by S. Prasad Ganti

Astrobiology represents the human quest to find life outside of our planet Earth – whether it is in our Solar system or elsewhere in our Universe. It could be simple life like microbes or intelligent life like us or maybe even higher than us. I read an interesting book on this topic recently “The secret life of the universe” by Nathalie Cabrol. 

Life appeared on Earth about 3 billion years ago. Conditions were favorable on Earth for formation of life. Recent discoveries suggest favorable conditions elsewhere too. Even on Earth life is found in extreme conditions like hydrothermal vents at the bottom of Oceans. But life has not yet been found anywhere else beyond our planet. It is difficult to believe that we are unique in our universe. By the same token as Enrico Fermi, the famous Physicist asked “Where is everybody” ?

Earth is at the right distance from our Sun. It is neither too cold nor too hot. It has abundant water in liquid form. It has plate tectonics which shift continents around like a jigsaw puzzle. In the process, they shuffle the nutrients from under the oceans to the land. A thick atmosphere to even out occurrences of extreme temperatures under light and darkness. A strong magnetic field, due to a molten iron core at the center,  to shield us from ultraviolet rays and other harmful radiation. Significant gravitational force which holds the atmosphere. An almost even temperature which holds the water in all the oceans without excessive evaporation into space. These are Goldilocks conditions in which we find ourselves thriving.     

The magnetic field on Mars was lost about 4 billion years ago. The atmosphere eroded due to the impacts from the solar wind and radiation. As a result, it is a barren land without any oceans. There is ice at poles. There could be microbial life beneath the surface or underneath the ice. But no evidence on the surface itself. Biosignatures indicating the presence of some chemicals could represent clues to the presence of life. One of them is methane found on Mars. It is still being generated. It can also be generated by geological processes, in addition to biological ones. It could be one or both of these processes at play. Further investigations are needed. 

Venus is a very hostile place for life to exist. It is a runaway greenhouse with very high temperatures. Acids are present in its thick atmosphere. Yet phosphene was detected in its atmosphere. It can be generated by geological or biological processes. Missions are planned for the future which will investigate Venus further.  

While Mercury is closest to the Sun and is hot, ice is still present today in the cold traps in those areas which are permanently shadowed from the Sun. Life could exist in such cold areas. 

Ceres is a dwarf planet in the asteroid belt between Mars and Jupiter. It contains a deep reservoir of salt-enriched water beneath a fractured icy crust. It has organic molecules as well. Life may exist in this interior. 

Jupiter’s moon Ganymede has plenty of water in oceans under an ice crust, a magnetic field, and a source of energy generated by tidal heating that triggers tectonic activity. Similarly Titan has an ocean of water under its surface. In addition, its thick atmosphere is rich in organic molecules. It does have oceans of liquid methane on the surface. While water would freeze at such temperatures, methane exists as liquid. 

Outside of our solar system, thousands of exoplanets have been found. TRAPPIST-1e, Kepler 186f and Kepler 452b are some of the planets closer to Earth in size and reside in habitable zones around their stars. Looking for atmospheres and biosignatures on these planets demands more powerful telescopes both on the ground and in space.  

All these investigations are a work in progress. They look for Goldilock conditions and biosignatures. Future missions like Europa  Clipper and JUICE to the moons of Jupiter, and Dragonfly to Titan are very exciting to follow.   

Organic molecules like methane, amino acids etc. have been found in various places in our Universe. In fact they were also created in an experiment conducted by Stanley Miller in the 1950s, simulating nature. The key question even for Earth is how the prebiotic chemistry consisting of organic molecules, turns into life.