Mercury, Venus, Earth, and Mars are all terrestrial planets — built primarily from rock and metal, the same basic ingredients, condensed out of the same warm inner region of the solar nebula. They're small compared to the gas giants farther out, dense, and solid all the way through.
Given that shared starting point, you might expect four similar worlds. Instead, you get an airless relic, a hothouse, a living planet, and a cold, mostly-dead desert that used to be wetter. This activity is about figuring out exactly what pulled them apart.
Mercury is so small and so close to the Sun that it can't hold onto a real atmosphere at all — any gas escapes almost as fast as it arrives. With nothing to trap heat or spread it around, Mercury swings wildly between a scorching day side and a frozen night side.
Venus kept its thick carbon dioxide atmosphere — 96% CO2, with clouds of sulfuric acid — and it never stopped trapping heat. The result is a runaway greenhouse effect: a surface hot enough to melt lead, hotter than Mercury even though Venus sits much farther from the Sun.
Mars tells the opposite story. Early Mars likely had a thicker atmosphere and flowing liquid water. But Mars is smaller and farther out, and it couldn't hold onto that thicker atmosphere — the CO2 needed for a real greenhouse effect was lost, temperatures dropped, and the remaining water froze. Earth is the one world that struck a balance, keeping just enough greenhouse warming to hold liquid water without losing control of it.
Click a planet to compare its "no-atmosphere" predicted temperature to what it actually is.
A planet's internal heat comes from two sources: leftover warmth from its violent formation, and heat released by decaying radioactive elements inside it. Bigger worlds hold onto that heat far longer — the same reason a large baked potato stays hot after a small one has already cooled off. That's why geological activity — volcanoes, plate tectonics, resurfacing — tracks closely with size.
A global magnetic field needs a specific engine: a molten, electrically conductive interior, stirred by the planet's own rotation. Earth has exactly that, and its magnetic field acts as a shield, deflecting the Sun's stream of charged particles — the solar wind — away from the atmosphere. Mars almost certainly had a magnetic field billions of years ago, but its small core cooled and mostly solidified, and the field shut off. Without that shield, the solar wind has been slowly stripping away what was left of Mars's atmosphere ever since. Venus, despite being Earth-sized, rotates far too slowly to generate a meaningful field of its own.
Everything above leaves fingerprints on each planet's surface — Activity 3.1.2 goes much deeper, but here's the preview:
| World | Signature surface feature |
|---|---|
| Mercury | Heavily cratered, Moon-like — no atmosphere or water to erode the scars |
| Venus | Volcanic plains hidden under permanent cloud cover, mapped mostly by radar |
| Earth | Constantly resurfaced by plate tectonics, water, and erosion |
| Mars | Olympus Mons (the solar system's largest volcano) and dry, ancient river valleys |
A newly discovered rocky exoplanet is Earth-sized but has essentially no atmosphere and no magnetic field. What would you predict about its surface?