Roger Weller, geology instructor email@example.com
last edited: 7/8/15
Water on Mars...Active Springs?
On Earth the concepts of Porosity and Permeability are important for explaining natural springs. A layer of sediment that is both porous and permeable that is also trapped between impervious layers will allow ground water to flow through it. If water completely fills the porous layer and the layer is inclined, the groundwater will flow down slope. In order to have a natural spring, the porous layer needs to be exposed on a cliff surface. The spring water will flow down the cliff surface; eventually creating a small gully by erosion is there are loose sediments.
On Mars there are exposures on the sides of cliffs where it appears that something is leaking from a layer. Below what may be a porous and permeable layer are gullies that run down the cliff slope.
Located above the layer where the gullies start is a light-colored layer. This may correspond to the overlying impervious layer in the diagram that describes how a natural spring on Earth operates.
Some of these gullies are hundreds of feet long.
In comparing photos of the surface of Mars taken several years apart, it has been discovered that some of these channels formed between the times the photos were taken. This means that the gully formation is still going right now, not something that happened millions of years ago. If it is water that is leaking out of the layer and causing the erosion, this means that there may be water available on the surface of Mars.
There are some puzzles still to be solved regarding the possibility of water creating these gullies. The atmosphere on Mars is so thin that water just above its freezing point would boil away and evaporate once it hit the thin atmosphere. In order to have water carve the gullies, a rather large quantity of water would have to leak out all at once. So much water would need to flow so that some of the water would make it all the way to the bottom of the cliff before it boiled entirely away. Another indicator of the quantity of water emerging from the porous layer are the large boulders that have been carried down the gullies.
Another problem is the cold surface temperature of Mars. It is so cold that most of the time that water could only exist in the form of ice like the permafrost ground in Alaska on Earth. The thick surface layer overlying the porous layer should act as an insulating blanket keeping the ice in the ground frozen solid. Where could the heat come from that would be melt the ice? Is there something below the ground like hot springs that is causing the ice to melt?
SAND BOX EXPERIMENT
Let's make a model that copies the process of making gullies like we see on Mars.
First we need to create something that acts like a porous and permeable layer of sediment. Take a shallow plastic box with a cover and fill it with gravel that is all about the same size. From our first experiment we saw that this type of gravel is both porous and permeable. The container needs some holes drilled on one edge of the pan, these holes will be the sources of our flowing springs. The top cover of the pan will be the upper impervious layer.
Next we need to be able to supply water to the layer after we bury the pan in wet soil.
We will attach a funnel to the top of the box and seal it well so that it doesn't leak when we add the water.
In a large, long plastic container filled with damp wet sand, we will build a cliff.
Why wet sand? Because wet sand sticks together, just like making sand castles. We will then bury the pan so that the edge with the holes in it is close to the edge of the cliff. Cover the pan with a thin layer of wet sand so that the pan is hidden.
Dust the cliff with a thin layer of dark dirt. This will help is see the gullies when they form.
We are all set to run the experiment. Slowly pour water into the funnel and watch for the springs to start flowing. Gullies should start to form on the cliff.