Prior to the Mars Exploration Rovers, the closest evidence we had of past life is the Alan Hills 84001 meteorite.

Based on chemical composition, this meteorite is known to have its origins from Mars. What prompted excitement with possible life was close (very close) examination of this meteorite:

To many, the image above is undeniable proof that at least bacterial life forms existed on Mars in the distant past. But this image is not without controversy. Two main pieces of evidence against this image:

• Microbe too small to contain DNA or RNA

• This "effect" could have been the result of sudden heat - i.e. entering Earth's atmosphere

A good unbiased article on ALH84001 can be found at SpaceRef.com.

Of course the biggest buzz regarding Mars is the Mars Exploration Rovers: Spirit and Opportunity. These two probes have demonstrated unparallel success as they have far exceeded expectations regarding operation and data collection. While the data collected by these two probes is enormous and will required decades of interpretation, the result so far is that:

Liquid water has existed on the surface of Mars.

As far as how long ago is still a question that needs an answer.

Images from recent Mars missions:

The first suggestion of water evidence is a rock called Escher. Based on comparing geography with Earth, this type of formation can only occur when the surface was wet, then dried:

This image below demonstrates geographical formation that could only occur as a result of shaping by liquid:

A mineral called magnetite is know to occur in only wet environments, and these "berries" are a footprint of magnetite formation:

Another bit of water evidence is the presence of another water forming mineral called goethite:

In order to determine composition, Astronomers use a variety of filters. The image below shows the basic composite of materials in Endurance Crater:

• Blue = basalt type rocks

• Green = iron oxide mixed with basalt

• Red = sulfates

We will close out this section on Mars with one of my favorite images:

This image is from the Mars Global Surveyor, and shows what looks like gullies - like those of which formed by liquid water. The two circles on the lower right are recent (within a few thousands of years) meteorite impacts.

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