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.
In 1947 Gerard Kuiper pointed the 82-inch reflector at McDonald Observatory in west Texas at Mars with a new kind of detector on the back of it: a lead-sulphide photoconductive cell, a wartime infrared technology far more sensitive at those wavelengths than the thermocouples astronomers had been working with. In the resulting spectrum sat two absorption bands, at 1.575 and 1.605 microns, far stronger than Earth's own atmosphere could account for. They were carbon dioxide.
That is the first molecule anyone ever confirmed in the atmosphere of Mars; the claims of water and oxygen going back to the 1860s had all come apart under scrutiny. Seventy years of drawing the planet had produced maps and arguments. One new detector produced a fact. Kuiper's estimate put at least as much carbon dioxide over the Martian surface as sits above a sea-level observer on Earth. He looked in the same data for oxygen and for water vapour and found neither. Carbon dioxide stayed the only confirmed constituent of the Martian atmosphere until 1963, when spectroscopists finally pulled water-vapour lines out of the planet's spectrum — and then only a trace, orders of magnitude drier than the driest desert on Earth.
Kuiper also aimed the instrument at the dark regions that Lowell had read as vegetation flushing with the seasons. 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 floated lichens instead — organisms that need no liquid water and could live off vapour. That was wrong, but it was wrong in the right currency: a specific spectroscopic prediction someone else could go and test, rather than a drawing.
What the spectroscopy did not fix was the pressure. Through the 1940s and 1950s the accepted figure for surface pressure on Mars was around 85 millibars, about a twelfth of Earth's, with nitrogen assumed to dominate by analogy with home. It was derived 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 not a component of the air but essentially all of it.
That error mattered more than it sounds. The crewed Mars designs drawn before 1965 — von Braun's included — assumed enough atmosphere to land with wings. There was not.