The habitability of planets and other natural satellites refers to the methods of measuring the propensity of the planets to have conditions that are hospitable to life. However, a habitable environment may not necessarily support life. The study of planetary habitability is still new, and it is considered to be significant for astrobiology because of several reasons, including that there are other planets and satellites other than earth that have conditions that are similar to earth’s, where life thrives (Dehant et al., 2019). This means that they may harbor some form of life. Scientists have identified several environmental factors that make it possible for life to be feasible and realized that they are present in other planetary bodies in our solar system. The planets and satellites with the highest probability to support life include Mars, the icy satellites of Jupiter and Saturn, known as Europa and Enceladus, and Ganymede and Callisto. However, if there is life the moons, the creatures would probably be restricted to subsurface habitats. Historically, the life on earth was believed to exist on its surface strictly, but recent researches have demonstrated that up to about half of the biomass of Earth’s may be living below the surface. Enceladus and Europa exist outside of the circumstellar habitable zone, which has defined the regions and boundaries that mark the limits of life within the solar system (Dehant et al., 2019). Within this is the zone water can exist in the liquid state, in which it can support life. Only Earth, Mars, and their moons lie within the habitable region of the Solar System, although other studies show that Mars and its moons are marginally out of the habitable zone. Areas outside this zone cannot sustain liquid water or an atmosphere. Scientists believe that tidal forces may play a crucial role in providing heat through stellar radiation. Life can only exist at specific temperature conditions.
Europa is one of Jupiter’s moons, together with Ganymede, Callisto, and Io. It was discovered by Galileo Galilei, a prominent astronomer, and still remains to be one of the largest moons in the solar system. Europa is smaller in comparison to the other moons of Jupiter, but also the most intriguing. Europa’s surface is frozen because a thick layer of ice covers it, but researchers believe that there is an ocean of liquid water under the surface. The white icy surface of the frozen water enables the moon to reflect a lot of light from the sun, making it among the most reflective satellites in the solar system. In 2012, the research team controlling the Hubble Space Telescope discovered a possible plume of water jetting from the southern polar region of the Europa moon (Chopra & Lineweaver, 2016). Another team of researchers repeated the exploration mission over the moon between 2014 and 2016 and confirmed that indeed its surface had water plumes. However, they warned that the jets of water were not yet fully proven, but there is enough to suggest the presence of water under the surface of the moon jetting out to the surface. A few spaceships have flown over Europa in the 1970s, including Voyagers 1 and 2 and Pioneers 10 and 11. Between 1995 and 2003, the Galileo space shuttle carried out a long-term exploration of Jupiter and its satellites (Dehant et al., 2019; Chopra & Lineweaver, 2016). The European Space Agency and NASA have both scheduled missions to Europa and its moons. The project is expected to launch this year.
The existence of liquid water under the frozen surface of the moon makes scientists believe that it is one of the places in the solar system with conditions in which life may exist. The moon’s ice depth is found to have vents that open up to the crust, like in the Earth’s oceans. Such ventilation can provide thermal conditions that are necessary to facilitate the evolution of life. If there is life on Europa, it may have been made possible by the comets deposits. During the early stages of the solar system, the comets may have deposited organic material from outer space to the moon. Also, a 2016 study reported that Europa generates about 10 times the amount of oxygen than hydrogen, similar to the situation on Earth. This might make its oceans to be more life-friendly and may not have to depend on tidal heating to produce adequate energy (Tamura, 2019). Instead, chemical reactions are sufficient to control the process.
Mars is a close neighbor of planet Earth and the fourth from the Sun. It is also the second smallest, after Mercury, and is known for its thin atmosphere which makes it impossible for liquid water to exist on its surface. Scientific research about the possibility of planet Mars supporting life has been underway for a long time now, and it seems that the process has become more objective over the years. The only thing that scientists believe Mars lacks is an atmosphere, which they have devised ways to create. Extensive studies on the surface of the planet have indicated that liquid water had once flowed on its surface, suggesting a possibility of the existence of water and life beneath.
On 7th June 2018, the National Aeronautics and Space Administration announced that organic compounds had been discovered in the sedimentary rocks by the Curiosity rover. These compounds were determined to date back up to 3 billion years, suggesting the presence of some of life’s essential building blocks sometime in the past (Dehant et al., 2019). The scientists also made another important discovery in 2018 of a sizeable sub-glacial water body, which still remains the first and the only mass of water discovered on the planet ever since. The lake is situated near the ice caps of the South Pole of the planet and sits about one and a half kilometers below the land surface and is approximately twenty kilometers long (Paine, 2018). Mars is the only planet in the solar system besides Earth, which has conditions that may support life. However, these conditions as they currently are, are still extreme such that nothing can live on the planet, except maybe under the surface. Nevertheless, in spite of its current habitability status, Mars indeed used to be a planet full of ocean water and suitable conditions for life. In 2001, NASA initiated the Mars Odyssey study that found large amounts of ice water under the surface of the planet, mainly at the top parts up to three feet or one meter deep (Chopra & Lineweaver, 2016). It is, however, difficult to determine whether there is more water underneath because the Rover cannot detect water any deeper.
The robotic rovers discovered clear evidence of the existence of rivers and lakes with liquid water distributed over the red surface of the planet thousands of years ago. This suggests that the atmosphere of Mars was at some stage in the far past thick enough to retain heat that could keep the water in the liquid state. Therefore, scientists believe that the current inhabitable conditions of Mars are temporary, and can be transformed through chemical reactions and other processes to make life there possible again.
Ganymede and Callisto
The two Jupiter moons, Ganymede and Callisto, are as large as other planets in the solar system and are made up of a combination of white ice and dark particles of dust. The interior of both moons is composed of a mixture of ice and rock or metal, although the interior of Ganymede seems to be divided by density while that of Callisto is usually mixed. Their sizes are also almost the same (Paine, 2018). Liquid water appears to exist below the surface of both moons in vast oceans that are sandwiched between the low-pressure ice on the surface and the high-pressure ice deep beneath its surface. The magnetic field of Ganymede is intrinsic and is produced through the dynamo action inside the inner core of molten iron. Callisto’s surface and some parts of Ganymede’s surface suggest that they have existed for a long time, because of many crater marks on its surface, as a result of the impact of asteroids bombardment, which occurred billions of years ago. The ice also decays slowly, subliming into the vacuum that surrounds them, leaving dark dust behind. Two-thirds of the surface of Ganymede was revitalized by intense stretching of the tectonic plates and probably cryovolcanism, during a tragic event in the history of the solar system.
Callisto and Ganymede are among the natural satellites that are on the list of places within the solar system, where there could be the existence of life. According to the data that the Galileo spaceship collected, the salty ocean of Callisto seems to interact with layers of rocks approximately 250 kilometers or 155 miles below the surface (Paine, 2018). This is an essential condition for the creation of life. The exosphere of the two moons has also been found to have substantial amounts of oxygen, which is a gas that supports life. Scientists, therefore, still continue to look for more evidence that would prove the habitability of these moons.