They estimate that a basin almost the size of the United States or Europe for billions of years covered part of Tharsis, a magmatically active bulge in the western hemisphere.

Tharsis landforms are a complex of towering volcanoes, lava flow fields, igneous plateaus, fault and rift systems, flood channels, vast canyon systems, and tectonic features. Most scientists believe that periodic release of internal planetary heat at Tharsis has for more than three billion years had a major impact on Mars' geology, hydrology and climate.

Parts of the aquifer may harbor near-surface water and possibly life, they add.

University of Arizona hydrologist James M. Dohm and his colleagues are reporting their basin/aquifer system hypothesis both in an article in the Journal of Geophysical Research - Planets and in a 3-dimensional animation on the Internet.

Collaborating in the research are Justin C. Ferris, Victor R. Baker and Robert G. Strom of the University of Arizona, Robert C. Anderson of the Jet Propulsion Laboratory in Pasadena, Calif., Trent M. Hare and Kenneth L. Tanaka of the U.S. G.S.-Flagstaff, Nadine G. Barlow of the University of Central Florida, and James E. Klemaszewski of Arizona State University.

They estimate that the 45 million cubic kilometer (11 million cubic mile) Tharsis basin ranged between 2 kilometers to 7 kilometers in depth (1.2 miles to 4.4 miles) and, if filled to an average depth of 5 kilometers (3.1 miles), would have a capacity of 12 billion billion gallons.

As lavas, sediments and volatiles (primarily water) partly infilled the basin early in Mars' history, the basin was transformed into a vast regional aquifer. This aquifer would serve as a potential source for water that carved what are believed to be the largest flood channels in the solar system, and helped fill lakes and oceans on ancient northern Mars.

If the terrestrial materials that filled the Tharsis basin are as porous as sediments and lavas on Earth, "then the potential volume of water contained in the aquifer would be more than equivalent to the volume of water required to create the putative ocean in the northern plains," the scientists wrote.

Baker, Strom, and others have long theorized that Mars' northern plains featured an ocean about a third as large as Earth's Indian Ocean and a smaller ocean the size of Earth's Arctic Ocean at least once in the ancient past.

Baker and colleagues have since developed this idea as the "MEGAOUTFLO" hypothesis. The theory says that Mars' history is punctuated by pulses of magmatic activity which trigger catastrophic floods, formation of oceans or lakes in the northern plains, and brief episodes of climate change lasting tens of thousands of years.

The scientists' 3-D visualization portrays how the Tharsis region landscape evolved over the past more than 3 billion years. The movie summarizes five geologic stages, with stage one depicting the ancient drainage basin and stage five depicting the present-day Tharsis landscape.

Dohm and colleagues based the sequentially reconstructed ancient terrains on geological and hydrological research. They synthesized analyses by many planetary scientists who began studying Viking data more than a decade ago and recently obtained high-resolution topographic data from the Mars Orbiter Laser Altimeter on the Mars Global Surveyor spacecraft.

The movie is "not a quantitatively accurate reconstruction of martian paleotopography at discrete time steps," the scientists wrote. "Such a reconstruction may well be possible at a future date when more data become available."

It is "an illustrated working hypothesis" that leads to the identification of an ancient, gigantic drainage basin that persists through much of the history of the region" and is consistent with diverse observations of martian geology, they said.

"Large topographic highs, including mountain ranges, an igneous plateau, topographic rises resulting from tectonism and other magmatic-driven processes, and large impact craters formed the margin of the gigantic drainage basin," Dohm said.

Magmatic and tectonic activity later fractured, deformed and, in places, exposed the stacked sequences of water-bearing layers in the aquifer, he added. The researchers interpret the layered canyon walls of Valles Marineris at the center of the proposed drainage basin, for example, to be basin fill comprised of layered flood lavas possibly laced with eroded lake and wind deposits.

Magmatic and tectonic energy also drove sediment-charged flood waters toward the northern plains and transferred water laterally so it collected at unmodified parts of the aquifer.

"The unmodified parts of the basin/aquifer system appear still to contain near-surface water reservoirs that may one day be sampled and analyzed by astronauts," Dohm said. He collaborated with Nadine Barlow of the University of Central Florida in recent research that suggests Mars today has such a "watering hole."

More, there may be hydrothermally active sites in the basin/aquifer similar to hydrothermally active sites on Earth now known to harbor life, Dohm said. These potential aqueous environments are prime candidates for hydrologic, mineralogic and "exobiologic" exploration, Dohm and his colleagues emphasize.

Before he joined the UA in 1999, Dohm worked more than a decade at the U.S. Geological Survey in Flagstaff as assistant coordinator of NASA-funded Mars and Venus mapping programs, now called the Planetary Mapping Program.

The JGR Planets Paper - PDF File

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MARSDAILY

Active Volcanism On Mars And The Search For Water
Reykjavik - May 10, 2001
The Tharsis Montes on Mars of Arsia Mons, Pavonis Mons and Ascraeus Mons, are gigantic volcano triplets of a youthful age with possible evidence of recent volcanism. Based on crater impact rates, Johann Helgason theorizes that a major fluvial erosive event took place within the past 10 million years.

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