Watching cartoons on a Saturday morning may cause you to believe that not only is there life on other planets, but it is quite abundant (and often highly antagonistic!). One of my personal favorites is Marvin the Martian (Figure 1 below). He and Bugs Bunny or Duck Dodgers often battle it out for the continuing and peaceful existence of Earth. I found it interesting that one of Marvin’s “weapons” was an old fashioned gum-ball machine with the label: “10,000 Instant Martians (Just add water)”. It seems that the general consensus of scientists today is that same slogan. Want to find life on other worlds? All you need is liquid water (and a few other easy to find elements).
Over the last few decades, scientific missions to Mars have found convincing evidence that liquid water did once flow on the surface of the red planet. Does this then mean that life existed on the now barren planet? Some scientists say yes, and claim that they have evidence to prove it – tiny fossil structures found inside a meteorite designated ALH84001 (Figure 2 below). This essay aims to be a general overview of the history of the fragment and evidence for and against the existence of life on Mars based on the composition and structure of the meteorite.
Two famous Martians.
Figure 1: Marvin the Martian1
Figure 2: Structures in ALH840012
The ALH84001 Meteorite
Before we even begin looking at the structures inside the meteorite, it is definitely worth showing that it has, indeed, come from Mars. ALH84001 was found in 1984 in Antarctica and was recognised to be an interesting sample (Figure 3 below). The designation indicates where it was found, in the Allan Hills region and when it was found, 1984. It was not until years later in 1993 that enough evidence had been put forward to re-label it as Martian3. It is thought to be part of the Martian crust, which formed 4.5 billion years ago. Isotopic abundances of oxygen in the rock do not conform to the Earth, but are the same as those that have been sampled on the surface of Mars4.
Figure 3: ALH84001 meteorite5
Computer programs have been used to model the dynamics of a collision between a large comet or meteor and Mars. These have found that it is possible for a large impact to emit crater ejecta at greater than the escape velocity of the planet6. Furthermore, due to gravitational interactions, it is possible for a piece of Mars ejected in this way to eventually reach the surface of the Earth. Recent research actually shows that possibly one fragment a month impacts the Earth, most of them are too small to make it to the surface, however7. So far about 26 different meteorites from Mars have been found (in 40 separate fragments).
The age of the meteorite has been estimated as being between 4.56-4.36 billion years by samarium and neodymium aging techniques. About 4 billion years ago the rock was heated by a large shock. This would fit in well with theories of solar system formation when there was a “big bombardment” phase. This is known from Argon-40 and Argon-39 isotopes and Rubidium and Strontium ageing8. These dates are important because the surface of the Earth undergoes continuous changes and hence there are no terrestrial rocks as old as ALH84001 (the closest in age on Earth are zirconium grains, not entire rocks)9.
3.9-1.4 billion years ago some CO2 rich fluids infiltrated cracks in the rock to create carbonate globules suggesting the existence of liquid water. It is also possible that the carbonate globules could have been formed by the shocks caused by large, nearby impacts10. It is fairly well determined now that at one stage there was liquid water flowing on Mars. Recent images and data from the Martian rovers and satellites show surface features that are highly indicative of water. Ice has also been seen on the surface (Figures 4 and 5 below).
Figure 5: Liquid water evidence in gullies on Mars12.
It seems likely that approximately 16 million years ago the fragment of Mars was ejected from its parent planet by a cometary or meteoric impact. The rock then travelled through space for between 10 and 20 million years. The evidence for these times comes from isotopic analyses of noble gases and examinations of cosmic ray traces13.
After its sojourn in space, about 13,000 years ago the meteorite landed in Allan Hills Antarctica where it lay until being found there. The time it has spent here on Earth can be determined from C-14, B and Cl decay signs14.
No scientist doubts that the meteorite fragment called ALH84001 originated from Mars. However, some of the contents of the rock are what has turned it into an exciting puzzle.
Life signs in ALH84001
Many scientists claim that there are signs of life in the meteorite. The three main proponents are David McKay, Everett Gibson and Kathie Thomas-Keprta. They have suggested over the years that there are four major signs that could be used as evidence of life in ALH8400115. There are many other signs, but due to time and space restraints I offer the major claims below:
Precipitated carbonate materials are much like those formed by biological activity on Earth.
Complex organic molecules interpreted as evidence of biomolecular breakdown.
Miniscule rodlike structures interpreted as microfossils.
Grains of magnetite that resemble the crystals formed within terrestrial bacterial cells.
Firstly, the carbonate materials. Carbonates show that there was water at one time to allow flowing water to leave behind deposits of carbon (see Figure 6 below). The carbonate globules in the meteorite have been suggested to imply that some sort of life was necessary to create the balance of chemical and shape seen16. There is no doubt that such globules can be formed with the assistance of life forms, however it not entirely necessary. More recent simulations have demonstrated that similar structures can be created abiotically (that is, with no life forms present)17.
Figure 6: Carbonate image18. In the carbonate globules there are magnetite and greigite mineral structures in contact with each other. This would require either highly alkaline water or biological activity. The “texture” seems to suggest a biological origin. However, the existence of the greigite has only tentatively confirmed, it is in such small quantities. The texture does not essentially require the addition of biological input, abiotic formation is still possible. Also they may not have actually formed near each other, just have been brought together at a later date, perhaps in one of the impact collisions that the meteorite is known to have withstood19.
The dating needs to be more accurate for a decision to be reached on the carbonates. If the globules are old then they could have formed at a period where it is thought that Martian conditions may have been suitable for life. If they are too young then it is doubtful that they formed at a time when life was possible on Mars. It seems almost certain that the rock was not inundated by water. Rather there was some time when there was water present, but it was not at the bottom of a river, lake or ocean20.
Organic Molcules (PAH)
Secondly, the complex organic molecules found in the meteorite. Polycyclic aromatic hydrocarbons (PAH) are found almost everywhere on earth. They are often formed through the breakdown of biological molecules. However, they have also been found in carbonaceous meteorites in the outer solar system! Being in Antarctica for thousands of years has led some terrestrial groundwater to intrude into the rock and some researchers claim that this is how the PAH materials entered the meteorite. However, most of the PAHs were found deep in the rock, where it is thought that no terrestrial material could have penetrated.
There are several ways to form PAH molecules without the need for life. Condensation of gases would do it, particularly as magnetite is a catalyst for the necessary reaction, and as we will see below there is plenty of magnetite in the rock21. This has been argued against, as experimental evidence shows that a slow cooling (as is evidenced by other geological signs) would not lead to the formation of the correct molecules. One other possibility explanation for abiotic formation is nearby impact shocks. The heating from a nearby impact (which the meteorite clearly suffered multiple times) can create PAHs22.
Other meteorites from Mars that definitely came after the existence of liquid water on the surface also have the PAH structures. This would imply that they do not need any form of life to create them. The PAHs also make up less than 1% of the organic carbon in the meteorite, and only about 20% of these are definitely Martian in origin. It is unlikely that the PAHs are biogenic due to conditions being unsuitable on the surface of the planet23.
Thirdly, the microfossils. When looking at Figure 2 above you can clearly see many shapes and structures, these have been described by many as microfossils of past bacterial life. The microfossil shapes seen in the meteorite are tiny, too small to be non-parasitic if found on Earth. They could not survive as individual life forms due to a lack of complexity. They are 10 to 100 times smaller than definitively confirmed terrestrial bacteria. Some scientists claim to have found terrestrial bacteria that are almost this small, but many argue that what they have found are not bacteria at all. They show no signs of the presence of DNA/RNA and they appear to have no cell wall structure. Perhaps they are very, very simple organisms. However, as is discussed below, similar structures can be formed by abiotic processes.
Magnetite is formed by a mix of oxygen and iron, often found in biological organisms. When formed in bacteria, it can be left behind, often in a crystalline form, when the bacteria die (Figure 7 below). The argument over whether the magnetite structures are formed biologically or abiotically continues today. Every time one group shows that one particular aspect of the forms is formed biologically, another group conducts experiments to show that it can be reproduced abiotically.
Images of Magnetite structures in ALH8400124
Figure 7: Magnetite structure.
Figure 8: Crystalline structure of the Magnetite.
There are numerous magnetite crystals in the meteorite. If the crystals are biologically created they should all be similar sizes, being formed by the same small bacteria, however many different sized crystals can be seen in the meteorite. The answer to that from the pro-biotic camp is that only some of the magnetite was formed biologically. Recent evidence shows flaws in some of the crystals that should only occur at ‘unliveable’ temperatures.
To form the magnetite biologically requires bacteria to synthesise the raw materials, upon the death of the organism, the magnetite crystal remains behind as the organism decomposes. After years of claim and counter claim, the current arguments focus on the shape of the crystals (Figure 8 above). Thomas-Keptra et al (2001) claim that the shape of these crystals has only ever been seen in organic life25. Treiman et al (2004) claim that they have found many ways of creating similarly shaped crystals inorganically26.
Treiman et al have provided two ways to form the crystals. Firstly by decomposition of iron-rich carbonates formed by inorganic hydrothermal precipitation, which they have done in laboratory experiments, and secondly via the cells of magnetotactic bacteria. They argue that the meteorite structures are different to both, therefore do not count as a robust biosignature. They conclude that the crystals follow logically from a volcanic or meteoritic impact thermal pulse. The pro-biotic contingent have suggested that Treiman et al have not really focused enough on the unique shape of the crystals.
There is no doubt that the signs given above could each point towards the possibility of life on Mars in the distant past. However, each of those signs could equally point towards ordinary geologic and physical processes. It is often at this point that Occam’s Razor is invoked, that the simplest answer is often the correct one. This is a difficult case, however. Those who believe that life is a simple by-product of the right chemicals, energy levels and liquid water would use Occam’s Razor to suggest that it makes sense that all of these signs are evidence that life existed as they can all be easily explained by the existence of bacteria. There are others that state that the number of planets in our universe that definitely have life on them is one, and that Occam’s Razor points to the simple solution that ordinary physical and chemical processes formed the crystals and other signs discussed above. The one thing that we do know for certain is that in terms of the discussion and research towards life on Mars: “That’s NOT all folks!”
1 Marvin the Martian image copyright Warner Brothers, image from http://looneytunes.warnerbros.com/web/stars/stars_marvin.jsp , downloaded March 17th 2007.
2 Structures in ALH84001 image from NASA, http://curator.jsc.nasa.gov/antmet/marsmets/alh84001/largephoto.cfm?photo=20 , downloaded March 17th 2007.
3 Mittlefehldt, D. W. 1994. ALH84001, a cumulate orthopyroxenite member of the Martian meteorite clan. Meteoritics Planet. Sci. 29:214-221
4 Clayton, R. N. 1993. Oxygen isotope analysis of ALH8400. Antarct. Meteorite Newsl. (JSC Curator's Office) 16:4.
5 ALH84001 Image from NASA, http://www-curator.jsc.nasa.gov/curator/antmet/marsmets/alh84001/photos.htm , downloaded March 17th 2007.
6 Melosh, H. J. 1995. “Cratering dynamics and the delivery of meteorites to the Earth.” Meteoritics Planet. Sci. 30:545-546.
7 Britt, 2002, “Why a Mars rock hits Earth every month.”, article from Space.com, http://www.space.com/scienceastronomy/solarsystem/mars_knocks_021107.html last accessed March 17th 2007.
8 Nyquist, L. E., D. D. Bogard, C.-Y. Shih, A. Greshake, D. Stoffler, and O. Eugster. 2001. “Ages and geologic histories of Martian meteorites.” Space Sci. Rev. 96:105-164
9 Wilde, S. A., J. W. Valley, W. H. Peck, and C. M. Graham. 2001. “Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.” Nature (London) 409:175-178
10Borg, L. E., J. N. Connelly, L. E. Nyquist, C.-Y. Shih, H. Wiesmann, and Y. Reese. 1999. “The age of carbonates in Martian meteorite ALH84001.” Science 286:90-94
112005, “Water ice in crater at Martian north pole.” ESA website, http://www.esa.int/SPECIALS/Mars_Express/SEMGKA808BE_1.html image downloaded March 17th 2007.
12 2000, “Evidence for recent liquid water on Mars: Seepage sites.” Mars Orbital Camera website, http://www.msss.com/mars_images/moc/june2000/ image downloaded March 17th 2007.
13 Goswami, J. N., N. Sinha, S. V. S. Murty, R. K. Mohapatra, and C. J. Clement. 1997. “Nuclear tracks and light noble gases in ALH84001: preatmospheric size, fall characteristics, cosmic ray exposure duration and formation age.” Meteoritics 32:91-96.
14 Jull, A. J. T., C. J. Eastoe, S. Xue, and G. F. Herzog. 1995. “Isotopic composition of carbonates in the SNC meteorites Allan Hills 84001 and Nakhla.” Meteoritics 30:311-318.
15 McKay, Gibson, Thomas-Keprta, Vali, Romanek, Clemett, Chillier, Maechling, Zare, “Search for past life on Mars: possible relic biogenic activity in Martian meteorite ALH84001” 1996, AAAS, Volume 273.
“Search for past life on Mars: Possible relic biogenic activity in martian meteorite ALH 84001”.
Science 273, 924-930
17 “An experimental study on kinetically-driven precipitation of Ca-Mg-Fe carbonates from solution: Implications for the low temperature formation of carbonates in Martian meteorite ALH84001.” by Golden D.C., Ming D.W., Schwandt C.S., Morris R.V., Yang S.V., and Lofgren G.E. (2000) Meteoritics and Planetary Science 35, 457-465.
18 Carbonate image from Corrigan and Harvey, 2004, “Carbonates in ALH84001: Part of the Story of Water on Mars.” From the Planetary Science Research Discoveries website, http://www.psrd.hawaii.edu/July04/carbonatesALH84001.html downloaded on 2nd April 2007.
19 Treiman, A. H. (1998) The history of Allan Hills 84001 revised; multiple shock events. Meteoritics and Planetary Science, v. 33, p. 753-764
20 Corrigan, C. M. and Harvey, R. P. (2004) Multi-generational carbonate assemblages in Martian meteorite Allan Hills 84001: Implications for nucleation, growth and alteration. Meteoritics and Planetary Science, v. 39, p. 17-30.
21 Anders E. (1996) Evaluating the evidence for past life on Mars (letter). Science 274, 2119-2121
22 An abiotic origin for hydrocarbons in the Allan Hills 84001 martian meteorite through cooling of magmatic and impact-generated gases. By Zolotov M.Y. and Shock E.L. (2000) Meteoritics and Planetary Science 35, 629-638.
23 “The origin of organic matter in the Martian meteorite ALH84001”, Becker et al, EPSL 167.
24 Images from NASA website: http://curator.jsc.nasa.gov/antmet/marsmets/alh84001/sample.cfm downloaded 2nd April 2007.
25 Thomas-Keprta, K. L., S. J. Clemett, D. A. Bazylinski, J. L. Kirschvink, D. S. McKay, S. J. Wentworth, H. Vali, E. K. Gibson, Jr., M. F. McKay, and C. S. Romanek. 2001. Truncated hexa-octahedral magnetite crystals in ALH84001: presumptive biosignatures. Proc. Natl. Acad. Sci. USA 98:2164-2169
26 “Evidence for exclusively inorganic formation of magnetite in Martian meteorite ALH84001” American Mineralogist 89, Golden DC; Ming DW; Morris RV; Brearley A; Lauer HV; Treiman AH; Zolensky ME; Schwandt CS; Lofgren GE; McKay GA, 2004.