Viking photographs showed gullies and deep valleys, appearing to
have been carved by running water. There were giant volcanoes, one
of which, Olympus Mons, rises 15 miles above the planet's surface,
and the solar system's largest canyon, Valles Marineris, which stretches
2,050 miles. But these dramatic features were natural - alas, the
artificial 'canals' observed by Lowell's generation had been an
optical illusion, created by the limitations of their 19th century
The absence of
sufficient atmosphere to sustain water now suggested that these
valleys and canyons had been formed in the planet's distant past,
when the amosphre was thicker, but with little geological signs
of erosion this meant in the tens of thousands of years, not millions.
lower gravity and the absence of tectonic motion enabled the proportions
of Martian volcanoes to far exceed those on the Earth. This absence
of tectonic crustal movement is crucial to my secondary hypothesis
and I will return to it later in this section.