Today a number of sources have been reporting on some amazing new images from the incredible Mars Reconnaissance Orbiter that show long, dark "tendrils" of liquid a few metres wide making their way down slopes on the surface of Mars. NASA, in today's news briefing called this the "best evidence we have to-date of liquid water forming on Mars".
So how does this fit into the rest of the evidence for life on Mars?
We've seen similar images before, but they showed only what looked like gullies left over from liquid flows, these new images actually show flows hundreds of meters long, often flowing around obstacles and sometimes splitting or merging. These flows seem to appear during the warmer months and disappear in the colder winter season on Mars. The temperatures are still too cold for pure water, but it's very plausible to consider that these flows are briny water solutions thawing out of muddy frozen icy hillsides.
Brine solutions with about the salinity of earth's oceans could thaw on Mars during the correct conditions, which means that plenty of very simple life could survive in this water. The additional challenges of surviving on Mars add to the difficulty for the survival of life, but micro-biologists consider it a strong possibility that Halophilic (salt loving) micro-organisms could survive in these conditions.
The amazing discoveries of the twin Mars rovers, Spirit and Opportunity, have provided a wealth of evidence that Mars was once a warmer and wetter planet with essentially very good conditions for micro-bacterial life. Part of that evidence - the discovery of large amounts of carbonate minerals - indicate that Mars once had chemically neutral water on the surface, which increases yet further the evidence that conditions suitable for life exist.
When all this is added to the evidence for methane on Mars, it seems that the more we look, the more the evidence grows for life on Mars.
The methane evidence was first found back in 2004 when telescopic spectroscopy data from Earth showed hints of methane in the atmosphere. In 2009 the evidence became more intriguing when new, more accurate studies showed that there are three discrete regions producing methane. Methane could only be produced by either geological activity or by life, however methane is broken down quickly by sunlight, so for it to be there now it must be being created right now by one of those processes. Given the otherwise dormant appearance of Mars' geology, geological activity seems an unlikely source.
Adding yet more to the evidence, the quantity of methane increases in the hemisphere experiencing the summer months and decreases in the cooler months. So, when there is liquid water do microbes thaw, become active and produce this methane? It's very tempting to believe so, and this theory seems to perfect fit the available evidence.
This new evidence for liquid on the surface brings us tantalisingly another step closer to confirming that theory.
I personally believe that there must be some sort of simple life on Mars. The evidence all seems to be pointing in that direction; we just need that first direct observation, which unfortunately remains illusive and probably will remain so, at least until we visit.
NASA's Curiosity rover will soon be heading to Gale Crater on Mars, a huge crater that (it is believed) once held large amounts of liquid water. Gale crater has a large mountain at it's centre which may make it a good spot to look for similar water flows. However, while Curiosity will have the sensitivity to detect methane being released in the immediate vicinity, NASA will not risk getting too close to water for fear of contamination with Earth bacteria.
Evidence that Mars may have conditions suitable for supporting life:
The Viking orbiter in 1976 found photographic evidence of features that looked like river valleys.
The Viking lander returned negative results for micro-organisms and organics (though a minority of scientists dispute this, positing that the evidence may in fact indicate a positive result for micro-organisms). Officially the results remain "inconclusive".
An ancient Mars meteorite (ALH84001) after analysis in 2002 was announced to contain structures resembling the mineralized casts of terrestrial bacteria. (Further analysis in 2009 prompted NASA to re-announce the claims, but many remain sceptical).
In 2004 methane was detected in the atmosphere.
Also in 2004 ESA announces that it's Mars Express rover has detected large quantities of Ice at the poles.
Hematite, an Iron Oxide that forms in the presence of water was discovered in 2004 by the Mars rover Opportunity.
NASA finds physical evidence using the Mars rovers for long term liquid on the surface such as "ripples" and "spherules" (the Hematite) in the rock.
In 2006 the Mars Global Surveyor photographed the first evidence that seasonal flows (of what may be salt water) flow on the surface.
In May 2007, the Spirit rover discovered Silica, another chemical compound that forms in the presence of water.
In 2008 The Phoenix lander found water ice, with neutral PH and "benign" salinity level, but the discovery of Perchlorate indicated difficult conditions for life.
In 2009 methane distributions where detected to be both regional and seasonal.
While stuck in soft soil on Mars in 2010, Spirit analyses the soil around it and discovers more evidence for water in it's composition.
Today (4th August 2011) NASA announces images that actually show liquid flowing on the surface.
So, to what conclusions does all this evidence point? Well, we now know for sure that there was once a lot of surface water on Mars, water that persisted for a long time, perhaps a few million years. There is currently a lot of water ice below and on the surface of Mars. The evidence also seems to indicate that under certain conditions this water can melt and run down slopes and across the surface. Seasonal methane releases coincide with the warmer regions when water ice has been seen to melt.
So, to tie it all together (almost the last piece in the puzzle): Are there micro organisms on Mars that become active in this melt water and create the methane? Does the carbon cycle on Mars convert carbon dioxide from the atmosphere into methane by means of a life form? If so, is this life form related to life on Earth, is it carbon based, DNA stamped life as we know it, or is it something completely different?
Fascinating questions! I can't wait for an answer!
 Launch is currently scheduled for November 25th 2011 riding an Atlas V from Cape Canaveral.