VENUS is too hot. Mars is too cold. But Earth is just right for organic life that is now more than 3 billion years old. How could three planets of roughly similar size that were formed from essentially the same materials in the inner part of the primordial solar nebula nearly 5 billion years ago be so different from one another?
This is the so-called ``Goldilocks problem'' of planetary science. It sets the overarching scientific theme for the Soviet Phobos mission nearing Mars, and America's Magellan radar-mapping mission to be launched toward Venus this spring. When scientists solve it, they expect to have a much better understanding of the basic factors that determine the general features of this planet's environment.
Venus, with 94.9 percent of Earth's radius and 81.5 percent of its mass, has sometimes been called this planet's twin by astronomers. But its 460 degree C (860 F) surface temperature would melt lead. Its largely carbon dioxide atmosphere bears down on that surface with 90 times Earth's sea-level air pressure. And it has lost so much water that it now has 100,000 times less of that vital element than does Earth.
Those Venusian characteristics reflect the past and present action of processes that geologists call plate tectonics. Studying them with Magellan's relatively fine-grain radar mapping should fill a crucial gap in scientists' knowledge of how these processes operate on inner solar system planets.
This activity is most noticeable on Earth, where the crust consists of a dozen or so large plates. These are renewed over a roughly 200 million-year cycle as new material wells up from inside the planet, mainly along ocean ridges, and old material is reabsorbed. This recycling and associated volcanic activity releases gases - including water and carbon dioxide - to the atmosphere. Meanwhile, water-driven weathering and erosion involve chemical reactions that change atmospheric composition and lock up some constituents, such as carbon, in rocks.
Plate tectonics died out early on small bodies such as Earth's moon. Their surfaces contain clues, frozen in time, of how those processes operated during the first few hundred million years of their - and Earth's - evolution. Earth's much younger surface shows what happened during the past 200 million years. Venus may show something in between.
Radar mapping by earlier Soviet and American craft and by ground-based radars suggests that Venus has a tectonicly active surface with an age of 300 million to a billion years. These maps, at best, resolve details no smaller than 1 to 2 kilometers (0.625 to 1.25 miles) across. Magellan is to map 90 percent of the planet at a resolution of a few hundred meters. That's fine enough to show plate-tectonic action clearly. Noting this, mission scientist James Head of Brown University says he is certain Magellan will provide ``another major data point'' in the evolution of the inner planets.
The Phobos mission is after a different kind of bench mark. The Soviet-sponsored international project has the Martian moon Phobos as its prime target. This moon may be a captured asteroid or may have formed from the same part of the primordial nebula as did Mars. In either case, scientists say they think it has preserved an unspoiled sample of the primitive material in its interior.
Detailed study of such small bodies should find clues to the earliest stages in planet formation and evolution. Thus the Phobos mission is pursuing what project scientist Alexander Zakharov of the Soviet Academy of Sciences Space Research Institute has called ``one of the most urgent goals of solar system exploration.''