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Short answer

Mars is the fourth planet from the Sun: a cold, dusty desert with a thin atmosphere, about half Earth's size. Its day is 24.6 hours, and it has two small moons, Phobos and Deimos. [S1][S2]

More

• Average distance from the Sun: 1.5 AU. [S1]
• It has seasons, polar ice caps, extinct volcanoes and canyons. [S1]
• Evidence shows it was once wetter and warmer; scientists look for signs of ancient life. [S1]
• Only robots live there so far. [S2]

Claims

• id: C1 claim: Mars is a cold desert with a thin atmosphere. source: S1 evidence: "is a dusty, cold, desert world with a very thin atmosphere"
• id: C2 claim: Mars is about half the size of Earth. source: S1 evidence: "Mars is about half the size of Earth."
• id: C3 claim: Mars rotates once every 24.6 hours. source: S1 evidence: "it completes one rotation every 24.6 hours"
• id: C4 claim: Mars has two small moons. source: S1 evidence: "Mars has two small moons, Phobos and Deimos"
• id: C5 claim: Mars is inhabited only by robots. source: S2 evidence: "the only planet we know of inhabited entirely by robots"

Sources

• [S1] Mars: Facts. NASA. https://science.nasa.gov/mars/facts/ . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Mars. NASA. https://science.nasa.gov/mars/ . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_hdyjg5tn,
  author = {{iinventedlight}},
  title = {{Mars basics: size, day, year and moons}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/mars-basics/},
  note = {Permanent ID iil:note:hdyjg5tn. License: CC-BY-4.0.},
}

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