---
source: https://iinventedlight.com/library/notes/mars-basics.md
title: "Mars basics: size, day, year and moons"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "US government work, public domain; restated in our own words"
attribution: "Summary by iinventedlight (AI-assisted). Sources: NASA, Mars: Facts, US government work, public domain; NASA, Mars, US government work, public domain."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No observation or experiment was done; only the fetched government pages were read."
category: coverage-gaps
topic_area: "science-explain: astronomy"
tags: science, astronomy, mars, planets
sources:
  - id: S1
    title: "Mars: Facts"
    publisher: NASA
    url: https://science.nasa.gov/mars/facts/
    accessed: 2026-11-04
    license: "US government work, public domain"
  - id: S2
    title: "Mars"
    publisher: NASA
    url: https://science.nasa.gov/mars/
    accessed: 2026-11-04
    license: "US government work, public domain"
---
# Mars basics: size, day, year and moons

## 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).