Researchers once thought Mercury to be much like Earth's moon, but MESSENGER has found many differences. For instance, unlike the moon, Mercury has huge cliffs with structures snaking hundreds of miles across the planet's face. The spacecraft also revealed impact craters that appear very different from lunar craters. One particularly curious crater has been dubbed "The Spider."
This formation never has been seen on Mercury before and nothing like it has been observed on the moon. It lies in the middle of a huge impact crater called the Caloris basin and consists of more than 100 narrow, flat-floored troughs radiating from a complex central region.
"The Spider has a crater near its center, but whether that crater is related to the original formation or came later is not clear at this time," said James Head, science team co-investigator at Brown University, Providence, R.I.