Darwin IV

"From an altitude of roughly 39,000 kilometers we had a splendid view of the planet we had come to explore. With an equatorial diameter of 6,563 km, Darwin IV is somewhat smaller than Earth. Its predominant color is dusky ochre, relieved by a sparse mottling of red and two crisply defined polar caps."

- Wayne Barlowe

The fourth planet in the Darwin Binary System, Darwin IV is the homeworld of an impressive diversity of native lifeforms.

Once an Earth-like world with vast oceans, Darwin IV was too small to hold its surface water for long: through the eras, its seas have gradually vanished. Although Darwin IV is technically considered a desert world for having little surface water and scarce precipitation, the planet is still thriving with life.

Astronomical data
Like its name suggests, Darwin IV is the fourth of six planets orbiting Darwin, an F-class binary star located in the Milky Way Galaxy, 6.5 light years from the Sol System. The two Darwin stars are located so close to each other that they often give the impression of being a single sun when seen from Darwin IV. This fact largely diminishes the odd daylight optical effects commonly found on planets orbiting binary stars.

The length of Darwin IV's year is about twice that of Earth. Its day lasts for 26.7 Earth hours. The planet is orbited by at least two small moons.

Physical aspects
In some aspects, Darwin IV is similar to the planet Mars. Both are similar in size - Darwin IV being slightly smaller - and are believed to have had large oceans in the past, which eventually evaporated. In both worlds, most of the remaining water is now frozen in the ice caps, which recede and advance with the seasons. Unlike Mars, however, Darwin IV has a dense atmosphere, rich in oxygen and water vapor. The combination of a relatively weak gravity (60% Earth's) and an oxygen-rich atmosphere has allowed Darwin IV's native fauna to reach gigantic sizes, even in airborne creatures.

Most of the surface of Darwin IV is covered by vast plains, especially on the areas which used to be the seafloor. Endless grasslands now dominate these areas, with scarce pocket forests of Plaque-bark Trees growing near the sparse lakes and rivers left. The planet's equatorial range is almost entirely encircled by a vast mountain range. Rain is virtually nonexistent on the planet. Although there are hurricanes and tropical storms, they differ from Earth's by being spawned from temperature changes rather than large bodies of water.

Surface water
"In a sense the succulent-rich savannas are the closest Darwin IV comes to true oceans, for the quantity of water trapped in the plants is vast."

- Wayne Barlowe

As stated above, Darwin IV currently has very little liquid water on its surface. Although there are lakes and streams, most of the planet's water is found in the atmosphere, frozen in the polar caps, or inside the bodies of living creatures.

On the northern hemisphere of the planet, there is a peculiar region called the Amoebic Sea, which is actually a huge living entity, a colony of billions of microscopic organisms. This ten-meter-deep gelatinous matrix covers about five percent of the planet's surface and retains large quantities of water inside itself.

Ecology
Darwin IV's plains are dominated by succulent plants, such as the Tube-grass and the Fodderball Weed. In the small pocket forests reside the largest floral organisms of the planet: the towering Plaque-bark Trees, which are home to creatures like the Daggerwrist and the Trunk-suckers, which feed on the trees' nourishing sap. Near the poles, the tundra region is blanketed by vast carpets of small gray-green lichen-like organisms.

Life in the skies of Darwin IV is as abundant as in the oceans of Earth. Countless tiny organisms, the aerophytes and microflyers, spend their whole lives floating in the skies of the planet and make up the main food source of most of Darwin IV's airborne fauna. These minute creatures are so numerous that they sometimes darken the skies with their numbers.

Many of the larger atmospheric denizens of Darwin IV are floaters, rather than flyers, having large hydrogen or methane-filled bladders to stay aloft. The largest of these floaters is the Ebony Blister-wing, which attains a wingspan of over 300 meters. Aerial predators like Skewers and Follow-wings propel themselves through the air by combusting methane. The small Jetdarters have a biological version of a ramjet engine – complete with a turbine of bone and gristle. The Eosapiens, large floating predators, are the only sapient species of the planet, yet their civilization is primitive and their technology, as of the 24th century, is limited to spears and clubs.

The ground-dwelling creatures display a huge diversity in morphology, though they all share some common characteristics. Darwin IV's terrestrial fauna is divided in four categories: monopedaliens (one leg), bipedaliens (two legs), tripedaliens (three legs) and quadrupedaliens (four legs). Nearly all predators on Darwin IV are liquivores: they secrete special enzymes to liquefy their meal before consuming it.

One of the most notable traits of Darwin IV's fauna is the ubiquitous lack of eyes. Instead, the organisms rely on echolocation, similar to Earth's bats. Despite being blind to visible light, the Darwinian animals are able to detect light in the infrared spectrum due to thermoreceptors similar to those of Earth's snakes. Many creatures of Darwin IV are bioluminescent, and the light they produce is visible both in the infrared and the normal spectrum. Most organisms are warm-blooded and have a very active metabolism. Both predators and herbivores are known for achieving great speed when running.

Besides plant-like photosynthesizers and animal-like consumers, there are also sessile predators on Darwin IV which blur the line between animal and vegetable, such as Electrophytes and Butchertrees.

History
An expedition to planet Darwin IV was carried out by Humans and Yma in the year 2358 and lasted about three Earth years. During this time the planet was explored and its unique lifeforms were studied. However, the most important objective of the mission was to have the littlest impact possible on the planet's natural environment.