The four planets closest to the sun are siblings of a sort. Mercury, Venus, Earth and Mars have similar core properties and densities, suggesting that they probably formed from the same dust cloud in the early solar system, but they have very different surfaces and atmospheres. Mercury is hot, has low gravity and almost no atmosphere, Venus is hot and dense, Mars is dry. Its average surface temperature is minus 55 C. Earth is the only planet covered with oceans.
But Earth and Mars may have once had something in common that would reverberate with us more directly than their core properties: life.
The search for extraterrestrial life has always aroused great interest, but usually more among the UFO-hungry public than scientists. That's now changing, and even distinguished academic journals such as Nature are discussing the topic, with one lighthearted article last month supposing that galactic law prevents aliens from making contact with Earthlings because we are too primitive and violent.
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