The North Polar Basin, more commonly known as the Borealis Basin, is a large basin in the northern hemisphere of

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

that covers 40% of the planet. Some scientists have postulated that the basin formed during the

impact Impact may refer to: * Impact (mechanics), a high force or shock (mechanics) over a short time period * Impact, Texas, a town in Taylor County, Texas, US Science and technology * Impact crater, a meteor crater caused by an impact event * Imp ...

of a single, large body roughly 2% of the mass of Mars, having a diameter of about 1,900 km (1,200 miles) early in the history of Mars, around 4.5 billion years ago. However, the basin is not currently recognized as an impact basin by the IAU. The basin is one of the flattest areas in the Solar System, and has an elliptical shape.

Large regions within the Borealis Basin

Because the Borealis basin covers 40% of the surface of Mars, and much of the Northern Hemisphere, many currently recognized regions of Mars lie within it: *

Acidalia Planitia Acidalia Planitia is a plain on Mars, between the Tharsis volcanic province and Arabia Terra to the north of Valles Marineris, centered at . Most of this region is found in the Mare Acidalium quadrangle, but a small part is in the Ismenius Lac ...

Arcadia Planitia Arcadia Planitia is a smooth plain with fresh lava flows and Amazonian volcanic flows on Mars. It was named by Giovanni Schiaparelli in 1882 after the Arcadia region of ancient Greece. It dates from the Amazonian period's Arcadia formation's ...

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 . Surrounding the high polar plain is a flat and featureless lowland plain called Vastitas Borealis which ...

Utopia Planitia Utopia Planitia (Greek and Latin: "Nowhere Land Plain") is a large plain within Utopia, the largest recognized impact basin on Mars and in the Solar System with an estimated diameter of . It is the Martian region where the ''Viking 2'' lander to ...

Vastitas Borealis (Latin 'northern waste') is the largest lowland region of Mars. It is in the northerly latitudes of the planet and encircles the northern polar region. Vastitas Borealis is often simply referred to as the northern plains, northern lowlands or ...

Borealis Impact

Formation of the Borealis Basin

One possible explanation for the basin's low, flat and relatively crater-free topography is that the basin was formed by a single large impact. Two simulations of a possible impact sketched a profile for the collision: low velocity— per second—oblique angle and a diameter of . Topographical data from ''

Mars Global Surveyor ''Mars Global Surveyor'' (MGS) was an American robotic space probe developed by NASA's Jet Propulsion Laboratory and launched November 1996. MGS was a global mapping mission that examined the entire planet, from the ionosphere down through t ...

'' are consistent with the models and also suggest that the elliptical crater has axes of length and , centered on , though this has been partially obscured by later volcanic eruptions that created the

Tharsis bulge Tharsis () is a vast volcanic plateau centered near the equator in the western hemisphere of Mars. The region is home to the largest volcanoes in the Solar System, including the three enormous shield volcanoes Arsia Mons, Pavonis Mons, and A ...

along its rim. There is evidence for a secondary rim as well. This would make the North Polar Basin by far the largest impact crater in the Solar System, approximately four times the diameter of the next largest craters:

, which is imbedded inside the North Polar Basin, the

South Pole–Aitken basin The South Pole–Aitken basin (SPA Basin, ) is an immense impact crater on the far side of the Moon. At roughly in diameter and between deep, it is one of the largest known impact craters in the Solar System. It is the largest, oldest, and dee ...

on the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

, and

Hellas Planitia Hellas Planitia is a plain located within the huge, roughly circular impact basin Hellas located in the southern hemisphere of the planet Mars. Hellas is the third- or fourth-largest known impact crater in the Solar System. The basin floor i ...

on Mars's southern hemisphere. This impact would have resulted in significant crustal melting and a general increase in the rate of crustal formation for a period of 40 million years following the impact. Such a large impact would have disturbed the

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

, altering the normal

convection currents Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...

and causing upwellings which further increase the amount of melting at the impact site. Overall, such an event would actually increase the rate of cooling of the Martian interior. The lack of magnetic anomalies observed in the northern hemisphere could be explained by such an impact, as the shock waves produced might have demagnetized the crust.

Potential formation of Phobos and Deimos via Borealis impact

The origin of Mars' moons, Phobos and

Deimos Deimos, a Greek word for ''dread'', may refer to: * Deimos (deity), one of the sons of Ares and Aphrodite in Greek mythology * Deimos (moon), the smaller and outermost of Mars' two natural satellites * Elecnor Deimos, a Spanish aerospace company * ...

(pictured right), is unknown and remains controversial. One theory is that the moons are captured asteroids. However, the moons' near circular orbits and low inclination relative to the Martian equator are not in agreement with the capture hypothesis. The detection of minerals on Phobos similar to those in the Martian

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years ...

, and the unusually low density and high porosity of Phobos, such that the moon would not be expected to remain aggregate if dynamically captured, suggest that the moons could have formed via

accretion Accretion may refer to: Science * Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity * Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucl ...

in Martian orbit, similar to how Earth's

formed. While estimates of the mass ejected by a large, Borealis-size impact vary, simulations suggest that a body approximately 0.02 Mars masses (~0.002 Earth Masses) in size is capable of producing a sizable debris disk in Martian orbit, on the order of 5×10²⁰ kg, with a significant fraction of the material remaining close to Mars. This figure lies within the estimated mass range necessary to form the two moons, as other data suggests that only 1% of the mass of an accretion disk successfully forms moons. There are several other large impact basins on Mars that could have ejected enough debris to form the moons.

Ancient tsunamis

Analysis of Mars Global Surveyor data found mineral deposits similar to

terminal moraine A terminal moraine, also called end moraine, is a type of moraine that forms at the terminal (edge) of a glacier, marking its maximum advance. At this point, debris that has accumulated by plucking and abrasion, has been pushed by the front edge ...

s on Earth along the southern rim of the northern lowlands. Scientists have developed several theories to explain their presence, including: volcanic activity, glacial activity, and a series of Martian

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater exp ...

s. The arrangement of the deposits resembles deposits observed in recent tsunami events on

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

, and other features of the deposits are inconsistent with the volcanic and glacial hypotheses. One recent investigation identified three impact craters in

as being the likely source of the hypothetical tsunamis, with the Lomonosov crater (pictured right) being the most likely candidate. Here, the tsunami generated by the impactor would have reached heights of 75 m (250 ft), and traveled 150 km (90 mi) past the southern rim. Dating techniques put the origin of the deposits sometime between the Late

Hesperian The Hesperian is a system (stratigraphy), geologic system and geologic timescale, time period on the planet Mars characterized by widespread Volcanology of Mars, volcanic activity and catastrophic flooding that carved immense outflow channels acr ...

and Early Amazonian periods, some 3 billion years ago, providing evidence to the presence of an

ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wor ...

during this period.

References