Martian Water, Water Everywhere…
Lurking just beneath the surface of Mars is enough water to cover the entire planet ankle-deep, says Los Alamos National Laboratory scientist Bill Feldman.
Feldman on Saturday released the first global map of hydrogen distribution identified by instruments aboard NASA’s Mars Odyssey spacecraft and offered initial minimum estimates of the total amount of water stored near the Martian surface. His presentation came at the annual meeting of the American Association for the Advancement of Science in Denver.
For nearly a year, Los Alamos’ neutron spectrometer has been carefully mapping the hydrogen content of the planet’s surface by measuring changes in neutrons given off by soil, an indicator of hydrogen likely in the form of water-ice, within about 35 degrees latitude of the north and south poles.
Five-Inches of Standing Water?
"It’s becoming increasingly clear that Mars has enough water to support future human exploration," Feldman said. "In fact, there’s enough to cover the entire planet to a depth of at least five inches, and we’ve only analyzed the top few feet of soil."
The new map is based on views of the red planet through more than half a Martian year of 687 Earth days, so researchers have been able to see both poles without obstruction by the seasonal polar caps of frozen carbon dioxide, dry ice. From about 55 degrees latitude to the poles, Mars has extensive deposits of soils that are rich in water-ice, bearing an average of 50 percent water by mass. In other words, Feldman said, a typical pound of soil scooped up in those polar regions would yield an average of half a pound of water if it were baked in an oven.
|Global water distribution in percentages found in the soil based on subsurface hydrogen readings from the orbiting neutron spectrometer. Click image for mosaic.
The tell-tale traces of hydrogen, and therefore the presence of hydrated minerals, also are found in lower concentrations closer to Mars’ equator, ranging from two- to 10-percent water by mass. Surprisingly, two large areas, one within Arabia Terra, the 1,900-mile-wide Martian desert, and another on the opposite side of the planet, show indications of relatively large concentrations of sub-surface hydrogen.
"The big reason we’re so confident now is that we have an absolute calibration of our results," Feldman said. He and his Los Alamos colleagues recently compared their neutron spectrometer readings as Odyssey flew over the north pole during early spring, when the dry-ice ground cover was thickest, against simulations of the spectrometer’s response to a thick layer of pure dry ice. This allowed them to calibrate the Odyssey readings with a known Martian soil type.
"We’re sure there’s dry-ice precipitation at the poles because the temperature of the ground cover is within the range for dry ice. And we can tell how thick the icecaps are, from the measured intensity of hydrogen gamma rays coming from underneath the icecaps," Feldman said. "We went from thick dry ice to a low-hydrogen abundance calibration when we applied our ‘neutron ruler’ at Mars’ equatorial latitudes. We were surprised to see such huge amounts of hydrogen at those lower latitudes: close to 10 percent in some places."
Hydrogen is only absorbed chemically near rock surfaces, but Mars geology appears to be rich in minerals as zeolites, clays and magnesium sulfate, all of which can retain significant amounts of water.
"This is material that has absorbed the hydrogen chemically and has retained it for millions of years," Feldman explained.
The team also studied meteor craters more than 250 miles across such as Schiaparelli and discovered that the water content in the crater’s center is reduced.
Tale of Two Springs: Polar Caps or Ancient Polar Tilt?
Scientists are attracted to two possible theories of how all that water got into the Martian soils and rocks.
The vast water icecaps at the poles may be the source. The thickness of the icecaps themselves may be enough to bottle up geothermal heat from below, increasing the temperature at the bottom and melting the bottom layer of the icecaps, which then could feed a global water table.
|South Pole water content by percentage found in subsurface layers.
On the other hand, there is evidence that about a million years or so ago, Mars’ axis was tilted about 35 degrees, which might have caused the polar icecaps to evaporate and briefly create enough water in the atmosphere to make ice stable planet-wide. The resultant thick layer of frost may then have combined chemically with hydrogen-hungry soils and rocks.
"We’re not ready yet to precisely describe the abundance and stratigraphy of these deposits at high latitudes, but the neutron spectrometer shows water ice close to the surface in many locations, and buried elsewhere beneath several inches of dry soils," Feldman said. "Some theories predict these deposits may extend a half mile or more beneath the surface; if so, their total water content may be sufficient to account for the missing water budget of Mars."
Hydrogen As the Orbital Divining Rod
Los Alamos’ neutron spectrometer, a more sensitive version of the instrument that found water ice on the moon five years ago, is one component of the gamma-ray spectrometer suite of instruments aboard Odyssey. Prof. William V. Boynton of the University of Arizona leads the gamma-ray spectrometer team.
The neutron spectrometer looks for neutrons generated when cosmic rays slam into the nuclei of atoms on the planet’s surface, ejecting neutrons skyward with enough energy to reach the Odyssey spacecraft 250 miles above the surface. Elements create their own unique distribution of neutron energy — fast, thermal or epithermal — and these neutron flux signatures indicate what elements make up the soil and how they are distributed. Thermal neutrons are low-energy neutrons in thermal contact with the soil; epithermal neutrons are intermediate, scattering down in energy after bouncing off soil material; and fast neutrons are the highest-energy neutrons produced in the interaction between high-energy galactic cosmic rays and the soil.
By looking for a decrease in epithermal neutron flux, researchers can locate hydrogen. Hydrogen in the soil efficiently absorbs the energy from neutrons, reducing their flux in the surface and also the flux that escapes the surface to space where it is detected by the spectrometer. Since hydrogen is likely in the form of water-ice at high latitudes, the spectrometer can measure directly, a yard or so deep into the Martian surface, the amount of ice and how it changes with the seasons.
Mars Odyssey was launched from Cape Canaveral Air Force Station in April 2001 and arrived in Martian orbit in late October 2001. During the rest of the spacecraft’s 917-day science mission, Los Alamos’ neutron spectrometer will continue to improve the hydrogen map and solve more Martian moisture mysteries.
In fact, a team of Los Alamos scientists has begun a research project to interpret the Mars Odyssey data and their ramifications for the history of Mars’ climate. The project seeks to develop a global Martian hydrology model, using vast amounts of remote sensing data, topography maps and experimental results on water loading of minerals.
But how did water vapor get into the subsurface soils and into rocks farther beneath the surface of Mars? The effort to answer that question and to reconstruct the Martian hydrologic cycle will occupy Feldman and his colleagues for years to come.
Two Mars Exploration Rovers are targeting what imagery indicates might have been ancient dry lake beds and other geologically interesting sites in early 2004.
Researchers working with Feldman on the Odyssey project include Tom Prettyman, Bob Tokar, Kurt Moore, Herb Funsten, David Lawrence and Richard Elphic. Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Odyssey mission for NASA’s Office of Space Science in Washington, D.C. Investigators at Arizona State University in Tempe, the University of Arizona in Tucson and NASA’s Johnson Space Center, Houston, operate the science instruments. Additional science partners are located at the Russian Aviation and Space Agency and at Los Alamos National Laboratories, New Mexico. Lockheed Martin Astronautics, Denver, the prime contractor for the project, developed and built the orbiter. Mission operations are conducted jointly from Lockheed Martin and from JPL.