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Let's go to Mars

Mariner 4 photographs a cratered, near-airless Mars

Twenty-one grainy frames replaced the canals with craters, and a radio occultation put the surface pressure near one percent of Earth's.

A square, coarsely dot-screened greyscale frame. A bright, almost featureless expanse — Mars — fills the upper part and gives way to black space below along a soft, gently curved diagonal edge, the planet's limb. Black registration marks are printed over the picture: a cross at the centre, brackets near the corners and edges. Ruled tick marks border all four sides, and a machine-printed frame code runs sideways up the right edge.
The first television picture returned from Mars: Mariner 4's frame 1, taken at 00:18 UT on 15 July 1965. It shows the limb of Mars against space near the boundary of Elysium Planitia and Arcadia Planitia; this file is a contrast-enhanced version of that frame. · Image: NASA/JPL-Caltech/Dan Goods

Mariner 3 launched on 5 November 1964 with a payload shroud whose inner fibreglass layer had delaminated from the outer skin; it would not jettison. The spacecraft carried that dead weight, never reached a proper trans-Mars trajectory, and drained its battery before the solar panels could open. A team from Lewis Research Center, Lockheed Missiles & Space Company and JPL modified the nose fairing in 17 days and got its twin, Mariner 4, off the pad on 28 November, near the end of the window.

Closest approach came at 01:00:57 UT on 15 July 1965, at 9,846 kilometres above the surface. The camera took 21 complete pictures plus 21 lines of a twenty-second one, each frame 200 by 200 pixels with six bits of grey per pixel. The radio link ran at eight and a third bits per second, so a single 240,000-bit frame took about eight hours to transmit. The whole mission returned 5.2 million bits. Engineers in JPL's telecommunications section tore off the printed strips of raw numbers, pinned them side by side on a display panel and coloured them in by hand with pastels from a nearby art shop.

The pictures showed craters and a battered, Moon-like surface: no canals and no oases. Percival Lowell's Mars ended that week; the public reaction cast the planet as dull and not worth the trouble. The radio occultation experiment, which watched the signal bend as the spacecraft passed behind the planet, gave a surface pressure of 4.1 to 7.0 millibars, roughly one percent of Earth's at sea level, with daytime temperatures near -100°C. At that pressure liquid water cannot exist on the surface. Mariner 4 also found no magnetic field, so the atmosphere and ground lie open to solar and cosmic radiation.

Mariner 4 imaged about one percent of the planet, and its track crossed the ancient, heavily cratered southern highlands, the oldest and least eventful ground on Mars; Tharsis and Valles Marineris lay outside the frame entirely.