Water, and Ancient Mars

In the last lecture, I discussed how we see both evidence for water channels on Mars, and at the same time recognize that the atmospheric conditions on present Mars preclude the possibility of liquid water lasting for any amount of time.

The resolution of this seeming inconsistency may lie in these two characteristics (water channels on one hand, low atmospheric pressure on the other) referring to different ages in the history of Mars. That is, the water channels may have been produced in a time when the atmosphere was denser, the temperature was warmer, and water could exist on the surface of Mars. In this case, the water channels (particularly the valley networks) would be a relic of a habitable time, that still may be seen on the hostile face of Mars.

This view is referred to as the warm, wet Mars scenario. The opposite point of view is described as the cold, dry Mars scenario. The warm, wet Mars possibility is more fun to think about, and it has definitely gotten a big boost from discoveries of this year, made with the Mars Exploration Rover and Mars Express spacecraft.

Once again, one of the reasons scientists are extremely interested in studying this is the possibility that, in the remote past, Mars may have been sufficiently benign for life to have gotten a start, even if it has not lasted until the present time.

To see why the surface conditions on ancient Mars may well be relevant to the question of the origin of life, let’s do some comparative planetology, and look at a diagram showing the geological history of Earth. This diagram is taken from the original in a 1989 article by Christopher McKay and Carol Stoker.

There are a number of possible surprises in this Earth timeline.

The original atmosphere of the Earth was not like the present atmosphere, but instead was primarily carbon dioxide, just like Mars (and Venus).
There is convincing evidence of the origin of life amazingly early, 3.5 billion years ago, or only about a billion years after the Earth formed. Some scientists believe there is even earlier evidence, back to 4.1 billion years (Gyr) ago, but that evidence is not entirely convincing. These first organisms were so-called prokaryotic cells, like bacteria.
The oxygen atmosphere built up slowly on Earth, and only began to become dominant in the last 2 billion years. The oxygen in the Earth’s atmosphere is a result of the life which originated and then expanded on Earth. Thus, the oxygen atmosphere of Earth is primarily a biological, rather than a physical or astronomical phenomenon.
The evolution and emergence of complex, multicellular animals only occurred at the time of the “Cambrian Explosion”, 550 – 500 million years ago.
The central message of this diagram is that life originated on Earth when it had a warm, wet, carbon dioxide atmosphere.

Now let’s talk about Mars, and look at a similar diagram for it.

Obviously, there is a lot missing on this diagram, since we don’t know as much about Mars. However, there are some things that are clear. Unlike Earth, Mars was apparently unable to hold on to its early, dense carbon dioxide atmosphere. Equally obviously, it never progressed to the point of evolving developing an oxygen atmosphere, so life, if it ever existed, didn’t get too far.

However, the interesting point of this diagram is the possible similarity with the early Earth, and not the differences.

Mars probably held on to its dense, carbon dioxide atmosphere until perhaps 3.5 to 3.0 billion years ago.
During this time, the valley networks formed, suggesting that liquid water could exist on the surface of Mars, even if it can’t now.
Thus, it looks as though conditions similar to those on Earth at the same time existed long enough for life to emerge on Mars as well as Earth.
Did it?

That is the big question at the present time.

Spacecraft exploration of Mars has approached this investigation in two ways.

(1) Direct biological experiments that attempt to detect microbes which may have evolved and adapted to the present conditions.

(2) Geological and mineralogical explorers which seek to test if an early-Earth-like environment (standing bodies of water, dense atmosphere, warm temperatures) existed on Mars in the first 1 – 1.5 billion years of its history. This is the approach of recent US spacecraft, such as the robots Spirit and Opportunity.

The announcement of March 2 was in the second category, and refers to the site of the second Mars Exploration Rover “Opportunity”. This site is shown below.

This picture is taken from the spacecraft after it has moved some distance from the landing craft, and is up in the rock outcropping near the landing site. Here you can see the edge of the crater it landed in, and the surrounding plain.

Remember that this landing site was chosen because there was strong evidence that there had been water at this site (existence of the mineral hematite). Nonetheless, one of the characteristics of science is independent evidence of a hypothesis, so confirming evidence of surface water was very important.

The results of interest come from inspection of the appearance of these rocks in the strata, and analysis of the minerals that can be done with instruments on the spacecraft. One of the rocks which was examined closely is shown below.

You can see a part of the spacecraft in the lower left corner of the image.

The evidence returned from Opportunity seems to strongly suggest that this site was submerged by water, not just in a transient fast flood, but for long periods of time as would be the case at the bottom of a lake or sea. As stated by one of the mainscientists, the site was “soaking wet”.

The evidence which was presented was as follows.

The instruments found the rocks contained high concentrations of sulfur in hydrated sulfur salts.
The rocks also contain significant amounts of jarosite, a hydrated iron sulfate mineral.
According to statements made, “ On Earth, rocks with as much salt as this Martian rock have either formed in water, or after formation, have been altered by long exposures to water”.
There are features of the rocks which are consistent with their having been submerged. A microscopic examination of the surface showed pits and indentations which seem to be caused by water eroding or dissolving minerals that had formed in these pits.

These results were announced earlier this year, so we will have to wait some time for a full response from the “cold, dry” scientists, which could take weeks or months. It may be that there are alternative interpretations of these data. But for the moment, it looks like a highly exciting development. The Spirit landing site, on the opposite side of Mars, initially seemed disappointing from a biological point of view, but slowly results emerged which seem to indicate that it, too, was submerged in water in the remote past.

Biological Experiments

As mentioned above, a second approach is to land biological experiments on the surface of Mars which could test for the existence of microbes. The idea here is that if conditions were conducive to the existence of life 3.5 to 4 billion years ago, perhaps that life has adapted to the present climate of Mars, and is still there.

At the moment, the only results we have in this area are from the Viking spacecraft, which landed on Mars almost 30 years ago. The plan was to have the European spacecraft Beagle 2 make tests of this sort (and improved relative to Viking) , but the spacecraft was lost in December.

The next lecture will give a summary of the results from Viking.