(photos: JPL/NASA/MSSC)

Present Reservoirs

            Exposed water reservoirs exist as water vapor and water ice in the atmosphere, seasonal water ice deposits at the surface and permanent water ice deposits at the polar caps.  Yet, considering the extent of fluvial erosion along with certain chemical evidence of the Martian surface, not can it only be expected that in the past the atmosphere was once thick enough to sustain warm enough temperatures for water flow but that there may be the presence of hidden water reservoirs whose combined masses should be much greater than the known water reservoirs that are exposed.  There are expectations that water is absorbed in the near-surface regolith (a layer of debris that makes up the soil that covers bedrock).  Experiments conducted by Viking surface rovers reveal that as much as 1% of the mass of the soil may be water.  Water may be contained in rocks in the form of hydrated minerals.  Some observations have revealed a presence of crystalline hematite.  Hematite (Fe2O3) is a mineral that grows from low temperature precipitation of water.  Other expectations include layered deposits surrounding the north and south poles that contain water ice and near surface ice in the ground all the way to latitudes as low as 20 degrees from the equator.  If you go deep enough (km depths) the Martian subsurface would eventually rise to temperatures above 0 degrees Celsius where liquid water would become stable.  So basically, the water that we know about is only a very small fraction of what we believe really exists on Mars.  It's "the tip of the iceberg."

Current Hydrology

            Hydrology is the study of the cycle of water movement based on its properties and distribution.  Knowing how water moves on Mars is essential to understanding it's distribution.

            Mars is a very cold and dry planet with a very thin atmosphere.  Both the atmosphere and the temperature affect the behavior of water.  The temperature only goes above 273 Kelvin (0 degrees Celsius or 32 degrees Fahrenheit) at low latitudes during the warmest periods of the day.  These cold temperatures favor solid forms of water.  Any liquid water would quickly evaporate due to the lack of pressure supplied by the thin atmosphere and be transported to colder locations where it would then freeze.  Therefore Mars is quite ineffective in transporting large quantities of water.  Water vapor observations from the Viking Mars Atmospheric Water Detector (MAWD) show that maximum abundances of water vapor are when the northern hemisphere is in summer and the north pole is exposed to the Sun.  These tests reveal that during this period the atmosphere can provide a water vapor precipitation of 100 microns only over parts of the northern latitudes.  (A micron is one millionth of a meter.  100 microns is 1/10 of a mm.  That's nothing.)  Normally it is less than 15 microns.

            Not much is known about Martian hydrology due to the lack of information providing physical and chemical properties of the Martian surface and subsurface.  The way sediments on Mars absorb water and how heat is transported through the planet are very important pieces of information to make models that can predict the behavior of water on Mars.  Therefore more detailed observations need to be conducted in order to become more knowledgeable about hydrology on Mars.

Paleohydrology  (For those who don't know, "Paleo" is a Greek prefix meaning old so paleohydrology is hydrology that is really old.)

            What's exciting about Mars is that when you look at it and see all this geologic stuff printed into the surface you know that something big was going on down there billions of years ago.  Geologic evidence such as giant outflow channels, runoff channels, sedimentary layers and gullies all suggest that water flowed on Mars in the distant past.  Also evidence such as contraction-cracked polygonal terrain, landforms associated with near-surface ground ice, debris covered glaciers, and ice-rich permafrost suggest more recent water flow from maybe the last few millions of years. 

            With such a vast range of features and vast range of time in which the landforms may have been created, it is very difficult to attempt to understand what really happened there millions and billions of years of ago.  There are many different kinds of theories that try to describe Martian paleohydrology but like the current hydrology, not much is known about the physical and chemical properties of the Martian surface (especially from the past) and the Martian geologic record to accurately define it.  Possible scenarios for the history of water on Mars range from a relatively dry past to one with a huge ocean which completely covered the northern hemisphere.  However, models to describe paleohydrology are improving and are becoming more consistent with the current observations of the atmosphere by MGS.