Evidence of past life on Mars in ALH84001
Brad E. Rhodes, HET618, Swinburne Astronomy Online
Introduction
Finding life on some distant world is dream of every astrobiologist. It just has to be life. It does not have to be complex. Simple microbes would suffice to prove that life exists somewhere other than Earth. Human beings might then have high hopes of finding their distant complex cousins some day. What scientists did not expect was to find possible evidence of extraterrestrial life on Earth. Approximately 13,000 years ago, a meteorite from Mars crash landed in Antarctica (LPI1). Given the simple name of ALH84001 for where it was found, this meteorite would turn the astrobiology world upside down in 1996. Scientists announced they had observed what appeared to be fossilized bacteria in ALH84001 (LPI2). This paper will briefly review the origins of ALH84001, the arguments for and against the possibility the meteorite contains fossilized Martian bacteria, and the steps the astrobiology community would expect to take to confirm or deny this discovery. ALH84001 may be the first evidence that life exists beyond the boundaries of Earth’s biosphere, but then again it may not.
ALH84001 – The Discovery.
In 1984, an unassuming meteorite was discovered in Alan Hills, Antarctica (LP1, WIKI1). Named ALH84001 for the location of discovery, the year of 1984, and 001 for the first object found, it was believed to be a Mars meteorite (WIKI1). The designation Mars meteorite comes from scientific analysis conducted indicating they “possess chemical, isotopic, and petrologic features consistent with data available from Mars” (WIKI2). Such data was available from the National Aeronautics and Space Administration (NASA) Viking Landers soil analysis experiments conducted in the mid-1970s (NASA1). To date, 34 Mars meteorites have been discovered on Earth. They are known as the SNC group of made up of “achondritic (stony) meteorites (by number): shergottites (25), nakhlites (7), and chassignites (2)” (WIKI2). ALH84001 is a shergottite (WIKI2). Its approximate age is 4.5 billion years, much older than other shergottite which only date to around 1.3 billion years (WIKI2). ALH84001 is an igneous rock, meaning that appears to have formed from the lava of an ancient Martian volcano (LPI1). Figure 1 shows the exterior of ALH84001 (WIKI1).
There may be some doubt as to whether ALH84001 is really from Mars. The right circumstances could knock a rock off of Mars and set it on a collision course with Earth. In the case of ALH84001, an asteroid with a diameter of 0.5 to 2.0 kilometers likely struck Mars (LPI1). This would have accelerated what would eventually become ALH84001 faster than Mars’ escape velocity of 5 kilometers per second, slinging the object into orbit around the Sun (LPI1). The influence of Jupiter and further interactions with other bodies set ALH84001 on track for its eventual landing in Antarctica (LPI1). Analysis of cosmic ray exposures shows that ALH84001 traveled in space for nearly 16 million years before coming to rest on Earth (LPI1). Trace amounts of gases analyzed from ALH84001 show consistency with the Martian atmosphere as sampled by the Viking landers (LPI1, NASA1).
After its discovery in 1984, ALH84001 lived a quiet life. That was until 1996 when a stunning announcement was made by NASA scientists (LPI2).
Arguments For “Life” in ALH84001
The detailed analysis of ALH84001 lead by David McKay had a simple goal: “look for signs of past (fossil) life within the pore space or secondary minerals of this
Martian meteorite” (SCI1). McKay’s team had a difficult task for two reasons. First, they only had a small piece of Martian crust to examine. Second, the team only had Earth-type “biomarkers” to use as reference. If the biomarker did not resemble something recognizable with similarities to known Earth fossils, something might be missed. Initial investigations were made along original fractures on the meteorite. The team found carbonate globules which under mass spectrometer microprobe showed the presence of polycyclic aromatic hydrocarbon (PAH) molecules (SCI1).
The presence of PAHs is what interested the scientists. According to the team’s publication in the August 1996 (Journal) Science: “On Earth, PAHs are abundant as fossil molecules in ancient sedimentary rocks, coal, and petroleum, where they are derived from chemical aromatization of biological precursors such as marine plankton and early plant life.” With a little luck, McKay had found what they were looking for: a recognizable biomarker. The carbonates found in ALH84001 seem to indicate that the Martian surface supported liquid water several billion years ago. Further microscopic investigation of ALH84001 revealed the presence of “ovoid” shapes that seemed to look like fossilized bacteria (SCI1, LPI2). Figure 2 shows a comparison of ALH84001 shapes and similar Earth-based fossils from the Columbia River, Oregon. PAHs have been shown to form during the decomposition of certain bacteria. Unfortunately, PAHs also occur from certain everyday human activities. To demonstrate that the PAHs found on ALH84001 had not occurred in the laboratory, the team did much to show that the molecules were legitimate. Also, the PAHs on ALH84001 were found in the inner part of the meteorite. Additionally, the team reviewed ice cores to show the difference between ALH84001 PAH levels and the Antarctic surface. The “contamination check” showed that the PAHs in ALH84001 did not come from the Earth’s surface. This evidence seems to support McKay’s theory that the PAHs were formed as the result of a biological process (SC1, LPI2).
From a strict scientific perspective, McKay and his team have done everything right. They isolated their samples using strict laboratory protocols. They showed that ALH84001 PAHs were not the result of Earth-based contamination. Finally, they have what appears to be fossilized evidence within the meteorite that can be directly correlated to Earth-type bacteria. Unfortunately, there was not direct proof that water ever existed on the surface of Mars. The data analysis from the Viking Landers indicated that Mars was a dry and desolate place. That was until 2004 and the Opportunity Rover. Operating on the Meridiani Planum, Opportunity discovered strong evidence that water once flowed on the surface of Mars. Using Opportunity’s x-ray spectrometer, NASA Jet Propulsion Laboratory (JPL) scientists examined sulfur formations that would have only formed in the presence of briny water (NASA2). Additionally, comparison photographs (figure 3) by the Mars Global Surveyor’s (MGS) Mars Orbiter Camera taken in 1999 and in 2005 seem to indicate that water still flows on the Martian surface occasionally (NASA3). These revelations add credence to the possibility that formations in ALH84001 may actually be the first evidence of life beyond Earth.
Even with credible scientific evidence from ALH84001 and recent indications that Mars’ past and present may be watery, doubts still exist.
Arguments Against “Life” in ALH84001
There are a variety of arguments against fossilized life in ALH84001. The most obvious is that in 1996, the best surface data from Mars came from the Viking landers. That data showed absolutely nothing. While the experiments have been criticized, Viking did not even find anything reactive in the soil that might indicate the possibility of life (NASA1). Using this as a basis, Mars appears to be a barren world. Therefore, the announcement of fossilized life in ALH84001 would seem to contradict all of the previous evidence. It is important to note that the latest observations gathered by the Opportunity rover and MGS seem to show water may be a more recent phenomenon on Mars (NASA2, NASA3). However, this data does not show presence of life, nor does it indicate conditions were ever able to support it.
Having environmental conditions on the Martian surface able to support life is critical to whether or not bacteria may have fossilized in ALH84001. The first environmental consideration is overall planetary temperature. The time at which ALH84001 formed (4.5 billion years ago), the Sun’s luminosity was thirty percent less than it is today (NASA4). This means that Mars’ surface temperature was even less than it is presently (20 degrees Celsius versus minus 60 degrees Celsius) (UNI1, PRSD1). While life has been found in extreme environments on Earth much had its beginnings in a consistently warm watery environment (UNI1). Mars was definitely not warm. There are additional factors to consider as to whether Mars supported life in the past. Probably the most interesting is Mars’ present lack of a magnetic field. As seen on Earth, a magnetic field is critical to protecting its fragile atmosphere (UNI1). Magnetometer readings from MGS show that Mars once had a magnetic field (eerily similar to the “striping” seen in Earth’s oceans) (NASA5). Yet for some reason, the mechanism that produced Mars’ magnetic field stopped. The result was the planet’s tenuous atmosphere was slowly stripped away. The Martian atmosphere was perhaps thick enough to support the processes for life, but did it exist long enough to support fossilization in ALH84001?
Beyond just the lack of evidence supporting the formation of life at the time ALH84001 was formed, there is much controversy surrounding the carbonates in the rock. In 2002, at the University of Hawaii’s Planetary Science Research Discoveries (PSRD), a team found additional carbonates besides PAHs (PRSD2). The mineral they found was called periclase (made of magnesium oxide (MgO)). Periclase and other carbonates tend to form in voids of magnesium rich rocks (as shown in figure 4). Additionally, the crystalline structures in the carbonates seem to be of natural formation instead of biological. The team also surmised that the “shock-heating” experienced by ALH84001 would have significantly changed the properties of biologically formed carbonates. It is also possible that carbonates formed as a result of shock heating (PRSD2, PRSD3).
All of this adds to the many questions about ALH84001. According to Andrew Knoll’s “A Martian Chronicle” in The Sciences, the evidence found in ALH84001 appears circumstantial. This does not make for good science. Clearly, the doubt regarding the “fossils” in ALH84001 means astrobiologists must continue to look! If only the fossil had been something recognizable such as plant leaf.
Next Steps – finding “Life” on Mars
ALH84001 captured the interest of the scientific community. If the findings are correct, this is the first evidence that life has existed beyond Earth. If the findings are not correct, ALH84001 is just another meteorite from Mars. There are at least four options:
1) Complete a Mars Sample Return Mission as proposed by NASA and European Space Agency (ESA). The challenge for such a mission is picking the most likely place where life might have formed/fossilized. These missions are not expected to begin until the middle of the next decade (NASA6, ESA1).
2) Accomplish human exploration of Mars and sample return. This the goal of NASA Constellation program spearheaded by the Ares V rocket. A manned Mars mission would face the same challenge as option one. Unfortunately seeing a human being on Mars will not occur until after 2020 (NASA6).
3) Another possibility suggested by Andrew Knoll is to look for subterranean water on Mars. If life still exists, perhaps it still finds safe harbor there (Knoll). Finding underground water on Mars implies that options one or two have been successful.
4) Finally, an easy solution to confirm ALH84001 would be to study existing samples for similar fossils.
Ultimately, the answer to life on Red-Planet lies on Mars. Whether fossilized or alive, astrobiologists will have to wait until a sample can be returned.
Conclusion
ALH84001 is no ordinary meteorite. The carbonates, PAHs, and shapes found on the interior of the rock certainly make an interesting case for past life on Mars. Unfortunately, the Viking landers, planetary environmental factors, and further analysis of ALH84001 seem to indicate the formations observed are not biological. Since 1996, the controversy over ALH84001 seems to have cooled. The next logical step is to find the “right” landing site on Mars and return a sample. There are two possibilities with a sample return: life (fossilized or live) existed or exists on Mars or it does not. If life is found, ALH84001 will be marked as a pathfinder. If life is not found, does it mean astrobiologists need to find other lines of work? No. It simply means that life did not form on Mars, but it may still be out there somewhere else!
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