A Natural Formation Mechanism Of The Abnormal Rotation Of Venus And Mercury

The slow and retrograde rotation state of Venus is very peculiar among the planets in the solar system. Most planets have fast and direct rotation with a small obliquity, typical rotation period is about or even less than one day; but the rotation period of Venus is 243 days and its rotation is retrograde, that is to say Venus, direction of spin and revolution is opposite. Since the unique rotation state of Venus was discovered, the formation mechanism of the character has been an important researchful question in the evolution of the solar system.It is still a research problem now that the present rotation state is originated with the formation of Venus or evolved from direct to retrograde rotation afer its formation, and what mechanism has made the evolution. It is generally believed that Venus originated with the typical fast and direct rotation. But in the late evolution process, as a result of atmospheric tides and core-mantle friction inside the planet, the rotation is changed gradually. While some others considered that the change of Venus'rotation perhaps due to the early giant planetesimal's collisions. All these view can explain some questions, but exist some difficulties, the probability would not be very large. At the same time, if occasional emergency really occured, even it can explain the retrograde rotation of Venus, can't explain the slow rotation of Mercury.In this letter we discuss another possible mechanism that may lead to the abnormal rotation of Venus, the effect of interplanetary matter. On the base of the corporate formation hypothesis of the planets,we calculate the effect on the terrestrial planets spin from the action of interplanetary matter which is asymmetrically distributed in the radial direction. Our results show during the evolution process Venus could have gain reverse angular momentum from the asymmetric collisions with planetesimals and other interstellar matter around, then its rotation evolved to the present state from the typical state.Using the same model we also can explain the slow rotation of Mercury. This paper consists of four chapters.In Chapter 1, we introduced the general situation of the planets in the solar system, showed three formation hypothesis of the solar system and introduced the history and the present study situation of Venus, and the significance and purpose of our study.In Chapter 2, we expatiated our first model and result. On the basis of the corporate formation hypothesis, we discussed a possibility that the effect of asymmetric interplanetary matter changed the rotation of Venus and Mercury since they formed completely. We assumed the density of interplanetary matter were fixed. Our result showed under the assumed conditions, the current rotation of Venus and Mercury could be achieved in 5 billion years and 2 billion years respectively.In Chapter 3, we introduced our ameliorated model. we improved the influence on terrestrial planets spin, including the loss of the angular momentum due to revolution and spin, and the density of interplanetary matter changed versus time. Our results showed all the terrestrial planets were in the typical fast prograde rotation state before 4.5 billion years, and the rotation states of Venus and Mercury remained steady before about 3.5 billion years.In Chapter 4, we made a conclusion about our work, pointed out the simplicities and problems to be modified in our future work.