The blazing hot surface of Mercury, 400 degrees Celsius during the day, might seen an unusual place to find water ice. But nighttime temperatures drop well below freezing, and that planet also has no axial tilt so the Sun is always above that planet's equator and thus is very low on the sky when viewed from the planet's north or south poles. Which means that the bottoms of deep craters near the poles are permanently shadowed and never see any sunlight, and thus are very cold. Cold enough to cause any water vapor there to freeze out at the bottom of these craters. But Mercury is airless and waterless, so there should be no ice there, right? Except for when an icy comet happens to impact the planet, which would create a hot plume of water vapor, most of which will escape the planet. But a tiny fraction of that escaping vapor will find its way to the polar craters where it freezes out, so these polar craters are slowly accumulating cometary water ice over time.
This expectation was recently confirmed by the Messenger orbiter at Mercury, which detected changes in the flux of neutrons from the planet's surface that is consistent with patches of water ice at the bottoms of these polar crater. Red areas in the above image of Mercury's north pole are the permanently shadowed regions, and yellow indicates where Earth-based radar imaging of this planet showed likely sites of water-ice deposits. For more information, visit the Messenger website for additional details.
Follow this link to see a truly amazing video showing all sorts of atmospheric lighting that is seen by the astronauts onboard the International Space Station: aurora, lightning, city lights, and background stars rising through Earth's atmosphere. This must-see movie was assembled by Alex Rivest using video acquired by astronaut Don Petit; see Alex's blog for more details.
This image of a circumstellar dust ring was recently acquired by C. Thalmann and colleagues at the 8 meter Subaru telescope that the Japanese operate on the summit of mount Mauna Kea in Hawaii. Although the dust ring is nearly circular, the line-of-sight to the ring is oblique which makes the ring appear elongated. To reveal the faint ring, the astronomers must subtract a model image of the very bright central star, but imperfections in the star-subtraction results in the radial residuals seen in the image. Nonetheless the dust ring is still quite prominent despite those residuals, and its radius is about 80 AU. This dust is thought to be the result of collisions occurring among unseen planetesimals also orbiting within the ring. These planetesimals probably resemble the comets that inhabit our Kuiper Belt, which is the Sun's outermost debris belt that is of radius 45 AU. Another interesting feature of this image is that the dust ring's center is offset slightly from the star. Although the origin of this offset is uncertain, an unseen extra-solar planet is implicated, because its gravitational influence can displace the ring's center. But if there is an exoplanet here, it is too faint to be seen or is obscured by the residual starlight. For more information about this circumstellar dust ring, see this press release.