Magellan Mirror
The spin-casting process for the first 8.4-meter (27-foot) diameter mirror for the Giant Magellan Telescope (GMT) began July 23 when The University of Arizona’s Steward Observatory Mirror Lab started pre-firing its huge spinning furnace and inspecting tons of glass for casting.

The GMT telescope primary mirror, scheduled for completion in 2016 at a site in northern Chile, will consist of six 8.4-meter off-axis mirrors surrounding a seventh, on-axis central mirror. (An off-axis mirror focuses light at an angle away from its axis, unlike a symmetrical mirror that focuses light along its axis.) This arrangement will give the GMT four-and-one-half times the collecting area of any current optical telescope and the resolving power of a 25.6-meter (84-foot) diameter telescope, or 10 times the resolution of the Hubble Space Telescope.

“Spin-casting” single-piece telescope mirrors that are giant, stiff yet lightweight is an ingenious, awesome process that was conceived and developed by University of Arizona Regents’ Professor of Astronomy J. Roger P. Angel. Casting giant monolithic mirrors is accomplished at only one place in the world ­ the Steward Observatory Mirror Laboratory.

The mirror’s mold, consisting of almost 50,000 pounds of hearth tiles, fiber tub walls and cores and pins, was loaded with 40,000 pounds of borosilicate glass. It heated for six days, achieving a temperature of 2,150 degrees Fahrenheit (1,178 Celsius) that caused the glass to flow like room-temperature honey as the furnace rotated. The thick liquid glass flowed between the hexagonal cores in the mold to create a “honeycomb” structure, which weighs about a fifth as much as would a solid glass mirror of its size.

The GMT mirror spun 5 times a minute, slower than the two 8.4-meter mirrors the Lab made for the Large Binocular Telescope (LBT), because the off-axis GMT mirror is to be a shallower, longer focal-length mirror than the symmetric Large Binocular Telescope primary mirrors.

Web sites:
Giant Magellan Telescope
Steward Observatory Mirror Lab

Permafrost Lakes
Jon Pelletier, UA assistant professor of geosciences, has discovered why the thousands of oval-shaped lakes that dot Alaska’s North Slope behave the way they do. They range from puddles to more that 15 miles in length, their skinny ends oriented northwest. How the lakes grow so fast, why they’re oriented in the same direction and what gives them their odd shape has puzzled geologists for decades.

The lakes’ unusual characteristics result from seasonal slumping of the banks when the permafrost thaws abruptly, Pelletier explains. The lakes grow when rapid warming melts a lake’s frozen bank, and the soggy soil loses its strength and slides into the water. Such lakes are found in the permafrost zone in Alaska, northern Canada and northern Russia.

A key ingredient for oriented thaw lakes is permafrost ­ the special mixture of soil and ice that forms the surface of the land in the Far North. When it warms gradually, the ice portion melts slowly, allowing the water to drain out of the soil and leave relatively firm sand or sediment behind. However, if an early heat wave melts the permafrost’s ice rapidly, the result is a soggy, unstable soil. When such rapidly thawed permafrost is part of the vertical bank of a lake, the bank slumps into the water, enlarging the lake. More of the bank collapses if the soil is fine-grained, rather than sandy.

Another ingredient in Pelletier’s explanation is a long, gentle slope. Because Alaska’s oriented lakes are embedded in a gently sloping landscape, the downhill end of a lake always has a shorter bank. According to Pelletier’s computer model, shorter banks melt more and have bigger slumps. Therefore when the lake experiences thaw slumping, Pelletier’s model says the lake grows more in the downhill direction than it does uphill, generating the lakes’ characteristic elongated-egg shape.

Web site: Jon D. Pelletier home page

Nova Residue
A UA scientist’s work confirms a discovery by NASA scientists of mineral grains called olivine that were plucked by a NASA research aircraft from Earth’s upper atmosphere. The grains were formed in an ancient supernova explosion and traveled in interstellar space for 4.5 billion years as part of a comet or asteroid.

Scott Messenger and Lindsay P. Keller of the NASA Johnson Space Center in Houston and Dante S. Lauretta of UA’s Lunar and Planetary Laboratory published their findings in the current June 30 issue of Science.

Messenger used a new kind of ion microprobe, called the NanoSIMS, to measure oxygen isotopes in the unusual grains. Results showed that the captured olivine doesn’t come from anywhere in our solar system, though the mineral is plentiful here. “The supernova grains have oxygen isotopic ratios that have never been seen before in meteorites or comet dust, but are predicted in astrophysical models of supernova explosions,” Messenger said.

Keller identified the mineral composition using a transmission electron microscope. The NASA scientists then asked Lauretta if the grain could possibly be a supernova grain.

Lauretta said his computational chemical analysis matched the grain’s actual isotopic and mineral composition “dead on.” Messenger’s isotopic ratios enabled the team to pinpoint where in the supernova explosion the grains formed.

Web site: Dante Lauretta home page

Burning Ruins
Arizona State Museum archaeologists faithfully recreated rooms found in a 13th to 14th century ruin in northern Arizona and then destroyed their work by setting the structure afire.

They, along with fire investigators, will use the resulting ruins to better understand evidence of ancient structural fires found at the Chevelon Pueblo, located near Winslow, Ariz. Burning, scientists say, is a common occurrence in the archaeological record of the Southwest but seldom has a systematic study of the role of fire been initiated.

While mapping and testing structures at Chevelon last year, archaeologists uncovered widespread burning. With the assistance of former FBI arson investigator Timothy Huff, they excavated two contiguous rooms revealing clues from burn patterns on wall plaster, surviving charred artifacts and wooden roof beams. The rooms also showed that the fire was set intentionally in the roof and left to spread along grass and small cottonwood beams of the roof. Probably the earthen part of the roof was removed to enable the fire to sustain itself.

“Our goal is to develop a better understanding and interpretation of ancient fires, which are so prevalent in the archaeological record,” said program director and archaeologist E. Charles Adams. “Better science will result from our collaboration with arson investigators.”

Adams and UA anthropology graduate student A.J. Vonarx led the team. The burn took place during the week of July 10 near the visitor center at the Homolovi Ruins State Park near Winslow.