Article: First Planet Finishes Last

On January 14, 2008, in a nondescript office of the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, scientists are huddled around computer screens, biting their nails. The team is waiting for an image to downlink from the MESSENGER spacecraft--the first image of the planet Mercury in 33 years. They are expecting to see a hemisphere of the enigmatic planet that humans have never before laid eyes on.


MESSENGER scientists react to the first image downlinked from the spacecraft on January 14, 2008.


Given the tension, when the image pops on screen, it’s startling. It shows a steely, crater-pocked half-orb not unlike Earth’s moon. “Oh my God,” a researcher exclaims. “Isn’t that spectacular?” asks another. “Wow!”

The image even provokes tears. “It’s very emotional for me,” says an overwhelmed Robert Strom, co-investigator on the Mariner 10 mission in 1974“1975, which mapped just 45 percent of Mercury’s surface. “I’ve waited a long time for this.” 

Seeing the Unseen

MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. It launched four years ago, and in 2011 it will be the first spacecraft to orbit the Solar System’s innermost planet. On January 14, MESSENGER made a critical first pass 200 km above Mercury, closer than Mariner 10 ever got. The flyby was an opportunity for the spacecraft to use the planet’s gravity to boost its orbit trajectory. It was also a chance for MESSENGER’s cameras to take images and for several other instruments to gather early data on the planet’s magnetic field, topography, and chemical composition.

MESSENGER’S first images of Mercury reveal intriguing geology, such as zigzagging ridges hundreds of kilometers long, dark-edged craters, and a strange formation dubbed “the Spider.” 

The picture quality far surpasses anything Mariner 10 returned. In 1975, Strom reminds us, there were no desktop computers. At that time, planetary photographs literally had to be cut and pasted into mosaics. With MESSENGER, “every picture that you look at is basically a new discovery,” says Strom with a palpable awe. “This is like looking at a whole new planet.”

Yet the images are only the beginning. Strom and the other mission scientists hope to use MESSENGER’s data to answer long-standing questions about Mercury and the early evolution of the Solar System.


MESSENGER’s first image of the unseen side of Mercury. Because the planet has no tectonic plates, everything that has happened to it in 4.6 billion years—cratering, surface deformation, volcanism—is preserved in its crust.

NASA/JHUAPL/Carnegie Institution

Cinderella Planet

“Mercury has been the Cinderella child of the Solar System for a very long time, caught in the shadow of the stepsisters Venus and Mars,” said MESSENGER lead investigator Sean Solomon in a press conference before the flyby. “That’s about to change.” Indeed, Mercury is the most unusual of the four terrestrial planets (those closest to the Sun). It’s also the most neglected. Until MESSENGER, it was the only terrestrial planet that hadn’t been comprehensively imaged.

Given Mercury’s proximity to the Sun, it has been difficult to observe and reach. So far, most of what scientists know about the planet they’ve learned from the three Mariner 10 flybys and from solar-glare-bathed telescope observations from Earth.

Researchers know that Mercury is dry and extremely hot, reaching 467ÂC (872ÂF) on its sunny side. It is the fastest and the smallest planet. It is also the densest. Scientists suspect Mercury’s heft results from an iron-rich core that is 75 percent of the planet’s radius and 42 percent of its volume. In contrast, Earth’s iron core is 42 percent of its radius and only 16 percent of its volume.

Differences like these are unusual because all four terrestrial planets share similar early histories. The story, theoretically, went like this: About 4.6 billion years ago, a massive cloud of dust, ice, and gas collapsed from its own gravity. It formed a rapidly spinning disk around the young Suna solar nebula. As the disk cooled, minerals that were in a hot vapor state condensed into rocky particles in the hot inner regions. These particles clumped together to eventually form Mercury, Venus, Earth, and Mars. Inside each of the inner planets, dense metals sank to form a central core.

“Mercury is, in some sense, the most extreme outcome of the planetary formation processes that must have governed all of the inner planets, including our own,” says Solomon. “And we want to understand how that process could have led to a mostly metal planet.”

It is possible that the Sun’s extreme heat vaporized Mercury’s outer layers of rock, leaving behind only the heavier core. Or perhaps a giant collision stripped off the outer layers. A third hypothesis is that the innermost regions of the early solar nebula became enriched with iron particles, leading to Mercury’s metal-heavy composition. Each hypothesis predicts a different chemical composition of Mercury’s crust and mantle. MESSENGER’s spectrometers will measure the planet’s composition, clarifying its history.

Indeed, data from MESSENGER may help rewrite textbooks on how Earth and the other terrestrial planets evolved. “Cinderella can finally go to the ball,” says Solomon.


Artist's impression of MESSENGER in orbit.

NASA/JHUAPL/Carnegie Institution

More Mercury Mysteries

MESSENGER scientists will also tackle other quandaries about Mercury. Of the terrestrial planets, only Earth and Mercury have global magnetic fields. Earth’s magnetic field arises from churning motions of its outer molten core. Mercury’s likely does, too, but it is unclear what fraction of its core is molten and what fraction is solidand what that means for Mercury’s magnetism.

One look at the planet’s ancient, inactive crust reveals the paradox of Mercury’s thermal state. The MESSENGER images show plenty of evidence of volcanism, but activity appears to have ceased four billion years ago. Internal heat from the mantle likely powered the ancient volcanoes.

“How to reconcile a dynamic interior and an ancient surface?” says Solomon. “We'll find out with MESSENGER.” The spacecraft’s laser altimeter will confirm the presence of a liquid outer core. Its magnetometer will offer more details on how the planet’s magnetic field is generated. And the stunning geologic images will further unravel Mercury’s volcanic history.

Mercury’s next flyby is in October 2008. Much of this second flyby will provide new views of the Mariner 10 side. MESSENGER will make a third flyby in September 2009 and then enter permanent orbit around Mercury in March 2011. By then, this neglected planet will be well on its way to being, finally, understood.