Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: (CNN) -- X-ray data from NASA's MESSENGER probe points to high levels of magnesium and sulfur on the surface of the planet Mercury, suggesting its makeup is far different from that of other planets, scientists say. The unmanned orbiter has been beaming back data from the first planet for a year and a half. Readings from its X-ray spectrometer point to a planet whose northern volcanic plains formed through upwellings of rocks more exotic than those often found on the Earth, the Moon or Mars, said Shoshana Weider, a researcher at the Carnegie Institution of Washington. "Before this MESSENGER mission, a lot of people assumed it was very like the Moon -- it's dark, it's grey," Weider said. But while the Moon's surface formed when light materials floated to the top of an ocean of molten rock, the low level of calcium on Mercury indicates that didn't happen there.NASA probe gives insight into Mercury's formationThe surface has sulfur levels 10 times higher than Earth'sThe MESSENGER orbiter has been circling Mercury since March 2011 
 Question:It's about 5% the size of _ and revolves around the sun every 88 days.

Earth