Posted on Saturday, July 29, 2017
By Kadri

The Twin Sister Planets Venus and Earth: Why are they so different?  by Robert J. Malcuit.

In this book the author looks at what makes the planets Venus and Earth different at present age, and also what might have been the reasons behind those differences despite some similarities between them.

One of the main differences is seen as the Earth having a moon and Venus not having any natural satellites. Given that the presence of the Moon has had a stabilizing effect on the Earth's axial tilt and a slowing effect on the rotation rate, a lot of attention in this book goes to looking at differences that might be due to one planet having a sizable moon and the other not.

For that reason the formation theories of the Moon are presented starting with the fission model that was popular until 1930s, continuing with the co-formation theory and various capture theories and ofcourse the now widely supported Giant Impact model.

A lot of attention in this book has gone to the capture theories and showing how various initial conditions would lead to a different outcome which might have become the differences between the Earth-Moon system and the moonless Venus.

As the capture model plays a significant role in the structure of the book, a lot of space is dedicated to the interpretation of observational data from that point of view.

A part of the book is written in a question-answer style, which does certainly give the impression of a well-organized book, but it breaks up the reading-experience to small chunks.

It is interesting that the book also features the idea of soft-body impacts as the explanation for circular lunar maria and for mass concentrations under its surface, something that could have only taken place for a very short time in lunar history.

Getting back to the main topic of the book though - why are Venus and Earth so different? After looking at Earth-Moon system the author continues with a hypothetical model where Venus captures a satellite (named Adonis) of about half lunar mass into a retrograde orbit. As Adonis orbits Venus and it orbit evolves, it circles in closer and the crust of Venus would be reworked because of tidal forces. As Adonis reaches the Roche limit, it would break up and eventually the pieces would collide with the planet.

The book focuses on the effect that a captured satellite of about lunar mass would have on an Earth-mass planet and how the effects would be manifested in the planets crustal motion, atmospheric composition etc.


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