Explore: Life on Mars
Life on Mars: Are We Alone?
Origin of Life?
Scientists believe that life on Earth may have begun as microscopic organisms in extreme underwater hydrothermal environments such as depicted here.
Credit: Lunar and Planetary Institute.
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?
How is NASA searching for life?
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
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.
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.
Deep Sea Hydrothermal Vent
One analog environment is a hydrothermal vent, a hot spring on the seafloor. It continuously spews super-hot, mineral-rich water that supports a diverse community of organisms. These vents occur along mid-ocean ridges (spreading seafloor) in all the Earth’s oceans, at an average depth of about 7000 feet (2100 meters). The creatures that live in darkness, from bacteria to tubeworms, may help us in identifying life beyond Earth.
Credit: NOAA (National Oceanic and Atmospheric Administration).
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.
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.
Stream drainage across the southern highlands of Mars. Viking Orbiter image 606A56. Credit: NASA.
Rover images of layers in the rocks at the martian surface. The thin layers are interpreted to be sediment deposited by flowing water. The “blueberries” are small, BB-sized deposits of hematite. Hematite is a mineral that typically forms in water. Credit: NASA.
Outflow channels cut by flood waters in Ares Vallis. The blocky “chunks” in the broad channel at the bottom of the image are displaced blocks of material pulled from the walls of the channel as the water rushed along. Credit: ESA/DLR/FU Berlin (G. Neukum).
Water and carbon dioxide ice (“dry ice”) occur in the southern polar ice cap of Mars.
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.
Scientists stand in the midst of three generations of NASA’s Mars rovers (Pathfinder’s Sojourner, MER’s Opportunity/Spirit, and MSL’s Curiosity). Curiosity is the largest and most technologically advanced rover to date. Credit: NASA.