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Distance spared Earth from Venus-like fate

AFP-JIJI

Similar in size and often referred to as twin planets, Earth and Venus evolved from common origins into two contrasting worlds — one dry and inhospitable, the other wet and teeming with life.

The reason has had science stumped, until now.

Writing in the journal Nature on Wednesday, a Japanese research team said the answer is to be found in the planets’ respective proximity to the sun. Though relatively close on a cosmic scale — Earth is 150 million km from the sun and Venus 108 million km — the planets most likely orbit on either side of a “critical distance” from their central star, the team wrote.

This would explain, the researchers contend, why two similar-sized planets, almost identical in their molten state at the moment of creation about 4.5 billion years ago, can look so different once solidified.

At some 12,000 km, Venus’ diameter is about 95 percent that of Earth, and its mass about 80 percent. It orbits between Earth and Mercury, the closest planet to the sun.

As for their differences: Venus has no surface water and a heavy, toxic atmosphere comprising almost exclusively carbon dioxide; its average surface temperature is a searing 477 degrees.

The study authors said a type I planet like the Earth, formed beyond the “critical distance” from its host star, would have time to solidify from its molten magma state within several million years, trapping water in rock and under its hard surface.

However, type II planets, of which Venus may be an example, would remain in a molten state for longer, as much as 100 million years, as it got more of the sun’s heat — with more time for any water to escape.

Venus has not yet been categorized because it is so near to the line of critical distance, though its dryness would be characteristic of a type II planet, the team said. The new method may be useful in the study of planets beyond our own solar system — helping to determine which ones would be most likely to host life, they added.

“The present results indicate that for habitable planets, rapid ocean formation would have occurred within several million years of planet formation,” they wrote.

  • pbrower2a

    I once did some calculations that showed that under conditions close to those known at both the coolest and warmest times in the fossil record, the Earth could always hold methane, ammonia, and water vapor — but not hydrogen or helium. Such is significant to the extent that biochemistry depends heavily upon those three substances. That methane and ammonia cannot exist in equilibrium with an oxygen-rich atmosphere matters little. Neither Mars nor Venus can hold any of those gases — but they can both hold carbon dioxide easily. (The calculations show similar effects in what gases could predominate on either Venus or Mars — carbon dioxide and nitrogen).

    Life says much about the early chemistry of Earth. Venus and Mars both lost those three critical substances necessary for life. Any methane, ammonia, or water vapor eventually rose to the top of the atmosphere of Mars or Venus and got stripped of its hydrogen atoms to form carbon dioxide or nitrogen water vapor by harsh solar radiation or floated away to more congenial places. On Earth such gases could stay low in the atmosphere. Whether the disappearance of those vital gases happened faster on Mars than on Venus may never be known.

    Yes, you may say — but water forms oceans. But over time all water eventually enters the atmosphere as vapor even if it eventually returns to the liquid or solid state as rain, snow, or ice.

    …Venus is still intensely hot because of its dense atmosphere. Mars is cold because of its thin atmosphere. To be sure, Venus may preserve a very hot early stage of its history to some extent — but put Earth where Mars is and it would be much warmer than Mars is. Venus’ location would be a different story because the Earth could not radiate heat fast enough to prevent it from getting as hot as Venus. The greenhouse effect would keep Venus nearly as hot as it is if it were where the Earth is — or even Mars. If Mars ever had life it probably had it for a very short time or else life has retreated to havens probably under the surface. Any evidence of life on Venus’ surface would of course have been long destroyed if it ever existed.