Venus, Earth, and Mars are the three rocky planets that sit in the middle of our solar system, orbiting the Sun at 0.72, 1.0, and 1.52 AU respectively. Despite being close neighbors made from similar raw materials, they turned out remarkably different. Venus became a furnace with crushing atmospheric pressure, Earth developed the only known habitable surface in the solar system, and Mars ended up as a cold, thin-aired desert roughly half Earth’s size.
Size, Mass, and Gravity
Venus and Earth are near twins in physical dimensions. Venus has an equatorial radius of 6,052 km compared to Earth’s 6,378 km, making it about 95% of Earth’s width. Its mass is 4.87 × 10²⁴ kg versus Earth’s 5.97 × 10²⁴ kg, so it packs about 82% of Earth’s heft. Stand on Venus, and you’d feel surface gravity of 8.87 m/s², only about 10% lighter than the 9.80 m/s² you’re used to on Earth. You’d barely notice the difference.
Mars is the odd one out. With an equatorial radius of just 3,396 km, it’s roughly half Earth’s diameter and about 53% as wide. Its mass is only a tenth of Earth’s at 0.64 × 10²⁴ kg. Surface gravity on Mars is 3.71 m/s², meaning you’d weigh about 38% of what you weigh on Earth. A 150-pound person would feel like they weighed 57 pounds.
Atmospheres and Surface Pressure
The atmospheres of these three planets could hardly be more different, even though Venus and Mars share a similar recipe. Both Venus and Mars have atmospheres made of more than 98% carbon dioxide and nitrogen. Earth’s atmosphere, by contrast, is 99% nitrogen and oxygen, with carbon dioxide making up a tiny 0.038%.
The real drama is in thickness. Venus has an atmosphere so dense that its surface pressure is 92 times that of Earth. Standing on Venus would feel like being nearly a kilometer underwater on Earth. Mars sits at the opposite extreme, with surface pressure of only a few millibars, less than 1% of Earth’s. The Martian atmosphere is so thin it offers almost no insulation and very little protection from radiation. Earth’s atmosphere, at 1 bar, sits in a sweet spot that keeps liquid water stable and shields the surface from most solar and cosmic radiation.
Temperature Extremes
Distance from the Sun matters, but atmosphere matters more. Venus orbits closer to the Sun than Earth, yet its extreme surface temperature of roughly 465°C (870°F) isn’t just a product of proximity. It’s the result of a runaway greenhouse effect. That thick blanket of carbon dioxide traps heat so efficiently that Venus is hotter than Mercury, despite being nearly twice as far from the Sun.
Earth’s average surface temperature hovers around 15°C (59°F), regulated by a greenhouse effect that stays in balance thanks to the water cycle, plant life, and relatively modest levels of carbon dioxide. Mars, farther from the Sun and barely wrapped in atmosphere, averages around minus 60°C (minus 80°F). Temperatures on Mars can swing wildly, dropping below minus 100°C at the poles in winter and occasionally climbing above freezing at the equator during summer afternoons.
Days and Rotation
Earth and Mars spin at nearly the same rate. Earth completes one rotation in just under 24 hours (23 hours 56 minutes for a sidereal day). Mars is remarkably similar at about 25 hours per solar day, which is one reason Mars has been attractive for exploration planning: its day-night cycle is close enough to ours that mission controllers can roughly sync with it.
Venus is the outlier in every sense. A single solar day on Venus lasts 5,832 hours, or about 243 Earth days. Venus also rotates backward compared to Earth and Mars, spinning from east to west. This means the Sun rises in the west on Venus. The reasons for this retrograde spin are still debated, but a massive ancient impact or long-term atmospheric tidal effects are the leading explanations.
Magnetic Fields and Radiation Protection
Of the three planets, only Earth has a strong global magnetic field. This magnetosphere acts as a shield, deflecting charged particles from the solar wind and protecting the atmosphere from being stripped away over billions of years. It’s generated by the circulation of liquid iron in Earth’s outer core.
Venus and Mars both lack global magnetic fields. Venus has a weak, induced magnetic field created by the interaction of the solar wind with its thick atmosphere, but nothing generated internally. Mars once had a global magnetic field early in its history, and patches of remnant magnetism are still detectable in its ancient southern crust, but the planet’s internal dynamo shut down billions of years ago. Without that magnetic protection, the solar wind gradually eroded much of Mars’s atmosphere, which helps explain why it’s so thin today.
Internal Structure
All three planets share the basic architecture of rocky worlds: a metallic core, a rocky mantle, and a crust. The details, though, vary significantly.
Earth’s core has two layers. The inner core is solid iron and nickel, and the outer core is molten. That liquid outer core is what drives Earth’s magnetic dynamo. NASA’s InSight lander confirmed that Mars also has a molten core, with a radius of about 1,830 km, proportionally larger relative to the planet’s size than scientists expected. Mars’s mantle extends about 1,560 km beneath the surface. Venus is thought to have a similar iron core to Earth’s, but without seismic data from the surface, the details remain uncertain. The absence of a magnetic field suggests its core may not have the same kind of convection pattern that powers Earth’s dynamo.
Volcanism and Tectonics
Earth is the only planet of the three with active plate tectonics, where large slabs of crust move, collide, and recycle into the mantle. This process drives earthquakes, builds mountain ranges, and recycles carbon through the rock cycle, playing a key role in regulating Earth’s climate over geological time.
Venus doesn’t have tectonic plates, but its surface is far from dead. Radar images from NASA’s Magellan mission revealed vast circular features called coronae, formed by plumes of molten rock pushing up from the mantle. Of 75 coronae studied, 52 appear to have buoyant mantle material beneath them that is actively driving surface deformation. Researchers have also spotted direct evidence of erupting volcanoes, including lava flows from several large volcanic peaks. A different kind of subduction may occur around some coronae, and a process called lithospheric dripping, where dense, cooled material sinks back into the hot mantle, could also be reshaping the surface. Venus appears to lose its internal heat through these localized processes rather than through the planet-wide conveyor belt of plate tectonics.
Mars was once volcanically active. Olympus Mons, the tallest volcano in the solar system at roughly 22 km high, formed because Mars lacks plate tectonics: the crust stayed stationary over a hotspot, allowing lava to pile up in one place for hundreds of millions of years. Most evidence suggests Mars’s volcanism is now extinct or nearly so, though some lava flows appear to be geologically recent, within the last few million years.
Water: Past and Present
Earth is the only planet of the three with stable liquid water on its surface, covering about 71% of the planet. The combination of moderate temperature, sufficient atmospheric pressure, and a protective magnetic field makes this possible.
Mars has abundant water ice at its polar caps and buried beneath the surface at various latitudes. NASA’s Mars Reconnaissance Orbiter found the strongest evidence yet that small amounts of liquid water may flow intermittently on present-day Mars. Dark streaks called recurring slope lineae appear on steep slopes when temperatures rise above about minus 23°C and fade in colder seasons. These flows are likely shallow, briny trickles rather than rivers, kept liquid by dissolved salts that lower the freezing point as far as minus 70°C. Billions of years ago, Mars almost certainly had rivers, lakes, and possibly an ocean, but it lost most of its atmosphere and surface water as its magnetic field disappeared.
Venus likely had water early in its history as well, but as temperatures climbed, any surface water would have evaporated. Ultraviolet radiation then broke apart water vapor in the upper atmosphere, and the hydrogen escaped to space. Today, Venus’s atmosphere contains only trace amounts of water vapor. The planet’s surface is far too hot and pressurized for liquid water in any form.
How They Diverged
What makes this comparison striking is that all three planets started with similar ingredients. They formed in the same region of the solar disk from the same cloud of gas and dust. Small differences in distance from the Sun, initial size, and timing of volcanic outgassing set them on wildly different paths. Venus was close enough to the Sun that its water evaporated, triggering a greenhouse feedback loop that never reversed. Mars was small enough that it couldn’t hold onto a thick atmosphere or sustain the internal heat needed to keep its magnetic dynamo running. Earth landed in the narrow zone where liquid water remained stable, plate tectonics kept cycling carbon, and a magnetic field held the atmosphere in place.
Together, the three planets form a natural experiment in how rocky worlds evolve. Venus shows what happens when greenhouse warming runs unchecked. Mars shows what happens when a planet is too small to retain its atmosphere and geological activity. Earth, at least for now, sits in the balance between those two extremes.