A fly-by by a Nasa unmanned space probe has revealed evidence of "widespread" volcanism on the planet Mercury.
The US Mercury Messenger spacecraft made a close pass of the first planet from the Sun on 14 January.
Evidence from the Mariner 10 probe launched in the 1970s had provided only tenuous evidence for volcanic activity.
In addition, only 45% of Mercury's surface had previously been mapped; Messenger has already covered a further 30% of the planet.
"This fly-by allowed us to see a part of the planet never before viewed by spacecraft, and our little craft has returned a gold mine of exciting data," said Dr Sean Solomon, Messenger's chief scientist.
On the evidence for volcanism, mission scientist Louise Prockter, from the Applied Physics Laboratory at Johns Hopkins University in Maryland, said: "I think there's little doubt on the part of our team that there has been widespread volcanism on Mercury."
The scientist cited a cratered basin which has been flooded with smooth material, probably representing volcanic flows.
The spacecraft also discovered a unique feature that scientists dubbed The Spider. This formation has never been seen on Mercury before, and nothing like it has been observed on the Moon.
The spider consists of more than 100 troughs
It lies in the middle of a large impact crater called the Caloris basin and consists of more than 100 narrow, flat-floored troughs radiating from a 40km-wide central crater.
"The Spider has a crater near its centre, 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 in Providence.
Robert Strom, professor emeritus at the University of Arizona in Tucson, said he thought The Spider might eventually be tied to the history of volcanism on the planet.