Oceans of the Solar System - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 228

and flash-heating had dehydrated them. Further study, though, indi-
cated that they were originally poor in water, but that instead they con-
tain a good deal of chlorine as a volatile phase—more than twice as
much as in equivalent rocks on Earth. 138 Volcanism, it seemed from
this, did not provide much of Mars's early water, which must there-
fore have originally come from the icy comets that crash-landed on
that planet early in its history.
Yet these same meteorites have recently been re-examined, and the
new studies focused on tiny crystals of apatite within them. These are
relatively strong, leak-proof crystals that formed early in these rocks'
history as they began to crystallize deep in the Martian magma cham-
ber. The composition of the apatites suggests that the magma was
relatively water-rich. The degassing then likely took place not in
impact-shock on hitting the Earth, but much earlier, as the original
magmas were ascending to the Martian surface. 139 So volcanic erup-
tions may have provided substantial amounts of water after all. (We
suspect this debate will rumble on for some time yet, as the detective
work continues.)
How long did that (presumed) Martian ocean last? Could the sur-
face of a youthful Mars have been warm and wet, at least periodically,
with a long-lived ocean and a hydrological cycle something like that
of Earth? This could only have taken place below a dense greenhouse
gas atmosphere, which could balance the reduced light and warmth
of a faint young Sun. It is difficult to see how Mars could have made—
and retained—such an atmospheric blanket. The evidence on the
ground does not support such an idea, either. One symptom of long-
lived water is the alteration of minerals to clay, both at the bottom of
a sea and on land (where erosion soon carries that clay into the sea).
Clays on the Martian surface have been identified and mapped by the
orbiting spacecraft from their particular spectral pattern—but they
are rare on the northern plains (although somewhat commoner in