Hellas Planitia

Hellas Planitia is the largest well-preserved visible impact structure on Mars and one of the largest multi-ring impact basins in the Solar System. Centered near 42.4° S, 70.5° E within the heavily cratered southern highlands, the main inner depression spans approximately 2,300 km east-to-west by 1,500 km north-to-south, while outer concentric structural rings and the surrounding uplifted ejecta plateau extend across a diameter of up to 4,000 km. Plunging 7,152 m below the Martian topographic datum (and more than 9 km below its raised annular rim crest), Hellas Planitia contains the lowest topographic point on Mars—located inside a 12-km sub-crater informally named Badwater Crater on the western basin floor—where the surface atmospheric pressure reaches 1,155 to 1,240 pascals (11.5 to 12.4 mbar), nearly double the standard Martian datum pressure.

Because the floor of Hellas Planitia frequently fills with bright water-ice fogs, seasonal carbon dioxide frost, or reflective yellow dust clouds contrasting against the dark surrounding highland plains of Syrtis Major and Tyrrhena Terra, the basin was among the earliest Martian albedo features mapped by telescope. Following the opposition of 1862, the British astronomer Richard Anthony Proctor named the bright circular region Lockyer Land in honor of Sir Joseph Norman Lockyer. During the opposition of 1877, Giovanni Schiaparelli replaced earlier nomenclature with classical Mediterranean geography, naming the bright plain Hellas (the ancient Greek name for Greece).

For nearly a century, planetary astronomers assumed Hellas was an elevated highland plateau because of its bright, frost-covered appearance. That misconception ended when NASA's Mariner 9 (1971–1972) and Viking orbiters (1976–1980) imaged the bounding scarps and performed ultraviolet and infrared atmospheric pressure sounding, proving that Hellas is an immense sunken impact basin. In the late 1990s, the Mars Orbiter Laser Altimeter (MOLA) and gravity radio-science experiments aboard Mars Global Surveyor mapped the full bathymetric relief and Bouguer gravity anomaly of the Circum-Hellas province.

Crater size-frequency distributions on the annular rim and outer rings constrain the formation of Hellas Planitia to the Early Noachian epoch, approximately 4.0 to 4.1 billion years ago, concurrent with the Late Heavy Bombardment across the inner planets. Hydrocode impact simulations in planetary science indicate that the basin was excavated when an asteroid or protoplanetary remnant roughly 200 to 300 km in diameter struck the southern Martian lithosphere at an oblique angle from the northwest, releasing ~5 × 10²⁷ J of energy and excavating a transient cavity over 1,200 km wide and 100 km deep.

This impact permanently transformed the crustal architecture of the southern hemisphere across three geophysical dimensions:

1. Excavation and Crustal Thinning: Combined MOLA topography and Mars Reconnaissance Orbiter gravity inversions demonstrate that the southern highland crust surrounding Hellas averages 55 to 65 km in thickness. Directly beneath the central floor of Hellas Planitia, the impact stripped away more than half of the crust, leaving a thin crystalline floor only 20 to 27 km thick above an uplifted mantle Moho plug that generates a pronounced positive Bouguer gravity anomaly (+400 to +500 mGal).
2. The Circum-Hellas Highland Plateau: Approximately 2.5 to 3.5 × 10⁹ km³ of crustal and upper-mantle rock was excavated from the transient cavity and deposited outward as a continuous annular ejecta blanket up to 2 to 4 km thick, extending more than 1,500 km from the basin rim. Distributed across this elevated Circum-Hellas plateau are ancient volcanic centers (Patera structures)—including Hadriacus Mons (Hadriaca Patera), Tyrrhenus Mons, Amphitrites Patera, and Peneus Patera—where Late Noachian and Early Hesperian magma rose along deep concentric impact ring fractures to erupt pyroclastic ash sheets and low-viscosity basalt flows.
3. Shock Metamorphism and Impactites: Orbital thermal infrared and near-infrared spectroscopy of peak-ring massifs and craters punching through the basin rim reveals orthopyroxene-rich noritic lower crust, olivine-bearing impact-melt breccias, impactite glass, and shocked plagioclase converted to isotropic maskelynite, analogous to the deep crustal signature exposed inside the lunar South Pole–Aitken Basin.

Because Hellas Planitia lies more than 7 km below the Martian reference datum—and nearly 10 km below the surrounding highland rims—the column of carbon dioxide atmosphere above the basin floor is thicker than anywhere else on the planet. Whereas the Martian zero-elevation datum is defined by the triple-point pressure of water at 610.5 Pa (6.1 mbar) and the 21.9-km summit of Olympus Mons experiences only ~30 to 72 Pa, the deepest floor of Hellas Planitia maintains a surface atmospheric pressure of 1,155 to 1,240 Pa (11.5 to 12.4 mbar), fluctuating by ~25% seasonally as CO₂ condenses onto and sublimates from the southern polar cap.

This elevated atmospheric pressure has direct thermodynamic consequences for water on Mars:

• Thermodynamic Stability Against Boiling: Across high-altitude Martian plateaus where atmospheric pressure sits near or below the triple point of H₂O (611.65 Pa at 273.16 K / 0.01 °C), pure liquid water cannot exist in thermodynamic equilibrium; ice sublimates directly into vapor upon warming, or transient meltwater boils into steam. On the deep floor of Hellas Planitia, the 1,150–1,240 Pa ambient pressure sits well above the triple point, meaning that pure liquid water is stable against boiling between 0 °C and approximately +10 °C, and eutectic perchlorate or sulfate brines remain liquid across an even wider thermal window.
• Global Dust Storm Nucleation: The 9-km topographic step between the cold, frost-covered southern highland rim and the sun-warmed, higher-pressure floor of Hellas drives strong katabatic slope winds and baroclinic eddies during southern spring and summer (near Mars perihelion). Convective dust lifting along the steep northern and western scarps of Hellas frequently coalesces into regional dust storms that expand out of the basin into planet-encircling global dust storms, such as the historic 1971 Mariner 9 and 2001 Mars Global Surveyor global dust events.

Throughout the Noachian, Hesperian, and Amazonian periods, Hellas Planitia served as a closed hydrological sink for a drainage basin spanning nearly 10⁷ km² of the southern hemisphere. Along the northeastern margin of the basin below Hadriacus Mons, four large Hesperian outflow channels—Dao Vallis, Niger Vallis, Harmakhis Vallis, and Reull Vallis—incise the highland slope and empty directly onto the eastern floor of Hellas. Dao and Niger Valles originate in steep-walled box canyons and collapsed plains where magmatic heating from Hadriacus Mons melted subterranean ground ice, triggering catastrophic mud and water floods that carved canyons up to 40 km wide and 1.5 km deep over lengths of 800 to 1,200 km.

Within the lowest interior plains of Hellas below −5,800 m elevation, HiRISE and CTX imagery reveals finely laminated lacustrine mudstones, rhythmic beds, polygonal desiccation cracks, and bench-like shoreline terraces near the −5.8 km and −3.1 km contours, indicating that runoff and outflow-channel discharges pooled into a deep Noachian–Hesperian Hellas paleolake. During the later Amazonian period, high-obliquity orbital cycles shifted water ice to mid-latitude highland slopes at 35° to 55° S. Consequently, the eastern and southern margins of Hellas Planitia host abundant lobate debris aprons (LDAs) and concentric crater fill. Subsurface radar sounding by the Shallow Radar (SHARAD) instrument aboard MRO confirmed that these lobate aprons up to 500–800 m thick consist of >80% to 90% pure water ice insulated beneath a meters-thick lag of rocky sublimation till.

Why is the lowest point on Mars located inside Hellas Planitia?

The combination of a giant Early Noachian impact that excavated ~35 to 40 km of southern highland crust and the subsequent lack of thick plate-tectonic sediment or ocean water to fill the cavity left a permanent 9-km-deep lithospheric depression. Inside the western floor of Hellas at 32.7° S, 62.2° E, a secondary 12-km impact crater (informally named Badwater Crater) punched deeper into the already depressed basin floor, reaching an elevation of −7,152 m (−23,465 ft) below the Martian datum.

Could liquid water exist on the floor of Hellas Planitia today?

Pure liquid water could remain transiently stable against boiling on the floor of Hellas Planitia during summer afternoons when surface temperatures rise between 0 °C and +10 °C because the ambient atmospheric pressure (~1,150–1,240 Pa) is roughly double the 611.65-Pa triple-point pressure of water. However, because the dry Martian atmosphere has a very low water-vapor partial pressure, any exposed pool of pure water would evaporate rapidly into the dry air unless stabilized by deliquescent perchlorate salts or sheltered beneath subsurface soil.

Did the Hellas impact trigger the volcanic activity of the Tharsis rise?

The center of Hellas Planitia (42.4° S, 70.5° E) lies nearly antipodal to the Alba Mons and northern Tharsis volcanic province in the opposite hemisphere. Several geodynamic models have proposed that focused seismic shock waves and mantle decompression antipodal to the Hellas impact helped localize deep mantle plumes beneath Tharsis, though crater-counting evidence indicates that Tharsis volcanism continued for over three billion years afterward.

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