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Let's go to Mars

Kuiper finds carbon dioxide, and no water

A lead-sulphide cell on the 82-inch at McDonald Observatory found the first molecule ever confirmed in the air of Mars: carbon dioxide. Water vapour stayed hidden until 1963.

A large reflecting telescope inside an observatory dome, its dark tube tilted steeply upward. The near end is a wide closed mirror cell with instrument housings and bundled cables around a central opening; the upper half of the tube is an open black steel framework, and two small guide telescopes are clamped along the solid section. A curved catwalk, strip lights and the pale ribbed dome wall stand behind it.
The 82-inch Otto Struve reflector at McDonald Observatory in west Texas, photographed in 2014. Kuiper's infrared spectrometer, built around a lead-sulphide cell, was attached at this telescope's Cassegrain focus, the lower end of the tube, when it recorded the Martian carbon dioxide bands in October 1947. · Image: Carol M. Highsmith

In 1947 Gerard Kuiper observed Mars with the 82-inch reflector at McDonald Observatory in west Texas, using a lead-sulphide photoconductive cell: a wartime infrared detector far more sensitive at those wavelengths than the thermocouples then in use. The spectrum held two absorption bands, at 1.575 and 1.605 microns, far stronger than Earth's own atmosphere could account for. They were carbon dioxide.

It was the first molecule confirmed in the atmosphere of Mars; earlier claims of water and oxygen, going back to the 1860s, had come apart under scrutiny. Kuiper's estimate put at least as much carbon dioxide above the Martian surface as sits above a sea-level observer on Earth. He found no water vapour. Carbon dioxide remained the only confirmed constituent of the Martian atmosphere until 1963, when spectroscopists pulled water-vapour lines out of the planet's spectrum — a trace only, orders of magnitude drier than the driest desert on Earth.

Kuiper also aimed the instrument at the dark regions Lowell had read as seasonal vegetation. Green plants on Earth have an unmistakable jump in near-infrared reflectance, the red edge. Mars did not show it. Kuiper concluded that whatever darkened those areas was not higher plant life, and proposed lower plant life instead, comparing it to lichens, organisms that need no liquid water and could live off vapour. He was wrong, but it was a spectroscopic prediction others could test.

Through the 1940s and 1950s the accepted surface pressure was about 85 millibars, a twelfth of Earth's, with nitrogen assumed to dominate by analogy with Earth. The figure came from how the planet scatters and polarises sunlight, and it was wrong by more than an order of magnitude. Mariner 4's radio occultation in July 1965 gave 4 to 7 millibars, and carbon dioxide turned out to be essentially all of the air rather than a component of it. The crewed Mars designs drawn before 1965, von Braun's included, assumed enough atmosphere to land with wings.