The Earth’s magnetic field, or magnetosphere, stretches from the planet’s core out into space, where it meets the solar wind, a stream of charged particles emitted by the sun. For the most part, the magnetosphere acts as a shield to protect the Earth from this high-energy solar activity.

But when this field comes into contact with the sun’s magnetic field — a process called “magnetic reconnection” — powerful electrical currents from the sun can stream into Earth’s atmosphere, whipping up geomagnetic storms and space weather phenomena that can affect high-altitude aircraft, as well as astronauts on the International Space Station.

Now scientists at MIT and NASA have identified a process in the Earth’s magnetosphere that reinforces its shielding effect, keeping incoming solar energy at bay.

By combining observations from the ground and in space, the team observed a plume of low-energy plasma particles that essentially hitches a ride along magnetic field lines — streaming from Earth’s lower atmosphere up to the point, tens of thousands of kilometers above the surface, where the planet’s magnetic field connects with that of the sun. In this region, which the scientists call the “merging point,” the presence of cold, dense plasma slows magnetic reconnection, blunting the sun’s effects on Earth.

“The Earth’s magnetic field protects life on the surface from the full impact of these solar outbursts,” says John Foster, associate director of MIT’s Haystack Observatory. “Reconnection strips away some of our magnetic shield and lets energy leak in, giving us large, violent storms. These plasmas get pulled into space and slow down the reconnection process, so the impact of the sun on the Earth is less violent.”

Foster and his colleagues publish their results in this week’s issue of Science. The team includes Philip Erickson, principal research scientist at Haystack Observatory, as well as Brian Walsh and David Sibeck at NASA’s Goddard Space Flight Center.

Mapping Earth’s magnetic shield

For more than a decade, scientists at Haystack Observatory have studied plasma plume phenomena using a ground-based technique called GPS-TEC, in which scientists analyze radio signals transmitted from GPS satellites to more than 1,000 receivers on the ground. Large space-weather events, such as geomagnetic storms, can alter the incoming radio waves — a distortion that scientists can use to determine the concentration of plasma particles in the upper atmosphere. Using this data, they can produce two-dimensional global maps of atmospheric phenomena, such as plasma plumes.

These ground-based observations have helped shed light on key characteristics of these plumes, such as how often they occur, and what makes some plumes stronger than others. But as Foster notes, this two-dimensional mapping technique gives an estimate only of what space weather might look like in the low-altitude regions of the magnetosphere. To get a more precise, three-dimensional picture of the entire magnetosphere would require observations directly from space.

Toward this end, Foster approached Walsh with data showing a plasma plume emanating from the Earth’s surface, and extending up into the lower layers of the magnetosphere, during a moderate solar storm in January 2013. Walsh checked the date against the orbital trajectories of three spacecraft that have been circling the Earth to study auroras in the atmosphere.

As it turns out, all three spacecraft crossed the point in the magnetosphere at which Foster had detected a plasma plume from the ground. The team analyzed data from each spacecraft, and found that the same cold, dense plasma plume stretched all the way up to where the solar storm made contact with Earth’s magnetic field.

A river of plasma

Foster says the observations from space validate measurements from the ground. What’s more, the combination of space- and ground-based data give a highly detailed picture of a natural defensive mechanism in the Earth’s magnetosphere.

“This higher-density, cold plasma changes about every plasma physics process it comes in contact with,” Foster says. “It slows down reconnection, and it can contribute to the generation of waves that, in turn, accelerate particles in other parts of the magnetosphere. So it’s a recirculation process, and really fascinating.”

Foster likens this plume phenomenon to a “river of particles,” and says it is not unlike the Gulf Stream, a powerful ocean current that influences the temperature and other properties of surrounding waters. On an atmospheric scale, he says, plasma particles can behave in a similar way, redistributing throughout the atmosphere to form plumes that “flow through a huge circulation system, with a lot of different consequences.”

“What these types of studies are showing is just how dynamic this entire system is,” Foster adds.

Tony Mannucci, supervisor of the Ionospheric and Atmospheric Remote Sensing Group at NASA’s Jet Propulsion Laboratory, says that although others have observed magnetic reconnection, they have not looked at data closer to Earth to understand this connection.

“I believe this group was very creative and ingenious to use these methods to infer how plasma plumes affect magnetic reconnection,” says Mannucci, who was not involved in the research. “This discovery of the direct connection between a plasma plume and the magnetic shield surrounding Earth means that a new set of ground-based observations can be used to infer what is occurring deep in space, allowing us to understand and possibly forecast the implications of solar storms.”

By Jennifer Chu, MIT News Office

The following is adapted from an announcement today by Fermilab.

Scientists looking for dark matter face a serious challenge: No one knows what dark matter particles look like. So their search covers a wide range of possible traits — different masses, different probabilities of interacting with regular matter.

Today, scientists on the Cryogenic Dark Matter Search experiment, or CDMS, announced that they have shifted the border of this search down to a dark-matter particle mass and rate of interaction that has never been probed.

The analysis, led by Adam Anderson, an MIT graduate student in physics, is the first dark matter result using a new sensor technology — developed, in part, at MIT — that shows much better rejection of background events than the previous generation of CDMS detectors. The work was presented today at the Symposium on Sources and Detection of Dark Matter and Dark Energy in the Universe, held at the University of California at Los Angeles.

“We’re pushing CDMS to as low mass as we can,” says Fermilab physicist Dan Bauer, the project manager for CDMS. “We’re proving the particle detector technology here.”

The result, which does not claim any hints of dark matter particles, contradicts a result announced in January by another dark matter experiment, CoGeNT, which uses particle detectors made of germanium, the same material as used by CDMS.

To search for dark matter, CDMS scientists cool their detectors to very low temperatures in order to detect the very small energies deposited by the collisions of dark matter particles with the germanium. They operate their detectors half a mile underground in a former iron ore mine in northern Minnesota. The mine provides shielding from cosmic rays that could clutter the detector as it waits for passing dark matter particles.

Today’s result carves out interesting new dark matter territory for masses below 6 gigaelectronvolts (GeV). The dark matter experiment Large Underground Xenon, or LUX, recently ruled out a wide range of masses and interaction rates above that with the announcement of its first result last October.  

Scientists have expressed increasing interest in the search for low-mass dark matter particles, with CDMS and three other experiments — DAMA, CoGeNT, and CRESST — all finding their data compatible with the existence of dark matter particles between 5 and 20 GeV. But such light dark-matter particles are hard to pin down. The lower the mass of the dark-matter particles, the less energy they leave in detectors, and the more likely it is that background noise will drown out any signals.

Even more confounding is the fact that scientists don’t know whether dark matter particles interact in the same way in detectors built with different materials. In addition to germanium, scientists use argon, xenon, silicon, and other materials to search for dark matter in more than a dozen experiments around the world.

“It’s important to look in as many materials as possible to try to understand whether dark matter interacts in this more complicated way,” says Anderson, who worked on the latest CDMS analysis as part of his MIT thesis. “Some materials might have very weak interactions. If you only picked one, you might miss it.”

Scientists around the world seem to be taking that advice, building different types of detectors and constantly improving their methods.

“Progress is extremely fast,” Anderson says. “The sensitivity of these experiments is increasing by an order of magnitude every few years.”
By News Office

Looking for first light

April 18, 2014

From a cosmic perspective, one could argue that we all come from stars.

Nearly 14 billion years ago, the Big Bang spawned the universe, yielding a primordial mixture of dark matter and gas within the first few minutes. The lightest elements in this gas — hydrogen and helium — were fused in the Big Bang itself, but the other elements that have since evolved into solar systems, planets, and living organisms were formed much later.

Scientists have traced production of these heavier elements, such as carbon and oxygen, to nuclear fusion within stars. But what exactly created the first stars, and when did that happen?

This question of stellar origin is the focus of Robert Simcoe, an associate professor of physics at MIT. Using telescopes on Earth and in space, Simcoe is peering far into the universe’s past, searching for a period when the first stars blinked on.

“What my field is progressing toward is [an understanding of] when and how the first stars turned on, and when galaxies started to look like they do today,” says Simcoe, who recently earned tenure. “We’re starting to see distant objects [with] very low chemical content. That’s one hint that you’re getting to an interesting time.”

Simcoe’s work indicates that something interesting may have taken place a mere 750 million years after the Big Bang, when the universe was only 5 percent of its present age. Last year, he was part of a team that observed the most distant known quasar — a galaxy that shines especially brightly because of an accreting black hole at its center; the light from this object has been traveling to Earth since then. Yet in the quasar’s immediate surroundings, the team found a large structure of diffuse hydrogen gas with no evidence of heavy metals. The finding suggested that this early quasar belonged to a period in which stars had yet to appear and pollute their surroundings. Simcoe says more distant quasars will have to be discovered and characterized before scientists can confirm exactly when the first stars arose — but the recent detection of pristine gas is a promising start.

“Right now it’s a pretty fuzzy boundary of what we’re looking for,” Simcoe says. “But you might as well think expansively.”

Expanding an astronomer’s view

Simcoe began thinking expansively as a child. He grew up in Westborough, Mass., a suburb west of Boston, where the night sky offered clear views of distant stars. A third-grade project on space sparked Simcoe’s interest in observation, and he saved up for a small telescope, which he used in his backyard to pick out constellations. But the telescope’s magnification was too weak to detect far-off galaxies.

One weekend, Simcoe’s father took him on a road trip to Vermont, where amateur telescope-makers had gathered to swap parts and discuss building techniques. The Simcoes came away with a piece of glass that they honed over a period of weeks in the family basement, grinding with progressively finer abrasives to shape the curvature of an optical telescope objective. They fitted the mirror into a cardboard tube, and pointed it at the sky. The payoff was worth the work: Suddenly, Simcoe had a view of multiple galaxies, including Andromeda, and gaseous nebulae around the Milky Way.

An avid musician in high school, Simcoe also contemplated a future as a pianist. But when an audition at the Tanglewood summer music festival in western Massachusetts fell through, he reconsidered. An advertisement in the back of an astronomy magazine convinced him to try astronomy camp, and he soon headed to Arizona, where he learned to observe the sky with even stronger telescopes.

The experience propelled him to Princeton University, where, as an undergraduate, he played a minor part in a major project: the Sloan Digital Sky Survey, the largest, most detailed map of the universe to date, depicting hundreds of thousands of galaxies and quasars.

“This was one of the biggest experiments to happen in astronomy in the last 20 years,” Simcoe recalls. “It was the first time people made a digital map of a good chunk of the sky, and that seemed exciting and ambitious to me.”

The beginnings of a star search

From Princeton, Simcoe headed west, to the California Institute of Technology, where, as a graduate student, he started digging into the mystery of the first stars. At the time, scientific theory held that heavy elements like oxygen and carbon would only be found near the stars and galaxies that produced them. But astronomers began to observe evidence of chemicals far from any galaxy — a puzzle that theories failed to explain.

In his thesis, Simcoe analyzed the distribution of chemicals throughout the universe, and came up with an estimate of the volume of chemicals that may have leaked out of galaxies. His work ultimately informed more realistic models for how galaxies evolve.

“We used to think of galaxies as little vacuum cleaners: They would suck in gas and turn it into stars,” Simcoe explains. “Instead, it’s almost like they’re a factory, taking in pure pristine gas from the Big Bang, polluting it with these other chemicals and spewing it back out.”

Looking for ‘generation zero’

In 2000, a group of universities commissioned the twin Magellan Telescopes in Chile, affording a view of extremely distant objects, like the old quasars and galaxies that help in identifying the era of the first stars. MIT is a shareholder in the telescopes, meaning the Institute’s researchers have regular access to the facility — an opportunity Simcoe seized.

He joined MIT’s physics department as a Pappalardo Postdoctoral Fellow in 2003, and soon after, accepted a faculty position. Since then, he has taken a handful of trips each year to Chile, spending a week at a time at the telescopes, in the remote Atacama Desert.

The observatory includes a dormitory, where scientists sleep during the day. Before nightfall, they rise to calibrate the telescopes before heading to the dining hall.

“Around the dinner table you can get all the scuttlebutt from different places,” Simcoe says. “It’s outside the conference circuit, where everyone’s more formal. Here, everyone’s tired, off their guard — it’s very much like camp.”

When the sun goes down, the work begins, as astronomers spend the night analyzing readouts of objects captured by the telescopes. Based on such observations, Simcoe has identified a number of far-off objects, including the quasar whose spectrum lacked heavy elements.

In the next few years, Simcoe will continue looking for signs of the universe’s first stars, using Magellan, other observatories on Earth, and the Hubble Space Telescope. Today’s stars are produced with “a little carbon and oxygen sprinkled in with gas,” he says. But immediately following the Big Bang, only hydrogen and helium were in any supply. Simcoe says the very first stars may have formed from molecular hydrogen cooling — a very different process than the atomic cooling from carbon and oxygen that gives rise to today’s stars. Detecting far-off stars composed of hydrogen alone, he says, remains a distant goal for astronomers.

“Once you’ve reached those, you know you’re at generation zero,” Simcoe says. “We don’t know exactly when that happened, or how we’re going to get there. But we think we’re in the right ballpark as far as how far back we need to look.”

By Jennifer Chu, MIT News Office

To get an idea of how the early solar system may have formed, scientists often look to asteroids. These relics of rock and dust represent what today’s planets may have been before they differentiated into bodies of core, mantle, and crust.

In the 1980s, scientists’ view of the solar system’s asteroids was essentially static: Asteroids that formed near the sun remained near the sun; those that formed farther out stayed on the outskirts. But in the last decade, astronomers have detected asteroids with compositions unexpected for their locations in space: Those that looked like they formed in warmer environments were found further out in the solar system, and vice versa. Scientists considered these objects to be anomalous “rogue” asteroids.

But now, a new map developed by researchers from MIT and the Paris Observatory charts the size, composition, and location of more than 100,000 asteroids throughout the solar system, and shows that rogue asteroids are actually more common than previously thought. Particularly in the solar system’s main asteroid belt — between Mars and Jupiter — the researchers found a compositionally diverse mix of asteroids.

The new asteroid map suggests that the early solar system may have undergone dramatic changes before the planets assumed their current alignment. For instance, Jupiter may have drifted closer to the sun, dragging with it a host of asteroids that originally formed in the colder edges of the solar system, before moving back out to its current position. Jupiter’s migration may have simultaneously knocked around more close-in asteroids, scattering them outward.

“It’s like Jupiter bowled a strike through the asteroid belt,” says Francesca DeMeo, who did much of the mapping as a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “Everything that was there moves, so you have this melting pot of material coming from all over the solar system.”

DeMeo says the new map will help theorists flesh out such theories of how the solar system evolved early in its history. She and Benoit Carry of the Paris Observatory have published details of the map in Nature.

From a trickle to a river

To create a comprehensive asteroid map, the researchers first analyzed data from the Sloan Digital Sky Survey, which uses a large telescope in New Mexico to take in spectral images of hundreds of thousands of galaxies. Included in the survey is data from more than 100,000 asteroids in the solar system. DeMeo grouped these asteroids by size, location, and composition. She defined this last category by asteroids’ origins — whether in a warmer or colder environment — a characteristic that can be determined by whether an asteroid’s surface is more reflective at redder or bluer wavelengths.

The team then had to account for any observational biases. While the survey includes more than 100,000 asteroids, these are the brightest such objects in the sky. Asteroids that are smaller and less reflective are much harder to pick out, meaning that an asteroid map based on observations may unintentionally leave out an entire population of asteroids.

To avoid any bias in their mapping, the researchers determined that the survey most likely includes every asteroid down to a diameter of five kilometers. At this size limit, they were able to produce an accurate picture of the asteroid belt. The researchers grouped the asteroids by size and composition, and mapped them into distinct regions of the solar system where the asteroids were observed.

From their map, they observed that for larger asteroids, the traditional pattern holds true: The further one gets from the sun, the colder the asteroids appear. But for smaller asteroids, this trend seems to break down. Those that look to have formed in warmer environments can be found not just close to the sun, but throughout the solar system — and asteroids that resemble colder bodies beyond Jupiter can also be found in the inner asteroid belt, closer to Mars.

As the team writes in its paper, “the trickle of asteroids discovered in unexpected locations has turned into a river. We now see that all asteroid types exist in every region of the main belt.”

A shifting solar system

The compositional diversity seen in this new asteroid map may add weight to a theory of planetary migration called the Grand Tack model. This model lays out a scenario in which Jupiter, within the first few million years of the solar system’s creation, migrated as close to the sun as Mars is today. During its migration, Jupiter may have moved right through the asteroid belt, scattering its contents and repopulating it with asteroids from both the inner and outer solar system before moving back out to its current position — a picture that is very different from the traditional, static view of a solar system that formed and stayed essentially in place for the past 4.5 billion years.

“That [theory] has been completely turned on its head,” DeMeo says. “Today we think the absolute opposite: Everything’s been moved around a lot and the solar system has been very dynamic.”

Clark Chapman, a senior research scientist at the Southwest Research Institute in Boulder, Colo., says the new map is a welcome update to the asteroid maps he and his colleagues developed in the 1980s, which included only those asteroids measuring 20 kilometers or more in diameter. In the past two decades, he says, scientists have made leaps in their understanding of asteroids’ dynamics and evolutionary history, which DeMeo and Carry have now put into context.

“What they have done is attempted to at least qualitatively describe how the unexpected relationships between asteroid size, distance from the sun, and composition fit into the current dynamical models and other insights from the past two decades,” Chapman says. “I’m very glad that this basic research has been done, and I think it is a most welcome contribution to understanding the solar system.”

DeMeo adds that the early pinballing of asteroids around the solar system may have had big impacts — literally — on Earth. For instance, colder asteroids that formed further out likely contained ice. When they were brought closer in by planetary migrations, they may have collided with Earth, leaving remnants of ice that eventually melted into water.

“The story of what the asteroid belt is telling us also relates to how Earth developed water, and how it stayed in this Goldilocks region of habitability today,” DeMeo says.

By Jennifer Chu, MIT News Office

For nearly as long as astronomers have been able to observe asteroids, a question has gone unanswered: Why do the surfaces of most asteroids appear redder than meteorites — the remnants of asteroids that have crashed to Earth?

In 2010, Richard Binzel, a professor of planetary sciences at MIT, identified a likely explanation: Asteroids orbiting in our solar system’s main asteroid belt, situated between Mars and Jupiter, are exposed to cosmic radiation, changing the chemical nature of their surfaces and reddening them over time. By contrast, Binzel found that asteroids that venture out of the main belt and pass close to Earth feel the effects of Earth’s gravity, causing “asteroid quakes” that shift surface grains, exposing fresh grains underneath. When these “refreshed” asteroids get too close to Earth, they break apart and fall to its surface as meteorites.

Since then, scientists have thought that close encounters with Earth play a key role in refreshing asteroids. But now Binzel and colleague Francesca DeMeo have found that Mars can also stir up asteroid surfaces, if in close enough contact. The team calculated the orbits of 60 refreshed asteroids, and found that 10 percent of these never cross Earth’s orbit. Instead, these asteroids only come close to Mars, suggesting that the Red Planet can refresh the surfaces of these asteroids.

“We don’t think Earth is the only major driver anymore, and it opens our minds to the possibility that there are other things happening in the solar system causing these asteroids to be refreshed,” says DeMeo, who did much of the work as a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

DeMeo and Binzel, along with former MIT research associate Matthew Lockhart, have published their findings in the journal Icarus.

Asteroid roulette

The idea that Mars may shake up the surface of an asteroid is a surprising one: As Binzel points out, the planet is one-third the size of Earth, and one-tenth as massive — and therefore exerts a far weaker gravitational pull on surrounding objects. But Mars’ position in the solar system places the planet in close proximity with the asteroid belt, increasing the chance of close asteroid encounters.

“Mars is right next to the asteroid belt, and in a way it gets more opportunity than the Earth does to refresh asteroids,” Binzel says. “So that may be a balancing factor.”

DeMeo, who suspected that Mars may have a hand in altering asteroid surfaces, looked through an asteroid database created by the International Astronomical Union’s Minor Planet Center. The database currently consists of observations of 300,000 asteroids and their orbits; 10,000 of these are considered near-Earth asteroids.

Over the past decade, Binzel’s group has tracked the brightest of these asteroids, measuring their colors to determine which may have been refreshed recently. For this most recent paper, the researchers looked at 60 such asteroids, mapping out the orbit of each and determining which orbits had intersected with those of Earth or Mars. DeMeo then calculated the probability, over the last 500,000 years, that an asteroid and either planet would have intersected, creating a close encounter that could potentially generate asteroid quakes.

“Picture Mars and an asteroid going through an intersection, and sometimes they’ll both come through at very nearly the same time,” Binzel says. “If they just barely miss each other, that’s close enough for Mars’ gravity to tug on [the asteroid] and shake it up. It ends up being this random process as to how these things happen, and how often.”

Refreshing the face of an asteroid

From their calculations, the researchers found that 10 percent of their sample of asteroids only cross Mars’ orbit, and not Earth’s. DeMeo explored other potential causes of asteroid refreshing, calculating the probability of asteroids colliding with each other, as well as the possibility for a phenomenon called “spin-up,” in which energy from the sun causes the asteroid to rotate faster and faster, possibly disrupting its surface. From her calculations, DeMeo found no conclusive evidence that either event would significantly refresh asteroids, suggesting that “Mars is the only game in town,” Binzel says.

Although 10 percent of 60 asteroids may not seem like a significant number, DeMeo notes that given Mars’ small size, the fact that the planet may have an effect on one out of 10 asteroids is noteworthy. “Mars is more powerful than we expected,” she says.

Vishnu Reddy, associate research scientist at the Planetary Science Institute in Tucson, Ariz., says the possibility that Mars may weather passing asteroids provides scientists with one of potentially many weathering sources in space.

“On each of the asteroids we have visited so far, every one of them has shown a different kind of space weathering,” says Reddy, who was not involved in the research. “So it appears that not only is composition an important factor, but also the location of the asteroid with respect to the sun.”

The researchers add that now that Mars has been proven to refresh asteroids, other planets, such as Venus, may have similar capabilities — particularly since Venus is closer in mass to Earth.

“You think about these asteroids going around the sun doing their own thing, but there’s really a lot more going on in their histories,” says DeMeo, who is now a postdoc at Harvard University. “This gives you a dynamic idea of the lives of asteroids.”

This work was supported by the National Science Foundation.

By Jennifer Chu, MIT News Office

Last week, scientists in MIT’s Department of Earth, Atmospheric and Planetary Sciences helped characterize three large, near-Earth asteroids, two of which measure about 12 miles in diameter — the largest asteroids to have been discovered in 23 years. The smallest of the three asteroids measures little more than a mile across, but it may pass within 3.4 million miles of Earth, making it a “potentially hazardous asteroid.”

The team made their measurements using NASA’s Infrared Telescope Facility in Hawaii as part of a project devoted to determining the compositions of new comets and asteroids relatively close to Earth. While these newest asteroids pose little danger to our planet in the near future, they possess some unusual features. MIT News spoke with Richard Binzel, a professor of planetary sciences, about his team’s measurements, and the likelihood of a close encounter.

Q: What are you able to tell about these three asteroids from your brief observations of them?

A: So far, we have made spectral color measurements of only the strangest one of these: 2013 UQ4. (These are names only scientists can love!) This object is coming from out beyond Pluto, from the region we call the Kuiper Belt. And to top it off, it is also orbiting in a backward direction compared to all the planets. Nearly all of the 1,000 currently known Kuiper Belt objects reside in orbits that at all times keep them at least as far away as Neptune. This new object, “UQ4,” is on an orbit that carries it closer to the sun than Mars, meaning that it comes rather close to the Earth. From our spectral measurements, we can estimate that its composition is likely carbon-rich, meaning the surface is very dark, reflecting only about 4 percent of the sunlight that hits it. Even though this object does not reflect very much light, the fact that we can see it in our telescopes implies that it must be rather large. From our measurements, we deduce it is nearly 20 kilometers, or 12 miles, across.

Q: The last large asteroid was detected 23 years ago. Why haven’t these new asteroids been detected until now? And what conditions made it possible for you to see them?

A: These newly found objects are in orbits that usually keep them rather far from the sun, meaning they are too faint to see. In addition to being far from the sun, they also spend most of their time way above or way below the plane where the Earth and other planets orbit — thus they are far from where astronomers concentrate most of their searches. So it has been a combination of these objects just happening to be getting close enough to the sun and the ongoing diligence of search teams that has revealed them to be there.

Q: What hazard, if any, do these near-Earth asteroids pose to our planet?

A: Fortunately, none of these objects poses any foreseeable hazard to Earth. Only one, 2013 UP8, approaches Earth’s orbit close enough to merit the categorization as “potentially hazardous.” All that means is that astronomers have a long-term interest to continue to track its orbit.
By Jennifer Chu, MIT News Office

On a clear night, the moon’s battered history comes into sharp relief: Even from 240,000 miles away, its largest craters are so massive as to be visible to the naked eye.

Scientists have long thought that such lunar craters arose during a period called the Late Heavy Bombardment (LHB), about 4 billion years ago. During that time, a hailstorm of giant asteroids pummeled the solar system, slamming into the moon, along with young planets like Mercury, Venus, Earth, and Mars.

Evidence for this theoretical period comes mostly from the moon itself. While most traces of Earth’s early history have been wiped away by erosion and tectonic activity, the moon remains as a nearby, relatively untouched, and easily observable relic of the early solar system. In particular, scientists have based most of their theories of that period on the impact basins found on the moon’s near side — the side always facing the Earth — assuming, from the size of its craters and basins, that the moon and other planets endured impacts from massive asteroids.

But now scientists from MIT, the University of Paris, and elsewhere have found that craters on the near side of the moon may not reflect the intensity of asteroid impacts from that period. Instead, much smaller asteroids likely created these craters — a finding that may redefine scientists’ picture of the LHB.

“This is very interesting, because we thought we knew the approximate sizes of impacting asteroids to make the bigger basins on the near side,” says Maria Zuber, MIT’s vice president for research and the E.A. Griswold Professor of Geophysics. “What [this work] indicates is that the flux of large impacting bodies during the Late Heavy Bombardment has been overestimated.”

Zuber and her colleagues publish their results this week in the journal Science.

Different faces of the moon

While massive impact basins pockmark the moon’s near side, its far side contains considerably smaller basins. The discrepancy in crater distribution has puzzled scientists for decades.

To investigate what may have caused this difference, the team obtained data from NASA’s twin GRAIL probes, which orbited the moon from January to December 2012. During its mission, the probes circled the moon, making measurements of its gravity. Zuber and her colleagues used this data to generate a highly detailed map of the moon’s crust, showing areas where the crust thickens and thins; in general, the group observed that the moon’s near side has a thinner crust than its far side.

Katarina Milijkovic, a postdoc at the University of Paris, generated computer simulations of asteroid impacts on the moon by plugging in crustal thickness data from GRAIL (which stands for Gravity Recovery and Interior Laboratory). Milijkovic also incorporated estimates of the moon’s early internal temperatures from thermal modeling, based in part on lava deposits that flooded the large impact basins on the moon’s near side. Scientists have observed that more volcanic activity occurred on the near side, generating higher internal temperatures than on the moon’s far side.

A less catastrophic bombardment

With crustal thickness and temperature data incorporated into the model, Milijkovic then simulated the effects, on both the moon’s near and far sides, of impacts by asteroids of the same size and velocity. She found that identical asteroids would have had very different impacts on the two sides, creating basins on the near side that were as much as twice as large as those on the far side — a result that matches  the size distribution of structures seen today.

Zuber says the near side’s thinner crust and higher temperatures may have made the surface more deformable than the thicker, cooler crust of the moon’s far side. These results, she says, suggest that the LHB may have involved less massive asteroids than scientists have thought.

“I’d certainly been a believer in the Late Heavy Bombardment from looking at those large impact basins. The idea of a Late Heavy Bombardment remains,” Zuber says, “but it will be have to be re-examined.”

Why the near side should have larger impact basins than the far side is puzzling, since previous work has shown that the impact flux on both sides should be about the same. According to the team’s results, the far side of the moon may better reflect the size distribution of asteroids that pummeled the early inner solar system.

“My simulations show that the largest lunar far side basin could have been formed by approximately the same size impactor as the largest impact basin on the lunar near side,” Milijkovic says — even though the latter basin is much larger. “In essence, the Late Heavy Bombardment should have been less catastrophic.”

William Bottke, a planetary scientist specializing in the study of asteroids at the Southwest Research Institute in Boulder, Colo., says scientists have questioned the difference in crater sizes on the moon since the 1960s, when researchers first imaged the moon. Bottke says the recent results may provide a resolution to this puzzle.

“I do think this work has intriguing implications for the earliest time of bombardment of the moon and other worlds, which will be interesting to explore,” Bottke says. “To really make progress, it would be useful to have more physical constraints from the moon itself. Ideally, this would come in the form of samples — rocks are great at telling the stories of worlds.”
By Jennifer Chu, MIT News Office

On the exoplanet Kepler 7b, the weather is highly predictable, an international team of scientists has found: On any given day, the exoplanet, which orbits a star nearly 1,000 light-years from Earth, is heavily overcast on one side, while the other side likely enjoys clear, cloudless weather.

The new work, by researchers from MIT and other institutions, is the first mapping of the distribution of clouds on an exoplanet. The scientists observed that one of Kepler 7b’s hemispheres is blanketed with a dense layer of clouds — far denser than any found on Earth, and so thick that it reflects a significant portion of its host star’s incoming light. This shield of clouds makes the planet cooler than others of its type, creating an atmosphere that encourages further cloud formation.

The team generated a low-resolution map of the planet’s clouds using optical data from NASA’s Kepler Space Telescope. The researchers also analyzed the light originating from Kepler 7b at various phases of its orbit, finding that much of the planet’s reflectivity is due to the presence of clouds, and that this cloud cover is unevenly distributed.

“There are a lot of different chemical processes that could take place to create this inhomogeneous cloud,” says Nikole Lewis, a postdoc in the Department of Earth, Atmospheric and Planetary Sciences (EAPS). “Kepler 7b is an important test-bed for the way circulation and cloud distribution work together in exoplanet atmospheres.”

Lewis and her colleagues have published their results in Astrophysical Journal Letters. Co-authors from MIT include postdocs Brice-Olivier Demory and Andras Zsom, graduate student Julien de Wit, and Sara Seager, the Class of 1941 Professor of Physics and Planetary Science.

Mapping clouds, slice by slice

Kepler 7b was among the first exoplanets identified by the Kepler spacecraft, which has since confirmed more than 130 planets outside our solar system. The planet is considered a “hot Jupiter,” as it is composed mostly of gas, and is about 50 percent larger than Jupiter (although it has only about half the mass of that planet).

In 2011, Demory analyzed Kepler 7b’s albedo, or reflectivity, and found that it is unusually bright for an exoplanet, reflecting about 50 percent of light from its star. At the time, the cause of such reflectivity was a mystery, but the new analysis, which makes use of Spitzer’s infrared observations, reveals that much of it is due to the presence of clouds in Kepler 7b’s atmosphere.

To reach this conclusion, the researchers looked through three years’ worth of Kepler light data, combined with recent thermal observations from the planet, taken with NASA’s Spitzer Space Telescope. Combining both datasets, the researchers compared the amount of light and heat given off by the planet at every phase of its orbit. The planet is tidally locked, presenting the same face to its star at all times. From Earth, the planet appears to wax and wane as it circles its star, much like the phases of our moon.

“You can reconstruct the information in terms of brightness, slice by slice,” de Wit says. “This is really fantastic, because though the planet is extremely small, there are techniques for getting spatial information about the planet.”

Clouds rolling in

The researchers analyzed Kepler 7b’s phase curves — measurements of light from the planet at every orbital phase, taken by the Kepler spacecraft. To determine whether these emissions stem from light or heat, the team looked at phase curves in the infrared, provided by Spitzer. They detected very little thermal energy emitted by the planet — a confirmation that most of Kepler 7b’s emissions are indeed reflected light.

But that finding wasn’t a sure indication of clouds on the planet. The group reasoned that the reflected light could instead be caused by a phenomenon called Rayleigh scattering, in which light from Kepler 7b’s star uniformly scatters around the planet, reflected by atoms or molecules much smaller than those in clouds — much as Earth’s atmospheric gases scatter sunlight, creating a blue sky.  

To distinguish between the two possibilities, the group looked again at Kepler 7b’s phase curves. If the planet’s reflectivity is due to uniform Rayleigh scattering, its light emissions should peak at the point at which the planet is behind the star, displaying its full dayside to an observer. But instead, the researchers found that the planet’s brightness peaked slightly after it had passed behind the star, indicating that its reflectivity is not uniform — a sign that the reflectivity was due to an uneven distribution of clouds.

It’s unclear exactly what conditions may give rise to such a stark contrast in cloud cover; Lewis says that investigating the possible causes will be a research focus in the future.

“Kepler 7b happens to be in this temperature range where you can form condensates high up in the atmosphere,” Lewis observes. “Compared to Jupiter, it has a lower gravity that allows you to keep particles lofted much more readily. So Kepler 7b is in this happy regime that allows the atmosphere to create this dense cloud deck. It will keep us busy for the next several years.”

“This detection of clouds on Kepler 7b is very startling, but also very convincing,” says Drake Deming, a professor of astronomy at the University of Maryland who was not involved in the research. “This illustrates that the Kepler data are a gold mine of information that we haven’t fully mined yet, even for atmospheric properties.”

This research was funded by NASA. Zsom was supported by the German Science Foundation, and de Wit received support from the Belgian American Educational Foundation and Wallonie-Bruxelles International. Lewis is supported by a Sagan fellowship.
By Jennifer Chu, MIT News Office

Two MIT professors — computer scientist Dina Katabi and astrophysicist Sara Seager — are among 24 recipients nationwide of 2013 MacArthur Fellowships, sometimes referred to as “genius grants.”

The fellowships, awarded annually, carry a five-year, $625,000 prize, which recipients are free to use as they see fit. Including today’s winners, 19 MIT faculty members, and three staff members at the Institute, have won MacArthur Fellowships, which were established in 1981.

Dina Katabi

Katabi, a professor of computer science and engineering, works at the intersection of computer science, electrical engineering and physics to improve the speed, reliability and security of data exchange. “That intersection can enable new things that we couldn’t have thought of if we just focused on one of those things alone,” Katabi says. “How can you rethink your mobile applications if you have information about how the signal is going to propagate over the air and the wireless medium? How can you design your signals differently if you know what the application wants?”

The MacArthur Foundation cited Katabi, 42, for her “ability to translate long-recognized theoretical advances into practical solutions that could be deployed in the real world. Through her numerous contributions, Katabi has become a leader in accelerating our capacity to communicate high volumes of information securely without restricting mobility.”

Although most of Katabi’s work has centered on wireless data transmission, her research has provided solutions for a range of networking issues — from protocols to minimize congestion in high-bandwidth networks to algorithms for spectrum analysis.

“When they called me, I thought they were joking, so I wasn’t paying attention,” Katabi says. But, she adds, a recognition like the MacArthur grant “brings a lot of general attention to the work which you don’t get when you publish your work in your own community. And that’s very important for scientists.”

Katabi joined the MIT faculty in 2003. She is director of the MIT Center for Wireless Networks and Mobile Computing, and a member of the Computer Science and Artificial Intelligence Laboratory, where she leads the Networks at MIT group.

Sara Seager

Seager, who is the Class of 1941 Professor of Physics and Planetary Science, is an astrophysicist and planetary scientist who has explored the possibility of life elsewhere in the galaxy. Specifically, she has adapted the principles of planetary science to the study of exoplanets — planets outside our own solar system.

The MacArthur Foundation cited Seager, 42, for “quickly advancing a subfield initially viewed with skepticism by the scientific community. A mere hypothesis until the mid-1990s, nearly 900 exoplanets in more than 600 planetary systems have since been identified, with thousands of more planet candidates known.”

Seager found out about the award a few weeks ago via a phone call, although the MacArthur Foundation had to try twice to deliver the good news.

“The first call did not get through because, as the Foundation put it, my ‘very professional and protective assistant’ screens my calls for anything unusual, as I try to avoid calls about UFOs and aliens,” Seager recalls. “This one sounded like a UFO call at first.”

The award, she says, represents the enormous support she’s received from friends and colleagues throughout her career.

“So many people in my professional life and personal life believe in me, and that
I can accomplish my very most ambitious dreams,” Seager says. “I did not fully appreciate this until the MacArthur Award.”

Seager adds that she is looking forward to the freedom that the fellowship provides — both for her research and her family life.

“As a single (widowed) mother, I will use all of the grant money on the home front, so my own brain can be free to think creatively, and I can preserve quality time
with my children,” Seager says.

Seager joined the MIT faculty in 2006, following appointments at the Institute for Advanced Study in Princeton, N.J., and the Carnegie Institute of Washington. She is the author of “Exoplanet Atmospheres” and “Exoplanets,” both published in 2010.

Other recent MacArthur Fellowship winners from MIT have included Junot Díaz, the Rudge and Nancy Allen Professor of Writing at MIT, in 2012; Nergis Mavalvala, the Curtis and Kathleen Marble Professor of Astrophysics, in 2010; Esther Duflo, the Abdul Latif Jameel Professor of Poverty Alleviation and Development Economics, in 2009; John Ochsendorf, the Class of 1942 Professor of Building Technology and Civil and Environmental Engineering, in 2008; and Marin Soljačić, a professor of physics, also in 2008.

By Jennifer Chu and Larry Hardesty, MIT News Office

The future of satellite technology is getting small — about the size of a shoebox, to be exact. These so-called “CubeSats,” and other small satellites, are making space exploration cheaper and more accessible: The minuscule probes can be launched into orbit at a fraction of the weight and cost of traditional satellites.

But with such small packages come big limitations — namely, a satellite’s communication range. Large, far-ranging radio dishes are impossible to store in a CubeSat’s tight quarters. Instead, the satellites are equipped with smaller, less powerful antennae, restricting them to orbits below those of most geosynchronous satellites.

Now researchers at MIT have come up with a design that may significantly increase the communication range of small satellites, enabling them to travel much farther in the solar system: The team has built and tested an inflatable antenna that can fold into a compact space and inflate when in orbit.


View of a CubeSat equipped with an inflated antenna, in a NASA radiation chamber. Photo: Alessandra Babuscia

The antenna significantly amplifies a radio signal, allowing a CubeSat to transmit data back to Earth at a higher rate. The distance that can be covered by a satellite outfitted with an inflatable antenna is seven times farther than that of existing CubeSat communications.

“With this antenna you could transmit from the moon, and even farther than that,” says Alessandra Babuscia, who led the research as a postdoc at MIT. “This antenna is one of the cheapest and most economical solutions to the problem of communications.”

The team, led by Babuscia, is part of Professor Sara Seager’s research group and also includes graduate students Benjamin Corbin, Mary Knapp, and Mark Van de Loo from MIT, and Rebecca Jensen-Clem from the California Institute of Technology. The researchers, from MIT’s departments of Aeronautics and Astronautics and of Earth, Atmospheric and Planetary Sciences, have detailed their results in the journal Acta Astronautica.

‘Magic’ powder

An inflatable antenna is not a new idea. In fact, previous experiments in space have successfully tested such designs, though mostly for large satellites: To inflate these bulkier antennae, engineers install a system of pressure valves to fill them with air once in space — heavy, cumbersome equipment that would not fit within a CubeSat’s limited real estate.

Babuscia raises another concern: As small satellites are often launched as secondary payloads aboard rockets containing other scientific missions, a satellite loaded with pressure valves may backfire, with explosive consequences, jeopardizing everything on board. This is all the more reason, she says, to find a new inflation mechanism.

The team landed on a lighter, safer solution, based on sublimating powder, a chemical compound that transforms from a solid powder to a gas when exposed to low pressure.

“It’s almost like magic,” Babuscia explains. “Once you are in space, the difference in pressure triggers a chemical reaction that makes the powder sublimate from the solid state to the gas state, and that inflates the antenna.”

Testing an inflating idea

Babuscia and her colleagues built two prototype antennae, each a meter wide, out of Mylar; one resembled a cone and the other a cylinder when inflated. They determined an optimal folding configuration for each design, and packed each antenna into a 10-cubic-centimeter space within a CubeSat, along with a few grams of benzoic acid, a type of sublimating powder. The team tested each antenna’s inflation in a vacuum chamber at MIT, lowering the pressure to just above that experienced in space. In response, the powder converted to a gas, inflating both antennae to the desired shape.

The group also tested each antenna’s electromagnetic properties — an indication of how well an antenna can transmit data. In radiation simulations of both the conical and cylindrical designs, the researchers observed that the cylindrical antenna performed slightly better, transmitting data 10 times faster, and seven times farther, than existing CubeSat antennae.

An antenna made of thin Mylar, while potentially powerful, can be vulnerable to passing detritus in space. Micrometeroids, for example, can puncture a balloon, causing leaks and affecting an antenna’s performance. But Babuscia says the use of sublimating powder can circumvent the problems caused by micrometeroid impacts. She explains that a sublimating powder will only create as much gas as needed to fully inflate an antenna, leaving residual powder to sublimate later, to compensate for any later leaks or punctures.

The group tested this theory in a coarse simulation, modeling the inflatable antenna’s behavior with different frequency of impacts to assess how much of an antenna’s surface may be punctured and how much air may leak out without compromising its performance. The researchers found that with the right sublimating powder, the lifetime of a CubeSat’s inflatable antenna may be a few years, even if it is riddled with small holes.

Kar-Ming Cheung, an engineer specializing in space communications operations at NASA’s Jet Propulsion Laboratory (JPL), says the group’s design addresses today’s main limitations in CubeSat communications: size, weight and power.

“A directional antenna has been out of the question for CubeSats,” says Cheung, who was not involved in the research. “An inflatable antenna would enable orders of magnitude improvement in data return. This idea is very promising.”

Babuscia says future tests may involve creating tiny holes in a prototype and inflating it in a vacuum chamber to see how much powder would be required to keep the antenna inflated. She is now continuing to refine the antenna design at JPL.

“In the end, what’s going to make the success of CubeSat communications will be a lot of different ideas, and the ability of engineers to find the right solution for each mission,” Babuscia says. “So inflatable antennae could be for a spacecraft going by itself to an asteroid. For another problem, you’d need another solution. But all this research builds a set of options to allow these spacecraft, made directly by universities, to fly in deep space.”
By Jennifer Chu, MIT News Office