Anthonares

In a series of papers published in the journal Science last December, teams of scientists headed by Mars Exploration Rover mission director Steve Squyers reported that the landing site of the rover Opportunity, Meridiani Planum, had abundant evidence of liquid water at the surface for extended periods of time. One of those lines of evidence is the “blueberries” pictured at the left (credit: NASA, JPL) that indicate possible accretion of minerals within a groundwater aquifer.

All data collected on NASA missions is made public, but as is typically done, the mission principle investigators get the first crack at interpreting the masses of data. Now, after a year of intense investigation, two papers have been published that challenge many of the interpretations of the MER mission scientists including the basic assumption that water was necessary during the formation of the rocks seen at Meridiani. These two papers are remarkable in that they both present serious challenges to water-based interpretations, and that they are completely independent of each other. One paper suggests volcanic origins for the rocks at Meridiani, the other hypothesizes that impacts were the cause.

Citations (online at CiteULike.org):
Knauth, P.L., Burt, D.M. and Wohletz, K.L. (2005). Impact origin of sediments at the Opportunity landing site on Mars. Nature 438 (7071), pp. 1123-1128.

McCollom, T.M. and Hynek, B.M. (2005). A volcanic environment for bedrock diagenesis at Meridiani Planum on Mars. Nature 438 (7071), pp. 1129-1131.

Synopsis:
Knauth and others
The mechanism of deposition that they suggest is the ground surge resulting from the impact of a massive iron meteorite. The picture at the right shows a nuclear test in Nevada with a surge moving along the ground at speeds near to the speed of sound. This surge picks up large quantities of rock, dust and gasses as it moves outward and turbulently mixes the combination. The authors note that the nuclear test shown here laid down deposits as much as 1 meter thick. Knauth and his co-authors follow a number of lines of reasoning to suggest that meteorite impact deposited the Meridiani Planum rocks, here I’ll mention the three most important arguments.

1) They note that the layering of sediments seen by Opportunity is similar to that seen in meteorite, volcanic, and nuclear surges here on Earth.
2) The presence of both highly water-soluble and insoluble salts in common locations at Meridiani challenges the assumption that they were formed in an evaporating lake. Here on Earth, evaporating lakes leave a distinct sequence of salts as the briny water becomes more and more concentrated. An impact, on the other hand, would stir up existing salt sequences, resulting in no particular separation in the final deposit.
3) Knauth and others note that the “blueberry” spheroids seen on Mars are particularly round and uniform, compared to water-created spheroids here on Earth. Impact spheroids tend to be much more uniform and spherical, however. Also, the high iron content in these spheroids is quite rare on the Martian surface, suggesting a localized event such as an impact by an iron meteorite.

McCollom and Hynek
These two authors look primarily at the elemental composition of the Meridiani rocks along with the layering features often attributed to water to argue that volcanic deposition is more likely than water-based deposition. Their primary composition argument is that the rocks at Meridiani are rich in sulfates, but not in the corresponding ions that would have been present if the sulfates were originally salt in a water solution. These ions, primarily magnesium and calcium, are relatively scarce, suggesting that the sulfates resulted from something other than being left behind from evaporating briny water. Volcanoes release large quantities of sulfur and produce sulfuric acids that could have reacted with the basalt rocks to produce sulfates. They also note that enormous quantities of water would be required to match the 500 meter to 1 km thickness of the rocks at Meridiani, but that volcanoes could easily deposit this much material.

Next the authors note that the layering seen in Martian rocks (pictured at the left, part a) is commonly attributed to water, but that volcanic deposits can contain very similar features (part b in the picture). The authors do explain the presence of the spherules as well, noting that they only seem to be present in the first few meters of rock. They conclude that reactions with the rock and the atmospheric oxygen over billions of years probably led to the formation of the spherules.

Context:
It is interesting to note that both articles point out problems with the original interpretations published last year, but that each points out different problems. I am not an expert in this field, so I will not pass judgement on which claim is more likely to be true, though I will say that the “true” answer is probably that all three mechanisms: water, volcanoes, and impacts, were active at some time in the history of the Meridiani Planum. There may be enough data to come to a definitive answer on which interpretation is most accurate, but I’m not sure. A way that this would be done on Earth is for each team to make a prediction about some feature that their theory says would be at the site. Whichever theory provides the most accurate prediction would be held the winner in this contest, since the predictive power of a theory is what makes it most valuable to scientists. But, until Marsnauts someday tour Opportunity’s old stomping grounds, we may never know for sure.

These two articles also illustrate that one set of data may lead to several competing interpretations. This ambiguity in science is very troubling to some who see it as incompetence, or even greed. But, given sufficient time (and funding), the correct answer almost invariably comes the forefront as scientists are never truly content to leave ambiguity unanswered.

Do these articles really, as they both suggest, mean that water was rare on early Mars? Probably not by themselves, no. As you may remember, a few weeks ago I reviewed a paper by Poulet and others that looked at the widespread presence of clay minerals on Mars and concluded that Mars was once very wet. As it dried out, they argued, the environment became more acidic, allowing for the sulfates to be deposited that we see at Meridiani. So, the Wet/Dry Mars question is not resolved one way or the other, but planetary scientists have much more to talk about at their upcoming meetings in the spring!

General Explanations
Mars probably was very wet in its earliest days. But how can a planet simply dry out? The answer lies with the sun, though not exactly as may seem initially the case. Because Mars lacks a strong geomagnetic field, energetic charged particles from the sun strike water molecules in the Martian atmosphere much more often than here on Earth. These particles shatter water molecules into hydrogen and oxygen gas (H2 and O2). The hydrogen gas is too light to be held by Mars’ gravity (or by the Earth’s for that matter), and the heat from the Sun provides the hydrogen gas with enough energy to gradually disperse into space. But, some of the water probably did not leave Mars this way, as the rocks in the Martian crust have cracks and pores that allow for water to percolate down into them. Water in the crust is referred to as an aquifer, and it may still be there today, frozen beneath a thick layer of rock and ice maybe as much as a mile thick in places. The Mars Express satellite is currently using a radar to try and detect this ice, and if they are lucky, maybe the water beneath.