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Mercury's Carbon-Rich Crust is Surprisingly Ancient

Before its planned crash into Mercury last year, NASA’s MESSENGER spacecraft gave scientists a parting gift.

Before its planned crash into Mercury last year, NASA's MESSENGER spacecraft gave scientists a parting gift.

In its final orbits, MESSENGER not only confirmed that Mercury's dark hue is due to carbon, but also revealed that the carbon wasn't deposited by impacting comets, as some researchers suspected.

Fuzzy to Clear: Space Robots Snap Solar System Into Focus

Instead, scientists now believe they are seeing remnants of the planet's primordial crust, which likely formed when a global ocean of super-heated magma cooled, allowing minerals to solidify.

Computer simulations and experiments show that most of these crystallized minerals would sink -- with one key exception. Graphite, the studies show, would float.

Scientists used an instrument on MESSENGER called a neutron spectrometer to make low-altitude measurements of the darkest regions on the planet's surface, which were suspected of having the most low-reflectance material (LRM.)

"The measurements showed enhanced fluxes of thermal neutrons over three areas of LRM, so only graphite as the darkening agent fits both the spectral reflectance observations and the neutron measurements," MESSENGER's lead scientist Sean Solomon, with Columbia University, wrote in an email to Discovery News.

NEWS: Mercury's Speedy Spin Hints at Planet's Insides

Scientists also were able to match the carbon-rich material with large impact craters, evidence that the material stemmed from deep within Mercury's crust and was exposed after an impacting body gouged out a crater.

"Because LRM deposits on Mercury are all associated with material excavated from depth by large impact craters, they must come from the mid to lower crust," Solomon said.

Scientists estimate the ancient crust was about .62 mile, or 1 kilometer, thick.

The crust of present-day Mercury has been bashed by impacts, covered with lava, melted and otherwise disturbed.

"The processes ... would dilute any primordial crust," physicist Patrick Peplowski, with Johns Hopkins University Applied Physics Lab, and colleagues write in a paper published this week in Nature Geoscience.

NEWS: Strange 'Hollows' on Mercury Revealed by NASA Probe

The finding also reinforces theories that Mercury's birthplace had higher concentrations of carbon than where the rest of the inner planets formed.

"This inference adds to our deepening appreciation that Mercury formed from a portion of the early solar nebula that was chemically much more reduced and was rich in other volatiles (such as sulfur, sodium, potassium and chlorine) compared with the portions of the nebula well sampled by Venus, Earth and Mars," Solomon said.

Enhanced color image highlights Mercury’s low-reflectance material, which appears blue in this image, and its association with impact-excavated material. The young rayed crater Degas appears near the center, and to the far left is the crater Akutagawa, whose extensive low-reflectance (dark blue) material was studied by MESSENGER and found to be carbon rich.

June 18, 2015 --

A few decades ago, getting a spacecraft to another world was a feat in itself. We take it for granted today, and demand more -- images and videos and science data that will tell us more about how that world came to be. And luckily for us, usually spacecraft succeed in delivering that. This week,

new images came from Pluto's vicinity

, showing us more surface detail on the dwarf planet. Click through to read more about Pluto and some other objects that got enhanced with spacecraft views, such as Mars, volcanic Io, and dwarf planet Ceres (complete with baffling bright spots).

Image: Pluto is getting big in the camera view of the New Horizons spacecraft, which will fly by the dwarf planet in July.

The first close-up view of Pluto is coming oh so soon; on July 14, the New Horizons spacecraft will whiz by the dwarf planet and its moons. What exactly is on its surface is a mystery. But looking at Pluto will give us a sense of what other icy objects far away in the solar system look like as well. Long-range observations of Pluto over the years (done by the Hubble Space Telescope) show a mottled surface that changes as the dwarf planet rotates.

MORE: Hubble Joins Hunt for New Target for Pluto-Bound Probe

Image: Even from afar, the Hubble Space Telescope was able to spot a mottled surface on Pluto in images released in 2010.

But it's New Horizons that is presenting more detailed questions. Is that a bright polar cap at the pole? Why are there dark and bright patches at the equator? We'll know more in just over a month.

MORE: NASA Probe Reveals More Detail in Pluto's Complex Surface

Image: Pluto's surface features are coming into sharper view for the New Horizons mission, which will fly by the dwarf planet in a few weeks.

What the heck are those white things? On Ceres, a dwarf planet in our asteroid belt, the Hubble Space Telescope spotted the bright regions in images released in 2005. "The bright spot that appears in each image is a mystery. It is brighter than its surroundings. Yet it is still very dark, reflecting only a small portion of the sunlight that shines on it,"

officials wrote at the time


MORE: Water Plume 'Unequivocally' Detected at Dwarf Planet Ceres

Image: A bright region on dwarf planet Ceres jumped out in these images released from the Hubble Space Telescope in 2005.

With the Dawn spacecraft now a few weeks into its mission at Ceres, there have been many

high-resolution images

of these spots -- yet astronomers remain puzzled. Are they ice? Are they salt? It will take more investigation to figure out what they are made of and how they arrived. As far as we know, NASA says, the phenomenon is unique in the solar system.

MORE: We STILL Don't Know What Those Bright Blobs on Ceres Are

Image: The Dawn spacecraft arrived at Ceres in 2015 and took closer-up images of several bright spots on the surface, but their nature is still unknown.

The first flybys of Mars happened to go by heavily cratered areas, leaving the impression that the planet looked a lot like the moon. The visions of life dancing in the public's heads faded, and the planet appeared a dead world until Mariner 9 did a global mission in 1971. From orbit, the NASA spacecraft spotted ancient volcanoes such as Olympus Mons and also discovered Valles Marineris, a vast canyon network stretching across most of the planet.

MORE: Epic Supervolcanoes May Have Decimated Mars

Image: Olympus Mons, a dormant volcano on Mars, emerges from a dust storm viewed by NASA's Mariner 9 in 1971.

Today we are lucky to have high-resolution images of Mars beaming back every day, so we can look in more detail at changes to the planet over time. The 2004 Mars Express image of the caldera (volcanic craters) on Olympus Mons is the first high-resolution image of them, according to the European Space Agency. The public also get involved through another Mars orbiter, NASA's Mars Reconnaissance Orbiter -- you can request photos of certain regions

via the HiRISE camera website


MORE: The Psychedelic Landscape of Mars

Image: The caldera (volcanic craters) on top of Olympus Mons, viewed by the European Space Agency's Mars Express in 2004.

From its perch in Earth orbit, the Hubble Space Telescope was instrumental in telling the

Rosetta spacecraft

how to get to Comet 67P/Churyumov-Gerasimenko. In 2003, the telescope pinned down the size and rotation period of the comet, and far-away measurements made it appear as though the comet was football-shaped.

MORE: Comet-Hunting Probe to Awaken, Zero In On Target

Image: Vague 3D images of Comet 67P/Churyumov–Gerasimenko from Hubble Space Telescope data in 2003.

Scientists and the public alike were enchanted in 2014 when Rosetta finally got close to the comet. An intriguing "rubber-duckie" shape emerged as pictures made it clear the comet is loosely held together at a joint. In November, the Philae spacecraft landed on the surface and got data for a few dozen hours there before hibernating; it just started re-communicating a few days ago.

MORE: Philae Rises! What's Next for Rosetta's Comet Lander?

Image: The odd rubber-duckie shape of Comet 67P/Churyumov–Gerasimenko became clear in pictures from the Rosetta spacecraft, which began orbiting the comet in 2014.

When the Voyager spacecraft flew by Titan in the early 1980s, there was only so much the pair could see. That's because they could only take images in visible light; an orange haze obscured the surface, leaving Titan's features mostly a mystery for decades. Much like peering through Venus' shroud, it would require images in light human eyes can't see to ferret out more of Titan's mysteries.

MORE: Titan's Hazy Sunsets Shed Light on Alien Atmospheres

Image: A visible-light image of Titan's haze taken by one of the Voyager spacecraft as it went through Saturn's system.

The Cassini spacecraft arrived at Saturn's system in 2004 and is well-equipped to see through the haze. For example, this 2014 image by Cassini's Visible and Infrared Mapping Spectrometer (VIMS) shows

seas of ethane and methane

. The spacecraft has found bodies of liquid near the poles and sand dunes closer to the equator. Also, the Huygens spacecraft landed on the surface and briefly sent back images in 2005.

MORE: Cassini Spies a Sunny Day on Titan's Seas

Image: In near-infrared light, seas pop out on Titan in this image from the Cassini spacecraft.

Mercury, the planet closest to the sun, escaped close scrutiny for much of the space age. A single probe flew a few times past the planet in 1974-5 and provided tantalizing glimpses of its surface. But a complete global map, and an idea of its insides and atmosphere, would have to wait several more decades.

PHOTOS: Mariner 10: Our First Mission to Mercury

Image: In 1974-5, Mariner 10 did just three flybys of Mercury and showed a heavily cratered surface.


just concluded the MESSENGER

(Mercury Surface, Space Environment, Geochemistry and Ranging) mission at Mercury in April. Some of MESSENGER's major finds include strong evidence of water ice at the poles, a tenuous atmosphere that changes with the seasons, and evidence of old volcanic deposits.

MORE: MESSENGER's Last Mercury Photo Before Crashing is Crazy Close-Up

Image: MESSENGER provided the first complete maps of Mercury, and provided evidence of ice in permanently shadowed craters near the pole. This image and data was released in 2012.

In contrast to our quiet moon, Io is an extremely volcanic moon at Jupiter. When Voyager 1 flew by the moon in 1979, its mottled, pizza-like appearance caused a stir -- not to mention the fact that Voyager spotted an eruption during its brief time in Jupiter's system.

MORE: Volcanoes on Jupiter's Moon Io Are All Wrong

Image: Voyager 1 captured this picture of a volcanic explosion on Io in 1979.

NASA's Galileo spacecraft followed up in the 1990s by taking global images of Io's surface and peering closely at the aftermath of explosions. Understanding how volcanoes work on Io can help us understand their processes more generally in the solar system.

MORE: Icy Europa Does Battle With Solar System's Most Hellish Moon

Image: Galileo was able to capture even more detail of Io's eruptions, such as this series of lava deposits shown in images released in 1999.