EPSC Abstracts,
Vol. 3, EPSC2008-A-00103, 2008
European Planetary Science Congress, © Author(s) 2008
Preservation of organic matter in the STONE 6 artificial meteorite experiment
J. Parnell and S. A. Bowden
School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK ([email protected] / Fax: +441224-272785)
The STONE 6 experiment
The STONE series of experiments, conducted by the
European Space Agency, are designed to test the
response of various rock types to entry into the Earth’s
atmosphere, i.e. to simulate meteorites. This includes
assessment of whether sedimentary rocks, and any
entrained organic materials, would survive. The rock
samples are attached to the exterior of a FOTON reentry capsule around the point of maximum heating,
and exposed to re-entry velocity of 7.6 km/s. The
STONE 6 experiment, conducted in September 2007
[1], included an organic-rich sedimentary rock, in
which the major objective was a detailed analysis of
the fate of the organic matter.
Experimental sample
The sample was a Devonian laminite (siltstone)
deposited in a lacustrine environment in the Orkney
Islands, Scotland, UK. It contained about 1.4 wt.%
organic carbon, and was also carbonate-rich. The
organic carbon and carbonate reflect algal
colonization and photosynthesis in the lake waters.
The sample was sculpted (Fig. 1) into a disc of 7cm
diameter and 2cm maximum thickness. Most of the
rock was ablated during the re-entry process, but about
26% survived, in which the original laminate structure
was preserved.
The rock is still black after the experiment and
preserves up to 0.5 wt.% organic carbon. However the
amount of extractable organic matter is very much
lower than pre-experiment. Organic geochemical
analyses show that recognisable biomarker molecules
such as hopanoids survive, but in greatly reduced
quantities. Using hydrocarbon potential as a frame of
reference, the compositions pre- and post-experiment
are comparable with organic-rich rocks in the zone of
oil generation and gas generation respectively (Fig. 3).
Measurements of the carbon isotope composition of
the organic carbon and carbonate carbon are about -25
per mil and 0 per mil respectively.
Glass
The glass is full of vesicles representing
devolatilization of the melted rock. Devolatilization
products are likely to have been water vapour and
carbon dioxide. The glass has partially recrystallized,
particularly near the contact between unmelted rock
and glass. Microprobe analysis of the crystallites show
them to consist of clinopyroxene, wollastonite and
melilite. The glass is consistently calcium-rich. This
mineralogy and chemistry is consistent with the
carbonate-rich nature of the original rock.
Fig. 1 Pre-flight (right) and Post-flight (left) samples, showing
substantial mass loss during experiment. Each sample 7cm diameter.
Composition of surviving sample
The surviving sample consists of unmelted rock
coated with a green-coloured glass.
Unmelted rock
The surviving rock retains the laminated fabric of the
original sample. X-ray diffraction data shows that the
primary mineralogy of quartz, feldspar and calcite is
preserved. However some of the calcite has thermally
decomposed to calcium oxide (requires ~900ºC), and
some calcium oxide subsequently rehydrated to the
hydroxide mineral portlandite (Fig. 2).
Fig. 2 X-ray diffraction data pre- and post-experiment. C, calcite;
CaO, calcium oxide; F, feldspar; P, portlandite; Q, quartz.
Preservation of evidence of life
Several aspects of the sample preserve evidence of life.
The preferential distribution of organic matter along
laminae suggests a biological mechanism for
concentration.
The surviving biomarkers include hopanes, for which
there is no known non-biological mechanism of
formation. Similarly, the isoprenoid phytane (Fig. 3) is
derived from chlorophyll and the cell membranes of
archaea [2].
The isotopic compositions of co-existing organic and
carbonate carbon show substantial differences
between them (about 25 per mil), which is interpreted
by most workers as reflecting a biological
fractionation process [3,4].
The combined evidence would be sufficient for an
intelligent alien, studying the rock as a meteorite
arrived at another planet, to deduce that there must be
life on the parent planet.
The successful measurements of biomolecular details
in the experimental sample offers hope that evidence
for life on other planetary bodies, such as Mars, could
survive the re-entry process and consequently could be
recorded in meteorites.
Fig. 3 Total ion chromatograms of extractable organic matter pre- and post-experiment. Hydrocarbon potential reference scale to left.
Ph = Phytane; Cx indicates carbon chain length of n-alkanes.
Acknowledgements
John Still is thanked for the microprobe data, Paula
Lindgren for the photograph, and Jim Marr for the
XRD data. The original sample was kindly
provided by Nigel Trewin. Rene Demets and Franz
Brandstätter recovered the sample in Kazakhstan,
and Frances Westall is the Principal Investigator for
the STONE 6 Project. The European Space Agency
is thanked for facilitating the experiment.
References
[1] Westall, F. et al. (2007) LPSC XXXIX, 1538.
[2] Peters, K E. et al. (2005) Biomarkers and
Isotopes in the Environment and Human History.
CUP, Cambridge.
[3] Schidlowski, M. (1992) Adv. Space. Res., 12,
101-110.
[4] Mojzsis, S. J. and Arrhenius, G. (1998). Jour.
Geophys. Res., 103, 28495-28512.