Mars appears red due to nanocrystalline red hematite dust particles in its thin atmosphere and coating the surface, scattering red light, though the redness is shallow.
The Red Planet: Why is Mars Red?
Introduction
- From space, Earth displays diverse colors: blue sky and oceans, brown/green continents, and white icecaps.
- Mars, however, is dominated by one color: red. The ground and atmosphere are red, with white icecaps tinted reddish.
- Surprisingly, the redness is shallow; beneath the surface, it vanishes.
The Redness of Mars Throughout History
- Mars's redness is its most prominent feature throughout recorded history.
- Sanskrit (Mangala) and ancient Egyptian (Har decher) names for Mars translate to "red."
- Photos from the space age confirm the Martian atmosphere has an intrinsically red color.
Why is Mars Red?
- Earth: Rayleigh scattering makes the sky blue, letting red light pass.
- Mars: Atmosphere is 0.7% as thick as Earth's, making Rayleigh scattering negligible.
- Dust Particles: Martian atmosphere is dominated by dust particles that:
- Absorb shorter (400-600 nm) wavelengths more than longer (600+ nm).
- Scatter longer wavelengths (red light) more efficiently.
The Composition of Martian Dust
- The suspended atmospheric dust is similar to:
- Highly reflective regions.
- Bright soil deposits.
- Iron-rich areas containing ferric oxides.
- The most common dust is nanocrystalline red hematite (α-Fe2O3), with particles 3-45 microns in diameter.
- Martian winds (~100 km/hr) continuously sweep dust into the atmosphere.
The Martian Surface
- Surface features change over time, with dark and bright areas shifting.
- Dark areas become brighter as they get covered in dust from brighter areas, then revert to being dark.
- Dark areas that change are typically lower elevation, have smaller slopes, and are surrounded by brighter areas.
The Role of Martian Dust
- Mars is covered with a thin layer of sandy dust driven by winds.
- The dust travels short distances more easily, moves from higher to lower elevations, and is blown away from steeper slopes.
- The red dust is only a few millimeters to centimeters thick, even in the thickest region (Tharsis plateau), it's about 2 meters thick.
Ferric Oxides on Mars
- Ferric oxides are present everywhere, including in the crust, lava outflows, and dust.
- The atmosphere contains carbon dioxide and water, providing oxygen for oxidizing iron-rich materials.
- Ferric oxide abundance is highest in northern and mid-latitudes, lowest in southern latitudes.
Topography and Ferric Oxides
- The southern hemisphere is at a higher elevation than the northern lowlands.
- The Tharsis region has the greatest elevation and is rich in ferric oxides.
- Lowlands east of Tharsis have lower ferric oxide abundance, possibly due to an ancient collision.
Hematite vs. Magnetite
- Red hematite (Fe2O3) is possibly the primary cause of Mars's redness.
- Magnetite (Fe3O4) is black.
- Global topography influences ferric oxide abundance, but isn't the only factor.
The Color of the Martian Surface
- There is a globally distributed dust that is swept into the atmosphere.
- The "settling of atmospheric dust" is only one factor in determining the surface color.
- The surface is more of an orangey shade of butterscotch, with rocks having brown, golden, tan, greenish, yellow, orange, and red colors.
- The color of a Mars rock depends on its mineral composition.
Hematite Formation
- The exact mechanism is still under investigation.
- Possibilities include reactions involving hydrogen peroxide (H2O2) or physical erosion.
- Tumbling magnetite powder with quartz can convert magnetite to hematite.
Conclusion
- Mars is red due to hematite, a red form of ferric oxide.
- Small dust particles in the atmosphere and coating the surface are responsible for the red color.
- If Martian dust settled, the sky would be very dark with a slight bluish tint due to the thin atmosphere.
- Only a tiny amount of Mars is actually red, but it is enough to make the entire planet appear red from an external perspective