Scientia Press

The significant presence of water on Earth has long stood out as an anomaly in the formation of the solar system.  As a light molecule, water was generally pushed by solar wind out beyond the "snow line" around 4.5 AU (Astronomical Unit--the distance of Earth to the Sun) where it can be found in abundance in the ice giants Uranus and Neptune as well as in moons and comets. 

A common explanation for the oceans--that Earth was bombarded by water-bearing comets--has never been substantiated and has always seemed hard to believe.  The ratio of deuterium to hydrogen in comets does not match the ratio in the water on Earth, except for those that formed close to the orbit of Jupiter.  Also, the flux of comets required would be several orders of magnitude larger than appears realistic.  Similar considerations hold for asteroids, many of which are not known to carry large amounts of water.  However, some asteroids from the asteroid belt and from the area between the early-stage Jupiter and Saturn may have delivered a small amount of water to Earth, and embryos from the outer asteroid belt might have delivered much more.(1)

A second model--that icy planetesimals from the outer solar system drifted into the early inner nebula--has been proposed to account for the presence and movements of water in the inner solar system in its nascent years.(2)  But no claim is made that this model actually accounts for the large oceans on Earth or for the discrepancy between them and the much smaller amount of water on Mars and Venus.

New theory and findings regarding the origin of Venus, however, can explain why Earth has so much water. 

We now have a commonsensical explanation of ancient myths that Venus emerged from Jupiter--that Venus was pulled from the outer solar system into the gravitational field of Jupiter and passed through the periphery of the gas giant, thereby heating up, losing its presumed ice, gaining a rock-filled cometary tail stripped from its own surface, and being steered into the inner solar system.  All this seems to have happened around 2500 B.C. when Venus first began to be mentioned as a comet by the ancients.  In addition, an Egyptian iconographic source clearly depicts Venus as a comet and can be considered probative.

The similar heating and resurfacing of the large moon of Jupiter Ganymede may be traceable to a similar peripheral passage of Jupiter, except that Ganymede lacked the momentum of Venus and so was unable to escape the gravitational field of the gas giant.

Jupiter's Pull

In the early years of the solar system, Jupiter's gravity is generally thought to have directed a high number of planetesimals into orbits on the fringes of the solar system but also some into the inner solar system.  So, too, we can suppose that at an early stage in Earth's history, it was orbiting outside of Jupiter--perhaps in the slot between Jupiter and Saturn, given the similarity of Earth's ratio of deuterium to hydrogen to that of near-Jupiter comets noted above--and then was pulled by Jupiter into the inner solar system around 4.5 billion years ago.(3)  Unlike Venus, Earth did not pass close enough to Jupiter to have its ice burned off. 

When this water-laden Earth reached the vicinity of its present orbit, it collided with a small, dry planet that had accreted from planetesimals in this area.  Out of this collision emerged the Moon.

The anomalously high level of neon in the Earth's atmosphere (4) may also be a legacy from the time of Earth's origin in the area beyond Jupiter.

While Jupiter's gravitational pull ensures that no large object can remain for more than perhaps 30 million years in the Jupiter-Saturn slot (5), rapid accretion in the early solar system would have permitted Earth  quickly to attain a high mass by impacts with planetesimals.  Meanwhile, we know that the regions around the outer planets are exceptionally clear of debris, suggesting that it was all swept up long ago by Saturn, Uranus, and Neptune.  But there is one exception.

The slot between Saturn and Uranus appears to contain zones where debris could have orbited from the beginning of the solar system until the present without being vacuumed up into these large planets.  However, this area  is also very clear of debris.  How to explain this?  One explanation would be that this was the slot of Venus, which cleaned up this debris until, around 2500 B.C., Saturn's gravitational pull, a collision, or some other cause steered it in the direction of Jupiter, which directed it into the inner solar system.  Until then, such a distant planet would not have been visible to skywatchers on Earth.

The residual water in the form of ice at the poles of Mercury suggests that Jupiter's gravity steered Mercury, too, into the inner solar system, where the nearby Sun burned away any ice or water not sequestered in the polar regions. 

Evidence of large, sudden water flows on early Mars suggests that it also originated in the outer solar system, then was steered by Jupiter--perhaps roughly at the same time as Earth--into its present orbit.  Its ice would have melted (assuming internal heat in its early years combined with a new location closer to the Sun--and perhaps an initially highly eccentric orbit that brought it close to the Sun).  Much of the resulting water would have evaporated and have been lost into space.

Various physical features of Venus--e.g., that it appears old yet has a new surface, that it contains 150 times as much deuterium relative to hydrogen compared to Earth (a sign of significant water in the past as well as perhaps of a near-Neptune original orbit), and that it seems to have a residual tail (the famous Black Drop)--match the explanation that it passed through the periphery of Jupiter into the inner solar system.  In light of this, the perception that Earth appears to have undergone a parallel process (but without coming too close to Jupiter) billions of years before should challenge scientists to hunt for similar kinds of evidence regarding the Blue Planet.

Conclusion

Why does this theory--the Outer Solar System Origin (OSSO) of the Terrestrial Planets--deserve credence?

First, OSSO helps explain the absence of debris in the outer solar system, and in particular in those zones between the orbits of Saturn and Uranus where such debris could have persisted from the early years of the solar system.

Second, it neatly accounts for many otherwise hard-to-explain features of Venus and for abundant related evidence from ancient times, as well as the similar features of Ganymede.

Third, OSSO powerfully explains the distribution of water on the terrestrial planets and above all the major anomaly of the large oceans on Earth. 

Notes:

1.  A. Morbidelli et al. (2000)  "Source regions and timescales for the delivery of water to the Earth".  Meteoritics & Planetary Science 35, 1309-1320

2. Cyr, Kimberly E. et al. (1998)  "Distribution and Evolution of Water Ice in the Solar Nebula:  Implications for Solar System Body Formation", Icarus 135, pp. 537-48

3.  The idea that the origin of the Earth as well as Venus was connected with Jupiter can be found in James P. Hogan.  Kicking the Sacred Cow:  Heresy and Impermissable Thoughts in Science.  Riverdale, New York:  Baen, 2004.  Hogan hypothesizes the emergence of the two planets from the gas giant.

4.  Morbidelli et al., Ibid.

5.  Stuart Ross Taylor.  Solar System Evolution:  A New Perspective.  New York:  Cambridge University Press, 1992, p. 27; see also the second edition (2001).  The details regarding the outer solar system are taken from Taylor and from Michael A. Seeds.  The Solar System.  5th ed.  Belmont, California:  Thomson, 2007.