lpetrich
Contributor
The numerous pictures returned by the Mariner 9 spacecraft over 1971 - 72 were a watershed in the exploration of planet Mars. They put to bed the Schiaparelli-Lowell canals, but they revealed a substitute: numerous riverbeds, places where rivers once flowed. Rivers usually thought to be rivers of liquid water, though Mars must have had a big greenhouse effect to have had such rivers, especially early in its history (the "faint young Sun paradox").
There is a lot of evidence of former oceans on Mars:
Mars ocean hypothesis (NASA Research Suggests Mars Once Had More Water Than Arctic Ocean | NASA). Oceans mainly in the northern hemisphere and in Hellas basin. Part of that evidence is evidence of shorelines and river deltas, though that evidence has the problem of varying in altitude in different places. A solution to that conundrum is True polar wander on Mars, where early in that planet's history, the north pole was 20d from its present position toward the Tharsis plateau. At that time, the Martian dichotomy boundary was on the planet's equator, with everything north of that being ocean-covered lowland and everything south of that being cratered highland.
But then, a mantle plume lifted up some land to make the Tharsis plateau, also making the Valles Marineris rift valleys and the four big Tharsis volcanoes. That made Mars's mass distribution off-center enough to make Mars's rotation shift enough to move the Tharsis plateau to near the equator, where it now is. The Distant Shores of Mars - Scientific American Blog Network notes Timing of oceans of Mars from shoreline deforation | Nature magazine about this scenario. After Tharsis formed some 4.0 - 3.5 billion years ago and Mars's poles moved, different parts of the planet had to bulge outward to make the planet's equatorial bulge, thus moving the former shoreline up and down in different places. I've also found [0902.0191] True polar wander driven by late-stage volcanism and the distribution of paleopolar deposits on Mars discusses mainly polar deposits, but it has similar conclusions.
As to how the Mars dichotomy originally formed, it may have been from a giant impact that stripped off some of Mars's crust. This induced some polar wandering that placed the stripping boundary at the equator, the most stable place for it.
Mars's oceans are long gone, with much or most of their water now gone to outer space (NASA's MAVEN Reveals Most of Mars' Atmosphere Was Lost to Space | NASA, Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover | Science, etc.).
There is a lot of evidence of former oceans on Mars:
Mars ocean hypothesis (NASA Research Suggests Mars Once Had More Water Than Arctic Ocean | NASA). Oceans mainly in the northern hemisphere and in Hellas basin. Part of that evidence is evidence of shorelines and river deltas, though that evidence has the problem of varying in altitude in different places. A solution to that conundrum is True polar wander on Mars, where early in that planet's history, the north pole was 20d from its present position toward the Tharsis plateau. At that time, the Martian dichotomy boundary was on the planet's equator, with everything north of that being ocean-covered lowland and everything south of that being cratered highland.But then, a mantle plume lifted up some land to make the Tharsis plateau, also making the Valles Marineris rift valleys and the four big Tharsis volcanoes. That made Mars's mass distribution off-center enough to make Mars's rotation shift enough to move the Tharsis plateau to near the equator, where it now is. The Distant Shores of Mars - Scientific American Blog Network notes Timing of oceans of Mars from shoreline deforation | Nature magazine about this scenario. After Tharsis formed some 4.0 - 3.5 billion years ago and Mars's poles moved, different parts of the planet had to bulge outward to make the planet's equatorial bulge, thus moving the former shoreline up and down in different places. I've also found [0902.0191] True polar wander driven by late-stage volcanism and the distribution of paleopolar deposits on Mars discusses mainly polar deposits, but it has similar conclusions.
As to how the Mars dichotomy originally formed, it may have been from a giant impact that stripped off some of Mars's crust. This induced some polar wandering that placed the stripping boundary at the equator, the most stable place for it.
Mars's oceans are long gone, with much or most of their water now gone to outer space (NASA's MAVEN Reveals Most of Mars' Atmosphere Was Lost to Space | NASA, Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover | Science, etc.).