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IN BRIEF

A new study is showing that carbon from Mercury's original crust is the reason the planet looks so dark. Data from the MESSENGER mission further supports this.

You might be surprised to hear that the closest planet to the sun has an immensely dark surface. For a long time, Mercury’s lack of sunlight left scientists puzzled. The planet’s surface isn’t littered with iron-rich materials, known to be a darkening agent. So just what makesMercury so dim?

A new study from Johns Hopkins University claims the answer is carbon. The element has ancient origins from deep within the planet itself.

This oblique image of Basho shows the distinctive dark halo that encircles the crater. The halo is composed of so-called Low Reflectance Material, which was excavated from depth when the crater was formed. Basho is also renowned for its bright ray craters, which render the crater easily visible even from very far away.

Credit: Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

QUESTIONING COMETS

Patrick Peplowski, of the Johns Hopkins University Applied Physics Laboratory, and his team used data from the MESSENGER mission to study the accumulation of carbon on Mercury. Prior to these studies, scientists proposed that the carbon came from comets that traveled into the inner Solar System.

Poplowski’s team came to a different conclusion. Rather than being delivered by comets, the carbon most likely originated deep below the surface, in the form of a now-disrupted and buried ancient graphite-rich crust, some of which was later brought to the surface by impact processes after most of Mercury’s current crust had formed.

Co-author and Deputy Principal Investigator of the MESSENGER mission, Carnegie’s Larry Nittler, explained: “The previous proposal of comets delivering carbon to Mercury was based on modelling and simulation. Although we had prior suggestions that carbon may be the darkening agent, we had no direct evidence. We used MESSENGER’s Neutron Spectrometer to spatially resolve the distribution of carbon and found that it is correlated with the darkest material on Mercury, and this material most likely originated deep in the crust. Moreover, we used both neutrons and X-rays to confirm that the dark material is not enriched in iron, in contrast to the Moon where iron-rich minerals darken the surface.”

MESSENGER obtained its statistically robust data via many orbits during its last year of operation. Repeated Neutron Spectrometer measurements showed higher amounts of low-energy neutrons, a signature consistent with the presence of elevated carbon, coming from the surface when the spacecraft passed over concentrations of the darkest material.

Figuring out exactly how much carbon was present required combining the neutron measurements with other MESSENGER datasets, including X-ray measurements and reflectance spectra.

Together, the data indicate that Mercury’s surface rocks are made up of as much as a few weight percent graphitic carbon, much higher than on other planets. Graphite has the best fit to the reflectance spectra, at visible wavelengths, and the likely conditions that produced the material.

THE ORIGINAL ANCIENT CRUST

When Mercury was in its infancy, much of the planet most likely consisted of an “ocean” of molten magma. Scientists believe that, as this magma cooled, most minerals that solidified would sink. Graphite though, would have been buoyant and floated to form the original crust of Mercury.

“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 thesurface we see today. 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,” concluded Nittler.

IN BRIEF

Scientists discover repeating fast radio bursts coming from the far edge of the galaxy, overthrowing notions that FRBs are non-repeating in nature.

SIGNALS IN THE NIGHT

While sifting through various observation results made using the world’s largest radio telescope, McGill University PhD student Paul Scholz discovered something very rare—repeating fast radio bursts.

Admittedly, it’s not as dramatic nor as action-packed as when Jodie Foster’s character in the 90s sci-fi movie Contact discovered radio transmissions from outer space, but Scholz’s discovery of ten additional bursts from the direction of FRB 121102 is certainly an exciting discovery in its own right.

Until Scholz’s new findings, repeating fast radio bursts (FRB) were—no pun intended—unheard of. All incidents of FRBs are recorded to be non-repeating in nature. This has led scientists to speculate that FRBs are caused by one-off, cataclysmic events such as a star exploding in a supernova or a neutron star collapsing into a black hole.

But, the data published in the March issue of Nature, showing the additional bursts having “dispersion measures and sky positions consistent with the original burst” of FRB 121102 from 2012, invalidates all existing theories on the nature of FRBs.

“I knew immediately that the discovery would be extremely important in the study of FRBs,” says Scholz.

Arecibo Radio Observatory in Puerto Rico. Credit: ARECIBO OBSERVATORY/NAIC/NSF

A NEW TELESCOPE CHIMES IN

And he’s right. Because not only does his data suggest that FRBs have physical origins—in fact, multiple physical origins—it also shows that it’s coming from a young, highly magnetized, extragalactic neutron star far beyond the edge of the Milky Way.

“Not only did these bursts repeat, but their brightness and spectra also differ from those of other FRBs,” says Laura Spitler, first author of the new paper and a postdoctoral researcher at the Max Planck Institute for Radio Astronomy in Bonn, Germany.

Of course, not everyone is sold on the results of Scholz’s study. And to reach a definitive conclusion on repeating FRBs, a radio telescope more powerful than Arecibo will be needed.

Enter Canada’s CHIME. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) “is a novel radio telescope consisting of cylindrical reflectors roughly the size and shape of snowboarding half-pipes with an array of radio receivers along the focus.”

The Canadian Hydrogen Intensity Mapping Experiment (CHIME). Credit: Mateus A. Fandiño – The CHIME collaboration, CC BY-SA 4.0

Its novel design allows it to detect multiple FRBs per day, allowing it to “produce a map of cosmic structure over the largest volume of the universe ever observed.”

“Once we have precisely localized the repeater’s position on the sky, we will be able to compare observations from optical and X-ray telescopes and see if there is a galaxy there,” says Jason Hessels, corresponding author of the paper with Scholz and associate professor at the University of Amsterdam and the Netherlands Institute for Radio Astronomy.

“Finding the host galaxy of this source is critical to understanding its properties.”

To be clear, no one’s suggesting that some vast extragalactic civilization, high up on theKardashev Scale, is using neutron stars or some other cosmic phenomenon to signal us from halfway across the universe. Still, it’s important to understand the origins of all radio emissions, for no loftier reason than to simply learn a new thing, but also to categorize and eliminate all the extraneous “noise.”

Then someday, perhaps, we really will detect that first deliberate radio transmission, and—like Jodie Foster’s character in Contact—realize that a universe devoid of alien intelligences really is just an awful waste of space.