Saturday, 3 April 2010

Johnald's Fantastical Daily Link Splurge

Johnald's Fantastical Daily Link Splurge

Why Volcanic Eruptions Can Spark Lightning

Posted: 02 Apr 2010 02:38 PM PDT


It's the ultimate love-at-first-sight story: In the middle of the desert, hundreds of miles from anything else, lonely sand grains meet up in a crowd and decide to electrify each other. Sparks fly.

sciencenewsPhysicists have long puzzled over why sand grains and other small particles can build up electrical charges as they collide with one another, sometimes to the point of discharging lightning in dust storms or plumes of volcanic ash. Now, a paper in an upcoming issue of Nature Physics suggests that particles transfer electrical charge vertically during a smashup, such that positive charges move downward and negative charges move up in the cloud.

The findings could help combat a wide variety of practical problems, such as the adhesion of charged dust to solar panels on a Mars rover or the generation of dangerous electrical discharges that sometimes occur when a helicopter takes off in the desert. Dust clouds can create problems in grain silos, where charge sometimes builds up and leads to explosions, and in the pharmaceutical industry, where particles of ground-up drugs can become charged and not mix properly, says Hans Herrmann, a materials researcher at ETH Zurich.

Herrmann says he became interested in the problem after watching lightning in swirling sands over dune fields at night. "Normally when particles collide, they neutralize," he says. "How could it be that charges increase?"

Working with ETH colleague Thomas Pähtz and Troy Shinbrot of Rutgers University's campus in Piscataway, New Jersey, Herrmann developed a model to explain how the charging happened. Before colliding, the grains have an overall neutral charge but are polarized by a background electric field, with a negative charge toward the top of the grain and a positive charge toward the bottom, relative to the ground. Upon colliding, the particles neutralize each other at the point of contact, but when they separate again they became further polarized, with additional charges building up on the grains' edges.

"Every time there's a collision you end up pumping charge from the top to the bottom," says Shinbrot. The researchers ran computer simulations and then a series of experiments with glass beads to confirm the theory.

Daniel Lacks, a materials physicist at Case Western Reserve University in Cleveland, says the new study could be identifying one of several mechanisms at work in particle clouds. In earlier work, Lacks showed that electrical charging depends on the size of particles in question, with smaller particles tending to charge negatively and larger particles tending to charge positively.

"The bottom line is that something is needed to break the symmetry when two particles of identical composition collide, in order for one particle to charge negatively and the other to charge positively," he says. For particles of different sizes, he says his mechanism might be in play; for identically sized particles, the new model might explain it.

Some challenges remain, such as explaining where the background electric field that charges the particles came from. But Herrmann says the work is philosophically satisfying, in answering a long-held question, and may yet have practical applications.

Image: Oliver Spalt/Wikimedia Commons

Inca Skeletons Show Evidence of Spanish Brutality

Posted: 02 Apr 2010 12:19 PM PDT


If bones could scream, a bloodcurdling din would be reverberating through a 500-year-old cemetery in Peru. Human skeletons unearthed there have yielded the first direct evidence of Inca fatalities caused by Spanish conquerors.

sciencenewsEuropean newcomers killed some Inca individuals with guns, steel lances or hammers, and possibly light cannons, scientists report online in the March 23 American Journal of Physical Anthropology.

Surprisingly, though, no incisions or other marks characteristic of sword injuries appear on these bones, according to a team led by anthropologist Melissa Murphy of the University of Wyoming in Laramie. Spanish documents from the 16th century emphasize steel swords as a favored military weapon.

inca_skeletonMany Spaniards who helped Francisco Pizarro conquer the Incas were fortune-seekers, not soldiers, "so the absence of sword injuries makes some sense," Murphy explains.

Skeletons in the Inca cemetery, as well as at another grave site about a mile away, display a gruesome array of violent injuries, many probably caused by maces, clubs and other Inca weapons, the researchers report. Those weapons may have been wielded by Inca from communities known to have collaborated with the Spanish, or might have been borrowed by the Spanish, they posit. "The nature and pattern of these skeletal injuries were unlike anything colleagues and I had seen before," Murphy says. "Many of these people died brutal, horrible deaths."

Little is known about early European dealings with the Inca, remarks anthropologist Haagen Klaus of Utah Valley University in Orem.

"Murphy's data show the types of violence that emerged from the first moments of contact between Spaniards and the Inca," Klaus says. Pottery and artifacts at the sites date to between 1470 and 1540, placing the deaths close to when Spaniards captured the Inca emperor around 1532. It took the invaders nearly another 40 years to control all Inca lands.

Murphy's team assessed skeletons of 258 Inca individuals, age 15 or older, excavated several years ago at the two cemeteries.

In one cemetery, bodies had been hastily deposited in shallow graves. One-quarter of 120 skeletons displayed head and body injuries inflicted at the time of death, as indicated by a lack of healed bone and other clues. That's a conservative estimate, Murphy notes, since soft-tissue damage doesn't show up on bones.

"I'm struck by the severity of violence in certain individual cases, where the skull was essentially crushed, repeatedly stabbed or struck, or shot through by gunshot," comments archaeologist Steven Wernke of Vanderbilt University in Nashville. Whoever killed these individuals wanted to intimidate survivors as well, he asserts.

One man's skull contained two holes and radiating fractures consistent with damage produced by early guns that shot ammunition at low velocities.

Another male skull sported three small rectangular openings in the back of the head. These injuries resemble those on skulls from a 1461 battlefield cemetery in England, Murphy says. Medieval weapons tipped with steel spikes or sharp beaks probably caused these wounds, she proposes.

Three other skeletons exhibited injuries likely due to Spanish weapons. Other skeletons contained head and body fractures probably inflicted by attackers bearing Inca weapons.

Individuals placed in this cemetery may have been slain in a documented 1536 Inca uprising against Spanish rulers in nearby Lima, Murphy suggests. Family members collected their bodies and buried them quickly near previously deceased relatives, she speculates.

At the second Inca cemetery, 18 of 138 skeletons showed definite signs of violent death, all from Inca weapons. That supports a scenario in which social turmoil around the time Spaniards arrived triggered conflicts between Inca communities, Murphy says.

Images: Melissa Murphy

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