The relative size of celestial bodies can be difficult to perceive from Earth. Observing the familiar Moon and the distant, reddish Mars does not immediately reveal their true scale. To accurately understand the comparison between a planet, like Mars, and a natural satellite, which is what the Moon is, we must examine their physical metrics.
Direct Size Comparison
Mars is significantly larger than the Moon. The equatorial diameter of Mars measures approximately 6,779 kilometers (4,212 miles), nearly twice the Moon’s diameter of 3,475 kilometers (2,159 miles).
The difference in volume is even more pronounced. Mars occupies approximately 7.4 times the volume of the Moon. It would take more than seven Moons to fill the space contained within Mars.
Placing Mars in Context
The Red Planet is often considered a small world, but only when compared to the gas giants or Earth. Mars is the second-smallest planet in the solar system, with only Mercury being smaller. Its diameter measures about 53% of Earth’s width.
Mars’s status as a full planet confirms its considerable scale within the inner solar system. It is a terrestrial world with diverse geological features, including the solar system’s largest volcano, Olympus Mons. The planet retains a surface area comparable to all the dry land on Earth.
Placing the Moon in Context
While dwarfed by Mars, Earth’s Moon is an exceptionally large satellite when measured against its host planet. The Moon’s diameter is about 27% of Earth’s diameter, a ratio that is unusually high among inner solar system moons. By comparison, Mars’s two tiny moons, Phobos and Deimos, are small, irregular, asteroid-like objects only a few kilometers across.
Its large size relative to Earth makes the Moon the fifth-largest natural satellite overall in the solar system. The Moon is much larger than any known dwarf planet, such as Pluto, highlighting its considerable mass.
Density and Gravitational Differences
The large size of Mars translates directly into a vastly greater mass, which governs its gravitational influence. Mars’s total mass is about 8.7 times greater than the Moon’s mass. This difference is compounded because Mars is slightly denser than the Moon, meaning it packs more material into its volume.
Due to its superior mass, Mars has a much stronger surface gravity, pulling objects down with about 38% of Earth’s force. This gravity is sufficient to retain a thin, permanent atmosphere composed mainly of carbon dioxide. In contrast, the Moon’s much weaker gravity, only about 16.5% of Earth’s, is too low to prevent atmospheric gases from escaping into space, leaving it airless.