Characteristics of Life

A Habitable Planet
Life exists in nearly every environment on Earth. Some environments have conditions so extreme that humans cannot live in them. These places might have extreme temperatures, high salinity levels, total darkness, or little water. Even though humans cannot live in these places, other organisms can. Despite the extreme conditions in which some organisms live, life on Earth needs the same basic things to survive: a source of energy, liquid water, and nutrients.

Humans have not yet found life anywhere else in the solar system. However, by studying the conditions that support life on Earth, they are learning about conditions that might support life elsewhere. Astrobiology is the Study of the origin, development, distribution, and future of life in the universe.

Source of Energy

The source of almost all energy on Earth comes from the Sun, a star. It is the perfect size to support life on Earth. Not every star in the universe is the right size to make life possible on its orbiting planets. The Sun is also the perfect distance from Earth, providing the right amount of energy for e to exist

Habitable Zone
The combination between the size of the Sun and Earth's distance from it make for the perfect place for life to flourish. Earth is located in what is called the habitable zone or “Goldilocks zone” of the Sun. The habitable zone is the area around a star where it's not too hot and not too cold for liquid water to exist on the surface of surrounding planets.

Extremophiles
Sunlight provides light and thermal energy. It also provides energy for plants, which are at the base of most food chains. However, a small percentage of organisms on Earth receives energy from chemicals or from Earth itself, such as the tubeworms that live in complete darkness near hot water vents in the ocean floor or bacteria that live in the Great Salt Lake. These organisms are called extremophiles. An extremophile is an organism that lives under extreme environmental conditions. These conditions can include extreme temperatures, high acidity levels, absence of oxygen, or a high level of salt.

Have you ever wondered what causes the many colors in the hot springs at Yellowstone? These colors are caused by extremophiles that live in the hot water. The hydrothermal features of Yellowstone are the ideal habitat for bacteria. They thrive on the chemicals and heat energy found in the springs.

Protective Atmosphere
Earth's moon receives about the same amount of sunlight as Earth. Yet conditions on the surface of the Moon are more extreme than they are on Earth. The Moon's surface temperature can rise to 100°G during the day and drop to ~150°C at night. Temperatures are extreme on the Moon because the Moon, unlike Earth, does not have an atmosphere.

Sustains Temperatures
Earth's atmosphere is like a blanket around Earth. It absorbs sunlight during the day and keeps heat from escaping into space during the night. Greenhouse gases in the atmosphere trap heat from the Sun, maintaining Earth's average surface temperature at a habitable 15°C. The natural process that occurs when certain gases in the atmosphere absorb and reradiate thermal energy from the Sun is known as the greenhouse effect. Gases such as carbon dioxide and water vapor trap heat in a similar way to the way that a blanket keeps you warm when you are feeling chilly.

Some planets have what is known as a runaway greenhouse effect, which occurs when the atmosphere is so absorbent that energy is radiated back to space from the upper atmosphere and not the surface of the planet. Venus has a runaway greenhouse effect.

Radiation Absorption
Have you ever spent the day outside playing soccer and come home with a painful sunburn? Sunburns are caused by the Sun's ultraviolet light. Even though you cannot see ultraviolet light, you can feel ts effects. The ozone layer, made up of the greenhouse gas ozone (Os), forms a protective layer around Earth that absorbs most of the Sun's ultraviolet radiation. The atmosphere also helps protect Earth from highly charged particles that erupt from the Sun, as well as X-rays and other potentially harmful energy from the Sun.

Air We Breathe
Earth's atmosphere is mostly made of nitrogen, but there are other gases that life depends on for survival. Much of life on Earth relies on oxygen and carbon dioxide. Plants release oxygen through photosynthesis, and we breathe it Earth's gravity keeps these gases from escaping into space

Earth essentially had an oxygen-free atmosphere until about 2.4 billion years ago. The evolution of blue-green algae known as cyanobacteria led to oxygen's existence i the atmosphere. These cyanobacteria evolved to use water to carry out photosynthesis, leading to their output of oxygen into the atmosphere.

Presence of Liquid Water
Liquid water is necessary for all life on Earth. Water dissolves minerals and transports materials in cells. Without liquid water, cells could not function, and life as we know it would not exist. Earth's atmosphere keeps temperatures on Earth's surface within a range where water can exist as a liquid.

Earth's unique among the terrestrial planets because it's the only planet with oceans of liquid water on its surface. Depending on temperature and pressure on Earth, water is solid, liquid, or gas. At sea level on Earth, 1 atmosphere (atm) of pressure, water is liquid between 0°C and 100°C. Without Earth's atmosphere, temperatures on Earth's surface would be too low for water to be liquid. Water would exist only as water vapor or ice. Scientists theorize that oceans may have given life a place to start before the development of the ozone layer. It acted as protection from the Sun's harmful ultraviolet radiation.

Nutrients
Living things are nourished by nutrients they take from the air, water, and land around them. Nutrients are used for energy, growth, and other processes, such as reproduction and cellular repair. Most molecules that provide nourishment for life on Earth contain carbon. They are organic molecules. Organic refers to a class of chemical compounds in living organisms that are based on carbon. Though it is possible that inorganic life could exist elsewhere, astrobiologists are most interested in places beyond Earth where liquid water is present and carbon is available.

Life Beyond Earth
Have you ever wondered what life beyond Earth would look like? You might imagine a planet filled with life identical to ours, or you might imagine humanoid aliens with large eyes and a big head. To find life beyond Earth, we must find conditions that could support life. Astrobiologists are constantly looking for signs of life on other objects in the solar system and out into the vast universe. Because liquid water is essential for life on Earth, scientists look for places where liquid water might exist or might have existed in the past.

Life in the Solar System
Scientists continue to look for further evidence of water on Earth’s Moon and on the moons of other planets. Even though temperatures in the outer solar system are extremely cold, scientists have found that as a satellite orbits a massive planet, the planet's gravity can cause the satellite’s interior to heat. There is enough energy to allow an ocean of liquid water to exist under the icy surface. Scientists suggest that the ridges on Europa, one of Jupiter's moons, could be cracks in the ice where liquid water has seeped to the surface and frozen solid. Enceladus, a moon of Saturn, erupts water from its subsurface ocean. Several other moons in the solar system, such as Triton, a moon of Neptune, show evidence of geysers.

Scientists have found that Mars once had liquid water flowing over its surface. Canyons and riverbeds on Mars's surface may have been formed by this flowing water. Today, ice can be found at both of Mars's poles. These ice caps are made of dry ice, or carbon dioxide. Scientists are also investigating the possibility of liquid water beneath the surface of Mars.

Life Beyond the Solar System

Liquid water is essential to life as we know it, and Earth is the only planet in our solar system with oceans of liquid water on its surface. Thousands of exoplanets have now been discovered around other stars. An exoplanet is any planet outside our solar system. Life on exoplanets could be much different than life as we know it, but it makes sense to start looking for life similar to ours. In order to target exoplanets that might have liquid water on the surface, astronomers look for exoplanets in the habitable zone of the star that they are Orbiting.