Emergence of two types of terrestrial planet on solidification of magma ocean

• Published in: Nature (London) 2013-05, Vol.497 (7451), p.607-610
• Main Authors: Hamano, Keiko, Abe, Yutaka, Genda, Hidenori
• Format: Article
• Language: English
• Subjects: 639/33/445/845; 704/445/845; Astrogeology; Atmosphere; Earth; Heat; Humanities and Social Sciences; Inventory; letter; Magma; multidisciplinary; Natural history; Net losses; Oceans; Planets; Plate tectonics; Science; Solar system; Solidification; Temperature; Terrestrial environments; Terrestrial planets; Water content
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature12163

Terrestrial planets can be divided into two distinct types on the basis of their evolutionary history during solidification from their initial molten state: type I planets (such as Earth) solidify within several million years and retain most of their water, and type II planets (possibly such as Venus), formed inside a critical distance, are desiccated by hydrodynamic escape. Earth and Venus are different class Accepted theories of planet formation commonly assume that planets with identical initial sizes and compositions would have the same early cooling history. But here Keiko Hamano and co-authors show that terrestrial planets fall into two distinct types based on their evolutionary history during solidification from an initially hot molten state. Type I planets, formed beyond a certain critical distance from the host star, solidify within several million years and retain most of their water, which forms the earliest oceans. On a type II planet, formed inside the critical distance, a magma ocean can be sustained for as long as 100 million years, and hydrodynamic escape desiccates these planets during this slow solidification process. Earth can be classified as a type I planet, but Venus formed close to the critical distance and its dry surface and mantle indicate that it might be a type II planet. Understanding the origins of the diversity in terrestrial planets is a fundamental goal in Earth and planetary sciences. In the Solar System, Venus has a similar size and bulk composition to those of Earth, but it lacks water 1 , 2 , 3 . Because a richer variety of exoplanets is expected to be discovered, prediction of their atmospheres and surface environments requires a general framework for planetary evolution. Here we show that terrestrial planets can be divided into two distinct types on the basis of their evolutionary history during solidification from the initially hot molten state expected from the standard formation model 4 , 5 . Even if, apart from their orbits, they were identical just after formation, the solidified planets can have different characteristics. A type I planet, which is formed beyond a certain critical distance from the host star, solidifies within several million years. If the planet acquires water during formation, most of this water is retained and forms the earliest oceans. In contrast, on a type II planet, which is formed inside the critical distance, a magma ocean can be sustained for longer, even with a larger initial amount of