- Hellas Planitia
NASAimage of Hellas Planitia
region=, south of
Planitia, also known as the Hellas Impact Basin, is a huge, roughly circular impact basinlocated in the southern hemisphere of the planet Mars. It is the second or third largest impact crater and the largest visible impact crater known in the Solar System. The basin floor is 3 km deeper than the moon's South Pole-Aitken basin, extends about 2,300 km east to west, [The part below zero datum, see Geography of Mars#Zero elevation] and its debris field could be interpreted as extending about 7,000 km across [The part above zero datum, The part below zero datum, see Geography of Mars#Zero elevation] [ [http://rst.gsfc.nasa.gov/Sect19/Sect19_12.html Remote Sensing Tutorial Page 19-12] , NASA] . It is centered at coord|-42.7|70.0|globe:Mars.
diameterof about 2,300 km (1,400 mi) [ Schultz, R.A., and H.V. Frey, A new survey of multiring impact basins on Mars, J. Geops. Res., 95, pp. 14175-14189, 1990.] , it is the largest unambiguous impact structure on the planet, though a distant second if the Borealis Basinproves to be an impact crater. The basin is thought to have been formed during the Late Heavy Bombardmentperiod of the Solar System, over 3.9 billion years ago, when a large asteroid hit the surface. [ Acuña et al., Science, 284, 790-793, 1999 [http://www.sciencemag.org/cgi/content/full/284/5415/790] ]
The altitude difference between the
rimand the bottom is ~9 km (30,000 ft). The depth of the crater (~7 km [http://www-star.stanford.edu/projects/mgs/sum/s0403210230.html Martian Weather Observation] MGS radio science measured 11.50 mbar at 34.4° S 59.6° E -7152 meters] (23,000 ft) below the standard topographic datum of Mars) explains the atmospheric pressure at the bottom: 1,155 Pa (11.55 mbar or 0.17 psi). This is 89% higher than the pressure at the topographical datum (610 Pa, or 6.1 mbar or 0.09 psi) and above the triple pointof water, suggesting that the liquid phase would be transient (would evaporate over time) if the temperature would rise above 0 °C (32 °F). [ [http://science.msfc.nasa.gov/headlines/y2000/ast29jun_1m.htm Making a Splash on Mars] , NASA, 2000-06-29]
Some of the low elevation outflow channels extend into Hellas from the volcanic Hadriacus Mons complex to the northeast, two of which Mars Orbiter Camera images show contain gullies:
Dao Vallisand Reull Vallis. These gullies are also low enough for liquid water to be transient around Martian noon, if the temperature would rise above 0 Celsius. [Heldmann et al., Jennifer L., para 3 page 2 Martian Gullies Mars#ReferencesDOI:10.1029/2004JE002261]
Discovery and naming
Due to its size and its light colouring, which contrasts with the rest of the planet, Hellas Planitia was one of the first Martian features discovered from Earth by
telescope. Before Giovanni Schiaparelligave it the name Hellas (' Greece'), it was known as 'Lockyer Land', having been named by Richard Anthony Proctorin 1867in honour of Sir Joseph Norman Lockyer, an English astronomer who, using a 6.25-inch (16-cm) refractor, produced "the first really truthful representation of the planet" (in the estimation of E. M. Antoniadi). [cite web|url=http://www.uapress.arizona.edu/onlinebks/mars/chap04.htm|title=The Planet Mars: A History of Observation and Discovery|accessdate=2007-08-20|author=William Sheehan]
List of plains on Mars
Geography of Mars
* J. N. Lockyer, [http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1863MNRAS..23..246L "Observations on the Planet Mars" (Abstract)] , Monthly Notices of the Royal Astronomical Society, Vol. 23, p. 246
* E. M. Antoniadi, "The Hourglass Sea on Mars", Knowledge, July 1, 1897, pp. 169-172.
* [http://www.spacedaily.com/news/mars-water-science-00i8.html The Hellas Of Catastroph] , Peter Ravenscroft, 2000-08-16, Space Daily
* [http://www.google.com/mars/#lat=-42.7&lon=70 Google Mars scrollable map] - centered on Hellas
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