June 28, 2000 WASHINGTON - The crust of the planet Mars may hold two to three times more water than scientists had previously believed. This finding is based on a study by Dr. Laurie A. Leshin of Arizona State University, comparing the amount of deuterium, an isotope of hydrogen, found in a meteorite of martian origin to the amount found in the martian atmosphere. Her report will be published in Geophysical Research Letters on July 15.
Deuterium, a heavier form of hydrogen, combines with oxygen to make "heavy" water. In today's thin Martian atmosphere, water has a deuterium-to-hydrogen ratio five times higher than is found in water on Earth. Previous research attributed this to the escape of hydrogen from the martian atmosphere over time. Because hydrogen is lighter than deuterium, it escapes more easily, leading to the high relative level of deuterium in the atmosphere of Mars today.
Scientists had previously assumed that before the deuterium level was enhanced by the escape of hydrogen, martian water more closely resembled that on Earth, with a comparable ratio of deuterium to hydrogen. In order to reach the current value of five times higher than Earth's water, they calculated that around 90 percent of the water in the Martian atmosphere and upper crust had been lost over the planet's history.
Leshin compared the deuterium level in the atmosphere with that in a meteorite known as QUE94201, found in Antarctica in 1994 and believed to have been blasted off Mars three million years ago. Tiny water-bearing crystals in the meteorite were analyzed by Leshin on the ion microprobe instrument at the University of California at Los Angeles. These crystals contain hydrogen from the martian interior, which was not affected by atmospheric escape. They revealed a smaller percentage of deuterium than current martian atmospheric measurements.
But instead of this ancient water demonstrating the same deuterium-to-hydrogen ratio as Earth water, as had been assumed, Leshin's research shows that Mars had a deuterium-to-hydrogen ratio nearly double that of Earth before any atmospheric escape could have occurred. Leshin suggests that this could have resulted from loss of hydrogen very early in martian history as a result of extreme ultraviolet radiation from the young Sun, a mechanism different than the current escape process. Alternatively, she writes, it could imply that comets, which share the same deuterium to hydrogen ration as martian interior water, supplied most of the water found on Mars today.
Since martian water originally contained higher deuterium levels than previously thought, Leshin concludes that the martian atmosphere has lost two to three times less water through the eons in order to arrive at the isotope's current atmospheric level. That water should still exist today on Mars, she says, located within the planet's crust. In fact, evidence from this and previous research on martian meteorites supports the idea that a significant martian groundwater reservoir currently exists.
Just how much water is there on Mars? Leshin cautions that her research does not provide the answer to that question, only that these latest findings suggest that there remains up to three times more water in the martian crust than previously thought. Future missions to Mars will have to study the martian soil, both in place and by returning samples to Earth, to arrive at a meaningful estimate of the actual amount of water remaining there.
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