NASA Viking Missions Mark 50 Years Since Historic Search for Life on Mars

Fifty years ago, NASA’s Viking missions conducted the only direct search for extraterrestrial life in history. Launched in 1975 and touching down on Mars in the summer of 1976, twin spacecraft deployed specialized biology experiments that continue to spark scientific discussion and speculation over half a century later.

Reaching the Martian Surface in 1976

Finding life beyond Earth remains one of the most significant pursuits in human thought. To tackle that goal, NASA developed the Viking program, consisting of twin orbiters and landers managed by the NASA Langley Research Center. Viking 1 achieved Mars orbit on June 19, 1976, and touched down on the Martian surface on July 20, 1976—coinciding with the seventh anniversary of the first human Moon landing. Viking 2 followed, landing at a northwest site on September 3, 1976.

These landers achieved humankind’s first two successful soft landings of operational spacecraft on another planet. Beyond the quest for alien organisms, the mission carried equipment to photograph the landscape, map heat energy, measure wind and weather, study surface chemistry, and analyze soil samples.

Atmospheric Discoveries and Seasonal Pressure Shifts

The data sent back by the Viking missions fundamentally changed how scientists understood Mars. Measurements of the atmosphere suggested that the planet once possessed a much denser atmosphere before losing it over time. The hardware also recorded winds lifting tiny dust particles into the air, creating a permanently pink sky.

The landers also revealed that surface atmospheric pressure changes with the seasons across the entire planet. Mars features frozen north and south poles comparable to Earth. During the Martian summer, frozen carbon dioxide at the poles sublimates directly into a gas, condensing back into a solid when winter arrives. This unique cycle shifts atmospheric pressure by altering the balance of carbon dioxide across the planet’s surface.

Inside the Sterilized Chambers of the Viking Biology Experiments

The centerpiece of the mission’s search for extraterrestrial biology involved three distinct experiments. Each trial brought a Martian surface soil sample into a sterilized test chamber, exposing it to different nutrients under varying atmospheric conditions. Researchers monitored whether the air inside the chamber changed due to metabolic processes like breathing from organisms consuming the nutrient.

To track these reactions, scientists utilized radioactive carbon, carbon dioxide, or carbon monoxide within the nutrients. By tracking rising or falling radioactivity levels, researchers could determine if soil microbes were active. Radio commands allowed the team on Earth to heat up the chambers still inside the lander on Mars, destroying any potential microorganisms and halting metabolic gas production.

Comparing the Pyrolytic and Labeled Release Tests

The first setup, known as the carbon assimilation or pyrolytic release experiment, simulated the Martian atmosphere inside a test chamber by adding radioactive carbon dioxide and carbon monoxide to observe interactions with the soil sample.

The second setup, called the labeled release experiment, involved directly injecting a soil sample with a nutrient containing radioactive carbon. Investigators subsequently monitored the chamber for radioactive carbon dioxide. In this second experiment, investigators observed results that could have originated from a biological source, keeping decades of debate alive among scientists attempting to answer whether life exists beyond Earth.