posted June 14, 2001
Outflow Channels May Make a Case for a Bygone Ocean on Mars
Written by Linda M.V. Martel
Hawai'i Institute of Geophysics and Planetology
High-resolution elevation data from the Mars Orbiter Laser Altimeter (MOLA) onboard the Mars Global Surveyor (MGS) spacecraft have been analyzed recently in Chryse Planitia to test the hypothesis that large outflow channels emptied into an ocean in this region of Mars. Researchers Mihail (Misha) Ivanov (Vernadsky Institute of Geochemistry and Analytical Chemistry) and James Head (Brown University) collected quantitative MOLA information on channel patterns, continuity, and elevations where those patterns change or disappear into the northern lowlands. Their recently published report describes how the channels end, or become more subtle, at elevations very close to a previously mapped geologic contact interpreted by some to represent a shoreline of an ancient ocean. Ivanov and Head hypothesize that the change in channel topography is consistent with flow of water from a river into a submarine environment with possible deposition of sediments by density currents deep into the North Polar basin.
||Chryse Planitia, chosen in the mid-1970s for the landing site of Viking 1, is a relatively flat, low, broad plain just north of the Martian equator. Because many of the largest Martian outflow channels converge here, Chryse Planitia is an ideal setting to study channel patterns and depositional environments. More importantly, researchers have noticed that the distinctive textures and teardrop-shaped islands inside the channels change and disappear near the margins of Chryse Planitia. These changes have led some people to hypothesize that debris-laden rivers may have emptied their loads into a lake at Chryse Planitia or into an ocean that occupied the northern lowlands. The actual timing of this bygone ocean is unknown, but may be Hesperian to Early Amazonian.
What the MOLA Data Reveal
Ivanov and Head cite five main lines of evidence from MOLA data that support the hypothesis that large outflow channels emptied into an existing standing body of water in the northern lowlands of Mars in Hesperian-Early Amazonian times.
- Chryse Planitia is not a locally closed basin but instead opens into the North Polar basin.
- The distinctive patterns of the six largest channels end at relatively the same elevation even though the channels are of different ages and are separated by hundreds of kilometers over a total lateral distance of more than 2500 kilometers.
- The Hesperian-aged channels end at elevations close to a previously mapped geologic contact interpreted as a shoreline of an ancient ocean.
||Mars has no actual sea level. The elevation designated as zero, therefore, is defined by the mean Martian radius, 3,382.9 kilometers, and the average atmospheric pressure of 6.1 millibars (6.1 thousandths of the Earth's atmosphere). If you were standing on the martian surface and the center of the planet were 3,382.9 kilometers beneath your feet, then you would be standing at 0 kilometers elevation, shown on this MOLA map in yellow.|
- Some of the channels continue out of Chryse Planitia for hundreds of kilometers into the North Polar basin, but their patterns are subdued and very different once past their recognized termini.
- The distinctive change in channel pattern is consistent with rapid loss of energy as when a river discharges into a shallow submarine environment.
Relationship of the Chryse Channels with the Hypothesized Ocean
Try to picture the floods. The crashing, tumultuous torrents were large and dirty, carrying rocks, ice, and sediments through the channels. Debris-laden floodwaters scoured the landscape, cutting through the underlying rock, and when they spread out, where did the floodwaters go? Did the water merely sink underground? Did it fill the North Polar basin to make an ocean? Did it enter an existing ocean? We can only guess what happened.
We need more information on the actual volumes of water and sediments involved in the floods and the timing of the floods. Ivanov and Head refer to different estimates of channel volumes (from Michael Carr, Victor Baker and others) to try to give us a better picture. Carr's estimate of the water volume of a single large flood in a Martian outflow channel is 300,000 km3. This is enough water to flood the entire North American continent by 30 meters! It would take at least 46 of these floods to fill the Martian northern lowlands to the level of Contact 2. Other estimates suggest that each channel may have filled the North Polar basin in separate events, requiring significant water loss between floods and refilling to essentially the same level. Ivanov and Head favor the hypothesis that the channels flowed into a preexisting standing body of water whose margins were already near the level of Contact 2 and cite the striking similarity of elevations where the channels end and the proximity of these elevations to the mapped Contact 2.
Mars Ocean Still Being Debated
Whether or not Mars had large bodies of standing water remains an unanswered question and not all investigators support the notion of a vast northern ocean. Photogeologic mapping of the proposed shorelines by Michael Malin and Kenneth Edgett in 1999 using high resolution images of Mars taken with the Mars Orbiter Camera (MOC) showed no features they would attribute to the action of water in a coastal environment. Other researchers contend that ridge networks in the northern lowlands are indicative of tectonic processes related to the Tharsis volcanoes. Tectonic features in this area of Mars, however, are not inconsistent with the possible presence of an ocean. Earth's ocean basins are prime examples of tectonic features.
Confirming the presence of large bodies of standing water in Martian history will require a multifaceted approach. We'll need laser altimiters (MOLA) for topographic data, cameras for photogeologic mapping, infrared spectrometers (such as onboard 2001 Mars Odyssey) for mapping the distribution of minerals on the surface, gamma ray spectrometers (also on Odyssey) for mapping the surface distribution and abundance of chemical elements, as well as mineral and chemical studies of meteorites and rocks returned from Mars (to check for the presence of salts, for example.) If there were standing bodies of water on Mars billions of years ago they may have influenced the planet's atmosphere and climate, geology, environmental chemistry, and ultimately its capacity to support the emergence of life. These are the reasons why scientists, including Ivanov and Head, seek evidence of ancient oceans on Mars. The search may quench our collective thirst for knowledge about the Red Planet.
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