They were lifted by convective currents for 400 kilometers.
Cosmonaut Anton Shkaplerov, the commander of the new ISS crew, who will arrive at the station in December 2017, said in an interview that bacteria appeared on the surface of the station, which "came from somewhere from space and settled on the outside of the casing." Earlier, soil bacteria from Madagascar were found on the surface of the station, apparently reaching it from Earth.
During spacewalks, the ISS crew collects samples of traces from the outer casing of the station with cotton swabs. Then the tampons in sealed containers are delivered to Earth and subjected to careful examination. As Shkaplerov notes, over time, bacteria were found there, which were not in the first samples. In other words, bacterial spores entered the ISS after the station was deployed in space. According to the cosmonaut, while new samples are being studied and, as far as can be understood, they do not pose any threat to the station itself or its inhabitants.
More interesting is the question of the source of the mysterious bacteria. Previously, DNA from Mycobacteria, typical of the heterotrophic marine bacterioplankton of the Barents Sea, as well as the extremophilic bacteria Delftria, were found in such samples. A number of strains close to the soil bacteria of Madagascar. DNA of plant genomes, archaea and fungi Erythrobasidium and Cystobasidium were also found. This biodiversity was recorded after only 19 samples from the outer skin of the ISS.
The mechanism of the rise of plant pollen, spores of fungi, bacteria and archaea to the altitude of the ISS orbit (about 400 kilometers) is not yet completely clear. They are supposed to rise with updrafts. The spores of many bacteria and fungi, as well as archaea, can withstand very low pressures (up to vacuum), as well as very high temperature changes and radiation.
The value of such finds is not only that they make it possible to realize the threat of biological pollution for objects in space, but also that they indicate the possibility of carrying DNA carriers in outer space, at least hundreds of kilometers. In addition, new data have raised a debate in the scientific community about where the upper boundary of the Earth's biosphere lies. Previously, it was considered a mark of 20 kilometers.
IVAN ORTEGA