Mercury in the fiction of Leigh Brackett
The planet Mercury appears frequently as a setting for many of the stories of Leigh Brackett, and Mercury and Mercurians are frequently mentioned in other stories of the Leigh Brackett Solar System. Brackett's Mercury shares some characteristics with the astronomical Mercury, but in other respects functions as a consistent fantasy world with recurring landmarks and characteristics that reappear from story to story. Some of these fantasy characteristics are of Brackett's own invention; others reflect some of the scientific theories about Mercury that were current before the mid-1960s. Readers of this article are advised not to depend upon it for scientific or astronomical information but instead to consult the article Mercury (planet).
Astronomical characteristics 
Mercury in the Solar System stories of Leigh Brackett is the closest planet to the Sun. Its most notable characteristic is that it is tidally locked to the Sun, that is, it makes a full rotation once for each of its 88-day orbits. As a result, it keeps the same face directed towards the Sun at all times, just as the same side of the Moon always faces the Earth. The sun-facing side is known as the Sunside; the side facing away from the sun is called the Darkside.
A Mercurian sidereal day (the period of rotation) lasts about 87.97 Earth days, but since Mercury does not rotate with respect to the Sun, there is no such thing as a solar day or night in usual sense. However, although the times of rotation and revolution are identical, due to Mercury's eccentric orbit, the subsolar point varies in position by up to 20° east or west of the mean. An observer standing on the boundary between Sunside and Darkside at perihelion could observe the sun set below the horizon, only to rise again at aphelion, stay in the sky for 44 Earth days, and then set again at perihelion (an observer on the other side of the planet would see the opposite sequence). Observers closer to the mean antisolar point would observe a shorter day, those closer to the subsolar point would observe a longer one. As a result, there is a strip, over 500 km wide at the equator and narrowing toward the poles, where there is an 88-day long day-night cycle; this region is known as the "Twilight Belt".
Although Mercury possesses a breathable oxygen atmosphere close to its surface, most of the planet is uninhabitable, as surface temperatures on Mercury range from metal-melting temperatures in the immediate subsolar point in the center of the side of Mercury facing the Sun (the Sunside) to cold enough to liquefy oxygen on the side of Mercury facing away from the Sun (the Darkside). In the Twilight Belt, however, temperatures fluctuate between Sunside heat and Darkside cold; in a few places, there is a habitable zone where temperatures are not too extreme for life.
Mercury also has a highly eccentric orbit, which slowly slowly precesses around the Sun. Some scientists hypothesize that another planet may exist in an orbit even closer to the Sun to account for this perturbation. This hypothetical planet has been named Vulcan.
Nothing is known of the pre-history of Mercury before the arrival of the Terrans. There is evidence of an ancient, long-lived race of silicon-based life-forms, but their day had passed long before the Terrans came (Shannach-The Last). The only intelligent forms of life found on Mercury were small furred humanoids, who lived a Stone-Age existence among the rocks of the Twilight Belt by hunting the rock-lizard.
The Terrans who came to Mercury were primarily involved in exploiting Mercury's rich mineral resources under the auspices of various companies, such as Mercury Metals and Mining and the similarly named (if not identical) Mercurian Metals. Few of them were adequately prepared for the rigors of the life; the brutal climate of Mercury, its swings between heat and cold, its electrical storms, and its severe exposure to solar radiation caused a terrible mortality. Terro-Mercurians tend to have ebony-black skin and dark hair, by which they can generally be recognized throughout the Solar System (Shadow over Mars; Queen of the Martian Catacombs).
Mineral resources 
Mercury possesses an abundance of rare and heavy metals that are rare on Earth and unknown in other parts of the Solar System, including: copper; Yttrium, used for armaments (The Demons of Darkside); pitchblende, from which Radium and Uranium can be extracted (A World is Born); and the rare radioactive crystal known as the sun-stone (Shannach – the Last).
Life in the Twilight Belt 
The inhabited regions of the Twilight Belt are not a single area, but a honeycomb of valleys, some containing hot springs, each separated from the others by sheer volcanic cliffs, taller than Earth's Mount Everest. As the atmosphere of Mercury is extremely shallow, communication between the valleys can only occur by those possessing a breathing apparatus, as the mountain peaks reach into virtually airless space. Mercury has only one notable concentration of population, in the Trade City of Solar City (aka Sun City). Due to the strong electromagnetic field surrounding Mercury and the storms generated by the sharp twilight transition between heat and cold, the Twilight Belt is marked by powerful electric discharges. (A World is Born). The storms are worst at perihelion; the Terro-Mercurian colonists use metal pylons and copper cables to channel and collect the electric power from the charged atmosphere. Metals tend to become highly magnetized under these conditions, and normal navigational instruments are rendered useless.
Mercurian aborigines 
Mercury is known to have evolved three forms of sentient life. Among these were a group of glacially-slow silicon-based life-forms that were almost extinct by the time the first settlers arrived; the crystalline life-forms of the Darkside (see below), that remained largely unknown to the settlers save as rumor; and the furry aborigines of the closed valleys of the Twilight Belt.
The furry aborigines were non-human, but humanoid, and had achieved a barely Stone Age existence before the arrival of the settlers. They were patriarchal and clannish, living by hunting rock-lizards. Male children hunted with wooden spears, and were forbidden certain quarries; when they came of age, if they were successful in the ritual of the Man's Hunt, they were awarded flint spears and became members of the community (Enchantress of Venus).
Although the aborigines had little physical power compared to the human settlers of Mercury, and many were ruthlessly massacred by the least scrupulous of the mineral exploiters, they were not to be underestimated. The aborigines inhabiting the cliff-caves of Arianrhod, near the Darkside, had developed a powerful form of telepathy, capable of discerning events across a planet; this skill was learned by the Terro-Mercurian Jaffa Storm (Shadow over Mars). They could be very friendly to humans on occasion, for instance adopting the orphaned Eric John Stark and raising him to adulthood.
Despite the harshness of its environment, Mercury has evolved its own remarkable fauna, including:
- Rock lizards, carnivorous reptiles growing to an immense size, hunted by the Mercurian aborigines.
- Flying lizards, jet-black pterodactyl-like creatures with yellow stomachs (Shannach – the Last).
- Electric beasts, a form of energy-life feeding on the lightning generated in the Mercurian atmosphere, and attracted to metal (A World is Born).
- Mercurian cave-cats or cave-tigers, twenty feet long with four pale eyes, eight legs and a tail armed with bone barbs (The Halfling).
- Mercurian hunting bats, metallic purple and green in color, with wings and fangs. They are attracted to salt, and their bodies are sheathed in silicate scales (similar to glass) that protect them from the lightning (Cube from Space).
The Darkside 
The cold and airless Darkside of Mercury is little known, as only a few travellers have entered that region and survived. Currents generated by Mercury's passage through the Sun's electromagnetic field prevent radio communication and would damage the instruments of any rocket that attempted to land there. Furthermore, for some time, those who returned were afflicted by a morbid psychosis that was fancifully attributed to "demons". This was eventually traced to a form of intelligent life inhabiting a crystalline matrix and possessing the power to affect mind-waves. These life-forms have since become quiescent and no longer pose a hazard (The Demons of Darkside).
See also 
Core Mercury stories 
- A World Is Born (Comet Stories July 1941)
- Shannach - The Last (novelette; Planet Stories November 1952)
Marginal Mercury stories 
- The Demons of Darkside (Startling Stories January 1941)
- Cube from Space (Super Science Stories August 1942)
Non-Mercury stories 
- The Halfling (novelette; Astonishing Stories February 1943)
- The Citadel of Lost Ships (Planet Stories March 1943)
- Shadow Over Mars (Startling Stories Fall 1944) published in book form as The Nemesis from Terra
- The Beast-Jewel of Mars (novelette; Planet Stories Winter 1948)
- Queen of the Martian Catacombs (Planet Stories Summer 1949) published in book form as The Secret of Sinharat
- Enchantress of Venus (novella; Planet Stories Fall 1949) also published as City of the Lost Ones
- Black Amazon of Mars (Planet Stories March 1951) published in book form as People of the Talisman