Next stop, Phobos: A bold new chapter after 50 years of Mars exploration

Mariner IV was the first spacecraft to provide close-up images of Mars. Arriving on 14 July, 1965, it swept past the planet, coming as close as 9,846 kilometres. 

Mariner’s trajectory missed all the major features on Mars’ surface, so its 22 images (covering just 1% of the surface), suggested that Mars was a lifeless, cratered world, like the Moon – not the inhabited, canal-covered planet of science fiction. 

NASA’s Tidbinbilla tracking station, near Canberra, played a major role in commanding the Martian flyby and receiving its images. This famous picture, image no. 11, shows Mariner crater, named for the spacecraft. 

Image credit: NASA

In November 1971, Mariner IX became the first spacecraft to go into orbit around another planet. 

At arrival, Mars was covered by a massive dust storm that didn’t end until early January. As the dust began to clear it revealed spectacular features never seen before, such as this view of Olympus Mons, the largest volcano in the Solar System, rising over 21km. 

Mariner IX mapped 85% of the Martian surface, producing 7,329 images including detailed views of Olympus Mon, the massive canyon system, Valles Marineris, that dwarfs the Grand Canyon, the polar caps, and the moons Phobos and Deimos.

Image credit: NASA

Mars may not have had the canals of science fiction, but Mariner IX’s views from orbit showed canyons, channels and other features that looked like river systems and water-carved landscapes on Earth. 

These suggested that Mars may have been different in the past, with a thicker atmosphere, enabling running water on the surface and possibly even microbial life, if not green-skinned Martians! 

This image shows Nirgal Vallis, one of the longest river valley networks on Mars, named for Nergal, the ancient Babylonian god of war. 

The suggestion of water encouraged NASA to develop the Viking lander program.

Image credit: NASA

Astronomer Carl Sagan, who helped plan the Viking missions and select the landing sites, gives scale to this Viking lander mock-up.

Powered by small radioisotope thermoelectric generators, the Viking landers were designed to investigate evidence for water on Mars, and look for possible signs of life. 

In addition to cameras, the 883kilogram landers carried scientific instruments and three biology experiments designed to look for possible signs of life. 

Unfortunately, these experiments were inconclusive in providing evidence for the presence of living micro-organisms. 

The Viking-1 Lander operated until November 1982, while Viking-2 ceased operation in April 1980. 

Image credit: NASA

The solar-powered Viking Orbiters carried the two landers to Mars, contained in aeroshells below the spacecraft that were released into the Martian atmosphere for descent. 

The Orbiters then continued in orbit around Mars, acting as communications relays for the landers and studying the planet’s atmosphere. 

Extensive photography of the surface from orbit, capturing images that showed strong evidence of geological forms that are usually produced by large amounts of water. 

Viking Orbiter-2 operated until July 1978, while Viking Orbiter-1 continued until August 1980, making 1,489 orbits of Mars. 

Image credit: NASA

Compiled from 102 images captured by Viking Orbiter-1.

This view of Mars shows one of its most spectacular geological features, the Valles Marineris (Valleys of Mariner) canyon system, over 3,000 km long and up to 8 km deep. 

To the left of Valles Marineris, the large dark spots mark the Tharsis volcanoes. Mars’ reddish colour comes from its surface dust, which is a mixture of different minerals, including iron oxides. 

The water-rich iron mineral ferrihydrite is believed to be the main reason for Mars’ rusty colour, providing another possible clue to a wetter and potentially more habitable past. 

Image credit: NASA/JPL-Caltech/USGS

The first complete photograph taken on the surface of Mars.

This was captured by Viking-1 just minutes after it landed successfully on 20 July, 1976, showing a surface covered with rocks and dust. 

Each Viking Lander carried two identical cameras that used a moveable mirror which scanned a vertical segment of the Martian scene, with photodetectors recording the amount of light reflected into the camera. 

A complete picture, or "image" of the surface built up by completing a vertical scan, and then rotating the camera slightly for the next scan. 

The cameras produced black and white, colour and infra-red images.

Image credit: NASA

Viking-2 landed on 4 September 1976 (Australian time) 6,460 kilometres from Viking-1. 

The colour in this view of the rock-strewn plain of Utopia Planitia, with the horizon about three kilometres away, was established by using the colour-calibration charts visible in the picture. 

Originally expected to be blue, colour calibration revealed that the Martian sky is salmon or butterscotch in colour, caused by dust particles suspended in the atmosphere. 

The Viking Landers’ instruments provided weather reports from Mars, with images revealing phenomena like clouds, morning frost covering the ground and the effects of dust storms. 

Image credit: NASA/JPL-Caltech

Sunrise and sunset on Mars have their own unique beauty. 

Viking-2 captured this Martian sunrise on June 14, 1978, with the Sun just breaking the horizon on the Lander's 631st sol (Martian solar day). 

While sunrise and sunset on Earth are red, on Mars the sky appears bluish, because the fine dust in the atmosphere scatters the light to enhance the blue wavelengths. 

The sky glow above the rising Sun is also caused by dust and ice scattering the first light above the horizon. 

The bands of colour are artefacts caused by limitations of the camera technology of the time. 

Image credit: NASA/JPL/LaRC

Images of Mars taken by the Viking Orbiters gave strong indications that Mars once had plentiful water flowing across its surface. 

They showed features similar to river channels, lakes, deltas and flood plains, suggesting that massive amounts of water flowed across the Martian surface. 

For example, this detailed view of the Maja Valles area (note that the image caption is misspelled shows streamlined islands in an area that shows evidence of a huge flood. 

Images taken by later Mars probes indicates that several flooding episodes may have occurred in this area.

Image credit: NASA

Volcanoes on Mars.

Viking Orbiter images provided wide area views of Mars, such as this photomosaic of the Tharsis region, with its three massive volcanoes that are dwarfed only by Olympus Mons. 

Martian volcanoes became so huge because there was no tectonic plate activity to move geological ‘hotspots’ of upwelling magma across the planet. 

On the right of the image, at the end of the Valles Marineris is a large area of “chaos terrain”, a jumble of canyons, mesas, and large blocks of terrain cut up by chasms. 

Volcanic activity may have released massive floods of underground water, creating this landscape.

Image credit: NASA/JPL

Martian Moons.

Since they were first discovered in 1877, Mars’ moons Phobos and Deimos have fascinated astronomers because they are so small compared to the size of the planet. 

Viking Orbiter images of the moons improved on earlier Mariner IX views due to better camera performance and closer encounter distances, giving support for the theory that they might be captured asteroids. 

The images of Phobos revealed the massive Stickney crater for the first time, while Deimos proved to be smaller than expected and covered with thick dust. 

The forthcoming MMX mission will help to reveal further information about these moons. 

Image credit: NASA