How was Olympus Mons on Mars discovered and measured?
Executive summary
Astronomers first recognized Olympus Mons as a bright albedo patch called Nix Olympica in 19th‑century telescopic maps, but its true size and nature — a 600–700 km wide shield volcano rising ~21–22 km above the Martian areoid — were confirmed by spacecraft imaging and topography, especially Mariner 9 and later laser altimetry from MOLA (Mariner 9 identification: [1]; dimensions and MOLA height: [2]; p2_s7). Modern work refines ages and activity: crater counts and high‑resolution orbiters place some flows as young as a few million to tens of millions of years, so Olympus Mons is scientifically treated as geologically young and possibly episodically active (age and recent activity: [3]; [4]; [3]3).
1. From “Nix Olympica” on a 19th‑century map to a mountain in 1971 — the discovery story
Telescopic observers in the late 1800s recorded light and dark markings on Mars and labeled a bright patch Nix Olympica; that naming established the feature but not its topography. The first unambiguous confirmation that this patch was an enormous volcano came when spacecraft reached Mars: Mariner 9’s imaging in 1971 revealed towering volcanoes and mapped the Tharsis region, turning a telescopic albedo feature into the recognized Olympus Mons volcano (early identification and Mariner 9 role: [1]; historical note on Nix Olympica: p1_s5).
2. How spacecraft measured the height and footprint — images, stereo, and laser altimetry
Mariner 9’s limb and surface photographs exposed the edifice’s vast flows, but precise elevation came from later missions. Mars Global Surveyor’s Mars Orbiter Laser Altimeter (MOLA) produced topographic maps that give Olympus Mons an elevation around 21.9–22 km above the Martian areoid; stereo cameras from missions such as Mars Express provided complementary 3‑D views and caldera detail (MOLA height and stereo imaging: [2]; Mars Express HRSC caldera and average elevation: p2_s7).
3. Why “how tall” is tricky — baseline, base definition and gravity effects
Different teams quote heights that vary (reports range from ~21.9 km to values near 25 km or larger) because “height” depends on whether you measure above the areoid, above the immediate surrounding plains, or from the base of the bounding cliffs. The areoid itself is distorted regionally by the mass of Tharsis and Olympus Mons, so local reference surfaces shift; authors note that measuring from cliff base or plain gives slightly different numbers, which is why various sources cite ~22 km, nearly 25 km, or higher depending on method (measurement range and baseline nuance: [5]; MOLA/areoid discussion and base vs areoid: [6]; public figures ~22 km and ~25 km: [2]; p2_s9).
4. Scope and volume — a planetary giant measured by area and comparative volume
Beyond peak elevation, scientists quantify Olympus Mons by its immense footprint and volume: the edifice spans roughly 600–700 km across and covers an area comparable to U.S. states or countries (Arizona/Italy/France comparisons), with volume estimates many tens to hundreds of times larger than Earth’s largest shield volcanoes — statements widely cited in mission summaries and reviews (diameter and area comparisons: [2]; [7]; volume comparisons and Mauna Loa context: [3]; [3]1).
5. Dating the flows — crater counts and remote mineralogy refine ages
Researchers use crater‑count statistics on high‑resolution orbiter imagery and mineral detections by spectrometers to date and characterize lava flows. Multiple studies and agency summaries report summit caldera resurfacing within tens to a few hundred million years and some flank flows dated from a few million to a couple hundred million years; ESA and other teams note caldera resurfacing within the last ~20 million years with possible flows as young as a few million years (crater counts and ages: [8]; ESA summary of recent volcanism and caldera resurfacing: [4]; earlier Space.com review of ages: [3]3).
6. Active, dormant or extinct? Competing readings in the literature
Some authors and mission teams treat Olympus Mons as geologically young with episodic or recent intrusive activity; crater counts and geophysical hints—plus discoveries of magma bodies beneath Tharsis—have led to cautious statements that the region could be dormant rather than permanently dead. Other pieces emphasize that surface eruptions are not observed and that “recent” in geological terms still spans millions of years, leaving debates about current activity unresolved (evidence for recent/episodic activity and magma basin reports: [4]; magma basin and potential reawakening claims: [9]; summary of volcanic episodes and youngest dates: [3]3).
7. Limits of the available reporting and what’s not settled
Available sources document discovery and measurement methods (telescopic mapping, Mariner 9 imaging, MOLA altimetry, stereo imaging, crater counts and spectral mineralogy) but do not provide a definitive, single “official” height because of method differences (measurement discrepancies and why numbers vary: [10]; p2_s8). Sources do not contain in situ summit rock samples that would nail eruption dates precisely — that gap remains in current reporting (not found in current reporting: in situ sample dating).
Sources cited above: historical and mission context (Nix Olympica, Mariner 9): [1]; mission altimetry and dimensions: [2]; MOLA and Mars Express caldera views: [11]; measurement nuances and areoid issues: [6]; size, volume and comparisons: [3]; crater counts and recent volcanism: [8]; ESA on recent activity: [4]; reports on ages and episodes: [12]; magma-basin reporting and renewed interest: [9].