Gallery: Mercury's Explosive Volcanic History Revealed
01young-crater
The solar system’s smallest and innermost planet has come into greater focus than ever before. Though only halfway through its first year in orbit around Mercury, NASA’s [MESSENGER spacecraft](http://messenger.jhuapl.edu/) has already revealed that the planet possesses a much more dynamic and fiery past than previously imagined. “The data obtained by the MESSENGER imaging system in a mere few months has revolutionized our thinking about volcanism on Mercury,” said geologist [James Head](http://www.planetary.brown.edu/html_pages/head.htm) of Brown University, a member of the spacecraft’s science team, during a NASA press briefing today. The team also published a set of seven papers in the journal *Science* with their new findings. Mercury is the densest of all the planets in the solar system, and is composed of around 60 percent iron — twice as much as Earth. How any why it ended up with so much iron is one of the planet’s main mysteries. Learning more about its formation and early history may give researchers a better understanding of both our own solar system and those around other stars. In the past, scientists were unsure how much of Mercury’s surface was shaped by volcanism. Flybys with the [Mariner 10 spacecraft](http://pds.jpl.nasa.gov/planets/welcome/m10.htm) in the 1970s could not definitively conclude whether formations on the surface were due to volcanic activity or impacts from craters. But now, with MESSENGER in orbit, the details of this tiny planet are finally in view. __Above:__ Young Crater ------------ The crater Debussy's long radial rays formed when an asteroid impact sent ejecta flying from the crater in all directions, forming secondary craters and uncovering younger terrain. Because of its brightness, Debussy is thought to be a relatively young crater, because the solar wind causes the planet’s surface to darken over time. MESSENGER is revealing that both impacts such as this and volcanic activity have been important in Mercury’s history. *Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington*
02volcanic-past
Volcanic Past ------------- The MESSENGER team has now found ample evidence for an explosive volcanic past on Mercury. Huge lava flows likely carved the long channels and teardrop-shaped hills shown in these images. The lava may have come from volcanic vents seen in the lower-left image. *Image: Science/AAAS*
03polar-ice
Polar Ice? ---------- That a planet so close to the blazing sun would have ice on it seems paradoxical, yet there is good evidence for this scenario. Ground-based radar has revealed very reflective material — much like water ice — sitting in deep craters at Mercury’s north pole. Recently, the MESSENGER spacecraft measured the length of shadows in these craters to estimate their depth and found they are deep enough to remain in permanent shadow, which could allow them to trap ice within. Radar measurements from the probe also corroborate the finding that there is highly reflective material in these impacts. “We can’t yet confirm this is water ice, but the polar cold trap theory is holding up,” said geophysicist Sean Solomon. *Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington*
04strange-hollows
Strange Hollows --------------- Oddly shaped dips surrounded by very bright material, shown here in Mercury's Raditladi basin, are a brand-new puzzle. Scientists have so far identified about three dozen of these markings inside impact craters at all latitudes and terrains on the planet. But how they formed is still a mystery. *Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington*
05bright-dips
Bright Dips ----------- The bright patterns shown here have many possible explanations, which is why scientists haven't pinned down their cause yet, said [geophysicist Sean Solomon](http://www.dtm.ciw.edu/scs/), of the Carnegie Institution of Washington, principle investigator for MESSENGER. They could be due to secondary impacts within the larger crater, volcanism, explosive outgassing, or some sort of sublimation process. The team has noticed that areas with the most sunlight are most affected, suggesting the bright material is being scattered as a result of solar heating. *Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington*
06northern-plains-2
Northern Plains --------------- Around 6 percent of Mercury’s northern hemisphere is covered in smooth contiguous plains. The relatively limited number of impact craters in the smooth area suggests it is much younger than surrounding regions and likely formed through more recent volcanic activity. *Image: Science/AAAS and Brown University*
07ghostly-crater
Ghostly Crater -------------- All over Mercury, MESSENGER has spotted “ghost” craters, such as the one at left in this image. These are older impacts that have been buried under subsequent lava flows later in the planet’s history. The volcanic flows must have been heavy, because the ghost craters can be as deep as 1.2 miles and are almost completely submerged. Researchers still don’t know where such torrents of lava could have come from. *Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington*
08magnetic-asymmetry
Magnetic Asymmetry ------------------ One of Mercury’s oddities is that it's the only inner planet other than Earth to have a global magnetic field, though this field is about a thousand times weaker than our own. Because the planet must possess a molten core to produce such a field, this finding has further bolstered the idea that Mercury once had a more active interior capable of volcanic and tectonic activity. But one of the strangest aspects of Mercury’s field is that it is highly asymmetrical. The northern hemisphere's magnetic field is 3.4 times stronger than the south's. This has implications for the internal dynamo that is producing this magnetic field, which could be very strangely shaped. *Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington*
09compositional-mystery
Compositional Mystery --------------------- Scientists have put forward two main theories for Mercury’s formation. One idea posits that the small planet was larger in the past, more like a Mars-sized object, and then early solar radiation boiled away most of its crust. The second theory suggests a large object may have impacted the protoplanet Mercury early in the solar system’s history, stripping it of most of its surface material. The MESSENGER team is finding more clues to the planet’s birth, including a few new twists. Despite being an overwhelmingly iron planet, the probe has seen little ferrous material on Mercury’s surface, which suggests the iron is mostly locked up in the interior or bonded to other elements. The spacecraft has also spotted significant amounts of elements such as sulfur and potassium, which are volatiles expected to be strewn off by heavy solar radiation or bombardment by a large body. The latest findings don’t exactly support either of the current models, suggesting the true explanation for Mercury’s formation may be more complex, said geophysicist Sean Solomon. *Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington*
