Oceans of the Solar System - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 219

transformed by the complex, ingenious, fragile space probes that,
since the 1960s, have been penetrating deep into its mysteries: from
Mercury as far as Neptune. As we write, the New Horizon satellite is
heading for Pluto (formerly known as a planet) and will make contact
in 2015—and then head for the Kuiper Belt beyond. These have been
real voyages of discovery, and have opened up worlds that are stranger
and more diverse—and in places far more ferocious—than the imag-
ined planets of the science fiction writers. Some have oceans perhaps
larger than Earth's. Some had oceans in the deep geological past.
Some might yet acquire them. We can explore them now, moving
from the regions close to the Sun into the chilly outer fringes of our
own star system.
The Space Exploration Era
Mercury, at its closest, orbits just 46 million kilometres from the
Sun (28.5 million miles), and lies far from the solar system's 'snow
line' (see Chapter 1). It is a rocky world of extremes, cratered like our
Moon, with temperatures swinging from a furnace-hot 425 degrees
Celsius during the day to nearly minus 150 degrees Celsius in its
planetary night. It certainly does not support anything that we
might call an ocean, no matter how hard we stretch the definition of
that word. Nevertheless, in the floors of craters near Mercury's north
pole, in patches that are permanently shaded from the Sun's heat,
scientists at Caltech and NASA's Jet Propulsion Laboratory in the
early 1990s used radar to identify bright spots that might be ice—
and, if so, perhaps relics left from the impact of ancient comets.
Recently, these bright patches have been confirmed by detailed radar
images from the MESSENGER spacecraft, which has been in orbit
around Mercury since 2008. It is still not certain, though, that the
bright spots do indeed represent water ice (they might be of another
volatile substance, such as sulphur). If it is water, it must—even