Our robotic probes of other planets have changed the way we see Earth. For the first time in our history we possess information about other planets that has enabled us to develop general theories about how our own planet works. Over the last fifty years planetary exploration has revealed many “laws” of planetary science. One of the most important of these laws is that a planet`s geologic evolution is determined by its size and composition.

Large planets are more differentiated than smaller ones, which heavier elements such as iron concentrated in their cores and lighter elements on their surfaces. This differentiation concentrates  heat-generating radioactive elements, which help drive volcanism , in their outer layers. Furthemore, large planets have more volume for their surface area and so they cool slowly, retaining some of the initial heat from the accretion process. Thus, we think that a planet`s size and composition determine the lifespan of the geologic heat engine on the planet.

Volcanism ended early on the smaller planets like Mars and Mercury, and on the Moon.  These smaller bodies have lost virtually all of their heat, and their surfaces have not appreciably changed in billions of years.  In contrast, Earth`s dynamic tectonic processes continually recreate the face of the planet. Most of Earth`s surface is younger than the surfaces of the Moon, Mars and Mercury.

But Earth is the only example we have for comparing large, dynamic planets with small, dead ones. If we are to develop theories about how Earth operates, we need more than one example. And that`s where our sister planet Venus comes in. Venus`s large size and high density, as well as its position in the solar system, suggest that it should be similar to Earth. Why, then, did Venus evolve so differently, with a thick blistering atmosphere and no oceans? Were the Earth and Venus more alike earlier in the history of the solar system? Could Earth become like Venus in the future?

We are getting to know our long separated twin sister planet a lot better recently. Spacecraft like Magellan and Galileo are teaching us more about Earth’s siblings and so about ourselves. Everyone should actively advocate the continuing exploration of the solar system.


Astronomy compels the soul to look upwards and leads us from this world to another. (Plato). Illustration: © Jorgensen

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