Climate Variability

The ocean is a significant influence on Earth's weather and climate. The ocean covers 70% of the global surface. This great reservoir continuously exchanges heat, moisture, and carbon with the atmosphere, driving our weather patterns and influencing the slow, subtle changes in our climate. The oceans influence climate by absorbing solar radiationand releasing heat needed to drive the atmospheric circulation, by releasing aerosols that influence cloud cover, by emitting most of the water that falls on land as rain, by absorbing carbon dioxide from the atmosphereand storing it for years to millions of years. The oceans absorb much of the solar energy that reaches earth, and thanks to the high heat capacity of water, the oceans can slowly release heat over many months or years. The oceans store more heat in the uppermost 3 meters (10 feet) that the entire atmosphere, the key to understanding global climate change is inextricably linked to the ocean.

Climate is influenced by storage of heat and CARBON DIOXIDE in the ocean, which depends on both physical and biological processes. Let's look at some of these processes. At the end of the last ice age, about 15,000 years ago, and the ice sheets melted away and climate warmed at that time. Ice sheets began to grow, and climate cool about 130,000 years ago at the beginning of the last ice age. About 130,000 years ago, fed by evaporation of ocean waters, the polar ice caps thickened and expanded Earth cooled by almost 12° C and global sea level to drop 130m below its current level. About 15,000 years ago, this process was reversed as more sunlight reached areas near the Arctic Circle, and Earth emerged from the ice age. Earth is about 8° Celsius (14° Fahrenheit) warmer today than it was then. Still recovering from the ice age, global sea level continues to rise. The past century alone has seen global temperature increase by 0.6 degree Celsius (1 degree Fahrenheit), and the average global sea level over the past decade has risen steadily. Is this just part of the natural cycle? How much of this warming is due to the burning of fossil fuels? Is human nature affecting Mother Nature? What should we do? Our response to the challenge of global warming begins by formulating the right set of questions.

The first step in addressing the issue of global warming is to recognize that the warming pattern, if it continues, will probably not be uniform. The term "global warming" only tells part of the story; our attention should be focuses on "global climate change." The real threat may not be the gradual rise in global temperature and sea level, but the redistribution of heat over the Earth's surface. Some spots will warm, while others will cool; these changes, and the accompanying shifts in rainfall patterns, could relocate agricultural regions across the planet. By studying the oceans from space, we can unlock a vast store of information about our changing environment.

Climate is effected by both the biological and physical processes of the oceans. In addition, physical and biological processes affect each other creating a complex system. Both the ocean and the atmosphere transport roughly equal amounts of heat from Earth's equatorial regions - which are intensely heated by the Sun - toward the icy poles, which receive relatively little solar radiation. The atmosphere transports heat through a complex, worldwide pattern of winds; blowing across the sea surface, these winds drive corresponding patterns of ocean currents. But the ocean currents move more slowly than the winds, and have much higher heat storage capacity. The winds drive ocean circulation transporting warm water to the poles along the sea surface. As the water flows poleward, it releases heat into the atmosphere. In the far North Atlantic, some water sinks to the ocean floor. This water is eventually brought to the surface in many regions by mixing in the ocean, completing the oceanic conveyor belt (see below). Changes in the distribution of heat within the belt are measured on time scales from tens to hundreds of years. While variations close to the ocean surface may induce relatively short-term climate changes, long-term changes in the deep ocean may not be detected for many generations. The ocean is the thermal memory of the climate system.

• Physical characteristics of heat transport and ocean circulation impact the Earth's climate system. Like a massive 'flywheel' that stabilizes the speed of an engine, the vast amounts of heat in the oceans stabilizes the temperature of Earth. The heat capacity of the ocean is much greater than that of the atmosphere or the land. As a result, the ocean slowly warms in the summer, keeping air cool, and it slowly cools in winter, keeping the air warm. A coastal city like San Francisco has a small range of temperature throughout the year, but a mid-continental city like Fargo, ND has a very wide range of temperatures. The ocean carries substantial heat only to the sub-tropics. Poleward of the sub-tropics, the atmosphere carries most of the heat.

• Climate is also influenced by the "biological pump," a biological process in the ocean that impacts concentrations of carbon dioxide in the atmosphere. The oceanic biological productivity is both a source and sink of carbon dioxide, one of the greenhouse gases that control climate. The "biological pump" happens when phytoplankton convert carbon dioxide and nutrients into carbohydrates (reduced carbon). A little of this carbon sinks to the sea floor, where it is buried in the sediments. It stays buried for perhaps millions of years. Oil is just reduced carbon trapped in sediments from millions of years ago. Through photosynthesis, microscopic plants (phytoplankton) assimilate carbon dioxide and nutrients (e.g., nitrate, phosphate, and silicate) into organic carbon (carbohydrates and protein) and release oxygen.

• Carbon dioxide is also transferred through the air-sea interface. Deep water of the ocean can store carbon dioxide for centuries. Carbon dioxide dissolves in cold water at high latitudes, and is subducted with the water. It stays in the deeper ocean for years to centuries before the water is mixed back to the surface and warmed by the sun. The warm water releases carbon dioxide back to the atmosphere. Thus the conveyor belt described below carries carbon dioxide into the deep ocean. Some (but not all, or even a large part) of this water comes to the surface in the tropical Pacific perhaps 1000 years later, releasing carbon dioxide stored for that period. The physical temperature of the ocean helps regulate the amount of carbon dioxide is released or absorbed into the water. Cold water can dissolve more carbon dioxide than warm water. Temperature of ocean is also impacted the biological pump. Penetrative solar radiation warms the ocean surface causing more carbon dioxide to be released into the atmosphere. Oceanic processes of air-sea gas fluxes effect biological production and consequentially impacting climate. But as plant growth increases, the water gets cloudy and prevents the solar radiation from penetrating beneath the ocean surface.

NASA Oceanography & Climate

NASA satellite observations of the oceans of the past three decades have improved our understanding of global climate change by making global measurements needed for modeling the ocean-atmosphere climate system. NASA uses several instruments to get sea-surface temperature (AVHRR or other), height

(altimeter), winds (scatterometers), productivity (MODIS), and salinity (future instruments). Global data sets available on time scales of days to years (and, looking ahead, to decades) have been and will be a vital resource for scientists and policy makers in a wide range of fields. Ocean surface topography and currents, vector winds (both speed and direction), sea-surface temperature, and salinity are the critical variables for understanding the ocean-climate connection.

Sea Winds

Scatterometers are used to measure vector winds. The SeaWinds scatterometer has provided scientists with the most detailed, continuous global view of ocean-surface winds to date, including the detailed structure of hurricanes, wide-driven circulation, and changes in the polar sea-ice masses. Scatterometer signals can penetrate through clouds and haze to measure conditions at the ocean surface, making them the only proven satellite instruments capable of measuring vector winds at sea level

day and night, in nearly all weather conditions.

Combined with data from Topex/Poseidon, Jason-1, and weather satellites, moorings and drifters, data from SeaWinds and its follow-on missions will be used to study long-term change. Earth's weather patterns such as El Niño, and the Northern Oscillation, which affect the hydrologic and bio-geochemical balance of the ocean-atmosphere system.

Ocean Surface Topography

Radar altimeters like those on the Topex/Poseidon and Jason missions, are used to measure ocean surface topography. Bouncing radio waves off the ocean surface and timing their return with incredible accuracy, these instruments tell us the distance from the satellite to the sea surface within a few centimeters - the equivalent of sensing the thickness of a dime from a jet flying at 35,000 feet! At the same time, special tracking systems on the satellites give their position relative to the center of mass of Earth also with an accuracy of a few cen timeters. By subtracting the height of the satellite above the sea from the height of the satellite above the center of mass, scientists calculate maps of the sea-surface height and changes in the height due to tides, changing currents, heat stored inthe ocean, and amount of water in the ocean.

By mapping the topography of the ocean we can determine the speed and direction of ocean currents. Just as wind blows around high- and lo w-pressure centers in the atmosphere, water flows around the high and lows of the ocean surface.

Maps of sea-surface height are most useful when they are converted to topographic maps. To determine topography of the sea-surface, height maps are compared with a gravitational reference map that shows the hills and valleys of a motionless ocean due to variations in the pull of gravity. The GRACE (Gravity Recovery and Climate Experiment) mission will provide very accurate maps of gravity that will allow us to greatly improve our knowledge of ocean circulation. GRACE has provided gravity measurements that are up to 100 times more accurate than previous values. This improved accuracy will lead the way to break-throughs in our understanding of ocean circulation and heat transport.
Two animations showing sea surface height (SSH) and sea surface temperature (SST) Anomalies in the Pacific Ocean from October 1992 to August 2002. The increase in temperature and height in the equatorial region west of South America illustrates the 1997-98 El Nino event.

Temperature & Salinity

Water is an enormously efficient heat-sink. Solar heat absorbed by bodies of water during the day, or in the summer, is released at night, or in winter. But the heat in the ocean is also circulating. Temperature & Salinity control the sinking of surface water to the deep ocean, which affects long-term climate change. Such sinking is also a principal mechanism by which the oceans store and transport heat and carbon dioxide. Together, temperature and salinity differences drive a global circulation within the ocean sometimes called the Global Conveyor Belt.

The heat in the water is carried to higher latitudes by ocean currents where it is released into the atmosphere. Water chilled by colder temperatures at high latitudes contracts (thus gets more dense). In some regions where the water is also very salty, such as the far North Atlantic, the water becomes dense enough to sink to the bottom. Mixing in the deep ocean due to winds and tides brings the cold water back to the surface everywhere around the ocean. Some reaches the surface via the global ocean water circulation conveyor belt to complete the cycle.

During this circulation of cold and warm water, carbon dioxide is also transported. Cold water absorbs carbon dioxide from the atmosphere, and some sinks deep into the ocean. When deep water comes to the surface in the tropics, it is warmed, and the carbon dioxide is released back to the atmosphere.

Salinity can be as important as temperature in determining density of seawater in some regions such as the western tropical Pacific and the far North Atlantic. Rain reduces the salinity, especially in regions of very heavy rain. Some tropical areas get 3,000 to 5,000 millimters of rain each year. Evaporation increases salinity because as evaporation occurs, salt is left behind thus making surface water denser. Evaporation in the tropics averages 2,000 millimeters per year. This denser saltier water sinks into the ocean contributing to the global circulation patterns and mixing. Ocean salinity measurements have been few and infrequent, and in many places salinity has remained unmeasured. Remotely sensed salinity measurements hold the promise of greatly improving our ocean models. This is the challenge of project Aquarius, a NASA mission scheduled to launch in 2008, which will enable us to further refine our understanding of the ocean-climate connection.

The Biological Pump

Life in the ocean consumes and releases large quantities of carbon dioxide. Across Earth's oceans, tiny marine plants called phytoplankton use chlorophyll to capture sunlight during photosynthesis and use the energy to produce sugars. Phytoplankton are the basis of the ocean food web, and they play a significant role in Earth's climate, since they draw down carbon dioxide, a greenhouse gas, at the same rate as land plants. About half of the oxygen we breathe arises from photosynthesis in the ocean.

Because of their role in the ocean's biological productivity and their impact on climate, scientists want to know how much phytoplankton the oceans contain, where they are located, how their distribution is changing with time, and how much photosynthesis they perform. They gather this information by using satellites to observe chlorophyll as an indicator of the number, or biomass, of phytoplankton cells.

Probably the most important and predominant pigment in the ocean is chlorophyll-α contained in microscopic marine plants known as phytoplankton. Chlorophyll-α absorbs blue and red light and reflects green light. If the ratio of blue to green is low for an area of the ocean surface, then there is more phytoplankton present. This relationship works over a very wide range of concentrations, from less than 0.01 ton early 50 milligrams of chlorophyll per cubic meter of seawater.

MESSENGER Gains Critical Gravity Assist for Mercury Orbital Observations

MESSENGER successfully flew by Mercury yesterday, gaining a critical gravity assist that will enable it to enter orbit about Mercury in 2011 and capturing images of five percent of the planet never before seen. With more than 90 percent of the planet’s surface already imaged, MESSENGER’s science team had drafted an ambitious observation campaign designed to tease out additional details from features uncovered during the first two flybys. But an unexpected signal loss prior to closest approach hampered those plans.

At approximately 5:55 p.m., the spacecraft passed by Mercury at an altitude of 142 miles and at a relative velocity of more than 12,000 miles per hour according to Doppler residual measurements logged just prior to the closet approach point. As the spacecraft approached the planet, MESSENGER’s Wide Angle Camera captured this striking view, which shows portions of Mercury's surface that had remained unseen by spacecraft even after the three flybys by Mariner 10 in 1974 and 1975 and MESSENGER’s two earlier flybys in 2008.

“This third and final flyby was MESSENGER’s last opportunity to use the gravity of Mercury to meet the demands of the cruise trajectory without using the probe’s limited supply of on-board propellant,” says MESSENGER Mission Systems Engineer Eric Finnegan of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md.

A portion of the complicated encounter was executed in eclipse, when the spacecraft is in Mercury’s shadow and the spacecraft – absent solar power – was to operate on its internal batteries for 18 minutes. Ten minutes after entering eclipse and four minutes prior to the closet approach point, the carrier signal from the spacecraft was lost, earlier than expected.

According to Finnegan, the spacecraft autonomously transitioned to a safe operating mode, which pauses the execution of the command load and "safes the instruments," while maintaining knowledge of its operational state and preserving all data on the solid-state recorder.

“We believe this mode transition was initiated by the on-board fault management system due to an unexpected configuration of the power system during eclipse,” Finnegan says. MESSENGER was returned to operational mode at 12:30 a.m. with all systems reporting nominal operations. All on-board stored data were returned to the ground by early morning and are being analyzed to confirm the full sequence of events.

“Although the events did not transpire as planned, the primary purpose of the flyby, the gravity assist, appears to be completely successful,” Finnegan adds. “Furthermore, all approach observing sequences have been captured, filling in additional area of previously unexplored terrain and further exploring the exosphere of Mercury.”

“MESSENGER’s mission operations and engineering teams deserve high commendation for their professional and efficient approach to last night’s spacecraft safe-mode transition,” says MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington. “They quickly diagnosed the initial problem, restored the spacecraft to its normal operating mode, and developed plans to recover as much of our post-encounter science observations as possible. Most importantly, we are on course to Mercury orbit insertion less than 18 months from now, so we know that we will be returning to Mercury and will be able to observe the innermost planet in exquisite detail.”

Additional information and features from this encounter will be available online at http://messenger.jhuapl.edu/mer_flyby3.html. Be sure to check back frequently to see the latest released images and science results!

NASA Ice Campaign Takes Flight in Antarctica

Early in the 20th century, a succession of adventurers and scientists pioneered the exploration of Antarctica. A century later, they're still at it, albeit with a different set of tools. This fall, a team of modern explorers will fly over Earth's southern ice-covered regions to study changes to its sea ice, ice sheets, and glaciers as part of NASA's Operation Ice Bridge.

Starting next month, NASA will fly its DC-8, a 157-foot-long airborne laboratory that can accommodate many instruments. The fall 2009 campaign is one of few excursions to the remote continent made by the DC-8, the largest aircraft in NASA's airborne science fleet.

The plane is scheduled to leave NASA's Dryden Flight Research Center in Edwards, Calif., on October 12 and fly to Punta Arenas, Chile, where the plane, crew and researchers will be based for through mid-November. For six weeks, the Ice Bridge team will traverse the Southern Ocean for up to 17 flights over West Antarctica, the Antarctic Peninsula, and coastal areas where sea ice is prevalent. Each round-trip flight lasts about 11 hours, two-thirds of that time devoted to getting to and from Antarctica.

Operation Ice Bridge is a six-year campaign of annual flights to each of Earth's polar regions. The first flights in March and April carried researchers over Greenland and the Arctic Ocean. This fall's Antarctic campaign, led by principal investigator Seelye Martin of the University of Washington, will begin the first sustained airborne research effort of its kind over the continent. Data collected by researchers will help scientists bridge the gap between NASA's Ice, Cloud and Land Elevation Satellite (ICESat) -- which is operating the last of its three lasers -- and ICESat-II, scheduled to launch in 2014.

The Ice Bridge flights will help scientists maintain the record of changes to sea ice and ice sheets that have been collected since 2003 by ICESat. The flights will lack the continent-wide coverage that can be achieved by satellite, so researchers carefully select key target locations. But the flights will also turn up new information not possible from orbit, such as the shape of the terrain below the ice.

"Space-based instruments like the ICESat lasers are the only way to find out where change is occurring in remote, continent-sized ice sheets like Antarctica," said Tom Wagner, cryosphere program scientist at NASA Headquarters in Washington, D.C. "But aircraft missions like Ice Bridge allow us to follow up with more detailed studies and make other measurements critical to modeling sea level rise."

Lasers and Radars

ICESat launched in January 2003 and since then, its sole instrument -- a precise laser altimeter -- has helped scientists map ice sheet elevation, calculate sea ice thickness, and monitor how both have changed.

"With ICESat, we have seen significant changes, things we wouldn't otherwise know were taking place," said Jay Zwally of NASA's Goddard Space Flight Center in Greenbelt, Md., and ICESat investigator on the mission. For example, shifts in surface elevation have previously revealed the draining and filling of lakes below Antarctica's ice.

After ICESat, scientists will rely on an airborne laser called the Airborne Topographic Mapper (ATM), developed at NASA Wallops Flight Facility in Wallops Island, Va. ATM pulses laser light in circular scans on the ground, and those pulses reflect back to the aircraft and are converted into elevation maps of the ice surface. By flying ATM over the same swath of ground covered by ICESat, researchers can compare the two data sets and calibrate them so that aircraft can continue the record keeping after the satellite data ends. They can also make more detailed elevation studies over dynamic areas, such as the Crane glacier on the Antarctic Peninsula, which sped up following the collapse of the Larsen Ice Shelf in 2002.

In addition, University of Kansas scientists will fly the Multichannel Coherent Radar Depth Sounder, which measures ice sheet thickness. It can also map the varied terrain below the ice, which is important for computer modeling of the future behavior of the ice.

The Laser Vegetation Imaging Sensor, developed at Goddard, will map large areas of sea ice and glacier zones. And a gravimeter, managed by Columbia University, will measure the shape of seawater-filled cavities at the edge of some major fast-moving major glaciers. Finally, a snow radar from University of Kansas will measure the thickness of snow on top of sea ice and glaciers, allowing researchers to differentiate between snow and ice and make more accurate thickness measurements.

Targets

The Antarctic continent may be remote, but it plays a significant role in Earth's climate system. The expanse is home to glaciers and ice sheets that hold frozen about 90 percent of Earth's freshwater -- a large potential contribution to sea level rise should all the ice melt.

How and where are Antarctica's ice sheets, glaciers, and sea ice changing? Compared to the Arctic, where sea ice has long been on the decline, sea ice in Antarctica is growing in some coastal areas. Snow and ice have been accumulating in some land regions in the east. West Antarctica and the Peninsula, however, have seen more dramatic warming and rapid ice loss.

"We don't see the same sea ice changes in Antarctica that we see in the Arctic, and the reason is that the system is more complex," said Thorsten Markus of NASA Goddard, the principal sea ice investigator for the mission. "But the fact that we don't see the same changes in Antarctica that we see in the Arctic doesn’t make it less important to study those changes. It's really important for us to understand the global climate system."

With the DC-8 limited to just a few hours over Antarctica on each flight, mission planners have carefully selected targets of current and potential rapid change.

One such target is West Antarctica's Pine Island Glacier. "That glacier is one of the great unknowns because its bed -- where the glacier contacts rock -- is below sea level," Martin said. "So if there's a surge or dramatic change, seawater could get under the glacier and we could be looking at very rapid change."

Other proposed targets along the Amundsen coast include the Thwaites, Smith, and Kohler glaciers and the Getz Ice Shelf. Researchers also intend to study the myriad glaciers and ice shelves on the Peninsula, which has been undergoing dramatic changes.

NASA Goddard Shoots the Moon to Track LRO

On certain nights, an arresting green line pierces the sky above NASA's Goddard Space Flight Center in Greenbelt, Md. It's a laser directed at the moon, visible when the air is humid. No, we're not repelling an invasion. Instead, we're tracking our own spacecraft.

28 times per second, engineers at NASA Goddard fire a laser that travels about 250,000 miles to hit the minivan-sized Lunar Reconnaissance Orbiter (LRO) spacecraft moving at nearly 3,600 miles per hour as it orbits the moon.

The first laser ranging effort to track a spacecraft beyond low-Earth orbit on a daily basis produces distance measurements accurate to about four inches (10 centimeters). For comparison, the microwave stations tracking LRO measure its range to a precision of about 65 feet (20 meters).

"Current lunar maps are not as accurate as we’ll need to return people safely to the moon," said Ronald Zellar of NASA Goddard, team lead for the LRO laser ranging system. "In order to make an accurate map, first you need to know where you are. Knowing the precise range to LRO is necessary for its instruments to produce much more accurate maps, with errors reduced to the size of humans or rovers."

"A further benefit of laser ranging to LRO is that it can improve knowledge of the moon's orientation and gravity, which are central to understanding its interior structure and to precision navigation," said Gregory Neumann, a Geophysicist at NASA Goddard.
Engineers use a telescope at the ground station on the Goddard campus to direct laser pulses toward LRO. The range to LRO is calculated by measuring how long it took the laser to reach the spacecraft.

The laser ranging to LRO is one way, meaning that the laser is directed at LRO, which records the time of arrival and sends the data back to ground stations on Earth by its radio telemetry link. This is the first time repeated, one-way tracking has been used for spacecraft ranging. Typical satellite laser ranging, used for spacecraft in low-Earth orbit, is two way, meaning the laser is simply reflected off the spacecraft and the time of flight recorded when it returns to the ground.

The advantage of LRO's one-way system is that a less expensive, lower-power laser system can be used -- especially important since the distance to LRO is hundreds of times greater than that to most Earth-orbiting spacecraft. Also, only a small receiver is needed on the spacecraft instead of a large retro-reflector array.

LRO's laser tracking presents unique challenges, however. First, there's the issue of avoiding interference. The laser pulses from Earth are received by a small telescope on LRO and transferred to the spacecraft’s laser altimeter instrument. The detector on this instrument performs double-duty, detecting both the laser ranging pulses as well as the pulses from its own laser reflected off the lunar surface. The instrument’s laser is used to build three-dimensional (topographic) maps of the lunar landscape and those pulses could hit the detector at the same time as the laser ranging pulses from Earth, confusing the data. So the pulses from Earth have to be carefully timed to avoid interfering with the instrument’s operation. Since the instrument sends laser pulses 28 times per second to the lunar surface, the laser ranging pulses are sent at the same rate but shifted in time to avoid interference. "It's like shooting at a spinning coin from a mile away and being able to hit it on the edge as it spins," said Neumann.

Another challenge is precise time measurement. Since the range to LRO is calculated by measuring how long it took the laser to reach the spacecraft, any variations in the time measurements will produce variations in the range estimates. LRO has a timing system that uses a crystal oscillator -- the heart of which is a vibrating crystal -- to measure time precisely. The oscillator is accurate to one part in a trillion over an hour. However, the rate at which the crystal vibrates changes with temperature, so the crystal is housed in a small oven which must be carefully controlled to maintain a stable temperature.

Then there's the difficulty of hitting a moving target. Since LRO is constantly moving in its orbit, the ground station must fire the laser pulses at a point in front of the spacecraft to compensate for the spacecraft's motion while the pulse is in-flight toward the moon. This is one of the reasons why LRO still relies on the traditional microwave tracking systems. They need the position of the spacecraft to know where to point the laser. The laser spot is 12 miles wide when it gets to the moon. Although this seems large on a human scale, it’s small in space and it would be easy to miss a tiny spacecraft moving 3,600 miles per hour. Even though the precision isn’t as great, without the microwave tracking system, the laser ranging system won’t work. This requires the LRO Mission Operations Center to track, predict, and communicate the position of the spacecraft to the laser ranging ground station.

Finally, there's the problem of bad weather; specifically, clouds. The laser can't penetrate thick cloud cover, so laser ranging is not available in those situations. Fortunately, there's plenty of opportunity to collect data over the course of LRO's one-year mission. "We're ranging to LRO whenever the moon is visible, 24 hours a day, 7 days a week," said Jan McGarry of NASA Goddard, ground system lead for laser ranging.

"Two-way satellite laser ranging (SLR) was developed at NASA Goddard in the 1960s," adds McGarry. "Since then, SLR has become a global effort, with about 30 countries participating and about 40 satellites carrying laser reflectors. NASA has eight SLR stations around the world, and Goddard is responsible for them. NASA is part of the global International Laser Ranging Service, an organization that provides a coordinated administration for all participating SLR stations and analysis centers." LRO's laser ranging effort is funded by the LRO project.

NASA's Spitzer Spots Clump of Swirling Planetary Material

PASADENA, Calif. -- Astronomers have witnessed odd behavior around a young star. Something, perhaps another star or a planet, appears to be pushing a clump of planet-forming material around. The observations, made with NASA's Spitzer Space Telescope, offer a rare look into the early stages of planet formation.

Planets form out of swirling disks of gas and dust. Spitzer observed infrared light coming from one such disk around a young star, called LRLL 31, over a period of five months. To the astronomers' surprise, the light varied in unexpected ways, and in as little time as one week. Planets take millions of years to form, so it's rare to see anything change on time scales we humans can perceive.

One possible explanation is that a close companion to the star -- either a star or a developing planet -- could be shoving planet-forming material together, causing its thickness to vary as it spins around the star.

"We don't know if planets have formed, or will form, but we are gaining a better understanding of the properties and dynamics of the fine dust that could either become, or indirectly shape, a planet," said James Muzerolle of the Space Telescope Science Institute, Baltimore, Md. Muzerolle is first author of a paper accepted for publication in the Astrophysical Journal Letters. "This is a unique, real-time glimpse into the lengthy process of building planets."

One theory of planet formation suggests that planets start out as dusty grains swirling around a star in a disk. They slowly bulk up in size, collecting more and more mass like sticky snow. As the planets get bigger and bigger, they carve out gaps in the dust, until a so-called transitional disk takes shape with a large doughnut-like hole at its center. Over time, this disk fades and a new type of disk emerges, made up of debris from collisions between planets, asteroids and comets. Ultimately, a more settled, mature solar system like our own forms.

Before Spitzer was launched in 2003, only a few transitional disks with gaps or holes were known. With Spitzer's improved infrared vision, dozens have now been found. The space telescope sensed the warm glow of the disks and indirectly mapped out their structures.

Muzerolle and his team set out to study a family of young stars, many with known transitional disks. The stars are about two to three million years old and about 1,000 light-years away, in the IC 348 star-forming region of the constellation Perseus. A few of the stars showed surprising hints of variations. The astronomers followed up on one, LRLL 31, studying the star over five months with all three of Spitzer's instruments.

The observations showed that light from the inner region of the star's disk changes every few weeks, and, in one instance, in only one week. "Transition disks are rare enough, so to see one with this type of variability is really exciting," said co-author Kevin Flaherty of the University of Arizona, Tucson.

Both the intensity and the wavelength of infrared light varied over time. For instance, when the amount of light seen at shorter wavelengths went up, the brightness at longer wavelengths went down, and vice versa.

Muzerolle and his team say that a companion to the star, circling in a gap in the system's disk, could explain the data. "A companion in the gap of an almost edge-on disk would periodically change the height of the inner disk rim as it circles around the star: a higher rim would emit more light at shorter wavelengths because it is larger and hot, but at the same time, the high rim would shadow the cool material of the outer disk, causing a decrease in the longer-wavelength light. A low rim would do the opposite. This is exactly what we observe in our data," said Elise Furlan, a co-author from NASA's Jet Propulsion Laboratory, Pasadena, Calif.

The companion would have to be close in order to move the material around so fast -- about one-tenth the distance between Earth and the sun.

The astronomers plan to follow up with ground-based telescopes to see if a companion is tugging on the star hard enough to be perceived. Spitzer will also observe the system again in its "warm" mission to see if the changes are periodic, as would be expected with an orbiting companion. Spitzer ran out of coolant in May of this year, and is now operating at a slightly warmer temperature with two infrared channels still functioning.

"For astronomers, watching anything in real-time is exciting," said Muzerolle. "It's like we're biologists getting to watch cells grow in a petri dish, only our specimen is light-years away."

Other authors are Zoltan Balog, Max Planck Institute for Astronomy, Germany; Paul S. Smith and George Rieke, University of Arizona; Lori Allen, National Optical Astronomy Observatory, Tucson; Nuria Calvet, University of Michigan, Ann Arbor; Paola D'Alessio, National Autonomous University of Mexico; S. Thomas Megeath, University of Toledo, Ohio; August Muench, Harvard-Smithsonian Center for Astrophysics, Cambridge; William H. Sherry, National Solar Observatory, Tucson.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

NASA Invites Media to Ames for LCROSS Impact Events

MOFFETT FIELD, Calif. -- NASA's Lunar Crater Observation and Sensing Satellite mission, known as LCROSS, will culminate with two lunar impacts at approximately 4:30 a.m. PDT on Oct. 9. The mission will search for water ice in the Cabeus A crater near the moon's south pole. Reporters are invited to observe the event and participate in pre-impact and post-impact media briefings Oct. 9 at NASA's Ames Research Center at Moffett Field, Calif.

The deadline for U.S. reporters to apply for accreditation is Monday, Oct. 5. International journalists planning to cover the LCROSS impacts from Ames must apply for accreditation no later than Friday, Sept. 25. Media representatives applying for credentials should submit requests to: ARC-Media-Accreditation@mail.nasa.gov.

Journalists should confirm they have been accredited before they travel. No substitution of credentials is allowed at any NASA facility.

Once approved, two forms of government-issued identification, one with a photo, will be required to receive an access badge for Ames to cover the pre- impact media briefing, impact event and post-impact conference. For further information about accreditation, contact Jonas Dino at 650-604-5612.