Planetary science: Martian plains as old as the hills

A series of buried impact craters has been discovered in the northern hemisphere of Mars, supporting the notion that this area of crust is a lot older than initially thought. The craters, detected by the MARSIS radar instrument aboard the European Space Agency's Mars Express spacecraft, are analysed by Thomas Watters and colleagues in this week's Nature.The martian crust is characterized by a topographic difference between the northern and southern hemispheres. The northern hemisphere is mainly lowland and sparsely cratered, whereas the southern hemisphere consists of heavily cratered ancient highlands. This hemispheric dichotomy remains one of the most intriguing features on Mars, and determining the age of the crust in the northern plains is a key factor in understanding the origin of the dichotomy and the evolution of the planet's interior. Various theories have been proposed as to why the northern crust appears so different from the southern highlands, including giant impacts and plate tectonics having operated on Mars, but only in the north.The authors have imaged 14 per cent of the northern lowlands, and their radar measurements reveal 11 buried impact basins ranging in diameter from about 130 to 470 kilometres. The basins show the same profile as other well known impact craters; parabolic shaped echoes reveal features such as rim walls, the peak ring and basin floor. The number of detected buried basins larger than 200 kilometres in diameter and the corresponding crater density of the regions studied suggest that the lowland crust is probably ancient - some 4,000 million years old - and therefore close in age to the highland crust, supporting estimates made from topographic data. The authors conclude that the crustal dichotomy formed during the early geological evolution of Mars.CONTACTThomas R. Watters (National Air and Space Museum, Smithsonian Institution, Washington, DC, USA) Tel: +1 202 633 2483; Email: watterst@si.edu