A5. Planetary Interiors & Fields

Interior, magnetism & heat flow

Interior structure:

Earth: layered silicate mantle over an iron core (liquid outer, solid inner); active convection.

Mars: basaltic crust and mantle over a liquid iron–sulfur core; cooler, smaller heat budget, no present global dynamo.

Magnetic field:

Earth: strong global magnetosphere from an active dynamo; shields surface from most solar/galactic radiation.

Mars: no global field; patchy crustal remanent magnetism; atmosphere/ionosphere provide limited shielding.

Heat flow & tectonics:

Earth: ~47 TW total heat flow; plate tectonics recycles crust.

Mars: lower heat flow; no plate tectonics—volcanism, impacts, and wind dominate surface evolution.

Seismology

Activity & magnitudes:

Earth: frequent earthquakes up to M9+; subduction/transform boundaries dominate.

Mars: marsquakes (generally M<5); likely driven by thermal contraction, crustal faulting, and impacts.

Crustal thickness:

Earth: highly variable (oceanic ~7 km; continental up to ~70 km).

Mars: thick, buoyant crust on average; major hemispheric dichotomy persists (southern highlands vs northern lowlands).

Geodesy & reference frames

Datums & coordinates:

Earth: geodetic latitude/longitude referenced to the WGS84 ellipsoid and geoid.

Mars: areocentric lat/long referenced to the Airy-0 prime meridian and an areoid (gravity-based datum).

Longitude sense:

Earth: east longitudes positive by convention.

Mars: modern areocentric system also uses east-positive longitudes.

Surface geology & geomorphology

Dominant processes:

Earth: active hydrology, glaciation, tectonics, biology reshape surface rapidly.

Mars: ancient water/ice activity + impacts + aeolian (wind) sculpting; slow change today.

Volcanoes:

Earth: many active, but height limited by gravity/plate motion.

Mars: giant shield volcanoes (e.g., Olympus Mons ~22–25 km above datum) aided by low gravity and long-lived hotspots.

Hydrology, cryosphere & volatiles

Liquid water at surface:

Earth: abundant; stable across wide T/P.

Mars: present climate/pressure do not permit stable liquid water; ice dominates (polar caps/permafrost); transient brines possible.

Polar caps:

Earth: H₂O ice; strong seasonal snow/sea-ice dynamics.

Mars: seasonal CO₂ ice forms and sublimates atop perennial H₂O ice—driving large annual pressure swings.

Ionosphere & radio

Layers & comms:

Earth: robust E/F layers enable HF skywave; storms can disrupt.

Mars: thinner, solar-controlled ionosphere; UHF/X/Ka deep-space links standard; conjunction outages near solar superior conjunction.

Acoustics & audibility

Speed of sound at surface:

Earth (air, 20 °C): ~343 m/s, weak frequency dependence.

Mars (CO₂, cold/low-P): ~240–250 m/s with frequency-dependent propagation (highs travel slightly faster than lows); voices sound quieter, crisper, and more attenuated with distance.

Background noise sources:

Earth: wind, water, biosphere, human activity.

Mars: wind over regolith, dust devils, mechanical rover/ISRU sounds; otherwise very quiet.

Gravity, motion & ergonomics

Surface gravity:

Earth: 1 g.

Mars: 0.38 g → longer hang times, higher jumps, lower traction; lifting feels easier but inertia unchanged.

Human movement:

A vertical leap that reaches 0.5 m on Earth would reach roughly 1.3 m on Mars (same takeoff speed), with ~1.6× airtime.

Aviation, EDL & wind loading

Flight & rotorcraft:

Earth: dense air → conventional aircraft feasible.

Mars: ultra-thin air → very high-RPM rotors/large disks required (demonstrated by Ingenuity); fixed-wing needs extreme wingspans/velocity.

Entry, Descent, Landing:

Earth: parachutes + atmosphere can dissipate most energy.

Mars: low density → supersonic parachutes + retroprop; dust entrainment and low dynamic pressure complicate design.

Illumination & sky optics (beyond raw irradiance)

Solar apparent size:

Earth: ~0.53° across.

Mars: ~0.35° average (smaller Sun).

Twilight behavior:

Earth: minutes to ~1 h depending on latitude/season.

Mars: long twilights (dust scattering keeps sky bright for up to hours); blue aureole at the Sun during dusk/dawn.

Auroras:

Earth: oval, magnetically guided.

Mars: diffuse and patchy over crustal-field regions; can be planet-wide under solar events.

Tides & “daily” environmental cycles

Gravitational tides:

Earth: strong lunar/ocean tides.

Mars: very weak solid/atmospheric tides (no oceans); thermal tides in the thin air dominate daily pressure cycles.

Atmospheric thermal tide:

Earth: modest daily pressure oscillation.

Mars: pronounced diurnal/semi-diurnal pressure cycles tied to solar heating and dust loading.

Radiation & habitability notes

Average surface dose:

Earth: low thanks to magnetosphere + atmosphere.

Mars: order-of-magnitude higher background; shielding and operations planning are mission-critical (storm shelters, regolith cover).

UV environment:

Earth: stratospheric ozone absorbs UVC/most UVB.

Mars: thin CO₂ atmosphere + little ozone → strong surface UV, especially UVB/UVC components.

ISRU-relevant chemistries

Atmospheric feedstocks:

Earth: N₂/O₂ readily available; CO₂ dilute.

Mars: CO₂-rich air for oxygen production (e.g., solid-oxide electrolysis); trace water vapor; N₂/Ar for buffer gases.

Regolith:

Earth: soils with organics/biology; diverse minerals.

Mars: basaltic regolith with perchlorates, fine dust, and abrasive grains—implications for toxicology, filtration, seals, and suits.

Communications geometry & cadence

One-way light time:

Earth internal: milliseconds–fractions of a second.

Earth↔Mars: ~3–22 minutes depending on orbital positions; blackout windows near solar conjunction; ~26-month synodic rhythm governs launch/transfer seasons.