The Martian Chronicles
Issue 9, Spring 2001

Searching for Life on Other Planets
Part I: Defining Life
by Elizabeth Tay

  "Searching for Life"... a galaxy  
"Searching for Life"... a galaxy

For centuries mankind has been scouring the skies in search of extraterrestrial life. But what is “life”? How do we know when we have found it? The Oxford Dictionary defines life as the “capacity for growth, functional activity, and continuous change until death”. But scientists fear that similar criteria for identifying life may not apply on other planets.

The first sign of life elsewhere will probably not be as obvious as little green men or purple eighty-legged animals. In fact, it is likely to be microscopic and maybe not at all similar to life on Earth. So, where and how should we search?

Criteria for Life

Before we begin our search, we have to work out what it is exactly that we are searching for. The first criterion for life (as we know it) is energy. Life requires energy to perform life-sustaining processes such as respiration. This energy can be in practically any form - geothermal heat, tidal energy, chemical energy, or sunlight. This does not appreciably narrow down the places where we should look.

The National Aeronautics and Space Administration (NASA) believes that the secret to life is liquid water, which translates into location. “Life cannot survive in hot conditions like on our sun. Life needs to be where it is not too hot and not too cold, and at a temperature at which liquid water can exist.” Thus, it is important to pay special attention to places which can support liquid water, either at the surface or subsurface of the celestial body.

The chemicals present in the atmosphere of a planet or a moon are important indicators of the presence of life. As per James Lovelock’s Gaia hypothesis, on any planet with thriving life, that life will have control over the atmospheric composition. This is because life will alter its surroundings in order to optimize them. For Earth-like life, this optimization takes the atmosphere far from the equilibrium state to a composition which cannot be achieved without the presence of life. Therefore, certain ratios of carbon dioxide, water vapor, and oxygen in the atmosphere are possible indicators that the astral body hosts life. While finding a non-equilibrium atmosphere is a sure sign of life, this is not a necessary condition. Oxygen appeared on Earth only about 500 million years ago, but we know that a whole world of bacteria thrived long before that, and life originated over 3.5 billion years ago.

Location, location, location

At 149.6 million kilometres from the Sun, Earth is pretty much in the perfect spot. As a result of its location and its significant greenhouse effect (CO 2 and water vapour in the atmosphere), Earth maintains an average temperature of 15 degrees Celsius – an ideal temperature for life to thrive.

Mars is the second most favourable place for life, and human habitation. This is due to its relative similarity to Earth – both are solid planets and are within the range of distances from the Sun which allow for liquid water conditions (the habitable zone). Similar to Earth, Mars also contains the elements required for Earth-like life - C, H, N, O, P, S, and water. Currently, Mars is too cold and has too thin an atmosphere to support liquid water on its surface or in the near subsurface. However, Mars is believed to have been very much like the Earth when the planets formed - and life on Earth arose. The reason behind the significant differences between the two planets today is thought to be Mars’ lack of plate tectonics. The carbon dioxide responsible for the greenhouse warming would have reacted with water to form carbonate rocks, but could not be recycled back into the atmosphere; therefore, Mars could not maintain a significant greenhouse warming.

Other celestial bodies with good prospects of hosting life (as we know it) include four of Jupiter’s moons - Europa, Enceladus, Titan and Io.

  An image of Io suggests volcanic activity -- a possible source of energy for life.  
An image of Io suggests volcanic activity -- a possible source of energy for life.

…Next: The search for life supporting planets in the search for extraterrestrial life…