicator of heat energy. By comparing changes in brightness Spitzer observed to the energy flow models, researchers realized an atmosphere with volatile materials could best explain the temperatures.

There are many open questions about 55 Cancri e, especially: Why has the atmosphere not been stripped away from the planet, given the perilous radiation environment of the star?

"Understanding this planet will help us address larger questions about the evolution of rocky planets," Hu said.

NASA's Jet Propulsion Laboratory, Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at IPAC at Caltech. Caltech manages JPL for NASA. For more information about Spitzer, visit:

http://spitzer.caltech.edu

https://www.nasa.gov/spitzer

Images (mentioned), Text, Credits: NASA/JPL/Elizabeth Landau.

Greetings, Orbiter.ch

How do you find a star cluster? Easy, simply count the stars












ESA - Gaia Mission patch.

16 November 2017

It's the perfect meeting of old and new. Astronomers have combined the latest data from ESA's Gaia mission with a simple analysis technique from the 18th century to discover a massive star cluster that had previously escaped detection. Now, subsequent investigations are helping reveal the star-forming history of our Galaxy, the Milky Way.

Gaia: How to find a star cluster

Video above: Video explainer: How to find a star cluster. Video Credit: ESA.

In the latter years of the 18th century, astronomers William and Caroline Herschel began to count stars. William called the technique "star gauging" and his aim was to determine the shape of our Galaxy.

Ever since 1609, when Galileo lifted his telescope to the misty patch of light known as the Milky Way and saw that it was composed of myriad faint stars whose light all blurred together, we have known that there are different numbers of stars in different directions throughout space. This means that our local collection of stars, the Galaxy, must have a shape to it. Herschel set out to find out what that shape was.

He used a large telescope, twenty feet (610 cm) in length, mounted between tall wooden frames to sweep out a large circle in the sky that passed through the Milky Way at right angles. He then split this circle into more than 600 regions and counted or estimated the number of stars in each.

With this simple technique the Herschels produced the first shape estimate for the Galaxy. Fast-forward to the 21st century and now researchers use star counts to search for hidden star clusters and satellite galaxies. They look for regions where the density of stars rises higher than expected. These patches are called stellar over-densities.


Image above: Gaia's first sky map. Image Credits: ESA/Gaia/DPAC. Acknowledgement: A. Moitinho & M. Barros (CENTRA – University of Lisbon), on behalf of DPAC.

Back in 1785, Herschel's circular track passed close to the brightest star in the night sky Sirius. Now, scientists mining the first data released from the ESA spacecraft Gaia have revisited that particular area of the sky and made a remarkable discovery.

They have revealed a large star cluster that could have been discovered more than a century and a half ago had it not been so close to Sirius.

The cluster was spotted by Sergey E. Koposov, then at the University of Cambridge (UK) and now at Carnegie Mellon University Pennsylvania (USA), and his colleagues. They have been looking for star clusters and satellite galaxies in various surveys for the past decade. It was natural for them to do this with the Gaia mission's first data release.

Gaia is the European Space Agency's astrometric mission. Collecting positions, brightnesses and additional information for more than a billion sources of light, its data allows nothing less than the most precise 'star gauging' ever.

Gaia scanning the sky

Video above: Gaia scanning the sky. Video Credits: ESA/Gaia/DPAC. Acknowledgement: B. Holl (University of Geneva, Switzerland), A. Moitinho & M. Barros (CENTRA – University of Lisbon), on behalf of DPAC.

These days the laborious task of counting the stars is done by computers but the results still have to be scrutinised by humans. Koposov was combing the list of over-densities when he saw the massive cluster. At first it seemed too good to be true.

"I thought it must be an artefact related to Sirius," he says. Bright stars can create false signals, termed artefacts, that astronomers must be careful not to mistake for stars. An early paper from the Gaia team had even discussed artefacts around Sirius using a nearby patch of sky to the one Koposov was looking at.

Although he moved on and found another over-density that looked promising, his mind kept wanting to return to the first one. "I thought, 'That's strange, we shouldn't have that many artefacts from Sirius.' So I went and looked at it again. And I realised that it too was a genuine object," he says.

These two objects were named: Gaia 1 for the object located near Sirius, and Gaia 2, which is close to the plane of our Galaxy, and both were duly published. Gaia 1 in particular contains enough mass to make a few thousand stars like the Sun, is located 15 thousand light years away, and spread across 30 light years. This means it is a massive star cluster.


Image above: The brightest star in this WISE image is Sirius. To the left of Sirius, and centred on this image, is Gaia 1, a massive star cluster discovered by scientists mining Gaia data. Image Credits: Sergey Koposov; NASA/JPL; D. Lang, 2014; A.M. Meisner et al. 2017.

Collections of stars like Gaia 1 are called open clusters. They are families of stars that all form together and then gradually disperse around the Galaxy. Our own Sun very likely formed in an open cluster. Such assemblies can tell us about the star formation history of our Galaxy. Finding a new one that can be easily studied is already paying dividends.

"The age is of great interest," says Jeffrey Simpson, Australian Astronomical Observatory, who conducted follow-up observations with colleagues using the 4-metre-class Anglo-Australian telescope at Siding Springs Observatory, Australia.

Identifying 41 members of the cluster, Simpson and colleagues found that Gaia 1 is unusual in at least two ways. Firstly, it is about 3 billion years old. This is odd because there are not many clusters with this age in the Milky Way.

Typically clusters are either younger than a few hundred million years – these are the open clusters – or older than 10 billion years – these are a distinct class called globular clusters, which are found beyond the main bulk of stars in our Galaxy. Being of intermediate age, Gaia 1 might represent an important bridge in our understanding between the two populations.


Images above: Star cluster Westerlund 2. Image Credits: NASA, ESA, the Hubble Heritage Team (STScI/AURA), A. Nota (ESA/STScI), and the Westerlund 2 Science Team.


Images above: Globular cluster 47 Tucanae. Image Credits: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration. Acknowledgment: J. Mack (STScI) and G. Piotto (University of Padova, Italy).

Secondly, its orbit through the galaxy is unusual. Most open clusters lie close to the plane of the Galaxy but Simpson found that Gaia 1 flies high above it before ducking down and passing underneath. "It might go as much as a kiloparsec (more than 3000 light years) above and below the plane," he says. About 90% of clusters never go more than a third of this distance.

Simulations of clusters with orbits like Gaia 1 find that they are stripped of stars and dispersed by these high velocity 'plane passages'. That puts it at odds with the age estimate.

"Our finding that Gaia 1 is three billion years old is curious as the models would have it not surviving anywhere near as long. More research is required to try and reconcile this," says Simpson.

To test a possible explanation, Alessio Mucciarelli, Universita' degli Studi di Bologna, Italy and colleagues investigated the chemical composition of Gaia 1. Such a study has the ability to see if the cluster formed outside of the Galaxy and has been caught in the act of falling in.

"The chemical composition of the stars can be considered a 'genetic' signature of their origin. If a stellar cluster formed in another galaxy, its chemical composition will be different with respect to that of our Galaxy," says Mucciarelli.

They found that the compositions were practically identical to those expected if Gaia 1 formed in the Milky Way – so the puzzle remains.

Now Mucciarelli hopes that the discrepancy might go away when Gaia releases more data. "Even if the orbital parameters seem to suggest a peculiar orbit, their uncertainties are large enough to prevent any firm conclusion. More accurate orbital parameters will be obtained with the second Gaia data release and we will better understand whether the orbit of Gaia 1 is peculiar or not," he says.

As well as finding new clusters, the Gaia data are proving useful for checking out the reality of previously reported associations of stars. "Using Gaia data I can see stars that share the same motion. So I can confirm which ones form real open clusters," says Andrés E. Piatti, Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina.

He recently published a study that showed ten out of fifteen previously published open clusters were not really star clusters at all, they were just statistical flukes where a lot of unrelated stars happened to be passing in different directions through the same region of space.

It is laborious but vital work. "No one wants to spend their life doing this," says Piatti, "but it is necessary. If we can determine the real size of the cluster population we can learn a lot about the processes that the Galaxy has suffered during its lifetime."

In astronomy, the most famous list of star clusters, nebulae and galaxies was compiled by astronomer and comet hunter, Charles Messier, in the 18th century. Unaware of the importance of these objects, he designed his catalogue to stop the frustration felt by him and other astronomers in mistaking one of these 'deep-sky objects' for a nearby comet.

That original catalogue ran to 110 objects. If it hadn't been for the glare from Sirius obscuring the view, Gaia 1 would have been bright and obvious enough to have made it onto that list too. And there is every reason to think that there are more to come, thanks to Gaia.

The next data release will give accurate proper motions and distances to an unprecedented number of stars, which can be used to more efficiently find star clusters that were buried too deep in the stellar field or were too diffuse or too distant to be seen before.

There is always the possibility to find something totally new too. "I hope with the next data release we can find some new classes of objects too," says Simpson.

For the astronomers ready to explore the Gaia data, the adventure has only just begun. Gaia's second data release is scheduled for April 2018. Subsequent data releases are scheduled for 2020 and 2022.

Background information:

A large pan-European team of expert scientists and software developers known as DPAC (Data Processing and Analysis Consortium), located in and funded by many ESA member states, is responsible for the processing and validation of Gaia's data with the final objective of producing the Gaia Catalogue.
http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=58277

Related publications:

"Gaia 1 and 2. A pair of new Galactic star clusters," by S. E. Koposov, V. Belokurov and G. Torrealba, is published in Monthly Notices of the Royal Astronomical Society, Volume 470, Issue 3, 21 September 2017, Pages 2702–2709, https://doi.org/10.1093/mnras/stx1182

"Siriusly, a newly identified intermediate-age Milky Way stellar cluster: A spectroscopic study of Gaia 1," by J. D. Simpson, G. M. De Silva, S. L. Martell, D. B. Zucker, A. M. N. Ferguson, E. J. Bernard, M. Irwin, J. Penarrubia and E. Tolstoy is accepted for publication in Monthly Notices of the Royal Astronomical Society, stx1892, https://doi.org/10.1093/mnras/stx1892

"Chemical composition of the stellar cluster Gaia1: no surprise behind Sirius," by A. Mucciarelli, L. Monaco, P. Bonifacio and I. Saviane, is published in Astronomy & Astrophysics, 603 (2017) L7, https://doi.org/10.1051/0004-6361/201731009

"On the physical reality of overlooked open clusters," by A. E. Piatti is published in Monthly Notices of the Royal Astronomical Society, Volume 466, Issue 4, 1 May 2017, Pages 4960–4973, https://doi.org/10.1093/mnras/stx081

First Gaia release (Gaia DR1): http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=58272

Second Gaia data release: https://www.cosmos.esa.int/web/gaia/release

Anglo-Australian telescope: https://www.aao.gov.au/about-us/AAT

ESA's Gaia mission: http://www.esa.int/Our_Activities/Space_Science/Gaia

Images (mentioned), Videos (mentioned), text, Credit: European Space Agency (ESA.

Best regards, Orbiter.ch

Fracture swarms on Mars












ESA - Mars Express Mission patch.

16 November 2017

Sirenum Fossae perspective view

These striking features on Mars were caused by the planet’s crust stretching apart in response to ancient volcanic activity.

The fractures in the Sirenum Fossae region in the southern hemisphere were imaged by ESA’s Mars Express in March. They extend for thousands of kilometres in length, far beyond the boundaries of this image.

The fractures divide the crust into blocks: the movement along a pair of faults causes the centre section to drop down into ‘graben’ several kilometres wide and a few hundred metres deep. Elevated blocks of crust remain between the graben when there is a parallel series of fault, as seen in this scene.

Sirenum Fossae in context

The Sirenum Fossae are part of a larger radial fracture pattern around the Arsia Mons volcano in the Tharsis region, which is situated some 1800 km to the northeast.

Tharsis is the largest volcanic province on Mars, its far-reaching fracture system testament to the powerful influence this impressive volcanic province had on the planet.

Sirenum Fossae fractures

Indeed, the Sirenum Fossae fracture system seen here is thought to be associated with tectonic stresses arising from ancient volcanic activity in the Tharsis region. For example, the graben could either be caused by the planet’s crust stretching apart as a magma chamber bulges the crust above it, or alternatively as the crust collapsed along lines of weakness as the magma chamber emptied.

It is also possible that each graben was associated with an ancient volcanic dike: a steep corridor within the rock along which magma from the interior of Mars once propagated upwards, causing cracking along the surface.

Sirenum Fossae topography

In this case the graben could represent a giant ‘dike swarm’ extending from the volcanic centre. Dike swarms are also seen on Earth, as in Iceland where they are observed with surface fractures and graben sets in the Krafla fissure swarm.

As with any geological feature that cuts into the surface of the planet, the graben systems make for a good window into the subsurface. They also provide steep surfaces for active processes occurring in more recent times.

Sirenum Fossae in 3D

For example, NASA’s Mars Reconnaissance Orbiter identified gullies on some of the steep slopes in Sirenum Fossae, along troughs and in the rims of impact craters. What material carves out the small channels is a topic of active research: they were initially thought to be related to flowing water, but recent proposals suggest that seasonal frozen carbon dioxide – dry ice – flowing downslope may be responsible.

Related links:

Mars Express: http://www.esa.int/Our_Activities/Space_Science/Mars_Express

Mars Express overview: http://www.esa.int/Our_Activities/Space_Science/Mars_Express_overview

Mars Express in-depth: http://sci.esa.int/marsexpress

ESA Planetary Science archive (PSA): http://www.rssd.esa.int/PSA

High Resolution Stereo Camera: http://berlinadmin.dlr.de/Missions/express/indexeng.shtml

HRSC data viewer: http://hrscview.fu-berlin.de/

Behind the lens... http://www.esa.int/Our_Activities/Space_Science/Mars_Express/Behind_the_lens

Frequently asked questions: http://www.esa.int/Our_Activities/Space_Science/Mars_Express/Frequently_asked_questions

Images, Text, Credits: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO/NASA MGS MOLA Science Team.

Best regards, Orbiter.ch

Our Living Planet Shapes the Search for Life Beyond Earth











NASA logo.

Nov. 16, 2017

Life. It's the one thing that, so far, makes Earth unique among the thousands of other planets we've discovered. Since the fall of 1997, NASA satellites have continuously and globally observed all plant life at the surface of the land and ocean. During the week of Nov. 13-17, NASA is sharing stories and videos about how this view of life from space is furthering knowledge of our home planet and the search for life on other worlds.

As a young scientist, Tony del Genio of NASA's Goddard Institute for Space Studies in New York City met Clyde Tombaugh, the discoverer of Pluto.

The Living Planet

"I thought, 'Wow, this is a one-time opportunity,'" del Genio said. "I'll never meet anyone else who found a planet."

That prediction was spectacularly wrong. In 1992, two scientists discovered the first planet around another star, or exoplanet, and since then more people have found planets than throughout all of Earth's preceding history. As of this month, scientists have confirmed more than 3,500 exoplanets in more than 2,700 star systems. Del Genio has met many of these new planet finders.

How to Find a Living Planet

Video above: The more we see other planets, the more the question comes into focus: Maybe we're the weird one? Decades of observing Earth from space has informed our search for signs of habitability and life on exoplanets and even planets in our own solar system. We're taking a closer look at what we'ver learned about Earth - our only example of a planet with life - to our search for life the universe.

Del Genio is now co-lead of a NASA interdisciplinary initiative to search for life on other worlds. This new position as the lead of this project may seem odd to those who know him professionally. Why? He has dedicated decades to studying Earth, not searching for life elsewhere.

We know of only one living planet: our own. But we know it very well. As we move to the next stage in the search for alien life, the effort will require the expertise of planetary scientists, heliophysicists and astrophysicists. However, the knowledge and tools NASA has developed to study life on Earth will also be one of the greatest assets to the quest.


Image above: Left, an image of Earth from the DSCOVR-EPIC camera. Right, the same image degraded to a resolution of 3 x 3 pixels, similar to what researchers will see in future exoplanet observations. Image Credits: NOAA/NASA, Stephen Kane.

Habitable Worlds

There are two main questions in the search for life: With so many places to look, how can we focus in on the places most likely to harbor life? What are the unmistakable signs of life -- even if it comes in a form we don't fully understand?

"Before we go looking for life, we're trying to figure out what kinds of planets could have a climate that's conducive to life," del Genio said. "We're using the same climate models that we use to project 21st century climate change on Earth to do simulations of specific exoplanets that have been discovered, and hypothetical ones."

Del Genio recognizes that life may well exist in forms and places so bizarre that it might be substantially different from Earth. But in this early phase of the search, "We have to go with the kind of life we know," he said.

Further, we should make sure we use the detailed knowledge of Earth. In particular, we should make sure of our discoveries on life in various environments on Earth, our knowledge of how our planet and its life have affected each other over Earth history, and our satellite observations of Earth’s climate.

Above all else, that means liquid water. Every cell we know of -- even bacteria around deep-sea vents that exist without sunlight -- requires water.

Life in the Ocean

Research scientist Morgan Cable of NASA's Jet Propulsion Laboratory in Pasadena, California, is looking within the solar system for locations that have the potential to support liquid water. Some of the icy moons around Saturn and Jupiter have oceans below the ice crust. These oceans were formed by tidal heating, that is, warming of the ice caused by friction between the surface ice and the core as a result of the gravitational interaction between the planet and the moon.

"We thought Enceladus was just boring and cold until the Cassini mission discovered a liquid water subsurface ocean," said Cable. The water is spraying into space, and the Cassini mission found hints in the chemical composition of the spray that the ocean chemistry is affected by interactions between heated water and rocks at the seafloor. The Galileo and Voyager missions provided evidence that Europa also has a liquid water ocean under an icy crust. Observations revealed a jumbled terrain that could be the result of ice melting and reforming.

As missions to these moons are being developed, scientists are using Earth as a testbed. Just as prototypes for NASA's Mars rovers made their trial runs on Earth's deserts, researchers are testing both hypotheses and technology on our oceans and extreme environments.

Cable gave the example of satellite observations of Arctic and Antarctic ice fields, which are informing the planning for a Europa mission. The Earth observations help researchers find ways to date the origin of jumbled ice. "When we visit Europa, we want to go to very young places, where material from that ocean is being expressed on the surface," she said. "Anywhere like that, the chances of finding evidence of life goes up -- if they're there."

Water in Space

For any star, it's possible to calculate the range of distances where orbiting planets could have liquid water on the surface. This is called the star's habitable zone.

Astronomers have already located some habitable-zone planets, and research scientist Andrew Rushby, of NASA Ames Research Center, in Moffett Field, California, is studying ways to refine the search. Location alone isn't enough. "An alien would spot three planets in our solar system in the habitable zone [Earth, Mars and Venus]," Rushby said, "but we know that 67 percent of those planets are not very habitable." He recently developed a simplified model of Earth's carbon cycle and combined it with other tools to study which planets in the habitable zone would be the best targets to look at for life, considering probable tectonic activity and water cycles. He found that larger rocky planets are more likely than smaller ones to have surface temperatures where liquid water could exist, given the same amount of light from the star.

Renyu Hu, of JPL, refined the search for habitable planets in a different way, looking for the signature of a rocky planet. Basic physics tells us that smaller planets must be rocky and larger ones gaseous, but for planets ranging from Earth-sized to about twice that radius, astronomers can't tell a large rocky planet from a small gaseous planet. Hu pioneered a method to detect surface minerals on bare-rock exoplanets and defined the atmospheric chemical signature of volcanic activity, which wouldn't occur on a gas planet.

Vital Signs

When scientists are evaluating a possible habitable planet, "life has to be the hypothesis of last resort," Cable said. "You must eliminate all other explanations." Identifying possible false positives for the signal of life is an ongoing area of research in the exoplanet community. For example, the oxygen in Earth's atmosphere comes from living things, but oxygen can also be produced by inorganic chemical reactions.

Shawn Domagal-Goldman, of NASA's Goddard Space Flight Center in Greenbelt, Maryland, looks for unmistakable, chemical signs of life, or biosignatures. One biosignature may be finding two or more molecules in an atmosphere that shouldn't be there at the same time. He uses this analogy: If you walked into a college dorm room and found three students and a pizza, you could conclude that the pizza had recently arrived, because college students quickly consume pizza. Oxygen "consumes" methane by breaking it down in various chemical reactions. Without inputs of methane from life on Earth's surface, our atmosphere would become totally depleted of methane within a few decades.

Earth as Exoplanet

When humans start collecting direct images of exoplanets, even the closest one will appear as a handful of pixels in the detector – something like the famous "blue dot" image of Earth from Saturn. What can we learn about planetary life from a single dot?

Stephen Kane of the University of California, Riverside, has come up with a way to answer that question using NASA's Earth Polychromatic Imaging camera on the National Oceanic and Atmospheric Administration's Deep Space Climate Observatory (DSCOVR). These high-resolution images -- 2,000 x 2,000 pixels – document Earth's global weather patterns and other climate-related phenomena. "I'm taking these glorious pictures and collapsing them down to a single pixel or handful of pixels," Kane explained. He runs the light through a noise filter that attempts to simulate the interference expected from an exoplanet mission.

DSCOVR takes a picture every half hour, and it's been in orbit for two years. Its more than 30,000 images are by far the longest continuous record of Earth from space in existence. By observing how the brightness of Earth changes when mostly land is in view compared with mostly water, Kane has been able to reverse-engineer Earth's rotation rate -- something that has yet to be measured directly for exoplanets.

When Will We Find Life?

Every scientist involved in the search for life is convinced it's out there. Their opinions differ on when we'll find it.

"I think that in 20 years we will have found one candidate that might be it," says del Genio. Considering his experience with Tombaugh, he added, "But my track record for predicting the future is not so good."

Rushby, on the other hand, says, "It's been 20 years away for the last 50 years. I do think it's on the scale of decades. If I were a betting man, which I'm not, I'd go for Europa or Enceladus."

How soon we find a living exoplanet really depends on whether there's one relatively nearby, with the right orbit and size, and with biosignatures that we are able to recognize, Hu said. In other words, "There's always a factor of luck."

Related links:

Earth: https://www.nasa.gov/topics/earth/index.html

Exoplanets: https://www.nasa.gov/content/the-search-for-life

Images, Video, Text, Credits: NASA/Tony Greicius/JPL/Elizabeth Landau/Alan Buis/Earth Science News Team, written by Carol Rasmussen.

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