May 28, 2009: A unique signal detected by NASA's Aqua satellite is helping researchers check the health and productivity of ocean plants around the world.

Fluorescent red light emitted by ocean phytoplankton and detected by Aqua reveals how efficiently the microscopic plants are turning sunlight and nutrients into food through photosynthesis.

"This is the first direct measurement of the health of the phytoplankton in the ocean," says Michael Behrenfeld, a biologist at Oregon State University who specializes in marine plants. "We now have an important new tool for observing changes in phytoplankton all over the planet."

Above: Phytoplankton -- such as this colony of chaetoceros socialis -- naturally give off fluorescent light as they dissipate excess solar energy that they cannot consume through photosynthesis. Credit: Maria Vernet, Scripps Institution of Oceanography

The findings were published this month in the journal Biogeosciences and presented at a news briefing on May 28th.

Single-celled phytoplankton fuel nearly all ocean ecosystems, serving as the most basic food source for marine animals from zooplankton to fish to shellfish. In fact, phytoplankton account for half of all photosynthetic activity on Earth. The health of these marine plants affects commercial fisheries, the amount of carbon dioxide the ocean can absorb, and how the ocean responds to climate change.

Over the past two decades, scientists have employed various satellite sensors to measure the amount and distribution of the green pigment chlorophyll, an indicator of the amount of plant life in the ocean. But with the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite, scientists have now observed "red-light fluorescence" over the open ocean.

"Chlorophyll gives us a picture of how much phytoplankton is present," says Scott Doney, a marine chemist from the Woods Hole Oceanographic Institution and a co-author of the paper. "Fluorescence provides insight into how well they are functioning in the ecosystem."

All plants absorb energy from the sun, typically more than they can consume through photosynthesis. The extra energy is mostly released as heat, but a small fraction is re-emitted as fluorescent light in red wavelengths. MODIS is the first instrument to observe this signal on a global scale.

Above: A global map of red fluorescent light emitted by phytoplankton. Credit: Aqua/MODIS/Mike Behrenfeld, Oregon State University [Larger image]

Red-light fluorescence reveals much about the physiology of marine plants and the efficiency of photosynthesis, as different parts of the plant's energy-harnessing machinery are activated based on the amount of light and nutrients available.

For example, the amount of fluorescence increases when phytoplankton are under stress from a lack of iron, a critical nutrient in seawater. The iron needed for plant growth reaches the sea surface on winds blowing dust from deserts and other arid areas, and from upwelling currents near river plumes and islands. Fluorescence data from MODIS has allowed the research team to study these dynamics.

The Indian Ocean was a particular surprise, as large portions of the ocean were seen to "light up" seasonally with changes in monsoon winds. In the summer, fall, and winter – particularly summer – significant southwesterly winds stir up ocean currents and bring more nutrients up from the depths for the phytoplankton. At the same time, the amount of iron-rich dust delivered by winds is reduced.

Right: A map of fluorescent light emitted by plankton in the Indian Ocean, where seasonal monsoons can limit the amount of iron nutrients in the water and stress the plankton to emit more light. Credit: Aqua/MODIS/Mike Behrenfeld, Oregon State University

"On time-scales of weeks to months, we can use these data to track plankton responses to iron inputs from dust storms and the transport of iron-rich water from islands and continents," says Doney. "Over years to decades, we can also detect long-term trends in climate change and other human perturbations to the ocean."

Climate change could mean stronger winds pick up more dust and blow it to sea, or less intense winds leaving waters dust-free. Some regions will become drier and others wetter, changing the regions where dusty soils accumulate and get swept up into the air. Phytoplankton will reflect and react to these global changes.

"NASA satellites are powerful tools," says Behrenfeld. "Huge portions of the ocean remain largely unsampled, so the satellite view is critical to seeing the big picture."

Have you ever wondered how you'd make your morning cup of java if you lived on another planet, or perhaps the moon? That steaming beverage would be a must on a cold lunar morning.

But with rare sunlight, no coal or wood to burn, and no flowing water for hydro-electrical power, how would you make that cup of coffee, much less cook breakfast, heat your abode, and power the life support equipment and tools you needed to live and work up there?

NASA, planning for a future lunar outpost, has been asking those same questions lately.

There's more than one way to generate power on the moon. Fission Surface Power is one of the options NASA is considering. If this method is chosen, an engine invented in the early 1800s by Scottish brothers Robert and James Stirling could help make it work.

Right: An artist's concept of a Fission Surface Power system in operation on the lunar surface.

[Editor's note: If you have questions about this technology, please contact Marshall Space Flight Center Public Affairs at 256 544 0034.]

The Stirlings were so proud of their creation that they made it their namesake – and with good reason. Over the years the Stirling engine -- the reliable, efficient "little engine that could" -- has earned a sterling reputation here on Earth, and it may one day prove its worth on the moon.

"Inhabitants of a lunar outpost will need a safe and effective way to generate light and heat and electricity," says Mike Houts of NASA's Marshall Space Flight Center. "The tried and true Stirling engine fits the bill. It's not only reliable and efficient, but also versatile and clean."

NASA is partnering with the Department of Energy to develop Fission Surface Power technology to produce heat and feed it into a Stirling engine, which, in turn, would convert heat energy into electricity for use by moon explorers.

It's not certain that this kind of power system will be adopted by NASA, but it does have some very appealing qualities. Houts explains: "A key advantage to this power system is that it wouldn't need sunlight to operate. An FSP system could be used to provide power any time, any place, on the surface of moon or Mars. It could be used at the poles and away from the poles, it could weather a cold lunar night, and it would do well in places like deep craters that are always shaded. Not even a swirling, sunlight obscuring, Martian dust storm could stop it."

Above: A Fission Surface Power system reference concept. Click on the image for more details. Credit: Mike Houts/NASA.

NASA's engine would only need to produce 40 kW or less power – just enough for a lunar outpost.

"This power level is high by space standards but extremely low by Earthly standards," says Houts. "It's about 1/20,000th of what a typical Earthly reactor puts out. We'd only need a tiny reactor on the moon – the fueled portion would be only about 10 inches wide by 1½ feet long."

It would provide more power with less mass than other power systems. The whole assembly, radiator on top of Stirling engine on top of reactor, could be stowed in a fraction of the lunar lander.

Before developing the final system, Houts and his team are testing with non-nuclear power for proof of concept.

"We're conducting tests in a thermal vacuum to learn about operating and controlling the system on the moon," says Houts. "We're using resistance heaters to simulate nuclear heat. Electrical resistance produces heat."

After the test system proves the viability of the concept, the team could be directed to build the "real thing," drawing heavily on US and international terrestrial reactor experience.

Above: An artist's concept of a Fission Surface Power System embedded in lunar regolith.

"It would be built from stainless steel and fueled by uranium dioxide. This combination has been used in terrestrial reactors throughout the world, so scientists and engineers are well-versed in its operation."

The unit would not be active at launch, but would be "turned on" once in place on the lunar surface, where it would be surrounded by shielding to prevent any hazard from the radiation emitted.

"It would be very safe," says Houts. "And the beauty of the system is that it would be practically self-regulating."

Here's how it would work: Inside the reactor is a bundle of small tubes filled with uranium. Outside the reactor are control drums -- one side of each drum reflects neutrons and the other side absorbs them, providing a way to control the rate that neutrons escaping the reactor core are reflected back in. To start up the unit, the absorbent side of each control drum is turned out, away from the reactor core, so the reflective material faces in and sends escaping neutrons back in to the core. The resulting increase in available neutrons enables a self-sustaining chain reaction, which produces heat.

A coolant (sodium potassium mixture)* flows through the passage-ways between the tubes, picks up the thermal heat produced by the reacting uranium, and transfers the heat to the Stirling engine. The Stirling engine then does its magic** to generate electricity. Meanwhile the coolant, which has "downloaded" some of its cargo (heat) to the Stirling engine, circulates back through the reactor core, where it picks up heat and is ready to repeat the entire cycle.

The system would use only a miniscule amount of fuel -- 1 kg of uranium every 15 years – and still have enough reactivity to run for decades.

"We give it a life expectancy of 8 years, though, because something else would falter before the fuel would run out."

After shutdown, radiation emitted by the system would decrease rapidly. A replacement system could easily be installed at the same site.

After all, coffee may be in high demand up there!

"The name of the planet is Theia," says Mike Kaiser, STEREO project scientist at the Goddard Space Flight Center. "It's a hypothetical world. We've never actually seen it, but some researchers believe it existed 4.5 billion years ago—and that it collided with Earth to form the Moon."

Right: An artist's concept of one of the STEREO spacecraft. [Larger image]

The "Theia hypothesis" is a brainchild of Princeton theorists Edward Belbruno and Richard Gott. It starts with the popular Great Impact theory of the Moon's origin. Many astronomers hold that in the formative years of the solar system, a Mars-sized protoplanet crashed into Earth. Debris from the collision, a mixture of material from both bodies, spun out into Earth orbit and coalesced into the Moon. This scenario explains many aspects of lunar geology including the size of the Moon's core and the density and isotopic composition of moon rocks.

It's a good theory, but it leaves one awkward question unanswered: Where did the enormous protoplanet come from?

Belbruno and Gott believe it came from a Sun-Earth Lagrange point.

Sun-Earth Lagrange points are regions of space where the pull of the Sun and Earth combine to form a "gravitational well." The flotsam of space tends to gather there much as water gathers at the bottom of a well on Earth. 18th-century mathematician Josef Lagrange proved that there are five such wells in the Sun-Earth system: L1, L2, L3, L4 and L5 located as shown in the diagram below.

When the solar system was young, Lagrange points were populated mainly by planetesimals, the asteroid-sized building blocks of planets. Belbruno and Gott suggest that in one of the Lagrange points, L4 or L5, the planetesimals assembled themselves into Theia, nicknamed after the mythological Greek Titan who gave birth to the Moon goddess Selene.

Above: Sun-Earth Lagrange points. The STEREO probes are about to pass through L4 and L5. Solar observatories often park themselves at L1 while deep space observatories prefer L2. [more]

"Their computer models show that Theia could have grown large enough to produce the Moon if it formed in the L4 or L5 regions, where the balance of forces allowed enough material to accumulate," says Kaiser. "Later, Theia would have been nudged out of L4 or L5 by the increasing gravity of other developing planets like Venus and sent on a collision course with Earth."

If this idea is correct, Theia itself is long gone, but some of the ancient planetesimals that failed to join Theia may still be lingering at L4 or L5.

"The STEREO probes are entering these regions of space now," says Kaiser. "This puts us in a good position to search for Theia's asteroid-sized leftovers."

Just call them "Theiasteroids."

Astronomers have looked for Theiasteroids before using telescopes on Earth, and found nothing, but their results only rule out kilometer-sized objects. By actually entering L4 and L5, STEREO will be able to hunt for much smaller bodies at relatively close range.

Right: This dynamical simulation shows how asteroids linger in the gravitational well of a Lagrange point of the Sun-Jupiter system. The principle of Sun-Earth Lagrange points is the same. Credit: Prof. Aldo Vitagliano/SOLEX.

"The search actually began last month when both spacecraft rolled 180 degrees so that they could take a series of 2-hour exposures of the general L4/L5 areas. In the first sets of images, amateur astronomers found some known asteroids and new comet Itagaki was imaged just a couple of days after the announcement of its discovery. No Theiasteroids however."

Hunting for Theiasteroids is not STEREO's primary mission, he points out. "STEREO is a solar observatory. The two probes are flanking the sun on opposite sides to gain a 3D view of solar activity. We just happen to be passing through the L4 and L5 Lagrange points en route. This is purely bonus science."

"We might not see anything," he continues, "but if we discover lots of asteroids around L4 or L5, it could lead to a mission to analyze the composition of these asteroids in detail. If that mission discovers the asteroids have the same composition as the Earth and Moon, it will support Belbruno and Gott's version of the giant impact theory."

The search will continue for many months to come. Lagrange points are not infinitesimal points in space; they are broad regions 50 million kilometers wide. The STEREO probes are only in the outskirts now. Closest approach to the bottoms of the gravitational wells comes in Sept-Oct. 2009. "We have a lot of observing ahead of us," notes Kaiser.

Readers, you may be able to help. The STEREO team is inviting the public to participate in the search by scrutinizing photos as they come in from the spacecraft. If you see a dot of light moving with respect to the stars, you may have found a Theiasteroid. Links to the data and further instructions may be found at sungrazer.nrl.navy.mil.

Let the hunt begin!