Undreamed Shores - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 262

Until then, astronomers have to be as ingenious as possible with
what remains of the global astronomy budget after the James Webb
has had its share. One way is to use hand-me-downs from rich rela-
tives. There is currently an offer of a couple of retired US spy satellites
to NASA. They are not ideal for imaging exoplanets (the mirrors,
although very high quality, are held in place by struts which, as luck
has it 'couldn't be in worse places', as one NASA scientist said). 177 Still,
beggars can't be choosers, and already there are plans to make
these useful by fitting them with carefully designed coronographs—
sunshades designed to block out the light from the star while care-
fully preserving that from an orbiting planet.
With current technology we cannot see the oceans themselves, for
the planets we are looking at are specks of light that are spatially unre-
solved—meaning you cannot see individual parts of them such as seas,
continents, icecaps and so on. But there are telltale signs that can be
used: colour, shade, and glint. Because oceans are dark and have differ-
ent colours from other surfaces such as land, variations in the colour of
a planet over time may betray the presence of a liquid water ocean.
Oceans are also smooth compared to other surface types, and they
can polarize light. Nevertheless, the light can then be scattered by the
actions of water vapour and aerosols in the atmosphere, so this is not
unproblematic. And there is glint, the specular reflection of light from
the ocean not long before the Sun sets. Where there is a large angle
between a planet's oceans and its star, then the planet should reflect
more light. 178 There are problems with this method too, because light
can bounce off clouds in the atmosphere in a similar manner, or it can
bounce off polar icecaps on an otherwise dry world such as Mars. 179
And so the search goes on. As you read this topic, new discoveries
will already have been made. The only certain thing is that these will
be surprises.