484 PART 4^ |^ THE SOLAR SYSTEM
Many fl ow features lead into the northern lowlands, and the
smooth terrain there has been interpreted as ancient ocean fl oor.
Features along the edges of the lowlands have been compared to
shorelines, and many planetary scientists conclude that the
northern lowlands were fi lled by an ocean when Mars was
younger. Large, generally circular depressions such as Hellas and
Argyre appear to be impact basins that also may once have been
fl ooded by water.
Mars Orbiter photographed the eroded remains of a river
delta in an unnamed crater in the old highlands (■ Figure 22-18).
Details show that the river fl owed for long periods of time, shift-
ing its channel to form meanders and braided channels as rivers
on Earth do. Th e shape of the delta suggests it formed when the
river fl owed into deeper water and dropped its sediment, much
as the Mississippi drops its sediment and builds its delta in the
Gulf of Mexico.
Did Mars once have that much water? Deuterium is 5.5 times
more abundant than normal (light) hydrogen in the Martian
atmosphere, and that suggests that Mars once had about 20
times more water than it has now. Presumably, much of the water
was broken up and the normal hydrogen mostly lost to space.
Th e remaining water on Mars could survive if it were frozen
in the crust. High-resolution images and measurements madeb Visual-wavelength imagea Visual-wavelength imageb Visual-wavelength imagea Visual-wavelength image■ Figure 22-1 7
These visual-wavelength images made by the Viking orbiters show some of
the features that suggest liquid water on Mars. (a) Outfl ow channels are
broad and shallow and defl ect around obstructions such as craters. They
appear to have been produced by sudden fl oods. (a) Valley networks resem-
ble drainage patterns and suggest water fl owing over long periods. Crater
counts show that both formations are old, but valley networks are older than
outfl ow channels. (NASA © Calvin J. Hamilton)
Visual-wavelength imageVisual-wavelength image■ Figure 22-18
This distributary fan was formed where an ancient stream fl owed into a
fl ooded crater. Sediment in the moving water was deposited in the still water
to form a lobed delta that later became sedimentary rock. Detailed analysis
reveals that the stream changed course time after time, and this shows that
the stream fl owed for an extended period and was not just a short-term
fl ood. Also, the shape of the fan is evidence that a lake persisted in the
crater while the fan developed. (NASA/JPL/Malin/Space Science Systems)