Mars Astrobiology: Recent Status and Progress - Planetary Exploration and Science: Recent Results and Advances - page 156

30 °

20 °

290 °

300 ° 310 °

km

− 8 − 4 0 4 8 12

Fig. 8.4 Kasei Vallis - a major outflow channel - seen in MOLA elevation data. Flow was from
the bottom left to right. Image is approximately 1,600 km across. The channel system extends
another 1,200 km south of this image to Echus Chasma - Mars Global Surveyor ( http://themis.
mars.asu.edu/feature/10 - NASA/JPL-Caltech/Arizona State University, 2006)

of water broke through dams, carved deep valleys, eroded grooves into bedrock,
and traveled thousands of kilometers. Areas of branched streams, in the southern
hemisphere, suggested that rain once fell. The numbers of recognized valleys have
increased through time. Research published in June 2010 mapped 40,000 river
valleys on Mars, roughly quadrupling the number of river valleys that had previously
been identified. Martian waterworn features can be classified into two distinct
classes: 1) dendritic (branched), terrestrial-scale, widely distributed, Noachian-age
valley networks and 2) exceptionally large, long, single-thread, isolated, Hesperian-
age outflow channels. Recent work suggests that there may also be a class of
currently enigmatic, smaller, younger (Hesperian to Amazonian) channels in the
mid-latitudes, perhaps associated with the occasional local melting of ice deposits
(Fig. 8.4 ).
Some parts of Mars show inverted relief. This occurs when sediments are
deposited on the floor of a stream and then become resistant to erosion, perhaps
by cementation. Later, the area may be buried. Eventually, erosion removes the
covering layer and the former streams become visible since they are resistant
to erosion. Mars Global Surveyor found several examples of this process. Many
inverted streams have been discovered in various regions of Mars, especially in the
Medusae Fossae Formation, Miyamoto Crater, Saheki Crater, and Juventae Plateau.
A variety of lake basins have been discovered on Mars. Some are comparable
in size to the largest lakes on Earth, such as the Caspian Sea, Black Sea, and Lake