|MadSci Network: Astronomy|
The question of the origin of life is currently a very hot topic. Currently, most people believe life originated in situ (on Earth) while others favor the idea that it developed on Mars and was transported to Earth early in the solar system. Venus is not seriously considered as a possible source for Earthly life. The reasons for this have largely to do with transportation issues.
First, to get a meteorite with life from one planet to another, an object like an asteroid or comet must impact with enough energy for some of the ejecta to escape the planet's gravity well. Venus is larger than Mars (meaning it has a much higher gravitational attraction) so a much more energetic impact is required for this. Hence, there would probably be more material ejected from Mars than from Venus. (Assuming that the number of impacts is the same; because Mars is closer to the asteroid belt, it might have had even more impacts, hence more chances for material to be blasted off it.)
Second, for a meteor large enough to transport life (say centimeter to a few meters in size) there are several processes which act to transport material inward (toward the Sun) but not outward (away from the Sun). For example, the Yarkovsky effect (significant for meter-scale bodies) is when photons hit the object and warm its surface. It rotates this warm spot to the afternoon side and radiates the heat away. Since radiation (photons) have momentum associated with them, this action serves to remove or add momentum to a meteor, depending on whether it rotates in a clockwise or counterclockwise direction with regard to the Sun. This would move it slowly outward or inward. This is a very slow process and moves meteors only a very short distance in a very long time. It may not contribute much to the direct delivery of a meteor to Earth, but it can move a body into a "resonance" where the larger gravitation of a planet could toss it to another part of the solar system. Jupiter does this often and there are regions of the asteroid belt which are devoid of objects because they have been cleaned out by resonances with Jupiter in this way. Mars is much closer to Jupiter and to these strong resonances, so objects likely to hit Mars have a greater chance of being transported to a resonance and then transported elsewhere by the much stronger effect of Jupiter's gravity.
Another effect is the Poynting-Robertson effect which acts on centimeter-sized particles. A particle orbits the Sun while the Sun's radiation goes straight out in all directions. In the particle's frame of reference, therefore, the photons seem to come preferentially from the forward direction. Again, photons have momentum so there is excess momentum hitting the forward direction and slowing the particle down, causing it to spiral inward.
These two effects mean that it is much more likely to transport meteors inward from Mars to Earth than outward from Venus to Earth. Very small dust particles are transported outward in the solar system by radiation pressure or the solar wind, but these particles would be unprotected from the Sun's high energy UV radiation and cosmic rays which over time can damage and kill even the most robust organisms.
The probability of getting a Venus meteorite to Earth is not zero, but it is very low---much lower than the probability of transporting a Mars meteorite to Earth. Currently Venus cannot sustain life. Its atmosphere is far too hot to support biological organisms, and the planet is devoid of water which life as we know it needs. In its early history, Venus could have had water delivered by comets, which may have been the source of Earth's oceans as well since it was probably too hot in the early solar nebula for the inner planets to form with any water. Also, the Sun was about 1/3 as luminous (meaning it gave off about 1/3 of the energy it does now). We really don't know what the early Venus was like. It could very well have been a congenial environment for life to begin. The primary reason for not considering it when talking about the origins of life is the difficulty of transporting life (or any material!) outward from Venus to Earth.
For more information on the Yarkovsky and Poynting-Robertson effects, see Moons & Planets by William K. Hartmann or any other introductory astronomy book with a chapter on celestial mechanics.
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