Giants Influence When the Planets Formed

Jupiter is ten times larger than Earth (and over 300 times its mass), and it is by far the most massive planet orbiting the sun. The origins of Jupiter and its neighbor Saturn (the "Jovian" planets) differ from those of other solar system bodies in that these planets grew largely by direct accretion of gas from the solar nebula in addition to accretion of solids. They are a more democratic sampling of the nebula, and accordingly, they have elemental compositions that are fairly similar to that of the sun, mostly hydrogen and helium. Jupiter formed very quickly, and its rapid growth had major effects on the interior planets, especially those that were trying to accumulate just inside its orbit. For example, a terrestrial planet was in the process of forming about halfway between Jupiter and the sun, but because Jupiter formed first, its development was aborted. This failed planet is now the asteroid belt, a region where some of the original planetesimals and their fragments still survive but were never assembled into a real planet. The largest of these planetesimals is the asteroid Ceres, a rounded object 1000 kilometers in diameter.

The asteroids are the source of meteorites, and detailed examination of these ancient rocks provides deep insight into the nature of planet formation. Most meteorites are ancient "rubble pile" mixtures of materials as old as the solar system. They are the oldest radiometrically dated rocks. The meteorites indicate that initially there was a period of growth when colliding materials led to accretion but that later, during most of the solar system's history, collisions have occurred with such high energy that they have led to erosion and disruption, not growth.

The effects that aborted the formation of a planet in the asteroid belt also severely affected the formation of Mars. Mars is often described as the most Earth-like planet, but in fact it is only half the size and one-tenth the mass of Earth. Presumably, both Mars and the asteroid planet would have grown to the size of Earth if the rapid growth of their giant neighbor had not occurred. If this had been the case, the solar system might have ended up with three truly Earth-like planets, each with oceans and advanced life forms living on or near their surfaces. If Mars had been as large as Earth, it probably would have retained a denser atmosphere, and, with the increased radioactive heat that comes with additional mass, it is likely that it would have been more volcanically active, perhaps driving plate tectonics. (Because of its smaller size, the volcanic activity on Mars is only a few percent of what occurs on Earth.) A larger Mars would also have had a larger core, presumably producing a larger magnetic field. One of the most critical shortcomings of the planet Mars, from the point of view of hospitality to life, is that it almost entirely lacks a global magnetic field. Thus electrically charged particles (the solar wind) flowing outward from the sun played a major role in sputtering the Martian atmosphere off into space. A substantial, Earth-like magnetic field deflects the solar wind and protects the atmosphere from erosion.

If Earth had been a little closer to Jupiter, or if Jupiter had had a somewhat larger mass, then the "Jupiter effect" that aborted the formation of the asteroid planet and nearly ruined the formation of Mars could also have affected Earth, rendering it a smaller planet. And if Earth had been smaller, its atmosphere, hydrosphere, and long-term suitability for life would surely have been less than ideal.

Following the discovery that Martian meteorites arrive at Earth at a rate of half a dozen each year, some investigators have suggested that Mars played a role in seeding Earth with life. The reasoning is that Mars is tougher than Earth to sterilize globally. Ironically, this aspect of habitability is caused by the lack of a Martian ocean. During the first half-billion years of the history of the solar system, during what is called "the period of heavy bombardment," the terrestrial planets were hit by projectiles larger than 100 kilometers in diameter. On Earth, impacts of such magnitude vaporized part of the ocean, and heat from the impact and the resulting greenhouse effect could warm the entire surface of the planet to sterilization temperature. On Mars, with no ocean, such an impact could cause great regional damage but would not sterilize the whole planet. With its thin atmosphere, surface heating would also be more rapidly radiated into space. The low total abundance of water on Mars may thus be the result of these giant impacts, coupled with the planet's lower mass and surface gravity. If the early Mars did have oceans, they may have been effectively ejected into space by impacts. Even if there were more water on the young Mars, most of the early impact history of Mars occurred on a planet that was dry compared to Earth. If life evolved on both planets, it may have been destroyed on Earth, once or even several times, while it survived on Mars.

There are sound reasons to believe that life may have formed during a limited window of opportunity. This window of time may have closed before the end of heavy bombardment on Earth. It is thus possible that the present life on Earth is of Martian origin, transported to Earth by meteorites ejected by major impacts. If Mars had been Earth-like, with oceans, then it too would have been sterilized by impact. If Mars had been larger and had had a denser atmosphere, it would also have been much more difficult for impacts to eject meteorites into space.

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