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growing sense of empty air ahead, as the far side of the caldera and
the rest of the great circle became visible. And then she was climbing down
onto the last ledge, a bench only some five meters wide, with a curved back
wall, shoulder-high: and below her dropped the great round chasm of Pavonis.
    This caldera was one of the geological marvels of the solar
system, a hole forty-five kilometers across and a full five kilometers deep,
and almost perfectly regular in everyway— circular, flat-floored, almost
vertically walled—a perfect cylinder of space, cut into the volcano like a rock
sampler’s coring. None of the other three big calderas even approached this
simplicity of form; Ascraeus and Olympus were complicated palimpsests of
overlapping rings, while the very broad shallow caldera of Arsia was roughly
circular, but shattered in every way. Pavonis alone was a regular cylinder: the
Platonic ideal of a volcanic caldera.
    Of course from this wonderful vantage point she now had, the
horizontal stratification of the interior walls added a lot of irregular
detail, rust and black and chocolate and umber bands indicating variations in
the composition of the lava deposits; and some bands were harder than those
above and below, so that there were many arcuate balconies lining the wall at
different elevations—isolated curving benches, perched on the side of the
immense rock throat, most never visited. And the floor so flat. The subsidence
of the volcano’s magma chamber, located some 160 kilometers below the mountain,
had to have been unusually consistent; it had dropped in the same place every
time. Ann wondered if it had been determined yet why that had been; if the
magma chamber had been younger than the other big volcanoes, or smaller, or the
lava more homogenous.... Probably someone had investigated the phenomenon; no
doubt she could look it up on the wrist. She tapped out the code for the
Journal of Analogical Studies, typed in Pavonis: “Evidence of Strombolian
Explosive Activity Found in West Tharsis Clasts.” “Radial Ridges in Caldera and
Concentric Graben Outside the Rim Suggest Late Subsidence of the Summit.” She
had just crossed some of those graben. “Release of Juvenile Volatiles into
Atmosphere Calculated by Radiometric Dating of Lastflow Mafics.”
    She clicked off the wristpad. She no longer kept up with all the
latest areology, she hadn’t for years. Even reading the abstracts would have
taken far more time than she had. And of course a lot of areology had been
badly compromised by the terraforming project. Scientists working for the
metanats had concentrated on resource exploration and evaluation, and had found
signs of ancient oceans, of the early warm wet atmosphere, possibly even of
ancient life; on the other hand radical Red scientists had warned of increased
seismic activity, rapid subsidence, mass wasting, and the disappearance of even
a single surface sample left in its primal condition. Political stress had
skewed nearly everything written about Mars in the past hundred years. The
Journal was the only publication Ann knew of which tried to publish papers
delimiting their inquiries very strictly to reporting areology in the pure
sense, concentrating on what had happened in the five billion years of
solitude; it was the only publication Ann still read, or at least glanced at,
looking through the titles and some of the abstracts, and the editorial
material at the front; once or twice she had even sent in a letter concerning
some detail or other, which they had printed without fanfare. Published by the
university in Sabishii, the Journal was peer-reviewed by like-minded
areologists, and the articles were rigorous, well researched, and with no
obvious political point to their conclusions; they were simply science. The
Journal’s editorials advocated what had to be called a Red position, but only
in the most limited sense, in that they argued for the preservation of the
primal landscape so that