Mercury’s dark surface was produced by a steady dusting of carbon from passing comets, a new study says.

Mercury reflects very little light but its surface is low in iron, which rules out the presence of iron nanoparticles, the most likely “darkening agent”.

First, researchers modelled how much carbon-rich material could have been dropped on Mercury by passing comets.

Then they fired projectiles at a sugar-coated basalt rock to confirm the darkening effect of carbon.

Their results, published in the journal Nature Geoscience, support the idea that Mercury was “painted black” by cometary dust over billions of years.

The effect of being intermittently blasted with tiny, carbon-rich “micrometeorites”, the team says, is more than enough to account for the mysterious, dull surface seen on Mercury.

via Mercury ‘painted black’ by passing comets – BBC News.

In this artist’s conceptual illustration, the moon Ganymede orbits giant planet Jupiter. NASA’s Hubble Space Telescope detected auroras on the moon controlled by Ganymede’s magnetic fields. Image released March 12, 2015.

Credit: NASA/ESA

A salty ocean is lurking beneath the surface of Jupiter’s largest moon, Ganymede, scientists using the Hubble Space Telescope have found.

The ocean on Ganymede — which is buried under a thick crust of ice — could actually harbor more water than all of Earth’s surface water combined, according to NASA officials. Scientists think the ocean is about 60 miles (100 kilometers) thick, 10 times the depth of Earth’s oceans,

NASA added. The new Hubble Space Telescope finding could also help scientists learnmore about the plethora of potentially watery worlds that exist in the solar system and beyond.

“The solar system is now looking like a pretty soggy place,” Jim Green, NASA’s director of planetary science, said during a news teleconference today (March 12). Scientists are particularly interested in learning more about watery worlds because life as we know it depends on water to thrive.

via Jupiter’s Moon Ganymede Has a Salty Ocean with More Water than Earth.

Have you ever seen an asteroid? An asteroid is a small rock in space, smaller than a planet yet larger than a meteoroid, the rocks which cause meteors to flash across the night sky.

While we know of millions of asteroids today, they were totally unknown to astronomers until 17th century astronomer Giuseppe Piazzi spotted Ceres, the largest and first known asteroid, in the early hours of New Year’s Day in 1801. Over the next six years, three more asteroids were discovered, but it would be another 38 years before another would be found. It wasn’t until photography began to be used for astronomy that the huge number of these tiny bodies became clear.

This week, third asteroid ever to be discovered will be well placed for observation. This is Juno, discovered in 1804 by German astronomer Karl L. Harding. Juno is quite a small body, measuring only 170 miles (274 kilometers) across. To give you some idea of scale, Juno is large enough to fit in between the cities of New York and Boston.

via How to See Asteroid Juno in the Night Sky with Binoculars.