Mars (Planet)
The fourth planet from the Sun — the Red Planet, named for the blood-red colour of iron oxide on its surface. Fourth brightest naked-eye object. Mythological counterpart: myth-ares. Crossing note: crossing-mars-ares.
Key Properties
| Property | Value |
|---|---|
| Orbital period | 687 Earth days (1.88 years) |
| Synodic period | ~780 days (opposition cycle) |
| Distance from Sun | 1.38–1.67 AU |
| Orbital eccentricity | 0.09 (second highest after Mercury) |
| Diameter | ~6,779 km (~half Earth's) |
| Surface gravity | ~0.38g |
| Day length | 24h 39m (nearly identical to Earth) |
| Moons | Phobos (Fear) and Deimos (Dread) |
The Red Colour
Iron oxide (rust) covers Mars's surface and fills its thin atmosphere with fine dust, giving everything a yellow-orange-red cast. The redness was visible to the naked eye in antiquity — every culture that saw it associated it with blood, war, or danger. The colour varies: when farthest from Earth, Mars appears more distinctly red; at closest approach it rivals Jupiter in brightness (magnitude −3.0).
Retrograde Motion — The Unpredictable Wanderer
As Mars approaches opposition (closest approach to Earth), it begins a period of apparent retrograde motion — it appears to reverse direction against the background stars for about 72 days, then resume forward movement. This happens every ~780 days.
To ancient observers with no orbital model, Mars would be tracking steadily eastward, then inexplicably stop, reverse, and resume — apparently at random. No other bright planet was as dramatically, visibly erratic. This is the astronomical fact at the heart of the crossing-mars-ares note.
Moons
Phobos and Deimos — Fear and Dread — the sons of Ares in Greek mythology, discovered in 1877 by Asaph Hall. The names were suggested by Henry Madan, science master at Eton. They are irregular captured objects, not formed from Mars itself.
Phobos orbits so close (9,376 km) that it completes an orbit in 7.65 hours — faster than Mars rotates — so Martian observers would watch it rise in the west and set in the east, moving across the sky three times per day.
Atmosphere and Habitability
Mars lost its global magnetic field ~4 billion years ago, after which solar wind stripped away most of its atmosphere. Mars's atmosphere is now ~1% of Earth's pressure — barely enough to support liquid water. The loss was measured directly by NASA's MAVEN mission: ~100 grams of atmosphere per second continue to be stripped away by the solar wind.
Evidence of ancient riverbeds, lake beds, and possibly a northern ocean suggests Mars was habitable billions of years ago. Whether life developed there remains an open question.