The smallest planet in the solar system, Mercury bears a strong resemblance to Earth's moon. Like the other three terrestrial planet, Mercury contains a core surrounded by a mantle and a crust. But Mercury's core makes up a larger portion of the planet than others in the solar system, hinting at a chaotic beginning.
The surface of Mercury
The first images of Mercury revealed a cratered, rocky planet that closely resembled Earth's moon. The early days of the solar system, soon after the rocky planet formed, were violent, with constant collisions, and conditions on Mercury preserved evidence of many of these impacts.
When NASA's MESSENGER orbiter visited the planet in 2008, it became the first spacecraft to glimpse the full spread of the Caloris basin, one of the biggest and youngest impact features in the solar system. The crater stretches about 960 miles (1,550 kilometers) across the planet's surface and is surrounded by a ring of mountains 1.2 miles (2 km) tall. Volcanic vents surrounding the basin's rim suggest that volcanism helped to shape the tiny world.
Other evidence for volcanism includes several plains that smoothed over some of the first craters. Most of the plains are covered with craters, suggesting that volcanism took place long ago. However, MESSENGER found that the floors of many craters have been tilted, and part of the floor of the Caloris basin has been raised above its rim. The discovery suggests that Mercury remained active well after its birth.
"It is not out of the question that Mercury is still active today, though I note that this is not very likely," Maria Zuber, a planetary scientist at the Massachusetts Institute of Technology told Space.com in 2012. "For sure we have not observed an active eruption or extrusion."
One of Mercury's youngest impact basin, Rachmaninoff, is only about a billion years old. The (290-km) diameter peak-ring impact basin has smooth plains on its floor suggestive of lava flows. The lowest point on the planet lies within the basin.
"We interpret these plains to be the youngest volcanic deposits we have yet found on Mercury," MESSENGER deputy project scientist Louise Prockter, of the Jet Propulsion Laboratory in California, said in 2010.
Although temperatures on the planet can reach as high as 801 degrees Fahrenheit (427 degrees Celsius), MESSENGER detected water-ice on its surface in the shaded portions of some of the polar craters, where the sun doesn't reach. According to NASA, a mysterious dark organic matter covers some of the ice, leaving scientists puzzled.
In addition to testifying as to the planet's early volcanism, the smooth plains also show evidence of wrinkle ridges, created as the planet squeezed together. This coming together most likely happened as the interior cooled. Although some compression is common among bodies in the solar system, the compression of Mercury as it pulled more tightly in on itself is the most significant yet seen. Scientists estimate that the radius of the planet shrank by 0.6 to 1.2 miles (1 to 2 kilometers) as it temperatures deep within dropped.
A small body like Mercury would have a difficult time holding on to an atmosphere in the best of circumstances. Because of the close distancebetween Mercury and the sun, Mercury also feels the brunt of the solar wind, which constantly sweeps away the thin atmosphere the planet does manage to gather. With only the most negligible of atmospheres, the temperatures on the night and day side differ dramatically.
The thin atmosphere allows most cosmic rays to bombard the planet, stripping neutrons from elements lying on the surface. MESSENGER studied material kicked up and found traces of potassium and silicon, suggesting that the elements lie on the planet's surface.
The crust of Mercury is likely very thin, thinner than Earth's. The outer shell is only about 300 to 400 miles (500 to 600 km) thick.
The planet has no plate tectonics, which is part of the reason the cratered surface has been preserved for billions of years.
The core of the matter
Although it's the smallest planet, Mercury is the second densest, topped only by Earth. Scientists used the calculated density to determine that Mercury holds a large metallic core. With a radius of 1,100 to 1,200 mile (1,800 to 1,900 km), the core makes up about 85 percent of the planet's radius. Radar images taken from Earth revealed that the core is molten liquid, rather than solid.
Mercury's core has more iron than any other planet in the solar system. Scientists think this had to do with its formation and early life. If the planet formed quickly, increasing temperatures of the evolving sun could have vaporized much of the existing surface, leaving only a thin shell.
Another alternative is that a larger Mercury was struck in its early life, during the violent, chaotic beginnings of the solar system. Such an impact could have stripped away much of its outer shell, leaving a core too big for remaining planet.
Mercury's iron core generates a magnetic field about one percent as strong as Earth's. The field is quite active, frequently interacting with the solar wind and funneling plasma from the sun to the planet's surface. The hydrogen and helium captured from the solar wind help create part of Mercury's thin atmosphere.
By precisely tracking MESSENGER, scientists were able to measure the planet's gravitational field. They determined that the rocky world has "mascons," massive gravitational concentrations associated with large impact basins.
"These were first discovered on the moon in 1968 and caused great problems in the Apollo program because they tugged low-orbiting spacecraft around and made navigation difficult," Zuber said.
"Subsequently, mascons were discovered on Mars, and now we find out that Mercury has them, so they appear to be a common feature of terrestrial planetary bodies."
But the planet has its own differences. Recent measurements of its magnetic field found it to be three times stronger at its northern hemisphere than at its southern. Researchers used this strange offset to create a model of the core.
Earth's iron core has an inner solid region and an outer liquid part. As the inner core grows, it provides the energy behind Earth's magnetic field. But the planet's strange magnetic field suggests that the iron turns from liquid to solid at the outskirts of the core.
"It's like a snow storm in which the snow formed at the top of the cloud and middle of the cloud and the bottom of the cloud, too," UCLA professor Christopher Russell said in a statement.
"Our study of Mercury's magnetic field indicates iron is snowing throughout this fluid that is powering Earth's magnetic field."
Both cores contain lighter elements along with the iron, keeping the whole thing from solidifying and powering the magnetic field. The whole thing is likely covered by a solid shell of iron and sulfur, creating a layering effect not known to exist on the other terrestrial planets.