Regardless of whether Abiogenesis did take place, or life was left behind by some intrepid explorers from an unknown star system, it was soon to end. Between 4.1 and 3.8 billion years ago the Moon, Mars, Mercury, and presumably Earth, and Venus were pockmarked by a massive wave of comets and asteroids. The cause is unknown, however some astronomers have theorized the bombardment could have been caused by the gas giants shifting positions.(1)R. Gomes et al. (2005) “Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets” Nature, Volume 435, Number 7041, Pages 466–469 This theory is far from accepted as the idea of gas giants shifting their orbits is difficult to accept, as it would involve a massive Delta-v (Δv) force, which has yet to be explained. Whatever the cause, this bombardment likely would have repeatedly sterilized any life in the inner solar-system.(2)H. Follmann and C. Brownson (2009) “Darwin’s warm little pond revisited: from molecules to the origin of life” Naturwissenschaften, Volume 96, Number 11, Pages 1265–1292 For hundreds of thousands of years the Earth was impacted by meteors and comets every century, causing craters of up to 5,000 kilometers (3,100 miles) in diameter.(3)G. Ryder (2000) “Heavy Bombardment on the Earth at ~3.85 Ga: The Search for Petrographic and Geochemical Evidence” Origin of the earth and moon edited by R.M. Canup et al. Pages 475-492 Some scientists speculate that life could have survived the Late Heavy Bombardment period in hydrothermal vents below the Earth’s surface.(4)Julie Steenhuysen (2009-05-20) “Study turns back clock on origins of life on Earth” Reuters
This era of bombardment is called the Late Heavy Bombardment theory, and is generally accepted by scientists, however the evidence for it happening is limited. All the hard evidence for the bombardment that has been collected has come from one source: the Moon. During the 1960s and 1970s the American Apollo Missions, and Soviet Luna Probes returned samples of Lunar rocks and soil that have been dated and support the Late Heavy Bombardment theory. Any direct evidence for the Late Heavy Bombardment on Earth would have been long ago erased by the movement of the continents, and the effects of wind, rain, and life on the surface of the Earth. The planets Mars and Mercury, as well as Jupiter’s moons Ganymede and Callisto show evidence of heavy bombardment, however until samples are collected from them and analyzed we Terrans are unable to corroborate the story the Moon is telling us about the Late Heavy Bombardment. Nevertheless it is accepted by the majority of scientists at this time, as there is no conflicting evidence.
This bombardment could have been triggered by several causes, including the theoretical moving planets theory, or passing too close to another star system. Another potential cause would be the intervention of an extraterrestrial civilization, however it it difficult to imagine why such massive bombardment would be required. Our limited knowledge of the earlier lifeforms is extremely finite and theoretical, however if Earth was the target of Direct Panspermia in its earliest period, it is possible Mars and Venus were too. If so, the Late Heavy Bombardment could have been an attempt to wipe out all traces of those colonies, either by an enemy civilization, or the colonists themselves as they ultimately abandoned the Solar System as it moved too far from their home system.
References [ + ]
|1.||⇑||R. Gomes et al. (2005) “Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets” Nature, Volume 435, Number 7041, Pages 466–469|
|2.||⇑||H. Follmann and C. Brownson (2009) “Darwin’s warm little pond revisited: from molecules to the origin of life” Naturwissenschaften, Volume 96, Number 11, Pages 1265–1292|
|3.||⇑||G. Ryder (2000) “Heavy Bombardment on the Earth at ~3.85 Ga: The Search for Petrographic and Geochemical Evidence” Origin of the earth and moon edited by R.M. Canup et al. Pages 475-492|
|4.||⇑||Julie Steenhuysen (2009-05-20) “Study turns back clock on origins of life on Earth” Reuters|