Explore: Life on Mars
Life on Mars: Are We Alone?
Within our solar system, Mars has always been in the forefront of our search for alien life. As missions shed new light on the Red Planet, we have new hopes for uncovering the fundamental conditions for life. More about Mars and its features is available at Explore Mars Inside and Out.
The new scientific field of astrobiology formed to investigate the origins, evolution, distribution, and future of life on Earth and beyond. Astrobiologists strive to address three questions:
- How does life begin and evolve?
- Is there life elsewhere in the universe?
- What is the future of life on Earth and beyond?
Through our efforts to understand how life began and evolved on Earth, we hope to determine where and how to best look for it elsewhere. The scientific field of astrobiology embraces the search for life both close to home (Earth) and far beyond. From laboratory and field investigations on Earth, to the exploration of Mars, the outer planets, and planets beyond our solar system, scientists are studying the potential for life to adapt and thrive beyond our home planet. This research requires partnerships among many fields of science, including molecular biology, ecology, planetary science, astronomy, information science, and space technologies.
How is NASA searching for life?
In 1998, in a concerted effort to address the challenges in finding life beyond Earth, the National Aeronautics and Space Administration (NASA) established the NASA Astrobiology Institute (NAI), competitively selected teams across the country that incorporate astrobiology research and training in their programs. For more information about the NAI and its teams, please visit: http://astrobiology.nasa.gov/nai/
What is Life?
Astrobiologists need a working definition of life — a set of criteria for something to be considered alive —in order to search for life. Defining life is not easy. Nonliving examples like viruses and computer programs display one or more of these same characteristics as life (such as making copies of themselves and using energy). Scientists have collaborated to develop a set of general characteristics of life:
- Life stores and uses energy
- Life engenders more life (reproduces and/or grows)
- Life responds to its environment (external stimuli)
- Life changes (evolves and adapts) over time
All Earth life is organized in essentially the same way: it is based on the chemistry of the element carbon, it requires liquid water, it engenders further life via DNA and/or RNA, it uses phosphate molecules to store energy, and it uses protein molecules to respond to and affect (influence) its environment. All life on this planet adheres to these basic principles.
What Does Life Need?
Life as we know it needs an energy source, nutrients (something to eat or consume), protection from the elements, and liquid water. These four main requirements have been the focus of our search for life in the universe. Scientists are looking for places in our solar system — and beyond — that have all the things that we know life needs.
Of the four identified necessities for life, the presence of liquid water is considered to be one of the most important and perhaps useful to scientists. We have only found living organisms where liquid water exists. Pure water is a liquid over a fairly wide range of temperatures — between 32°F (0°C) and 212°F (100°C). Under special circumstances, however, water can remain a liquid beyond this range: at high pressures (like at the bottom of the ocean or deep in the Earth’s crust), water can remain a liquid at higher temperatures, and saline water (water containing salt, like our ocean water) can remain a liquid at lower temperatures. Scientists are interested in identifying locations in the universe that possess water — especially liquid water — to better narrow their search for life beyond Earth! (More information about water is at Explore Ice Worlds “All About Water” and “Ice in the Solar System”.)
All organisms require some form of energy to run their life processes (like growing, moving, and reproducing). The organisms that we are familiar with primarily use light energy or chemical energy. Plants get their energy from light. Light energy diminishes with a planet’s distance from the Sun, and with distance below a planet’s surface. If light energy is absent, then there must be an alternate energy source. Microbes at Earth’s deep-sea vents get their energy by breaking down chemical compounds dissolved in water.
All organisms also require nutrients, the minerals and other chemicals used to maintain and grow their bodies and structures. Plants get nutrients from soils and the atmosphere. Animals get their nutrients as food by eating plants or other animals. Life must have a continuing source of nutrients, not only for an individual plant or animal, but over long periods of time so that the plant-animal communities can continue.
Finally, all organisms require protection from the extremes of the environment. This protection may provide the environmental stability necessary for the development and continuation of life. Rock layers and deep water can protect life from dangerous radiation from the Sun and some impacts; many organisms on Earth live underground or deep in the ocean. (More information about the Sun’s dangerous radiation is available in the background information for Explore UV Kid). A planet’s atmosphere can provide some protection from hazards (like ultraviolet radiation and extreme temperature variations) and allows access to sunlight as a major source of energy. However, to serve as an effective shield or insulator, an atmosphere has to be fairly substantial, such as those on Earth, Venus, or Saturn’s moon Titan. A small-sized body such as Pluto or Earth’s Moon has too little gravity to hold onto a significant atmosphere, making life on or near the surface difficult. On Mars there is very little atmosphere to protect living things from the Sun’s radiation.
What can life tolerate? Extremophiles
Much of the research taking place in astrobiology emphasizes the environment and habits of extremophiles — organisms that thrive in conditions that we would consider “extreme” and life-threatening (e.g., very high or low temperatures, very salty or acidic water). Extremophiles can live where most organisms cannot because they have adapted special mechanisms for survival. Any life beyond Earth may be found in harsh conditions. By studying analog sites on Earth — places that have similar environmental conditions to those beyond Earth (such as Mars) — scientists are exploring the processes that allow these resilient organisms to survive.
Extreme environments may include extreme depths, pressures, alkaline or saline waters, or severe radiation conditions. The majority of these extremophiles are microbes which closely resemble fossilized remains of earliest life on Earth and thrive in environments very similar to the conditions that scientists think fostered the origin of life as we know it.
Life on Mars?
All life as we know it requires liquid water. There is good evidence that liquid water once flowed and ponded on the surface of Mars, so it is possible that life could have become established there. The first evidence for life on Earth is in rocks that formed about 3.5 billion years ago. Life may have taken up to a billion years to become established on Earth, although it may have happened more quickly, and so scientists consider this to be a reasonable timeline for Mars as well.
Conditions on much of Mars would have been suitable for life for about a half billion years, before the martian environment changed to colder and drier. However, Mars’ features suggest that there were occasional warmer and wetter periods after the first half billion years, and there may have been refuges for life, such as moist areas near warm volcanic regions. Given the harsh conditions, and lack of evidence, it is unlikely that life evolved into complex multicellular forms, like it did on Earth between 1 and 500 million years ago. Life on Mars — if it exists or existed in the past — would most likely have been in the form of microbes.
In the 1990s NASA scientists announced the presence of organic molecules, mineral features that could have been formed by biological activity, and possible microscopic fossils of primitive, bacteria-like organisms in a martian meteorite. They interpreted the features to have formed on Mars more than 3.6 billion years ago, and to be evidence that life existed on Mars. The results have been hotly debated in the scientific community. Many scientists believe the structures could have been formed by chemical processes, rather than biologic; such chemically formed features are known to exist. Others suggest that the organic signature is contamination from Earth. At present, few scientists are convinced that the features are evidence of life. Debate is a healthy part of the scientific process, and it has served an additional purpose — it has helped scientists better identify the “signals of life” and develop more tools in the identification process being used by astrobiologists today.
Losing the Atmosphere
Early Mars probably had a thicker atmosphere with more carbon dioxide and water vapor, provided by vigorous volcanic activity. This Mars was warmer and wetter, and the higher atmospheric pressure permitted flowing water at the surface. However, by about 4 billion years ago, Mars’ environment became cold and dry, as it is now. As Mars’ interior cooled, the gases and water vapor from the volcanism gradually dwindled and the magnetic field disappeared. Left unprotected, the atmosphere was worn away by the solar wind, and the martian surface was bathed in radiation.
Early Mars was wetter and warmer. Images obtained by Mars orbiters have revealed that the ancient southern highlands are covered by networks of stream channels similar to gently meandering river channels on Earth. The Mars Exploration Rovers and the Curiosity rover have found structures in the rocks that are created by flowing water, and minerals formed in salty, acidic water.
Some scientists have calculated that Mars may have had a global layer of water that was about 394 feet (120 meters) thick. About 4 billion years ago, things changed; Mars became cooler and drier. The thin atmosphere and low air pressure no longer permitted liquid water to exist at the surface, and the water may have been sequestered underground, either as a liquid or as ice. Occasional warm periods in Mars’ history resulted in melting of the subsurface ice and gigantic floods, recorded by outflow channels which form from catastrophic floods of water.
Much of Mars’ water is underground, either as a liquid or as ice. Mars’ northern and southern ice caps also contain water ice, as well as carbon dioxide ice. Mars’ northern ice cap is mostly water ice.
Missions to Mars: The Search for Signs of Life — Past and Present
Scientists will continue to work to identify where the conditions might be right for life on Mars. NASA has successfully conducted both orbital and lander missions to the Red Planet. The first successful missions, Mariner 4, 6, 7, and 9, launched over the course of the 1960s and early 1970s, were the first spacecraft to acquire and return close range images of Mars.
In the 1960s, a group of NASA scientists, engineers, and technicians designed an ambitious robotic mission to Mars, named Viking. The Viking mission was composed of four spacecraft (two orbiters and two landers) whose principal objective was to look for evidence of life. The landers dug soil samples from the frozen surface and looked for signs of respiration –– an indication of biological activity. Although the initial results were thought promising, Viking found no conclusive signs of life. However, it is important to note that these experiments were not very sensitive by modern standards.
Following the successes — and disappointments (no confirmed life) — of the Viking mission, NASA’s Mars Exploration program sent a series of missions to explore the surface features and history of Mars as well as its geology and water, but these missions did not search for signs of life.
The Mars Exploration Rovers, named Spirit and Opportunity, landed on the Red Planet in January 2004 as a part of three-month missions to look for signs of past water activity on Mars. Both rovers far exceeded their mission goals and expectations, making important discoveries about wet environments on Mars in the past and possibly at the present.
The latest mission to Mars, Mars Science Laboratory (MSL), is looking for the precursors (building blocks) of life and evidence of past habitable environments. MSL’s Curiosity rover is studying rocks, soils, and the local geologic setting in order to detect chemical building blocks of life (e.g., forms of carbon) on Mars in order to assess what the martian environment was like in the past.
The Mars Science Laboratory rover, Curiosity, is continuing the exploration of Mars and is specifically searching for signs that habitable environments existed on Mars in the past. Future missions include Mars 2020, a rover that will collect soil and rock samples in preparation for return to Earth by a future mission.