"Habitable" isn't one number — it's a balance of several factors, and a real habitability argument weighs all of them together rather than picking a winner off just one dramatic fact. Three of these five factors you already have from Activities 3.1.1 and 3.1.2. Two are new here.
Surface gravity is usually given as a fraction of Earth's own (1 g). It's never come up yet in this unit because it doesn't affect a planet's geology or atmosphere the way size and heat do — but it matters enormously for an actual human visitor. Long-term exposure to low gravity is linked to bone density loss and muscle atrophy, among other effects; nobody yet knows exactly how a human body would adapt to spending years at a fraction of Earth gravity, since no one has done it.
| World | Surface gravity |
|---|---|
| Mercury | 0.38 g |
| Venus | 0.90 g |
| Earth | 1.0 g |
| Mars | 0.38 g |
| The Moon | 0.17 g |
Water is one of the heaviest, most expensive things to launch from Earth — so a world that already has some is a genuinely huge practical advantage for any future mission, both for drinking and for splitting into hydrogen and oxygen as fuel and breathable air.
The surprise: water ice has been confirmed not just on Mars, but in permanently shadowed craters at both poles of the Moon — and at Mercury's poles too. Two airless worlds that bake under direct sunlight are, in specific hidden pockets that never see the Sun at all, cold enough to have held onto ice for billions of years.
Use the scorecard below to pull real numbers for any two worlds — this is exactly the data your own project deliverable will draw on.
Pick any two worlds to compare — this is the exact data you'll need for your own report.
| Factor | Mars | Moon |
|---|---|---|
| Gravity | 0.38 g | 0.17 g |
| Distance | 1.52 AU | ~1.0 AU (orbits Earth) |
| Day length | ~24.6 hours | ~29.5 Earth days |
| Atmosphere | Thin CO₂, <1% Earth's pressure | None |
| Temperature | ~-63°C average | -173°C to 127°C swings |
| Magnetic field | None today | None |
| Water | Polar ice caps, some subsurface ice | Ice in permanently shadowed polar craters |
| Geology | Ancient highlands next to young volcanic Tharsis | Ancient cratered highlands + younger volcanic maria |
Weigh Mars against the Moon as a target for a first long-term human outpost.