🔴 Mars — The Red Planet & Its Lost Climate −60°C mean surface temp 95.3% CO₂ atmosphere 0.006 bar surface pressure
Range: −125°C (winter poles) to +20°C (summer equator midday)
Less than 1% of Earth's; too thin for liquid water (below triple point) at most temps
Yet surface is cold — CO₂ provides only ~5 K of greenhouse warming (thin atmosphere)
Receives only ~43% of Earth's solar flux (589 W/m²); long cold winters
Remarkably similar to Earth's day — one of the few shared traits
Similar to Earth's 23.4° — gives Mars seasons; but tilt varies chaotically 0–60° over millions of years
Largest volcano in the solar system; 600 km wide; 3× the height of Everest
Without magnetic shielding, solar wind stripped the atmosphere over ~500 Myr
★ From Warm and Wet to Cold and Dead — Mars's Climate Collapse
Mars is arguably the most climate-relevant planet after Venus for understanding Earth's long-term trajectory — but for opposite reasons. While Venus shows what happens when a planet gets too hot, Mars shows what happens when a planet loses its climate system entirely. Early Mars (~4.1–3.5 billion years ago) was likely warm enough for rivers, lakes, and possibly oceans — as evidenced by ancient valley networks, deltaic sediments, and mineralogical signatures of water-rock interaction. Today, Mars is a frozen desert with a wisp of an atmosphere and a surface sterilised by UV radiation.
The key events that stripped Mars of its habitable climate: (1) loss of its global magnetic dynamo ~4.2 Ga, exposing the atmosphere to solar wind sputtering; (2) atmospheric stripping by the solar wind over ~500 million years (confirmed by NASA MAVEN measurements showing Mars loses ~100 grams of atmosphere per second even today); (3) cooling of the mantle, ending volcanism that could have replenished the atmosphere; and (4) carbonate formation and regolith trapping of CO₂.
Mars vs. Earth — Planet Comparison
Temperature Distribution — Mars Seasonal Cycle
Atmospheric Pressure Seasonal Variation (Pa)
Martian Atmosphere — Why It's So Thin
Solar wind stripping
Without a global magnetic field (lost when Mars's iron core solidified ~4.2 Ga), the solar wind directly interacts with the Martian ionosphere. Charged particles from the Sun transfer momentum to atmospheric ions, which then escape to space. NASA's MAVEN mission measured this process directly: Mars currently loses ~100 g/s of atmosphere. Extrapolating back over 3.5+ billion years, solar wind stripping is responsible for the loss of a significant fraction of Mars's early, thicker atmosphere.
CO₂ condensation cycle
Mars is cold enough that CO₂ — the main atmospheric gas — actually freezes out at the winter poles. Each Martian winter, ~25% of the entire atmosphere condenses onto the polar caps as dry ice (CO₂ ice). This is why Mars's atmospheric pressure varies seasonally by ~25% as measured by Viking landers — the atmosphere literally freezes and thaws with the seasons.
Weak gravity
Mars's surface gravity (0.38 g) is insufficient to retain a dense atmosphere against Jeans escape — the thermal leaking of light molecules (H, H₂, He) from the upper atmosphere. Combined with UV photolysis of water vapour, this mechanism has stripped all of Mars's water over geological time.
★ Ancient Mars — Oceans, Rivers, and a Possible Biosphere
The evidence that Mars once had liquid water is now overwhelming: valley networks etched across ancient highland terrain, river delta deposits (including the spectacular Jezero Crater delta being explored by Perseverance), mineralogical evidence of clays, carbonates, and sulphates requiring water-rock interaction, and radar evidence of possible present-day subglacial liquid water under the south polar ice cap. The critical question is no longer "was there water?" but "for how long, how deep, and was there life?"
Mars Water History Timeline
Evidence for Ancient Water — Key Findings
Valley networks
Branching valley systems cut across ancient (~3.5–4 Ga) highland terrain — consistent with surface runoff from precipitation (rain or snowmelt). ~40,000 km of ancient valleys mapped by Mars Global Surveyor.
Jezero Crater delta (Perseverance site)
A 45-km-wide impact crater that once held a lake fed by a river delta. Perseverance has found organic molecules and conditions that were habitable ~3.5 Ga. Sample return mission planned to bring these rocks to Earth.
Clay minerals (phyllosilicates)
Widespread deposits of clay minerals (smectites, serpentine) detected by OMEGA (Mars Express) and CRISM (MRO) — these minerals only form in neutral-to-alkaline liquid water. Found in Noachian (~4 Ga) terrain globally.
Subglacial liquid water?
MARSIS radar (Mars Express) detected a ~20 km wide reflective zone 1.5 km beneath the south polar layered deposits — potentially a subglacial liquid water lake (Orosei et al. 2018, Science). Subsequent analysis is debated; could be liquid brine or conductive clay layers.
Mars Seasonal Temperature — Equator vs. Poles
Global Dust Storms — Mars's Extreme Weather
Why Mars has global dust storms
Mars is the only known planet with regular global-scale dust storms. The thin atmosphere and low gravity mean dust can be suspended for months. As sunlight heats dust particles, they warm the atmosphere locally, creating convection that lifts more dust — a positive feedback that can scale to planet-encircling storms within weeks. These global storms occur every 2–3 Martian years (4–6 Earth years), during southern hemisphere summer when Mars is closest to the Sun (perihelion).
Impact on temperature and solar flux
A global dust storm can reduce surface solar flux by 99% — reducing sunlight reaching the surface to near zero. Surface temperatures may drop 20–30°C during a storm as dust blocks insolation, while the atmosphere itself warms as dust absorbs solar radiation. The 2018 global dust storm (Mars Year 34) was responsible for ending NASA's Opportunity rover mission — solar panels could no longer generate sufficient power.
Dust devils and regional storms
Even outside global storm season, convective dust devils (vortices up to 8 km tall, detected by InSight) and regional dust storms are common. They are important for redistributing Martian regolith and maintaining the background atmospheric dust loading that persists year-round.
Mars Exploration & the Question of Past Life
Mars is humanity's primary target for interplanetary exploration and is the most likely place in the solar system (other than Earth) where life may have existed. Current missions are systematically building the case — or finding its limits — for a habitable ancient Mars.
Active Mars Missions (2024–2026)
Terraforming Mars — The Climate Science Case
Terraforming Mars (making it habitable) is a multi-millennial engineering challenge. The fundamental barriers are: (1) insufficient CO₂ reservoir to create a thick enough atmosphere even if all polar and regolith CO₂ were released (~+15 mbar — a fraction of what's needed); (2) no global magnetic field to protect against solar wind stripping any terraformed atmosphere; (3) insufficient solar flux at 1.52 AU; (4) extremely low nitrogen inventory. Elon Musk's "nuke the poles" concept would release perhaps 1–5% of the CO₂ needed and is not a scientifically viable terraforming pathway.