TANPOPO: Astrobiology Exposure and Micrometeoroid Capture Experiments
By Akihiko YAMAGISHI
, Hajime YANO
2, 3, 4)
, Kyoko OKUDAIRA
, Kensei KOBAYASHI
, Makoto TABATA5
, Hideyuki KAWAI
, Masamichi YAMASHITA
, Hiroshi NARAOKA
, Hajime MITA
1) Department of Life Sciences, Tokyo University of Pharmacy and Life Sciences, 1432-1Horinouchi, Hachioji-shi, Tokyo 192-0392, Japan
2) Department of Planetary Science, ISAS/JAXA, 3-1-1 Yoshinodai, Sagamihara-shi,Kanagawa 229-8510, Japan
3) Program Office & Research and Development Office, JAXA Space Exploration Center,JSPEC/JAXA, 3-1-1 Yoshinodai, Sagamihara-shi, Kanagawa 229-8510, Japan
4) Department of Space and Astronautical Science, Graduate University for AdvancedStudies, 3-1-1 Yoshinodai, Sagamihara-shi, Kanagawa 229-8510, Japan
5) Department of Space Information and Energy, I SAS/JAXA, 3-1-1 Yoshinodai,Sagamihara-shi, Kanagawa 229-8510, Japan
6) Department of Chemistry and Biotechnology, Yokohama National University,Hodogaya-ku, Yokohama 240-8501, Japan
7) Graduate School of Science and Technology, Chiba University, 1-33 Yayoi-cho,Inage-ku, Chiba-shi, Chiba 263-8522, Japan
8) Department of Physics, Chiba University, 1- 33 Yayoi-cho, Inage-ku, Chiba-shi, Chiba263-8522, Japan
9) ISAS/JAXA, 3-1-1 Yoshinodai, Sagamihara-shi, Kanagawa 229-8510, Japan
10) Department of Earth Sciences, Okayama University, Tsushima- Naka 3-1-1, Okayama,700-8530 Japan
11) Department Enviromenta Life Scinece, Fukuoka Institute of Technology, 3-30-1Wahakutou, Higasihiku, Hukuoka, 811-0295 Japan
There is a long history of the microbe-collection experiments at high altitude. Microbes have been collected using
balloons, aircraft and meteorological rockets. Spore forming fungi and Bacilli, and Micrococci have been isolated in these
experiments. It is not clear how high do microbes go up. If the microbes might have been present even at higher altitudes,
the fact would endorse the possibility of interplanetary migration of life.
TANPOPO, dandelion, is the name of a grass whose seeds with floss are spread by the wind. We propose the analyses of
interplanetary migration of microbes, organic compounds and meteoroids on Japan Experimental Module (JEM) of the
International Space Station (ISS). Ultra low-density aerogel will be used to capture micrometeoroid and debris. Particles
captured by aerogel will be used for several analyses after the initial inspection of the gel and tracks. Careful analysis of the
tracks in the aerogel will provide the size and velocity dependence of debris flux. The particles will be analyzed for
mineralogical, organic and microbiological characteristics. Aerogels are ready for production in Japan. Aerogels and trays
are space proven. All the analytical techniques are ready. The TANPOPO mission was accepted as a candidate experiments
on Exposed Facility of ISS-JEM.
Key Words: Astrobiology, micrometeoroid capture experiments, interplanetary migration of microbes, ISS-JEM, organic compounds
There has been a hypothesis on the origin of life called
“panspermia” (, ). According to this hypothesis, life has
migrated between the Earth and other extra terrestrial objects.
The finding of life-like structure in a meteorite originated
from Mars recalled the possibility. There is also a possibility
that the life on the Earth may have ejected from the Earth by
volcanic eruption or by meteorite impact. We have been
analyzing the presence of microbes at high atmosphere by
aircraft and balloons. Microbes have been captured by these
experiments. The microbe-sampling experiments could be
extended to lower Earth orbit by using ISS. It is also
important to test if the microbe ejected from the Earth may
survive during the voyage to other planets. We also propose
the survival test of microbes on ISS.
Another important subject on the origin of life is related to
the pre-biotic production of organic compounds. The
extra-terrestrial and outer-solar area can be the place for the
pre-biotic organic compound synthesis. To test the possible
pre-biotic organic compound synthesis, simulation has been
conducted. However, more direct evidence could be obtained
by the intact meteoroid capture experiment. It is also
important to know what kind and degree of denaturation
could occur on the complex organic compounds which are
expected to be formed in extra-terrestrial area. To test the
kind and degree of denaturation process, simulated complex
organic compounds are proposed to be exposed on ISS.
The development of extra-low density aerogel is an
important subject for the micrometeoroid capture
experiments. Silica aerogel is made of SiO
and is transparent
solid. Aerogel have been used for the collection of artificial
debris and interplanetary dust. For the proposed ISS project,
we developed extra-low density aerogel and will test the
aerogel on ISS. The developed extra-low density aerogel will
provide proof of the applicability of the aerogel, which can
be used for the next generation sample return mission in the
Our debris capture mission will collect many types of
debris. They will include debris of artificial objects, exhaust
form ISS, micrometeoroid, and micro particles from the
Earth. Much important information will be obtained from the
analysis of the many types of particles collected on ISS.
2. Collection of Microbes in Space
There is a long history of the microbe-collection
experiments at high altitude. Microbes have been collected at
the altitude from 3 to 58 km, using balloons, aircraft and
meteorological rockets from 1936 to 1976 (-). Spore
forming fungi and Bacilli, and Micrococci have been isolated
in these experiments. However, the experiments have been
done before the development of modern molecular biology
and only the taxonomic affiliation has been analyzed on the
isolates. It is not clear how high do microbes go up. If the
microbes might have been present even at higher altitudes,
the fact would endorse the possibility of interplanetary
migration of life.
Previously, we have conducted microbe-sampling
experiments using aircraft. Microbes were isolated from the
particles collected at the altitudes form 0.8 to 12 km. The
genes of the isolated microbes were analyzed. The analysis
revealed that the isolates belong to spore formers
(Streptomices, Bacillus and Paenibacillus) and Deinococcus
related species. Previously Deinococcus radiodurans has
been classified to be Micrococcus. Accordingly, the
Micrococcus strains isolated at high altitude in previous
experiments may be or may include Deinococcus species.
Deinococcus radiodurans is the species that is known to
be most radio-resistant. Then, we have analyzed the UV
resistance of high altitude isolates. Two of the high
atmosphere isolates showed the UV resistance similar to or
higher than Deinococcus radiodurans. The flux of UV light
at high atmosphere is expected to be much higher than
ground surface. Accordingly, it is reasonable that the
microbes isolated at high atmosphere shows high UV
We have also conducted microbe-sampling experiments
using balloons. The sampling device consists of vacuum
pump and filtration system. The air was incorporated by a
vacuum pump and passed through an ultra-membrane filter.
The air intake was controlled by a valve placed in front of the
filter. About 10 m
(corresponding mass at SPT) air was
sampled at the altitude from 20 to 35 km. Four strains were
isolated from the balloon sampling experiments. Analysis of
the isolates is now on progress.
To extend the sampling altitude we propose the microbe
collection experiment on ISS-JEM. The microbe/particle
collection on ISS needs totally different strategy. We are
going to use ultra low-density aerogel for the sampling
experiments. If there are microbes at ISS altitude, they have
to have the earth orbit velocity. The expected mutual velocity
of the microbes against aerogel is up to 16 km/s depending
on the direction of the microbe relative to the ISS movement.
We have tested the possibility of microbe sampling using a
two-stage light-gas gun.
Another point which has to be considered is the survival of
microbes in the environment where the UV dose is high. The
single cell of microbe is not expected to survive under the
high UV dose. However, if the microbes are present in the
mineral particles, there will be much higher possibility of
survival. We have tested the possible survival of microbes
using the montmorillonite particles containing microbial cells.
The particles were accelerated to 4 km/s by a two-stage
light-gas gun. Microbes were pre-stained with fluorescent
pigment. The particles were targeted to aerogel. The aerogel
was inspected by fluorescence microscopes. The fluorescent
particles were detected in the aerogel. Now, we are trying to
assure that the fluorescent particles are really microbial cells
we have used.
3. Survival of Microbes in Space
To evaluate the possibility of migration between planets
including the Earth and Mars, the survivability of microbes
in space must be tested. As a part of this project, we plan to
test the survivability of microbes in space by direct exposure
The exposure experiment of microbes in space has been
performed (e.g. , ). Most of experiments were not
direct exposure experiments: windows that shield EUV were
used to cover microbes. Therefore, these experiments might
underestimate the effects of light on the microbial cells.
In such exposure experiments, Bacillus sp., which is
spore-forming bacteria, was used (). Spore is a stage of
cells that is most tolerant to extreme environments. On the
other hand, bacterial species such as Deinococcus
radiodurans have been known to show extreme tolerance to
the UV-light and gamma radiation. Recently, we have
isolated several bacterial species from the high-altitudes (ca.
10 km) (Itahashi, Yang, Yokobori, & Yamagishi, manuscript
in preparation). They showed higher tolerance to the
UV-light than D. radiodurans. Such extreme UV-tolerant
bacteria might be able to survive at higher altitudes.
The microbes that are directly exposed to the space have
lower possibility of survival. However, some microbial cells
may survive, if the cells were shielded by other microbe cells
and/or clay minerals. UV light will not reach several tens to
hundreds of micrometers in depth (cf. ).
In our project, various microbial cells including D.
radiodurans and our newly isolated UV-resistant bacteria
will be used for exposure experiments. Cells will be
freeze-dried with/without clay mineral. In the space
environment, the cells are expected to be freeze-dried
(dehydrated, in another term). The cells will be dehydrated in
a hole of a metal plate. The dehydrated cells will be tightly
fixed to the metal plate without any covers that might shield.
The cell will be exposed to the space at least for 1 year. The
cells in the metal holder of the exposure apparatus of
TANPOPO will be retrieved and will be returned for the
The returned samples will be used for various tests for
checking the survival of microbes. Most direct test is the
cultivation of exposed microbes. The possible contamination
of microbial cells during the operation will be tested by PCR
analysis of genes. The method will tell whether the recovered
bacterial colonies are those of original species or not.
4. Collection of Organic Compounds in Space
A wide variety of organic compounds have been found in
such extraterrestrial bodies as meteorites (carbonaceous
chondrites) and comets. Chyba and Sagan estimated that
more than 100 kt of carbon had been delivered to the Earth
on extraterrestrial bodies . It could be an important
source of carbon for the first biosphere on the Earth.
Bioorganic compounds like amino acids and nucleic acid
bases were detected in the hot water extracts from
carbonaceous chondrites. Cronin and Pizzarello reported that
Murchison meteorite contains more L-isomers of some
amino acids than D-isomers , and such enantiomeric
excesses could be a seed for homochirality of bioorganic
compounds in our world. Nakamura-Messenger et al. have
found proto-cell-like organic globules in Tagish Lake
Meteorite . Complex organic compounds were also
found in comets by the direct analysis of the dust from
Comet Halley with mass spectrometer on spacecraft .
Preliminary organic-compound analysis of cometary dusts
returned by Stardust mission also showed the presence of
complex organic compounds in Comet Wild 2 .
There is the possibility that organic compounds in
interplanetary dusts (IDPs) have contributed more for the
generation of terrestrial life than those in meteorites and
comets because of the following reasons: (i) much more
organic compounds could be delivered by IDPs than by
comets and meteorites, (ii) organic compounds in IDPs could
be delivered to the Earth less destructive while those in
comets and meteorites could be destroyed on their impacts.
Though many IDPs have been collected in deep-sea and in
Antarctica, there is high probability of terrestrial
contamination. We propose the collection of IDPs for the
analysis of organic compounds as a part of TANPOPO
4.1 Simulation Experiment of High-Velocity Impact
For the analysis of organic compounds on IDPs, the major
problem is how to collect IDPs with ultra high velocity. We
are going to use aerogel with ultra low density. We have
performed simulation experiments by using the two-stage
light-gas gun at ISAS / JAXA. The following samples were
used for the analysis. (i) Porous silica gel (PSG) used as a
blank, (ii) powder of R-2-aminobutyric acid (AABA) with
sizes from 220 to 350 μm, (iii) AABA adsorbed to PSG.
AABA was chosen due to its low possibility of accidental
detection from contamination, since it is non-proteinous
amino acid and scarcely found in living organisms. Each
sample was placed in a “sabot” made of polycarbonate, and
targeted to aerogel at 4 km/s. The aerogel with a track made
by the impact was digested with 5 M HF - 0.1 M HCl mixed
acid at 383 K for 24 h, hydrolyzed with 6 M HCl at 383 K
for 24 h, desalted with AG-50X-X8 cation exchange resin,
and then analyzed with an amino acid analyzer (Shimadzu
LC-10A). AABA detected in (iii) was much more than that
in blank (i). AABA was not detected, however, in (ii). The
results suggest that amino acid itself is not stable against
impact even if aerogel is used, but amino acid associated
with inorganic matrix is more stable.
4.2 Alteration of Organic Compounds in Space
Where did organic compounds in meteorites, comets and
IDPs come from? Greenberg  proposed the following
scenario. The Solar System is formed in a dense cloud
(molecular cloud). Since temperature in dense clouds is as
low as 10-20 K, most molecules are frozen onto surface of
interstellar dusts (ISDs). Such “ice mantle” of ISDs contains
such molecules as water, carbon monoxide, methanol and
ammonia. When frozen mixtures simulating the ice mantle
were irradiated with high-energy particles or ultraviolet light,
amino acid precursors (molecules which give amino acids
after hydrolysis) were formed (-). Organic molecules
containing amino acid precursors formed in dense clouds
would be altered by cosmic rays and ultraviolet light before
they were incorporated in parent bodies of meteorites and/or
comets. They were again altered in the Solar System small
bodies. IDPs seem to have been made from the small bodies,
and organic compounds in IDPs were irradiated with cosmic
rays and strong solar ultraviolet light before fallen into the
It is of interest to examine how organic compounds alter in
actual space environments. There have been a great number
of experiments on radiochemical and photochemical
alteration of organic compounds. In these experiments, either
a light source or a radiation source was used on ground. In
addition, extreme ultraviolet light (EUV) has never been used
on ground, since there are no appropriate windows to pass
EUV. ESA has conducted several exposure experiments in
space, e.g., LDEF and BIOPAN, but target samples were still
covered with windows and space EUV did not reach to the
We are planning to expose organic compounds on
ISS-JEM Exposure Facility. The exposure unit is set next to
the aerogel unit. Here, samples such as amino acids and
“simulated interstellar organic compounds” will be adsorbed
onto a metal substrate and there will be no covering. The
simulated interstellar organic compounds will be made from
possible interstellar molecules like carbon monoxide,
ammonia and water by proton irradiation . The proposed
setup could make it possible to irradiate samples with cosmic
radiation and ultraviolet light including EUV simultaneously,
and could give information how cometary / meteoritic
organic compounds alter in IDPs. These samples will be
made of isotopic atoms to avoid the faulty identification of
the compounds for the exposure experiments as organic
compounds of ISD.
5. Micrometeoroid Intact Capture
Meteoroid observation and collection have been conducted
to study their parent bodies in planetary science. Several
types of analyses have been conducted on meteoroids.
Zodiacal dust cloud could be observed from the Earth surface.
Cosmic spherules have been found in Antarctic ice core.
Stratospheric interstellar dust particles have been captured by
aircraft. Meteoroid impacts have been noted on the surface of
LEO spacecrafts. Analysis of these particles has provided
information on their origin and the parental bodies. However,
intact and contamination-less collection of micrometeoroids
Measurement and modeling of debris flux have another
importance: It is important to know the debris flux to
evaluate the risk of LEO spacecraft. Debris with relatively
lager sizes has been monitored by ground observation. Debris
with smaller sizes have been detected and analyzed on the
surface of retrieved parts of spacecraft. Retrieved spacecraft
have been analyzed in the following missions: LDEF (84-90,
NASA/ESA), EuReCa (92-93, ESA), HST (89-93,
NASA/ESA), SFU (95-96, JAXA) (-). Passive
particle collection apparatus have been also deployed and
analyzed: Euro-Mir (96-97, ESA), ODC (97-98, NASA),
MPAC-SEED (02-05, JAXA). However, the number of
debris is increasing and continuous monitoring is needed.
The key technology for TANPOPO is the particle intact
captures using aerogel. The aerogel is amorphous SiO2 with
low bulk density (below 0.03 g/cm3), is optically transparent
and thus most suitable for hypervelocity particle capture
experiments. The aerogel is also an excellent thermal
insulator: thermal conductivity is about 0.017 W/mK.
Accordingly, aerogel is suitable for space utilization. The
aerogel tiles have been used for EuReCa, Euro-Mir, ODC,
MPAC-SEED, Stardust, etc, and thus can be said space
For example, the aerogel has been used in Stardust project
(, ). In the Stardust project, cometary and interstellar
dust were captured intact and the samples were returned for
analysis. The spacecraft was launched on 1999. Interstellar
dust was collected on 2002. The cometary particles were
collected from the Comet Wild-2 coma during the fly-by on
2004. The samples were returned on January 2006. The
particles with hypervelocity, 6.1 km/s, were successfully
captured. The initial analysis of the overwhelming success
has been reported.
Japanese aerogel has been also used in ISS-MPAC-SEED.
In the mission, sampling devices were launched on 2001
placed on the Russian Service Module of ISS in LEO of
about 400 km. Sampling devices were retrieved sequentially
on 2002, 2004 and 2005. However, the sampling device was
exposed only on one face of ISS. It is known that the type of
the particles collected depend on the face of the exposure
relative to the direction of ISS movement. East face, which is
the direction of ISS orbital, has the highest possibility of
capturing debris of man-made origin. West face and space
face are most suitable for collecting interplanetary dust.
One of the TANPOPO teams has already tested the
possibility of collecting hypervelocity meteoroid by aerogel.
CM2 Murchison powder was accelerated to be 6.2 km/s by a
two-stage light-gas gun and targeted on aerogel . The
track of the particle in the aerogel was inspected and the
particle was found at the tip of the track. The thin section of
the particle was inspected by an electron microscope. The
peripheral area of the particle consisted of amorphous
nodules that are typical to melting phenomena. However,
crystal structures were retained at the central part
representing the original mineral structures of the Murchison
powder. The result shows that it is possible to capture
hypervelocity meteoroid partially intact.
The aerogel tiles are going to be attached on several faces
of integrated experimental rack that will be placed on
ISS-JEM Exposure Facility. No signal-connection is
required during the whole exposure period. After more than
1-year exposure in LEO, the trays will be retrieved manually
by EVA crew and sealed in the ISS pressurized module for
the Soyuz Earth return.
Interface has been designed based on the discussion with
6. Aerogel for Microparticle Collection
Silica aerogel is an amorphous solid with a void volume up
to 99.5%. Aerogel has been widely used as optical radiators
for PID (particle identification) devices in high energy and
nuclear physics experiments, since it is optically transparent
and has very low refractive index among solids. One of the
advantages of our aerogel is that it is possible to make
aerogel hydrophobic, which makes the handling easier. We
have been developing aerogel in cooperation with the High
Energy Accelerator Research Organization (KEK) and
Matsushita Electric Works, Ltd. in Japan . At present, we
are able to produce aerogel at a wide range of densities i. e.
0.01-1.2 g/cm3 . The aerogel that were made with our
production method were actually used in MPAC-SEED
which was a Japanese contributory experiment exposed at
ISS Russian Service Module. Because of the extremely low
bulk densities, transparencies and thermal insulation
properties, aerogel is the most suitable medium for the
nondestructive capture of hypervelocity particles in space.
In TANPOPO mission, it is important to reduce the
thermal metamorphism of micro particles captured in Earth
orbit upon impact. The key to the successful scientific
analysis is the performance of aerogel with extremely low
densities. For this purpose, we developed an aerogel with the
lowest density (0.01 g/cm
or less) in our current production
method. Until now, typical density of aerogel employed in
space is 0.03 g/cm
. The density is suitable for easy mounting
and handling. However, for micro particles with relative
incident velocities over 6 km/s, the density would not be low
enough for nondestructive collection. We developed the way
of installing the ultra low-density aerogel in a module. The
layer of the extremely low-density aerogel for micro particle
capture was cast on a base layer of higher density. We have
already succeeded in developing a monolithic aerogel block
consisting of multiple layers with different densities.
Our primary purpose is to obtain information not only on
meteoroids and space debris but also on migration of organic
compounds and microbes through space. In addition, we aim
to demonstrate the ability of the aerogel-based micro particle
collecting apparatus in the space environment. Aerogel units
with different density structures will be tested on ISS
Japanese Experiment Module, which will extend a reach of
the micro particle capture operation. Extra low-density
aerogel used in TANPOPO will provide an innovative
technique for planetary expedition in the future.
The group member of Chiba University in TANPOPO
mission has the ability to produce ultra low-density aerogel.
The aerogel made in the method has been used on ISS
Russian Service Module for particle sampling experiment.
The basic design of aerogel tray has been also tested in the
JAXA mission MPAC-SEED. The TANPOPO mission
needs neither signals nor mechanism during the whole
exposure period. The TANPOPO mission is selected as one
of the second stage experiment of ISS-JEM EF. After more
than 1-year exposure in LEO, the trays will be retrieved
manually by EVA crew and sealed in ISS.
The TANPOPO mission teams already have long
experiences in bacterial analysis, organic compound analysis
and micrometeoroid analysis. Accordingly, all the analytical
techniques are ready.
Interface between TANPOPO apparatus and ISS-JEM EF
have been basically designed based on the coordination of EF
designers and EVA directors. This coordination must be
completed within the development time frame of ISS-JEM
EF coordination experiment project. However, all of above
requirements are feasible.
1) S. Arrhenius, Worlds in the Making-the Evolution of the
Universe (translation to English by H. Borns) (1908)
Harper and Brothers Publishers, New York.
2) F. Crick, Life Itself (1981) Simon & Schuster, New
3) M. J. Burchell, R. Thomson and H. Yano, Planet. &
Space Sci., 47 (1999) 189.
4) Y. Kitazawa, A. Fujiwara, T. Kadono, K. Imagawa, Y.
Okada and K. Uematsu, J. Geophys. Res., 104 (E9)
5) H. Yano et al., Adv. in Space Res., 25 (2000) 293.
6) H. Yano, Earth, Planets & Space, 51 (1999) 1233.
7) H. Yano: Meteoroid and space debris impacts on
telescopes in space, in “Preserving the Astronomical
Windows”, (Eds. S. Isobe and Y. Hirayama), Astro. Soc.
of the Pacific, p65-86, (1998).
8) H. Yano and Y. Kitazawa, Proc. the 21st Int'l Symp. on
Space Tech. and Sci.(ISTS), Hakushinsha., Tokyo,
Japan, (1998) 1819.
9) H. Yano et al., Adv. in Space Res. 20 (1997) 1489.
10) H. Yano, Proc. the 19th Int'l Symp. on Space Tech. and
Sci.(ISTS), (Eds. M. Hinada, et al.), Hakushinsha.,
Tokyo, Japan, (1994) 1017.
11) H. Yano, H.J. Fitzgerald and W.G. Tanner, Planet. &
Space Sci., 42 (1994) 793.
12) D. Brownlee et al., Science 314 (2006) 1711.
13) M. E. Zolensky et al., Science 314 (2006) 1735.
14) T. Noguchi et al., Meteorit. Planet. Sci., Vol. 42, No. 3.
15) I. Adachi et al., Nucl. Instr. and Meth. A355 (1995) 390.
16) M. Tabata et al., 2005 IEEE Nucl. Sci. Symp.
Conference Record, Puerto Rico, (2005) 816.
17) C. Chyba, and C. Sagan, Nature 355 (1992) 125.
18) J. R. Cronin, and S. Pizzarello, Science 275 (1997) 951.
19) K. Nakamura-Messenger et al., Science 314 (2006)
20) J. Kissel, and F. R. Krueger, Nature 326 (1987) 755.
21) S. A. Standford, et al., Science 314 (2006) 1720.
22) J. M. Greenberg, and A. Li, Adv. Space Res. 19 (1997)
23) T. Kasamatsu, et al., Bull. Chem. Soc. Jpn. 70 (1997)
24) G. M. Munoz Caro, et al., Nature 416 (2002) 403.
25) M. Bernstein, et al., Nature 416 (2002) 401.
26) M. Hegedüsa, et al., J. Photochem. Photobiol. B, 82
27) Y. Takano et al., Earth Planet. Sci. Lett. 254 (2007) 106.
28) L. A. Rogers and F. C. Meier, Natio. Geographic Soc.
Stratosphere Series. 2 (1936) 146.
29) C. W. Bruch, In: Airborne microbes: symposium of the
society of general microbiology (Gregory, P. A. and
Monteith, J. L. Eds.), Vol. 17 (1967) 385. Cambridge
30) A. A. Imshenetsky et al., On microorganisms of the
stratosphere. Life Scie. Space Res. 14 (1976) 359.
31) W.L. Nicholson et al., Microbiol. Mol. Biol. Rev. 64
32) G. Horneck et al., Orig. Life Evol. Biosph. 31 (2001)