Two Scorched Warnings

Mercury and Venus may be inhospitable, but they are scientifically invaluable:

• Mercury's giant iron core teaches us how rocky planets form
• Venus shows what happens when a greenhouse effect spirals out of control
• Mercury's orbital precession helped confirm Einstein's general relativity
• Venus's cloud tops near 50 km have Earth-like temperature and pressure, a possible site for floating habitats
• Neither planet has a moon, hinting at the dynamics of the inner solar system

These two scorched worlds reveal what could go wrong — and almost went wrong — with Earth.