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Geosciences Column: The quest for life on Mars

Geosciences Column: The quest for life on Mars

Understanding where we come from and whether Earth is the only habitable planet in the Solar System has been a long standing conundrum in science. Partly because it is our nearest neighbour, partly because of its past and current similarities with our own home, Mars, the red planet, is a likely contender in the quest for extra-terrestrial life. In this guest blog post, James Lewis, a PhD student at Imperial College London, takes a brief look at the findings of his recent research. Strap up, we are rocketing over to Mars!

Mars has always been at the forefront of our imaginations when we picture alien life and the discoveries planetary science has made in recent decades reveal that the idea of our neighbouring world having once been inhabited is not so far-fetched. Mars appears to have once been a habitable world, the question is did life ever exist there? This is one of the questions that Curiosity Rover is attempting to shed more light on but results so far have been inconclusive. One potential problem is that the mineralogy of Mars might seriously disrupt experiments looking for evidence of ancient microscopic Martians. Chlorine salts have already been proven to be problematic and in research, published today, and summarised in the following article I have shown that a salt containing iron, sulfur and oxygen, known as jarosite, can also be added to the list of problematic minerals for life detection experiments.

Eberswalde Delta on Mars, evidence for an ancient persistent flow of water over an extended period of time on the Martian surface. Image Credit: NASA/JPL/MSSS.

Eberswalde Delta on Mars, evidence for an ancient persistent flow of water over an extended period of time on the Martian surface. Image Credit: NASA/JPL/MSSS.

The satellites, landers and rovers sent to Mars have started to unravel many of the mysteries of the red planet. Perhaps their most exciting discovery is that ancient Mars may have been a habitable environment for life. The Martian surface at present is extremely cold, exceptionally dry and bombarded by ultraviolet radiation. The atmosphere is at such a low pressure that liquid water would instantly vaporise. However, characteristic landforms and the presence of minerals that we know only form in water have revealed that ancient Mars had persistent surface or near surface liquid water. The presence of liquid water is exciting because it is a precursor for life and for it to persist on the surface would require a warmer thicker atmosphere.

This potentially habitable liquid water existed billions of years ago, so how can we investigate if life ever existed in these environments? If ancient Martians existed they would likely be microscopic organisms like bacteria on Earth. We could look for the fossils they might leave behind but these features would be extremely small and there are many non-biological processes that can form similar structures. The least ambiguous evidence would be to find chemical compounds that only life leaves behind. As biological molecules contain carbon they fall under a chemical class called organic compounds. However, not all organic compounds are biological. For example, asteroids and comets contain non biological organic compounds that formed in the early Solar System.

Comets and asteroids have been impacting Mars throughout its history so when we send missions to Mars we would expect to see the organic molecules delivered by impacts from outer space. The strange thing is that we haven’t. If we can’t detect compounds we know should be there, what are our chances of detecting possible organic compounds indicative of life? All that has been detected so far are very simple organic compounds with chlorine attached. Their origin is uncertain as similar compounds are used as cleaning agents on Earth and sometimes as reagents inside the rovers, so they could just be contamination. However, recent discoveries have complicated things even further; in 2008 a mineral called perchlorate was discovered on Mars. Perchlorate is very rare on Earth as it is only stable in very arid environments such as the Atacama Desert and the Dry Valleys of Antarctica. Perchlorate has now been discovered by multiple Mars’ missions so it would appear it is widespread in the extremely arid present day Martian surface.

The Phoenix Lander made the first detection of perchlorate on Mars in 2008. Dusty Martian soil can be seen in the background and on the Lander’s frame. Image Credit: NASA/JPL-Caltech/University of Arizona/Texas A&M University.

The Phoenix Lander made the first detection of perchlorate on Mars in 2008. Dusty Martian soil can be seen in the background and on the Lander’s frame. Image Credit: NASA/JPL-Caltech/University of Arizona/Texas A&M University.

Perchlorate is a big complication in our search for organic compounds on Mars. The most common technique used to analyse samples for the presence of organic compounds is to heat materials in an inert atmosphere until organic compounds break down and go into the gas phase. The chemical composition of this gas can then be analysed. For example, on the Curiosity rover the gas passes from the sample oven into a and then a mass spectrometer, which separates out the constituent gases and identifies them. The problem with perchlorate is that it breaks down at low temperatures, in fact just at the temperatures that organic molecules would start to break down and be detectable. Perchlorate releases oxygen and chlorine when it thermally decomposes. Oxygen will react with, and break down, organic compounds into carbon dioxide and water. So it will greatly reduce the instrument’s ability to detect organic molecules if it is present in the sample heating oven. The simultaneous release of chlorine by perchlorate could also chemically alter the products of heating experiments. This may explain why so far we have only detected simple chlorinated organic molecules on Mars.

Like previous missions to Mars, Curiosity is detecting only simple organic compounds with chlorine attached. Image Credit: NASA/JPL-Caltech/MSSS

Like previous missions to Mars, Curiosity is detecting only simple organic compounds with chlorine attached. Image Credit: NASA/JPL-Caltech/MSSS

I wanted to investigate the question as to whether perchlorate is the only mineral that might have a negative influence on our search for organic compounds on Mars. I analysed a group of minerals called sulfates. They contain sulfur and oxygen in the form SO4 and include common minerals such as gypsum. When sulfates thermally break down they release sulfur dioxide and oxygen, so they have the potential to be problematic like perchlorate. However, most break down at very high temperatures (above 1000 °C), which is sufficiently high not to interfere with the release of organic molecules from samples during heating experiments. However, iron sulfates start to break down at dramatically lower temperatures. They can decompose to give off sulfur dioxide and oxygen from around 500 °C. This is around the same temperatures that large complex organic molecules might start to break down and be detectable. I was particularly interested in an iron sulfate called jarosite, as it has been detected on Mars, including recent detections by Curiosity Rover, and forms in wet acidic conditions. It’s therefore indicative of ancient wet environments that existed on Mars and may have once been inhabited by microorganisms, as similar environments on present day Earth, such as Río Tinto in Spain are a habitat for acid resistant bacteria.

I conducted fieldwork on a small island in the south of the United Kingdom called Brownsea Island. If you walk along the southern coast of Brownsea you will often see crusts of a soft yellow mineral on the short cliffs. This is jarosite, it grows here because the clay rich rocks that make up the cliff face contain the iron and sulfur mineral pyrite, pyrite reacts with water and the atmosphere to form jarosite. The geology here is a perfect case study as the rocks also contain a tough form of organic matter called lignite, a low rank of coal. I crushed the sample into a powder so that I had a mix of jarosite, clay and organic compounds. I then heated this powder at different temperatures to see if I would be able to detect the organic compounds contained in the sample. Unfortunately all I could detect was carbon dioxide, carbon monoxide, water and sulfur dioxide. The first three are compounds that you would expect to detect if organic matter was breaking down and reacting with oxygen and the sulfur dioxide indicated that the jarosite was thermally decomposing. When a sulfate breaks down we know that sulfur dioxide is paired with oxygen but when I heated this sample the oxygen wasn’t detectable. It had been consumed by reacting with organic compounds and breaking them down. From these results jarosite can now be added to the list of problematic minerals on Mars, alongside perchlorate.

Jarosite is a soft yellow mineral and can be seen growing on the clay rich cliffs of Brownsea Island, UK. As it is an iron mineral it can rust if exposed at the surface long enough in wet conditions. The orange-brown layer at the base of the cliff and the dark patches in the hand sample are rust. Image Credit: James Lewis.

Jarosite is a soft yellow mineral and can be seen growing on the clay rich cliffs of Brownsea Island, UK. As it is an iron mineral it can rust if exposed at the surface long enough in wet conditions. The orange-brown layer at the base of the cliff and the dark patches in the hand sample are rust. Image Credit: James Lewis.

Jarosite is indicative of environments that may have been habitable for life so simply avoiding it is not a satisfactory solution. Though it has a major negative influence on organic detection experiments some interpretation may still be possible. If sulfur dioxide and carbon dioxide peak at the same time in Curiosity Rover data, from a sample known to contain jarosite, it may be evidence that organic matter was present and reacting with oxygen. Unfortunately it is not always the case that a carbon dioxide peak means the presence of organic matter. Minerals known as carbonates contain carbon and oxygen in the form CO3. When carbonates thermally decompose they produce carbon dioxide. Therefore the chance of a carbonate being the source of carbon dioxide seen in Curiosity Rover data must be considered. Fortunately Curiosity has the ability to perform an assessment of the mineralogy it is adding to its heating ovens for analysis, so the presence of carbonates can be checked.

Identifying which rock units on Mars might contain abundant organic compounds would be of great use to future missions that might return samples to the Earth where a whole suite of laboratory techniques can be employed on samples without the tight space and energy constraints of a rover or lander.

My research is published online today in the journal of Astrobiology and will be free for all to read once the open access application is processed.

By James Lewis,  PhD Researcher at Imperial College London

References

Atreya, S.K., Mahaffy, P.R., and Wong, A.: Methane and related trace species on Mars: Origin, loss, implications for life, and habitability, Planetary and Space Science, 55, 358-369, doi:10.1016/j.pss.2006.02.005, 2007.

Glavin, D.P., Freissinet, C., Miller, K.E., et al.: Evidence for perchlorates and the origin of chlorinated hydrocarbons detected by SAM at the Rocknest aeolian deposit in Gale Crater, Journal of Geophysical Research, Planets Vol. 118, 1955-1973, doi:10.1002/jgre.20144, 2013.

Mahaffy, P.: Exploration of the Habitability of Mars: Development of Analytical Protocols for Measurement of Organic Carbon on the 2009 Mars Science Laboratory, Space Sci. Rev 135, 255-268, doi:10.1007/978-0-387-77516-6_18, 2008.

Ming, D.W., Archer, P.D., Glavin, D.P., et al.: Volatile and Organic Compositions of Sedimentary Rocks in Yellowknife Bay, Gale Crater, Mars. Science, 343, 1245267, doi:10.1126/science.1245267, 2013.

 

 

Geosciences Column: Shifting the O in H2O

Wherever you are in the world’s oceans, you can identify particular bodies of water (provided you have the right equipment) by how salty they are. You can get a feel for how productive that part of the ocean is by measuring a few chemical components in the water column. And, year on year, you will see a recurring pattern in how things like temperature, salinity and oxygen content vary with depth. This last property – the oxygen content – is vital for life in the oceans, but recent decades have seen shifts in the amount available.

There is always more oxygen at the surface than there is at depth. When waves break they mix an abundance of tiny air bubbles into the water, providing oceans with their oxygen supply, which is mixed into the deep through large-scale ocean circulation and storms over winter. At the surface, algae make the most of the abundant light to photosynthesise, beginning the base of the marine food web and adding a little more oxygen to the water in the process. These microscopic plants are eaten by animal plankton (zooplankton), which are, in turn, eaten by other plankton, crustaceans, fish, and a plethora of other predators – none of which contribute to the ocean’s oxygen. Instead, they, and a multitude of microbes, slowly use up more and more of the supply as they respire and there comes a point in the water column where there is no longer enough oxygen for these aerobic animals to survive – the oxygen minimum zone (OMZ).

The surface ocean, where oxygen begins its journey to the deep. (Credit: Anna Lourantou, distributed via imaggeo.egu.eu)

The surface ocean, where oxygen begins its journey to the deep. (Credit: Anna Lourantou, distributed via imaggeo.egu.eu)

What marks the boundary of this zone is dependant not on the properties of the water, but the life that lives there – it is the point when marine organisms experience hypoxic stress, usually an oxygen concentration in the range of 60–120 μmol kg−1. Below this, life in the marine environment is very different indeed. Anaerobic microbes thrive below the OMZ, making the most of life in an environment where there is very little oxygen in each litre of seawater.

The boundary between oxygen-rich water and the OMZ is known as the oxygen limiting zone (OLZ), and during the day many small swimming species take refuge here to avoid their predators. In the Eastern Pacific, you reach the OLZ when there’s 60 μmol kg−1 oxygen in the water, and the OMZ when there’s a mere 20 μmol kg−1.

Waves are key to mixing oxygen into the ocean. When they break at the surface they mix air bubbles into the water, taking oxygen from the atmosphere into the sea. (Credit: NOAA Okeanos Explorer Program)

Waves are key to mixing oxygen into the ocean. When they break at the surface they mix air bubbles into the water, taking oxygen from the atmosphere into the sea. (Credit: NOAA Okeanos Explorer Program)

The depth of the OMZ depends on temperature. Because warmer water is capable of containing less dissolved gas than cold, the OMZ is found at shallower depths in the tropics, and occurs at shallower depths in the summer than it does over winter. Winter weather allows more oxygen to be mixed into the deep ocean as storms break down the sea’s stratification, bringing nutrients to the surface and replenishing supplies closer to the sea floor. However, when there’s a lot of production at the surface (which draws down the oxygen) and the replenishment at depth is slow, large oxygen minimum zones persist from year to year.

Recently though, the upper boundaries of these zones have been shifting to shallower depths, resulting larger hypoxic regions in the ocean. Since the 1960s, the OLZ in the Gulf of Alaska, for example, has shifted some 100 metres shallower. Why?

The oceans are absorbing more heat in response to climate change. Because high temperatures reduce oxygen solubility, they reduce the amount of dissolved oxygen at the surface. The increase in surface heat also creates stronger stratification in the ocean, making it harder for oxygen to be mixed deep into the water column, and reducing dissolved oxygen at depth. Ocean circulation systems are also in a state of change, with systems like the Atlantic meridional overturning circulation in decline. Such changes in ocean circulation will also affect the amount of oxygen that’s mixed into the deep sea.

Atlantic meridional overturning circulation, better known as AMOC. Red arrows show warm water circulation in the upper 1100 m and blue arrows show the southward flow of cold, deep water. (Credit: Smeed et al., 2014)

Atlantic meridional overturning circulation, better known as AMOC. Red arrows show warm water circulation in the upper 1100 m and blue arrows show the southward flow of cold, deep water. (Credit: Smeed et al., 2014)

Working out whether this is part of a long-term trend is a difficult task, as records of deep ocean oxygen only stretch back to 70 years ago. Only a longer record of observations will help determine the trend, but for now we can be sure that shoaling oxygen minimum zones will change the amount of habitat available to species either side of the line between oxygen-rich and oxygen-poor.

By Sara Mynott, EGU Communications Officer

References:

Gilly, W. F., Beman, J. M., Litvin, S. Y., & Robison, B. H.: Oceanographic and biological effects of shoaling of the oxygen minimum zone. Annual Review of Marine Science, 5, 393-420, 2013

Smeed, D. A., McCarthy, G. D., Cunningham, S. A., Frajka-Williams, E., Rayner, D., Johns, W. E., Meinen, C. S., Baringer, M. O., Moat, B. I., Duchez, A., and Bryden, H. L.: Observed decline of the Atlantic meridional overturning circulation 2004–2012, Ocean Sci., 10, 29-38, 2014