Posted by Mars is the only other planet whose surface features you can pick out through a backyard telescope on a steady night, and that accident of nearness explains why almost everything we know about Mars the planet arrived in layers: first as smudges of light and dark that mapmakers argued over, then as grainy flyby frames, and lately as seismic traces recorded by a dome sitting on volcanic plains in Elysium Planitia. It is a world a little over half Earth’s diameter, with roughly a tenth of Earth’s mass and 38 percent of its surface gravity, carrying a thin carbon dioxide atmosphere, two small lumpy moons, and both the tallest volcano and the longest canyon system anyone has found in the Solar System.
What follows is a portrait of the place itself. How long the day runs, why the seasons come out lopsided, what the air can and cannot do, what the rock is doing underneath.
A Day That Fits, a Year That Doesn’t
A Martian solar day, called a sol, lasts 24 hours, 39 minutes and 35 seconds. Those extra forty minutes sound trivial until someone lives on them: rover teams on Mars time slide their shifts later every day, and within a fortnight they are eating breakfast at midnight. Bodies notice.
The year is where the family resemblance stops. Mars needs about 687 Earth days, roughly 669 sols, to get around the Sun, so each season stretches to nearly twice its terrestrial length.
The axial tilt is the real surprise. At about 25.2 degrees it sits within spitting distance of Earth’s 23.4, which is why Mars has proper seasons rather than one uniform climate. The orbit, though, is stretched: from roughly 207 million kilometres at perihelion out to about 249 million at aphelion, with southern summer falling near the close approach. The calendar comes out in unequal parts: northern spring runs something like 194 sols while northern autumn is squeezed into about 143. Southern summers are short and comparatively fierce, southern winters long and savage.
None of it is locked in place, which is the strangest thing about Mars the planet. Earth’s oversized Moon holds our tilt steady to within a degree or so, and Phobos and Deimos are far too small to do anything of the kind. Calculations by Jacques Laskar and colleagues indicate that Martian obliquity wanders chaotically over millions of years, plausibly between roughly 15 and 35 degrees. At high tilt the poles take in more annual sunlight than the equator and ice migrates towards the middle latitudes; orbiters have since found glacier-like deposits lying where that picture says they should be. Nobody can reconstruct the exact sequence, though. The pattern is solid, the timeline is not.
Air You Could Almost Ignore
The atmosphere is about 95 percent carbon dioxide, with a few percent nitrogen and argon between them and a whisper of oxygen. Mean surface pressure runs near 6 millibars, under one percent of Earth’s sea level value, roughly what you would find 30 kilometres up in our own sky.
That figure puts Mars uncomfortably close to the triple point of water. A glass of it poured out on the plains of Chryse would boil and freeze at more or less the same moment.
The air also comes and goes. Each winter something like a quarter of the entire atmosphere freezes onto the winter pole as carbon dioxide frost and snow, then returns to gas in spring. The Viking landers measured the cycle directly in the late 1970s, watching surface pressure climb and drop by about 25 percent over a Martian year. A whole planet’s air condensing out and re-inflating on schedule has no terrestrial equivalent.
Temperatures follow the thinness. The global average sits near minus 63 degrees Celsius, and a summer afternoon at the equator can push the ground above freezing while the air a metre higher stays tens of degrees colder, because there is so little gas around to move heat about. Curiosity’s weather station in Gale Crater logs daily swings that would take terrestrial machinery apart. Polar winter nights get down near minus 143.
The Bulge That Cracked a Hemisphere
One region dominates the geology. Tharsis is a volcanic province some 5,000 kilometres across, standing up to 7 kilometres above the surrounding datum, carrying a cluster of enormous shield volcanoes: Arsia, Pavonis and Ascraeus Mons in a tidy diagonal row, with Olympus Mons off to the northwest. The load was heavy enough to flex the crust and tear it, throwing fractures radially outward for thousands of kilometres.
Olympus Mons rises roughly 22 kilometres and spreads about 600 across, ringed in places by a scarp several kilometres tall. Standing on it, you would never know. The flanks average around five degrees of slope, so the mountain would read as a gently rising plain that runs past the horizon. It reached that size because Mars has no plate tectonics. On Earth the Pacific plate drags over the Hawaiian hotspot and smears the output into a chain of islands; Martian crust stays put, and every eruption piles onto the same address for billions of years.
Then there is the gash.
Valles Marineris runs more than 4,000 kilometres along the equator, up to 200 wide and 7 deep in the worst of it. The name comes from Mariner 9, which arrived in November 1971 to find the planet buried under a global dust storm; controllers waited weeks for the murk to settle, with only the volcano summits poking above it. The canyon is not a river valley. It is structural, a rift system tied to the Tharsis uplift, later widened by colossal landslides and probably reworked by water and ice.
The hemispheres do not match either: smooth low plain across the northern third, battered highland several kilometres higher in the south. The cause of that crustal dichotomy, one giant impact or slow internal convection, is still being fought over. The deepest hole is Hellas Planitia, about 2,300 kilometres wide and seven deep, where atmospheric pressure reaches its planetary maximum.
Two Different Kinds of Ice
The polar caps are not made of one substance. The residual northern cap, the part that survives summer, is water ice about 1,000 kilometres across. The southern one keeps a thin veneer of frozen carbon dioxide, a matter of metres, sitting on water ice beneath, which is much of why the poles behave so differently.
Under both lie the polar layered deposits: kilometres of alternating ice and dust, stacked like tree rings. Radar sounders on Mars Express and Mars Reconnaissance Orbiter have looked straight through and traced single layers across hundreds of kilometres. Read properly, it is a climate archive written by those obliquity cycles. Reading it properly is the hard part, and the ages pinned to the layers keep moving.
Rust, and the Planet That Vanishes
Dust is the connective tissue of Martian weather. The grains run a few microns across, light enough to stay aloft for months. Storms start local, sometimes grow regional, and every few Martian years one goes planet-encircling and hides the surface completely. The 2018 event did exactly that and finished off the Opportunity rover, which had been driving for fourteen years on a ninety-sol warranty; its last transmission came in June of that year.
They are less violent than they look. Winds reach perhaps 100 kilometres per hour, but in air one percent as dense as ours the push behind them is feeble, nothing like the opening scene of The Martian. What they really do is starve solar panels and drive the temperature down.
The colour comes from oxidised iron in that same dust, and the specifics have been reopened lately. Hematite, which forms in dry conditions, was the long-standing assumption. A 2025 analysis led by Adomas Valantinas argued that ferrihydrite, a poorly crystalline oxide that needs cool water to form, matches the orbital and laboratory spectra better. If that holds, the red is a fossil of wet chemistry rather than dry weathering. It is not settled.
What InSight Heard
All of the above is surface. The interior stayed largely guesswork until InSight set down on Elysium Planitia in November 2018 with a seismometer sensitive enough to register ground motion narrower than a hydrogen atom.
It logged more than 1,300 marsquakes before dust on its solar panels ended the mission in December 2022. The largest, in May 2022, came in near magnitude 4.7 and rang the planet for hours. The waves showed a crust thinner and more layered than expected beneath the lander, and a core that is liquid and large: early estimates put its radius near 1,830 kilometres, at a density low enough to demand a hefty fraction of light elements such as sulphur and oxygen. A later reanalysis found a molten silicate layer wrapped around the core, which shrinks it and makes it denser. The honest position is that the size of the Martian core is still being negotiated.
Not everything worked. The heat-flow probe meant to hammer five metres down never got past the topsoil, which behaved nothing like the models. That is a result too, just a maddening one.
Read enough about Mars the planet and a pattern shows up: the numbers that felt solid a decade ago are the ones most likely to move next. New radar profiles of the caps, another revision of the core, whatever the coming dust season decides to do. If you would rather follow those arguments while they are still arguments, SETIworld covers planetary science alongside the wider search for life beyond Earth, and it is decent company for your own questions.
