Astrobiology - the Potential for Extraterrestrial Life

Astrobiology:

The Potential for Extraterrestrial Life

Michael Russo

Professor Nieter

Science Inquiry: Geoscience

Introduction

Astrobiology is the area of study dealing with alien life, the likelihood of finding it, where it might be found, how it evolved, its morphology, its culture, etc. It's a huge field that overlaps all the sciences but especially the geosciences, evolutionary biology, chemistry and bio-chemistry and astronomy. In the following report I focus on astrobiology as it applies to geology, or what kinds of planets, stars and solar systems are the most likely to play host to life in some form or another.

Basic Assumptions

Earth is the only planet in the universe we know supports life and the life indigenous to Earth is the only life we know of in the universe. Although it is possible, and in fact likely when considering the size of the universe, that life exists in forms that we can not at this point conceive of, we are limited in this field by what we know for certain, and that is that all life we've studied is carbon based, evolved in an environment rich in oxygen, nitrogen hydrogen and carbon, and uses water as a solvent to facilitate biochemical reactions. Although alternative biochemistries are entirely possible, and life that is not biochemical but rather electrical or mineral may also be possible, if we open the door to these possibilities this report might never end. For the sake of brevity the information I present here will be based on the assumption that life that life is organic, carbon based, is dependent on some sort of energy source, either tidal, geothermal or solar and requires some sort of solvent, either water, liquid ammonia or liquid hydrocarbons.

Division

Issues of habitability, that is, "the measure of an astronomical body's potential for developing and sustaining life" (http://en.wikipedia.org/wiki/Planetary_habitability), can be placed into three groups, planetary, solar and galactic (or other). Planetary has to do with the conditions present on the planet itself, which includes the presence or absence of natural satellites. Solar conditions have to do with the primary star and the conditions of the solar system and galactic the solar system's place in the galaxy (namely our galaxy, currently the only galaxy known to support life).

I. Planetary

The factors that are believed to be crucial determinants for the development of life are:

Mass- The planet must be the proper size to support life. Although the upper limit for planet sizes, if one exists, has not yet been determined because the Earth and Venus are the largest known terrestrial planets (although certain planets ten times the size of Earth and larger have been found in other solar systems, we have been unable to determine so far if these planets are terrestrial or gas giants). The lower limit for mass is somewhere between that of Earth and that of Mars. The mass of a planet determines its gravity and its geological activity (the higher the surface area to volume area of a planet the quicker it gives up its heat and becomes "geologically dead"). It's gravity in turn determines how well it can retain an atmosphere. Earth's is high enough to keep atmospheric particles from attaining escape velocity unless the particles are extremely light such as helium and hydrogen. An atmosphere allows for biochemical reactions to occur outside of an ocean, provides insulation and allows for better heat transfer as well as offers some protection against meteors and radiation. Geological activity, which requires a molten core, provides evolutionary forces, releases climate regulating carbon dioxide into the atmosphere and the dynamo effect, which requires a liquid iron core, generates the magnetic field that deflects the dangerous solar wind that would otherwise strip Earth's atmosphere and irradiate the planet.

Orbit- The orbit of a planet is its path around its primary star. The difference between the planet's nearest and farthest approach is called its orbital eccentricity. Earths orbital eccentricity is 0.02, very small, so its orbit is almost a perfect circle. In fact all the planets in out solar system with the exception of Pluto have very small orbital eccentricities, however this is not the norm. A study of the known planets in the galaxy has revealed that the average planets orbital eccentricity is 0.25. That means that the planet at it's closest is 25% closer to its primary than its average orbit and at its farthest is 25% farther away from its primary than its average (http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1611). Such a variable orbit would have a dramatic effect on the planet's weather patterns, the highest and lowest yearly temperatures probably being outside the range most organisms can maintain homeostasis at.

Rotation and Tilt- The day night cycle on a planet must not be too long, otherwise one side will overheat while the other freezes, such as would happen if the orbital eccentricity was too great. The tilt of the planet in relation to its equator is also a factor. The tilt of a planet determines seasons and the more pronounced the tilt the more drastic the seasons are, while the reverse is also true. Actually a less pronounced tilt is probably more harmful than a more pronounced tilt because climatic changes have played a major role in evolution.

Planetary Chemistry- For life as we understand it to exist it requires chemical building blocks, a solvent or medium for biochemical reactions to occur in and a way to store energy in chemical bonds. For Earth the chemical building blocks for all life are carbon, oxygen, nitrogen and hydrogen which can form into a great variety of complex structures known as amino acids, carbon being the key element for the complex bonds that allow for macromolecules to exist. On Earth our solvent is water and all life forms either exist in water or maintain an aqueous environment in and between their cells. Also, all life on Earth uses carbohydrates to store energy, and release the stored energy by breaking the strong hydrogen bonds in the molecules. A planet will have to have enough of certain elements either in its crust, oceans or atmosphere for life to evolve. It has been suggested that alternate biochemistries might exist, and even be better suited to life, that those that we know of on Earth. Such possible alternatives