Mercury and Venus

Sunward of Earth lie two rocky worlds. Mercury is small, grey and pitted, baked on one side and frozen on the other. Venus is almost Earth's twin in size, yet its surface would melt lead and crush a submarine. Together they show how a planet's distance from the Sun and its atmosphere can produce utterly different fates from similar beginnings.

Mercury: the scorched messenger

Mercury is the smallest planet and the closest to the Sun, orbiting at an average of 58 million kilometres, barely more than a third of Earth's distance. It races around in 88 days, which is why the Romans named it after their fleet-footed messenger god. Yet it spins extraordinarily slowly, once every 59 days, so that a single cycle of sunrise to sunrise takes 176 Earth days, twice as long as its year.

With almost no atmosphere to hold heat, the surface swings from about 430 degrees Celsius at noon to minus 180 at night. Permanently shadowed craters at the poles never see the Sun at all, and radar and spacecraft have found water ice hiding in them, delivered by comets and preserved for billions of years just a few hundred kilometres from the blazing dayside.

Mercury's surface looks like the Moon, covered in craters from the heavy bombardment of the early solar system, including the Caloris Basin, an impact scar 1,550 kilometres wide. The planet is unusually dense; a huge iron core takes up about 85 percent of its radius, suggesting a giant collision early in its history stripped away much of its rocky outer layers. Only two spacecraft have visited: Mariner 10 in the 1970s and MESSENGER, which orbited from 2011 to 2015. Europe and Japan's BepiColombo arrives in 2026.

Venus: Earth's evil twin

Venus is nearly Earth's size, 95 percent of its diameter and 82 percent of its mass, and made of similar rock. Early astronomers imagined steaming jungles beneath its brilliant clouds. The truth, revealed by Soviet and American probes in the 1960s and 1970s, was far stranger.

The atmosphere is almost pure carbon dioxide, 92 times thicker than Earth's. That blanket traps the Sun's heat so efficiently that the surface sits at about 465 degrees everywhere, day and night, pole to equator, hotter than Mercury despite being twice as far from the Sun. This is the greenhouse effect at full throttle: the same process that keeps Earth comfortably warm, run out of control. Venus probably had oceans once. As the young Sun brightened they boiled away, the water vapour trapped more heat, and carbon dioxide that on Earth is locked in rocks baked out into the air.

The clouds are made of sulphuric acid, and they reflect so much sunlight that Venus is the brightest object in our sky after the Sun and Moon. Beneath them, the surface is a landscape of volcanic plains, cracked highlands and more than 1,600 major volcanoes, with surprisingly few craters. Something resurfaced the whole planet with lava a few hundred million years ago, and recent evidence suggests some volcanoes are still active.

A backward, slow world

Venus is odd in another way. It rotates backwards compared to most planets, and so slowly that one rotation takes 243 Earth days, longer than its 225-day year. On Venus the Sun rises in the west, if you could see it through the clouds, and a single day lasts longer than a year. Nobody is sure why; a giant impact early in its history is one possibility.

Visiting the inferno

Landing on Venus is one of the hardest things engineers have ever done. The Soviet Venera probes were the only craft to send pictures from the surface, and the longest survived just over two hours before the heat and pressure destroyed it. Their photographs show flat, cracked rocks under an orange sky. NASA and ESA both plan new missions in the 2030s, including orbiters that will map the surface and a probe that will sample the atmosphere on the way down.

Why it matters

Venus is the clearest warning in the solar system of what a runaway greenhouse effect can do. Earth and Venus began with similar ingredients, and a change in sunlight and atmosphere turned one into a paradise and the other into an oven. Understanding exactly how and when that happened helps us understand the stability of our own climate, and tells us how to judge the habitability of the thousands of Earth-sized planets now being found around other stars.