Lyell Crater

Location of Lyell Crater. This map has its craters appear oval due to foreshortening from the view at the equator
Planet	Mars
Region	Mare Australe quadrangle
Coordinates	70°06′S 15°36′W / 70.1°S 15.6°W / -70.1; -15.6Coordinates: 70°06′S 15°36′W / 70.1°S 15.6°W / -70.1; -15.6
Diameter	131 km
Eponym	Charles Lyell

Lyell Crater is an impact crater in the Mare Australe quadrangle of Mars, located at 70.1°S latitude and 15.6°W longitude. Lyell is also inside Noachis Terra and sits on the edge of the Sisyphi Planum. It is 131.0  km in diameter and was named after Charles Lyell, and the name was approved in 1973 by the International Astronomical Union (IAU) Working Group for Planetary System Nomenclature (WGPSN).^[1]

Nearby named craters include Mellish to the southwest, Daly to the north-northwest, further north the smaller Sarh and further northeast is Wegener, that crater in the southeast end of the Argyre quadrangle. Southeast is a hollow formation of Sisyphi Cavi. Also the crater is in the area where the sun shines 24 Martian hours in late spring and early summer, and all day dark in late autumn and early winter as the crater is south of the 65th parallel.

This picture was taken in the spring season on Mars when the temperature was rising. During the winter, much frost accumulates. When the temperature goes up in the spring, the frost disappears in the thin Martian atmosphere and leaves behind the dark ground. The surface appears covered with dark spots when this defrosting process is occurring.^[2] The dark spots can be seen in some of the pictures below.

Many gullies are present in Lyell Crater.

Gullies[edit]

Martian gullies are small, incised networks of narrow channels and their associated downslope sediment deposits, found on the planet of Mars. They are named for their resemblance to terrestrial gullies. First discovered on images from Mars Global Surveyor, they occur on steep slopes, especially on the walls of craters. Usually, each gully has a dendritic alcove at its head, a fan-shaped apron at its base, and a single thread of incised channel linking the two, giving the whole gully an hourglass shape.^[3] They are believed to be relatively young because they have few, if any craters. A subclass of gullies is also found cut into the faces of sand dunes which themselves considered to be quite young.
On the basis of their form, aspects, positions, and location amongst and apparent interaction with features thought to be rich in water ice, many researchers believed that the processes carving the gullies involve liquid water. However, this remains a topic of active research.
As soon as gullies were discovered,^[3] researchers began to image many gullies over and over, looking for possible changes. By 2006, some changes were found.^[4] Later, with further analysis it was determined that the changes could have occurred by dry granular flows rather than being driven by flowing water.^[5][6][7] With continued observations many more changes were found in Gasa Crater and others.^[8]
With more repeated observations, more and more changes have been found; since the changes occur in the winter and spring, experts are tending to believe that gullies were formed from dry ice. Before-and-after images demonstrated the timing of this activity coincided with seasonal carbon-dioxide frost and temperatures that would not have allowed for liquid water. When dry ice frost changes to a gas, it may lubricate dry material to flow especially on steep slopes.^[9][10][11] In some years frost, perhaps as thick as 1 meter.

• 
  

  

  
  

  
  

  Lyell (Martian crater), as seen by CTX camera (on Mars Reconnaissance Orbiter).
  

  
  

  
  


• 
  

  

  
  

  
  

  Gullies in Lyell crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of the previous image of Lyell crater.
  

  
  

  
  


• 
  

  

  
  

  
  

  Defrosting taking place in Lyell crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Dark areas are where frost has disappeared thereby showing dark ground. Note: this is an enlargement of a previous image of Lyell crater.
  

  
  

  
  

See also[edit]

• Climate of Mars


• Geology of Mars


• Geyser (Mars)


• List of craters on Mars: H-N


• List of people with craters on Mars named after them


• Martian gullies


• Ore resources on Mars


• Planetary nomenclature

References[edit]