Mars vs Earth: How the Two Worlds Compare

Mars is often called Earth's "sister planet," but the resemblance is only skin-deep. Here's exactly how the two worlds compare — and what that means for the humans who hope to one day visit.

What Mars Has in Common With Earth

• A 24-hour day. A Mars day (called a "sol") is just 39 minutes longer than ours.
• Tilted axis = real seasons. Mars's 25° tilt is remarkably close to Earth's 23.4°, producing genuine spring/summer/autumn/winter cycles.
• Polar ice caps. Mars's caps are a mix of water ice and frozen CO₂ ("dry ice").
• Evidence of ancient liquid water. Dry riverbeds, deltas, and lake sediments. Mars almost certainly had a thicker atmosphere and surface oceans in its first billion years.
• Roughly the same total land area as Earth's continents — Mars has no oceans, so its dry surface area equals Earth's continental area.

What Makes Mars Lethal Without a Spacesuit

The pressure problem: Mars's atmosphere is only 0.6% as dense as Earth's. At that pressure, water in the human body would boil at body temperature — including the moisture in your lungs, eyes, and skin. Death from ebullism would occur within minutes.

The cold: Average surface temperature is −63°C, with winter polar lows below −140°C. Even on the warmest summer day at the equator, temperatures rarely exceed +20°C and plunge to −73°C overnight.

The radiation: With no global magnetic field and only a thin atmosphere, Mars's surface gets ~50× more cosmic and solar radiation than Earth's. A 500-day surface stay would expose an astronaut to roughly the lifetime cancer-risk limit set by NASA.

Why Mars Looks Red

Mars's surface is rich in iron oxide — literally rust. When sunlight illuminates the regolith, the same chemistry that gives rust its color gives Mars its distinctive rusty appearance. Storms can lift this dust into the atmosphere, sometimes blanketing the entire planet for weeks.