Oceans of the Solar System - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 230

(or more: some estimates go as high as 1,700 metres ) could then have
rapidly formed on the northern plains.
Such an outpouring of water might have shifted the Martian cli-
mate, albeit briefly—from a cold, dry desert to a warmer, wetter
state—restoring an Earth-like hydrological cycle with rainwater, riv-
ers, deltas, and seas. The surface water would have released water
vapour and carbon dioxide into the atmosphere—both greenhouse
gases. As the Martian climate warmed, positive feedback mechanisms
would have kicked in, when carbon sources long held within the
Martian regolith and ice were released by melting. Thus, the Martian
climate may, for a time, have warmed even more.
All too soon, cold would have returned. The increased moisture in
the atmosphere would give rise to more snow at high latitudes: the
icecaps would grow in size, and reflect more of the Sun's heat and
light to cool the climate. Any increased rainfall, too, would have
'scrubbed' carbon dioxide from the atmosphere and allowed it to
react with rocks, in silicate weathering, to cool the climate still fur-
ther. A cold desert returned, perhaps to be punctuated again by sub-
sequent ocean events, until Mars's internal engine cooled and volcanic
activity dissipated, and the Oceanus Borealis finally shrank and died.
What were the waters of that short-lived ocean—or successive,
temporary oceans—like? They were certainly salty—probably salty
enough to act as a powerful antifreeze, remaining liquid at tempera-
tures well below zero. But the brines were not like those of the Earth's
oceans, because the dried-out salt deposits that remain include only
tiny amounts of carbonates. There is nothing on Mars's surface resem-
bling a limestone, despite the dominance of carbon dioxide in the thin
atmosphere of Mars today. But there are large deposits of iron oxides
and sulphates, a little like those that form in the strongly acidic water
which oozes today out of coal and metal mines. Hence the oceans of
Mars, it seems, over time became charged with sulphuric acid, and in