ALH84001: Origins and History


The ALH84001 meteorite is one of some 7000 meteorite fragments which have been recovered from Antarctica since 1972. (Figure 1.1)



Figure 1.1: ALH 84001. a): cut sample. b) Whole sample as photographed. Both images are from the Mars Meteorite Website (in


Meteorites are rocks with extraterrestrial origins. They have been known since ancient times (the first documented meteorite sightings date back to ancient Greece and China), but a non-Earthly source for these objects was not scientifically accepted until the late 18th and early 19th centuries, when E.E.F. Chaladni and others in England, Germany and France compared the physical and chemical characteristics of two meteorite Falls (stones observed to have fallen from the sky as fireballs in Wold Cottage in Wales, and L'Aigle in France) with those of previous reported falls as well as Finds - rocks of meteoritic character which had been discovered at various places around the world over time.

The Antarctic Meteorite Location and Mapping Project (AMLAMP), supported by NASA and the US National Science Foundation, has been active in the collection of meteorites in the south polar regions since 1975. Meteorites are easily collected in Antarctica for two reasons:

1) As they are dark objects, often with distinctive Fusion Crusts (a veneer of melted material formed by frictional heating of the meteorite as it fell through the atmosphere), they are easily seen on snow and ice.

2) The slow buildup and movement of Antarctic glacial ice transports meteorites from the center of the continent outward. Where strong polar winds evaporate and "sandblast" away glacial ice as it flows upward against mountain ranges, meteorites from the center of the continent entrained in the ice will collect, forming a unique sort of "lag" deposit. (Figure 1.2)



Figure 1.2: a) Map of Antarctica. Stars represent meteorite collection sites. b) Method of meteorite concentration via Antarctic ice. Glacial flow carries ice and entrained meteorites to the edges of mountains, where winds abrade and ablate away the ice, leaving behind a meteorite "lag." Both diagrams from Meteorites and their Parent Planets by McSween (1984)


The name ALH84001 arises from the scheme used to catalog meteorites found by U.S. scientists. The three letters refer to the Allan Hills collecting region in Antarctica, the first two numbers to the collecting season in which it was located (1984) and the last three numbers relate to when it was collected that season (i.e., it was the first meteorite collected in 1984 in the Allan Hills).


Where do Meteorites come from? The current scientific belief is that most meteorites represent materials dating to the very beginnings of our Solar System. These objects may be debris related to the collision of 500 to 1000 km-sized bodies called Planetisemals, which through a process called Accretion (basically just lots of collisions between small bodies, in which the colliding objects "stick" due to gravitational attraction), eventually become large planets. A key piece of evidence for these contentions lies in the ages of meteorites: based on radiometric dating, all but about 20 of the meteorites currently known appear to be around 4.55 billion years old, the oldest objects we have seen our Solar system. We think that essentially all of these ancient meteorites come from the Asteroid Belt - a region between Mars and Jupiter in which thousands of small (<500 km diameter) objects orbit the Sun. Gravitational interactions with the giant planet Jupiter can perturb small asteroids into lower, Earth Crossing Orbits, in which they stand a small but measurable chance of hitting the Earth at some point in time.

The twenty or so meteorites that do not date to the beginnings of our Solar System are believed to come from other planetary bodies. Seven of these are identical in look and makeup to rocks returned from the Moon by the Apollo Program, and thus are believed to come from the Moon. The other thirteen rocks, called as a group the SNC meteorites (this acronym comes from the names of three of the first to be found - Shergotty, Nahkla, and Chassigny) are believed to have originated on Mars.


ALH84001 was initially mis-identified as a relatively ordinary meteorite (called a Diogenite) and was thus ignored for some ten years after its collection, But in 1994 Dave Mittlefehldt, a meteorite specialist at NASA, discovered this mistake and reclassified ALH84001 as a unique variety of SNC meteorite, composed largely of the mineral Enstatite (Mg2Si2O6). Subsequent mineralogical studies of ALH84001 demonstrated that it was mineralogically unique as well, as it is the only meteorite known which contains significant amounts of carbonate minerals. These carbonates, which vary in composition from Magnesite (MgCO3) through Siderite (FeCO3) to Calcite (CaCO3), are found as "globules" and fracture fill deposits in the "crush zone" of brecciated material that runs through the center of the stone (Figure 1.3.). These carbonates are widely believed to be products of water-rock exchange, and a variety of ideas for their origins were proposed, most of them presuming a high-temperature origin consistent with the igneous character of the rest of the meteorite. �



Figure 1.3: a) 40X crossed-polars photomicrograph of crush zone in ALH 84001, showing carbonate minerals (golden specks) filling spaces in fractures and between grains. b) Photomicrograph of carbonate globules from ALH 84001 showing characteristic rimmed structure. Darker mineral near rim is magnetite. Image on left taken by the author, image on the right from the Mars Meteorite Website (

In 1996, both in a press release and in a paper published in the journal Science, David MacKay and coworkers reported what they described as "evidence for past life" which they discovered through careful micro-analysis of the carbonate minerals in the fractures and crush zones of ALH84001. This paper and announcment sparked a debate about possible microbial life on Mars that continues today!


Summary of the Mars Pathfinder/Global Surveyor Missions:


The United States' return to the exploration of Mars came with the Mars Pathfinder and Mars Global Surveyor missions. Mars Global Surveyor was launched on November 7, 1996, and Mars Pathfinder on December 6, 1996. However, despite the relatively close launch times of the two vehicles, the Pathfinder and Global Surveyor probes arrived at Mars several months apart: Pathfinder reached Mars on July 4, 1997, while the Global Surveyor arrived on September 12, 1997. Both probes followed a similar orbital trajectory, playing "catch up" with Mars as the planet moved away from its closest approach to the Earth. But each probe moved at a different velocity so as to stagger their arrivals: Pathfinder landed in the summertime of the Martian northern hemisphere, the best time for a landing. The timing of the Global Surveyor arrival was less critical, as the Surveyor was to undergo a long period of orbit adjustment and aerobraking in the Martian atmosphere to move it from an elliptical orbit around the Martian equator to the cirumpolar orbit necessary for planetary mapping.







Figure 2.1a) Trajectory of Mars Pathfinder. b) Trajectory and orbital adjustment strategies for Mars Global Surveyor. Diagrams are from Mars Pathfinder/Mars Global Surveyor homepages in the Jet Propulsion Laboratory website. (


Both the Mars Pathfinder and the Mars Global Surveyor missions are part of the NASA "Discovery" program, which specializes in low-cost (less than 150 million dollars for construction, launch, and the primary mission) and fast (maximum 3 year turnaround time for mission results) exploration missions in the Solar system. The Mars Pathfinder and Global Surveyor were both launched by Delta II-7925 rockets, the first time this "workhorse" rocket was used to launch an interplanetary probe. Science equipment onboard these probes was limited by comparison to past missions: the Mars Pathfinder included an imager (basically a high-resolution camera), magnets for measuring magnetic properties in the soil, wind socks, and an atmospheric science instrument/meteorology experiment package (ASI/MET). The most unique science instrument in the Pathfinder package was its rover called Sojourner, a 65 cm (2 foot) long, 10.6 kg (25 lbs.) mobile explorer with an independent guidance computer and an alpha protron x-ray spectrometer (APXS) for measuring the chemical compositions of rocks and soils, as well as three cameras.


Initial results from the Mars Pathfinder program first reached print in the December 5, 1997 issue of the journal Science, but results were reported on a day-by-day basis on the Jet Propulsion Laboratory's Mars Pathfinder website - the first time the general public had ever had such direct access to events in an ongoing NASA mission. Interest was so heavy that reflector sites were set up all over the world to accomodate the "hits."


For more detailed information about the Mars Pathfinder, and its rover Sojouner, read the Mars Pathfinder Mission Overview, included in this module, and consult the pre-landing portion of the Mars Pathfinder Archival Website (look under Past Missions in the Jet Propulsion Lab website ( Some major achievements of the Pathfinder program are listed below




Figure 2.2 a) "Monster" panoramic image of Sagan Memorial landing site in Are Vallis, with named features (from Golombek et al. 1997)




Figure 2.2b) Soil types and rocks identified in the Sagan Memorial Site based on visual and physical properties. From Golombek et al. (1997).


Despite a "small" budget and a limited retinue of scientific equipment, the Mars Pathfinder mission has provided us with a tremendous amount of information about the makeup of Mars, and about conditions at its surface. The vast majority of this information comes to us in the form of imagery - basically, just pictures which scientists interpret, using their understanding of earth materials, landforms, and processes, to infer the sorts of things that might have brought Mars to the conditions we find there today.


In your own Web searches and readings on this mission, use the information you find to try and imagine what this piece of Mars we call Are Vallis really is like. Ask yourself questions, such as: how would you dress for the weather? How big are the rocks that Sojourner was traversing (hint: how big is Sojourner?) What is the terrane like - is it flat, rolling, rugged? These are exactly the kinds of thought experiments that the JPL scientists in charge of the Pathfinder mission have been doing. Inferences derived from these though exercises, and tested against the data, are how we draw our conclusions about the modern surface of Mars.


The Phase Chemistry of H2O, and Its Relevance to Martian Climate History


Question: The Mars Pathfinder found a cold and dry world, with a very thin atmosphere, and no evidence for water. If so, how could life have existed on this planet?


The answer to the question above may lie in the many channels and hydrologic features of Mars, and their implications. As we've seen, the surface of Mars shows abundant evidence for the presence of liquid water - dendritic streams, river-sized channels, and many different sorts of flood deposits and water-mediated slide and subsidence features. All of these point to surface temperatures and conditions that were very different in the Martian past.


Figure 3.1: A pressure-temperature diagram depicting the physical states of the compound H2O. Temperatures are in degrees Celsius. Pressure units: 1 atmosphere = 1 bar = 760 mm. 4.579 mm = 0.006 bars. Diagram is taken from Ehlers (1972)


Figure 3.1 is a Pressure-Temperature Phase Diagram for H2O that sheds light on how conditions on Mars must have differed in the past. The phase field labeled "Water" represents all the pressures and temperatures at which H2O will exist in a liquid form. The chord at 760 mm (=1 bar) pressure represents terrestrial conditions - thus, the intersection between this chord and the ice-water boundary curve is 0°C, and the intersection with the water-vapor curve is 100°C. Point A is calle d the Triple Point of H2O, because at this specific pressure (0.0061 bars) and temperature (0.0099°C) H2O will coexist as a solid, liquid and gas. Below this pressure and this temperature, liquid water will not exist - it will either freeze, or boil away as steam!


1) What are the pressures and temperatures of Mars today, and where do they plot on this diagram?

2) How much do pressure AND temperature have to increase on Mars to allow liquid water to exist?

One can thus infer: the existence of channels and other water-cut features on Mars indicates that long ago, pressures and temperatures were high enough to permit the existence of liquid water!


The Development of Planetary Atmospheres, and The Climates of Planets

Additional Readings:

Chapter 11, Planetary Atmospheres, from Moons and Planets, by W.K. Hartmann.


An Atmosphere is the envelope of gases that surrounds a planet. Some planets, such as the gas giants like Jupiter and Saturn, might, strictly defined, be mostly atmosphere, but for our purposes we shall focus on the development of atmospheres around the "terrestrial" inner planets. Of these, Venus, the Earth, and Mars, all have substantial atmospheres.


How do planetary atmospheres begin? Initially stray gases in the Solar system were probably attracted to planetary bodies by gravity, and became their "primitive" atmospheres. These stray gases most likely were jetted out by the Sun, and so they consisted largely of hydrogen and helium. Jupiter, Saturn, Uranus and Neptune all have hydrogen/helium dominated atmospheres today - probably, these are their initial atmospheres, now larger and denser due to 4.5 billion years of gravitational gas collection (Table 1)



Table 4.1: Atmospheric Inventories of Selected Planets
























































































































* pressure and temperatures at cloudtop level.

g = acceleration due to gravity, which is proportional to the mass of each planet.

Data in table from Consolmagno and Schaefer:Worlds Apart - A Textbook in Planetary Sciences (1996), p. 91


The inner planets, as well, probably had H-He rich primitive atmospheres. However, the proximity of these planets to the Sun made them vulnerable to the stripping effects of the Solar Wind. Especially in the early history of the Sun, when it is believed to have undergone a violent "T-Tauri" phase associated with the ignition of its internal fusion generator, any atmospheric gases around the inner terrestrial planets would probably have been stripped away. Thus, all the inner planets lost their primitive atmospheres early in the history of the Solar System.


However, volcanism associated with the internal differentiation of the terrestrial planets offered another means of producing at atmosphere. All volcanic eruptions release gases from a planet's interior - in fact, the gas is the "fizz" that drives lava to the surface, much in the same manner as a shaken up bottle of soda. Gases released by volcanic eruptions collect slowly near the planet's surface and eventually build up to become a Secondary Atmosphere. These atmospheres tend to be rich in CO2 and H2O, with significant abundances of a range of minor gases: NH3, SO2, H2S, and noble gases such as Argon. Many of these minor gases will react with one another and with materials on the planet's surface, especially in the presence of water, and the end result is a secondary, volcanogenic atmosphere that is dominated by CO2 and contains significant proportions of N2.


Of these gases, two - CO2 and H2O - are Greenhouse Gases, which means they help to retain heat energy in the atmosphere. The presence of these gases allows the atmosphere to hang onto more of the heat energy reaching the planet from the Sun, and thus permits the surface to warm. So a CO2-rich atmosphere will lead to a warm er planet - how warm depends o n that planet's proximity to the Sun, and the mass of CO2 in the atmosphere.


Table 11-2 in Hartmann (1999) outlines current estimates of the respective volatile inventories of Mars, Earth and Venus. Note that abundances of CO2 and N2 on Earth and Venus are believed to be very similar - as these planets are the same size, they are believed to have begun with similar inventories of gases trapped in their interiors. Abundances on Mars are lower in proportion to its much smaller mass (about 10% that of Earth) but the proportions are similar. While all of these numbers are highly uncertain (especially estimates of fixed gas components in planetary crusts!), it is clear that if volcanism was the source of these gases, that all three of these planets would probably develop an initial CO2-rich secondary atmosphere early in their histories.


Examining Table 1, we see that both Venus and Mars preserve "volcanogenic" proportions of atmospheric gases, if not actual abundances - atmospheric pressures on Mars are 105 times lower than on Venus. The Earth, however, has a very different atmospheric makeup - dominantly nitrogen-oxygen, with a little water vapor and argon, and only 300 ppm (or 0.03%) CO2. If , as is believed, the Earth's atmosphere started out with similar gas proportions as those of Mars and Venus, then we need to answer some questions:


1) Where is all the CO2 that outgassed from the Earth to be found today?

2) Where did all this oxygen come from - what processes on Earth put oxygen into the atmosphere?


3) What conditions do the two sets of processes above have in common - temperature constraints? A specific pressure range? Something in terms of composition? What basic things do these processes need to "go"?

The Atmosphere of Venus:

Initial atmospheric conditions on Venus would probably have been significantly warmer than on Earth - as it is closer to the Sun, the mean temperature on Venus, even in a thinner, 1-3 bar CO2 atmosphere, might well have been _100°C. Under such conditions, H2O probably could not have persisted as a free liquid, which means that water-mediated processes such as carbonate formation and biological carbon uptake may not have been able to operate. Thus, CO2 exhaled from volcanoes would progressively build up in the atmosphere, which means the planet would get progressively hotter. Eventually water vapor in the atmosphere would break down due to the heat, and/or react with rocks on the planet's surface. The Venusian atmosphere, then, is a water-free "runaway greenhouse," which developed largely because liquid water never persisted on the Venusian surface.


The Atmosphere of Mars:

The current Martian atmosphere might be called a Residual Atmosphere, because much of its original inventory of gases has been removed either by planetary processes, or by progressive loss to interplanetary space. The average atmospheric pressure at the Martian surface hovers around 6 mbars, which is curious, as that is the lowest pressure at which liquid water can exist. However, Mars today is far too cold for liquid water, averaging -50°C and never exceeding 0°C even in the midst of summer at the Mars Pathfinder landing site. Today such H2O as there is on Mars must be bound up as ice in thick permafrost regions below the planet's surface, and at the polar caps. Nonetheless, the Martian surface is rich in landforms that indicate the presence of abundant liquid water in the past. So on Mars, it is reasonable to hypothesize about possible climate conditions that might allow for the generation of these landforms.



Figure 4.1: Plot of surface temperature versus surface pressure on Mars as a function of incident solar energy. Dotted horizontal line marks the freezing point of water, and the three solid curves represent three possible options on the strength of incident solar energy. Assuming no relative change in incident sunlight, the Martian atmosphere would have to be at least twice as dense as that of the Earth, and predominantly CO2, in order to raise planetary temperatures above the freezing point of water. Diagram from McKay and Stoker (1989)


Figure 4.1 provides some constraints on possible atmospheric conditions for early Mars. To allow liquid water to persist at the surface, atmospheric pressures at least twice that of the Earth's atmosphere are necessary, assuming similar incident solar energy. And even under these conditions, with atmospheric CO2 abundances 10,000 times higher than on Earth, temperatures probably only creep slightly above the freezing point. Near-freezing conditions for at least some period of early Martian history are certainly consistent with the abundant "chaos terranes" and outwash deposits, which most closely resemble landforms on Earth that are associated with catastrophic glacial melting. The dendritic channel patterns and meandering channels on Mars, however, point to the long-term existence of water in liquid form.


Early Mars, and Possible Life: McKay and Stoker (1989) published a review on possible scenarios for the development of life on early Mars that still influences scientific thinking today. (Figure 4.2). While recent discoveries have antiquated some of their hypothetical timeline (for example, volcanic activity on Mars probably has persisted until some 30 million years ago, based on SNC meteorite ages, and accretion must have ended sooner, as the ALH 84001 meteorite represents a piece of Martian crust formed very close to 4.5 Ga), the general scenario they presented is still viewed as plausible. The record of planetary formation on Earth and on Mars must necessarily be similar, and if life documentably existed on Earth as early as 3.7 Ga (Schopf, 1983), then it is possible that it existed on Mars.



Figure 4.2: Hypothetical geologic timeline for the development of Mars, compared to the timeline for the development of life on Earth. From McKay and Stoker (1989).


The ~3.5 Ga time period during which Mars may have had a warm climate correlates broadly to possible dates for the shock events that formed the crushed zones in the ALH 84001 meteorite (Treiman 1996; McKay et al. 1996). That these crush zones are where the carbonate minerals are found in this meteorite is even more compelling in light of this probable timing - carbonates (basically, a version of limestone), are forming in this Martian rock at a critical time - a key reaction from the terrestrial carbon cycle was happening to some degree on Mars as well!

Question: Is there any evidence for abundant free oxygen on Mars? (not the little bit in the modern atmosphere, but for LOTS of oxygen, such as might be produced by a large biomass?

But, if Mars had a 2-3 bar atmosphere rich in CO2 which allowed the existence of liquid water and the possible existence of life, what happened to it? Some 99.9% of this atmospheric CO2 would have to be removed somehow. McKay and Stoker (1989) proposed a hypothesis that may eventually be testable by future Mars probes - they suggested that much of this CO2 is bound up as carbonate minerals in the Martian crust. They hypothesized that a process of inorganic limestone precipitation might proceed on Mars so long as it was possible for liquid water to exist and mediate the reactions. Liquid water could only persist so long as atmospheric pressures were high enough, and temperatures were warm enough, both of which depend on the abundance of atmospheric CO2. Thus, as CO2 contents in the atmosphere declined, the carbonate making process would proceed more slowly, but it would proceed, only ceasing when it became impossible for liquid water to exist on the planet - when mean atmospheric pressures dropped to around 6 mbars! Obviously, other processes, such as the adsorption of CO2 by clay minerals, and the development of "dry ice" in the polar regions, would also remove atmospheric CO2, but the McKay and Stoker idea is appealing for its simplicity. A key presumption of their hypothesis is that for whatever reason, life did not thrive and persist on Mars - the climate may have been too cold to allow living things to develop en masse and modify the atmosphere, as happened on Earth. This idea, considered rather wild at the time, has gained new credence with the discovery of abundant ancient carbonate minerals in the ALH 84001 Martian meteorite.

Thus, in the atmospheric evolution of Venus, Earth, and Mars, we have a "porridge" analogy: all three started with similar, volcanogenic atmospheres, but due to varying temperature condtions that affected whether water could exist as a liquid, they all took very different paths - Venus to the "hothouse," and Mars eventually to the "icebox, " while the Earth was "just right" for life to blossom! Whether there was ancient life on Mars is still an open question, one we may not answer until we go there personally. But ancient life there appears to have been possible, at least in terms of planetary raw materials and climatic conditions.