Introduction
Since the first discovery by a Japanese expedition in 1969 (Yoshida et al. Reference Yoshida, Ando, Omoto, Naruse and Ageta1971), over 25 000 meteorite specimens have been collected (Harvey Reference Harvey2003). Meteorites that fall onto the Antarctic polar plateau are rapidly buried in accumulating snowfall and become part of the ice sheet. Flow of the ice can bring them to stagnant zones, often associated with mountain ranges that block the flow of the ice. Here the polar ice is ablating leaving behind meteorites on the surface (Cassidy et al. Reference Cassidy, Harvey, Schutt, Delisle and Yanai1992).
Antarctic meteorite finds include many samples of Martian origin (Harvey Reference Harvey2003) including ALH84001 which was collected at the Allan Hills and was suggested to contain evidence of life on Mars (McKay et al. Reference Mckay, Gibson, Thomas-Keprta, Vali, Romanek, Clemett, Chillier, Maechling and Zare1996). However, these suggestions for evidence of life have not garnered widespread support. One issue raised with respect to ALH84001 was weathering and alteration when exposed to the environment on the surface of the ice.
Meteorites on the ice surface are warmed by sunlight and can melt the surrounding ice opening up the possibility of contamination and chemical alteration. Gibson & Andrawes (Reference Gibson and Andrawes1980) discuss the importance of solar heating on rocks on the Antarctic ice, noting that the temperature of the rocks can exceed the local air temperature. They report observations by U.B. Marvin of small puddles in rocks on ice when air temperatures were below freezing.
The suggestion by McKay et al. (Reference Mckay, Gibson, Thomas-Keprta, Vali, Romanek, Clemett, Chillier, Maechling and Zare1996) that the ALH84001 meteorite contained evidence consistent with life on Mars motivated considerable research on this meteorite. Bada et al. (Reference Bada, Glavin, Mcdonald and Becker1998) studied the ALH84001 meteorite and found trace amounts of glycine, serine, and alanine in concentrations from 0.1 to 7 parts per million. Primarily L enantiomers were detected but low concentrations of D-alanine were also reported. Bada et al. (Reference Bada, Glavin, Mcdonald and Becker1998) concluded that the amino acids in ALH84001 appear to be terrestrial in origin and similar to those in Allan Hills ice consistent with melting of the ice in contact with the meteorite. Becker et al. (Reference Becker, Glavin and Bada1997) suggested that Antarctic meteorites, in particular ALH84001, are contaminated with terrestrial organics probably derived from Antarctic ice meltwater that had percolated through the meteorite.
Burns et al. (Reference Burns, Burbine, Fisher and Binzel1995) studied weathering processes in meteorites expressed as ferric iron content. They concluded that the amount of weathering is dependent not on the total time the meteorite has been on Earth but the time that the meteorite has been exposed to the surface of the ice. Similar results were also obtained by Krähenbühl et al. (Reference Krähenbühl, Noll, Döbeli, Grambole, Herrmann and Tolber1998). They concluded that ALH84001 was exposed to surface weathering effects for < 500 years out of its total residence time on Earth of 13 000 years. Steele et al. (Reference Steele, Goddard, Stapleton, Toporski, Peters, Bassinger, Sharples, Wynn-Williams and Mckay2000) reported the presence of an Antarctic endolithic organism, Actinomycetes, in ALH84001 and thus provided microbiologic evidence for colonization of this Martian meteorite by terrestrial biota.
Kopp & Humayun (Reference Kopp and Humayun2003) also pointed out the possibility of Antarctic weathering and suggested that the plausibly biogenic minerals (such as the single-domain magnetite of characteristic morphology and sulphide) reported from the rims may be the products of terrestrial microbial activity.
Understanding the liquid water production surrounding meteorites is important for understanding the contribution of the surrounding ice, and the potential for microbial growth and contamination. These factors must be taken into account when searching for extraterrestrial organics and life in meteorites (e.g. Martins et al. Reference Martins, Alexander, Orzechowska, Fogel and Ehrenfreund2007).
Results and discussion
We recorded temperature data for a rock placed on ice at the meteorite collection site near the top of the Beardmore Glacier (84°30′S, 162°00′E), an area unofficially named “Foggy Bottom” at an altitude of c. 2000 m. We used a dark dolorite rock from the Antarctic Dry Valleys. The rock was approximately spherical shape with a diameter of about 10 cm and a specific density of 2.9.
Rock and ice temperatures were measured by Stowaway XTI temperature loggers (Onsett Computer) with external thermistor sensors. Air temperature was measured with the same type of unit equipped with an internal sensor, shielded from direct sunlight by a housing of aluminized mylar and placed 1 m above the ice surface. In the temperature range for the data reported in this paper the maximum error was ±0.5°C but solar heating of the shield may have resulted in high reading in the maximum air temperature measurements by 2 or 3°C. Sensor calibration at the freezing point was checked in a stirred ice-water bath before deployment.
We measured the temperature of the upper and lower surfaces of the rock, the air, and the temperature within the ice cover at 50 cm depth. We found that during the summer of 1997–98, air temperature was above freezing for nine hours, whereas the upper rock surface was above freezing for 80 hours, and the lower surface, in direct contact with the ice, for 42 hours. Figure 1 shows temperatures of the rock and air for the two warmest days of the 1997–98 Antarctic summer: under bright sun conditions the rock was warmed to about l0–15°C above air temperature, reaching 15°C. This is the same level of warming measured for dark-crusted sandstone rocks in the nearby McMurdo Dry Valleys at 1600 m elevations (McKay & Friedmann Reference Mckay and Friedmann1985) and is theoretically explained by surface energy balance models (Nienow et al. Reference Nienow, Mckay and Friedmann1988).
Temperature on the top and bottom surfaces of a 10 cm dark dolerite rock on the Antarctic polar plateau. Also shown are the air temperature and the temperature about 50 cm beneath the surface of the ice. The freezing point is indicated by a dashed line.
Conclusions
Direct measurement of temperatures considerably above freezing for significant periods of time indicates that meltwater can be present around rocks and meteorites on the Antarctic Plateau. Interaction between liquid water and these meteorites on the ice surface has also been strongly suggested by the presence of evaporites and by chemical weathering studies. The most significant warming probably occurs during a few singularly warm days each summer; during these times, liquid water is probably produced by the melting of any adhering snow, but the underlying ice will contribute as well as the bottom temperatures also rise above melting. Liquid water could facilitate microbial growth and activity, as well as transporting contaminants into the meteorite and leaching material out. Thus interaction between Antarctic meteorites and liquid water is certain to occur, and the possibility of terrestrial microbial contamination must be taken into account when apparent evidence of extraterrestrial life in these specimens is evaluated.
Acknowledgements
This field study was suggested by the late E.I. Friedmann and this paper is dedicated to his memory. David W. Mittlefehldt deployed and retrieved the sensors - his encouragement and support of this study are appreciated. The field experiment benefited from suggestions and advise from Ralph Harvey and David McKay.
