Mars (Planet)

Mars is the fourth planet from the Sun and the outermost of the four terrestrial planets in the Solar System, orbiting at a mean heliocentric semi-major axis of a = 1.523680 astronomical units (AU; 2.2794 × 10⁸ km) with an orbital period of 686.980 Earth days (1.8808 Julian years). With an equatorial radius of R_eq = 3,396.2 ± 0.1 km (0.5320 R_⊕), a polar radius of R_pol = 3,376.2 km, and a mass of M = 6.4171 × 10²³ kg (0.10745 M_⊕), Mars has roughly one-tenth of Earth's mass, a surface area (1.444 × 10⁸ km²) nearly equal to Earth's dry land area, and a surface gravitational acceleration of g = 3.72076 m s⁻² (~37.9% of Earth's gravity). Its lower bulk density (ρ = 3.9335 g cm⁻³) compared to Earth, Venus, and Mercury reflects a smaller metallic core mass fraction and an FeO-enriched (14–18 wt%) silicate mantle. Dynamical models such as the Grand Tack hypothesis attribute Mars's small mass to early gravitational depletion of the 1.5-AU feeding zone by migrating Jupiter.

Mars rotates prograde with a sidereal period of 24 hours, 37 minutes, and 22.66 seconds, yielding a mean solar day—termed a sol—of 24 hours, 39 minutes, and 35.244 seconds (88,775.244 s). Its current axial obliquity of 25.19° drives seasonal cycles similar to Earth's, modulated by its orbital eccentricity (e = 0.0934, perihelion 1.381 AU to aphelion 1.666 AU). Because Mars is orbited only by two small irregular moons—Phobos and Deimos (R ≈ 11.3 km and 6.2 km)—rather than a large stabilizing Moon, secular spin–orbit resonances drive chaotic obliquity variations between 0° and >45°–60° over 10⁵ to 10⁷ years, periodically mobilizing polar water ice to mid-latitudes.

Global Crustal Dichotomy, Tharsis, and Valles Marineris

Laser altimetry from Mars Global Surveyor (MOLA) and gravity mapping show that Martian topography is organized around a hemispherical Martian dichotomy:

1. Southern Highlands (~60% of surface): Ancient, heavily cratered Noachian crust standing 1 to 4 km above the datum (crustal thickness 50–70 km), indented by giant impact basins including Hellas Planitia (2,300 km diameter, >7.1 km deep), Argyre Planitia, and Isidis Planitia.
2. Northern Lowlands (Vastitas Borealis, ~40% of surface): Smooth, lightly cratered plains lying 3 to 6 km below datum atop a thinner crust (~25–35 km), interpreted as an ancient planetary-scale oblique impact scar (Borealis basin, ~8,500 × 10,600 km) or degree-1 mantle convection structure.

Straddling the dichotomy boundary is the Tharsis volcanic rise, a 5,000-km-wide volcanic plateau (~3 × 10⁸ km³ of basalt) built by long-lived mantle plumes beneath a stationary stagnant-lid plate. On its western flank stands Olympus Mons, a shield volcano ~624 km wide rising 21.9 km above the datum (~26 km above surrounding plains), alongside the three Tharsis Montes. Flexural uplift and dike intrusion from Tharsis fractured the eastern highland crust to open Valles Marineris—a tectonic graben canyon system over 4,000 km long, up to 200 km wide, and 7 to 10 km deep.

InSight Seismology and the Liquid Metallic Core

Between 2018 and 2022, NASA's InSight lander recorded over 1,300 marsquakes and core-transmitted (SKS) / core-reflected (ScS) seismic phases, proving that Mars possesses a fully liquid metallic core with a radius of 1,830 ± 40 km (or 1,650–1,680 km beneath a ~150-km basal molten silicate layer). Its low core density (~6.0–6.5 g cm⁻³) requires 15 to 20 wt% light elements (S, O, C, H), preventing inner-core solidification.

Orbital spectroscopy (Mars Express OMEGA, MRO CRISM) and surface rovers (Spirit, Opportunity, Curiosity at Gale Crater, and Perseverance at Jezero Crater) divide Martian history into three epochs:

1. Noachian Epoch (4.1 to 3.7 Ga — The Phyllosilicate Era): Records widespread liquid water on Mars, including branching dendritic valley networks, open-basin lakes, and river deltas (such as Jezero Crater). Neutral-to-alkaline weathering altered basalt into Fe/Mg phyllosilicates (smectite clays) and carbonates, while Curiosity at Gale Crater documented neutral-pH lacustrine mudstones containing CHNOPS bioessential elements and preserved organic molecules.
2. Hesperian Epoch (3.7 to 3.0 Ga — The Acidic Sulfate Era): Volcanic outgassing of SO₂ and H₂S acidified surface waters, precipitating layered hydrated sulfates (gypsum, kieserite, and acidic jarosite, KFe³⁺₃(SO₄)₂(OH)₆, at Meridiani Planum), while overpressured subsurface aquifers burst forth to carve giant flood outflow channels (Kasei and Ares Valles).
3. Amazonian Epoch (3.0 Ga to Present — The Anhydrous Ferric Oxide Era): Three billion years of cold, hyper-arid oxidation coated the surface in wind-blown nanophase anhydrous ferric oxides (hematite, α-Fe₂O₃) alongside the North and South Polar Layered Deposits (~2–3 km thick H₂O ice caps with seasonal CO₂ frost).

Crustal paleomagnetic anomalies detected by Mars Global Surveyor across the southern highlands—and their absence inside the ~4.0–3.9 Ga Hellas, Utopia, and Argyre basins—prove that Mars's core dynamo shut down around 4.0 to 4.1 Ga. Exposed directly to the solar wind and coronal mass ejections, the unmagnetized upper atmosphere underwent ion pickup, sputtering, and photochemical escape exceeding Mars's 5.03 km s⁻¹ escape velocity. NASA's MAVEN orbiter confirmed via heavy-isotope enrichment (³⁸Ar/³⁶Ar, ¹³C/¹²C, and D/H ≈ 5–8 × VSMOW) that Mars lost 65% to >90% of its early volatile atmosphere to space, leaving today's 6.1-mbar (610 Pa) atmosphere of 95.32% CO₂, 2.7% N₂, 1.6% Ar, and 0.13% O₂.

Why is Mars red?

Mars gets its reddish-orange color from global dust rich in nanophase ferric iron oxides (Fe³⁺), primarily hematite (α-Fe₂O₃) and goethite. Beneath this millimeter-to-meter dust veneer, Martian bedrock is dark gray ferrous (Fe²⁺) volcanic basalt.

Can liquid water exist on the surface of Mars today?

Pure liquid water is unstable at the surface because the mean pressure (~6.1 mbar) sits at the triple point of water (6.11 mbar) and temperatures average ~210 K (−63 °C). However, deliquescent perchlorate salts (Mg(ClO₄)₂ and Ca(ClO₄)₂) depress the freezing point of brines to ~200–225 K, while ~5 million km³ of water ice is stored in the polar caps and subsurface glaciers.

Why did Olympus Mons grow three times taller than Mount Everest?

Because Mars operates in a stagnant-lid single-plate regime without horizontal plate drift and has lower surface gravity (g = 3.72 m s⁻²), a stationary mantle plume beneath Tharsis erupted lava onto the same location for over three billion years, building Olympus Mons to 21.9 km height atop a thick elastic lithosphere.

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