Mars' North Pole – Chronicle of the Martian climate
A colour mosaic of Mars' north polar cap was created from 32 images acquired by the HRSC camera
In this perspective view, the terraced structure of Mars’ polar cap can be clearly seen. The ice blanket on the North Pole is over two kilometres thick in places. The processes that led to the several-hundred-metre-deep valleys between the spiral-shaped surfaces of ice are not yet clear. This may be due to the prevailing wind directions in this region. Credit: ESA/DLR/FU Berlin, NASA MGS MOLA Science Team
Mars' northern hemisphere is currently experiencing winter. A newly generated colour mosaic depicts the extraordinary, almost perfectly symmetrical pattern of Mars' north polar ice cap, 1100 kilometres in diameter. Studying the North Pole cap will tell us more about climate development on our planetary neighbour. The mosaic consists of 32 image strips acquired by the High Resolution Stereo Camera (HRSC) on board the European Space Agency (ESA) Mars Express spacecraft during the same number of overflights. The HRSC has been operated by the German Aerospace Center (Deutsches Zentrum für Luft- und raumfahrt; DLR) for 13 years.
The north polar cap is composed of a mixture of water ice and frozen carbon dioxide, the main element of Mars' atmosphere. The dust deposited into the frozen carbon dioxide by Martian winds explains the spectacular colour changes. Stretching from the North Pole to the south right up to a latitude of 80 degrees, the ice cap permanently covers a surface of approximately one million square kilometres – roughly one quarter the size of the summer ice cap on Earth's North Pole. The prominent Martian ice cap was discovered by Dutch astronomer Christiaan Huygens in 1672, and as early as 1681 Friedrich Wilhelm Herschel observed that both polar caps change throughout the seasons on Mars.
Dry ice ‘snowfall’ at below -125 degrees Celsius
The north polar ice cap has an estimated volume of 1.2 million cubic kilometres, around half the size of Earth's Greenland ice sheet. The ice cap has an average thickness of approximately two kilometres. Temperatures fall to below -125 degrees Celsius during the Martian winter, which is twice as long as Earth's due to Mars' two-year orbital period and is characterised by long polar nights due to the tilt of its rotational axis.
Throughout the winter season, a significant proportion of the carbon dioxide in Mars' thin atmosphere (estimated to be up to one third) condenses to ice, crystallises in the atmosphere and then falls onto the surface. Throughout this process, the winter ice cap spreads to a latitude of 68 degrees. The additional ice cover, however, is only one to two metres thick. It quickly sublimates with the onset of spring, directly transforming from solid to gaseous state. The significant temperature differences between polar and temperate latitudes give rise to storms on Mars with speeds of up to 400 kilometres per hour.
Canyons in the North Pole as a climate archive
The characteristic features of the north polar cap include dark troughs that spiral outwards from the pole centre like a pinwheel in a counterclockwise direction. Chasma Boreale, the ‘northern canyon’, is a particularly striking trench up to two kilometres deep, 500 kilometres long and 100 kilometres wide located at approximately 300 degrees east. Layers that, in a similar way to tree rings, reflect the seasonal changes in ice accumulation and dust coverage due to Martian storms can be seen at its steep slopes. By examining these layer sections, scientists hope to obtain information about the development of the Martian climate. Why Chasma Boreale was cut into the ice cap of the North Pole at this precise location through the forces of erosion is not clear. The canyon could have formed due to prevailing wind directions or by a gigantic outflow of a lake beneath the ice cap.
The three-dimensional structure of the entire north polar ice cap was investigated using, among others, the MARSIS radar experiment on Mars Express and the SHARAD radar on NASA's Mars Reconnaissance Orbiter. These showed that the entire ice cap is composed of numerous individual layers of ice and dust. The layered north pole deposits are a valuable archive of the Martian climate of the last million years. Some radar data suggests that the spiral troughs and incisions were created by the wind carrying dust that was then deposited here.
The HRSC experiment
The High Resolution Stereo Camera was developed at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and built in collaboration with partners in industry (EADS Astrium, Lewicki Microelectronic GmbH and Jena-Optronik GmbH). The science team, which is headed by principal investigator (PI) Ralf Jaumann, consists of over 40 co-investigators from 33 institutions and ten countries. The camera is operated by the DLR Institute of Planetary Research in Berlin-Adlershof. The images shown here were generated by the Institute of Geological Sciences at FU Berlin in conjunction with the DLR Institute of Planetary Research in Berlin.
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