1919 Solar Eclipse

The Eclipse That Revealed the Universe[1]

Einstein presented his general theory of relativity 100 years ago, August 2017.

In 1919, British astronomers photographed a solar eclipse and proved that light bends around our Sun—affirming Einstein’s theory of general relativity.





A view of the 1919 solar eclipse, observed in Sobral, Brazil. Arthur Eddington set out to verify Einstein’s prediction that gravity could affect the course of starlight. Credit Arthur Eddington







So, this is what it is like to play cosmic pinball. The worlds move, and sometimes they line up. Then you find yourself staring up the tube of blackness that is the Moon’s shadow, a sudden hole in the sky during a total solar eclipse.

Such moments have left their marks on human consciousness—like the monoliths in the classic movie, “2001: A Space Odyssey”—since before history was recorded.

Few eclipses have had more impact on modern history than the one that occurred on May 29, 1919, more than six minutes of darkness sweeping across South America and across the Atlantic to Africa. It was during that eclipse that the British astronomer Arthur Eddington ascertained that the light rays from distant stars had been wrenched off their paths by the gravitational field of the Sun.

That affirmed the prediction of Einstein’s theory of general relativity, ascribing gravity to a warp in the geometry of space-time, that gravity could bend light beams. “Lights All Askew in the Heavens,” read a headline in this newspaper.

Eddington’s report made Einstein one of the first celebrities of the new 20th century and ushered in a new dynamic universe, a world in which space and time could jiggle, grow, warp, shrink, rip, collapse into black holes and even disappear. The ramifications of his theory are still unfolding; it was only two years ago that a rippling of space-time—gravitational waves produced by colliding black holes—was discovered.

The British astronomer Arthur Eddington, who discovered that light rays from distant stars would bend from the gravitational field of the Sun, an affirmation of one of the more dramatic predictions of Einstein’s theory of general relativity. Credit Oxford Science Archive, via Getty Images





But the first step wasn’t easy. How it happened illustrates that even the most fundamental advances in science can be hostage to luck and sometimes divine inspiration.

The bending of light by gravity was the most stunning and obvious prediction of Einstein’s theory. Astronomers had been trying to detect the effect at solar eclipses since before he had even finished formulating the theory. Nature and politics did not always cooperate.

One of the earliest to try was Erwin Finlay-Freundlich, an astronomer at the Berlin Observatory who was to become a big Einstein booster. Freundlich led an expedition to the Crimea in 1914 to observe an eclipse, but World War I began and he was arrested as a spy before the eclipse occurred. A team from the Lick Observatory in California did make it to the Crimean eclipse—but it rained.

“I must confess that I never before seriously faced the situation of having everything spoiled by clouds,” said William W. Campbell, the team’s frustrated leader. “One wishes that he could come home by the backdoor and not see anybody.”

Worse, Lick’s special eclipse camera was impounded by the Russians and not returned in time for the next eclipse, in Venezuela in 1916.

The next big chance to prove Einstein correct came in 1918, when the Moon’s shadow tracked right up the Columbia River between Washington State and Oregon. Lick sent another team of observers, but their camera was still not back from the Crimea and their improvised optics fell short, leaving the stars looking like fuzzy dumbbells as darkness fell.

So, the universe was still up for grabs in March 1919, when Eddington and his colleagues set sail for Africa to observe the next eclipse. Astronomically, the prospects were as good as they could get. During the eclipse, the Sun would pass before a big cluster of stars known as the Hyades, so there ought to be plenty of bright lights to see yanked askew.

Eddington was the right man for the job. A math prodigy and professor at Cambridge, he had been an early convert to Einstein’s new theory, and an enthusiastic expositor to his colleagues and countrymen.

A story went that he was once complimented on being one of only three people in the world who understood the theory. Admonished for false modesty when he didn’t respond, Eddington replied that, on the contrary, he was trying to think of who the third person was.

Instruments used during the solar eclipse expedition in Sobral, Brazil. Credit SSPL, via Getty Images







General relativity was so obviously true, he said later, that if it had been up to him he wouldn’t have bothered trying to prove it.

But it wasn’t up to him, due to a quirk of history. Eddington was also a Quaker and so had refused to be drafted into the army. His boss, Frank Dyson, the Astronomer Royal of Britain, saved Eddington from jail by promising that he would undertake an important scientific task, namely the expedition to test the Einstein theory.

Eddington also hoped to help reunite European science, which had been badly splintered by the war, Germans having been essentially disinvited from conferences. Now, an Englishman was setting off to prove the theory of a German, Einstein.

According to Einstein’s final version of the theory, completed in 1915, as their light rays curved around the Sun during an eclipse, stars just grazing the Sun should appear deflected from their normal positions by an angle of about 1.75 second of arc, about a thousandth of the width of a full Moon.

According to old-fashioned Newtonian gravity, starlight would be deflected by only half that amount, 0.86 second, as it passed the Sun during an eclipse.

A second of arc is about the size of a star as it appears to the eye under the best and calmest of conditions from a mountaintop observatory. But atmospheric turbulence and optical exigencies often smudge the stars into bigger blurs.

So, Eddington’s job, as he saw it, was to ascertain whether a bunch of blurs had been nudged off their centers by as much as Einstein had predicted, or half that amount—or none at all. It was Newton versus Einstein.

No pressure there.

And what if Eddington measured twice the Einstein deflection?, Dyson was asked by Edwin Cottingham, one of the astronomers on the expedition. “Then Eddington will go mad and you will come home alone,” Dyson answered.

To improve the chances of success, two teams were sent: Eddington and Cottingham to the island of Principe, off the coast of Africa, and Charles Davidson and Andrew Crommelin to Sobral, a city in Brazil. The fail-safe strategy almost didn’t work.

In Sobral, the weather was unusually cloudy, but a clearing in the clouds opened up only one minute before totality, the moment the Moon fully eclipsed the Sun. On Principe, it rained for an hour and a half on the morning of the eclipse, and Eddington took pictures through fleeting clouds, hoping that some stars would show up.

A few blurry stars were visible on a couple of his photographic plates, and a preliminary examination convinced Eddington that the positions of the stars had moved during the eclipse. He turned to his colleague and said, “Cottingham, you won’t have to go home alone.”

In the end, there were three sets of plates from which the deflection of starlight could be measured. How Eddington and his colleagues played them off against one another sealed the fate of Einstein’s theory.

The best-looking data had come from an Irish telescope at Sobral. The images indicated a deflection of 1.98 seconds of arc—more than Einstein had predicted.

Another Sobral telescope, known as an astrograph, also produced lots of star images, but they were blurred and out of focus, perhaps because heat from the Sun had affected the telescope mirror. The images gave a value of 0.86 for the deflection, about in line with Newton’s formula, but with large uncertainties.

Finally, there was the Principe telescope, which recorded only a handful of stars, from which Eddington heroically derived a reading of 1.61 seconds of arc.

Which result should Eddington use? If he averaged all three, he would wind up in the unhappy middle ground between Newton and Einstein.

If he just depended on the best telescope, as the astronomers and historians John Earman and Clark Glymour pointed out in an influential essay in 1980, the figure of 1.98 would have cast doubt on Einstein’s theory of general relativity.

In the end, Eddington wound up throwing out the Sobral astrograph data on the grounds that it was unreliable. Both of the remaining plates “point to the full deflection 1”.75 of Einstein’s generalized relativity theory,” Dyson and his colleagues wrote in their official report.

“Dear Mother, joyous news today,” Einstein wrote when he got wind of the result.

Astronomers and historians have argued ever since about whether Eddington’s belief that he already knew the answer led him to fudge the eclipse analysis by leaving out the warped astrograph.

In 2007, however, Daniel Kennefick, an astrophysicist and historian at the University of Arkansas, con-cluded after a long study of the records of the eclipse expedition that it was Dyson, the astronomer royal, who had decided to exclude the results from the astrograph. Dyson was well known to be skeptical of Einstein’s new theory.

Eddington and Dyson were right. The experiment was repeated during an eclipse in 1922 and at many other eclipses over the years, always with the same Einsteinian result. With improvements in technology, today even small universities can do the requisite observations.

During the eclipse coming this month, Bobby E. Powell, a physicist at the University of West Georgia, will be redoing the experiment with his students at a site near Lexington, SC. His is only one of half a dozen universities that will try it. He hopes to develop a lab manual for individuals or schools that might want to do it during the next eclipse in American skies, in April 2024.

In modern times, some of the most precise measurements of light-bending have come from radio observations of distant galaxies. In 2009 Edward Fomalont, of the National Radio Astronomy Observatory in Charlottesville, Va., and his colleagues used a set of antennas known as the Very Long Baseline Array to obtain results that supported Einstein’s predictions to within 0.02 percent.

Astronomers in the meantime have learned to use the light-bending and amplifying abilities of immense galaxies as telescopes—Einstein’s own telescope, if you like—to study exploding stars on the other side of the cosmos and to map the mysterious dark matter that pervades the universe.

In November 1919, news of Einstein’s triumph was announced to the world with all due pomp and circumstance at a joint meeting of Royal Society and the Royal Astronomical Society in London.

Presiding over the meeting, the physicist J. J. Thomson called general relativity one of the highest achievements of mankind, describing it “as a whole continent of new scientific facts.” Black holes and the Big Bang were still in the future.

Indeed, what emerged from the Moon’s shadow that cloud-speckled day in May was an entirely new universe.

[1] Dennis Overbye, “Out There,” New York Times (July 31, 2017). A version of this article appears in print on August 1, 2017, on Page D4 of the New York edition with the headline: “When We Knew Light Could Bend”. Downloaded August 6, 2017


Ancient Asteroid and Birth of the Solar System

Ancient Asteroid and Birth of the Solar System[1]


  • Illustration by NASA’s Goddard Space Flight Center Conceptual Image LabAsteroids are more than just dinosaur missiles. They’re the remaining clues to the birth of the Solar System.









The Solar System’s origins have been a point of controversy among planetary scientists. But, an international team tipped the debate (August 3, 2017) in Science with the discovery of one of the oldest known asteroid families. Their work uncovers how some of the first asteroids formed 4 billion years ago and point to how planets like Earth came to be.

“This family is like a big missing piece of a puzzle that we found,” Marco Delbo, a planetary scientist at the Observatoire de la Côte d’Azur in Nice, France, and the study’s lead investigator, said.

In 2012, Delbo and his team launched an asteroid belt treasure hunt. They wanted to learn as much as possible about dark asteroid families—fragments from asteroid collisions that tend to orbit as a collection—in the region of the belt closest to Earth and Mars. NASA’s OSIRIS-REx mission will visit this region, when it stops at the near-Earth asteroid Bennu and collects a sample. Bennu is almost certainly a member of one of these dark asteroid families, Delbo said.

Because smaller chunks of asteroids drift from the point of collision faster than larger pieces do, these asteroid families become shaped like the letter “V.” Smaller pieces spread out far and wide from the original impact site, while larger fragments remain condensed at the point. The legs of the V can resemble the straight wings of a swift or the narrow angle of a vulture in flight.

Older asteroid families are harder to find, but this V signature can expose the age of these clusters. The longer it’s been since the initial collision, the more time smaller pieces have had to spread out into the belt. But this pattern also makes it more difficult to know which pieces belong to a certain family.

Delbo and his team focused on the inner side of the asteroid belt, the one closer to Mars. Then, rather than looking at all of the asteroids in this region, they narrowed their search to dark, carbon-rich asteroids, which are not as common as bright asteroids in this part of the belt.

Using this specific homing method, Delbo and his team identified a collection of asteroids with a unique V-shape, a new family. Given the angle of this family’s V-shaped leg, they estimate this primordial asteroid family is 4 billion years old.

“We discovered this family that is more ancient than anything we know,” Delbo said.

The OSIRIS-REx probe is headed to Bennu, a roughly spherical asteroid measuring about 1,614 feet (492 meters) in diameter. Photo by NASA’s Goddard Space Flight Center Conceptual Image Lab






They double checked their discovery by paging through old science papers. Members of an asteroid family, which originate from a single asteroid, have similar traits, like how dark or bright they are. Their curation found every astral body in the new family looked alike.

Beyond the edge of the newly identified family, however, is a void. A few orphan asteroids populate this area.

“This is the holy grail of the asteroids,” Delbo said. These orphans must have formed in different manner than those that belong to the new family. The orphans are the original settlers, the report found, they existed in the inner belt before anything else.

These orphan asteroids are large, ranging from 21 to around 93 miles across. Their size matches up with predictions from theoretical models of how large original asteroids might have been 4 billion years ago, when they initially formed.

Their size suggests the Solar System was likely formed by gravitational collapse, according to a recent proposal from the Max Planck Institute. This hypothesis posits that the Solar System began 4.5 billion years ago with grains of space dust that pooled into eddies. After half a billion years, gravity had rapidly pulled them together into large objects, around 62 miles in diameter. In the past, people thought this space dust had aggregated over a much longer period of time to create the myriad-sized objects in the Solar System.

Delbo’s study provides evidence for the Max Planck Institute’s gravitational collapse hypothesis by suggesting the oldest asteroids started out large, and then became smaller through collisions and other destructive forces happening in the ancient Solar System. Planetary scientists have been debating this hypothesis for nearly a decade.

“It’s strong evidence, but it’s just one piece of the puzzle,” Francesca DeMeo, a planetary scientist at the Massachusetts Institute of Technology, who was not involved in the study, said. “It’s probably not the final say in asteroid sizes.”

In particular, DeMeo points out that Delbo and his team were looking at one type of asteroid family—the dark asteroids on the inner side of the belt. Whether their results can also apply to bright asteroids or dark asteroids found in other regions of the asteroid belt remains to be seen.

For Delbo’s part, he dreams of tying his discoveries of the asteroid belt to the piece of Bennu due to be delivered by OSIRIS-REx. He is also grateful for the scientists who came before him and without whom his discovery would not have been possible.

[1] Roni Dengler, “This ancient asteroid family reveals clues about the birth of the Solar System,” PBS News Hour (August 3, 2017), downloaded August 3, 2017. Roni Dengler is a 2017 AAAS mass media science & engineering fellow. She recently earned a doctorate in molecular, cellular and developmental biology from the University of Colorado Boulder. Beyond the lab bench, she acted as editor-in-chief for a graduate student-run blog Science Buffs and co-organized several science and science communication symposiums, including the upcoming ComSciCon Rocky Mountain West.

Vulcan, the Vanishing Planet

Vulcan, the Vanishing Planet[1]

Total solar eclipses, like the one that will be visible across the US on 21 August, have historically provided astronomers with unique opportunities for observing the Sun or near-Sun objects. This is the story of a near-Sun planet that was, in fact, not there at all.

In the early 1800s, everyone in the scientific community was standing on the shoulders of such giants as Galileo, Kepler, and Newton. Their works gave astronomers the means and the mathematics to predict the motions of planetary bodies almost exactly. When predictions deviated from observations, astronomers adjusted their models, sometimes adding more planets to the Solar System; that is the story behind Neptune’s discovery in 1846. When astronomers turned their telescopes to Mercury to monitor and model its orbit, they discovered it was not quite what they expected and turned once again to their trusted giants to understand why.

Precession, or the slow gyration of a planet’s orbital major axis, was the source of the problem. Applying Newtonian mechanics to the celestial bodies had modeled this phenomenon very nicely, but Mercury’s orbit did not follow the predictions all of the time. After the French astronomer Urbain Jean Joseph Le Verrier had successfully used similar perturbation methods on Uranus’ orbit to theorize Neptune’s existence, he applied the same approach to Mercury. In September 1859, he announced that a planet or a group of asteroids must exist between Mercury and the Sun. He hypothesized that it orbited about halfway between Mercury and the Sun, was about the same size as its planetary neighbor, and would most likely be seen during a total solar eclipse. Meanwhile, Edmond Lescarbault, a country doctor and amateur astronomer, had witnessed what he thought was a transit of an unknown planet in March 1859. It wasn’t until December that he thought to contact Le Verrier about his findings. Le Verrier trekked out to the countryside to learn what he could of the man and his purported sighting. Judging it legitimate, Le Verrier announced that Lescarbault had discovered a new planet. By February 1860, he christened it Vulcan, after the Roman god of fire. Attempts to see the planet from Spain and Algiers during the 1860 eclipse failed, much to everyone’s disappointment. For the next decade, claims of Vulcan sightings were ubiquitous, but verification eluded everyone, leaving many to doubt its existence.

During the July 1878 eclipse, two prominent American astronomers, James Craig Watson and Lewis Swift, claimed to have seen Vulcan. Newspapers spread the word, but soon critics produced evidence that what Watson and Smith had seen were two well-known stars, and their detection was deemed an error. So, if this planet that seemingly predicted the precession of Mercury’s orbit failed to exist, what was causing this disturbance in the force?

Enter Albert Einstein with his General Theory of Relativity in 1915. Einstein proved that Newton didn’t have all the answers. The latter’s theory required the existence of a non-existent planet while relativity posited that a massive object warps the surrounding space and time, changing the paths of light rays passing nearby. Einstein applied his theory to the orbit of Mercury and discovered that it accounted for the inexplicable perturbation perfectly. During the May 1919 eclipse, Sir Arthur Eddington successfully tested Einstein’s theory using images of stars. This proved that Mercury was not being affected by some elusive planet, but was moving through the distorted space-time around the Sun. Einstein forced Vulcan to its final vanishing act.

Further Reading:

[1] Teresa Wilson, Michigan Technological University, “This Month in Astronomical History: Relativity and Vulcan’s Vanishing Act,” American Astronomical Society. Downloaded August 3, 2017. Each month as part of this new series from the Historical Astronomy Division of the AAS, an important discovery or memorable event in the history of astronomy will be highlighted. This month, we look at the non-existent planet Vulcan and its connection to relativity.

What Are Mars’ Moons Made Of?

What Are Mars’ Moons Made Of?[1]

Phobos as seen by Mars Express. [G. Neukum (FU Berlin) et al., Mars Express, DLR, ESA; Peter Masek]

Where Did Phobos and Deimos Come From?

Phobos and Deimos, Mars’ two small moons, were initially believed to be the result of interplanetary kidnapping. Many moons in our Solar System appear to be captured objects, and the featureless reflectance spectra of Phobos and Deimos hint that they might be D-type asteroids. However, captured objects tend to have highly eccentric orbits, and both Phobos and Deimos orbit Mars in a nearly circular fashion. More recently, it has been proposed that both moons are the result of a massive impact 4.3 billion years ago—instead of having been captured from interplanetary space, they could have coalesced from the debris disk generated by the impact. Past research has shown that the masses and orbits of Phobos and Deimos can be explained by this method. This theory could also explain the presence of Borealis basin, an extended low-altitude region spanning Mars’ north pole, which can be seen in Figure 1.

Topographical map of Mars. Borealis basin is the low-lying (blue) region in the northern hemisphere. It encompasses many officially-named regions, such as Vastitas Borealis and Utopia Planitia. [Adapted from this image, which is made from data from the Mars Orbiter Laser Altimeter aboard Mars Global Surveyor]





In this paper, the authors use smoothed particle hydrodynamics (SPH) simulations to learn more about the thermodynamical and structural properties of the debris disk generated in the proposed impact. SPH is a common method used when simulating astrophysical fluids, or systems with a large number of particles that can be treated as a fluid, like stars in colliding galaxies. In SPH (which has been detailed in previous Astrobites like this one and this one), the properties at a given location within a fluid are extracted by weighting the properties of the particles near that point using a smoothing function (sometimes called a smoothing kernel), which is often simply a Gaussian.

Building Baby Moons

To generate a debris disk, the authors modeled a collision between young Mars and an impactor with 3% the mass of Mars. The first 20 hours of their simulation are shown in the figure below.

Snapshots from the first 20 hours after the simulated impact. Top row: Positions of particles over time. Red points are Mars particles, yellow are particles that fall on to Mars, white are disk particles, and cyan are particles that escape the system. Bottom row: Temperature of the particles. Shock heating in the moments after impact liquefies much of the material.[Hyodo et al. 2017]

The material ejected in the collision is so hot that it becomes molten, but quickly cools into roughly 1.5-meter solid droplets. The initially eccentric orbits of the disk particles precess over the next 30–40 years, resulting in a collisional torus of material around the planet. The collisions within this torus heat the material once more, again rendering it molten. Most of the material cools into 100-micron-sized droplets, but a small fraction of the silicates in the disk vaporize and condense into 0.1-micron-sized particles that can coat the larger particles. Although the solid-to-gas transition is inefficient, the process of gas-to-solid condensation generates the fine silicate particles that could be responsible for the observed, asteroid-like spectral properties.

The figure above shows the cumulative fraction of Mars-originating disk particles as a function of the depth below Mars’ surface from which they originated. Beyond 4 Mars radii (solid line), there is a higher percentage of particles originating from > 50 km below the surface than in the disk as a whole (dashed line). [Hyodo et al. 2017]

Another important finding from this work is that regardless of the angle of the impact, the disk contains material from both the impactor and young Mars. The graph shows the distribution of disk particles as a function of how far below Mars’ surface they originated. Regardless of the impact angle, the disk as a whole contains at least 35% Martian material by mass. In the outer disk, beyond 4 Mars radii, this fraction rises to ~70%. What’s more, the Martian material largely comes from the mantle, about 50–150 km below the surface. This means that although the angle of impact and the radial distance at which the moons form will determine how much Mars material they contain, all formation scenarios lead to the moons being composed of a mixture of the impactor and Martian mantle. Although we currently have rovers scratching the surface, our best hope of learning about the material beneath Mars’ crust could be by studying its moons.

Know Before You Go: How Can This Help Future Mars Missions?

Now that we have some idea what to expect if Phobos and Deimos were formed from a debris disk, how can we use this knowledge? These results will be valuable for the planning of future Mars-system sample-return missions, like JAXA’s planned Martian Moon eXploration (MMX) mission, which is set to launch in the early 2020s. MMX is slated to make close observations of both Phobos and Deimos before collecting a sample from one of the moons and returning it to Earth. Performing simulations like these in advance of future sample return missions will help scientists interpret their findings to learn about the origin of these two moons as well as the interior of Mars itself.

[1] Susanna Kohler at AAS Nova (August 1, 2017) from Astrobites. Downloaded August 3, 2017. Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we repost astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org! The original cites Title: On the Impact Origin of Phobos and Deimos I: Thermodynamic and Physical Aspects. Author: Ryuki Hyodo,Hidenori Genda, Sébastien Charnoz, and Pascal Rosenblatt. First Author’s Institution: Tokyo Institute of Technology, Japan Status: Accepted to ApJ, open access. About the author, Kerrin Hensley: Hensley is a third year graduate student at Boston University, where he studies the upper atmospheres and ionospheres of Venus and Mars. He is especially interested in how the ionospheres of these planets change as the Sun proceeds through its solar activity cycle and what this can tell us about the ionospheres of planets around other stars. Outside of grad school, you can find him rock climbing, drawing, or exploring Boston.

Weird Galaxies

Identifying Weird Galaxies[1]

The image above (click for the full view) shows PanSTARRS observations of some of the 185 galaxies identified in a recent study as “ring galaxies”—bizarre and rare irregular galaxies that exhibit stars and gas in a ring around a central nucleus.





Hoag’s Object, an example of a ring galaxy. [NASA/Hubble Heritage Team/Ray A. Lucas (STScI/AURA)]





Ring galaxies could be formed in a number of ways; one theory is that some might form in a galaxy collision when a smaller galaxy punches through the center of a larger one, triggering star formation around the center. In a recent study, Ian Timmis and Lior Shamir of Lawrence Technological University in Michigan explore ways that we may be able to identify ring galaxies in the overwhelming number of images expected from large upcoming surveys. They develop a computer analysis method that automatically finds ring galaxy candidates based on their visual appearance, and they test their approach on the 3 million galaxy images from the first PanSTARRS data release. To see more of the remarkable galaxies the authors found and to learn more about their identification method, check out the paper below.


Ian Timmis and Lior Shamir 2017 ApJS 231 2. doi:10.3847/1538-4365/aa78a3

[1] Susanna Kohler, “Featured Image: Identifying Weird Galaxies.” AAS NOVA (July 31, 2017), accessed at http://aasnova.org/2017/07/31/featured-image-identifying-weird-galaxies/, July 31, 2017.

Water in the Moon’s Interior

Water in the Moon’s Interior[1]

A new study of satellite data finds that numerous volcanic deposits distributed across the surface of the Moon contain unusually high amounts of trapped water compared with surrounding terrains. The finding of water in these ancient deposits, which are believed to consist of glass beads formed by the explosive eruption of magma coming from the deep lunar interior, bolsters the idea that the lunar mantle is surprisingly water-rich.

Scientists had assumed for years that the interior of the Moon had been largely depleted of water and other volatile compounds. That began to change in 2008, when a research team including Brown University geologist Alberto Saal detected trace amounts of water in some of the volcanic glass beads brought back to Earth from the Apollo 15 and 17 missions to the Moon. In 2011, further study of tiny crystalline formations within those beads revealed that they actually contain similar amounts of water as some basalts on Earth. That suggests that the Moon’s mantle—parts of it, at least—contain as much water as Earth’s.

“The key question is whether those Apollo samples represent the bulk conditions of the lunar interior or instead represent unusual or perhaps anomalous water-rich regions within an otherwise ‘dry’ mantle,” said Ralph Milliken, lead author of the new research and an associate professor in Brown’s Department of Earth, Environmental and Planetary Sciences. “By looking at the orbital data, we can examine the large pyroclastic deposits on the Moon that were never sampled by the Apollo or Luna missions. The fact that nearly all of them exhibit signatures of water suggests that the Apollo samples are not anomalous, so it may be that the bulk interior of the Moon is wet.”

The research, which Milliken co-authored with Shuai Li, a postdoctoral researcher at the University of Hawaii and a recent Brown Ph.D. graduate, is published in Nature Geoscience. The work was part of Li’s Ph.D. thesis.

Detecting the water content of lunar volcanic deposits using orbital instruments is no easy task. Scientists use orbital spectrometers to measure the light that bounces off a planetary surface. By looking at which wavelengths of light are absorbed or reflected by the surface, scientists can get an idea of which minerals and other compounds are present.

The problem is that the lunar surface heats up over the course of a day, especially at the latitudes where these pyroclastic deposits are located. That means that in addition to the light reflected from the surface, the spectrometer also ends up measuring heat.

“That thermally emitted radiation happens at the same wavelengths that we need to use to look for water,” Milliken said. “So in order to say with any confidence that water is present, we first need to account for and remove the thermally emitted component.”

To do that, Li and Milliken used laboratory-based measurements of samples returned from the Apollo missions, combined with a detailed temperature profile of the areas of interest on the Moon’s surface. Using the new thermal correction, the researchers looked at data from the Moon Mineralogy Mapper, an imaging spectrometer that flew aboard India’s Chandrayaan-1 lunar orbiter.

The researchers found evidence of water in nearly all of the large pyroclastic deposits that had been previously mapped across the Moon’s surface, including deposits near the Apollo 15 and 17 landing sites where the water-bearing glass bead samples were collected.



Colored areas indicate elevated water content compared with surrounding terrains. Yellows and reds indicate the richest water content. Milliken lab / Brown University




“The distribution of these water-rich deposits is the key thing,” Milliken said. “They’re spread across the surface, which tells us that the water found in the Apollo samples isn’t a one-off. Lunar pyroclastics seem to be universally water-rich, which suggests the same may be true of the mantle.”

The idea that the interior of the Moon is water-rich raises interesting questions about the Moon’s formation. Scientists think the Moon formed from debris left behind after an object about the size of Mars slammed into the Earth very early in Solar System history. One of the reasons scientists had assumed the Moon’s interior should be dry is that it seems unlikely that any of the hydrogen needed to form water could have survived the heat of that impact.

“The growing evidence for water inside the Moon suggest that water did somehow survive, or that it was brought in shortly after the impact by asteroids or comets before the Moon had completely solidified,” Li said. “The exact origin of water in the lunar interior is still a big question.”

In addition to shedding light on the water story in the early Solar System, the research could also have implications for future lunar exploration. The volcanic beads don’t contain a lot of water—about .05 percent by weight, the researchers say—but the deposits are large, and the water could potentially be extracted.

“Other studies have suggested the presence of water ice in shadowed regions at the lunar poles, but the pyroclastic deposits are at locations that may be easier to access,” Li said. “Anything that helps save future lunar explorers from having to bring lots of water from home is a big step forward, and our results suggest a new alternative.”

The research was funded by the NASA Lunar Advanced Science and Exploration Research Program (NNX12AO63G).

[1]Scientists spy new evidence of water in the Moon’s interior,” News from Brown (July 24, 2017), PROVIDENCE, R.I. [Brown University]

Mystery of Quasars

The Mystery of Quasars[1]

In the early 1960s, astronomer Maarten Schmidt had a problem. Along with other researchers, this fixture of the California Institute of Technology had been studying mysterious radio sources discovered in the 1950s.

These strange objects, the two most notable designated 3C 458 and 3C 273, appeared tiny on the sky but were extremely energetic sources of radio waves. They didn’t fit any logical explanation of what astronomers understood at the time. (The designation 3C came from the Third Cambridge Catalog of Radio Sources, produced at Cambridge University.)

A precise position of 3C 273, using the 200-inch Hale Telescope on Palomar Mountain in California, finally allowed Schmidt to record the object’s spectrum, the signature of its light, for the first time. This, in turn, produced a 1963 paper declaring that the strange radio object lay at the impressive distance of 2.4 billion lightyears. Yet in Earth’s sky, the object appeared merely as a faint star, leading Schmidt to name this new thing a “quasi-stellar object,” or quasar.

How could something so far away be so incredibly energetic? At first, the notion completely baffled astronomers.

And then the mystery deepened. Over the years to come, astronomers found a series of strange, distant, highly energetic objects far beyond the Milky Way. Using a wide range of the electromagnetic spectrum, a rogues’ gallery of super-energetic, distant objects emerged. They came to include quasars, Seyfert galaxies, BL Lacertae objects (or “blazars”), and radio galaxies. For nearly a whole generation, this array of weird objects seemed to represent a complex puzzle of unrelated oddities in the astrophysical zoo.

Eventually, astronomers learned that these strange high-energy objects were similar beasts viewed from different angles. They were all some form of high-energy galaxy, called active galactic nuclei (AGNs) with centers harboring super-massive black holes. Material cascaded around the black holes-but not swallowed up—was slingshot outward for astronomers to see.

And the first step in resolving the mystery of quasars was complete.

[1] David J. Eicher, “The Mystery of Quasars,” Astronomy (45, 5, May 2017, p. 8)