Hellas Planitia is the largest obvious impact basin on Mars and one of the largest in the solar system. It is about 2,300 km across. The floor lies more than 7 km below the areoid, which makes it the deepest regional surface on the planet and the other end of the topographic range that starts at Olympus Mons.
The basin sits in the southern highlands. Its rim is a broken ring of massifs and old cratered ground. Plains inside the basin have been resurfaced by wind, volcanic deposits, and, in places, ice-related debris. The fact sheet puts the whole planet's topographic range at about 30 km, and Hellas is the deep end of that range. [1]
Air at the bottom
Because the floor is so low, the air column above it is thicker than the air over the high volcanoes. Pressure can climb toward the top of the planetary range quoted on Atmosphere of Mars. That still does not make a stable sea. It does mean frost and cloud behave differently here than on the Tharsis summits.
Why it matters
Hellas is the cleanest giant scar on the dichotomy's highland side. Its age, depth, and the way later deposits filled it are part of how geologists time the bombardment that also shaped the Moon. See Geography of Mars for how the basin sits against the lowlands.
Engineering screen
The floor is more than 7 km below the areoid, and that is as precise as this sheet gets. Site dossiers leaves the elevation blank on purpose. The 7 km button on Pressure and the areoid is a round screen, not a survey of this floor. No landed mission is listed for the basin.
See also
What links here
References
- NASA Goddard Space Flight Center. Mars Fact Sheet. Topographic range about 30 km. Updated 19 May 2025. https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html