Venus Science Background

Human History of the Planet Venus

Venus lives an undercover life. To the naked eye, it glows brilliantly in the twilight sky—the Morning Star, the Evening Star. Through a telescope, it looks silky smooth, perpetually covered in clouds that envelop the entire world. To the ancient Greeks, it was the star of Aphrodite, the goddess of love. The Romans adopted the Greek gods and named it Venus.
But the Israelites had a different name for it: Lucifer. It was the fallen star for the fallen angel, thrown for his insolent pride down from the heavens to the horizon below. There he was doomed to remain for eternity, never to rise again, fading quickly in the morning light.
Perhaps the Israelites knew it best. For hidden beneath Venus' featureless mask is a surface belying a violent past: scarred by vicious impacts, crushing pressures, and temperatures hot enough to boil water—of which there's hardly any left.
Yet, for all of its mythological import, Venus also ushered in the scientific revolution. When Galileo looked at it through a telescope in the 17th century, he saw not the pinpoint of a star, but a disc, one that waxed and waned with phases like the moon. This told him that Venus must orbit the sun—and so must all the other planets, Earth included. It was one of the leading pieces of evidence that fueled the Copernican Revolution—the revelation that the sun, not the earth, was the center of the solar system.
Along with it came the additional realization that Venus and all the other planets that had been observed wandering the skies weren't just peculiar stars, but each a world of their own. In the 18th century, when astronomers observed a faint glow encompassing Venus and deduced that it had an atmosphere, they couldn't help but wonder if it, too, contained life.
By the midpoint of the twentieth century, it was known that the surface of Venus was hidden from view by a global, perpetual shroud of cloud cover. What lay below was still a mystery; scientists speculated that underneath those clouds were conditions resembling prehistoric Earth—a lush, swampy world that science fiction writers eagerly populated with fantastical creatures and beings.
But again, mythology and speculation gave way to knowledge, and by the 1960s, the first wave of interplanetary probes sent back observations that revealed a grim truth: the temperature on the surface of Venus was over 450° C (840° F)—far too hot for liquid water, and thus, life as we know it.
Today, we know that the atmospheric conditions on Venus make its surface a very alien world. The air is so dense that a 3 km/hr (2 mph) breeze would strike you with the equivalent force of a 150 km/hr (90 mph) hurricane. The atmospheric pressure is 92 times that on earth, crushing scientific instruments with a pressure equivalent to that nearly 1000 meters below the surface of Earth's oceans and eating away at them with the acidic gases of sulfuric and hydrochloric acid. No spacecraft that has dared to descend into the molasses of Venus' atmosphere has survived more than a few hours before going silent; very few have actually made it to the surface. Clearly, life as we know it could not be there today.
But that's not the end of the story—we still don't know how Venus got that way. Our models of how the solar system formed tell us that all of the inner planets—Mercury, Venus, Earth, and Mars—were formed out of the same cloud of material that included hydrogen and oxygen, the buildings blocks of water.
Did Venus once have oceans on its surface, sitting beneath clear, blue skies? Could Venus have once had life? Results from spacecraft sent to Mars are increasingly suggesting that our red neighbor was once a watery world, covered in oceans that could have harbored and sustained life. Could the same be true of Venus? If we are to understand the history of water in the solar system, we must understand the history of water on Venus.

Why Deuterium Matters

We hope to contribute to the understanding of Venus' possible watery past by making one important measurement of Venus' upper atmosphere: the amount of deuterium ("heavy" hydrogen - one proton + one neutron, which we label as "D") compared to the amount of protium ("regular" hydrogen, which we generally just label as hydrogen, or "H"). We write this crucial measurement as the ratio D/H. Both varieties of hydrogen are found in water vapor, but because hydrogen is lighter (atomic mass = 1), it leaks out into space faster than deuterium (atomic mass = 2). If we can measure the current value of D/H in Venus' atmosphere, we can work backwards to determine approximately how much water used to be on Venus—if there were once oceans, or if Venus has always been an arid, hellish place.