Here Be Dragons

THE INCREDIBLE SHRINKING MARTIANS
layers of the Sun, in the martian atmosphere, on the martian surface,
in the Earth's atmosphere, or somewhere along the way. The correct
location can in principle be determined by making comparisons between
observations of different targets: for example, bands that show
up when observing Mars but not when observing the Moon are likely
due to elements present on Mars or in its atmosphere. Spectroscopy
can often determine not only the existence but also the amount of a
compound that is present.
Some early spectroscopic observations—for example those conducted
by V.M. Slipher at the Lowell Observatory in 1907—suggested
the presence of abundant water vapor and oxygen in the martian atmosphere,
circumstances obviously very favorable to the existence of
life. But subsequent studies came up with lower and lower estimates.
By 1926, oxygen was down to "undetectable" levels, and water vapor
was present at no more than 3 percent of its terrestrial abundance. In
spite of these developments, belief in the existence of plant life on
Mars persisted and even strengthened. In 1947, a Dutch-born
American astronomer, Gerard Kuiper, correctly detected the telltale
bands of carbon dioxide in the infrared region of Mars's spectrum.
This raised the possibility of photosynthesis, since photosynthetic
plants use carbon dioxide as their carbon source. Kuiper failed to detect
chlorophyll, but he suggested that martian plant life might resemble
terrestrial lichens, some of which use other kinds of molecules to
capture light. Then, in 1957, William Sinton of Harvard University reported
that he had detected absorption bands further into the infrared,
which he (wrongly) ascribed to the organic components of plant life
(including carbohydrates). Therefore, he concluded, there was an extensive
cover of vegetation on Mars.
During the 19605, three Mariner spacecraft flew by Mars and sent
back pictures of a rugged, cratered terrain without any signs of life.
They also sent back data about the atmosphere and surface that made
clear just how hostile to life Mars really is. According to the latest
measurements, the average atmospheric pressure at the surface is less
than one hundredth the pressure on Earth. The atmosphere is 95 percent
carbon dioxide and 3 percent nitrogen, with traces of other gases.
There is almost no oxygen, and so little water vapor that, if it were all
squeezed out, it would form a layer of liquid water only about one hundredth
of a millimeter thick on the planet's surface. Even so, water-ice
clouds do sometimes form. Mars is bone-chillingly cold: even at the
equator, the average temperature is only about minus 6o°c, and the
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