Venus is Earth's twin in many ways, so its lack of liquid water oceans has perplexed scientists. A new study suggests that Venus might be about 7 million miles too close to the sun.
The northern hemisphere of Venus is displayed in this view of the planet's surface. A runaway greenhouse effect makes liquid water impossible on Venus's scorching surface, but a new study suggests most of Venus's water might might have been gone even before the planet cooled.
JPL/AP/File
EnlargeTwo planets ? Earth and Venus ? share similar sizes, bulk compositions, and underlying structures. They are the nearest of any two planetary neighbors in the solar system. So why does one have oceans while the other one doesn't?
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Therein lies a steamy tale of early planetary evolution, one whose different endings were determined from the outset by location, rather than by processes that sent the two on divergent paths later in their histories, according to a new study.
If the analysis holds up to further scrutiny, it not only could help answer the Earth-Venus riddle. It also could help scientists studying extrasolar planets pin down more precisely a star's habitable zone, or help them identify rocky planets in habitable zones that are still working their way through their molten youth, some researchers say.
"Of all the planetary-science questions we have, the question of why are the Earth and Venus different is the most gigantic and fundamental unanswered question we've got," says Lindy Elkins-Tanton, director of the Carnegie Institution of Washington's Department of Terrestrial Magnetism and a researcher who studies planet evolution.
If scientists want to say they know anything about what makes for a habitable planet, she says, "We'd better be able to answer that one."
Generally, ideas about how the differences came about fall into two broad categories, she explains. One envisions both planets starting out as dry. After they solidified, they accumulated water through comet and asteroid impacts. The other envisions both starting as planets with steamy atmospheres.
In both cases, Venus lost its water through a runaway greenhouse effect based on its closer proximity to the sun and the copious amounts of heat-trapping water vapor in its early atmosphere, reinforced by the lack of a carbon cycle, which partitions and recycles heat-trapping carbon dioxide (CO2) among oceans, living things, and rocks. Earth retained its supply of water because it has these and other features.
Each general explanation, however, presumes the planets had first cooled to host solid crusts.
The new work represents "the first model that suggests that the planets accreted with the same wet material, but Venus lost its water as it was solidifying, not afterwards," says Dr. Elkins-Tanton, who was not part of the research team.
The story, as set out by a trio of Japanese scientists led by the University of Tokyo's Keiko Hamano, begins with the generally accepted picture of rocky planets building from primordial, rocky chunks that dominated the inner regions of the disk of dust and gas that surrounded the young sun some 4.6 billion years ago.
Growth often was a violent process, aided by collisions with other massive objects trying to become planets. These collisions generated heat sufficient to periodically cover the planets with relatively deep oceans of magma.
Meanwhile, water was ubiquitous in clouds of gas and dust that gave rise to stars and planets. The recurring collisions that kept the crust molten before Earth and Venus solidified released the water bound up in the once-solid minerals as steam.
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