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A Low Temperature Transfer of ALH84001 from Mars to Earth

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Science  27 Oct 2000:
Vol. 290, Issue 5492, pp. 791-795
DOI: 10.1126/science.290.5492.791

Abstract

The ejection of material from Mars is thought to be caused by large impacts that would heat much of the ejecta to high temperatures. Images of the magnetic field of martian meteorite ALH84001 reveal a spatially heterogeneous pattern of magnetization associated with fractures and rock fragments. Heating the meteorite to 40°C reduces the intensity of some magnetic features, indicating that the interior of the rock has not been above this temperature since before its ejection from the surface of Mars. Because this temperature cannot sterilize most bacteria or eukarya, these data support the hypothesis that meteorites could transfer life between planets in the solar system.

Large-body impacts are the only known natural processes capable of ejecting a rock from Mars. It has been suggested that some rocks could be ejected without being shocked and heated (1, 2), and laboratory shock experiments have spalled lightly shocked material moving at about 20% of Mars' escape velocity (3). Thermal conductivity calculations (4) demonstrate that passage through Earth's atmosphere will not heat the interior of meteorites larger than ∼0.3 cm above 100°C.

ALH84001 is a meteorite composed of ∼95% orthopyroxene that accumulated in a magma chamber on Mars ∼4.5 billion years ago (Ga) (5). Carbonate blebs, which may contain evidence for ancient life on Mars (6), formed in its fractures at about 4 Ga (7). During its first few billion years, ALH84001 experienced several shocks, probably from minor planet impacts (8). It was launched from the surface of Mars at ∼15 million years ago by another impact (9) and after wandering through space, landed in Antarctica at about 11 thousand years ago (ka) (10).

Transmission electron microscopy imaging of the rims of the carbonate blebs in an ultrathin section prepared by focused ion beam detected single domain (SD) and superparamagnetic (SP) (11) magnetite (Fe3O4) and monoclinic pyrrhotite (Fe7S8) with characteristic lattice fringes. Because magnetic minerals have not been positively identified outside the carbonate bleb's rims, the blebs probably carry most of the magnetization in ALH84001. Other studies (12) have shown that this magnetite is stoichiometric (impurities <0.1%). After exposure to a 5-T field at room temperature, a 20-mg pyroxenite grain from ALH84001,236 containing multiple carbonate blebs exhibited a remanence transition at 112 K and a possible weaker remanence change at ∼35 K, diagnostic of low-Ti magnetite (Fe3–zTizO4with z < ∼0.01) and pyrrhotite, respectively. The grain's magnetization increased during cooling and then recovered ∼90% of its original magnetization upon warming to room temperature, indicating the presence of SP and SD crystals and a lack of multidomain (MD) crystals. Anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) acquisition and demagnetization experiments on this grain also provide evidence of pyrrhotite (it acquires an IRM up to and beyond 1000 mT) and a small fraction of pseudo–single domain (PSD) crystals (alternating-field demagnetization is more effective at removing an IRM than an ARM). X-ray maps obtained by electron microprobe analysis detected Fe-sulfide crystals dispersed through the pyroxene matrix, suggesting that pyrrhotite may also be present outside the carbonate. We have thus detected two major magnetic minerals in ALH84001, located in the carbonate blebs and also probably in the pyroxene: magnetite and pyrrhotite ranging in size between SP, SD, and PSD. This confirms a previous identification of these magnetic minerals (13) and argues against the presence of titanomagnetite (14).

Kirschvink et al. (13) suggested that the interior of ALH84001 has been cooler than 110°C since before the formation of the carbonate. To obtain more precise thermal constraints, we imaged the perpendicular (east/west in the meteorite orientation system) component of the magnetic field of eight oriented slices of ALH84001 (15) using the Ultrahigh Resolution Scanning SQUID Microscope (UHRSSM). This magnetometer has a sensitivity of better than 0.1 nT and is capable of making two-dimensional images of the magnetic field of materials at room temperature with a resolution of 500 μm (16).

The fusion crust on the top-south surface of ALH84001,228b formed during its high-temperature passage through Earth's atmosphere and magnetic field, and is associated with an intense magnetic anomaly (Fig. 1). Moment magnetometry measurements of this and two additional samples with fusion crusts indicate that the meteorite initially came to rest in the ice with its east-southeast axis pointing up. Much weaker (∼1% intensity) positive and negative magnetic features are present at distances of <5 mm in from the surface, implying that the heat pulse from atmospheric deceleration did not travel further than this into the meteorite. This shallow depth of heating is typical of most meteorites of this size (4). This suggests that the heterogeneous magnetization in the interior (Fig. 2) predates arrival at Earth.

Figure 1

(Left) Photograph of oriented slice ALH84001,228b that contains a small piece of the surface fusion crust (marked with arrows) oriented about normal to the image plane. This crust formed during entry through Earth's atmosphere. (Right) UHRSSM image of the same slice, displaying the east (out-of-the-page) component of the magnetic field at 250 μm above the slice. North is to the right and top is toward the top of the page, as demonstrated by the compass registered to the Johnson Space Center curatorial orientation system. An outline of the slice, corresponding to the edge of the slice in the photograph at left, is drawn around the sample. The color bar gives the field intensity in nT, with red and yellow (blue) corresponding to eastward (westward) magnetization. The fusion crust has been remagnetized by Earth's field in the east direction, with a peak intensity of ∼1200 nT. A few millimeters toward the interior of the meteorite (i.e., to the lower right) the original heterogeneous pattern of magnetization is observable with amplitudes similar to those elsewhere inside the rock (Fig. 2). Scale bars, 2 mm.

Figure 2

UHRSSM images of oriented slice ALH84001,232e. (A) NRM of the sample at room temperature before heating. Fracture surfaces on which two dipolar features sit are labeled with dashed lines. A fracture defining a distinctly magnetized grain is labeled with a solid line. A carbonate bleb containing magnetite and pyrrhotite was found 100 μm below the surface at the location labeled “Cb.” (B) Magnetization of the sample after being heated to 40°C for 10 min. The arrows in (A) and (B) indicate features that show significant decrease in intensity. Scale bars represent 2 mm and the color bar is in nT. (C) Magnetic intensity variations along a line connecting points x and y, as observed in the unheated sample and after heating to 40° and 200°C. This is a three-column running average centered on the line of pixels joining x and y.

Using the same technique, we made multiple magnetic images of ALH84001,232e, which was extracted from the interior of the meteorite. The first image of the natural remanent magnetization (NRM) at room temperature (Fig. 2A) reveals a spatially heterogeneous pattern of magnetization like that observed in the interior portion of 228b. We also observed a similar, stable (17) pattern in UHRSSM images of four other cm-sized slices taken from the interior of the meteorite. Several positive (eastward) and negative (westward) features are present (black arrows), some of which are dipolar in character. The strongest of these is centered on a carbonate bleb containing magnetite and pyrrhotite located 100 μm below the surface of the slice. In some regions, the dipolar features are centered on fractures (Figs. 2 and 3), while elsewhere fractures form the boundary between pyroxene fragments that are magnetized in different directions. The latter has been observed in previous studies of individual ALH84001 grains (13,14).

Figure 3

Backscattered scanning electron microscopy image of top half of oriented slice ALH84001,232e, showing fractures (dark black lines), orthopyroxene (light gray), feldspathic glass (dark gray), and chromite (white). Both fractures labeled as dashes lines inFig. 2A are visible and identified by a set of arrows. In one of them, a carbonate bleb containing magnetite and pyrrhotite was found 100 μm below the surface at the location labeled “Cb” (as in Fig. 2A). The carbonate bleb was observed after removal of a 100-μm-thick layer from the original surface from which the magnetic map was obtained. The uniformly magnetized clast (defined by solid line in Fig. 2A) is visible at left. Scale bar, 1 mm.

After producing the UHRSSM image of the NRM, slice 232e was then heated to 40°C for 10 min, cooled in an eastward 15-μT field, and then reimaged. It was then heated to 40°C for 10 min in a magnetically shielded furnace and cooled in a zero field (<10 nT) and reimaged. This cycle of heating and cooling was continued in 10° or 20°C steps up to 200°C. Oxidation of the iron-bearing phases was inhibited by first immersing the ALH84001 slices in mineral oil containing an organic antioxidant (α-tocopherol) and by removing trapped air under vacuum before each heating cycle. Following the heating to 40°C, the magnitude of many magnetic features decreased (including the dipole centered on the carbonate) (Fig. 2, B and C) [first frame of Web fig. 1 (18)], completely erasing some that were present in the NRM scan (Fig. 2A). These features never regained the intensities present in the NRM image (Fig. 2C and Web fig. 1). At each temperature step above 40°C, different randomly oriented features weakened or strengthened in magnetization (Web fig. 1). The magnetization that unblocked at 40°C should have been mostly from pyrrhotite: our high-resolution transmission electron microscope (TEM) compositional maps showed that in the carbonate rims, magnetite is only two to three times more abundant than pyrrhotite and both minerals have similar crystal size distributions, while the theoretical range of pyrrhotite crystal sizes with blocking temperatures between room temperature and 40°C is ∼1000 times larger than that of magnetite.

With rare exceptions, most rocks magnetize uniformly in the direction of the local magnetic field when they cool below their Curie temperatures or when their magnetic minerals crystallize out of a low-temperature solution. We conclude that ALH84001 should have been capable of isotropically acquiring a thermoremanent magnetization (TRM) during our heating experiment and throughout its 4.5–billion year history (19). This conclusion is supported by two experiments. In the first, we subjected the large pyroxene grain studied by Kirschvink et al.(13) to three 100-mT magnetic pulses at room temperature to give the grain an IRM, which we measured after each pulse. After the first and second pulses, the grain was rotated 90° along the north and east directions, respectively, while keeping the field direction constant. In all three cases the magnetic phases in the grain remagnetized with the same intensity to within 1° of the field direction. In our second experiment, we compared the magnetization image obtained after heating to 200°C and cooling in the eastward 15-μT field with that obtained after heating to the same temperature and cooling in the zero field (Fig. 4). Almost the entire slice remagnetized in the eastward direction (20). Thus, if the meteorite was previously heated, many of the features that weakened after the laboratory heating to 40°C in the zero field (Fig. 2 and Web fig. 1) would have remagnetized isotropically and in the direction of the background field. Because the meteorite is even more likely to have isotropically acquired a TRM when it first cooled (i.e., before it had been shocked and deformed), it is also likely that the meteorite was originally magnetized in the direction of the local field.

Figure 4

Intensity difference between spatially registered UHRSSM images of oriented slice ALH84001,232e after heating to 200°C: image taken after cooling in an eastward 15 μT minus image taken after cooling in a zero field. Prior to subtraction, images were registered by maximizing their cross-correlation. The carbonate bleb identified in Figs. 2 and 3 is also labeled here (“Cb”). Scale bar, 2 mm. The background intensity gradient in the vertical direction is an artifact from instrument drift.

At least three processes can have produced the spatial heterogeneity of the magnetization. The first is a brecciation event, in which fragments of the meteorite were rotated and translated with respect to one another without being heated to 40°C. Such an event could explain the association between the magnetization pattern and rock fragments (e.g., the clast on the left side of 232e in Figs. 2 and3). The second process is the deposition of several generations of magnetic minerals over a period during which the relative orientation of the local magnetic field changed. The latter could explain the dipolar and other magnetic features centered on fractures (e.g., those centered on the dashed cracks in Fig. 2). A third process is a series of shock events, which could have heterogeneously heated the rock on submillimeter scales, selectively remagnetizing isolated regions in the rock. Treiman (8) has identified five shock deformational events and a carbonate formation event. The earliest deformation, which he labeled D1, mobilized and rotated fragments of the rock, forming mm-thick cataclastic granular bands. Later events (D2b and D3) produced brittle fracture surfaces that transect the granular bands (8). However, these events may have been too hot to produce a spatially heterogeneous magnetization (8, 21). The fractures from D1 and D2b were then partly filled with the magnetized carbonate blebs (event Cγ) and also phyllosilicates (22). We have correlated at least one dipolar feature centered over a crack with a carbonate bleb (Figs. 2 and 3). Some heterogeneity could also have been produced when the D3 event broke up and translated some of these carbonates (8, 21). A low-temperature D4 event also may have brecciated the meteorite late in its history (8).

Regardless of which particular event(s) are responsible for randomizing the magnetization, it is fairly certain that ALH84001 was not brecciated during or after its ejection from Mars. Such a brecciation is highly unlikely to occur for orbital dynamical reasons (10,23). Furthermore, cosmic ray exposure studies demonstrate that ALH84001 was never shielded in a larger body in space (24). The latter indicates that the meteorite lacked appreciable self-gravity and so could not also have been broken up and reassembled during the ejection process. Although there is evidence that terrestrial water has flowed through the meteorite (25,26), the carbonates and their associated magnetic minerals originated on Mars (6). It is unlikely that significant quantities of magnetic minerals were deposited in ALH84001 while on Earth, because the meteorite was mostly encased in ice in subzero temperatures and no secondary minerals have been identified other than those from Mars (8, 22).

Using recently measured magnetization parameters (27), we calculated a Pullaiah diagram (28) for monoclinic pyrrhotite, which gives the time required for an SD crystal to magnetize (in the presence of a field) or demagnetize (in zero field) at a given temperature (Web fig. 2). Although the diagram is not always a reliable indicator of the behavior of natural crystal assemblages (29), it nevertheless suggests that any viscous remanent magnetism (VRM) acquired by ALH84001 while in Antarctica is unlikely to have affected pyrrhotite crystals with 10-min blocking temperatures of 40°C and above (30).

The impactors capable of removing rocks from Mars, which are likely several km in diameter (23) and moving at a Keplerian velocity of ∼10 km s−1, would produce a shock pulse of duration ∼0.1 s (31) during collision with the martian surface. The duration of thermodynamically irreversible heating produced during this process can be orders of magnitude larger than this shock pulse time (31). The Pullaiah diagram demonstrates that crystals that magnetize after heating to 40°C for 10 min would magnetize after heating to 125°C for >10−4s or to 60°C for >1 s. Thus, if during its ejection, ALH84001 had been heated to 40°C and then cooled in a zero field, no magnetic features would have changed in intensity during our 10-min heating experiment. If the meteorite had been heated to 40°C and instead cooled in a nonzero field, features magnetized in only one of the two directions would have weakened during our experiment, while the oppositely magnetized features strengthened. Because eastward and westward features (i) weakened by as much as 100% during demagnetization at temperature steps between 40° and 200°C and (ii) ALH84001 magnetizes isotropically in the direction of an applied field, ALH84001 has most likely not been heated even to 40°C since before it left Mars.

This means that ALH84001 was only lightly shocked during its ejection (1, 2) and therefore originates from near the planet's surface. Our data support recent suggestions (32, 33) that shock-implanted gases in ALH84001 did not enter the meteorite during ejection, but instead sample a martian atmosphere that is much older than 15 million years. Our results also indicate that major impact events are capable of moving rocks from the surface of Mars to the surface of Earth without subjecting them to temperatures high enough to cause thermal sterilization of eukarya or bacteria. Dynamic simulations of martian impact events (2, 10, 23, 34) indicate that materials can be launched into a wide variety of orbits. Although most of the ∼1 ton of martian rocks which land on Earth each year have spent several million years in space, one in 107 of the arriving rocks will have made the journey in less than a year (23). Every million years, ∼10 rocks larger than 100 g are transferred in just 2 to 3 years (23). Bacterial spores, as well as microorganisms within rocks, can survive in deep space for more than 5 years (35). These results indicate that it may not be necessary to protect Earth's present biosphere by quarantining rocks retrieved by a Mars sample return mission (36). Conditions are appropriate to allow low-temperature rocks—and, if present, microorganisms—from Mars to be transported to Earth throughout most of geological time.

  • * To whom correspondence should be addressed. E-mail: bweiss{at}gps.caltech.edu

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