Astrobiologists suggest looking for signs of life on the planets closest to Earth and their satellites. There are ecological niches suitable for the habitation of microorganisms on Venus and Mars, Europa and Enceladus. We talk about where in the solar system the most likely meeting with alien creatures.
Atmosphere of venus
The space at an altitude of 51-65 kilometers from the surface of Venus can be habitable, British scientists say. They found that the temperature there ranges from minus 20 to plus 65 degrees Celsius. The atmosphere consists of water vapor highly saturated with sulfuric acid aerosol. In such conditions, extremophile bacteria that feed on sulfur survive. For example, the archaea Picrophilus, which lives in the hot brines of the island of Hokkaido.
On Earth, extremophiles occupy the most lifeless places - hot underground springs, oxygen-free reservoirs, permafrost. Some microbes have adapted to feed on inorganic substances, assimilating them with the help of solar energy. Cyanobacteria survive at temperatures up to 70 degrees - at higher values, chlorophyll in their cells is destroyed.
A weak magnetic field does not protect Venus from a stream of high-energy particles of galactic radiation. Solar flares are more dangerous there than on Earth, whose orbit is further away. On the other hand, the atmosphere of the "planet of crimson clouds" is several times denser than the earth's and better detains radiation. However, there is still no chance of surviving on the surface of Venus, scientists say.
Liquid oceans and biosphere may have been on early Venus, but the ensuing greenhouse effect turned it into a dry, sterile desert. It is unlikely that life has survived under the surface of the planet, in the ground. The only place where it can glow is among the clouds high in the atmosphere. For example, the bacteria Mesophilic Deinococcus radiodurans survive even when exposed to ten kilograms of radiation. And the hyperthermophilic archean Thermococcus gammatolerans multiplies at 88 degrees and withstands a radiation dose of three kilogram.
Archaea Nanopusillus acidilobi lives in the very acidic hot springs of Yellowstone Park (USA) / Photo: Wurch et al. / Nature Communications 2016.
Promotional video:
Underground ocean of Mars
Many scientists believe that Mars and Earth three to four billion years ago were similar and equally rich in water. American geologist Timothy Parker made out in the Viking images the contact zone of land and sea around the Northern Wasteland (Vastitas Borealis) - a vast lowland in the North Pole of the planet.
Judging by the preserved relief, the Martian ocean was a swampy lowland periodically replenished with moisture. It is estimated that about 2.3x107 cubic kilometers of water could have accumulated there. A third of it evaporated, a third went to the formation of the polar ice cap, and another third could be preserved as a layer of ice underground.
Unlike Earth, early Mars had a cold climate. However, the Red Planet could be the cradle of life, scientists from NASA believe. The discovery in Gale Crater of sediments similar to those that form in geysers has greatly inspired the proponents of the panspermia hypothesis, according to which comets brought life to Earth.
The drying up of oceans and lakes does not mean the disappearance of life. It is known that some of its forms are capable of extracting moisture from the atmosphere or soil. In addition, Mars had conditions suitable for life five million years ago. The planet's orbit was then tilted at 45 degrees, which means that the poles received twice as much solar heat as they do now. About the same as the polar regions of the Earth.
Mars' polar caps are not melting now, but five million years ago they resembled today's Antarctica. The closest analogue of the south pole of Mars is dry frozen rocks of the alpine University Valley in the south of the continent. They are similar to those found by the Phoenix spacecraft on the Red Planet.
Microorganisms were found in the permafrost of the University Valley, which does not melt even in summer. Scientists have isolated pieces of DNA from several strains of bacteria, archaea and lower fungi. All of them were at rest or died. In the laboratory, several species have been awakened to life.
Fungi of the order Chaetothyriales, found on the rocks of the University Valley. Photo: CDC / Sherry Brinkman.
Icy ocean orbiting Saturn
The sixth-largest moon of Saturn, Enceladus, is covered with 40 kilometers of ice. In 2011, the Cassini spacecraft recorded the release of water, sodium chloride, ammonia and carbon dioxide at the south pole of Enceladus. This led scientists to speculate that a salty ocean is hidden under the ice. This means that the satellite has internal sources of heat. According to calculations, the temperature of the water in the ocean can be 26 degrees Celsius, although on the surface - minus 170.
The warm salty ocean is a suitable place to live. The ice shell protects against cosmic radiation. True, it does not allow sunlight to pass through, but this is not critical, since many groups of microbes live in complete darkness. For example, organotrophs are bacteria that exist due to the decomposition of organic matter, or chemotrophs that use the energy of redox reactions.
Enceladus' conditions are close to the subglacial lakes of Antarctica. The most accurate analogue is considered to be the relict Lake Vostok under a four-kilometer layer of ice. However, living organisms have not yet been found in it.
But Lake Untersee is encouraging. The glacier above it never melts and only lets in five percent of the light. Despite this, a high content of microbial methane was found there, and powerful bacterial mats and stromatolites were found at the bottom.
Colonies of the cyanobacterium Trichodesmium thiebautii color tropical waters red. But red stripes were found along the faults on the icy moon of Jupiter - Europe / Photo: FWC Fish and Wildlife Research Institute.
Tatiana Pichugina