VOA慢速英语 2008 0408b（在线收听）

Enceladus, photographed by the Cassini spacecraft

Saturn is best known for the rings of icy material that surround the planet. But Saturn's moons interest scientists because some may hold liquid water and other materials necessary for life.

Recently, the American space agency NASA ordered the Cassini spacecraft to visit one of Saturn's most interesting moons. NASA jointly operates Cassini with the European Space Agency and the Italian Space Agency.

The moon is Enceladus. It is not Saturn's largest moon. It is only five hundred kilometers across. But the forces that affect the surface of Enceladus are very active.

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Periodically, huge amounts of material shoot up from the surface. NASA officials have called these events geysers, like the hot water that is forced out from under the ground on Earth. Cassini first captured pictures of such an event three years ago. The pictures have proved so scientifically important that NASA made changes to its plans for Cassini just to study the geysers.

On March twelfth, the space agency directed Cassini to pass only about fifty kilometers from the surface of Enceladus. Cassini got so close that it passed through material shooting out of the moon. The spacecraft was traveling at a speed of fifteen kilometers a second.

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What Cassini found has only increased scientists' interest in the moon. New maps of temperatures on Enceladus show that an area on the southern part of the moon is ninety-three degrees below zero Celsius. Temperatures on Enceladus are normally about one hundred thirty degrees below zero.

John Spencer is a scientist at the Southwest Research Institute in Boulder, Colorado. He says the new temperature information makes it more likely that there is liquid water not far below the surface.

Liquid water is believed to be one of the things needed for life. Organic material is another. Cassini also found that the geysers are releasing organic material.

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Hunter Waite is an investigator for the Cassini Ion and Neutral Mass Spectrometer at the Southwest Research Institute in San Antonio, Texas. The spectrometer is a device that helps identify the chemistry of substances.

Mister Waite says the chemicals gathered from the geysers of Enceladus are much like those found on comets in our solar system. Cassini found water, carbon dioxide, carbon monoxide and also organic material shooting from the geyser.

It is not known what causes the geysers on Enceladus. Cassini's deputy project scientist, Linda Spilker, says scientists know that heat causes the geysers to shoot from the surface of Enceladus. But, she says, it is not known what causes the heat.

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Gravity from Saturn and the moon Dione are known to affect Enceladus. But it is not clear if this gravitational force is enough to cause the moon's energetic geysers.

The geysers are powerful. The material is leaving the surface at four hundred meters a second. And there is a link between the geysers and the objects for which Saturn is most famous. Material from Enceladus helps form the E-ring, the most distant of Saturn's many beautiful rings.

The most recent visit is only the beginning of close study of Enceladus. Scientists will have another chance to observe Enceladus when Cassini passes very near the moon again in August.

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Scientists think they are a step closer to a new drug to treat schistosomiasis. More than two hundred million people suffer from this parasitic worm disease. Most live in developing nations. About ten percent of victims become seriously disabled from internal bleeding, iron loss, organ damage or other effects.

A team in the United States found that chemical compounds known as oxadiazoles can attack an enzyme needed for the survival of Schistosoma. This is the group of flatworms that cause schistosomiasis.

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The scientists tested oxadiazoles on laboratory mice. They found that one compound killed the parasite at every level of development. The study also showed that the compound was active against all three major kinds of Schistosoma worms that infect human beings.

America's National Institutes of Health supported the research. Nature Medicine magazine reported on the study by scientists from Illinois State University and the Chemical Genomics Center at N.I.H.

David Williams led the research. He says the Schistosoma parasite needs oxygen to survive. Oxygen use produces oxygen-free radicals that can destroy an organism. The worm has a protective enzyme. But Professor Williams says the experimental drug disables this enzyme, causing the worm to self-destruct.

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Each year, two hundred eighty thousand people die of schistosomiasis, also known as bilharzia or snail fever. The microscopic worms infect snails, which produce infected eggs. People become infected when they enter fresh water where the snails live.

The worms dig through skin to enter the body. They move into blood passages that supply the intestinal and urinary systems. Then, if worm eggs in human waste enter fresh water, more snails and people become infected.

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Since the nineteen eighties, doctors have used one main drug to treat schistosomiasis. Public health experts worry that the worms will become resistant to this drug, praziquantel.

More studies are needed on the experimental drug. The scientists say the results in mice were better than all the targets set by the World Health Organization for new schistosomiasis compounds. They hope the drug will be ready for testing in humans in four to five years.

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Grand Canyon

Scientists in the United States say the Grand Canyon is nearly three times as old as earlier estimates. They say they found evidence that the Grand Canyon began forming seventeen million years ago. That is eleven million years earlier than other studies have shown.

Geologists at the University of New Mexico carried out the new study. Their findings were published last month in Science magazine.

Other scientists say the findings fit with earlier theories about how the Grand Canyon may have been formed. But some experts on Earth's development disagree. They say the study fails to support earlier findings.

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The Grand Canyon is a popular stop for visitors to the southwestern United States. It stretches up to twenty-nine kilometers wide and nearly two kilometers deep. Yet its age has long been an issue of scientific debate.

Scientists have often used geologic events to describe the history of the Grand Canyon. Such events have included rock flows and sedimentary rock, or rock formed from other rocks. Generally, this method is only able to confirm ages of rock formations up to one million years ago.

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Instead, the American geologists used a uranium-lead dating method that finds ages of minerals back tens to hundreds of millions of years. They dated minerals from caves at different depths of the canyon’s walls. Minerals from openings on hillsides are less likely to suffer damage from water or other causes of erosion.

The uranium-lead dating system helped the geologists estimate water levels over time as river water cut through the rock to form the Grand Canyon. Their findings suggest that the rate of erosion was much slower in the western canyon than in the eastern part.

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Today the Colorado River runs along the four hundred forty-six kilometer long canyon. Based on their findings, the geologists believe a separate river began the formation of the Grand Canyon. They say the canyon started instead from the west by a river about seventeen million years ago. Another river began forming a canyon from the east. Over time, the rivers connected to each other. The geologists estimate the two canyons joined together about six million years ago.

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This SCIENCE IN THE NEWS was written by Brianna Blake, Jill Moss and Mario Ritter. Brianna Blake was also our producer. I'm Bob Doughty.

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And I'm Barbara Klein. Read and listen to our programs at voaspecialenglish.com. Join us again next week for more news about science in Special English on the Voice of America.