Scientists measure Martian mountains with observations collected by spacecraft orbiting the planet. NASA’s Mars Orbiter Laser Altimeter (MOLA) timed laser pulses reflected from the surface; researchers combined those ranges with the spacecraft’s position to build topographic maps. Because Mars has no ocean to define sea level, MOLA elevations use a gravity-based reference surface called the MOLA areoid. A mountain’s elevation above that datum is not the same as its height above nearby terrain.
How does a spacecraft measure the surface?
MOLA sent laser pulses toward Mars and measured how long the reflected light took to return. That travel time gives the distance between the instrument and the point on the ground beneath its orbital track. With the spacecraft’s position and orbit known, scientists can calculate the surface location and elevation. NASA describes this pulse-and-return method in its Mars elevation measurement profile.
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As the spacecraft moves, repeated laser returns form surface profiles along its ground tracks. Researchers adjust and combine observations from those tracks, then use them to create regional and global topographic products. MOLA’s main mapping observations were collected aboard Mars Global Surveyor; NASA Goddard says its surface-altimetry collection continued until June 30, 2001. The NASA Goddard MOLA overview and NASA Planetary Data System MOLA metadata provide instrument and data-product context.
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On Earth, sea level is a familiar reference for elevation. Mars has no global ocean, so scientists use the MOLA areoid: a gravity-based equipotential reference surface. A NASA technical report defines it as the gravitational equipotential whose average value at the equator equals the mean radius determined by MOLA. The USGS explains that its map elevations were converted from planetary radii to elevations above an areoid derived from a Martian gravity-field solution.
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The areoid gives mapmakers a consistent zero for comparing elevations across the planet. It is not an ocean surface, and it is not the foot of a particular mountain. So “above the MOLA areoid” describes a summit’s elevation relative to a planetary datum—not how far the mountain rises above the surrounding plains.
Olympus Mons: a summit elevation is not local relief
A NASA-hosted technical report gives the peak of Olympus Mons as 21.2 km above the MOLA areoid. That figure is a datum elevation. It should not be described as the volcano’s rise above the ground around it.
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“Height” can refer to different measurements: a summit’s elevation above the areoid, its relief above a nearby plain, or its total rise from a defined base. For a broad shield volcano such as Olympus Mons, the base can be difficult to define consistently. A local-relief figure therefore depends on which surrounding terrain or boundary is chosen. The available official figures cited here do not establish a directly comparable value for Olympus Mons’s rise above a precisely defined base.
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What can a MOLA topographic map tell you—and what can it not?
A map pixel is not necessarily an individual laser measurement. The USGS’s 2003 documentation for its MOLA-derived Mars map describes more than 600 million measurements collected from 1999 to 2001, adjusted for consistency and converted to elevations above the areoid. The map combines track observations into a gridded digital elevation model (DEM); where tracks leave gaps, the DEM uses interpolation. Its reported specifications apply to that map product, not automatically to every Mars elevation dataset.
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| Map specification | What the USGS reports for this product |
|---|---|
| Input observations | More than 600 million measurements gathered during 1999–2001 |
| Grid spacing | 0.015625 degree per pixel (64 pixels per degree), about 926 m per pixel at the equator |
| Track gaps | Gaps of 1–2 km are common; some near the equator reach 12 km. The DEM fills gaps by interpolation. |
| Elevation uncertainty | At least ±3 m total elevation uncertainty for this map, including global and regional areoid uncertainty |
| Original point accuracy | About 100 m in horizontal position and about 1 m in radius, as reported for the input points |
| Polar coverage | Especially sparse above 87° north and below 87° south |
These figures measure different things. Grid spacing describes sampling in the gridded map; track gaps describe where observations were not collected; horizontal position accuracy describes location; and total elevation uncertainty describes uncertainty in mapped height. None should be used as a substitute for another. The USGS’s Topographic Map of Mars (I Map 2782) documents the product’s methods and limitations.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare mountain heights fairly
Before comparing two reported heights, check that they refer to the same kind of measurement. A datum elevation, local relief, and total rise from a defined base can produce different figures for the same mountain. Also check whether a number comes from a track profile or a gridded map, whether grid cells include interpolated areas, and which product’s uncertainty and processing apply.
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That distinction matters when comparing Mars with Earth, too: a Martian summit elevation above the MOLA areoid is not directly equivalent to an Earth mountain’s height above local ground or sea level. State the baseline alongside any figure, rather than presenting “height” as if it had only one meaning.
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