Hellas Planitia - General Introduction
Hellas Planitia, sometimes known as the Hellas Impact Basin, is a huge impact crater which dominates the southern hemisphere of Mars, and is immediately visible on any topographic map of the planet, such as those available at Google Mars. The word 'Hellas' is Latin for 'Greece', and 'planitia' means 'plains', so Hellas Planitia means something like 'Plains of Greece'. The crater was previously called 'Lockyer Land' after an English astronomer, Sir Joseph Norman Lockyer, before Giovanni Schiaparelli named it Hellas1 The crater has a diameter of approximately 2,300km, and is 7km in depth below the topographic datum (the equivalent of the Martian 'sea level'). The distance between the rim and the bottom of the crater is about 9km. The crater was probably formed in the Late Heavy Bombardment period of the Solar System1, approximately 3.8 to 4.1 billion years ago2.
Introduction to Research
I have downloaded a number of research papers on Hellas Planitia from the Lunar and Planetary Institute website at http://www.lpi.usra.edu/. I'm going to attempt to summarise them here, as the reports on the site are full of highly scientific language, and I wish to clarify it into plain English, both for my benefit, and hopefully for the benefit of the E2 community as a whole, particularly those interested in planetary science. The research does give some insight into the way planetary scientists analyse information and photographs from spacecraft in an effort to better understand the processes which occur on planetary surfaces. Added to that is the fact that all this research should hopefully be of benefit in understanding the geography (areography) of Mars, and the processes at work there. A good map of the whole of Hellas is available as part of the report at http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1446.pdf
Eastern Hellas Planitia
The eastern rim of Hellas Planitia, is of particular interest to scientists. Here there are three major outflow channels3,4, probably created by flowing water: the Dao, Harmakhas, and Reull Valles, or valleys. The Niger Vallis is a smaller valley east of the Dao Vallis. Dao Vallis flows NE-SW, is about 1200km long, and is immediately recognisable by its two-pronged appearance, the eastward fork being Niger Vallis. Dao Vallis is part of the volcanic region known as Hadriaca Patera, and it is suggested3 that interactions between volcanoes and ice may have formed the outflow channels. Harmakhis Vallis is approximately 800km long, and runs parallel to and south of Dao Vallis3. The Reull Vallis has been studied in detail by Leth and Treiman5,especially with regard to possible sheet floods emanating from Pomona Vallis. Reull Vallis is about 1500km long, and is made up of three main segments, running N-S (Pomona Vallis), NE-SW, and SE-NW. In the final segment it merges with the Harmakhis Vallis3.
Eastern Hellas has also been studied with regard to the incidence of so-called volatiles6,7, compounds with low boiling points, for example hydrogen, water, carbon dioxide, ammonia, and methane8, These compounds would be important for any potential life on Mars. Examples of evidence for volatiles includes "fluvial" valleys, the Vallis systems, and recent ice-related features6. The slopes in Hellas Mensae have been analysed in some detail9, showing volcano-ground ice interaction, "mass-wasting" processes (the movement of rocks under gravity10, and glacial processes. The European Space Agency's Mars Express mission has recently discovered an 'Hourglass crater' 11, formed from two linked craters with evidence of flowing material between them.
Moore and Wilhelms 12 proposed the hypothesis that Hellas might once have held ice-covered lakes, such as those though to exist on Europa and Titan, and thought also to to be a possible habitat for life to evolve in. They state that geological evidence backs up possible lakes at depths of 5.8km and 3.1km (remember that the Hellas crater is about 6.9km deep at its lowest point). The evidence suggests that water-laid and ice-rich sediment was transported by channels like the valleys described above, carrying water released by volcanism and other geothermal activity. The flows might have resembled water-rich mudflows, and the lakes would quickly have frozen over, with ice several hundred metres thick, which gradually sublimed to the atmosphere, leaving only sediment.
Dust devils have also been studied in the Hellas basin13, as they provide an explanation for the amount of dust in the Martian atmosphere, which contributes to the Martian climate. Many dust devil tracks have been found in Hellas and in Argyre Planitia. These tracks have been studied in detail, partly because it is difficult to photograph the dust devils systematically from orbit. The studies suggest that dust devils form more often in Argyre than in Hellas, especially in the summer, but that these are covered or erased more quickly in Argyre. Further, there are more tracks in Hellas at lower altitudes, because of lower surface pressure.