Construction of the World's Largest Neutrino Observatory Completed: Antarctica's IceCube

The last of 86 holes had been drilled and a total of 5,160 optical sensors are now installed to form the main detector--a cubic kilometer of instrumented ice--of the IceCube Neutrino Observatory, located at the National Science Foundation's Amundsen-Scott South Pole Station.

From its vantage point at the end of the world, IceCube provides an innovative means to investigate the properties of fundamental particles that originate in some of the most spectacular phenomena in the universe.

In the deep, dark, stillness of the Antarctic ice, IceCube records the rare collisions of neutrinos--elusive sub-atomic particles--with the atomic nuclei of the water molecules of the ice. Some neutrinos come from the sun, while others come from cosmic rays interacting with the Earth's atmosphere and dramatic astronomical sources such as exploding stars in the Milky Way and other distant galaxies. Trillions of neutrinos stream through the human body at any given moment, but they rarely interact with regular matter, and researchers want to know more about them and where they come from.

The size of the observatory is important because it increases the number of potential collisions that can be observed, making neutrino astrophysics a reality.

The completion of construction brings to a culmination one of the most ambitious and complex multinational scientific projects ever attempted. The National Science Foundation (NSF) contributed $242 million toward the total project cost of $279 million. NSF is the manager of the United States Antarctic Program, which coordinates all U.S. research on the southernmost continent.

The University of Wisconsin-Madison, as the lead U.S. institution for the project, was funded by NSF to manage and coordinate the work needed to design and build the complex and often unique components and software for the project.

The university designed and built the Enhanced Hot Water Drill, which was assembled at the physical sciences lab in Stoughton, Wisconsin. The 4.8- megawatt hot-water drill is a unique machine that can penetrate more than two kilometers into the ice in less than two days.

After the hot water drill bores cleanly through the ice sheet, deployment specialists attach optical sensors to cable strings and lower them to depths between 1,450 and 2,450 meters. The ice itself at these depths is very dark and optically ultratransparent.

Each string has 60 sensors at depth and the 86 strings make up the main IceCube Detector. In addition, four more sensors sit on the top of the ice above each string, forming the IceTop array. The IceTop array combined with the IceCube detector form the IceCube Observatory, whose sensors record the neutrino interactions.

The successful completion of the observatory is also a milestone for international scientific cooperation on the southernmost continent. In addition to researchers at universities and research labs in the U.S., Belgium, Germany and Sweden--the countries that funded the observatory--IceCube data are analyzed by the larger IceCube Collaboration, which also includes researchers from Barbados, Canada, Japan, New Zealand, Switzerland and the United Kingdom.

"IceCube is not only a magnificent observatory for fundamental astrophysical research, it is the kind of ambitious science that can only be attempted through the cooperation--the science diplomacy, if you will--of many nations working together in the finest traditions of Antarctic science toward a single goal," said Karl A. Erb, director of NSF's Office of Polar Programs.

"To complete such an ambitious project, both on schedule and within budget, is a tribute to the fine work of the University of Wisconsin-Madison and its partner institutions, but it's also a reflection of the excellence of the personnel and infrastructure of the U.S. Antarctic Program," he added. "Science like IceCube is done in Antarctica because it is a unique global laboratory. I am very gratified that the U.S. Antarctic Program is equal to the challenge of supporting such a project."

IceCube is among the most ambitious and complex scientific construction projects ever attempted.

To build the observatory, all project personnel, equipment, food, and detector components had to be transported to Antarctica from various places around the globe. Everything then had to be flown in ski-equipped C-130 cargo aircraft from McMurdo Station near the Antarctic coast to the South Pole, more than 800 air miles away.

Working only during the relatively warm and short Antarctic summer--from November through February, when the sun shines 24 hours a day--drill and deployment teams worked in shifts to maximize their short time on the ice each year.

An international team of scientists, engineers and computer specialists have been working on development and construction of the detector since November 1999, when the first proposal was submitted to NSF and partners in Belgium, Germany and Sweden.

In the 1950's, Nobelist in physics Frederick Reines and other particle physicists realized that neutrinos could be used as astronomical messengers. Unlike light, neutrinos pass through most matter, making them a unique probe into the most violent processes in the universe involving neutron stars and black holes. The neutrinos IceCube studies have energies far exceeding those produced by manmade accelerators.

Unlike many large-scale science projects, IceCube began recording data before construction was complete. Each year since 2005 following the first deployment season, the new configuration of sensor strings began taking data. Each year as the detector grew, more and better data made its way from the South Pole to the data warehouses in the University of Wisconsin and around the world.

"Even in this challenging phase of the project, we published results on the search for dark matter and found intriguing patterns in the arrival directions of cosmic rays. Already, IceCube has extended the measurements of the atmospheric neutrino beam to energies in excess of 100 TeV," said Francis Halzen, principal investigator for the project. "With the completion of IceCube, we are on our way to reaching a level of sensitivity that may allow us to see neutrinos from sources beyond the sun."

Funding agencies outside of the U.S. that contributed to the construction of the IceCube Observatory are:

• in Belgium: Fonds de la Recherche Scientifique (FRS-FNRS) and Fonds voor Wetenschappelijk Onderzoek (FWO);
• in Germany: Federal Ministry of Education and Research (BMBF) and Deutsches Elektronen-Synchrotron Project (PD-DESY): and
• in Sweden: Swedish Research Council (VR), and the Knut and Alice Wallenburg Foundation.

Sensor descends down a hole in the ice as part of the final season of IceCube. Icecube is among the most ambitious scientific construction projects ever attempted. (Credit: NSF/B. Gudbjartsson)

Universe's Most Massive Stars Can Form in Near Isolation, New Study Finds

This is the most detailed observational study to date of massive stars that appear (from the ground) to be alone. The scientists used the Hubble Space Telescope to zoom in on eight of these giants, which range from 20 to 150 times as massive as the Sun. The stars they looked at are in the Small Magellanic Cloud, a dwarf galaxy that's one of the Milky Way's nearest neighbors.

Their results, published in the Dec. 20 edition of The Astrophysical Journal, show that five of the stars had no neighbors large enough for Hubble to discern. The remaining three appeared to be in tiny clusters of ten or fewer stars.

Doctoral student Joel Lamb and associate professor Sally Oey, both in the Department of Astronomy, explained the significance of their findings.
"My dad used to fish in a tiny pond on his grandma's farm," Lamb said. "One day he pulled out a giant largemouth bass. This was the biggest fish he's caught, and he's fished in a lot of big lakes. What we're looking at is analogous to this. We're asking: 'Can a small pond produce a giant fish? Does the size of the lake determine how big the fish is?' The lake in this case would be the cluster of stars.

"Our results show that you can, in fact, form big stars in small ponds."
The most massive stars direct the evolution of their galaxies. Their winds and radiation shape interstellar gas and promote the birth of new stars. Their violent supernovae explosions create all the heavy elements that are essential for life and the Earth. That's why astronomers want to understand how and where these giant stars form. There is currently a big debate about their origins, Oey said.

One theory is that the mass of a star depends on the size of the cluster in which it is born, and only a large star cluster could provide a dense enough source of gas and dust to bring about one of these massive stars. The opposing theory, and the one that this research supports, is that these monstrous stars can and do form more randomly across the universe -- including in isolation and in very small clusters.

"Our findings don't support the scenario that the maximum mass of a star in a cluster has to correlate with the size of the cluster," Oey said.

The researchers acknowledge the possibility that all of the stars they studied might not still be located in the neighborhood they were born in. Two of the stars they examined are known to be runaways that have been kicked out of their birth clusters. But in several cases, the astronomers found wisps of leftover gas nearby, strengthening the possibility that the stars are still in the isolated places where they formed.

The research is funded by NASA and the National Science Foundation.

Left: Star 302, as viewed from the ground. Right: Star 302 as viewed through the Hubble Space Telescope, which can zoom in roughly 40 times closer. From the ground, everything within the circle appears to be one star. (Credit: Courtesy of Joel Lamb)

Raindrops Reveal How a Wave of Mountains Moved South Across the Country.

About 50 million years ago, mountains began popping up in southern British Columbia. Over the next 22 million years, a wave of mountain building swept (geologically speaking) down western North America as far south as Mexico and as far east as Nebraska, according to Stanford geochemists. Their findings help put to rest the idea that the mountains mostly developed from a vast, Tibet-like plateau that rose up across most of the western U.S. roughly simultaneously and then subsequently collapsed and eroded into what we see today.

The data providing the insight into the mountains -- so popularly renowned for durability -- came from one of the most ephemeral of sources: raindrops. Or more specifically, the isotopic residue -- fingerprints, effectively -- of ancient precipitation that rained down upon the American west between 65 and 28 million years ago.

Atoms of the same element but with different numbers of neutrons in their nucleus are called isotopes. More neutrons make for a heavier atom and as a cloud rises, the water molecules that contain the heavier isotopes of hydrogen and oxygen tend to fall first. By measuring the ratio of heavy to light isotopes in the long-ago rainwater, researchers can infer the elevation of the land when the raindrops fell.

The water becomes incorporated into clays and carbonate minerals on the surface, or in volcanic glass, which are then preserved for the ages in the sediments.

Hari Mix, a PhD candidate in Environmental Earth System Science at Stanford, worked with the analyses of about 2,800 samples -- several hundred that he and his colleagues collected, the rest from published studies -- and used the isotopic ratios to calculate the composition of the ancient rain. Most of the samples were from carbonate deposits in ancient soils and lake sediments, taken from dozens of basins around the western U.S.

Using the elevation trends revealed in the data, Mix was able to decipher the history of the mountains. "Where we got a huge jump in isotopic ratios, we interpret that as a big uplift," he said.

"We saw a major isotopic shift at around 49 million years ago, in southwest Montana," he said. "And another one at 39 mya, in northern Nevada" as the uplift moved southward. Previous work by Chamberlain's group had found evidence for these shifts in data from two basins, but Mix's work with the larger data set demonstrated that the pattern of uplift held across the entire western U.S.

The uplift is generally agreed to have begun when the Farallon plate, a tectonic plate that was being shoved under the North American plate, slowly began peeling away from the underside of the continent.

"The peeling plate looked sort of like a tongue curling down," said Page Chamberlain, a professor in environmental Earth system science who is Mix's advisor.

As hot material from the underlying mantle flowed into the gap between the peeling plates, the heat and buoyancy of the material caused the overlying land to rise in elevation. The peeling tongue continued to fall off, and hot mantle continued to flow in behind it, sending a slow-motion wave of mountain-building coursing southward.

"We knew that the Farallon plate fell away, but the geometry of how that happened and the topographic response to it is what has been debated," Mix said.

Mix and Chamberlain estimate that the topographic wave would have been at least one to two kilometers higher than the landscape it rolled across and would have produced mountains with elevations up to a little over 4 kilometers (about 14,000 feet), comparable to the elevations existing today.

Mix said their isotopic data corresponds well with other types of evidence that have been documented.

"There was a big north to south sweep of volcanism through the western U.S. at the exact same time," he said.
There was also a simultaneous extension of the Earth's crust, which results when the crust is heated from below, as it would have been by the flow of hot magma under the North American plate.

"The pattern of topographic uplift we found matches what has been documented by other people in terms of the volcanology and extension," Mix said.

"Those three things together, those patterns, all point to something going on with the Farallon plate as being responsible for the construction of the western mountain ranges, the Cordillera."

Chamberlain said that while there was certainly elevated ground, it was not like Tibet.
"
It was not an average elevation of 15,000 feet. It was something much more subdued," he said.
"
The main implication of this work is that it was not a plateau that collapsed, but rather something that happened in the mantle, that was causing this mountain growth," Chamberlain said.

Mix presented results of the study at the American Geophysical Union annual meeting in San Francisco on Dec. 17.

Total Lunar Eclipse and Winter Solstice Coincide on Dec. 21

Early in the morning on December 21 a total lunar eclipse will be visible to sky watchers across North America (for observers in western states the eclipse actually begins late in the evening of December 20), Greenland and Iceland. Viewers in Western Europe will be able to see the beginning stages of the eclipse before moonset, and in western Asia the later stages of the eclipse will be visible after moonrise.

From beginning to end, the eclipse will last about three hours and twenty-eight minutes. For observers on the east coast of the U.S. the eclipse lasts from 1:33am EST through 5:01 a.m. EST. Viewers on the west coast will be able to tune in a bit earlier. For them the eclipse begins at 10:33 p.m. PST on December 20 and lasts until 2:01am PST on Dec. 21. Totality, the time when Earth's shadow completely covers the moon, will last a lengthy 72 minutes.

While it is merely a coincidence that the eclipse falls on the same date as this year's winter solstice, for eclipse watchers this means that the moon will appear very high in the night sky, as the solstice marks the time when the Earth's axial tilt is farthest away from the sun.

A lunar eclipse occurs when the Earth lines up directly between the sun and the moon, blocking the sun's rays and casting a shadow on the moon. As the moon moves deeper and deeper into the Earth's shadow, the moon changes color before your very eyes, turning from gray to an orange or deep shade of red.

The moon takes on this new color because indirect sunlight is still able to pass through Earth's atmosphere and cast a glow on the moon. Our atmosphere filters out most of the blue colored light, leaving the red and orange hues that we see during a lunar eclipse. Extra particles in the atmosphere, from say a recent volcanic eruption, will cause the moon to appear a darker shade of red.

Unlike solar eclipses, lunar eclipses are perfectly safe to view without any special glasses or equipment. All you need is you own two eyes. So take this opportunity to stay up late and watch this stunning celestial phenomenon high in the night sky. It will be the last chance for sky watchers in the continental U.S. to see a total lunar eclipse until April 15, 2014.

Path of the Moon through Earth's umbral and penumbral shadows during the Total Lunar Eclipse of Dec. 21, 2010. (Credit: Fred Espenak/NASA's Goddard Space Flight Center)