1. NSF STAR Program at the SETI Institute,
Pamela Harman and John Keller; NASA
Exobiology Program, Dr. Michael New.
The fundamental redox conversion, which
produces H2 in the matrix of minerals, also
generates peroxy defects at a 1:1 ratio.
To quantitatively assess the concentration of
H2 we can develop techniques to measure
the peroxy concentrations. This, however, is
also not a simple task. We’ll keep working
on this scientifically exciting question.
Finding the answer will have implications for
questions such as the slow but inextricable
oxidation of the early Earth through the
injection of O2 during global weathering and
the evolution of early Life in an environment
that became slowly ever more oxidized.
Evidence has become overwhelming that a
small (but non-neglible) fraction of oxygen
anions in minerals in typical igneous and
high-grade metamorphic rocks has
converted from their usual 2– valence state
to the 1– (peroxy) state. The underlying
process is a solid state redox reaction by
which pairs of hydroxyl in the mineral
structure, i.e. dissolved “water”, convert to
molecular H2 plus peroxy, O3Si-OO-SiO3.
This reaction introduces molecular H2 into
the rock column. H2 is of astrobiological
interest as a potentially inexhaustible energy
source for deep microbial communities.
There are many additional geophysical and
geochemical aspects to this amazing redox
reaction, which deserve wider attention.
We approach this project from several fronts.
(i) Crushing rocks as in the earlier study [4],
using a stainless steel chamber which we
can evacuate or flush with gases, measuring
H2 with either a Residual Gas Analyzer, RGA
or a Hydrogen Leak Detector (H2000), or
(ii) A wide range of electrical measurements
to determine the number of peroxy in rocks.
The experiments indicate that significant H2
concentrations exist in mineral grains. This
seems to confirm that the rock column
contains comparatively large amounts of H2,
formed in the matrix of nominally anhydrous
minerals. This H2 can be expected to slowly
diffuse out of the mineral grains, saturate in
intergranular water films, and be available to
sustain deep microbial communities.
[1] R. Martens, H. Gentsch, F. Freund, H2
release during the thermal decomposition of
Mg(OH)2 to MgO. Catalysis 44, 366 (1976).
[2] F. Freund, Hydrogen and carbon in solid
solution in oxides and silicates Phys. Chem.
Minerals 15, 1 (1987)
[3] M. Wheelies,, Principles of Modern
Microbiology, Jones & Bartlett Publishers,
(2007) 164-171.
[4] F. Freund, Dickinson, T., and Cash, M.,
“Hydrogen in rocks: An energy source for
deep microbial communities” Astrobiology 2
(2002) 83-93.
G. Benjamin 1, I. Kulahci 1, G. Cyr 2,
R. Dahlgren 1,2, and F. Freund 1,2,3
1.The SETI Institute, Mountain View, CA, USA.
2. Physics, San José State University, San José, CA, USA.
3. NASA Ames Research Center, Moffett field, CA, USA.
Acknowledgements
References
Future Work
Conclusion
Results
Each run produced powders with a large
portion of fine grains and a few remaining
large pieces. Despite of this, moderate
differences in the amount of H2 released per
gram of rock were found. At least 70 nmol of
H2/g diffused out of crushed andesite,
equivalent to 5,000 cm3 of H2/m3 of rock at
standard pressure and temperature.We used a 30 ton press to crush 20-25 g rock
samples between two stainless steel pistons.
We observed a rapid release of H2. The
H2000 gave essentially the same values in
dry and moist gas, suggesting that the H2
release is not due to reactions of H2O with
the fresh fracture surfaces. After crushing, H2
continued to evolve, indicating outdiffusion of
H2 molecules from the grains. For technical
reasons neither RGA nor H2000 were very
suitable to continuously record H2 evolution
over days. More work will be needed.
We do not report on electrical measurements.
Equipment
Methodology
The first organisms to inhabit Earth are
believed to have been chemoautotrophs.
Chemoautotrophs still dominate the genera
that populate the rock column down to at
least 3 km. They generally use H2 as a
primary energy source.
H2 can be produced when H2O reacts with
fresh mineral surfaces oxidizing Fe2+ to Fe3+.
This reaction depends upon the exposure of
fresh rock surfaces and works well only at
elevated temperatures. It is highly unlikely
that this reaction can supply a steady H2 flux
supporting deep microbial communities over
billions of years.
If H2 exists inside mineral grains in igneous
or metamorphic rocks, the volume of the
rocks thus becomes a source, from where H2
can be drawn – not just fracture surfaces,
The big question is how much H2 there is.
In an earlier study [4] a lower limit was
established by crushing rocks in a non-
metallic mortar to a coarse, unsorted
powder, measuring the H2 release by means
of H2 gas chromatography: up to 5000 cm3
H2 per m3 rock, equivalent to just ~5 ppm.
Realistically, if the amount of solute H2O in
nominally anhydrous minerals is in the range
of 500-5000 ppm, and if a significant fraction,
up to 90%, of the solute hydroxyl converted
to H2 plus peroxy, the expected H2 content in
rocks should be ~300-3000 ppm.
Incidentally, an H2 concentration of 315 ppm
is equivalent to the number density of H2
molecules in the gas phase a 1 bar pressure.
Abstract Astrobiological Connection
Hydrogen in Rocks, an Inexhaustible Energy Source for the Deep Biosphere
[P23C-0394] [Tue 12/15/09 1:40PM]Contact: friedemann.t.freund@nasa.gov
Background Story
Setup with stainless steel vacuum chamber
to crush rock samples and two hydrogen
sensors, an Adixen H2000 H2-specific H2
detector and an RGA, Residual Gas Analyzer.
H2000
Upper
piston
Sample
Piston
PC
F
F
RGA PC
The redox conversion of hydroxyl pairs was
discovered during a study of OH– in MgO
[1]. Up to 20,000 ppm H2 were measured
with very finely divided, OH––rich MgO. The
same type of redox reaction was later
recognized in silicate minerals [2]:
O3Si/OH
OH/SiO3 <—> O3Si/OOSiO3 + H2
The consequences of this reaction, which
occurs during cooling through the 600-
400°C window, are far-reaching.
(i) The amount of solute “water” cannot be
obtained by measuring the hydroxyl content
by means of IR spectroscopy.
(ii) Peroxy equivalent to excess O atom in
the mineral structure redox-balanced by H2.
Rocks release H2
upon crushing.
H2 per gram rock is lowest in marble
and basalt but highest in labradorite.