Why doesn't Venus have oceans? Study offers intriguing new theory.

Two 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?

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.

How much water did that degassing deliver? In 1986, another team of planetary scientists in Japan calculated an amount comparable to the water contained in the modern world's oceans, lakes, ice, rivers, and in the atmosphere as water vapor. Since then, researchers have refined those estimates somewhat – and collisions with comets and asteroids have helped Earth maintain its inventory of water – but the findings essentially stand. In other words, a day at the beach means swimming in the distilled remains of Earth's early atmosphere.

The researchers then built a model of the processes that would affect an early Venus or Earth – such as the melting and cooling of a basaltic crust, the water content of the rock, the transport of heat from magma oceans to the greenhouse atmosphere, and the transfer of heat from the sun to a planet at Earth's and at Venus's distance.

Even before the planets solidified, location made the big difference in this first-cut estimate. Earth's crust cooled substantially faster than did Venus's, according to the study.

Despite the powerful greenhouse effect from all of the water vapor in the atmosphere, Earth shed more heat than it was receiving from the sun (which was some 20 to 30 percent fainter than it is today). As the planet cooled, so did the atmosphere. Water vapor condensed and formed oceans. The oceans formed faster than ultraviolet light from the sun could destroy the water molecules feeding the ocean.

Meanwhile, Venus's position closer to the sun meant its atmosphere was warmer, slowing the pace at which the molten surface cooled, according to the model. This kept large quantities of water vapor in the atmosphere far longer than than on Earth, exposing more of it to destruction from the sun's ultraviolet radiation. The radiation cut the bonds between the hydrogen and oxygen, leaving the lighter hydrogen atoms to be swept away by the solar wind.

Whereas Earth solidified in a few million years in this experiment, a similar planet at Venus's distance and with far more water than Earth's oceans contain would have cooled over perhaps 100 million years. The amount of water left would have been only about 10 percent of the Earth's oceans, mostly tied up in rocks in the planet's interior.

The team suggests that the break point between a Venus outcome and an Earth outcome sits at about 0.8 astronomical units (AU), or about 74.4 million miles from the sun. Venus orbits at about 0.70 AU.

"This is a first-order look at a really new idea," Elkins-Tanton says, and as such, it leaves additional factors to fresh iterations of the model. For instance, the model doesn't account for the effect of the sun's increasing intensity with time on the processes the model describes.

This has important implications for figuring out if a newly discovered extrasolar planet is habitable, adds Ralph Lorenz, a planetary scientist at the Johns Hopkins University's Applied Physics Laboratory in Laurel, Md. A star like the sun intensifies with time, shifting the location of the habitable zone outward.

The carbon cycle, which makes Earth habitable, "relies on there being a lot of liquid water around. If you have a world that cooks out all of its water at the beginning, then even if its surface temperatures fall to nice levels later, it might not have enough water to have your warm little ponds" that serve as incubators for simple forms of life.