A4. Planetary Atmospheres & Climates

Quick takeaway: Earth’s thick, N₂/O₂ atmosphere, hydrologic cycle, and magnetosphere produce warm, wet, and shielded conditions. Mars offers a thin CO₂ atmosphere, large day–night temperature swings, frequent dust activity (from dust devils to planet-encircling storms), dimmer sunlight (~43% of Earth’s), seasonal “breathing” of its atmosphere with the polar caps, and significantly higher radiation at the surface—all central design constraints for Martian habitats and operations.

Atmosphere & pressure

Composition (by volume, dry air):

Earth: ~78% N₂, ~21% O₂; trace Ar, CO₂ (~420 ppm), etc

Mars: ~95% CO₂, ~2.6% N₂, ~1.9% Ar; trace O₂, CO, H₂O (≈10² ppm), noble gases.

Mean surface pressure:

Earth: ~1014 mbar (101.4 kPa)

Mars: ~6.36 mbar (0.636 kPa) at mean radius; seasonally varies roughly 4–9 mbar and by ~25% year to year

Scale height (how fast air thins with altitude):

Earth: ~8.5 km

Mars: ~11 km (thinner but “puffier” column)

Total atmospheric mass:

Earth: ~5.1 × 10¹⁸ kg

Mars: ~2.5 × 10¹⁶ kg

Temperatures & diurnal swing

Global mean near-surface temperature:

Earth: ~288 K (15 °C)

Mars: ~214 K (–59 °C)

Typical day–night range at reference sites:

Earth: ~10–20 °C (varies by region/season)

Mars: Viking site range ~184–242 K (–89 to –31 °C); equatorial summer afternoons can reach ~20 °C, but nights plunge far below freezing

Insolation & daylight “feel”

Top-of-atmosphere solar irradiance (mean):

Earth: ~1361 W/m²

Mars: ~586 W/m² (~43% of Earth); surface values vary strongly with dust and season.

Perception: Earth has bright blue skies; Mars daylight is dimmer and often butterscotch due to dust, with blue-tinted sunsets near the Sun

Seasons & large-scale cycles

Axial tilt (drives seasons):

Earth: ~23.44°.

Mars: ~25.19° (Earth-like seasons but more asymmetric because of higher orbital eccentricity)

Seasonal pressure “breathing” (Mars only): ~25–30% atmospheric mass exchange as CO₂ freezes onto and sublimes from the winter pole; this drives global pressure swings each year

Weather & winds

Typical near-surface winds:

Earth: 0–100 m/s (tropical cyclones, jet streaks)

Mars: a few m/s in fair weather; 17–30 m/s during dust storms at lander sites; localized gusts and dust devils are common

Global dust storms (Mars): episodic planet-encircling events can raise optical depth τ above ~10, dimming noon to deep twilight (e.g., 2018 storm that ended Opportunity)

Clouds, precipitation, & surface volatiles

Earth: robust hydrologic cycle with liquid precipitation; diverse clouds (H₂O droplets/ice).

Mars: water-ice and CO₂-ice clouds; CO₂ frost and seasonal polar caps; liquid water is unstable at the surface at present pressures/temps (brines may be transient)

Sky color & twilight

Earth: Rayleigh-scattered blue daytime sky; red/orange sunsets with aerosols.

Mars: dust-tinted butterscotch days; blue around the Sun at dusk/dawn because fine dust preferentially lets blue light penetrate in forward scattering

Radiation & shielding

Earth: strong global magnetosphere and thicker atmosphere → lower surface GCR dose.

Mars: no global magnetic field; thin air → ~0.67 mSv/day average surface dose measured by Curiosity’s RAD (cruise to Mars ~1.8 mSv/day). Auroras can occur across wide latitudes.

“Wind + dust” vs. mechanical effects

Earth: dense air imparts large dynamic pressure; wind-blown sand/ash can erode rapidly.

Mars: despite high speeds, low air density means lower force on structures; however, dust accumulation and abrasion are persistent engineering problems

Notable environment-shaping facts

Triple-point constraint (Mars): mean pressure sits near water’s triple point, so stable liquid water is rare at the surface today (favoring ice/sublimation)

Polar reservoirs: both planets host large polar ice deposits; on Mars, the seasonal CO₂ cap condenses/sublimes each year atop perennial H₂O ice

Energy & reflectivity (context for climate/insolation)

Solar irradiance at top of atmosphere (mean):

Earth: ~1,361 W/m²

Mars: ~586 W/m² (~43% of Earth’s)

Bond albedo / geometric albedo:

Earth: 0.294 / 0.434

Mars: 0.250 / 0.170

Reference elements (J2000; for ephemerides)

Earth (mean orbital elements): semimajor axis 1.00000011 AU; e = 0.01671022; i = 0.00005°; Ω = −11.26064°; ϖ = 102.94719°; L = 100.46435°

Mars (mean orbital elements): semimajor axis 1.52366231 AU; e = 0.09341233; i = 1.85061°; Ω = 49.57854°; ϖ = 336.04084°; L = 355.45332°

Notes. Definitions of terms (eccentricity, sidereal/tropical periods, etc.) follow standard conventions. Values are mean/standard references and can vary slightly with epoch and model.