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

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:

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. �

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!

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.

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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 (www.jpl.nasa.gov). Some major achievements of the Pathfinder program are listed below

• The use of "off-the-shelf" (almost) equipment as components in the Pathfinder and Sojourner appears to have been a resounding success, as the three months of operation of the lander and the rover were the and twelve times greater than predicted, respectively.
• APXS analysis of several rocks and of soils in the "Sagan Memorial Landing Site" in Are Vallis demonstrated greater diversity in Martian surficial materials than was believed to exist from SNC meteorite and Viking Lander data, including the first evidence for quartz-rich rocks on Mars.
• The first documented evidence for Martian clouds, from Pathfinder imagery and meteorological experiments.
• Detailed stereoscopic imagery of the Are Vallis landing area for use in studies of surface composition, mineralogy, and physical properties (Figure 2.2).

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.

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 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!

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!

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)

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:

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.

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!

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.