How do Mercury, Venus, Earth, and Mars differ?

Rocky does not mean identical

These four worlds are called terrestrial planets. They have a solid rocky surface, a mantle of rock beneath it, and a metallic core deeper down. This is a shared structural plan, rather than four identical copies of Earth. The sizes and physical states of the layers, and the way the interiors release heat, can differ. NASA: the structure of Earth and its terrestrial relatives.

Start with size. Earth is the largest of the four; Venus has about 95% of its diameter. Mars has about 53%, Mercury 38%. Mars is therefore larger than Mercury, although both are much smaller than Earth. All the disks below share one scale, while the temperatures are reference mean values.

Notice two pairs. Venus and Earth are similar in size but different in conditions. Venus and Mars have similar atmospheric CO₂ percentages but different amounts of atmosphere. That already shows why one number cannot explain an entire world.

Mercury: a scorching day and a cold night

Imagine a rock in sunlight. It absorbs light, warms up, and emits energy at the same time. After sunset, incoming sunlight stops, but emission continues. Mercury has almost no dense layer of air to carry heat between sunlit and dark regions or change how it escapes to space.

In place of a familiar atmosphere, it has an exosphere, an extremely tenuous envelope of individual atoms. Its slow cycle of day and night gives the surface a long time to heat up and a long time to cool. NASA gives temperatures reaching roughly +430 °C by day and −180 °C at night. These describe extreme conditions in different places and at different times, rather than two ways of writing the +167 °C mean. NASA: Mercury’s surface and exosphere.

Mercury resembles the Moon, with many impact craters and plains. Its interior, however, is distinctive: the radius of its metallic core is about 85% of the planet’s radius. That is a fraction of radius, not mass or volume. A small diameter does not mean all planets have the same internal proportions.

Venus: nearly Earth's size, with a very different atmosphere

Imagine two rocky surfaces receiving equal sunlight: above one there is almost no gas; above the other, a dense atmosphere that absorbs thermal radiation. Their temperatures need not match even under equal illumination. The real Venus and Mercury also receive different amounts of sunlight.

A warm surface emits mostly infrared radiation, an invisible form of electromagnetic waves. Carbon dioxide absorbs some of this radiation; the atmosphere also emits radiation of its own, including downward. This exchange raises surface temperature compared with conditions without that greenhouse influence. This is the greenhouse effect. Energy still escapes to space: the atmosphere does not lock it away forever. NASA: the natural greenhouse effect.

Venus has a very dense atmosphere, about 96% CO₂, with a surface pressure of around 92 bar. A bar is a unit of pressure; Earth’s sea-level pressure is about 1 bar. The large amount of gas produces a powerful greenhouse effect. Venus’s surface remains extremely hot at night, so its mean of about +464 °C exceeds even Mercury’s quoted daytime maximum. NASA: Venus’s atmosphere, NSSDCA: pressure and composition.

Clouds also reflect some sunlight. An explanation therefore needs both sides of the energy exchange: how much energy is absorbed and how it returns to space. Proximity to the Sun is only part of the answer.

Earth: oceans above a moving rocky shell

Earth’s atmosphere consists mainly of nitrogen and oxygen. Smaller constituents can still have a large effect: water vapor and CO₂ contribute to the greenhouse effect. Natural atmospheric warming helps maintain conditions for liquid oceans. A “rocky surface” need not be dry: water can cover much of it. NASA: Earth, NASA: atmospheric composition and greenhouse warming.

Earth’s rigid outer shell is divided into moving tectonic plates. Their interactions reshape the surface, building mountains, spreading the ocean floor, and causing earthquakes. Understanding a planet therefore requires its internal history as well as its distance from the Sun.

Volcanism is not exclusive to Earth. In radar images of Venus taken eight months apart in 1991, researchers identified a change in the shape and size of a volcanic vent. Their paper appeared in 2023: this is evidence of activity during the imaging period, rather than an eruption observed in 2023. The similar sizes of Earth and Venus do not make their surfaces or ways of releasing internal heat identical. Herrick and Hensley: the study in Science.

Mars: carbon dioxide without a powerful greenhouse

Mars is farther from the Sun and has a much thinner atmosphere than Earth or Venus. Although about 95% of its gas is CO₂, surface pressure is only about 0.006 bar, with seasonal and local differences. A high percentage of a very small amount does not provide a Venus-like quantity of gas. There is therefore no contradiction between “almost all the gas is CO₂” and a mean of −65 °C. NSSDCA: Mars’s atmosphere.

Oxidized iron-bearing minerals in its rocks and dust give Mars its rusty color. Alongside volcanoes and craters, there are ancient river valleys, deltas, and lake deposits: evidence that liquid water once flowed and collected on the surface. They do not establish that all of Mars was always a warm ocean world. Water ice exists there today, but present surface conditions do not sustain long-lived exposed bodies of liquid water. NASA: Mars’s surface and water.

Data from MAVEN helped establish that substantial amounts of gas escaped to space. The solar wind, a stream of charged particles, and solar radiation contribute to processes that give upper-atmosphere particles the energy to escape. Measurements of gas composition help reconstruct that history; exact ancient conditions require models. JPL: how MAVEN investigated atmospheric loss.

Mars currently lacks an internally generated global magnetic field. But “no such field means the entire atmosphere disappears” is an incorrect rule: Venus also lacks an internally generated global field, yet its atmosphere is very dense. Gravity, gas temperature and composition, replenishment from the interior, and losses over time also matter. A magnetic field affects interaction with the solar wind, but cannot replace this whole explanation. NASA: Venus’s magnetic environment.

What can you now explain yourself?

A distant planet is described as “rocky, with an atmosphere containing 95% CO₂.” Can we already decide that it is as hot as Venus? Try naming the missing information before returning to the comparison.

We need to know at least how much gas there is in total, how much sunlight reaches the planet, how much it reflects, and how its atmosphere transmits thermal radiation. Mars shows why the CO₂ percentage alone is insufficient; Mercury and Venus show why distance alone is insufficient.

Mercury is a small world with a large metallic core and a surface exposed to sharp temperature swings. Venus is nearly Earth’s size, with a dense atmosphere and a very powerful greenhouse effect. Earth has oceans, a nitrogen–oxygen atmosphere, and moving plates. Mars is a smaller, cold world with a thin atmosphere and traces of a watery past. Their shared rocky structure defines the family; size, illumination, gaseous envelopes, and geological history explain their different conditions.

Return to the map of the Solar System and find these four worlds before the asteroid belt. Their order is now the start of an explanation of their differences, rather than a ready-made temperature scale.