Mars seems small beside Earth. Its imply radius is about 3,390 kilometres, a little bit over half Earth’s, and its quantity is simply about 15 per cent as giant. But if the entire Martian globe had been unwrapped onto a map, its space would come surprisingly near all of the land uncovered above Earth’s oceans.
The figures are roughly 145 million sq. kilometres for Mars and about 149 million sq. kilometres for Earth’s land. Relying on the reference radii and land dataset used, Mars has round 97 per cent as a lot floor space because the dry a part of Earth.
It’s a splendidly compact comparability, however it’s simple to misinterpret. Mars isn’t virtually as giant as Earth, and it isn’t a planet completely stripped of water. The similarity seems as a result of the global ocean covers 71 per cent of Earth, whereas present-day Mars has no confirmed everlasting lakes, rivers or seas on its floor.
Why half the diameter doesn’t imply half the world
The floor space of a sphere is 4 occasions pi occasions the radius squared. Halve a planet’s radius and its floor space falls to 1 quarter, not one half. Mars isn’t precisely half Earth’s dimension, so the calculation is barely bigger than 1 / 4.
Utilizing imply radii of about 3,390 kilometres for Mars and 6,371 kilometres for Earth, the radius ratio is roughly 0.532. Sq. it and Mars has about 28.3 per cent of Earth’s complete floor space. That produces a Martian space close to 144.5 million sq. kilometres, usually rounded to 145 million.
Earth’s complete floor covers roughly 510 million sq. kilometres. If 71 per cent is ocean, the remaining 29 per cent is near 148 million sq. kilometres. Extra detailed geographical datasets normally place world land space round 149 million sq. kilometres, relying on how coastlines and inland waters are handled.
The coincidence is now seen. Mars gives about 28 per cent as a lot complete space as Earth, whereas dry land occupies about 29 per cent of Earth. NASA was already illustrating the comparison in its 1984 guide to Mars and Earth, putting maps of the 2 worlds on the similar scale.
“All of the continents” is helpful quite than actual
The acquainted wording treats “the continents” as shorthand for Earth’s uncovered land. A strict geologist might object. Continental crust continues beneath shallow seas, islands aren’t at all times counted as elements of continents in unusual speech, and Antarctica stays land even the place kilometres of ice conceal its rock.
Coastlines aren’t mathematically fastened boundaries both. Tides, altering sea degree, river mouths and the decision of a map all alter a land-area estimate. Mars additionally has mountains, crater partitions and canyon slopes whose true textured space exceeds that of a clean reference sphere.
None of these issues breaks the comparability. They clarify why “almost as a lot” is the best phrase. The declare isn’t an equality carried to the final sq. kilometre. It’s a scale comparability between Mars’s world floor and the world typically labeled as land on Earth.
It could be improper to show that into “Mars is nearly as huge as Earth.” Earth’s submerged seafloor continues to be planetary floor. Drain the oceans and Earth would retain its full 510 million sq. kilometres, greater than three and a half occasions the reference space of Mars.
No everlasting floor water isn’t the identical as no water
NASA’s current Mars overview says liquid water cannot exist for long on the surface. The reason being a troublesome mixture of low strain and low temperature. Common Martian floor strain is near the triple-point strain of water, under which liquid water isn’t a secure bulk section. Strain additionally adjustments sharply with elevation and climate.
In a lot of the local weather, uncovered water freezes. If sunshine warms it sufficient to soften, the skinny environment encourages fast evaporation or boiling. Salt can decrease the freezing level and may permit tiny portions of brine to persist briefly below beneficial native situations, however that could be a good distance from a secure lake or flowing river.
Seasonal darkish streaks as soon as attracted consideration as attainable briny flows. Later topographic work discovered that many finish on the similar steep angles as avalanching sand. NASA’s account of that analysis describes granular flow as a better fit, whereas permitting that small quantities of water might nonetheless assist set off or modify some options.
Mars nonetheless holds giant shops of water in different varieties. Water ice makes up a lot of the everlasting northern polar cap and lies beneath the floor throughout broad high-latitude areas. Frost seems seasonally. Hydrated minerals retain water inside their buildings, and buried ice happens effectively past the seen caps.
Potential liquid reservoirs under the floor are a separate query from everlasting our bodies on the bottom. Shiny radar reflections beneath the south polar ice had been interpreted as briny lakes, however various explanations involving rock, clay or uncommon ice stay credible. New NASA radar observations reported in 2025 made the lake interpretation harder to sustain, with out establishing that liquid water is unimaginable in all places underground.
The dry planet carries the structure of water
Trendy Mars has no ocean to interrupt its land space, however a lot of that steady floor was formed by liquid water. Orbiters have mapped branching valley networks, flood channels, deltas and basins that when held lakes. Rovers have examined rounded pebbles, cross-bedded sediments and minerals fashioned or altered in water.
The distinction is especially clear within the planet’s color. As SpaceDaily recently reported, a 2025 study linked much of the red dust to ferrihydrite, an iron mineral usually related to cool liquid water. If that interpretation holds, even the color of the fashionable desert carries proof of wetter situations.
How a lot open water existed without delay continues to be debated. Some reconstructions place an ocean throughout parts of the northern lowlands, maybe masking roughly a 3rd of Mars. Shoreline candidates are troublesome to hint as a result of volcanism, impacts, erosion and motion of the crust have altered the file.
The environment then thinned, a lot water escaped to area and one other portion grew to become ice or entered the crust. SpaceDaily’s look at InSight-based estimates of water in the Martian mid-crust explored one attainable vacation spot. These estimates depend upon how seismic velocities are interpreted and don’t describe an accessible underground ocean.
A planet-sized expanse isn’t uniformly usable
The land-area comparability could make Mars sound like one huge accessible continent. In actuality, its floor is split by formidable terrain. Valles Marineris drops kilometres under surrounding plateaus. Olympus Mons rises far above the reference degree. Craters, dunes, boulder fields, mud deposits and polar ice complicate motion and touchdown.
Elevation additionally adjustments the environment. Low basins have increased strain than mountain summits, although nowhere approaches Earth’s sea-level air. Temperature, daylight, buried ice, radiation publicity and line of sight to orbiting relays all fluctuate by location. Probably the most scientifically attention-grabbing website is probably not the most secure or most sensible place for a spacecraft.
Neither is uncovered space equal to liveable space. Earth’s continents sit beneath a thick environment, take part in an energetic water cycle and connect with oceans that retailer warmth and transfer moisture across the globe. Mars’s skinny carbon-dioxide environment gives little thermal buffering and solely restricted safety from radiation.
The statistic is subsequently helpful for greedy mapping scale, not for estimating how a lot “actual property” a future settlement might occupy. Each sq. kilometre on the Moon is uncovered too. Accessibility and habitability require way over the absence of an ocean overhead.
Earth is the bizarre a part of the comparability
Seen from one other angle, the arithmetic reveals one thing simply neglected about Earth. A lot of the stable planet beneath human life is hidden below water. The continents dominate political maps as a result of individuals stay on them, however they’re a minority of the globe.
Mars reverses the visible steadiness. Nearly each mapped location is uncovered land, but it’s the smaller world. Its complete space almost matches the portion of Earth that human geography has skilled the attention to deal with because the planet.
The similarity is mathematical, whereas the distinction is planetary historical past. Mars gives a dry floor roughly equal in space to Earth’s land as a result of Earth retained a worldwide ocean and fashionable Mars didn’t. The lacking blue between Martian landscapes is what makes two very different-sized worlds look briefly comparable on a map.
