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Earth has a bond albedo of 0.3, Mars (presumably because of fewer clouds) is 0.25, and the moon comes in at 0.12 (which is darker than it seems from Earth). Water has a very low albedo, but clouds have a very high albedo, being white and reflective.

I'm going to assume it therefore needs to be a habitable desert planet with a narrow habitable band around a smaller volume of water than on Earth (but not so small as to be unstable), so as to minimize cloud coverage. The rest of the planet would be inhospitable desert.

My question is: is there any rock or substance this desert could be made of which could be darker even than the moon or is that the limit for naturally occurring materials? I'm looking for a way to have a very dark grey planet.

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  • $\begingroup$ albedo is a human measurement of light reflectivity that assumes sight in the human eye range. If you postulate alien/animal (different visible spectrum) vision, an alien 'albedo' would probably see brightness differently. $\endgroup$ Nov 12, 2021 at 14:55
  • $\begingroup$ @MarkRipley: The albedo measures the diffuse reflectivity across the entire spectrum of electromagnetic radiation coming from the sun. It is not related to human perception of light. It is a radiometric property, not a photometric property. $\endgroup$
    – AlexP
    Nov 12, 2021 at 15:12

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The general category of rocks to consider would be those derived from ultramafic lava.

enter image description here

Such rocks contain a large proportion of dark minerals, such as olivine and peridotite and dunite.

A typical rock type would be komatiite.

Soil derived from such lavas tend to have high amounts of nickel and chromium and thus can be toxic to plants. Distinctive vegetation grows on such soils.

Examples are the ultramafic woodlands and barrens of the Appalachian mountains and piedmont, the "wet maquis" of the New Caledonia rain forests, and the ultramafic forests of Mount Kinabalu and other peaks in Sabah, Malaysia. Vegetation is typically stunted, and sometimes includes endemic species adapted to the soils.

Komatiite has a low albedo. In laboratory experiments komatiite has been used as an analog for the dark surfaces of the planet Mercury. Depending on the type of komatiite used and the wavelength of light, the reflectance is similar to or less than the albedo of the Moon.

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  • $\begingroup$ So komatiite is a good choice then. Thanks. I suppose this could occur across a large portion of the planet if there were huge outflows in the past. Would a planet that had large surfaces of this material be darker than the moon? $\endgroup$
    – Axion
    Nov 12, 2021 at 15:03
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    $\begingroup$ See edit, at the end regarding Mercury. The reflectance of komatiite is similar to the albedo of the Moon. $\endgroup$
    – user81881
    Nov 12, 2021 at 16:12
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I think you should have one just looking out of your window: asphalt

enter image description here

Tar is a naturally occurring substance, as it can be seen from the various ponds filled with it existing here and there on our planet.

Just have some process spreading it on the surface of the desert, and you will have a nicely dark environment.

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  • $\begingroup$ Asphalt may be a naturally occurring substance (if your definition of natural includes biology), but it's not a rock. And if asphalt, why not coal? $\endgroup$
    – jamesqf
    Nov 12, 2021 at 17:46
  • $\begingroup$ And asphalt shingles have a gravel coating to protect the waterproof layers from friction by branches and animals -- it's that which determines the color (and albedo) of asphalt shingles. I've read that the Moon itself is about the same albedo as slightly worn asphalt pavement -- which is common crushed rock bonded with asphalt between the stone fragments. $\endgroup$
    – Zeiss Ikon
    Nov 12, 2021 at 17:50
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The Moon has an albedo of 0.14, which should be an average of its lighter and darker areas.

Other solar systems bodies have albedos as high as 0.95 (Eris) and even 0.99 (Enceladus). Many solar system objects have low albedos.

Many small objects in the outer Solar System[21] and asteroid belt have low albedos down to about 0.05.[22] A typical comet nucleus has an albedo of 0.04.[23] Such a dark surface is thought to be indicative of a primitive and heavily space weathered surface containing some organic compounds.

Different parts of the moon Iapetus have highly different albedos.

The difference in colouring between the two Iapetian hemispheres is striking. The leading hemisphere and sides are dark (albedo 0.03–0.05) with a slight reddish-brown coloring, while most of the trailing hemisphere and poles are bright (albedo 0.5–0.6, almost as bright as Europa). Thus, the apparent magnitude of the trailing hemisphere is around 10.2, whereas that of the leading hemisphere is around 11.9—beyond the capacity of the best telescopes in the 17th century. The pattern of coloration is analogous to a spherical yin-yang symbol or the two sections of a tennis ball. The dark region is named Cassini Regio, and the bright region is divided into Roncevaux Terra north of the equator, and Saragossa Terra south of it. Before optical observations could be made by deep space probes, theories about the reason for this dichotomy included an asteroid shearing off part of the moon's crust.[23] The original dark material is believed to have come from outside Iapetus, but now it consists principally of lag from the sublimation of ice from the warmer areas of Iapetus's surface.[24][25][26] It contains organic compounds similar to the substances found in primitive meteorites or on the surfaces of comets; Earth-based observations have shown it to be carbonaceous, and it probably includes cyano-compounds such as frozen hydrogen cyanide polymers.

https://en.wikipedia.org/wiki/Iapetus_(moon)

Of course the dark surface material of Iapetus is likely to be some type of dust or ice instead of what is usually called rock. And I don't know how to produce such materials in the surface conditions on a planet habitable for Earth life.

But that might give you some ideas on how to darken your planet.

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