In addition to its enormous volcanoes, huge rift valley systems, and dried-up crater lakes and river valleys, the ice caps at the north and south poles of Mars have been the subject of intensive scientific investigations. These ice caps, which grow in the winter and shrink in the spring and summer, are also a distinctly aesthetic sight. This image, created using data acquired by the German Aerospace Center (Deutsches Zentrum fur Luft- und Raumfahrt; DLR) High Resolution Stereo Camera (HRSC) on board the European Mars Express orbiter, shows the planet's South Pole with its ice cover, which is approximately 600 kilometres across. It is springtime in the Martian southern hemisphere, marking the end of the 343-day polar night darkening the surface south of the Arctic Circle, and the increased solar irradiation will soon shrink the ice cap to its summer diameter of 400 kilometres.

Mars Express, the first planetary mission implemented by the European Space Agency (ESA), has been orbiting Mars since the 25 December 2003. Along with NASA's 2001 Mars Odyssey orbiter, 'MEX' is the spacecraft that has been exploring the Solar System for the longest period of time. One of its seven experiments is the HRSC camera system, developed at DLR in collaboration with German industry, which has been mapping the planet in high resolution since January 2004, both in colour and in 3D. The stereo image data recorded by the camera is used to create digital terrain models from which the topography of Mars can be derived.

This vertical plan view of the South Pole was created using data acquired almost exactly nine years ago, on 17 December 2012 during orbit 11,404, from altitudes of between 1330 and 1700 kilometres. The image resolution is approximately 200 metres per pixel. The raw image data were processed by the HRSC team at the DLR Institute of Planetary Research in Berlin-Adlershof. The image was created using data acquired by the blue, green and near infrared sensors, and approximates the true colour impression that the human eye would observe from a spacecraft in orbit. The colour differences in the ice cover result from alternating layers of ice and dust.

Temperatures fall far below minus 100 degrees Celsius
In 1672, Dutch astronomer Christiaan Huygens discovered the Martian polar ice caps. Just over 100 years later, in 1781, William Herschel observed that the polar ice caps are not centred at exactly 90 degrees north and south from the equator, respectively, but have a slightly offset placement. He also discovered that the ice caps grow and shrink seasonally. In 1976, NASA's Viking 2 spacecraft found that the Martian northern polar cap is comprised of at least 50 percent water ice, with the remaining material being frozen carbon dioxide. The southern polar cap has only a small amount of water ice mixed with carbon dioxide ice. The southern polar cap is up to 1500 meters thick and, like the ice sheet at the North Pole, has a volume of approximately 1.6 million cubic kilometres. Together, these two poles have slightly more ice than the Greenland ice sheet, which has a volume of just under three million cubic kilometres.

During the polar night in the winter months, temperatures at the poles drop below minus 100 degrees Celsius. Mars is an average of 70 million kilometres further away from the Sun than Earth and thus receives less light and warmth - only about 600 watts per square metre (Earth receives approximately 1360 watts per square metre at the top of its atmosphere). The Martian atmosphere is much thinner than Earth's and can therefore only generate a much smaller natural greenhouse effect and thus cause the storage of correspondingly less energy.

Dry ice 'snows' onto Mars in the winter
Because the South Pole of Mars is several thousand metres higher in altitude than the North Pole, temperatures there are even lower during the polar night and can fall to below minus 130 degrees Celsius. This extreme cold causes carbon dioxide to crystallise out of the atmosphere and fall onto the Martian surface as dry ice. This new 'snowfall' causes the polar caps to extend far over the surface of Mars; in the south, this fresh ice layer cover extends to latitudes of 60 to 50 degrees. On Earth, this would be equivalent to Antarctica extending to reach Cape Horn at the southern tip of Chile in South America during the southern winter.

The Martian atmosphere consists of 95 percent carbon dioxide, together with small quantities of nitrogen and argon, and only traces of water vapour. Calculations indicate that up to one third of the carbon dioxide precipitates out of the atmosphere during the course of a Martian year in the relevant winters, first to one pole, before sublimating again at the beginning of spring, then falling and re-sublimating again at the other pole later in the year. The resulting mass shifts have been detected by multiple observations.

The somewhat spiral geometry of the ice layers is striking. Its origin is not yet completely understood, but it is thought to be caused by the Coriolis effect, which also acts on Earth. The surface rotation speed of a planet is fastest at its equator and decreases to zero towards the poles. As a result, the warmer air masses flowing from the equator towards the poles are deflected in one direction, while the colder air masses flowing from the poles to the temperate latitudes are deflected in the reverse direction. This leads to characteristic rotational flow patterns, giving rise to eroded structures on the ground. The spiral pattern is much more pronounced at the North Pole of Mars than at the South Pole.

Seasons on Mars and Earth are similar
From an astronomical point of view, the seasons on Mars are very similar to those on Earth. Mars is only half the size of Earth, but the three parameters that lead to this almost identical annual cycle as on Earth are easily comparable. Crucial for the occurrence of seasons is the fact that Mars, like Earth, has a rotational axis that is tilted compared to its orbital plane around the Sun. Its axis of rotation is inclined by 25.2 degrees with respect to the ecliptic, which is the plane of Earth's orbit around the Sun. For Earth, the axis tilt is 23.4 degrees. This results in the Martian poles being slightly more shifted to the south and north, respectively. Additionally, the length of a day on planets are comparable; on Earth the rotation period is 24 hours, while on Mars the rotation period, referred to as a 'sol', lasts only 37 minutes longer.

In addition, the length of a year - the period of time that passes while Earth of Mars complete a 360-degree journey around the Sun - is easily comparable between both planets. For Earth it is 365 days (and an additional quarter of a day - resulting in a leap year every four years), and for Mars it is 687 days. The Martian year is therefore 1.9 times longer than that of Earth, which means that all four seasons on Mars last almost twice as long as they do on Earth.

The High Resolution Stereo Camera (HRSC) was developed at the German Aerospace Center (DLR) and constructed in cooperation with several industrial partners (Airbus, Lewicki Microelectronic GmbH and Jena-Optronik GmbH). The science team, led by Principal Investigator (PI) Ralf Jaumann, consists of 50 co-investigators from 35 institutions and 11 nations. The camera is operated by the DLR Institute of Planetary Research in Berlin-Adlershof. Staff from the Department of Planetary Sciences and Remote Sensing at Freie Universitat Berlin used the data acquired by the camera system to create the image shown here.

A high-resolution version of this image and additional images from HRSC can be found in the Mars Express image gallery on flickr.

Related Links
DLR High Resolution Stereo Camera (HRSC)
Mars News and Information at MarsDaily.com
Lunar Dreams and more

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