The hypothesis called "panspermia" proposes an interplanetary transfer of life. Experiments have exposed extremophilic organisms to outer space to test microbe survivability and the panspermia hypothesis. Microbes inside shielding material with sufficient thickness to protect them from UV-irradiation can survive in space. This process has been called "lithopanspermia," meaning rocky panspermia. We previously proposed sub-millimeter cell pellets (aggregates) could survive in the harsh space environment based on an on-ground laboratory experiment. To test our hypothesis, we placed dried cell pellets of the radioresistant bacteria Deinococcus spp. in aluminum plate wells in exposure panels attached to the outside of the International Space Station (ISS). We exposed microbial cell pellets with different thickness to space environments. The results indicated the importance of the aggregated form of cells for surviving in harsh space environment. We also analyzed the samples exposed to space from 1 to 3 years. The experimental design enabled us to get and extrapolate the survival time course to predict the survival time of Deinococcus radiodurans. Dried deinococcal cell pellets of 500 μm thickness were alive after 3 years of space exposure and repaired DNA damage at cultivation. Thus, cell pellets 1 mm in diameter have sufficient protection from UV and are estimated to endure the space environment for 2-8 years, extrapolating the survival curve and considering the illumination efficiency of the space experiment. Comparison of the survival of different DNA repair-deficient mutants suggested that cell aggregates exposed in space for 3 years suffered DNA damage, which is most efficiently repaired by the uvrA gene and uvdE gene products, which are responsible for nucleotide excision repair and UV-damage excision repair. Collectively, these results support the possibility of microbial cell aggregates (pellets) as an ark for interplanetary transfer of microbes within several years.

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[1] We retrieved CO 2 volume mixing ratios (VMRs) from solar absorption spectra in the 1.6-mm CO 2 (30012-00001) band measured with a ground-based high-resolution Fourier transform spectrometer (FTS) at Tsukuba, Japan, using profile retrieval and scaling retrieval algorithms. We derived the time series of the column-averaged CO 2 VMRs (X CO2 ) from December 2001 to December 2007. The average difference between the X CO2 values obtained with the two algorithms was approximately 0.8 ppm. This difference was attributed to differences between the column averaging kernels of the algorithms. We corrected the distortion effect of an instrumental line shape (ILS) on X CO2 retrieval by determining information about the ILS simultaneously with X CO2 . Aircraft in situ measurements were made simultaneously with the FTS measurements over the FTS site on 10 August 2004 and 30 March 2005. The differences between the X CO2 values derived from the FTS measurements and from the aircraft in situ measurements complemented by model data were less than 1%. The diurnal variations of X CO2 derived from the FTS measurements demonstrated that X CO2 could be retrieved with a precision of $0.2%. The retrieved X CO2 values were compared with CO 2 VMRs obtained from aircraft sampling measurements up to 7 km altitude over Sagami Bay, Japan. The seasonal amplitude of the retrieved X CO2 agreed within 1 ppm with those of the CO 2 VMRs obtained from the aircraft sampling measurements above 3 km. The average seasonal amplitude of X CO2 over Tsukuba was $8 ppm. In addition, X CO2 showed an increasing trend, with a growth rate of $2 ppm/a.