Geophysics
As noted on the fact sheet page, Melidora is larger than Terra, with twice the radius and three times the mass. However, due to its larger radius, it actually has a lower surface gravity (75% of Terra's), although its escape velocity is higher because it is more massive.
Melidora's density is a whopping 20.6 g/cm3! Terra, the densest object in our solar system, has a density of just 5.51 g/cm3 in comparison. The high density of Melidora can be explained by its large iron core. This core drives powerful mantle convection, which in turn leads to increased tectonic activity compared to Terra. Currently, Melidora has seven major continents: Hesperia, Ys, Nova Lemuria, Avalon, Lamarckia, Notios, and Glacialis.
Landforms
Volcanoes
Melidora has high levels of volcanic activity. Frequent volcanic eruptions have led to a high concentration of greenhouse gases in the atmosphere, which has warmed the planet over time. Volcanic hotspots are especially prevalent along the rift valley between Notios and Glacialis, the eastern part of Ys, and the island of Vulcanesia. Hot springs around volcanoes are particularly interesting places to search for life on Melidora, as there are numerous types of thermophilic microbes that thrive in the extreme conditions.
Mountain Ranges
Due to the lower surface gravity, mountains on Melidora can reach far greater heights than their Terran counterparts. The tallest mountains in the world lie in the Dividing Range, which separates the continents of Lamarckia and Avalon. Some peaks in this range can reach heights of up to 11,280 meters (37,000 feet), dwarfing Everest! Although these mountains are high, they are teeming with life, as the Melidoran atmosphere is thicker, so the thinner mountain air has a higher partial pressure than would be expected and thus is sufficiently oxygenated.
Plains
As rivers flowing from the high mountains deposit silt on their banks, a sort of soil forms, becoming the bedrock for a plain. These plains (referred to as Planitia on Melidora) are a typical feature of the lowland regions. Dapsilis Planitia, one of the most notable Melidoran plains, covers much of the Lamarckian continent and is comparable to the Terran Serengeti in its grandeur and biodiversity.
Deserts
Deserts typically form in the horse latitudes, around 30 degrees north or south of the equator. Here, bands of high pressure air prevent clouds from forming, thus starving the land of moisture. Another type of desert are rainshadow deserts, formed when high mountain ranges effectively prevent rainfall from reaching into the interior of a continent. The Dividing Range has this effect on the Avalonian Plateau. The Avalonian Plateau is parched and deprived of any moisture, the only sources of water being salt lakes scattered across the landscape. Only the hardiest of Melidoran creatures can survive in such deserts.
Swampland
Swamplands typically form when a river or lake is gradually filled with debris over time. The coastal regions of Melidora, flooded by rivers and the ocean, are fringed by marshy deltas and swamplands. With their warm, humid climates, the swampy environments are a paradise for life. They are arguably the richest terrestrial environments in Melidora in terms of biota.
Oceans
Oceans cover nearly 80% of Melidora's surface. The high sea levels are a direct result of Melidora's hothouse climate.
Vast shallow seas have expanded over the continental margins. They were formed through plate tectonics. As continental plates squeezed together, water was forced out of mid-ocean trenches and pushed to the surface, where it flooded low-lying land areas.
However, the Melidoran oceans can also reach great depths. The deepest parts of the oceans lie some 30 kilometers (19 miles) below the surface. Here, the pressure is crushing. So much so, in fact, that water begins turning into Ice VI. This is a special form of ice with a tetragonal crystal structure, different from regular ice's hexagonal crystals. It forms a thick frozen layer just above the seabed.
The ice is not completely solid. Even down here, there are hydrothermal vents, and their heat melts the ice from within, hollowing it and creating an intricate network of tubes and caverns. Chemosynthetic microbes live around these vents, creating simple carbohydrates from the sulfur compounds produced by the vents. They form the basis of a strange ecosystem, isolated from the surface world. Powered by chemical energy from the vents, these tube systems harbor a surprisingly rich biota. In fact, life on Melidora originated around hydrothermal vent tubes like these.
Atmosphere
Melidora's atmosphere has three times the density of Terra's. Since atmospheric density roughly corresponds with surface pressure, this also means that sea level pressure on Melidora is three times greater than its Terran value. To an observer from Terra, walking on Melidora would be the equivalent of swimming 30 feet (98 meters) underwater!
Although the Melidoran atmosphere contains only 10% oxygen, the actual partial pressure of oxygen in the atmosphere is close to 32% due to the denser atmosphere. This means that the Melidoran atmosphere is highly oxygenated. Nitrogen gas makes up the remaining 90% of the atmosphere, and there are also other gases such as carbon dioxide and water vapor present in small quantities.
Climate
As mentioned earlier, Melidora is a hothouse world. The predominant climates throughout much of the world are warm and tropical, and there are no polar ice caps. Climatic regimes throughout the world are dictated by Hadley cells, belts of atmospheric circulation. On Terra, the Hadley cells are largely concentrated around equatorial latitudes, but on Melidora they extend well into temperate regions, creating tropical conditions even near the poles.
One of the most important factors that affects Melidora's climate is the planet's axial tilt, its inclination from the orbital plane. Melidora's axial tilt, at 27 degrees, is comparable to that of Saturn and nearly 4 degrees higher than that of Terra. This means that Melidoran seasons are, on average, harsher than their Terran counterparts. Extreme weather events, such as droughts and megamonsoons, are also more common on Melidora. It also causes ocean storms to be extremely violent, especially in the northern seas where there is little land to impede them. The effects of the axial tilt are especially pronounced in the polar regions, which experience tropical summers and bitterly freezing winters.