What is the possibility of life in our Solar System?
Updated: Oct 19, 2022
When we consider the critical factors that make our own home, Earth, a habitable planet we realize that it is the lucky combination of our stable atmosphere, vast oceans, rain and inland water, and rocky continents that set the stage for life. Furthermore, life on our blue planet can only exist in all of its startling variety and complexity because the global climate falls within the temperature threshold that can sustain life as we know it. We are just the right distance from our powerhouse, the sun, for all these crucial factors to work in concert to allow life to flourish.
Our rocky neighbors
There are several other planets in our solar system that are a similar distance away from the sun as we are. Would they, too, be able to support life?
Our closest neighbor is Mars, another rocky planet like Earth and yet today it is too cold and too dry to support the myriad of life we have here. Evidence points to the probability that Mars was once a temperate planet with surface water, an atmosphere, and the means to sustain some form of life in its distant past. There remains the chance that some types of primitive microbes adapted to extreme conditions may still be clinging to life on its barren surface or more likely, not far below. Likewise, our neighbors Venus and Mercury are rocky planets but being closer to the sun, their temperatures are far too hot to be able to sustain any life that we would be familiar with.
What about the gas giants?
Looking further out into the solar system we find the gas giants, Jupiter, Saturn, Uranus and Neptune. While life is unlikely on these planets themselves, their moons propose interesting possibilities that need to be investigated. Water is one of the most vital factors required for sustaining life and while many of the moons orbiting these gas planets are very cold and covered in ice, there is evidence that some of them have oceans of water below the surface.
The most promising targets for mankind to search for signs of life in our solar system would have to be these moons. One of Saturn’s larger moons, Enceladus, is entirely covered with ice but NASA has photographs that reveal frozen geysers spouting from cracks in the surface ice which points to the possibility of liquid water lying in places beneath the surface. Analysis reveals the presence of a wide variety of mineral compounds like salts, ammonia and even organic molecules that can be the basis of living organisms, no matter how primitive. NASA’s discoveries on Enceladus hint at hydrothermal activity deep beneath the surface that could possibly provide the heat source necessary to support life.
Saturn’s largest moon, Titan, also poses the intriguing possibility of conditions capable of supporting primitive life, either now or in its past. Although extremely cold, Titan is known to have liquid lakes of hydrocarbons like ethane and methane (which is essentially liquid natural gas), and a nitrogen-rich atmosphere that is similar chemistry to the atmosphere we find here on Earth. Titan also has an ocean of water deep below the surface, and it has a weak gravitational field. The rich and complex organic chemistry making up the moon’s surface may be the raw materials necessary to support life even if it is in a form not present on Earth. In 2027, NASA is launching a Dragonfly drone mission to investigate Titan’s atmosphere.
Jupiter shows promise
Jupiter has many moons and several of these are of interest due to their potential for life, past or present. Foremost among these is Europa, which may have vast liquid oceans albeit, lying beneath surface ice more than 10 miles thick. This makes the water difficult for us to investigate and no light can penetrate that deep, so the processes of photosynthesis that powers most life on Earth could not occur. However, even on Earth, rare examples of extreme life forms can exist through chemical processes different from photosynthesis. If there is some source of geothermal heat on Europa, coupled with the ingredients for life that we have already identified near the surface, life could find a way.
Two of Jupiter’s moons, Ganymede and Callisto, may also have liquid oceans deep below the surface. However, they may be buried at least 60 miles below a rocky surface. Despite being extremely cold, Callisto does have a thin but promising atmosphere with chemistry similar to that on Earth, including oxygen, carbon dioxide and hydrogen.
Ganymede is Jupiter's largest moon and like the others, it is covered in a layer of ice. However, beneath the ice are vast saltwater oceans with the potential to host life. This moon even has a thin oxygen atmosphere and a magnetic field, which is vital to help protect it from the sun’s harmful radiation.
Neptune also has a moon that might present possibilities in the search for extraterrestrial life. Triton is Neptune's largest moon and it is known to be geologically active, with geysers that spit forth sublimated nitrogen gas. While the surface of the moon is predominantly frozen nitrogen, its crust is composed of ice made of water, however, it is so extremely cold that it is hostile to the sustaining of life in any form that we know.
The European Space Agency’s JUICE project will be conducting flyby studies of Jupiter’s moons Europa, Callisto and Ganymede during its mission to the Jovian system, and an exploratory mission called Clipper, is heading for Europa in 2024.
Promising indicators of the raw ingredients necessary for life are scattered throughout our solar system and it remains for the curiosity of humankind to drive the future exploration of our neighbors to track down any traces of life, either past or present. These exciting possibilities are out there, just waiting for us to uncover them.