Long ago 4.5 billion years ago, right after the Earth formed our planet was hot. It formed by essentially getting slammed billions of times by asteroids, and all that energy of impact was enough to keep the Earth broiling.
Eventually heavy elements like iron and nickel sank to the core, and lighter ones rose to the surface. This differentiation created the Earth we see today: An inner, solid nickel/iron core that's nearly as hot as the surface of the Sun; an outer, liquid iron core; the solid, rocky mantle; and the thin crust topping it off.
The Outside/In[box]: Was The Origin Of Life A Singular Event?
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Mars is dead. As corpses go, it’s pretty interesting: volcanos so big they might have altered the planet’s rotation; enough atmosphere to support seasonal weather patterns; a cleft in its surface that makes the Grand Canyon look like a roadside ditch; polar ice caps; tantalizing, suggestive geological features. It’s interesting enough that humanity has spent some time and effort getting to know it.
As of this writing there is a little plutonium-powered robot scootching around on the floor of one of Mars’ craters, getting to know the planet still further.
Curiosity is the latest in a line of celebrated Mars expeditions, which began with the Vikings in the 1970s and continue to be despatched, with intermittent success, up to the present day. Our robots, however, are just the vanguard. Eventually, someone will make the trip to an alien world, and it’s incomprehensible to think that their destination will be anything other than the rusty corpse that is Mars.