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
Figure 1 - ALH84001 (WIKI1).
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
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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
Figure 2 - Comparison of ALH84001/Columbia River Basalt
microscopic investigation of
(http://www.astrobiology.com/adastra/its.dead.jim.html).
ALH84001 revealed the presence
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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
Figure 3 - Possible Water Deposit on Mars (NASA3)
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).
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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”
Figure 4 - ALH84001 cracks that display a plagioclase, silica, and
experienced by
other carbonates (PRSD3).
ALH84001 would have
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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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