Valles Marineris

Valles Marineris (Latin for "Mariner Valleys") is a vast equatorial system of tectonic troughs and canyons on Mars located east of the Tharsis volcanic province (spanning roughly 0° to 18° S latitude and 250° to 320° E longitude). Extending more than 4,000 km in length—nearly one-quarter of the equatorial circumference of Mars and comparable to the continental width of the United States—the canyon system reaches widths of 200 to 600 km and vertical depths of 7 to 10 km below the surrounding plateau rim. Roughly ten times longer, twenty times wider, and four to five times deeper than Earth's Grand Canyon, Valles Marineris ranks among the largest tectonic rift structures in the Solar System, surpassed in total length only by the terrestrial mid-ocean ridge system and certain chasmata belts on Venus.

Before spacecraft orbiters reached Mars, ground-based observers recorded a dark, seasonally variable albedo marking south of the Martian equator named Coprates. When NASA's Mariner 9 entered Martian orbit in November 1971 during a global dust storm, the deep troughs of Valles Marineris initially remained concealed beneath thick dust clouds while the high summits of Olympus Mons and the Tharsis Montes protruded above the haze. As the dust cleared during early 1972, Mariner 9 television cameras unveiled an interconnected chain of steep-walled canyons cutting across the elevated Thaumasia and Lunae Planum plateaus. In 1973, the International Astronomical Union named the canyon system Valles Marineris in honor of the Mariner 9 science and engineering team.

Unlike the Grand Canyon in Arizona—which was incised primarily by fluvial downcutting of the Colorado River through an uplifting plateau—Valles Marineris originated as a crustal tectonic rift and collapse structure during the Late Noachian and Early Hesperian epochs (~3.8 to 3.4 billion years ago). Only after lithospheric extension, radial dike intrusion, and normal faulting opened deep structural graben did subsurface groundwater release, sapping, lacustrine sedimentation, and giant wall-rock landslides widen and deepen the individual troughs into their modern configuration.

Valles Marineris exhibits a systematic west-to-east morphological progression across three interconnected structural domains that reflect decreasing elevation down the eastern flank of the Tharsis rise:

1. Noctis Labyrinthus (Western Uplands)

At its westernmost extremity (near 7° S, 260° E) high atop the Syria and Pavonis Planum domes at elevations of +7 to +10 km above the Martian datum, Valles Marineris begins in Noctis Labyrinthus ("Labyrinth of the Night"). This 1,200-km-wide complex consists of an intricate network of intersecting arcuate graben, fault-bounded valleys, and coalescing circular-to-elliptical pit-crater chains. Noctis Labyrinthus formed where radial and concentric extensional stress fields above Tharsis magma plumes intersected shallow subterranean ice-rich cryosphere reservoirs and magmatic dikes; withdrawal of magma and melting of ground ice caused the overlying basaltic crust to collapse into a maze of steep-walled pits.

2. The Central Parallel Chasmata

East of Noctis Labyrinthus, the canyon system organizes into immense, east-southeast-trending linear troughs termed chasmata (singular chasma, defined as a deep, elongated, steep-sided depression), separated by flat-topped horst plateaus preserving ancient Hesperian lava plains:

• Ius and Tithonium Chasmata: Two parallel western troughs separated by a narrow horst ridge; the floor of Ius Chasma displays prominent fault scarps and sapping theater-headed valleys (Louros Valles), while Tithonium Chasma exposes wind-eroded layered sulfate mounds.
• Melas, Candor, and Ophir Chasmata: The widest central basin complex of Valles Marineris, where the coalesced troughs reach a combined north-to-south span of nearly 600 km and plunge to depths exceeding 9 to 10 km below the surrounding plateau rim. These central basins contain some of the thickest stacks of sedimentary and volcanic interior layered deposits (ILDs) on Mars alongside overlapping multi-kilometer landslide aprons.
• Coprates Chasma: Extending nearly 1,000 km southeast of Melas Chasma, the dual parallel graben of Coprates Chasma exhibit classic rift-valley geometry bounded by linear normal-fault escarpments 7 to 9 km high whose triangular faceted spurs and basal fault scarps demonstrate repeated kilometers-scale vertical displacement.

3. Eastern Chaos Terrains and Outflow Channels

At its eastern terminus, Coprates Chasma and the northern troughs of Hebes and Juventae Chasmata transition into depressed expanses of chaos terrain—including Eos Chaos, Capri Chaos, Aromatum Chaos, and Ganges Chaos. Within these zones, the plateau surface foundered into jumbled arrays of tilted mesas and knobs when pressurized sub-cryosphere aquifers ruptured during the Hesperian period. Vast volumes of liquid water discharged from these eastern chaos basins to carve the circum-Chryse catastrophic flood outflow channels—including Simud Vallis, Tiu Vallis, Shalbatana Vallis, and Ares Vallis—which transported sediment northward across the Martian dichotomy into Chryse Planitia.

Geophysical modeling by Schultz (1998) and Andrews-Hanna (2012) demonstrates that Valles Marineris originated from the interplay of three lithospheric processes centered on the Tharsis volcanic rise in planetary science:

1. Tharsis Flexural Uplift and Membrane Stress: Emplacement of hundreds of millions of cubic kilometers of basaltic magma within the Tharsis province flexed the Martian lithosphere and generated a hemispherical membrane stress field. Along the radial axis running ESE from the crest of Tharsis, north–south horizontal tensile stresses exceeded the brittle fracture strength of the upper crust, nucleating deep normal faults that dropped keystone blocks down by 5 to 9 km.
2. Giant Radial Dike Swarms and Thermokarst Collapse: As mantle plumes ascended beneath Syria Planum and central Tharsis, giant radial mafic dike swarms propagated thousands of kilometers eastward through the mid-crust, melting ground ice within the thick Martian cryosphere and coupling tectonic faulting to subterranean volume loss (exemplified by enclosed troughs such as Hebes Chasma).
3. Deep Crustal Exposure and Giant Landslides: In the lower walls of Coprates and Melas Chasmata, normal faulting exposes an unweathered 8-km vertical cross-section of Hesperian basalt lava flows overlying fractured Noachian basement enriched in low-calcium pyroxene, olivine, and Fe/Mg-phyllosilicates. Giant rotational and long-runout translational landslides (up to 5,000 km³ in single failure events) subsequently widened the troughs to triple their initial tectonic fault spacing.

Rising 5 to 8 km above the chasma floors are freestanding sedimentary mountains termed Interior Layered Deposits (ILDs) or mensae (such as Candor Mensa and Juventae Mensa). Near-infrared orbital spectroscopy by OMEGA aboard Mars Express (Gendrin et al., 2005) and CRISM aboard MRO (Murchie et al., 2009) established that these rhythmically bedded strata consist of hydrated magnesium and calcium sulfate minerals—including monohydrated kieserite (MgSO₄·H₂O), polyhydrated sulfates (Mg/Fe-SO₄·nH₂O), and gypsum (CaSO₄·2H₂O)—interbedded with crystalline gray hematite and basaltic ash. Deposited in deep ancestral chasma paleolakes and upwelling groundwater spring mounds tied to water on Mars, these sulfate and opaline silica beds represent premier astrobiological targets for preserved Martian biosignatures.

Was Valles Marineris carved by a river like the Grand Canyon?

No. Whereas the Grand Canyon was carved primarily by the erosive action of the Colorado River cutting through uplifting sedimentary strata, Valles Marineris was formed primarily by crustal rifting and normal faulting as the Tharsis volcanic bulge stretched and fractured the Martian lithosphere. Liquid water played a major secondary role by pooling in ancestral chasma lakes, depositing layered sulfates, sapping canyon walls, and bursting out of the eastern chaos terrains to carve the circum-Chryse outflow channels.

Could you see from one side of Valles Marineris to the other while standing on the rim?

In narrower graben segments such as eastern Coprates Chasma (60 to 100 km wide), an observer standing on the 8-km-high northern rim on a dust-free day could see the opposing southern wall across the abyss. However, across the central coalesced basin of Melas, Candor, and Ophir Chasmata (where the canyon spans 300 to 600 km wide), the opposite rim lies far beyond the planetary horizon.

What is Hebes Chasma, and why is its missing rock volume notable?

Hebes Chasma is an enclosed, 315-km-long, 125-km-wide, 8-km-deep trough just north of the main Valles Marineris system that has no surface inlet or outlet channels, yet contains a 7-km-tall central plateau of layered sulfate deposits (Hebes Mensa). Because no rivers could carry the ~100,000 km³ of missing rock out over the rim, the trough collapsed downward into subterranean voids created when magmatic heat melted underground ice lenses and drained the resulting water and mud through deep fracture networks.

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