Back in 2005, NASA's Deep Impact probe slammed a hunk of copper into the comet Tempel 1 to determine what was under its surface, as well as to see what happens in a hypervelocity collision.
The copper block vaporized in the high-energy impact, but the spacecraft lived on. It's now on an extended mission called EPOXI, and one part of that is EPOCh: Extrasolar Planet Observations and Characterization, designed to look back at the Earth and see what a habitable and inhabited planet looks like from a distance. The idea is to see what we can observe about our own world that can be used to look at worlds orbiting other stars.
One hope was that the spacecraft would see sun glints; flashes of light from standing water on Earth (much like Cassini did with Titan). And see them it did! Check out the video below. It's short, so you can watch it a few times; there are several glints, and I labeled the region of Earth where they occur.
That video was put together by Don Lindler (my old boss back in the STIS/Hubble days) using images from Deep Impact. It shows a full rotation of the Earth as seen above the north pole, taken when Deep Impact was still 18 million kilometers (11 million miles) from home (that's 75 times the distance to the Moon!). Another video, from a different part of Deep Impact's travels, shows the view as seen from the south. Due to the geometry of the Sun, Earth, and spacecraft, the glints all appear on the same place on the Earth's face, though the location on the Earth's surface changes as it rotates.
These images were taken in the infrared, where the contrast between land and water is highest. There may come a day when our spacecraft observe other planets orbiting other stars, and glints like these may be the tell-tale signs of liquid on their surface. In those cases, the planet may not be more than an unresolved dot, but the sudden increase in brightness may be the giveaway we're looking for.
There's been speculation lately that some extrasolar planets may be water worlds. We can't know for sure just yet, but EPOXI may be showing us one way we might be able to find out.
Related post: HOLY FRAK! Moon transits Earth!