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.