Habitability and Biology

What are the Properties of Life?

It is possible that life on other planets could be very different from what we are accustomed to on Earth. But in order to look for life in the universe, or even in our own Solar System, researchers must have a set of criteria by which to define what life is. Based on our current level of understanding, the best approach is to look for characteristics or traits that are common among many different living organisms.

Biologists have identified at least six properties that are shared by all living organisms on Earth:

1. Order: Molecules in living things are arranged in specific structures.
2. Reproduction: Living things have the ability to reproduce their own kind. Simple life forms, such as bacteria, reproduce by dividing and making almost exact replicas of themselves. More complex organisms reproduce sexually, so that their offspring have genetic material from two individuals. Offspring with traits from both parents have a greater chance of survival because they are better able to adapt.
3. Growth and Development: Living organisms grow and develop in patterns determined by heredity, the traits passed to offspring by parents.
4. Energy Utilization: Living things need to capture and use energy, a process known as metabolism. An example of such a process is photosynthesis, whereby plants convert sunlight into energy.
5. Response to Stimuli: Living organisms respond to changes in their environment.
6. Evolutionary Adaptation: Living things evolve in such a way that future generations are adapted to unique situations in their surroundings. For example, the hammerhead shark, considered to be perhaps the most highly evolved species of shark, has superior vision and sensory perception due to its hammer-shaped head. Organisms that cannot adapt to a changing environment decline or become extinct.

It is important to remember that some objects may have some of these properties but still not be a living organism. For example, fire uses energy, can grow, and responds to its environment (such as when it spreads rapidly in response to winds), but fire is not a living thing.

The Habitable Zone

Since the telescope was first invented in 1608, technological advances have made it possible for us to look at other planets in our solar system, and even far beyond to other galaxies. One of the primary motivations for exploring our solar system is to answer the question, "Are we alone?".

When considering what places in our Solar System might harbor life now or in the past, scientists must look at the conditions that make a planetary surface habitable. Most scientists agree that the presence of liquid water is the primary requirement for life. But can liquid water exist anywhere in the solar system?

Earth's distance from the Sun allows its surface to be within a precise temperature range that makes it possible for liquid water to exist. If Earth's temperature were much warmer, liquid water would evaporate or be lost to space; if it were colder, liquid water would freeze. The region of the Solar System where temperatures allow liquid water to exist on a planetary surface is called the habitable zone.

Several factors determine surface habitability. First, a planet's distance from the Sun affects not only surface temperatures, but also processes that lead to water evaporation. Due to its tremendous greenhouse effect resulting in extremely high surface temperatures, Venus is not considered to be habitable because it is too hot for liquid water to exist. At the other end of the spectrum, the planets and moons in the outer reaches of our solar system, like Pluto, are too cold to have liquid water on the surface.

Lastly, processes that lead to atmospheric loss play a key role in habitability. Most researchers agree that early Mars had a much thicker atmosphere, but it gradually lost its atmosphere over billions of years, which was directly related to the planet's dramatic cooling. In addition, many scientists believe that plate tectonics are a requirement for habitability, because they drive the carbon cycle. This cycle is a complex series of processes where much of the Earth's carbon is exchanged between the crust, oceans, and atmosphere. Earth is the only planet currently known to have active plate tectonics.

The inner boundary of our solar system's current habitable zone lies somewhere between the orbits of Earth and Venus. Optimistic estimates state that the outer boundary lies just beyond Mars, while more conservative estimates say that it lies between Earth and Mars. In theory, if Mars were larger and had retained its thick atmosphere, it could have a habitable surface today. Therefore, the outer boundary of the habitable zone could lie beyond the orbit of Mars.

But what about a planet or moon that lies outside the habitable zone? Is it possible for life to exist there? It is important to remember that the habitable zone, by definition, is the region where a planet could potentially have surface temperatures that would support liquid water, but a more accurate name for this zone might be the "zone of liquid water," as scientists now believe that habitability can occur outside this zone in certain conditions. For example, subsurface groundwater, which may still exist on Mars today, could harbor life, as well as subsurface oceans like the one believed to exist on Jupiter's moon Europa. Recent images taken by NASA's Cassini Spacecraft show what may be liquid water reservoirs that erupt in Yellowstone-like geysers on Saturn's moon Enceladus.

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