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The past year’s other huge space stories

The end of 2011 through fall of 2012 had a lot of important discoveries. Here are the ones that didn’t quite make the top ten.
Unfortunately, we can fit only the past year’s top 10 astronomy stories in the pages of Astronomy magazine. Deciding which of the big discoveries announced in the past 12 months to include is always a difficult task. We also wanted to showcase the next 10 important finds announced from the end of 2011 through the end of 2012. So, here they are in chronological order.
01.Sungrazing-comet
The Solar and Heliospheric Observatory imaged a comet (arrow) as it dove toward the Sun on July 6, 2011. This was the first time scientists were able to watch such an object disintegrate as it neared our star’s surface. // SOHO (ESA/NASA)
Observatories spy a comet’s death
By Karri Ferron
On July 6, 2011, a “sungrazing” comet met its demise when it passed too close to our star, as so many others have done before it. But this event was different for astronomers on Earth. For the first time, they were able to watch the comet’s death as it happened. The results appeared in the January 20 issue of Science.

Scientists first identified the comet using the Solar and Heliospheric Observatory (SOHO). They expected the object to eventually disappear from SOHO images, as well as those from NASA’s Solar Dynamics Observatory (SDO). Instead, they saw it evaporate into the Sun’s outer atmosphere as it passed within 62,000 miles (100,000 kilometers) of our star’s surface.

Based on these observations, the astronomers could figure out the comet’s properties. They determined that it was between 150 and 300 feet (45 and 90 meters) long and weighed about the same as an aircraft carrier. It also had a glowing gas tail that appeared to pulse during its journey toward the Sun, which suggests it was gradually losing chunks of material as it fell apart due to our star’s heat.

But with all these observations, one big question remains. “Normally, a comet passing in front of the Sun absorbs the light from the Sun,” says Dean Pesnell, SDO project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We would have expected a black spot against the Sun, not a bright one. … So one of the really big questions is: Why do we see it at all?”
Answering such a question will provide scientists with information not only about this now-dead comet, but also about the Sun’s atmosphere.

02.Mercury
This global mosaic of Mercury covers more than 99.9 percent of the planet’s surface. Scientists combined more than 22,000 individual images taken during the first 6 months of the MESSENGER mission to create the picture. // NASA/JHUAPL/CIW
Mercury’s newest surface features
By Richard Talcott
When planetary scientists announced their latest results from Mercury in the September 30 issue of Science, more than 6 months had come and gone since the MESSENGER spacecraft started orbiting the innermost planet. By that time, MESSENGER (which stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) had experienced 2 full Mercury years. Even more importantly, the probe had witnessed 1 complete Mercury day (equal to 176 Earth days), so it had seen the entire planet’s surface under all possible lighting conditions.

Intriguing surface features figure in two of the most interesting recent results. Although three flybys by MESSENGER (in 2008 and 2009) confirmed that volcanic deposits exist on the planet’s surface, no one expected the vast expanse of lava that flooded the north polar region long ago. (Those flybys occurred over Mercury’s equatorial region and didn’t deliver good views of either pole.)

The north polar volcanic plains cover more than 6 percent of the planet’s surface and extend to extreme depths. “If you imagine standing at the base of the Washington Monument, the top of the lavas would be something like 12 Washington Monuments above you,” says James Head of Brown University in Providence, Rhode Island. Head, lead author of one of the Science papers, says the deposits appear typical of flood lavas, such as those found in the northwestern United States.

The now-solidified lava is so deep that it buries many of the openings from which the lava gushed, but apparently not all. MESSENGER scientists discovered volcanic vents up to 16 miles (25 kilometers) long that likely were sources of some of this lava. The huge volumes carved valleys and created teardrop-shaped ridges in the underlying terrain.

Images also revealed apparently unique features on the floors and central peaks of some impact craters. Although previous flybys revealed these areas to be brighter than the rest of Mercury, no one knew what they were. High-resolution images show them to be small, shallow, irregularly shaped depressions, which the science team dubbed “hollows.” These features appear to be common, and many have bright interiors and halos, says David Blewett of Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, lead author of another Science paper.

“Analysis of the images and estimates of the rate at which the hollows may be growing lead to the conclusion that they are actively forming today,” says Blewett. “The conventional wisdom was that ‘Mercury is just like the Moon.’ But MESSENGER is showing us that Mercury is radically different from the Moon in just about every way we can measure.” The probe will have a chance to do even more — NASA just extended the mission until March 2013.

03.water
Astronomers have found tremendous amounts of cool water vapor around the planet-forming dusty disk surrounding orange dwarf star TW Hydrae, as shown in this artist’s illustration. // NASA/JPL-Caltech
Oceans of water surround protoplanetary cloud
By Bill Andrews
Water, water everywhere — at least, within a certain young star system. With fresh data from the Herschel Space Observatory, a team of scientists published a paper in the October 21 edition of Science describing a huge amount of water vapor surrounding a dusty disk around the young star TW Hydrae.

“Our observations of this cold vapor indicate enough water exists in the disk to fill thousands of Earth oceans,” says lead author Michiel Hogerheijde of Leiden Observatory in the Netherlands. The imagery is appropriate because astronomers believe the water in this system could end up on earthlike planets within it through cometary impacts. This is the first detection of water vapor far from the warm inner regions of a protoplanetary disk, which suggests water-filled planets like Earth may be widespread throughout the universe.

Lying some 175 light-years away in Hydra, TW Hydrae is an orange dwarf star about 10 million years old. Hogerheijde’s team suspects the water it found starts with tiny grains of dust coated with water ice in the outer layers of the dusty disk. The energetic ultraviolet light from the central star probably knocks some of the water molecules free of the dusty ice, causing the thin layer of vapor to form. TW Hydrae and its disk should evolve over the next few million years into a more familiar system with planets, asteroids, and ice-laden comets.

04.Eris
Eris, shown in this illustration, recently passed in front of a star, allowing astronomers to determine its size and other details with new precision. Oddly, Eris appears to be virtually the same size as Pluto, the most famous dwarf planet. // ESO/L. Calçada
Eris in the spotlight
By Bill Andrews
Astronomers have taken advantage of distant dwarf planet Eris passing in front of a star to analyze it in better detail than ever before. A paper in the October 27 issue of Nature describes its likely surface composition and newly determined size, which is almost identical to its fellow dwarf planet Pluto.

Eris’ discovery in 2005 helped bring about the new class of objects called dwarf planets, ultimately leading to Pluto’s demotion. According to the latest findings, Eris is almost perfectly spherical, with a diameter of 1,445 miles (2,326 kilometers), plus or minus 7.5 miles (12 km). Pluto’s diameter measures somewhere between 1,400 and 1,500 miles (2,300 and 2,400 km); its atmosphere makes it difficult for scientists to measure its diameter, so Eris’ size is better known, despite its being about 3 times farther away than Pluto.

Comparing Eris’ more accurate size to its mass, astronomers determined its density to be about 157 pounds per cubic foot (2.52 grams per cubic centimeter). “This density means that Eris is probably a large rocky body covered in a relatively thin mantle of ice,” says co-author Emmanuel Jehin of the University of Liège in Belgium.

The team also found Eris’ surface to be extremely reflective, shining back 96 percent of the light that falls on it. Even fresh snow on Earth isn’t as shiny, making Eris one of the most reflective objects in the solar system, likely due to a thin layer of ice.

“It is extraordinary how much we can find out about a small and distant object such as Eris,” says lead author Bruno Sicardy of the Paris Observatory in France. “Five years after the creation of the new class of dwarf planets, we are finally really getting to know one of its founding members.”
05.ocean-worlds
Europa (left) and Titan, moons of Jupiter and Saturn, respectively, hold a substantial amount of subsurface water, according to discoveries made this past year. // Europa: NASA/JPL/University of Arizona; Titan: NASA/JPL-Caltech/Space Science Institute
More worlds have liquid water
By Liz Kruesi
The search for life elsewhere in the universe focuses largely on finding liquid water, which is crucial for life on Earth. So, when planetary scientists racked up strong evidence this past year that large amounts of water exist on two worlds in the solar system, it brought the hunt for life even closer.

In November 2011, a team of researchers published its study of bumpy and irregular-shaped textures called “chaos terrains” on the jovian moon Europa from 1998 Galileo spacecraft data. By comparing these two structures to similar ones on Earth, the team presented a description of how those on Jupiter’s satellite may have formed. The model indicates that ice near the surface melted due to pressure from below, breaking apart the ice, and then the water refroze with the ice fragments in different positions, taking on jagged and irregular shapes. The importance of this four-step model in the search for life is that the collapsing ice sheets create a mechanism to circulate nutrients on the surface into the water below. “The potential for exchange of material between the surface and subsurface is a big key for astrobiology,” says Wes Patterson of Johns Hopkins University in Laurel, Maryland, a co-author of the study. The team suggests that the lakes some 2 miles (3 kilometers) below the surface are about the size of North America’s Great Lakes, and many more could be scattered across Europa.

Meanwhile, on Saturn’s largest moon, Titan, a global ocean may exist 60 miles (100km) below the surface. Another group of scientists used Cassini data from six close Titan flybys to study how Saturn’s gravity stretches and squeezes the satellite. These tidal changes ranged up to 30 feet (10 meters), about 10 times as large as expected if the moon were composed of solid rock. “Casssini’s detection of large tides on Titan leads to the almost inescapable conclusion that there is a hidden ocean at depth,” says Luciano Iess of the Sapienza University in Rome.

Both discoveries offer compelling indirect evidence of liquid water in our neighborhood, but to prove that it exists on these moons, a spacecraft would need to land on the worlds and drill many miles below the surface —no small feat. The Jupiter Icy Moon Explorer mission, set to launch in 2022, will orbit Europa, but not land on the moon. Scientists will likely have to wait a few more decades to know with certainty if Europa and Titan hold liquid water.

06.dark-energy
Fluctuations from the early universe remain intact, giving cosmologists a ruler (illustrated by white circles) to measure structure. The Baryon Oscillation Spectroscopic Survey project uses this ruler to trace cosmic structure across the universe. // Eric Huff/SDSS-III Team/South Pole Telescope Team/Zosia Rostomian
Scientists trace dark energy’s effects
By Liz Kruesi
In 1998, scientists discovered that our universe’s expansion appears to be accelerating, and since then the search has been on to figure out what’s causing it. Either a mysterious substance, dubbed “dark energy,” is speeding up the expansion, or our best model of gravity — Albert Einstein’s general theory of relativity — is wrong. The first step to learning if dark energy does in fact exist is to map the distribution of galaxies across the universe’s history and thus the rate at which the cosmos expanded. On March 30, a team of cosmologists released the most precise measurements yet of galaxy distances.

To make these measurements, researchers need a ruler, and the early universe provides one. Density fluctuations (such as acoustic, or sound, waves) in the primordial soup of material left impressions on the cosmic microwave background (CMB) as variations in temperature. Those fluctuations eventually grew to the large-scale structure observed today. And the imprints from those “baryon acoustic oscillations” are a consistent size — about 150 megaparsecs (some 500 million light-years) wide.

When observing the cosmic structure out to billions of light-years away, scientists see the 500-million-light-year signatures. If they measure the angle that this “ruler” takes up in the sky, they can determine the distance — to about 1 percent accuracy.

A team using the Baryon Oscillation Spectroscopic Survey (BOSS), the largest component of the third Sloan Digital Sky Survey located in New Mexico, recently released the project’s first 1.5 years of data: three-dimensional positions of 264,283 massive galaxies.

“We’ve made precision measurements of the large-scale structure of the universe 5 to 7 billion years ago — the best measure yet of the size of anything outside the Milky Way,” says David Schlegel of the Lawrence Berkeley National Laboratory in California and BOSS’ principal investigator. This time period is when cosmologists say dark energy first “turned on.” The survey will continue compiling data until 2014, and project scientists expect BOSS to map close to 1.5 million galaxies in total.
07.red-giant
Middleweight stars like our Sun eventually will become red giants, as depicted in this illustration. Scientists have recently figured out a way to explain how these behemoths can spew out much of their mass without it burning up in the process. // David Aguilar/CfA
New insights into stellar death
By Bill Andrews
Whether in humans or stars, death is a big — and largely unknown — part of the life cycle. A Nature study published April 12 may start to change that, however, as it describes events surrounding the demise of Sun-like stars in unprecedented detail. “We are now a big step further in understanding this cycle of life and death,” says co-author Albert Zijlstra of the University of Manchester in England.

When intermediate-mass stars like the Sun reach the red giant phase at the end of their lives, they begin to emit a “super wind” millions of times stronger than ordinary stellar wind. Over some 10,000 years, this process removes about half of the star’s mass, but astronomers weren’t sure exactly how it worked. They had suspected that the uppermost stellar layers formed tiny dust grains, which the star’s radiation then pushed outward. The problem was that such grains would theoretically burn up before traveling far.

The researchers solved this mystery by studying three red giants at exceptionally high resolution. They found surprisingly large dust grains (about 600 nanometers across) less than a stellar diameter away from each giant, suggesting that the dust didn’t absorb the star’s light directly, instead likely reflecting it.

Not only does this explain the super wind’s behavior, and thus shed light on how stars like our Sun will one day die, but it also helps scientists understand how planets like Earth form. “The dust and sand in the super wind will survive the star and later become part of the clouds in space,” says Zijlstra. “Our own Earth has formed from star dust.”

08.cosmic-rays
The search for the source of ultra-high-energy cosmic rays has led to scientists crossing off one leading theory: Gamma-ray bursts do not rev up these particles to their extreme energies. // NSF/J. Yang
Gamma-ray bursts not responsible for extreme cosmic rays
By Liz Kruesi
Ever since scientists discovered ultra-high-energy cosmic rays (particles from space) in the mid-19th century, they’ve been searching for what revs up these particles to such extreme energies. Theories proposed that either the explosive death of an extremely massive star (resulting in a gamma-ray burst [GRB]) or jets shot out from supermassive black holes could accelerate cosmic rays to energies 1 million to 1 billion times those created in the largest Earth-based accelerators. Now, a study published in the April 19 issue of Nature suggests that GRBs are not responsible for ultra-high-energy cosmic rays, thus ruling out one of the leading possibilities.

The team analyzed data from IceCube, a cubic-kilometer detector embedded in the Antarctic ice. The IceCube collaboration looked for neutrinos — particles that interact weakly with matter and have little mass — that are produced as ultra-high-energy cosmic rays decay into other particles. The researchers compared the positions of more than 200 GRBs to IceCube neutrino data.

“According to a leading model, we should have expected to see 8.4 events corresponding to GRB production of neutrinos in the IceCube data,” says Spencer Klein of the Lawrence Berkeley National Laboratory in California and a member of the IceCube collaboration. “We didn’t see any, which indicates that GRBs are not the source of ultra-high-energy cosmic rays.”
Ruling out one method does not confirm that the other leading theory (jets from active supermassive black holes) is the answer to this decades-old puzzle. Scientists will use IceCube and other particle detectors to continue searching for the cause of ultra-high-energy cosmic rays.
09.lunar-crater
The Moon’s Shackleton Crater is a great target for researchers interested in lunar ice formation. Here, a detailed elevation map of the crater appears at left, with a shaded relief of it at right. // NASA/M. T. Zuber, et al., Nature, 2012
Lunar crater is full of ice
By Bill Andrews
The Moon might be an icier place than scientists had first realized, based on recent data from NASA’s Lunar Reconnaissance Orbiter (LRO). In particular, the walls of Shackleton Crater near the lunar south pole may be 5–10 percent ice by weight (30 percent by volume), according to a July 28 paper in Geophysical Research Letters.

“The interior of this crater lies in permanent shadow and is a ‘cold trap’ — a place cold enough to permit ice to accumulate,” says co-author Ben Bussey of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. He’s also the principal investigator of the LRO’s Mini-RF radar, which measured the ice concentration. “The radar results are consistent with the interior of Shackleton containing a few percent ice mixed into the dry lunar soil.”

The crater had already attracted attention earlier in the summer, when LRO’s laser altimeter team announced that Shackleton had significant concentrations of ice after mapping it more precisely than ever before. About 2 miles (3 kilometers) deep and some 12.5 miles (20km) wide, the crater has an ice composition in keeping with other recent lunar analyses, such as the radar measurements of India’s Chandrayaan-1 spacecraft and the plume of debris the Lunar Crater Observation and Sensing Satellite studied in 2009.

The research makes it easier for scientists to understand the Moon’s composition and behavior.
10.Pheonix-Cluster
The conglomeration of galaxies known as the Phoenix cluster, seen in a composite photo at left, is unusual because its central region is home to a tremendous amount of stellar creation, as illustrated at right (hot gas appears red while cooler star-forming gas is blue). // X-ray: NASA/CXC/MIT/M. McDonald; UV: NASA/JPL-Caltech/M. McDonald; Optical: AURA/NOAO/CTIO/MIT/M. McDonald; Illustration: NASA/CXC/M. Weiss
Unusual galaxy cluster furiously forms stars
By Bill Andrews
Galaxy clusters, as the name implies, are among the universe’s largest objects. They can shed light on how the cosmos expands and how galaxies evolve. But an August 16 Nature paper describes a newly found cluster that might change how researchers think about these structures.

The recently discovered object is officially named SPT-CLJ2344-4243, but is nicknamed the Phoenix cluster both because it resides within that constellation and because of its unusual properties. “While galaxies at the center of most clusters may have been dormant for billions of years, the central galaxy in this cluster seems to have come back to life with a new burst of star formation,” says lead author Michael McDonald of the Massachusetts Institute of Technology in Cambridge. “The mythology of the Phoenix, a bird rising from the dead, is a great way to describe this revived object.”

Lying about 5.7 billion light-years distant, the Phoenix cluster has a huge pool of hot gas near its central galaxy, like other clusters. Unlike them, however, Phoenix’s gas forms about 740 solar masses’ worth of stars per year — the highest rate ever seen in a galaxy cluster’s center. The Perseus cluster, for instance, creates only about 37 solar masses per year, likely because its central galaxy’s supermassive black hole disrupts the formation process. Astronomers had recently thought such disruptions occurred in all galaxy clusters, but the Phoenix cluster proves that’s not the case.

Researchers think they might have caught this cluster and its central star city at an opportune time. “The galaxy and its black hole are undergoing unsustainable growth,” says co-author Bradford Benson of the University of Chicago. “This growth spurt can’t last longer than about a hundred million years.” Continued study will reveal more insight into not only galactic clusters but also fundamental processes like star formation.

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