Hellas Planitia

Hellas Planitia spans a colossal diameter of roughly 2,300 kilometers across the Martian surface. The crater plunges to a depth of about 7,000 meters below the surrounding topography. This makes it not only the largest visible impact crater on Mars but also one of the largest in the entire Solar System.

Because the crater floor sits so incredibly deep below the average Martian elevation, the atmospheric pressure at the bottom is significantly higher than at the surface level. In fact, the pressure is high enough that liquid water could theoretically exist there during certain times of the Martian day. This unique microclimate makes it a prime target for astrobiological research.

The extreme depth of the basin causes unique meteorological phenomena, including frequent, massive dust storms and dense morning fog. During the harsh Martian winter, the crater floor becomes cold enough for carbon dioxide frost to accumulate. Spacecraft have observed this seasonal freezing and sublimation process repeatedly over the years.

The basin was formed approximately 4 billion years ago during the Noachian epoch, when a massive asteroid slammed into the southern highlands of Mars. The sheer force of the impact blasted a massive amount of crustal material out of the basin. This ejected debris is still visible today as rough, hilly terrain surrounding the crater's vast perimeter.