Mars may owe its lopsided face to a Pluto-sized collision
The northern half of Mars is low and smooth, the southern half high and battered by craters. One explanation is a strike by a body the size of Pluto some four billion years ago, which may have gouged out the Borealis basin, covering 40 percent of the planet and four times the size of the Moon's largest crater.
Mars, the fourth planet, is about half Earth's diameter at 6,779 kilometres, yet its surface area roughly matches all of Earth's dry land. With 11 percent of Earth's mass, it offers about 38 percent of our gravity. Its air is thin carbon dioxide, surface temperatures swing from minus 153 to 20 degrees Celsius, and water survives only as ice, frost, fog and wispy clouds. The red-orange colour comes from iron oxide dust, though the ground can also look butterscotch, golden, brown or greenish depending on minerals. A tilt of 25 degrees gives it seasons, a year lasts 687 Earth days, and a sol runs 24.6 hours.
The planet holds superlatives. Olympus Mons, an extinct volcano, rises 21.9 kilometres, and the Valles Marineris canyon runs 4,000 kilometres. Mars is not geologically dead: lava flows in Athabasca Valles are around 200 million years old, water ran through the Cerberus Fossae less than 20 million years ago, and the InSight lander recorded more than 450 marsquakes and related events by 2019. Seismic data reveal a crust averaging 42 to 56 kilometres thick, a rigid mantle and a partly molten iron core; whether a solid inner core exists is still debated, with 2023 and 2025 studies reaching opposite conclusions.
Its history divides into three eras named for regions: the heavily cratered Noachian, when liquid water probably flooded the surface; the Hesperian, dominated by volcanism and huge outflow channels; and the Amazonian, continuing today. Chlorine, phosphorus, sulfur and other elements that boil off easily are more plentiful there than on Earth, probably pushed outward by the young Sun's wind. A 2023 study of the tilt of Deimos's orbit suggests Mars may once have had rings, with Phobos a leftover of a destroyed moon.
Exploration began badly: Mars 1 lost contact in 1963. Mariner 4 made the first successful flyby in 1965, and in 1971 Mariner 9 became the first spacecraft to orbit another planet, while Mars 3 made the first landing. Probes have operated there continuously since 1997. The Phoenix lander found the soil mildly alkaline, at pH 7.7, but laced with perchlorate at levels toxic to humans. Whether Mars ever hosted life is still being investigated.
Source: Mars
