Oceans of the Solar System - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 229

the highlands). This does not chime with a long-lived Martian sea
floor that was being supplied with clay by long-lived river systems. In
a cold and wet Mars though, where a cold ocean is fringed by ice, clay
production and transport would be inhibited. 140 That ocean, and its
attendant hydrological cycle, may also have been short-lived or inter-
mittent. The erosional valleys of Mars are impressive, but there are
few signs that those rivers were long-lived. A mature river system
develops particular characteristics such as meander belts, and while
there is one beautifully preserved—indeed iconic—ancient river
meander on Mars, such structures are rare in general.
Those widespread early valley systems could rather be the product
of brief, more violent, events. If most of the water on Mars has always
been in the form of permafrost (perhaps with large aquifers beneath),
this could have been a source for floods of water, following the largest
of the many impact and volcanic events of this early era. Torrential
outflow of such subsurface water may have created collapse struc-
tures and undercuts where the water exited the ground, to help shape
the canyons and chaotic ground that make up a large part of those
dramatic and puzzling present-day Martian landscapes.
Of these signs of cataclysmic outbursts of water, the greatest are
concentrated in the Chryse Trough on the eastern margin of the Thar-
sis Bulge. The Tharsis Bulge is reminiscent of the giant flood basalt
terrains on Earth, such as the Deccan Traps of India, some 65 million
years old, and the even larger Siberian Traps that are some 250 million
years old. The basalt lavas in these terrestrial examples erupted so
quickly that both have been implicated in mass extinction events. If
the Tharsis Bulge formed in a similar manner, the rapid eruption of
huge quantities of volcanic lava at the Martian surface may have
released large amounts of water. 141 The lava would have caused local
heating of the crust, melting permafrost and so releasing yet more
water to the surface. An ocean perhaps several hundred metres deep