Carbonate Globules

ALH84001 contains secondary carbonate minerals in the form of globules from 1 to 250 ^m across, that have radiogenic Rb-Sr ages of 3.9 ± 0.04 billion years (Borg et al., 1999) (Fig. 1). This is a distinct feature of ALH84001, since other

Figure 1. Evidence of Martian life suggested by McKay et al. (1996). Center: ALH84001 after removal of some samples (Photo Credit: NASA/JSC). Top left: carbonate globules (examples marked by arrows) in the meteorite. Scale bar is 200 ^m (Photo Credit: Jacek Wierzchos and Carmen Ascaso). Top Right: structure of some of the PAHs identified in the meteorite. Bottom Left: bacteria-shaped-objects <100 nm in length (Photo Credit: NASA/JSC). Bottom Right: magnetite crystals that resemble those synthesized by magnetotactic bacteria on Earth. Scale bars are 20 nm. (From Thomas Keprta et al., 2000.)

Figure 1. Evidence of Martian life suggested by McKay et al. (1996). Center: ALH84001 after removal of some samples (Photo Credit: NASA/JSC). Top left: carbonate globules (examples marked by arrows) in the meteorite. Scale bar is 200 ^m (Photo Credit: Jacek Wierzchos and Carmen Ascaso). Top Right: structure of some of the PAHs identified in the meteorite. Bottom Left: bacteria-shaped-objects <100 nm in length (Photo Credit: NASA/JSC). Bottom Right: magnetite crystals that resemble those synthesized by magnetotactic bacteria on Earth. Scale bars are 20 nm. (From Thomas Keprta et al., 2000.)

SNC meteorites (Shergottite, Nakhlite, Chassigny), contain only trace carbonate phases. The conditions in which the carbonate globules formed and their thermal history have been a subject of heated debate, as they are central to the discussion surrounding the evidence of life in ALH84001. The maximum temperature at which life has been found on Earth is 121°C (Kashefi and Lovley, 2003), and if the carbonate globules formed at substantially higher temperatures, then the life hypothesis would be seriously undermined. Petrographic and electron microprobe studies (Mittlefehldt, 1994; Scott et al., 1997, 1998) suggest that the carbonates formed at temperatures in excess of 500°C, whereas the stable oxygen isotope data indicate that the carbonates formed between 0°C and 80°C (Romanek et al., 1996), a range of temperatures compatible with life. The magnetic properties of pyroxene grains within the meteorite seem to reflect primary features of the ancient Martian magnetic field. These paleomagnetic imprint can be erased at temperatures >200°C, which indicates that the rock could not have been substantially heated after its formation, and thereby supporting a low temperature origin of the carbonate globules (Kirschvink et al., 1997). However, thermochronometry analyses indicate that the meteorite may have been shocked multiple times, reaching peak temperatures of 400°C in localized areas (Min and Reiners, 2007). A large number of studies based on a variety of techniques have followed one another since 1996, arguing for and against a low temperature origin of the carbonate globules (i.e. Harvey and McSween, 1996; Warren, 1998; Golden et al., 2001; Kent et al., 2001). However, none of these studies has provided conclusive evidence for the conditions in which the globules formed and evolved. Currently there seems to be a consensus that the carbonates did form at temperatures below 300°C. This temperature is still incompatible with life as we know it, but being an upper-end value it cannot be used to rule out the biogenic hypothesis either.

0 0

Post a comment