Thursday, March 10, 2011

Our Solar System Part 2: Mars

Mars, the fourth planet from the Sun, is named after the Roman god of war and agricultural guardian. This is a mosaic image of the planet taken by Viking 1 in 1980. It was first seen close up by Mariner 4 as it flew by the planet in 1965. Until then, many researchers believed the light and dark patches and long channels visible in this image were signs of liquid water in the form of seas and rivers, possibly even irrigation channels, suggesting that intelligent life lives there. We now know that Mars is far less hospitable than we once thought and yet, after decades of intensive study, Mars still intrigues us with its many mysteries. 


Water does, in fact, exist on Mars. The lines and depressions you see in the image above are optical illusions. But, there is a large quantity of water ice at the poles, as revealed by radar data from two current missions, Mars Express and Mars Reconnaissance Orbiter. In fact, there is so much water ice in the southern polar cap that, if melted, it would cover the whole planet in water 11 meters deep. However, liquid water can not exist for long on the surface of Mars because any liquid water would quickly freeze and sublimate into the atmosphere. As well, the surface temperature is, on average, too cold for water in its liquid state and the atmospheric pressure is far too low (it's about equal to the air pressure on Earth at 35 km above sea level, that’s about 3 times higher up than a typical jet's cruise altitude) to support liquid water. At this extremely low pressure it just sublimates and disperses into space, although a very tiny amount can and does exist as water vapour in Mars' thin atmosphere.

These are two colour images taken by NASA's Phoenix Mars Lander in 2008. The white patches reveal water ice just beneath the planet’s surface regolith.

The right image was taken 4 days after the left one. If you look closely you can see that some ice has sublimated away during this time.

In the following NASA image you can see the northern polar cap of Mars very clearly.

This ice is not to be confused with the water ice I just mentioned. This is dry ice, or frozen carbon dioxide, CO2

On Mars, during a pole's winter, the pole exists in continuous darkness. The surface becomes very cold, averaging around -87°C (compare this to the absolute coldest Earth temperature on record at -89°C) at Volstok Station in Antarctica in 1983). At this temperature, about a third of Mars' atmosphere condenses out (freezes) into solid dry ice. When Martian spring arrives, the dry ice will sublimate back into the atmosphere as it is exposed to sunlight and warms up. This seasonal cycle drives powerful winds, as fast as 400 km/h, sweeping off the poles and gives rise to high cirrus clouds, shown below as bluish wisps in this beautiful photograph of the Martian sky just before sunrise, taken by the Imager for Mars Pathfinder in 1996.

The thin atmosphere on Mars consists of almost all carbon dioxide, 95%, with 3% nitrogen and traces of argon, oxygen, water vapour and other gases. The atmosphere extends almost twice as high into the Martian sky as it does on Earth. This is because Mars has much lower gravity to keep the atmosphere contained, about 38% that of Earth.

Methane Mystery

Interestingly, Mars also has a tiny bit of methane in its atmosphere. Although methane is a bit chemically unstable, each methane molecule on Mars should last for hundreds of years, maintaining a fairly stable atmospheric level. However, observations show that its levels fluctuate yearly, and they coincide with the seasonally changing presence of atmospheric water vapour. It is estimated that Mars produces about 270 tons of methane per year and there is currently some heated speculation about where it is coming from. Asteroid impacts should contribute less than 0.8% of this amount per year. There is also the nagging question of what is destroying the methane because it also seems to have a strangely high turnover rate in the atmosphere. Methane can come from volcanic activity, comet impacts, microbial activity or mineral processes such as serpentinization. In this case, a mineral called olivine, which is abundant on Mars, reacts with water and carbon dioxide to create serpentine, magnetite (these are two minerals) and methane gas. There are other possible geological sources of methane on Mars as well.

None of these possibilities has yet been ruled out. Until recently, it was widely believed that Mars has not been volcanically active for billions of years. Yet new atmospheric data suggests that Mars might have experienced at least one volcanic eruption, with resultant water flowing on its surface, within the last 100 million years. For some researchers, in particular the European Space Agency, the coincidence of methane levels and water vapour hints at a biological methane source, especially since these two gases seem to be concentrated in three equatorial regions. The significance of this will be explained in a moment. But first things first: how can life on Mars exist? The surface of Mars is very inhospitable to life. It is constantly blasted with deadly UV radiation, micrometeorites and solar radiation because Mars has no protective magnetosphere to shield it (this is also why Mars' atmosphere is so thin; we will get into this in more detail in a moment). However, recent images from the imaging system onboard NASA's Mars Odyssey orbiter reveal seven possible cave entrances on the flanks of the Arsia Mons volcano. These caves, known as the seven sisters, might provide a haven for possible methanogenic microbes. Researchers at the European Space Agency believe that perhaps these caves are deep enough and warm enough to sustain some liquid water, and being caves they could shield microbes from radiation and micrometeors. To test this hypothesis, NASA will launch the Mars Science Laboratory in 2011 (landing on Mars about 9 months later). It will measure the isotopic proportions of carbon-12 and carbon-14 in Martian methane. The idea behind this measurement is that living cells absorb carbon-14 at the same rate as they absorb carbon-12 into their tissues. Their living cell ratio of carbon-12 to carbon-14 is the same as that of their surroundings. However, when the cells die, the ratio of carbon-14 to carbon-12 decreases as the unstable carbon-14 decays. Meanwhile the ratio in nonliving things, such as the atmosphere and rocks, stays the same. The two ratios can be compared and the once-living fossil can be dated. This dating method, though, is not perfect and it has upper and lower reliable time limits. As well, the atmosphere on Mars is not equivalent to Earth’s atmosphere, making this kind of test potentially unreliable. So, the ratio of ethane to methane will also be tested. A ratio less than 0.001 suggests a biological methane source, whereas nonbiological chemical reactions produce nearly equivalent amounts of ethane and methane. Perhaps when these results come back, we can put to rest one way or another the question of life on Mars. Personally, I hope there is life, even microbial life. Such a discovery would suggest that life on other planets in our and other solar systems might be more likely than we ever imagined.

Martian Meteorites

Meanwhile, NASA has amassed a catalogue of 34 Martian meteorites. So far, these are the only physical samples of Mars we have because no probes sent to Mars have yet included return missions. That makes these meteorites extremely valuable to researchers. Scientists are fairly certain these meteorites are of Martian origin because they have the same elemental and isotopic compositions as rocks and gases as those analyzed on Mars. Most of these meteorites are quite young and this, along with the new atmospheric data mentioned above, suggests that Mars might have been volcanically active more recently than once believed. Rocks could have been spewed from volcanoes and launched into space, released from Mars' low gravity. For example, 7 nakhlite meteorites have been found so far. These unassuming-looking meteorites are named after El-Nakhla in Egypt where a large (10kg) meteorite was discovered in 1911. About a dozen of them fell as a meteor shower in the area. One is shown here.

These are all igneous rocks formed from basaltic magma about 1.3 billion years ago and ejected from Mars by an asteroid impact about 11 million years ago. The interesting thing about these meteorites is that they are all about the same age in terms of their formation age (1.3 billion years old) and their cosmic ray exposure age (11 million years). This points to a single origin of these rocks: a single location on Mars and a single impact. Perhaps most significantly for us is that the rocks formed 1.3 billion years ago, meaning that a volcano must have existed as recently as then. More recent data from the European Space Agency’s Mars Express orbiter suggests that some lava flows on Mars are as recent as 2 million years. All of this means that while Mars dos not appear to have ever had plate tectonic activity, it has, up to quite recently, been geologically active.

The most intriguing, and controversial, Martian meteorites contain what appear to be fossilized Martian microbes. The most convincing meteorite fell about 13,000 years ago in Antarctica. This is what it looks like.

It was discovered in 1984 and it made quite an international stir in 1996 when scientists announced it might contain fossilized Martian microbes. The rocks itself is very old; it is thought to have formed from molten rock about 4 billion years ago and later blasted off the Martian surface about 15 million years ago in an asteroid impact. It then floated in space until it landed in Antarctica. A rock this old may have come from a young wet Mars. The structures are very small, 20-100 nanometres (nm) in diameter, smaller than any known Earth bacteria, which are on average about a micrometer (1000 nm) in diameter. However, some very recently discovered round bacteria are as small as 400 nm. This bacteria, called Nanoarchaeum equitans, lives in boiling hot hydrothermal vents and has a simple archaic-appearing genome never seen before. Here is what the Martian candidates look like under a scanning electron microscope:

If they are tiny microbes, they are the first concrete evidence of extraterrestrial life. Researchers have done many tests on the rock to look for organic compounds, which would point to the presence of life processes. They have found some possible organic signatures of life such as amino acids and polycyclic aromatic hydrocarbons, but these compounds could be nonbiological in origin, or they could be the result of contamination by organic compounds within the Antarctic ice. As well, researchers have come up with both biological and nonbiological mechanisms by which these bacteria-like shapes could be formed. I will continue this debate in the following section.

Mars in The Beginning

Let’s now focus on early Mars – how was it different from today and could early Mars have supported possible life?

So far, I have hinted at an early Mars with liquid water (and an implied significant atmosphere) and active volcanic activity. Mars Express orbiter and Mars Reconnaissance orbiter have both found clay minerals that are signatures of a wet environment on Mars, at least in its southern highlands, where surface rocks are about 4 billion years old. Ancient dried up valley networks, and chaotic flood plains are also evident on Mars’ surface. This evidence of water along with the possible microbial fossils of a similar age in the meteorite mentioned above suggest that simple microbial life may have gotten a very early foothold on the young planet, possibly much earlier than life on Earth, which is thought to have begun about 3.5 billion years ago . However, we must keep in mind that a nonbiological origin of the “fossils” can also be argued. And we must add to this extensive evidence on the Martian surface of a meteorite bombardment about 3.9 billion years ago, at the time that our young Moon was bombarded by impacts. This makes a case for early tenuous Martian life to have been obliterated during this period of intense meteorite bombardment before it had any chance to evolve. Keeping in mind these scientific pros and cons, a large question remains: How did Mars ever hold onto liquid water (and an atmosphere) in the first place? What conditions existed then and not now?

 If you have read some of the other planetary articles in this series you may by thinking in the direction of magnetosphere. Mars doesn’t have one, and without a magnetosphere, any atmosphere formed on Mars would be rapidly ionized by solar UV radiation and then picked up and swept away by magnetized solar wind (solar wind is magnetized by the Sun’s very powerful magnetic field). In 1989, the Soviet Phobos probe directly measured Mars’ atmospheric erosion. When the data is extrapolated backwards 4 billion years (and changes in solar wind taken into consideration) it fully accounts for the planet’s lost atmosphere. You might argue that, for liquid water to exist on Mars, it must have also once had an atmosphere and it, therefore, must have once had some kind of magnetosphere to protect that atmosphere from solar erosion. You would be right: In 1998, magnetometers on NASA’s Mars Global Surveyor discovered the remnants of one. A series of magnetic loops are arrayed across the southern hemisphere. In these areas the surface magnetic field is about as strong as that on Earth. However, elsewhere the magnetic field is 100 to 1000 times weaker. The southern magnetic fields harbour localized pockets of gases ionized by solar UV radiation. Earths’ magnetosphere is created by an active dynamo, the result of electrical currents circulating in its liquid metal core. A similar dynamo once churned inside Mars, and we now even have evidence for when it stopped. A giant impact basin, about 4 billion years old, is demagnetized. This means that the crust that reformed after the impact was not under the influence of any magnetic field. This, in turn, means that the dynamo must have stopped before then. The question we are left with is why? Mars’ crust points to a possible answer that explains both the formation and the demagnetization of the impact basin. Its northern hemispheric crust is much thinner and lower in elevation than the crust in the southern hemisphere. This could be the site of what would be largest impact crater in the solar system, roughly the area of Europe, Asia and Australia combined, on a planet about one tenth the mass of Earth. Mars could have been struck by a meteor one tenth to two thirds the size of the Moon, depending on the velocity of the impact. The impact would have to have been violent enough to blow off a significant amount of crust off the northern hemisphere and not enough to melt the whole planet, as was the case with early Earth in which a planetoid is believed to have impacted early Earth and formed the Moon. It would also have to have been violent enough to disrupt Mars’ core dynamo. This is currently a hypothesis based on computer modeling, and as promising as it is, the scientists themselves admit that it needs more verification. Scientists are also currently looking at other possible mechanisms by which Mars lost its early magnetosphere.

We are left with a somewhat haunting image of a young newly formed planet, possibly endowed with all the ingredients necessary for life to take hold and evolve, violently jarred from its future path onto a different path that leads to dry dead planet, whose past is now a puzzle for us, the inhabitants, the evolutionary products of a much luckier planet, to solve. I leave you with this Cosmic Journeys 25 minute video called "Mars World That Never Was." It offers great imagery and a good recap of all that was discussed here.

Next up: Part 3: Venus

No comments:

Post a Comment