The Shape of Stalactites

Classic stalactites, the formations that hang from the roofs of caves, all tend toward the same singular shape, UA scientists have found.

“... . The difference is one of magnification ­ it’s either big or it’s small, but it’s still the same shape,” said researcher Martin Short of the University of Arizona. “It’s an ideal shape in nature and in mathematics that had not been known before,” said Raymond Goldstein, a UA physics professor and senior author on an article detailing the findings. Other authors are James C. Baygents, of chemical and environmental engineering; J. Warren Beck, department of physics; David A. Stone, a doctoral candidate in soil, water and environmental science; and Rickard S. Toomey, III, Arizona State Parks.

The group took a field trip to the famed Kartchner Caverns State Park and were floored by the variety of forms, especially the ripples many structures possess. Goldstein suggested that Short, his student, investigate the formation of ripples on stalactites. Short said he first had to learn about the underlying dynamics of stalactite growth.

Stalactites grow when water laden with carbon dioxide and calcium carbonate drips from cracks or holes in the cave’s ceiling. As a water droplet hangs from the crack, the carbon dioxide escapes, depositing a tiny bit of solid calcium carbonate. As each successive drip flows over the minute mineral deposit, the sequence repeats, ultimately forming a stalactite.

The group developed an equation of motion to describe how a stalactite’s shape evolves, plugged the equation into a computer and asked it to “grow” some shapes. No matter what shape was used, the computer’s formations lengthened and thickened in a universal manner.

Then they returned to Kartchner Caverns and projected a pair of green laser dots as a scale bar onto the stalactites and then took pictures of the stalactites. The actual stalactites, compared to the ideal form predicted by the mathematics, differed by less than 5 percent.

Now, Short and Goldstein say, they finally know enough to figure out what gives stalactites their ripples.

The Research Corporation and the National Science Foundation funded the research.

Deep Impact

If all goes as planned, NASA’s Deep Impact will become the first mission to slam into a comet, giving astronomers worldwide something far better than any other fireworks show on July 4, ­ the first look inside a comet at the most primitive material left in the solar system.

“The idea is that the best way to find what’s inside a comet is to blast a hole in it,” University of Arizona Regents’ Professor and Deep Impact science team member H. Jay Melosh said. “Other comet-rendezvous missions have proposed sampling less than a foot into the upper surface. But that doesn’t get at the ices in the interior, which scientists believe are early solar system materials that have been kept in the deep freeze for the past 4.5 billion years.”

Melosh’s 1989 book on planetary impact cratering is still the universal reference for scholars, expert or novice. Melosh’s research interests relate to the origin and evolution of the early solar system. Deep Impact could add chapters to that story.

Deep Impact spacecraft is planned to reach comet Tempel 1 beyond the orbit of Mars on July 4. Deep Impact will deploy an 820 pound (372 kilogram) copper probe into the comet at about 23,000 mph (37,000 kph).

Melosh carefully calculated the abundances of “critical” elements scientists might expect to see vaporize on impact. Critical elements are those which scientists want to measure because they are important in early solar system processes. Melosh also modeled how much mass of each of the different elements would be vaporized on impact so scientists can know how much vaporized material comes from the comet and how much from the spacecraft.

The Deep Impact probe is made mostly of copper because “copper is an element that no geochemist or cosmochemist trying to work out the origin of the solar system cares about. It’s not characteristic of any particular solar system process,” Melosh said.

University of Maryland astronomy professor Michael A’Hearn leads the mission. Kitt Peak National Observatory Astronomer Emeritus Michael Belton of Belton Space Exploration Initiatives, Tucson, is another member of the Deep Impact science team. Ball Aerospace & Technologies in Boulder, Colo., built NASA’s Deep Impact spacecraft.

Find more information on Deep Impact at http://www.nasa.gov/deepimpact.

Drought Linked to Climate Warming

Severe drought in the West in recent years may be linked to climate warming trends, say researchers who analyzed aridity in the western United States over the past 1,200 years and found that elevated aridity may be a natural response to climate warming.

The study’s authors used tree-ring records to reconstruct evidence of drought and also looked at a number of independent drought indicators, ranging from elevated charcoal in lake sediments to sand dune activation records. The team then used published climate model studies to explore mechanisms that link warming with aridity in the western United States.

The study reveals that a 400-year-long period of elevated aridity and epic drought occurred in what is now the western United States during the period A.D. 900-1,300. This corresponds broadly to the so-called “Medieval Warm Period,” during which paleoclimate records indicate unusual warmth over much of the Northern Hemisphere. The authors of the new study argue that there are climate mechanisms involved that make warming climate conditions likely to lead to increased drought in the western, interior region of North America.

The four-year drought “pales in comparison with some of the earlier droughts we see from the tree-ring record,” said co-author David M. Meko of the UA Laboratory of Tree-Ring Research. “What would really put a stress on society is decade-long drought.”

“If warming over the tropical Pacific Ocean promotes drought over the western U.S., this is a potential problem for the future in a world that is increasingly subjected to greenhouse warming,” said Edward R. Cook of Lamont-Doherty Earth Observatory.

The research is published in the current edition of Science Express, the early online edition of the journal Science. Connie Woodhouse and C. Mark Eakin of the NOAA National Climatic Data Center and David W. Stahle of the University of Arkansas are co-authors on the article. The research was supported by NOAA and the National Science Foundation. The researchers also created a CD-ROM called “North American Drought Atlas,” which can be obtained by contacting Edward R. Cook at Lamont-Doherty Earth Observatory, drdendro@ldeo.columbia.edu.

Cancer Treatment and Cognition

Marissa Carey, an assistant research scientist in the UA College of Medicine department of pediatrics, is conducting a study to investigate cognitive difficulties in pediatric cancer survivors.

Acute lymphoblastic leukemia (ALL), brain tumors and lymphoma are the most common types of childhood cancer, and their treatments have been linked with long-term difficulties in cognitive and academic function.

The cognitive problems often reported in childhood cancer survivors have been compared to Nonverbal Learning Disability (NVLD) which is characterized by difficulties in arithmetic, reading comprehension and subject areas that require complex problem solving, as well as difficulties in social skills.

NVLD is believed to result from central nervous system (CNS) white matter damage or disease. Among children with cancer who receive CNS treatment, white matter damage can occur as a result of radiation or chemotherapy.

To test the white matter model of CNS injury in children, white matter volumes will be gathered from structural magnetic resonance images (MRI) of the brain and will be correlated with tests of cognitive, academic and social function.

“This study will help us better understand how cancer and its treatment effects the brain of childhood CNS cancer survivors,” says Carey. “It’s important because it may help identify the underlying causes of cognitive, academic, and social problems in children following treatment for CNS cancer. If we can identify the underlying mechanisms involved, we can develop specific interventions for, and provide services to, children treated. Ultimately, we want to improve their quality of life.”

The study seeks children between the ages of 6-18 who are survivors of ALL, lymphoma or brain tumor. They should be at least one year post-diagnosis. The study also seeks ALL, lymphoma or brain tumor survivors 18-30 years of age, diagnosed before the age of 21. Healthy children and young adults 6-30 years of age may also participate as a comparison group.