Rocky planet
Mars
Mars looks familiar enough to invite comparisons with Earth: it has days, seasons, clouds, mountains, and ice. Then the differences start adding up. The air is extremely thin, the surface is cold, and the landscape preserves a long history of water that is mostly gone from the surface today.
Updated · About 6 minutes to read
- Place in the Solar System
- Fourth planet from the Sun
- Mean radius
- About 3,390 km
- Average orbital distance
- About 228 million km, or 1.52 AU
- Solar day, called a sol
- About 24 hours 39 minutes
- Year
- About 687 Earth days
- Surface gravity
- About 38% of Earth's
- Moons
- Phobos and Deimos
- Main atmospheric gas
- Carbon dioxide
Rounded reference figures: NASA reference data. Distances are orbital averages, not a live position; temperatures describe the stated layer or context.
A smaller world with an enormous landscape
Mars has roughly half Earth's diameter. Its rusty color comes from iron-bearing material that has oxidized, but close-up images include browns, tans, and darker rocks as well as red dust. Olympus Mons and Valles Marineris show how misleading a simple globe can be: the planet contains a huge volcanic mountain and a canyon system thousands of kilometers long. Their dimensions need a stated reference level, especially when comparing a Martian mountain with a mountain measured from sea level on Earth. The basic point is clear even without turning it into a tallest-mountain contest: this is a small planet with very large geological features.
A Martian day is close to ours; the year is not
A solar day on Mars is a sol, lasting about 24 hours and 39 minutes. Its rotation relative to distant stars is slightly shorter, so those two measurements should not be swapped. A Martian year lasts about 687 Earth days. An axial tilt of roughly 25 degrees gives Mars seasons, but the orbit is more stretched out than Earth's, making the seasons unequal in length. Always check whether a timeline uses Earth days or sols. A rover's mission day count is useful for following its work, but it is not a date in an Earth calendar.
The thin air changes almost everything
Mars has an atmosphere dominated by carbon dioxide, with surface pressure much lower than Earth's. Pressure varies with elevation and season; one quoted number cannot describe every place. The thin air does not make dust unimportant. Dust can spread through the atmosphere, change heating, and affect spacecraft operations. Conditions also differ from one time of day and location to another. A warm reading at one spot is not evidence that the planet has a comfortable Earth-like climate. Temperature, pressure, radiation, and available water have to be considered together.
The clearest water story is written in the rocks
Jezero Crater was chosen for Perseverance because the landscape preserved evidence of an old lake and river delta. Sediments can record where water moved, what it carried, and how the environment changed. Rover observations let scientists compare that large-scale pattern with the chemistry and texture of individual rocks. A delta is particularly interesting because flowing water can bring material from a wider area and deposit it together. A habitable environment means conditions could have supported life; it is not, by itself, a finding that life was there.
Water remains, but headlines need context
There is water ice on Mars, including in polar regions and below parts of the surface. Instruments can also detect hydrogen that is interpreted in terms of water-bearing material. Those findings are not automatically photographs of an underground lake. For example, the Trace Gas Orbiter's work in Valles Marineris identified a hydrogen-rich region, and researchers considered what form the associated water could take. When reading a discovery, check what was actually measured, how deep the instrument can probe, and which explanation remains uncertain. 'Water on Mars' can refer to ice, vapor, hydrated minerals, or evidence of ancient liquid water.
How a planet loses part of its atmosphere
MAVEN studied the Martian upper atmosphere and its interaction with the Sun. Its observations helped show how gas can escape into space, contributing to the explanation for Mars becoming a much drier world. Solar wind and radiation are part of that history, but a single process does not explain every detail of the planet's climate. Measurements of different isotopes and escaping particles help researchers test possible histories rather than simply guessing from the present landscape.
The water story is still being revised
In September 2026, NASA described Perseverance findings from Jezero's Margin Unit that point to several episodes of water interacting with rock, including hot groundwater. That is a useful example of why a planetary history is not settled by one photograph or one mission milestone. New evidence can change the interpretation of a familiar location. These pages are starting points, so the linked original reports matter when you want the details or a later update.
Two moons and a different view of home
Phobos and Deimos are small, irregular moons rather than a close match for Earth's large Moon. Their origin remains a research question, so describing them as definitely captured asteroids would go too far. Mars gives us a way to compare how rocky planets develop without pretending they started alike or followed one simple path. Keep the distinction between evidence for old habitable environments and evidence for life. They are connected questions, but they require different kinds of proof.
For the wider context, read Rocky planets, super-Earths, sub-Neptunes, and gas giants.
Reading a landscape shaped by changing water
A branching valley, a layered sedimentary deposit, and a mineral that forms in water are different kinds of evidence. Together they can support a history of flowing or standing water on ancient Mars. They do not automatically tell us that a lake lasted for millions of years or that the whole planet was warm at the same time.
Rovers add local measurements to the broad context supplied by orbiters. A rover can examine grain sizes, mineral chemistry, textures, and the relationships between exposed layers. An orbiter can show how that outcrop fits into a crater, river system, or regional deposit. Connecting the two scales helps researchers test whether a feature formed in water, wind, volcanic activity, or later alteration.
Modern Mars has low surface pressure, cold conditions, seasonal frost, and a lively dust cycle. Water ice remains important, but a report of water-bearing minerals is not a report of a present surface river. Ancient habitability, modern environmental conditions, and evidence of life are three separate research questions. A promising rock can motivate further study without settling the biological question.