Planum Boreum

Planum Boreum (Latin: "the northern plain") is the northern polar plain on Mars. It extends northward from roughly 80°N and is centered at 88°00′N 15°00′E / 88.0°N 15.0°E / 88.0; 15.0. Surrounding the high polar plain is a flat and featureless lowland plain called Vastitas Borealis which extends for approximately 1500 kilometers southwards, dominating the northern hemisphere.

The main feature of the Planum Boreum is a large fissure or canyon in the polar ice cap called Chasma Boreale. It is up to 100 kilometres (62 mi) wide and features scarps up to 2 kilometres (1.2 mi) high. By comparison, the Grand Canyon is approximately 1.6 kilometres (1 mi) deep in some places and 446 kilometres (277 mi) long but only up to 24 kilometres (15 mi) wide. Chasma Boreale cuts through polar deposits and ice, such as those present at Greenland.

Planum Boreum interfaces with Vastitas Borealis west of Chasma Boreale at an irregular scarp named Rupes Tenuis. This scarp reaches heights of up to 1 km. At other places, the interface is a collection of mesas and troughs.

Planum Boreum is surrounded by large fields of sand dunes spanning from 75°N to 85°N. These dune fields are named Olympia Undae, Abalos Undae, Siton Undae, and Hyperboreae Undae. Olympia Undae, by far the largest, covers from 100°E to 240°E. Abalos Undae covers from 261°E to 280°E and Hyperboreale Undae spans from 311°E to 341°E. See also List of extraterrestrial dune fields.

The North Polar Layered Deposits (NPLD) are a kilometer-thick part of Mars's north polar ice cap. They are mostly water ice (about 95%) mixed with layers of dust. They form a geological record of Mars's climate over millions of years. Layers form because the climate has gone through many cycles of atmospheric ice and dust deposition.

Planum Boreum is home to a permanent ice cap consisting mainly of water ice (with a 1 m thick veneer of carbon dioxide ice during the winter). It has a volume of 1.2 million cubic kilometres and covers an area equivalent to about 1.5 times the size of Texas. It has a radius of 600 km. The maximum depth of the cap is 3 km.

The spiral troughs in the ice cap are formed by katabatic winds that entrain surface ice eroded from the equator-facing sides of the troughs, likely aided by solar ablation (sublimation), which is then redeposited on the colder pole-facing slopes. The troughs are roughly perpendicular to the wind direction, which is shifted by the Coriolis effect, leading to the spiral pattern. The troughs gradually migrate towards the pole over time; the central troughs have moved about 65 km in the last 2 million years. Chasma Boreale is a canyon-like feature older than the troughs, and in contrast is aligned parallel to the wind direction.

The surface composition of the northern ice cap in middle spring (after a winter's accumulation of seasonal dry ice) has been studied from orbit. The outer edges of the ice cap are contaminated with dust (0.15% by weight) and are mostly water ice. As one moves toward the pole, the surface water ice content decreases and is replaced by dry ice. The purity of the ice also increases. At the pole, the surface seasonal ice consists of essentially pure dry ice with little dust content and 30 parts per million of water ice.