Viking Orbits Mars

Viking Orbits Mars

Although most press coverage of the Viking mission will stress the landing attempt, exploration started weeks earlier with both the basic research and attempt to find a safe landing site.

Viking discoveries began even before the spacecraft reached orbit. Far encounter pictures, made at about the moon`s distance from Earth, showed Mars half illuminated (a view never seen from Earth). Viking approached from the morning side of Mars, and photos showed light, hazy veils and bands over more of the planet than expected by some scientists. Low-floored impact basins, such as Hellas and Argyre, showed startlingly bright patches of frost and/or low haze as they emerged from Martian night into morning. By midday (the phase most clearly seen from Earth), such frost and haze “burns off” in the Martian sun.

Source of the photo: Nasa

Other photos showed unexpected details among the four huge volcanic mountains of the Tharsis region. All four mountains showed up as very dark spots – similar to early Mariner 9 views when the dark, lava cove red volcanos protruded through bright clouds of the 1971 Martian dust storm. But this year there was no dark storm! Why where the mountains so dark? They may have been protruding through morning mists that lighten the general tone of lower topography. Alternatively, they may have been stripped of light dust or coated with dark dust as a result of Mars’ active winds. Arsia Mons, the southernmost volcano, was especially altered since 1971-1972, with lobes of dark material extending north-east and south-west from its base. Again, there are probably products of dust removal or deposition.

The far encounter pictures were especially useful in filling a gap in Mars data: Previous missions had produced either “postage stamp” close-ups too detailed to show global patterns, or global images from too far away to reveal relations between dark marking and geologic detail.

Following For encounter studies, Viking I entered Mars orbit on June 19, 1976, with a 38 minute rocket burn. This slowed the spacecraft into a 43 hour orbit ranging from 930 miles (1,500 km) to 31,000 miles (50, 600 km). A three minute burn on June 21, 1976, brought the vehicle into an orbit that passes over the landing site every 24.6 hours, ranging from the same low point to a high of 20, 400 miles (32,800 km).

Early orbiter measurements indicated a greater quantity and diversity of water vapor that had been expected in the Martian atmosphere. Viking scientists interpreted this as evidence of daily exchange of water from soil to atmosphere, as might happen with diurnal melting of ice. The finding suggested that Viking may have arrived at a good time to look for biological activity, which is presumed to require water.

The prime choice for a Viking I landing site – selected many months before Viking neared the planet – was a relatively smooth plain known as Chryse (rhymes with icy). It is several thousand feet lower than most Martian terrain, meaning that the air pressure is higher than average. Though this pressure may be only one percent of that found at Earth’s sea level (less than that outside the cabins of the highest commercial jets), it is high enough to allow liquid water, should the temperature rise high enough. Liquid water, of course, would favor biological activity if there is any life on Mars – and the search for life is a prime motivation of the Viking mission.

A second favorable aspect of the Chryse region is that it is only 20 degrees north of Martian equator. On warm afternoons, temperatures should exceed the melting point of water. (The highest temperatures actually recorded by Viking through June 21 were around -31 degrees Fahrenheit, but these were morning temperatures).

Third and most important, Chryse lies at the mouths of four of the largest channels of Mars, named Shalvatana Vallis, Siums Vallis, Tiu Vallis, and Ares Vallis. These channels are believed to be beds of ancient rivers that ran with water perhaps 100 million years ago. If so, the water must have emptied into the Chryse region, perhaps forming a temporary sea or possibly evaporating rapidly. Thus, the Chryse region might have a good chance of presenting ancient bio-chemical effects that required water.

For these reasons, as Viking approached Mars, there was intense interest in seeing what detailed, close-up pictures of the Chryse landing site would reveal. Would Chryse be as smooth as had been hoped? The maximum slope that Viking could land on was about 20 degrees, and the maximum size rock that would clear the undercarriage without punching a hole in the lander was about 10 inches. Could such obstacles be avoided?

The first pictures to help provide an answer came on the night of June 22-23, after Viking went into orbit around Mars. Due to improved camera optics and electronics, they showed more detail than anything provided by Mariner 9. What they showed was somewhat frightening to Viking mission planners. Scientists are concerned, on the one hand, that the small scale structure of Mars is unexpectedly rough – but, on the other hand, there is great excitement at the new detail discovered. Some pictures have about 10 times the resolution of Mariner 9 frames. Cinder cones and other volcanic features have been found. Unknown craters have been recognized in regions that were obscured by haze during previous missions. Most exciting is confirmation of Mariner 9’s finding features such as apparent ancient shore lines along Martian channels, indicating that abundant water flow once occurred and carved out major landforms in some regions. Explanations of the geologic processes or climate changes that produced water in Mars’ ancient past will be a major goal of Viking explorations.

This color rendition of Mars as it would appear to a person approaching the planet was made from three separate black and white pictures taken through color filters – red, violet and green – on June 17, 1976 A Viking 1 closed to within 348,000 miles (560,000 km) of the red planet. The black and white frames were taken just seconds apart by one of the Viking orbiter`s two television cameras and radioed back to Earth. Corrections were made in a computer for the color response of the camera; the color photograph was then reconstituted on a TV screen. The Tharsis Mountains are clearly seen – a row of three huge volcanos standing about 12 – ½ miles (20 km) above the surrounding plain. Toward the top of this picture is Olympus Mons, Mars’ largest volcano. The circular whitish feature at the bottom of the disk is a large impact basin, Argyre. The area around Argyre is slightly brighter than elsewhere, probably because of the presence of discontinuous thin carbon dioxide ice on the surface. Several atmospheric features are faintly visible. West of the southernmost Tharsis volcano (left) is an irregular white area (which has been seen on two successive days by Viking 1) that has been interpreted as a water ice surface frost or ground fog; the faint, curved bands in the lower half of the picture probably are thin cirruslike clouds. The yellos at the edge of the planet’s limb is somewhat artificial, caused by extreme variation of brightness in the violet photograph. Source of the photo: NASA

By William K. Hartmann (Viking Orbits Mars, a text in Astronomy magazine, July, 1976)