Hagal is the informal name of a dune field on Mars located below the north pole of Mars. Its name derives from the sand dunes in Frank Herbert's novel Dune and the fictional planet Hagal. It is located at coordinates 78.0° N latitude, 84.0° E longitude, and consists of linear and round dunes with a southeast slipface orientation. It was one of the dune formations targeted for imaging by the HiRISE camera, onboard the Mars Reconnaissance Orbiter, at the rate of one image every six weeks. in the third year of its seasonal expedition. It is also known as the "Martian Morse Code" due to the linear and rounded formations of its dunes, which have the appearance of dots and dashes.
Formation
Although normally it is possible to obtain information about the wind direction from the orientation and form of sand dunes, the complexity of shapes of the Hagal dunes makes it difficult to determine the direction of the forming winds. In the case of the Hagal dunes, it is theorised that a local circular crater, probably formed due to meteorite impact and filled with sand, has decreased the quantity of dune-forming sand; this, in turn, impacted the localtopography, causing a change in wind patterns. The linear dunes were formed through the action of bidirectional winds, acting perpendicular to the line of the sand dune, causing a funneling effect directing the sand to accumulate along the linear axis of the dune. The round-shaped dunes were formed when the winds that caused the linearly-shaped accumulations were interrupted. The round dunes are classified as "barchanoid dunes". However, the exact mechanism of either formation is still unknown and this is the reason the area was chosen for imaging by the HiRISE mission. Veronica Bray, HiRISE camera targeting specialist, commented that there are similarly shaped dunes in other locations on Mars, but the Hagal field provides better images of these shapes due to the uncommon characteristics of its topography. Bray also decoded the "Morse Code" of a formation as "NEE NED ZB 6TNN DEIBEDH SIEFI EBEEE SSIEI ESEE SEEE !!".
