Terrestrial Planets

One family, four worlds

The four planets closest to the Sun — Mercury, Venus, Earth, and Mars — are the terrestrial planets, from the Latin terra, meaning land. NASA groups them together because they are the inner planets with solid surfaces made of rock and metal, in contrast to the giant planets beyond them, which lack hard surfaces. They are also small: the largest, Earth, is about 12,756 kilometers across, far smaller than the gas giants. And they share a layered interior: each has a dense metal core, a rocky mantle, and a solid crust. Venus and Earth have broadly similar bulk compositions, roughly one-third iron-bearing metal and two-thirds silicate rock, and Mars and Mercury follow the same general recipe, with Mercury unusually rich in metal.

Distance and size differ from world to world. Mercury orbits at 0.4 astronomical units (AU), where one AU is Earth's average distance of about 150 million kilometers from the Sun. Venus sits at 0.72 AU, Earth at 1 AU, and Mars at 1.52 AU. In diameter, Mercury is about 4,880 kilometers — a little more than a third of Earth's width — Mars about 6,780 kilometers, and Venus about 12,104 kilometers, close enough to Earth that the two are often called near twins.

Each planet carries a signature. Mercury's surface resembles the Moon: heavily cratered, with day temperatures near 430 C and nights near -180 C. Venus hides a surface near 467 C — hot enough to melt lead — beneath a thick, mostly carbon-dioxide atmosphere. Earth is the only known planet with liquid water on its surface, its oceans covering about 71 percent, and the only known world to host life. Mars shows the largest volcano in the solar system (Olympus Mons, more than 40 kilometers tall), the largest canyon (Valles Marineris, about 3,870 kilometers long), polar ice caps, and dry channels carved by ancient water.

What drives the differences

Comparative planetology is the study of planets as a group, using each world as a natural experiment for the others. Four factors do most of the work among the terrestrial planets. Distance from the Sun sets how much sunlight a world receives. Size matters twice: a larger planet holds more internal heat, which powers geological activity, and its stronger gravity holds a thicker atmosphere. Atmosphere matters most for climate, because greenhouse gases control how much heat escapes. And geological activity shapes the surface: active worlds erase old craters and build new landscapes, while quiet worlds keep their ancient records.

Mercury is the extreme case: small, close to the Sun, and almost airless. With no atmosphere to carry heat, its surface swings wildly between day and night, and its craters have survived for billions of years. Venus shows what a thick atmosphere can do. It is nearly Earth's size, but its dense carbon-dioxide atmosphere traps heat so effectively that Venus is the hottest planet in the solar system, with surface pressure about 93 times Earth's sea-level pressure. Mars is the opposite: about half Earth's diameter, with a thin carbon-dioxide atmosphere that cannot hold much heat, so the planet is cold even though it receives about half the sunlight Earth does. Earth sits between the extremes, with a moderate atmosphere and surface temperatures that keep water liquid.

Three climates, three outcomes

The three inner worlds that resemble Earth most — Venus, Earth, and Mars — started from similar rocky material, yet their climates diverged completely. Venus is the textbook case of a runaway greenhouse effect. Carbon dioxide traps outgoing heat; as the surface warmed, more gas and water vapor entered the atmosphere, trapping still more heat, until the planet settled into a scorching equilibrium near 467 C. Mars went the other way. Its thin atmosphere lets warmth escape easily, a situation sometimes called a runaway refrigerator, leaving the cold, dry desert we see today. Only Earth kept a climate in which liquid water is stable at the surface. The greenhouse effect operates here too — the natural greenhouse raises Earth's average surface temperature to about 15 C, more than 30 degrees warmer than it would be without an atmosphere — but on Earth it is balanced rather than runaway.

Earth: the only known home

Life as we know it needs liquid water, usable energy, and a supply of the right chemical elements, and it needs those conditions to persist. Earth is the only planet known to satisfy them. NASA notes that life began here about 3.8 billion years ago, in oceans that cover most of the planet. Its atmosphere is about 78 percent nitrogen and 21 percent oxygen, shields the surface from much of the Sun's harmful radiation, and burns up most incoming meteoroids. These are general conditions rather than a formula: scientists cannot yet say exactly which combinations of features make a world habitable. That is why Venus and Mars matter. They are the nearest test cases for how easily a planet like Earth can stay habitable — and for what happens when it does not.