Friday, April 3, 2015

The Conditions Needed for Other Life in Our Solar System

Does life exist elsewhere within our Solar System? Many people believe that life must exist somewhere else within our universe, but the question of whether other life exists within our Solar System remains unanswered. It is already known from studies on Earth that life can exist in extremely harsh conditions. Some microorganisms, known as extremophiles, have been found to thrive in conditions previously thought to be unsuitable for life. These organisms can live in water buried by sheets of ice in the Antarctic, or near incredibly hot thermal vents on the floor of the ocean. The fact that life can exist in such conditions gives new hope to the possibility of other life within our Solar System. At the same time, however, there are certain features required for extraterrestrial life to exist.

There are several characteristics regarding the composition of planets and moons that are required for them to sustain life. Perhaps most importantly, water is believed to be necessary. Life as we know it is based upon molecules and molecular interactions, and water is a medium that effectively allows life-sustaining chemical reactions to take place. Additionally, water’s high heat capacity and ability to expand when frozen allows it to help maintain a stable environment in which life can exist. In addition to water, however, many astrobiologists believe that carbon and other heavy elements must be present. Carbon’s unique ability to form a variety of chemical bonds and therefore create a diverse set of organic molecules allows for the establishment and growth of life.

In addition to the chemical properties of the planet or moon, it is also required that it be made of rock and be of a specific size. As opposed to a gaseous environment, a solid planet or moon has the potential for tectonic plates that create geothermal energy and recycle the raw materials on the surface. Relatedly, the size of the planet or moon is also extremely important. A planet that is too small will not have a molten core with tectonic plates or a magnetic field, nor will it have an atmosphere capable of protecting inhabitants from radiation. A planet that is too large, however, will have too heavy an atmosphere, which may prevent liquid water from existing. Planets or moons that support life will have a size within an optimal range, which astronomers estimate is somewhere between 0.2-10 Earth masses.

It is not only its composition that determines a planet’s or moon’s ability to host life, however. The position of the object in space is also vital. It is important that the planet exists within a “habitable zone” of its stellar system, or near another large planet. This habitable zone exists a certain distance from the central star, where planets and moons are far enough from the star to avoid high temperatures and radiation, but not so distant that they are extremely cold and frozen. Within our Solar System, astronomers estimate that the habitable zone ranges approximately from .95 to 1.4 astronomical units from the sun. Within the habitable zone, however, many planets or moons can still fail to sustain life. It is possible, for instance, that a planet can be tidally locked in orbit. In this case the side of the planet facing the central star would be too hot to support life, while the other side would be too cold. It is possible, however, for moons orbiting extremely large planets that are outside of the habitable zone to sustain life. In some cases the strong gravitational forces exerted by the planet on its moon cause it to have a molten core, and therefore support subsurface life, such as in salty oceans covered in ice. Therefore, while the habitable zone is a good indicator of where life could potentially exist, it remains a possibility for life to develop outside of it.

In our continuing search for life within our Solar System, we have determined that life can only exist in certain places. We know that there must be a specific composition of the planet or moon to be able to support life, and that it likely will exist within the habitable zone or near a large planet. Research within our Solar System has suggested that there is potential for life on Mars, Europa, and Enceladus, but further studies must be done before we can make any conclusions. Nevertheless, our expanding knowledge about what is required for life to exist gives us a basis off of which we can narrow and perfect our search for extraterrestrial life.

- Zac Ettensohn