Early studies of the effects of large comet and asteroid impacts on the atmospheric evolution on Earth suggest that impacts could erode atmosphere from Mars with high efficiency. Indeed, if early melting, differentiation, and outgassing of Mars occurred simultaneously with terminal accretion, it is quite difficult to avoid loss of more than 99% of the primordial atmosphere. This idea had been explored in detail using hydrodynamic codes appropriate for large explosions and has found that such erosion is quantitatively very important for Mars because the low planetary escape velocity permits easy escape of shocked gases. Such atmospheric-eroding events were also found to be effective at removing strong rocks from the planet, thus offering a reasonable explanation of the ejection of SNC meteorites from Mars. The most interesting aspect of explosive blowoff as an escape mechanism is that it is entirely mass independent. The blowoff process is hydrodynamic and occurs on a time scale (minutes) too short for mass-dependent fractionation of gases.
Syrtis Major - Point Perspective
Mosaic of the Syrtis Major hemisphere of Mars projected into a point perspective, a view similar to that which one would see from a spacecraft. The viewer's distance is 2,000 kilometers above the surface of the planet.
The large bright colored area, located in the upper left area of the image is known as Arabia. The dark area to the right of Arabia, called Syrtis Major Planus, is a low-relief volcanic shield of probable
basaltic composition. Bright white areas to the south, including the Hellas impact basin at extreme lower right, are covered by carbon dioxide frost. Regions to the west and south of Syrtis Major are heavily cratered and relatively old. The dark feature coming around the western horizon is known as Sinus Sabaeus.