Wednesday, December 23, 2009
12/23/2009 05:23:00 PM Publicado por Alquimia
There is more to the snowflake than its ability to delight schoolchildren and snarl traffic.
The structure of the frosty flakes also fascinate ice chemists like Purdue University's Travis Knepp, a doctoral candidate in analytical chemistry who studies the basics of snowflake structure to gain more insight into the dynamics of ground-level, or "tropospheric," ozone depletion in the Arctic.
"A lot of chemistry occurs on ice surfaces," Knepp said. "By better understanding the physical structure of the snow crystal - how it grows and why it takes a certain shape - we can get a better idea of the chemistry that occurs on that surface."
His work on snowflake shape and how temperature and humidity affect it takes place in a special laboratory chamber no larger than a small refrigerator. Knepp can "grow" snow crystals year-round on a string inside this chamber. The chamber's temperature ranges from 100-110 degrees Fahrenheit down to minus 50 degrees Fahrenheit.
Knepp, under the direction of Paul Shepson, professor and head of Purdue's Department of Chemistry, is studying snow crystals and why sharp transitions in shape occur at different temperatures. The differences he sees not only explain why no two snowflakes are identical, but also hold implications for his ozone research in the Arctic Ocean region.
"On the surface of all ice is a very thin layer of liquid water," Knepp said. "Even if you're well below the freezing point of water, you'll have this very thin layer of water that exists as a liquid form. That's why ice is slippery. Whenever you slip, you're not slipping on ice, you're slipping on that thin layer of water."
This thin, or quasi-liquid, layer of water exists on the top and sides of a snow crystal. Its presence causes the crystal to take on different forms as temperature and humidity change.
For example, the sides of a crystal growing in a warmer range of 27-32 degrees Fahrenheit expand much faster than the top or bottom, causing it to take on a platelike structure. Between 14 and 27 degrees Fahrenheit, crystals look like tall, solid prisms or needles.
"As you increase the humidity, you'll get more branching," Knepp said.
Snow crystals transition to other shapes, and sometimes even back and forth, as the temperature and humidity change.
"The bottom line is that the thickness or the presence of this really thin layer of water is what dictates the general shape that the snow crystal takes," Knepp said. "By altering the quasi-liquid layer's thickness, we changed the temperature at which the snow crystal changes shape.
"Until now, nobody knew that the quasi-liquid layer had such a significant role in determining the shape of snow crystals. Our research clearly shows this to be the case."
This knowledge has application for Knepp and his colleagues in their ozone work.
"Most people have probably heard of ozone depletion in the North and South Poles. This occurs in the stratosphere, about 15 miles up," Knepp said. "What people don't know is that we also see ozone levels decrease significantly at ground level."
Ground-level ozone is very important. It gives the atmosphere the ability to clean itself. However, it also is toxic to humans and vegetation at high concentrations, like those found in smog, Shepson said.
Complex chemical reactions regularly take place on the snow's surface. These reactions, which involve the thin layer of water found on the surface of snow crystals, cause the release of certain chemicals that reduce ozone at ground level.
"How fast these reactions occur is partially limited by the snow crystals' surface area," Knepp said. "Snow crystals with more branching will have higher surface areas than non-branched snow crystals, which will allow the rate of reaction to increase."
The need to understand these intricate chemical reactions and their implications for ozone reduction drive the researchers to continue studying snow.
"As the impact of emissions from human activities continues to grow, we need to be able to understand the impact of global average ozone," Shepson said. "Understanding ice and snow is part of that."
(Photo: Purdue University photo/Shepson Lab)
12/23/2009 05:22:00 PM Publicado por Alquimia
The first observations with the world's newest planet-hunter instrument on the Subaru Telescope, HiCIAO (High Contrast Instrument for the Subaru next generation Adaptive Optics), have revealed a companion to the Sun-like star GJ 758. With an estimated mass of 10–40 times Jupiter's mass, GJ 758 B is either a giant planet or a lightweight brown dwarf.
Its orbit is comparable in size to Neptune's, and its temperature of 600 K makes it the coldest companion to a Sun-like star ever resolved in an image. A second companion with a similar mass at the Uranus's orbit is also suggested. The presence of such massive planets at these large distances challenges standard assumptions about planetary system formation based on the Solar System. Since a strategic search for exoplanets and their formation sites has just started, further observations will eventually answer questions about whether the Solar System is ubiquitous or not.
The number of planets known outside of our Solar System has recently surpassed 400. Most of those were discovered with indirect methods, such as Doppler spectroscopy (determining the "wobble" of the parent star due to the orbiting planet's gravitational tug) or transit photometry (measuring the ever-so-slight dimming of the parent star when the planet passes through the line of sight, obscuring part of the star). Actually resolving the planet's faint speck of light against the host star's overwhelming glare is much more challenging, yet ultimately more rewarding. It grants access to invaluable information about the planet's orbit and the temperature and composition of its atmosphere, and after all, "seeing is believing".
Direct imaging of exoplanets has been a hot topic in recent years. In 2005, a 4 Jupiter-mass companion around a young brown dwarf and several 10-a few 10s Jupiter-mass companions at a large distance of ~100 AU or more were discovered, but the results have been controversial. In 2008, 3 A-type stars (stars with about twice of the Sun's mass) were reported to be circled by planets. Among them, 3 planets around HR8799 have been confirmed by others including the Subaru Telescope. They are 7-10 Jupiter-mass at 24-68AU. However, there have been no reports on planet candidates orbiting close enough around the Sun-like stars (G-type stars).
The new planet candidate, GJ 758 B, was discovered around a G star JG758A at a distance of 50 light-years from the Sun by an international team composed of members from Germany, Japan, and the USA. Interestingly, its temperature is only a balmy 600 K; it is the coldest companion around G stars imaged so far.
Two epoch observations in May and August, 2009 established that the candidate is not a background star. It is found at a projected separation of 29 AU (Neptune's semi-major axis). The companion mass is estimated from its brightness and age. Although there is some uncertainty about the age of the host star, the mass of the companion is about 10 Jupiter-masses with an age of 700 million years old. Therefore, the companion can certainly be called a giant planet candidate.
In other August 2009 observations, another candidate with a similar mass at only 18 AU (Uranus's semi-major axis) was also detected, suggesting that it may be part of a planetary system with at least two giant planets at Neptune and Uranus orbits. However, this hypothesis should be confirmed with other observations.
All of these observations were conducted with the 8.2-meter Subaru Telescope at the summit of Mauna Kea in Hawaii, using the new instrument HiCIAO with its state-of-the-art adaptive optics to correct for the blurring effect of Earth's turbulent atmosphere in real time. The data and findings reported here were obtained during the commissioning phase of HiCIAO.
Although the Subaru Telescope has been equipped with its previous coronagraph CIAO (Coronagraphic Imager with Adaptive Optics) since 2000, the new planet/disk-hunting instrument HiCIAO employs not only the standard coronagraph but also various differential techniques (wavelength, polarization, angular differential imaging) to suppress the noise due to the residual stray-light halo of the host star. After five years of development funded by the MEXT grants-in-aid for Priority Area, its performance has demonstrated that it is more than 10 times better than CIAO.
The current discovery obtained by direct imaging provides information that cannot be acquired with the Doppler method, and it gives a much deeper understanding of planets around the Sun-like stars. In order to directly detect and observe exoplanets and disks, we need special instruments such as coronagraphs to block the light from a nearby bright host star, so that nearby faint objects can be viewed. A total eclipse, like the one seen in some regions of Japan in July 2009, is a "natural coronagraph" in which the Moon suppresses the bright light of the Sun. By using such direct imaging, we can answer the question, "Is the Solar System unique in the Universe?"
The Subaru Telescope recently began its first strategic observations for direct exoplanet hunting in October 2009. Over the next five years, its project SEEDS (Subaru Strategic Exploration of Exoplanets and Disks with HiCIAO/AO188) will search for exoplanets and circumstellar disks around about 500 stars. SEEDS is an international project composed of about 100 members from Japan, the USA, Germany, and the UK. By detecting more planet candidates similar to the ones reported here, we are able to not only answer questions about whether our Solar System is unique but also to understand how planets form from protoplanetary disks.
(Photo: Subaru Telescope)
National Astronomical Observatory of Japan
12/23/2009 05:21:00 PM Publicado por Alquimia
An international team of scientists has rescued visual function in laboratory rats with eye disease by using cells similar to stem cells. The research shows the potential for stem cell-based therapies to treat age-related macular degeneration in humans.
A team led by Dennis Clegg, of UC Santa Barbara, and Pete Coffey, of University College London (UCL), published their work in two papers, including one published in the journal PloS One. The first paper was published in the October 27 issue of the journal Stem Cells.
The scientists worked with rats that have a mutation which causes a defect in retinal pigmented epithelial (RPE) cells and leads to photoreceptor death and subsequent blindness. Human RPE cells were derived from induced pluripotent stem cells –– embryonic stem cell-like cells that can be made from virtually any cell in the body, thus avoiding the controversy involved in using stem cells derived from embryos. Pluripotent means that the cells can become almost any cell in the body.
In experiments spearheaded by UCL's Amanda Carr, the team found that by surgically inserting stem cell-derived RPE into the retinas of the rats before photoreceptor degeneration, vision was retained. They found that the rats receiving the transplant tracked their visual focus in the direction of moving patterns more efficiently than control groups that did not receive a transplant.
"Although much work remains to be done, we believe our results underscore the potential for stem-cell based therapies in the treatment of age-related macular degeneration," said Sherry Hikita, an author on both papers and director of UCSB's Laboratory for Stem Cell Biology.
Dave Buchholz, first author of the article in Stem Cells, explained that by using induced stem cells that can be derived from patients, the scientists avoid immune rejection that might occur when using embryonic stem cells.
According to Buchholz, "RPE cells are essential for visual function. Without RPE, the rod and cone photoreceptors die, resulting in blindness. This is the basic progression in age-related macular degeneration. The hope is that by transplanting fresh RPE, derived from induced pluripotent stem cells, the photoreceptors will stay healthy, preventing vision loss."
(Photo: George Foulsham, Office of Public Affairs, UCSB)
University of California, Santa Barbara