Largest impact basin on Mars

The largest impact basin on Mars is the Hellas Basin (also known as the Hellas Planitia), located in the southern hemisphere of the planet. This ancient impact crater has a diameter of 2,299.16 km (1,428 miles) and is 7.152 km (4.44 miles) deep (measured from Martian zero elevation).

An impact basin is one of the ways in which the scar of a planetary impact presents on the surface of a planetary body. The most common type is what's called a simple crater, which consists of a circular depression ringed by a raised rim of ejected material. The Barringer Crater in Arizona, USA, is an example of a simple crater. Impact craters can present in a few different ways – those that result from larger impacts (called complex craters) typically have a raised peak of rebounded material in the centre.

The largest impact formations are called impact basins. These take the form of a broad, shallow depression with several concentric circular ridges. These features are formed by impactors with greater kinetic energy, which cause more extensive deformation of the surface. The exact mechanism by which impact basins form is not fully understood.

It has been suggested that the entire northern hemisphere of Mars is a massive impact basin, known as the Borealis Basin, or perhaps a set of large basins. While this is a plausible hypothesis (it would explain why the northern hemisphere of Mars consists of a flat plain some 5 km (3.1 mi) lower than the southern hemisphere, and why the crust is thinner there), there are no unambiguous surface structures to confirm the existence or extent of the proposed basins. As a result, the Borealis Basin is not currently a recognized feature according to the International Astronomical Union.