Dark Stains on Mercury Reveal Its Ancient Crust

Expanded-color image of Mercury's 52-km Degas crater, showing an abundance of low-reflectance material (LRM).

Ever since the MESSENGER spacecraft entered orbit around Mercury in 2011, and indeed even since Mariner 10‘s flyby in 1974, peculiar “dark spots” observed on the planet’s surface have intrigued scientists as to their composition and origin. Now, thanks to high-resolution spectral data acquired by MESSENGER during the last few months of its mission, researchers have confirmed that Mercury’s dark spots contain a form of carbon called graphite, excavated from the planet’s original, ancient crust.

Commonly found within and around impact craters and volcanic vents, the dark spots on Mercury—also referred to as “low-reflectance material,” or LRM—were originally suspected to contain carbon delivered to the planet by comets.

Data from MESSENGER’s Gamma-Ray and Neutron Spectrometer (GRNS) and X-ray instruments confirmed the LRM to contain high amounts of graphitic carbon, but likely originating from within Mercury itself. It’s thought that Mercury was once covered by a crust composed of graphite, when much of the planet was still molten.

“Experiments and modeling show that as this magma ocean cooled and minerals began to crystallize, minerals that solidified would all sink with the exception of graphite, which would have been buoyant and would have accumulated as the original crust of Mercury,” said Rachel Klima, co-author of a recent study on LRM and a planetary geologist at the Johns Hopkins University Applied Physics Laboratory. “We think that LRM may contain remnants of this primordial crust. If so, we may be observing the remains of Mercury’s original, 4.6-billion-year-old surface.”

See more MESSENGER images of LRM on Mercury here.

Although similar in visible coloration and covered in craters, cracks, and mountains, any similarities between Mercury and other smaller worlds in our Solar System—including our Moon—end there. Mercury has a formation history all it own and is compositionally unique among the planets.

These data revealing such a relatively high concentration of graphite in Mercury’s crust only adds to those differences, and also tell us about the various elements that were present around the Sun when the planets were forming.

“The finding of abundant carbon on the surface suggests that we may be seeing remnants of Mercury’s original ancient crust mixed into the volcanic rocks and impact ejecta that form the surface we see today,” said Larry Nittler, research paper co-author and Deputy Principal Investigator of the MESSENGER mission. “This result is a testament to the phenomenal success of the MESSENGER mission and adds to a long list of ways the innermost planet differs from its planetary neighbors and provides additional clues to the origin and early evolution of the inner Solar System.”

On Earth graphite is used in industry to make bricks that line refractory furnaces and increase the carbon content of steel. It’s also widely used in fire retardants, batteries, and lubricants, and is mixed with clay in various amounts to create the “lead” in pencils (which, by the way, contain no actual lead.)

These findings have been published in the March 7, 2016 Advanced Online Publication of Nature Geoscience.

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) was a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and entered orbit about Mercury on March 17, 2011 (March 18, 2011 UTC). On April 30, 2015, after four years in orbit MESSENGER‘s mission and operational life came to an end when it impacted the surface of Mercury in its northern polar region.

Source: Carnegie Science and JHUAPL

 

Image credits:  NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

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Massive Planet Gone Rogue Discovered

In this artist's conception, a rogue planet drifts through space. Credit: Christine Pulliam (CfA)

A massive rogue planet has been discovered in the Beta Pictoris moving group. The planet, called PSO J318.5338-22.8603 (Sorry, I didn’t name it), is over eight times as massive as Jupiter. Because it’s one of the few directly-imaged exoplanets we know of, and is accessible for study by spectroscopy, this massive planet will be extremely important when piecing together the details of planetary formation and evolution.

Most planets outside our solar system are not directly observable. They are discovered when they transit in front of their host star. That’s how the Kepler mission finds exoplanets. After that, their properties are inferred by their gravitational interactions with their star and with any other planets in their system. We can infer a lot, and get quite detailed, but studying planets with spectroscopy is a whole other ball game.

The team of researchers, led by K. Allers of Bucknell University, used the Gemini North telescope, and its Near-Infrared Spectrograph, to find PSO’s  radial and rotational velocities. As reported in a draft study on January 20th, PSO J318.5338-22.8603 (PSO from now on…) was confirmed as a member of the Beta Pictoris moving group, a group of young stars with a known age.

The Beta Pictoris moving group is a group of stars moving through space together. Since they are together, they are understood to be formed at the same time, and to have the same age. Confirming that PSO is a member of this group also confirmed PSO’s age.

Once the age of PSO was known, its identity as a planet was confirmed. Without knowing the age, it’s impossible to rule it out as a brown dwarf, a “failed star” that lacked the mass to ignite fusion.

This new rogue planet is 8.3 + or – 0.5 times the mass of Jupiter, and its temperature is about 1130 K. Spectra from the Gemini scope show that PSO rotates at between 5 to 10.2 hours, and that its radial velocity is within the envelope of values for this group. According to the researchers, determining these properties accurately means that PSO J318.5338-22.8603 is “an important benchmark for studies of young, directly imaged planets.”

PSO is in an intermediate position in terms of other planets in the Beta Pictoris moving group. 51 Eridani-b is another directly imaged planet, only slightly larger than Jupiter, discovered in 2014. The third planet in the group is Beta Pictoris b, which is thought to be almost 11 times as massive as Jupiter.

Rogue, or “free-floating” planets like PSO J318.5338-22.8603 are important because they are not near a star. Light from a star dominates the star’s  surroundings, and makes it difficult to discern much detail in the planets that orbit the star. Now that PSO is confirmed as a planet, rather than a brown dwarf, studying it will add to our knowledge of planetary formation.

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