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Return to the Iron Planet: An ungainly stack of satellites is set to double the number of spacecraft that have visited Mercury.

It's small and gray and covered with craters--and, no, it's not the Moon. At first glance, Mercury looks a lot like Earth's natural satellite. Both have lava plains and rugged cratered terrain, with bright streaks radiating out from powerful impact scars on their drab, airless surfaces. But the Sun-scorched innermost planet is a very different place, from its inside out.

In contrast to the Moon's puny core, Mercury's iron heart takes up some 80% of the planet's radius. It's still at least partially molten and generates a weak, global magnetic field. Moreover, the planet's ongoing, slow solidification affects its surface: As the core freezes, it shrinks, and the crust wrinkles like a drying plum, creating ridges that cut right across big craters.

Look closely at those craters, and you'll find some have inexplicably dark rays, rather than bright ones. The floors and peaks of others appear to be moth-eaten, with "hollows" of missing rock. Mercury's dayside might be baked by the Sun to temperatures reaching 427[degrees]C (800[degrees]F), but even that isn't hot enough to evaporate its surface rock.

These hollows, like so many other features of Mercury, remain an enigma. All of what we know about this world comes from Earth-based observations and a couple of spacecraft. More than 50 years into the age of interplanetary travel, only two missions have targeted Mercury: NASA's Mariner 10, which flew past the planet three times in 1974 and 1975, and NASA's Mercury Surface, Space Environment, Geochemistry, and Ranging (Messenger) spacecraft, which also zoomed by three times before settling into a highly productive orbital mission from 2011 to 2015 (SScT. Apr. 2012, p. 26). Messenger's data upended scientists' theories for how the planet formed and left scientists grappling with its results. In October or November of this year, the joint European-Japanese BepiColombo mission will launch to pick up where Messenger left off, beginning a 7-year journey to the innermost planet to solve some of Mercury's persistent mysteries.

Under New Scrutiny

Mercury hasn't been as high a priority for exploration as other places, such as Mars. That's partly because it's harder to put an orbiter around Mercury than it is to get to Pluto: The tiny planet is deep in the Sun's gravity well, and it takes numerous flybys of Earth, Venus, and Mercury itself to reduce a spacecraft's velocity enough to settle it into a Mercurian orbit.

Two space agencies are taking on the challenge together. Their joint mission will deliver two spacecraft into orbit at Mercury: Europe's Mercury Planetary Orbiter (MPO), and Japan's Mercury Magnetospheric Orbiter (MMO, nicknamed Mio). The pair will loop around Mercury in polar orbits, as Messenger did, but with crucial differences that will give the BepiColombo mission a much more thorough view.

Unlike Messenger's path, which took it close to the north pole and far from the south one, MPO's orbit will be nearly circular and close to the planet, giving its imagers and spectrometers a detailed look at all latitudes. MMO will follow a more elongated orbit that takes it out farther than MPO to sample more regions of the magnetic field. Both spacecraft will always orbit in the same plane, enabling scientists to coordinate observations.

The set of instruments that BepiColombo will bring to bear at Mercury is broadly similar to Messenger's, only more capable. MPO has color cameras, infrared spectrometers, a laser altimeter, and particle detectors. MMO has a magnetometer, two instruments to study plasma in the magnetic field, plus an exosphere imager and a dust monitor. What will these spacecraft learn that Messenger could not?

Dark Surface, Light Elements

The most surprising discovery made by Messenger is Mercury's surface composition. It took the orbiter's gamma-ray and neutron spectrometers until the end of the mission to accumulate enough data for quality compositional maps. The result: Mercury's crust is about 5% sulfur by mass. This is roughly 100 times sulfur's abundance in Earth's crust and was totally unexpected. The crust is also 1.4% carbon. The abundance of these two elements directly contradicts formerly favored ideas about how Mercury's bulk composition could be so very metal-rich.

Sulfur and carbon are both relatively "volatile" elements and were not expected to condense in the hottest, innermost portion of the solar nebula as our solar system formed. Prior to Messenger, some scientists had hypothesized that modern-day Mercury is little more than the core of a preexisting larger planet whose mantle was blasted off by a large oblique impact, or perhaps that its lighter materials were blown away by an early, hot Sun. But sulfur and carbon should have been scarce in the aftermath of either scenario. Their abundance confounded mission scientists.

Messenger did confirm ground-based studies that the surface contains little iron or titanium. These are the elements that make the Moon's basalt-filled maria dark, and without them, scientists had difficulty explaining why Mercury's surface is so dark. The unexpectedly high abundance of carbon offers a clue: It might be that graphite (a crystalline form of carbon), not iron, darkens Mercury, mixed among the silicate rocks of its crust.

Mercury has patches of what's been dubbed low-reflectance material that's even darker than the usual crust. It's generally exposed by impacts, which dig into the crust and excavate deep material, throwing it onto the surface. Mercury likely once had a magma ocean, as the just-formed Moon did after its violent birth (S&T: Aug. 2018, p. 26). But whereas the Moon's magma ocean developed a crust of floating low-density silicates, some planetary scientists suggest that Mercury developed a crust of buoyant graphite. After more than 4 billion years of impacts and volcanism, that primordial crust is now buried or disrupted (perhaps, blending with other crustal rocks to darken them). But surviving patches occasionally make themselves known when impacts punch through to a remnant.

The surface's low iron content hampered Messenger's ability to discern the composition of Mercury's rocks in fine detail. Near-infrared spectroscopy relies on the presence and positions of spectral features associated with iron to separate rocks by composition. So BepiColombo's instrument suite includes a thermal emission spectrometer, MERTIS, which is more sensitive to variations in surface composition beyond iron content. The hope is that MERTIS can determine exactly what Mercury's enigmatic dark material is. The instrument's ability to see in the dark--imaging the surface using the heat emitted by its rocks during Mercurian night--will produce a whole new kind of data for the planet and reveal some of the secrets of its poles. And, maybe, better compositional information will help scientists solve the mystery of why Mercury's core is so disproportionately big.

Puzzling Geology

Planetary scientists determine the ages of surface units by counting the numbers and sizes of craters that they have accumulated, but geologic processes can reset that clock. Mercury has fewer small craters than the Moon does, suggesting a major "resurfacing event" at some point early in its history. Geologists intentionally employ the bland word "resurfacing" to avoid attributing a cause, because we don't know if the resurfacing was caused by volcanism, impacts, tectonism, some combination, or none of the above.

Of particular interest are the relatively flat plains interspersed between the large craters (called, appropriately, intercrater plains). They appear to be volcanic flows, but are they? Are they all the same age, or is there a lengthy history recorded in the rock? Messenger couldn't tell. Maybe BepiColombo will. It will deliver much finer-scale pictures of much more of the planet, particularly the southern hemisphere. The more smaller craters that geologists can count, the better they can estimate the ages of small surface areas --and the finer distinctions they will be able to draw between the histories of different parts of the surface.

Higher-resolution pictures will also enable geophysicists to track Mercury's global shrinkage to smaller scales. Mercury shrinks because it has a partially molten interior that is slowly solidifying, and solid rock is denser than liquid rock and thus occupies less volume. Mariner 10's distant images of Mercury showed scarps that recorded 1 to 2 kilometers (about 1 mile) of planetary shrinkage. After Messenger, we now think that all of Mercury contracted by as many as 9 km s (6 mi). If there are more scarps at finer scales than Messenger could see, there may have been even more shrinkage. Mercury is still cooling, so there should still be compression along those faults even today. Can BepiColombo find evidence for geologically recent fault motion? Can it map different amounts or types of crustal crumpling in rocks of different ages to discover how Mercury shrank with time?

A Hot Planet's Ice

We know that the planet closest to the Sun has deposits of nearly pure water ice at its poles. This seems outlandish, given how strongly the Sun beats down on Mercury's surface (up to some 10 times more intensely than at Earth). Even more amazing is that we first discovered that polar ice in 1991 using radio telescopes on Earth, which detected deposits near the pole that looked bright in radar images (S&T: Jan. 1992, p. 35). Messenger proved that the round radar features near the north pole all lie inside deep impact craters whose floors and north-facing slopes never see the Sun, and whose interiors plummet to about -200[degrees]C (-330[degrees]F).

Later in the mission, Messenger scientists commanded its camera to shoot long-exposure images of those permanently shadowed crater floors. Using light reflected off the sunlit south-facing rims, the spacecraft was able to reveal the radar-bright deposits. Unexpectedly, these images showed some of them to be as black as coal, while others are bright. The interpretation: The cold, shadowed craters contain ice deposits that are tens of meters thick at most, mantled with a varying amount of dark, carbon-rich material.

How does this stuff get to Mercury's poles? Water and most carbon-containing organic molecules are both highly volatile at Mercury--once delivered by asteroids or comets, they don't sit around on the hot surface; instead, they turn into gas and float off. Gravity might bring them back down, but they can't remain stuck to the planet's hot surface. However, any volatile molecule floating in Mercury's thin atmosphere that happens to touch down on the incredibly cold surface of a permanently shadowed floor can become trapped there forever, unless it's disturbed by an impact.

The Moon also has polar ice deposits, but they are patchy and impure. So why are these two worlds' polar ices so different? Could it be that Mercury's ice looks cleaner because it's fresher, delivered recently in a single impact event?

If that's the case, then the impact should have left a crater we can see. You can tell which craters are relatively young on Mercury (or the Moon, for that matter) by the presence of bright rays. And there is no more impressive rayed crater in the solar system than Mercury's Hokusai, a 114-km-wide scar near 60[degrees]N with bright rays stretching across the face of the planet, making Mercury look like a gray watermelon.

A team led by Carolyn Ernst (Johns Hopkins University Applied Physics Laboratory) has calculated what kind of impacting body would create Hokusai. Observing its horseshoe-shaped peak ring and the large volume of formerly molten rock that fills its floor, they calculated that a reasonable-sized comet or asteroid impactor (25 km) traveling a reasonable speed (less than 30 km per second) could have produced Hokusai and easily delivered sufficient water to account for everything now at Mercury's north pole.

As for the south pole, recent Arecibo observations suggest that radar-bright deposits there cover roughly double the area that their northern counterparts do. Moreover, the southern pole region is more heavily cratered. We don't know what surprises might still hide there, but BepiColombo will get the first close views of this region.

Embedded in the Stellar Wind

Much of Messenger's mission focused on the environment around the planet--particularly its magnetic fields and the charged and neutral particles residing there. No other world in the solar system has the intense relationship with the Sun that Mercury does. At Venus and Earth, the solar wind's interactions are primarily with their ionospheres, far above the ground. But at times the solar wind can interact directly with Mercury's rocky surface. One bizarre implication is that, while aurorae appear high in the atmospheres of Earth and Venus, there could be aurora-like emissions at ground level on Mercury, at least at X-ray wavelengths.

Mercury's churning molten outer core generates a global magnetic field, albeit one that's only 1% as strong as Earth's at the surface. A weird (and still unexplained) aspect of Mercury's magnetic field is its offset toward the north pole by roughly 500 km, some 20% of the planet's radius. The planet's weak magnetosphere can hold off solar radiation -sometimes. But when coronal mass ejections blast toward Mercury, the incoming plasma can compress the field drastically enough to let solar radiation smash directly into Mercury's surface. The south pole gets bombarded more because of the northward offset of the field.

When solar wind particles slam into Mercury's surface, they knock atoms from surface rocks and into Mercury's tenuous exosphere. It's not a proper atmosphere--individual atoms rarely encounter each other, so it has no wind or weather--and spacecraft like Messenger and BepiColombo can fly directly through it unimpeded, tasting atoms that until recently were part of the planet's surface.

Solar wind, intrinsic magnetic field, and neutral and charged particles are all interlinked and dynamic on very short time scales. For example, when the solar wind's magnetic field lines connect with a planet's dayside magnetic field, the latter's field lines peel back around to the planet's nightside, where they reconnect. On Earth, this process takes about an hour; on Mercury, magnetic field lines can shift from one side of the planet to the other in a matter of minutes. It's so dynamic that it's difficult to understand all the interactions with only one spacecraft in one location at one time, especially because many of the relevant measurements are performed in situ, with the spacecraft directly measuring field strength or ion composition. With two spacecraft in different orbits, BepiColombo will be able to measure the magnetic field and particle environment at two points simultaneously. MPO will operate at a distance where the planet's influence is stronger, while MMO will explore a region where the solar wind takes over. Together, they'll watch how the whole system responds to solar storms.

BepiColombo's Odyssey

To get MPO and MMO to Mercury, ESA built a third spacecraft, the Mercury Transfer Module (MTM). The MTM has huge solar arrays to power ion thrusters that will accomplish the hard work of setting up BepiColombo's rendezvous with Mercury. This is the first time that ESA will use solar electric propulsion on an interplanetary mission. MTM employs the full power-generating capability of its solar panels only when it's farther from the Sun than Venus's orbit; once it passes inside Venus's distance, it has to tilt its solar panels at an angle to the Sun to avoid overheating and also to limit damage from solar energetic particles.

Spacecraft propulsion alone won't be enough. BepiColombo will perform one flyby of Earth, two of Venus, and six of Mercury to tweak the shape, size, and orientation of its heliocentric orbit before settling in at Mercury in December 2025. The MTM will be dropped two months before the combination craft enters orbit. On arrival, MPO will hit the brakes so that Mercury can capture the paired craft. After many smaller maneuvers, the duo will reach MMO's desired orbit. MMO will separate, and MPO will drop to its own low-altitude circuit of the planet in March 2026. The science mission is planned to begin shortly thereafter.

Unfortunately, the stacked spacecraft structure limits BepiColombo's science capability during the long cruise. MMO in particular will be hidden beneath a European-built heat shield and unable to employ its instruments. It won't be able to do any science at all until it's been captured by Mercury, where it will jettison the shield and spin up to a 4-second rotation period, using that spin to deploy its booms and redistribute heat so that it can bear the high temperatures close to the Sun.

The use of ion propulsion also constrains science observations during the voyage. The engine has to operate nearly continuously for months on end. The thrust direction and solar-panel orientation dictate the direction that the spacecraft points, so there's no possibility of pointing instruments to do observations--except when the engine is off and the craft is coasting (see diagram above).

The next seven years will not just be a waiting game, though. BepiColombo's science teams are particularly eager to test their instruments at Venus. MPO's remote-sensing instruments mostly have their eyes pressed firmly against the transfer module, but a couple of instruments (notably MERTIS) have sideways-pointed channels that will be able to take some data. And measurements of fields and particles --by the magnetometer, neutral and ion spectrometers, radio science experiment, accelerometer, and others --can be made even while the spacecraft remain stacked. Perhaps, when BepiColombo passes Venus, it will be able to do coordinated observations with another Japanese mission, Akatsuki, training its senses on one enigmatic world en route to another.

* S&T Contributing Editor EMILY LAKDAWALLA is Senior Editor and Planetary Evangelist for The Planetary Society, and author of the recent book The Design and Engineering of Curiosity: How the Mars Rover Performs Its Job.

MERCURY

FAST FACTS

0.01[degrees] (Earth: 23.5[degrees]) Axial tilt

58 million km (0.39 astronomical unit] Mean distance from Sun

-223[degrees]C to 427[degrees]C (coldest in shadowed polar craters) (Earth range: -88CC to 58[degrees]C) Surface Temperature

Bepi Colombo, the Man

Giuseppe "Bepi" Colombo (1920-84) was an Italian mathematician and engineer who discovered that the time it takes Mercury to rotate around its axis is two-thirds as long as its year. Before his work, astronomers had thought the planet's day was the same length as its year, 88 days. He also proposed putting NASA's Mariner 10 spacecraft in a 176-day-long solar orbit that would bring it past Mercury repeatedly, enabling the mission's three flybys.

Caption: IRON PLANET This 66-image Messenger mosaic is roughly centered on the rayed crater Kuiper. just south of Mercury's equator. Long rays striate the globe, many tracing back to Hokusai along the limb at upper right.

Caption: ALL HEART Mercury's solid-iron inner core and liquid outer core of iron, sulfur, and silicates together dominate the planet's interior. In comparison. Earth's inner and outer cores take up far less of our world's total volume. Cutaways are roughly to scale.

Caption: CALORIS BASIN This mosaic of Mercury's signature, 1,500-kmwide impact structure combines enhanced-color and topographic data to reveal differences in geologic features. Lava flows appear orange. Several smaller, subsequent craters have punched through the surface lava to expose what geologists term low-reflectance material (blue). This material is likely part of the original basin floor (see close-up in center). Based on the craters, the volcanic layer appears to be between 2 1/2 and 3 1/2 km thick.

Caption: MESSENGER VS. BEPICOLOMBO BepiColombo's components will follow polar orbits, as Messenger did, but they won't fly as far from the planet and will be oriented differently to the planet's spin axis.

Caption: MYSTERIOUS DARK STUFF This enhanced-color mosaic shows three craters within much larger Caloris Basin: Munch (left), Sander, and Poe. Sander, the smallest of the three, is about 50 km across. They've excavated low-reflectance material, oddly dark stuff that might be rich in graphite. The same craters lie toward the top of the image at left.

Caption: SHRINK MARKS The giant thrust fault Carnegie Rupes slashes through the 132-km-wide crater Duccio, the wall it creates rising nearly 2 km above the lower terrain. Scarps like these form as Mercury's interior cools, causing the planet to contract. By tallying up all the planet's known scarps, researchers calculate that the planet's circumference has shrunk by at least 7 km--and perhaps 2 km more before its crust was solid enough to form wrinkles.

Caption: THERE'S ICE IN THEM CRATERS Radar observations made from Arecibo Observatory reveal bright deposits (tinted yellow) in several shadowed craters at Mercury's north pole, which have been revealed by long-exposure Messenger images. Scientists suspect that these deposits consist of fairly pure water ice.

Caption: HOKUSAI During a flyby of Mercury in 2008, Messenger recorded this prominent 114-km-wide crater and its bright splash pattern. The impact that created Hokusai might have delivered water that migrated to the planet's poles and formed ice deposits on the floors of permanently shadowed craters there.

Caption: A DAY ON MERCURY The innermost planet takes 58 Earth days to turn once around its axis, and 88 Earth days to orbit the Sun. But as seen from the planet's surface, the Sun takes far longer - 176 Earth days--to return to the same point in Mercury's sky. Thus the planet experiences one "day" every two "years."

Caption: COMBO SPACECRAFT Top: Artist's impression of BepiColombo in cruise configuration. Bottom: An expanded view of BepiColombo's components. The Mercury Planetary Orbiter and Mercury Magnetospheric Orbiter (nested in the sunshield) will ride to the innermost planet with the ion-powered thrust of the Mercury Transfer Module.

Caption: SCENIC ROUTE? To reach the innermost planet, BepiColombo has to lose orbital energy--a lot of it. The spacecraft will use gravity assists with Earth, Venus (twice), and Mercury itself (six times) to slow down enough to gently enter orbit around the iron planet in 2025.

Caption: HOLLOWS Strange pits pockmark Mercury's surface, sometimes appearing in clumps several kilometers across. Individual hollows can be as small as a couple of hundred meters wide. Scientists think that the vaporization and escape of volatile compounds in the surface might explain the marks.
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Title Annotation:EXPLORING MERCURY
Author:Lakdawalla, Emily
Publication:Sky & Telescope
Date:Nov 1, 2018
Words:3630
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