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Let's go to Mars
c. 4.5 billion years ago

The Borealis impact splits Mars in two

The northern lowlands sit 5 km below the southern highlands. The boundary traces an ellipse 10,600 km across — plausibly the largest impact scar in the solar system.

Two colour-coded elevation maps of Mars's hemispheres; the cratered southern half stands kilometres higher than the smooth northern plains.
Laser altimetry of the crustal dichotomy — the northern lowlands lie several kilometres below the southern highlands. · Image: NASA/JPL

The northern lowlands cover roughly two-fifths of the martian surface: smooth, low, thinly crusted, nearly free of large craters. The south stands 5 to 6 kilometres higher, is saturated with impacts, its crust 20 to 30 km thicker. Mariner 9 and the Viking orbiters saw the split in the 1970s; explaining it took another thirty years and is not finished.

Tharsis obstructed the measurement: the volcanic province erupted later, burying part of the boundary and warping the gravity field elsewhere. In 2008 Jeffrey Andrews-Hanna, Maria Zuber and Bruce Banerdt modelled the load and subtracted it. The boundary underneath is an ellipse roughly 10,600 by 8,500 km, about 40 percent of the planet, four times wider than Hellas, the next largest martian basin.

An ellipse implies a glancing blow. In the same issue of Nature, Margarita Marinova, Oded Aharonson and Erik Asphaug modelled it: a body 1,600 to 2,700 km across striking at 6 to 10 km/s, at 30 to 60 degrees, delivering 3 to 6 × 10²⁹ joules — Pluto-scale, arriving while Mars was barely finished forming.

A collision that violent should melt much of the planet and flood the new basin with fresh rock, erasing its own evidence — a standing objection to impact models since the idea was floated in 1984. Marinova's team answered that at low velocity and a glancing angle the melt stays inside the crater. Francis Nimmo's group, in the third of those papers, modelled a head-on strike and argued the lowlands crust is itself impact melt, from a deep, depleted mantle source matching the shergottite meteorites. The two do not agree.

The competing explanation skips the impact: a single vast mantle upwelling, degree-one convection, thickening the southern crust over hundreds of millions of years. It struggles to produce an elliptical boundary, and neither side has won.

Most valley networks in the southern highlands drain into the low half, some into Hellas and Argyre. If Mars held an ocean, this basin is the bowl.