Hellas Planitia

How big is Hellas Planitia?

About 2,100 to 2,300 km (1,300 to 1,400 mi) across, depending on the source, and up to about 9 km (6 mi) deep. NASA’s Mars Orbiter Laser Altimeter (MOLA) team measured it at 2,100 km (1,300 mi) across and nearly 9 km (6 mi) deep.[4] DLR, Germany’s aerospace center, which operates the HRSC camera on ESA’s Mars Express, gives about 2,200 km (1,370 mi) and up to 9 km.[3] ESA gives 2,300 km (1,430 mi) and more than 7 km (4 mi).[1] The USGS Gazetteer of Planetary Nomenclature lists the named plain at 2,200 or 2,299 km, depending on the reference map.[5]

Hellas also has no single rim. DLR describes several rings of mountains around it; the floor lies more than 4,000 m (13,000 ft) below the innermost ring and about 9,000 m (30,000 ft) below the peaks of the outermost.[2] Around the whole basin, a broad ring of material rises about 2 km (1.25 mi) above the surrounding land and reaches out to 3,200 km (2,000 mi) from the basin’s center, NASA says.[4] Mars is 6,779 km (4,212 mi) wide,[11] so Hellas is about a third as wide as the planet itself.

Hellas in numbers

Sources measure Hellas from different edges and reference levels, so their figures differ. Mars has no sea level; heights are measured from a reference surface.[1][2][3][4][5][6][13][14]

Is Hellas the lowest point on Mars?

Yes. The Hellas basin contains the lowest point on Mars, NASA says.[7] NASA’s 1999 MOLA maps put the deepest point roughly 8,200 m (5.1 mi) below Mars’s mean equatorial height.[6] NASA has called Hellas “deep enough to swallow Mount Everest.”[4] The deepest parts are in the western half of the basin.[1][2] Mars has no sea level, so heights there are measured from a modeled reference surface.[2]

How did Hellas form?

A huge asteroid, probably well over 100 km (60 mi) across, hit the young crust of Mars about 4 billion years ago. DLR dates the impact to about 4.1 billion years ago;[2] ESA gives 3.8 to 4.1 billion years.[1] It happened near the end of the Noachian period, when asteroid impacts on the inner planets were most frequent. Earth was hit too, but its active crust has long since erased those scars.[2]

Material dug out of Hellas helped raise Mars’s southern hemisphere and build up the scarp along its boundary with the northern lowlands, the MOLA team reported in 1999.[8] Since the impact, wind, ice, water and volcanoes have changed the basin.[1] Wind blew dust in, glaciers and streams carried in sediment, and volcanoes spread runny lava across the floor.[2] Even so, DLR calls Hellas the deepest and best-preserved large impact basin on Mars.[2]

How does Hellas compare with other impact basins?

It is one of the largest in the solar system, ESA says.[1] DLR names two of similar size: the South Pole–Aitken basin on the far side of the Moon, and the Valhalla basin on Jupiter’s moon Callisto.[2][3] South Pole–Aitken, the largest and oldest recognized basin on the Moon, is roughly 2,500 km (1,550 mi) across[16] and more than 8 km (5 mi) deep.[17]

Could liquid water exist at the bottom of Hellas?

At times, perhaps, because the air is thicker down there. Mars’s air is thin. At the planet’s mean radius, air pressure is about 6.36 millibars, 0.6% of Earth’s.[11][12] NASA says that is too thin for liquid water to last long on the surface.[10] More on the atmosphere of Mars.

At the deepest points of Hellas, the pressure is almost twice that of the nearby highlands, DLR says; ESA puts it about 89% higher.[1][2] At times, pressure and temperature there rise above what is known as the triple point of water, so in some places water on the surface could be stable.[2] A 2001 study by scientists at NASA’s Ames Research Center combined Viking pressure and temperature data with MOLA heights. It found liquid water would be stable for part of the Martian year in the Hellas and Argyre basins, and over parts of the northern lowlands, if any were present.[9]

Is there ice in Hellas?

Yes, as glaciers buried under rock and dust. In 2008, scientists using the shallow radar on NASA’s Mars Reconnaissance Orbiter reported glaciers of water ice under blankets of rocky debris in the Hellas region. One feature they studied is three times the size of Los Angeles and up to half a mile (800 m) thick. The glaciers may be left over from an ice sheet that covered Mars’s middle latitudes in a past ice age.[13] In three craters in eastern Hellas, the radar measured buried ice about 250, 300 and 450 m (820, 980 and 1,480 ft) thick.[14]

Mars Express images of the western rim from Dec 6, 2015, show giant terraces, steps that faults cut into the ground during and just after the impact. They also show frost on low ground as southern winter spreads north, and a narrow valley carved by a rock glacier: ice covered by boulders and debris.[3] Water on Mars covers ice across the planet.

Do dust storms start in Hellas?

Some do: the planet-wide dust storm of 2001 began there. On Jun 21, 2001, NASA’s Mars Global Surveyor watched a small dust storm surge into Hellas from the southwest. A day later it had circled a third of the way around the basin, a sign of strong winds. Between Jun 25 and 26 it burst north across the equator. Within a day, dust was rising thousands of kilometers away: the start of that year’s global dust event.[15]

Dust often hides the floor: a clear view is unusual, ESA says.[1] DLR describes fierce dust storms blowing off the highlands and the arc of mountains over the plain.[2] Weather on Mars covers dust storms in general.

Can you see Hellas from Earth?

Yes, through a telescope, as a large, light, almost round patch in Mars’s southern hemisphere. It is one of the few features that are fairly easy to pick out, DLR says. It looks light because winds have covered its floor with a light coat of dust from the highlands.[2] The name dates from telescope maps of Mars’s light and dark markings: the Gazetteer lists it as a “classical albedo feature name,” adopted by the International Astronomical Union in 1973.[5] Planitia is the official term for a low plain.[20] Mars tonight shows where to find the planet in the sky.

Hellas in pictures

ESA’s Mars Express has taken many of the clearest views.

• The deepest part. On Dec 17, 2013, Mars Express caught an unusually clear view of craters in the deepest, western part of the basin, at about 15 m (49 ft) per pixel. The larger crater is about 25 km (16 mi) across. Flow features around it most likely come from glacial ice covered in debris.[1][2]
• Reull Vallis. A channel almost 1,500 km (930 mi) long that runs toward the floor of Hellas. Running water probably carved it long ago; ice and loose debris later flowed along it, ESA says.[18]
• Dao and Niger Valles. Outflow channels on the northeast side, where Hellas meets the volcanic region Hesperia Planum. Dao Vallis is 2,400 m (7,900 ft) deep.[19]

Hellas is one of the big landmarks on the surface of Mars, along with the canyon system Valles Marineris.

Sources

1. ESA — Mars Deep downPrimary · checked Oct 6, 2026
2. DLR (German Aerospace Center) — Seeing Hellas PlanitiaPrimary · checked Oct 6, 2026
3. DLR (German Aerospace Center) — The icy terraces of the giant Hellas basinPrimary · checked Oct 6, 2026
4. NASA Goddard Space Flight Center — Mars Topography (MOLA)Primary · checked Oct 6, 2026
5. USGS / IAU — Gazetteer of Planetary Nomenclature: Hellas PlanitiaPrimary · checked Oct 6, 2026
6. NASA Scientific Visualization Studio — A Rotating False Color View of the Martian South Pole from MOLAPrimary · checked Oct 6, 2026
7. NASA Science (Photojournal) — The Specters of Mars (PIA21784)Primary · checked Oct 6, 2026
8. Science (AAAS) — The global topography of Mars and implications for surface evolution (Smith et al., Science, 1999)Primary · checked Oct 6, 2026
9. PNAS — Use of spacecraft data to derive regions on Mars where liquid water would be stable (Lobitz et al., PNAS, 2001)Primary · checked Oct 6, 2026
10. NASA Science — Mars: FactsPrimary · checked Oct 6, 2026
11. NASA NSSDCA — Mars Fact SheetPrimary · checked Oct 6, 2026
12. NASA NSSDCA — Earth Fact SheetPrimary · checked Oct 6, 2026
13. NASA JPL — NASA Spacecraft Detects Buried Glaciers on MarsPrimary · checked Oct 6, 2026
14. NASA JPL — Three Craters (PIA11433)Primary · checked Oct 6, 2026
15. NASA Science — The 2001 Great Dust Storms – Hellas/Syrtis MajorPrimary · checked Oct 6, 2026
16. NASA Science (Photojournal) — The Moon’s Largest Impact Basin (PIA13496)Primary · checked Oct 6, 2026
17. Lunar Reconnaissance Orbiter Camera team, Arizona State University — The Moon’s largest impact basinPrimary · checked Oct 6, 2026
18. ESA — Reull Vallis: a river ran through itPrimary · checked Oct 6, 2026
19. ESA — The eroded valleys of Dao and Niger VallesPrimary · checked Oct 6, 2026
20. USGS / IAU Gazetteer of Planetary Nomenclature — Descriptor Terms (Feature Types)Primary · checked Oct 6, 2026