For years, scientists have wondered why Mercury is so dark. It doesn’t have as much iron and titanium as the Moon, so it should be brighter. A team at Johns Hopkins’ Applied Physics Laboratory finally has the answer, though. Thanks to a spectral analysis of color images from NASA’s Messenger probe, they’ve determined that carbon (specifically, graphite) is the probable culprit behind Mercury’s dim look. Most likely, what you’re seeing is the aftermath of the tiny planet growing up. As the young Mercury’s magma ocean cooled 4.6 billion years ago, the graphite would have floated to the top and formed the original crust — there just happens to be enough of it left to affect visibility.
The data might not just solve one riddle. Besides explaining some of Mercury’s early history, it could also give a sense of the materials that were swirling close to the Sun as the solar system formed. The exact blend of minerals still isn’t known, but this one insight could easily pay dividends. And given that Messenger collected much more data, it won’t be shocking if there are more answers in the near future.
It quickly became clear this was not your typical gas cloud, since it contains gas with a very wide range of speeds. Researchers also found the cloud has an elliptical shape and consists of two components: a compact but low density component with a very wide velocity dispersion of 100 km/s, and a dense component extending 10 light-years with a narrow velocity dispersion.
They ruled out a supernova explosion, after X-ray and infrared observations did not find any compact objects in the cloud.
In this stunning example of celestial good timing on Sept. 24, Cassini snapped Enceladus drift in front of larger moon Tethys in near-perfect alignment. Interestingly, the size difference between the two moons closely reflect their relative differences in diameter. Enceladus is 313 miles (504 kilometers) across whereas Tethys is 660 miles (1,062 kilometers) across.
At the time of the moon transit, Cassini was 1.3 million miles (2.1 million kilometers) from Enceladus and 1.6 million miles (2.6 million kilometers) from Tethys, meaning the 2 moons were only approximately 300,000 miles (500,000 kilometers) apart.