As Earth’s “sister planet,” many have long speculated about the nature of Venus and the possibility of life existing there in some form. Through much of the nineteenth century observers harbored hopes that Venus might be a place teeming with life. As R.A. Proctor wrote in 1870, because of its similarity to Earth “on the whole, the evidence we have points very strongly to Venus as the abode of living creatures not unlike the inhabitants of earth.”
In the latter part of the nineteenth century, however, a series of astronomical observations suggested Venus might be much less conducive to life similar to that seen on Earth than previously expected. R.G. Aitken, an astronomer at Lick Observatory, argued admitted that the possibility of life there “must be utterly desolate.”
Even so, and perhaps surprisingly, in the first half of the twentieth century a popular theory held that the sun had gradually been cooling for millennia and that as it did so, each planet in the solar system had a turn as a haven for life of various types. Although it was now Earth’s turn to harbor life, the theory suggested that Mars had once been habitable and that life on Venus was now just beginning to evolve. Beneath the clouds of the planet, the theory offered, was a warm, watery world and the possibility of aquatic and amphibious life. “It was reasoned that if the oceans of Venus still exist, then the Venusian clouds may be composed of water droplets,” noted JPL researchers as late as 1963; “if Venus were covered by water, it was suggested that it might be inhabited by Venusian equivalents of Earth’s Cambrian period of 500 million years ago, and the same steamy atmosphere could be a possibility.”
This theory was popularized by Svante Arrhenius, a Nobel Prize-winning chemist who reached millions with popular lectures and publications. Arguing for a tropical environment of more than 37.8 Celsius (100 degrees Fahrenheit) Arrhenius posited a strikingly wet atmosphere on Venus, one conducive to the rise of aquatic and amphibian life. He wrote:
We must therefore conclude that everything on Venus is dripping wet…A very great part of the surface of Venus is no doubt covered with swamps, corresponding to those on the Earth in which the coal deposits were formed…The constantly uniform climactic [sic] conditions which exist everywhere result in an entire absence of adaptation to changing exterior conditions. Only low forms of life are therefore represented, mostly no doubt belonging to the vegetable kingdom; and the organisms are nearly of the same kind all over the planet. The vegetative processes are greatly accelerated by the high temperatures.
Arrhenius speculated that more complex life forms might have evolved at the Venusian poles since the temperatures would not be quite as hot there, and with that “progress and culture…will gradually spread from the poles toward the equator.”
The director of the Smithsonian Observatory, Charles Greeley Abbot, took these ideas even further in the 1920s. He argued that Venus’s “high reflecting power seems to show that Venus is largely covered by clouds indicative of abundant moisture, probably at almost identical temperatures to ours.” Abbot even speculated that Earth might make contact with life on Venus, evincing his excitement at coming “into fluent communication by wireless with a race brought up completely separate, having their own systems of government, social usages, religions, and surrounded by vegetation and animals entirely unrelated to any here on earth. It would be a revelation far beyond the opening of Japan, or the discoveries of Egyptologists, or the adventures of travelers in the dark continent.”
Thus the debate over the climate of Venus portended a larger debate over the possibilities of life in the solar system. Venus’s atmosphere, the pressures it had, the presence or absence of atmospheric oxygen, H2O, and CO2 fundamentally informed this debate. It led to a succession of planetary theories concerning Venus. Measurement of these climate characteristics constrained theoretical models of planetary evolution while also restraining some of the more exotic speculations about Martian and Venerean life.
By the 1930s the detection of carbon dioxide in its thick atmosphere forced scientists grudgingly to abandon the idea that Venus contained a carboniferous swamp. The scientists investigating Venus replaced the pre-Cambrian environment, as Carl Sagan noted in 1961, for “an arid planetary desert, overlain by clouds of dust from the wind-swept surface.” They continued to search for water vapor, but failed to find it. What scientists found was carbon dioxide, a lot of it; a layer of gas roughly equivalent to a two mile deep ocean at a pressure similar to that of Earth. In 1939 astronomer Rupert Wildt postulated a “greenhouse effect” with temperatures far above what was present on Earth. As astronomer Ronald A. Schorn concluded, “By 1940 there was good reason to believe that conditions on Venus were harsh and life impossible.” Charles Greeley Abbot, for one, refused to change his perspective. He wrote in 1946 that “the conditions may possibly be as favorable for life there as on our earth.”
A few others agreed with Abbot. For example, in 1955 Soviet astronomer G.A. Tikhoff commented:
Now already we can say a few things about the vegetation of Venus. Owing to the high temperatures on this planet, the plants must reflect all the heat rays, of which those visible to the eye are the rays from red to green inclusive. This gives the plants a yellow hue. In addition, the plants must radiate red rays. With the yellow, this gives them an orange color. Our conclusions concerning the color of vegetation on Venus find certain confirmation in the observation…that in those pats of Venus where the Sun’s rays possibly penetrate the clouds to be reflected by the planet’s surface, there is a surplus of yellow and red rays.
Subsequent measurements largely overturned these idea about Venus as a planet teeming with life even before the dawn of interplanetary travel.
So bleak did the situation appear by 1961 that even Carl Sagan thought it unlikely that the planet has ever harbored life. He concluded:
At such high temperatures, and in the absence of liquid water, it appears very unlikely that there are indigenous surface organisms at the present time. If life based upon carbon-hydrogen-oxygen-nitrogen chemistry ever developed in the early history of Venus, it must subsequently have evolved to sub-surface or atmospheric ecological niches. However, since, as has been mentioned, there can have been no appreciable periods of time when Venus had both extensive bodies of water and surface temperatures below the boiling point of water, it is unlikely that life ever arose on Venus.
After carrying out ground-based efforts in 1961 to view the planet using radar, which could “see” through the clouds, and learning among other things that Venus rotated in a retrograde motion opposite from the direction of orbital motion, both the Soviet Union and the United States began a race to the planet with robotic spacecraft to learn the truth about the planet and its prospects for life. The Soviets tried first, launching Venera 1 on February 12, 1961. Unlike lunar exploration, however, the Soviets did not win the race to Venus; their spacecraft broke down on the way. The United States claimed the first success in planetary exploration during the summer of 1962 when Mariner 1 and Mariner 2 were launched toward Venus. Although Mariner 1 was lost during a launch failure, Mariner 2 flew by Venus on December 14, 1962, at a distance of 21,641 miles.
It investigated the clouds, estimated planetary temperatures, measured the charged particle environment, and looked for a magnetic field similar to Earth’s magnetosphere (but found none). Most important, it confirmed that the planet’s surface was an inferno. A report stated:
Earth-based measurements of microwave emissions from Venus had indicated a temperature of about 600 °F., but researchers did not—and could not—know whether the emissions came from the surface, from cloud layers in the atmosphere or from a dense ionosphere high overhead. The question was answered by a microwave radiometer aboard Mariner 2, which revealed “limb-darkening” (weaker emissions at the edge of the planet’s disk than at the center). The conclusion was not only that the surface was the hot part, but that, at about 800 °F., it was even hotter than the earth-based data had implied. An infrared radiometer, meanwhile, took temperatures high in the atmosphere, revealing, to the scientists’ disappointment, no breaks in the clouds.
Certainly, such an environment made unlikely the theory that life—at least as humans understood it—had ever existed on Venus.
Subsequent planetary spacecraft revealed that Venus was superheated because of the greenhouse effect of the cloud layer, and that the pressure on the surface was about 90 atmospheres, far greater than even in the depths of the oceans on Earth. Add to this the observations of James Pollack and others using aircraft-based near-infrared spectroscopy in 1974 that found on Venus a cloud sheet made predominantly of sulfuric acid, and the possibilities of life on the planet appeared as remote as they had ever been.
Although one would think that evidence from the spacecraft sent to Venus would be conclusive, overwhelmingly altering most of the beliefs held as recently as a generation ago about Venus as an abode of life, such was only partially true. For example, data from the Pioneer Venus mission suggested that in the distant past Venus had an ocean that may have existed for as long as a billion years, certainly enough to spawn primitive life, before sublimating the moisture into space. Planetary scientist Thomas M. Donahue reported that he and his team of researchers had found traces of water molecules in the upper atmosphere of Venus in 1993: “The data indicate that Venus was a pretty wet planet.” Notwithstanding, any beliefs held about Venus as a tropical, proto-organic planet have proven a bust.