SCIENCE
Notebook

Monday, July 9, 2007

Mars Rover at Crater's Edge

The unexpectedly long-lived Mars rover Opportunity was scheduled to make a dramatic and potentially dangerous descent into a crater last weekend, but the worst Martian dust storm in years put the plan on hold. NASA officials said late last week that the winds were calming down and the fine dust was beginning to settle, leaving them optimistic that they might be able to try again to enter Victoria Crater later this week.

NASA announced the imminent descent on June 27, and principal investigator Steven Squyres of Cornell University said the wind and dust began blowing soon after. He said that the dust measurements on both Mars rovers climbed higher than ever recorded and that long-term coverage could damage the rovers' ability to move and operate. Although the dust is more like smoke than a sandstorm, he said, it can rise high into the atmosphere and blot out the sun's rays -- leaving the rovers' solar panels without a power source.

The rover is perched near an outcropping called Duck Bay on the rim of Victoria Crater, which is by far the deepest one that a Mars rover has neared. NASA researchers have been eager for Opportunity to enter the crater since it arrived there last year, but the rover first had to explore along the rim for the best way down. Squyres said the team was especially interested in exposed rock formations down in the crater that may give clues to the planet's history.

The two rovers landed on Mars more than 1,000 days ago and were expected to last only three months. Both, however, defied predictions and remain in operation. Dust storms such as the one that postponed the descent into Victoria Crater have also been partially responsible for the longevity of the rovers, since the high winds have worked to periodically clean the solar panels to a degree unpredicted before the rovers landed.

-- Marc Kaufman

A Rainbow From a Test Tube

Chemists at the University of California at Riverside have found a way to make a test tube of clear water turn into every color of the spectrum.

The trick is achieved by harnessing the capacity of crystals to absorb or reflect specific wavelengths of electromagnetic energy.

A crystal is an orderly packed array of units such as atoms or molecules. The layers of units can split light into different wavelengths and colors, and the overall architecture of the crystal determines which colors are absorbed and which are reflected.

Yadong Yin and his colleagues created a "colloidal crystal," a crystal suspended in another substance, in this case water. Its units were round particles with cores of magnetite, an iron-based compound that can move in response to a magnetic field. Encasing each core was a batterlike coating of polyacrylate plastic.

Polyacrylate has an electrostatic charge. The coating's outer surface was strongly attracted to the water, the inner surface similarly attracted to the magnetite. This combination of forces pulling in different directions made each particle slightly compressible.

Added to water in the proper proportion, the particles formed a quivering crystal.

When the researchers brought a magnet close to the liquid, they found they could compress or expand the crystal's invisible lattice by altering the strength of the magnetic field.

The distance between layers of particles varied from 169 to 274 nanometers -- a lot, in this world. It allowed enough change in the diffraction and reflectance properties of the crystal that the liquid could be made to take on every hue from violet to red.

This property of "tunable" color in liquid may prove useful in sensing devices, electronic display screens, even disappearing inks. The research appears in Angewandte Chemie International Edition.

-- David Brown

Antidepressants, Suicide Tries

Two new studies in the American Journal of Psychiatry this month show that fewer people attempt suicide in the month after they start antidepressant treatment than in the month before they start taking the drugs. The studies add a new layer of evidence to the controversy over whether medications such as Prozac and Paxil cause people to become suicidal.

The studies examined the records of more than 330,000 patients in Group Health Cooperative, aSeattle-based health-care system, and of veterans who received care through the Department of Veterans Affairs.

Greg Simon of the Seattle health-care system and Robert Gibbons of the University of Illinois at Chicago, who analyzed the records of veterans, said the decline in suicide attempts following the start of treatment was seen among all age groups. Similar benefits were also detected after the introduction of other kinds of treatment, including psychotherapy.

The Food and Drug Administration has asked the makers of antidepressants to place "black box" warnings on the drugs, following evidence from placebo-controlled trials that suggested that patients getting the drugs displayed more suicidal behavior than patients who got sugar pills. Antidepressant prescription rates have fallen steeply since the warnings went into effect. Some experts have wondered whether the drugs might increase suicidal behavior among some individuals while lowering the risk among others.

-- Shankar Vedantam