Viking 1 Mars Landing (1976): First Pictures From Mars

Viking 1 was the first spacecraft to land successfully on Mars and carry out a long scientific mission there. Its lander touched down on the plain of Chryse Planitia on 20 July 1976 and within minutes sent back the first clear photograph from the Martian surface. Together with its twin, Viking 2, it searched directly for microbial life, and the debate over its results has continued for nearly half a century.

Background

By the early 1970s Mars had been visited only by brief flybys and by orbiters. The American Mariner 4 had returned the first close-up images in 1965, revealing a cratered, Moon-like surface, and Mariner 9 mapped the whole planet from orbit in 1971–1972, finding giant volcanoes and dry river channels. The Soviet Union had tried to land several probes, but none had returned useful data from the surface: Mars 3 touched down in December 1971 and fell silent within about 20 seconds.

NASA approved the Viking programme in 1968. It was managed by the Langley Research Center in Virginia, with James S. Martin Jr. as project manager and Gerald Soffen as project scientist. Each of the two Viking spacecraft consisted of an orbiter, built by the Jet Propulsion Laboratory, and a lander, built by Martin Marietta. The total cost was roughly US$1 billion at the time.

Launch and the search for a safe site

Viking 1 was launched from Cape Canaveral on 20 August 1975 aboard a Titan IIIE-Centaur rocket, and Viking 2 followed on 9 September 1975. After a ten-month cruise, Viking 1 entered orbit around Mars on 19 June 1976.

The team had hoped to land on 4 July 1976, the American bicentennial. But the orbiter’s photographs showed that the chosen site in Chryse Planitia was rougher than expected, and the landing was delayed while scientists searched for safer ground. A new site farther west on the same plain was selected.

The landing and the first pictures

On 20 July 1976, seven years to the day after the Apollo 11 Moon landing, the lander separated from the orbiter. It slowed through the thin atmosphere with a heat shield, a parachute and finally three throttled rocket engines, and landed gently on the plain.

Because radio signals took about 19 minutes to travel from Mars to Earth, the descent was fully automatic. Minutes after touchdown the lander began transmitting its first image, a black-and-white picture of one of its own footpads resting on rocky soil. A panoramic view followed, showing a stony desert stretching to the horizon.

The first colour image, released the next day, showed a reddish-orange surface. Engineers at first adjusted the colours so that the sky looked blue like Earth’s, but after recalibration it became clear that the Martian sky is a pinkish or butterscotch colour, tinted by fine dust.

The search for life

The lander carried a compact biology laboratory with three experiments designed to detect the metabolism of microbes in the soil:

• Gas Exchange, led by Vance Oyama, which looked for changes in gases above a soil sample given nutrients and water.
• Labeled Release, led by Gilbert Levin, which added nutrients tagged with radioactive carbon and watched for radioactive gas given off.
• Pyrolytic Release, led by Norman Horowitz, which tested whether anything in the soil absorbed carbon from a simulated Martian atmosphere.

A robotic arm scooped up soil and delivered samples to the instruments. The results were surprising. The Labeled Release experiment showed a burst of radioactive gas, much as living organisms would produce, and the signal disappeared when samples were heated, which looked like a biological response.

However, a separate instrument, the gas chromatograph-mass spectrometer, found no organic molecules at the levels it could detect. Without organic material, most scientists concluded that the results were caused by highly reactive, oxidising chemicals in the soil rather than by life. Viking 2, which landed in Utopia Planitia on 3 September 1976, produced similar results.

A debate that continues

Levin and his co-investigator Patricia Straat maintained for decades that the Labeled Release had detected living microbes. In 2008 NASA’s Phoenix lander found perchlorate salts in Martian soil. Some researchers argued that perchlorate could have destroyed organic molecules when Viking heated its samples, so that the absence of organics was less conclusive than once thought. The majority view remains that Viking did not find life, but its results helped shape later missions that looked first for habitable environments and preserved organic matter.

What else the landers found

The biology experiments drew the headlines, but the landers did much more. Each carried a weather station that recorded temperature, pressure and wind every day. They showed that air pressure on Mars rises and falls by roughly a third over the year, because a large part of the carbon dioxide atmosphere freezes onto the winter polar cap and is released again in spring.

An X-ray instrument analysed the soil, which proved rich in iron and similar in composition at both sites, suggesting that dust is spread across the planet by global storms. Viking 2 later photographed a thin layer of frost on the ground during the Martian winter. Each lander also carried a seismometer, but Viking 1’s failed to unlock after landing, and Viking 2’s recorded mostly wind.

Meanwhile the orbiters photographed huge outflow channels and branching valleys, strong evidence that large amounts of water had flowed across Mars in the distant past.

Key people

Gerald Soffen led the scientific effort and became a public face of the mission. Thomas Mutch headed the lander imaging team, and after his death in a climbing accident in 1980 the Viking 1 lander was renamed the Thomas A. Mutch Memorial Station. The astronomer Carl Sagan, a member of the imaging team, helped popularise the mission’s discoveries.

Myths and facts

Myth: Viking photographed a carved face on Mars. Fact: On 25 July 1976 the Viking 1 orbiter imaged a mesa in the Cydonia region that, in low-resolution light and shadow, resembled a face. NASA itself pointed out the likeness at the time. Higher-resolution images from Mars Global Surveyor and later orbiters showed an ordinary eroded hill.

Timeline

• 1975, 20 August: Viking 1 launches from Cape Canaveral.
• 1975, 9 September: Viking 2 launches.
• 1976, 19 June: Viking 1 enters orbit around Mars.
• 1976, 20 July: The Viking 1 lander touches down in Chryse Planitia and returns the first surface image.
• 1976, 3 September: Viking 2 lands in Utopia Planitia.
• 1980, August: The Viking 1 orbiter is switched off after running out of attitude-control gas.
• 1982, November: Contact with the Viking 1 lander is lost after a faulty command.

Legacy

The Viking orbiters photographed almost the entire surface of Mars, and the landers measured its weather, soil chemistry and seismic activity for years. The Viking 1 lander worked for more than six years, a record for a Mars surface mission until the Opportunity rover passed it in 2010.

After Viking, no spacecraft landed on Mars for two decades, until Mars Pathfinder in 1997. Later missions, including the Curiosity and Perseverance rovers, built directly on Viking’s experience and on the scientific questions it raised.

Why it matters

Viking 1 turned Mars from a distant disc into a real landscape that people could see. It was also the first serious attempt to test for life on another world, and the unresolved questions it left behind continue to guide the exploration of Mars.