Emily LakdawallaMay 17, 2018

#Mercury2018: From MESSENGER to BepiColombo and beyond

About 130 Mercury fans gathered in Maryland this month for a meeting about “Mercury: Current and Future Science of the Innermost Planet.” The Mercury community is eagerly anticipating the launch of BepiColombo this fall, so it was a perfect time to take stock of what we learned (and in many areas, are still learning) from NASA’s MESSENGER. This meeting will ultimately help the BepiColombo team focus their scientific investigations on questions that MESSENGER wasn’t capable of answering.

Before I begin this very lengthy post, I want to congratulate the BepiColombo team on the shipping of their spacecraft to the launch site! Here it is in Kourou, preparing for final integration and stacking onto its rocket.

This post will be a long summary of meeting notes. Right at the top I want to thank several scientists who participated in a collaborative notetaking activity: Caleb Fassett, Cesare Grava, Steve Hauck, and Indhu Varatharajan. All of our notes from the conference can be found here. I'm also grateful to some scientist friends who graciously hosted me in their home so I could take a trip that The Planetary Society would otherwise not have been able to afford to send me on.

A lot of the conference focused on aspects of Mercury science that are out of my expertise (magnetic fields and energetic particles), and the notes below are heavily weighted toward geology and geophysics. For other areas, I refer you to program and abstracts.

Introduction to MESSENGER and BepiColombo

To review: MESSENGER was a Discovery mission, which is NASA’s least expensive mission line. It launched in 2004, flew past Mercury three times in 2008 and 2009, entered orbit in 2011, and crashed into Mercury after an incredibly productive mission in 2015.

Artist’s concept of MESSENGER at Mercury
Artist’s concept of MESSENGER at Mercury A depiction of the MESSENGER spacecraft is shown flying over Mercury's surface displayed in enhanced color. The crater ringed by bright orange is Calvino crater. The enhanced color imagery of Mercury was obtained during the mission's second Mercury flyby in 2008.Image: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

MESSENGER carried: a laser altimeter to measure topography; a color camera system for global mapping; energetic particle, plasma, gamma, x-ray, and neutron spectrometers for measuring composition of surface and exosphere; a magnetometer; and it used its radio system for gravity measurements. It had an elliptical orbit that passed close over the north pole and far from the south pole. This was great for exosphere and magnetosphere science, but meant that the quality of MESSENGER’s data on topography, gravity, and surface composition of Mercury is good in the northern hemisphere and either not as good or nonexistent in the southern hemisphere. It got a great global photographic map of Mercury, which will be the base map for future investigations of the planet for decades.

Global color map of Mercury from MESSENGER
Global color map of Mercury from MESSENGER This mosaic shows Mercury's colors as viewed by placing images from MESSENGER's 1000 nm (infrared), 750 nm (infrared), and 430 (blue) nm narrow-band filters in the red, green, and blue channel respectively. This is not what Mercury would look like to the human eye. By using a wider range of wavelengths, color differences on Mercury's surface due to varying mineralogy can be accentuated beyond those visible to a person. A much higher resolution version is available here.Image: NASA / JHUAPL / CIW

BepiColombo is a much bigger mission than MESSENGER. When you consider the ESA and JAXA contributions together, it’s flagship-class, with a price tag of 1.65 billion Euro (about $2 billion). At Mercury there will be two main spacecraft, ESA’s Mercury Planetary Orbiter (MPO, but which they seem to be nicknaming Bepi) and JAXA’s Mercury Magnetospheric Orbiter (MMO, but it will be renamed, like all JAXA spacecraft, after a successful launch). There are two more components to the full BepiColombo stack: a sun shield for MMO that will be jettisoned after Bepi places MMO into orbit, and a Mercury Transfer Module with enormous solar arrays and solar-electric propulsion system that will take BepiColombo to Mercury and be jettisoned after orbit insertion.

BepiColombo Fact Sheet
BepiColombo Fact Sheet See also the ESA website.Image: ESA/JAXA

Bepi's orbit will not quite be circular, but its periapsis will be over the equator, so it will approach close to Mercury at all latitudes. MMO’s orbit is more elliptical. Both spacecraft orbits will always be in the same plane, making coordinated observations easy and productive.

Relationship of BepiColombo and MESSENGER orbits to Mercury's magnetic field
Relationship of BepiColombo and MESSENGER orbits to Mercury's magnetic field Image: Courtesy Sebastian Besse, ESA

Bepi is focused on Mercury’s surface. It has a laser altimeter, an accelerometer for gravity, a magnetometer, a spiffy multi-camera color imaging system, radiometer/thermal imaging spectrometer, an x-ray imaging spectrometer, as well as neutron, gamma-ray, x-ray, ion, and neutral spectrometers, and a solar wind instrument. While many of these are similar in concept to MESSENGER instruments, they’re all more capable and will be operating in an orbit that’ll better reach the whole planet.

Mercury Planetary Orbiter
Mercury Planetary Orbiter Image: ESA/ATG medialab

MMO carries a wide variety of instruments focused on Mercury’s magnetosphere and the charged particles that zip around in it, an imager designed to see the exosphere from its sodium ions, and a dust monitor.

Mercury Magnetospheric Orbiter Above Mercury
Mercury Magnetospheric Orbiter Above Mercury Image: ESA/ATG medialab; Mercury: NASA/JHUAPL/CIW
BepiColombo cartoons
BepiColombo cartoons As they did for Rosetta and Philae, ESA has created cartoon personalities for the different spacecraft components of BepiColombo. The three tweet from separate Twitter accounts: @JAXA_MMO, @ESA_Bepi, and @ESA_MTM. See them in their first cartoon here.Image: ESA

Mercury’s Interaction with the Solar Wind

After a couple of talks introducing the BepiColombo spacecraft, Jim Slavin gave an overview of Mercury’s relationship to the solar wind. Mercury has an internally generated magnetic field, a dipolar one like Earth’s, but it’s much weaker than Earth’s. Slavin described Mercury as a perfectly conducting iron sphere with only a few hundred kilometers of poorly conducting rock scattered across the top; the connection between Mercury’s field and the solar wind is only a couple thousand kilometers above that (the altitude varies as solar weather varies). At the magnetic field cusps, which are usually near the poles, charged particles from the solar wind travel down field lines to slam directly into Mercury’s surface; the collisions knock atoms right off of Mercury and into space in a process called sputtering, generating its tenuous exosphere. The process also causes X-ray emission, which an x-ray imager can see as a brightening on Mercury’s nightside where the cusp of the magnetic field intersects the surface. This is the same process that makes aurorae on Earth and other planets with atmospheres; at Mercury, which lacks an atmosphere, the aurora happens at the surface! A few talks following Slavin’s discussed developments in computer modeling of magnetospheres in advance of BepiColombo.

Jim Raines talked about how, being so close to the Sun, Mercury bears the brunt of coronal mass ejections, which bring showers of charged particles and strong magnetic fields. Coronal mass ejections exert intense pressure on Mercury’s magnetic field. They can compress field lines and cause something called “reconnection,” a shredding of Mercury’s magnetic field where field lines that used to be closed (both entering and exiting Mercury) merge with open field lines, tearing up the field and increasing the amount of Mercury’s surface that’s directly exposed to the solar wind. Coronal mass ejections can exert so much pressure on Mercury’s relatively weak magnetic field that they can compress the dayside magnetic field right into the iron core, which would have weird results that I didn’t quite understand.

Mercury's magnetic field
Mercury's magnetic field Mercury’s magnetic field is distorted by the solar wind, which compresses the magnetic field on the dayside and stretches it out to form a long tail on the nightside. Interaction between the solar wind and the planetary field creates the bow shock and magntopause boundaries, as well as features noted in the figure. Two representative MESSENGER orbits are shown in green. When the spacecraft is closest to the planet at night the orbit takes the form of the solid green ellipse. One-half Mercury year later, the spacecraft is closest to the planet during the daytime and the orbit takes the form of the dashed green ellipse. The MESSENGER orbit carries the spacecraft across the magnetospheric boundaries and through each region of Mercury’s magnetic field. The challenge for MESSENGER scientists is to extract telltale signatures that indicate how Mercury’s magnetic field would look without the solar wind.Image: Courtesy of Science / AAAS

Mercury’s Core

The Tuesday afternoon session traveled to the opposite extreme of Mercury science, into its deep interior. Mercury is an unusually dense terrestrial planet, meaning that it’s mostly metal. Of course, we can’t see the core directly. There are three main ways we get information about the interior of Mercury. The easiest is its density, which we can derive from its volume and mass. We can also use radio tracking of spacecraft in orbit to determine its gravity field, which tells us how the mass is concentrated within the planet. Finally, we can track features on Mercury’s surface to see how it librates and precesses in its orbit, which also depends on the distribution of mass in the interior as well as on the state of the matter in the interior (solid or liquid). All of this requires serious physics.

People familiar with the history of relativity may remember that you have to take relativity into account in order to accurately predict Mercury’s orbit. Even so, Antonio Genova explained that Mercury’s orbital path is highly uncertain compared to other planets. Another factor they have to correct for is the change in solar mass with time. To put that another way, a scientific result of the MESSENGER mission was one of the first measurements of the solar mass loss rate, a measurement that was done as a byproduct of trying to use MESSENGER gravity data to understand Mercury’s interior. Wow. (They also verified the Strong Equivalence Principle. Like I said, serious physics.)

Producing better gravity maps of Mercury is work that is still actively continuing. The varying altitude of MESSENGER’s orbit presents a particular challenge for gravity scientists. They’ve recently generated new gravity maps and new maps of crustal thickness and new proposed interior structures. Genova says their results suggest there is a solid inner core of about half the radius of the whole core; the rest of the core is liquid. Both parts are made of a mixture of iron and silicon. The crustal density is on the low side, about 2600 to 2800 kilograms per cubic meter; like the Moon, it probably has a higher grain density, but eons of bombardment have left it with a fractured crust that is porous. The mantle density is also on the low side, about 3300 kilograms per cubic meter.

If there’s a solid inner core, that means it’s frozen out of a core that was once all liquid. Ludovic Huguet talked about how that might happen on a small scale. He said that most pure metals supercool before crystallizing, and that while simulations can get Mercury’s core to cool past its freezing temperature, it can’t seem to get cool enough for crystals to begin growing. Instead, he thinks that the core is crystallizing from the top down; iron crystals nucleate and grow into dendrites that may get to tens of meters long before they are big enough to sink through the liquid and snow into the supercooled core. Pause for a moment and think about spaceship-sized iron feathers slowly drifting downward through molten metal.

Comparing the interiors of Earth and Mercury
Comparing the interiors of Earth and Mercury Mercury is a far more metal- and sulfur-rich planet than Earth is. Its metallic core dominates its interior. The exact size of the core, how much of it is liquid, and what kind of material exists at the core-mantle boundary are still not well understood; BepiColombo hopes to understand Mercury's interior better.Image: Case Western Reserve University / Steve Hauck

Mercury’s Magnetized Crust

Unfortunately, I didn’t understand these talks well enough to summarize them except to say that Mercury’s crust does have some magnetization here and there, but it’s hard to tell if it’s remanent (that is, if the fields reside in permanently magnetized rocks) or if it’s a field that’s induced in conductive materials located in the crust. Another question for BepiColombo, likely MMO, to answer.

Mercury’s Crustal Composition

One of the major MESSENGER discoveries was that its crust is incredibly rich in sulfur and low in iron – its crust contains something like 5 weight percent sulfur, about 100 times as much as Earth. The incredibly high quantity of sulfur (as well as other lighter elements like potassium) completely invalidated all the previously existing hypotheses for how Mercury formed. To get so much sulfur, Shunichi Kamata proposed a new model. The large amount of sulfur indicates that Mercury formed in a chemically reducing environment, with low oxygen fugacity. (I had to look up fugacity; for the purpose of this blog post, just think of it as the oxygen’s potential to react with iron. Low fugacity basically means oxygen atoms might be around, but for some reason they don’t want to react with iron). If it turned out that the innermost solar system was rich in carbon, then the carbon would scoop up all the oxygen to form gaseous carbon monoxide, leaving little oxygen available to react with iron. So instead of getting iron and other metal oxides, you’d get sulfides, and that’s what Mercury formed from. Why would the innermost solar system be so rich in carbon? Maybe when particles come streaming in from the outer solar system, all the water and other ices come off the particles first, so all that was left when they got to the innermost solar system was carbon. This explanation sounded arm-wavey to me, but the point is that the evidence suggests that there were a lot more sulfides in the neighborhood when Mercury formed than there were at greater distances from the Sun. (I do find myself wondering if Venus’ composition is intermediate between Mercury and Earth – if it’s much more sulfur-rich than Earth, too. That might help explain why its lavas are so runny. Hmmmmmm.) Anyway, Kamata said what we really need is a sample from Mercury. (Sadly, there are no known Mercury meteorites, though physics predicts that there should be at least one in our meteorite collections that we just haven’t recognized.) Next research steps, Kamata said, involve understanding the isotopic composition and the abundance of carbon on Mercury.

Unfortunately, around this time of the meeting, I was getting very sleepy and my attention was wandering. There were a couple of talks on what all that sulfur would be doing, petrologically, inside Mercury, and whether any sulfides that formed would float or sink, and where they would be today, but I’m afraid I can’t tell you much about what they said.

Mercury's northern volcanic plains
Mercury's northern volcanic plains Mercury's northern region is dominated by expansive smooth plains, created by huge amounts of volcanic material flooding across Mercury's surface in the past. The volcanic lava flows buried craters, leaving only traces of their rims visible. Such craters are called ghost craters, and there are many visible in this image, including a large one near the center. Wrinkle ridges cross this scene and small troughs are visible regionally within ghost craters, formed as a result of the lava cooling. The northern plains are often described as smooth since their surface has fewer impact craters and thus has been less battered by such events. This indicates that these volcanic plains are younger than Mercury's rougher surfaces.Image: NASA / JHUAPL / CIW

Mercury’s Surface Geology and Geologic History

One of the (to me) most striking discoveries of the MESSENGER mission was the northern plains, now called Borealis Planitia, a massive deposit of lava flows covering 10% of Mercury’s surface, centered near the north pole. (Because they are primarily polar, they weren’t visible during Mariner 10’s equatorial flybys, leaving them discoverable by the later mission.) Geologic mapping of MESSENGER images has now yielded an estimate that 30% of Mercury’s surface is covered by smooth plains – comparable, actually, to the amount of the Moon that’s covered by mare basalts. Mercury’s smooth plains are very likely also basalt, but they don’t contrast as much to Mercury’s “intercrater plains” because the intercrater plains are also basalt, whereas on Moon the stuff that’s not basalt is much brighter anorthosite.

Mercury's north polar topography from MESSENGER
Mercury's north polar topography from MESSENGER Measurements from MESSENGER's laser altimeter instrument mapped the topography of Mercury's northern hemisphere in great detail. This is an interpolated shaded relief map of these data. The lowest regions are shown in purple, and the highest regions are shown in red. The difference in elevation between the lowest and highest regions shown here is roughly 10 kilometers! Among the prominent features visible here are the smooth northern volcanic plains and the enigmatic northern rise. The low-lying craters near the north pole host radar-bright materials, thought to be water ice. Linear artifacts can be seen in some areas of this map. These are due to individual MLA tracks that need minor adjustments in order to fit the rest of the data. Crossover analysis and better knowledge of the spacecraft position can be used to adjust these tracks and improve the map.Image: NASA / JHUAPL / CIW

MESSENGER mapping has shown that while the smooth plains deposits represent the last widespread volcanism on Mercury, it was a long time ago, about 3.6 billion years ago. What caused them to form is still a mystery. James Roberts explained that any formation hypothesis has to account for four key observations. First: the volcanism was either long-lived or episodic, because not all smooth plains units formed at the same time. Second: the composition also varies from unit to unit. Third: Mercury has a very thin silicate mantle (compared to other terrestrial planets), so there isn’t much source material to work with. Fourth, Mercury has undergone substantial global contraction, at least 7 kilometers (and maybe more). The contraction is a real problem for magmatism, because the compressional stresses imposed on the crust by contraction would tend to seal fractures that magma would like to rise through. Roberts finished by saying that geologists always invoke impacts when they can’t explain something (they’re the “last refuge of the scoundrel,” in his words), “but in my defense, Caloris exists” and clearly had some influence on the timing and position of at least some of the smooth plains. More work is needed. BepiColombo could really help here with its more thorough mapping of the southern hemisphere, and better surface composition information from MERTIS to help discern rock units from each other and tease out the timing of these volcanic events.

While most smooth plains are extensive and old, C Malliband gave a talk about smaller smooth plains that locally abut Mercurian lobate scarps. It demonstrates, he said, that there was late effusive volcanism outside impact basins.

Another of the cool discoveries of MESSENGER was pyroclastic vents. These are irregularly shaped pits without rims (so they don’t look like impact craters), surrounded by haloes of material that show up interesting colors. Sebastien Besse gave a talk that described how to do automated classification of surface regions that show evidence of pyroclastic deposits. He found that they have spectra that have a sharp downturn in ultraviolet wavelengths and a very steep slope in the visible (making them appear redder than their surroundings). By mapping these two parameters, he’s found that pyroclastic deposits extend farther from vents than is obvious in imaging. These algorithms can be applied to future BepiColombo data.

Butterfly vent
Butterfly vent This color view of Mercury's surface highlights what may be an old volcanic vent. Just to the right of center, within a brown-red patch, is an irregularly shaped 30-km depression depression, in this case the irregular shape is a bit like a butterfly. While most of these suspected volcanic vents are located within impact craters (like this one), the butterfly-shaped vent is not clearly associated with any crater. It also appears that it may be older than other vents, resulting in more subdued details and colors.Image: NASA / JHUAPL / CIW

When scientists worked with Mariner 10 data, they developed a chronology for Mercury that was analogous to the time scale for the Moon, because Mercury looks sort of like the Moon with its heavily cratered surface and recent rayed craters. Maria Banks spoke about efforts to make better estimates of absolute ages in Mercury's history than "sort of like the Moon." In general, they find that Mercury's more recent historical periods are younger than the analogous ones for the Moon. In particular, it looks like Mercury's "Calorian" age, which is named for the Caloris impact basin and is analogous to the Imbrian age on the Moon, may have lasted for 2 billion years, squeezing Mercury's most recent Kuiperian age (like the Moon's Copernican) and next-most-recent Mansurian age (like the Moon's Eratoshenian) into much more recent periods. They estimate that the Mansurian began about 1.7 billion give or take 200 million years ago, and the Kuiperian about 280 million give or take 60 million years ago. This work should be dramatically improved after BepiColombo completes mapping.

Geologic time scales for Mercury, Moon, Mars, Venus, and Earth
Geologic time scales for Mercury, Moon, Mars, Venus, and Earth The geologic histories of terrestrial planets are divided into time periods based on key events in each planet's history. For all but Earth, the time period boundaries are based on major impact or volcanic events. They are tied to specific chronological years based on crater counting, but for places other than Earth and the Moon, there is a lot of uncertainty about absolute ages of each geologic era.Image: Jean-Pierre williams

Phew, that was just Tuesday and I'm already at more than 2500 words. On to Wednesday.

Mercury's polar deposits

This session contained one of the coolest (ha) stories I heard at the meeting about the age of Mercury's polar deposits.

Ariel Deutsch compared polar ice deposits on Mercury and the Moon. At Mercury, MESSENGER's neutron spectrometer revealed nearly pure water ice in many permanently shadowed regions on crater floors and walls. By contrast, at the Moon, Lunar Reconnaissance Orbiter's similar LEND instrument found only patchy ice deposits. Why the difference? Deutsch counted craters on the Mercury ice deposits and -- with disclaimers about small-number statistics -- found them to be quite young, 50 million years old or so. How about at the Moon? They looked for a relationship between the patchiness of lunar ice deposits and the ages of the craters they lie in, and found that young lunar craters contain no evidence of surface ice, middle-age craters (like Shackleton and Sverdrup) have more spatially coherent ice, and the oldest craters (Shoemaker, Haworth, Amundsen) have patchy ice. Deutsch's conclusion: the Moon's polar ice was delivered a long time ago, more than 2.5 billion years ago, and this explains the difference between Moon and Mercury polar ice deposits.

In Chesterton's permanent shadow
In Chesterton's permanent shadow Average MDIS mosaic of Chesterton crater (37 km diameter) with: (a) Arecibo radar image in yellow [1]; (b) WAC broadband-filter, 10-ms-exposure image overlain, showing details of the permanently shadowed surface within the crater; (c) both radar (yellow) and WAC broadband filter images.Image: NASA / JHUAPL / CIW / NAIC, Arecibo Observatory / Nancy Chabot

The Moon's ice being 2.5-ish billion years old is intriguing, because that makes ice emplacement coincident with mare volcanism, Caleb Fassett pointed out to me. He mentioned work by Debra Needham that suggests lunar ice could've come out of its volcanoes.

The Moon's Geologic Time Scale (with numbers)
The Moon's Geologic Time Scale (with numbers) Image: Emily Lakdawalla after Tanaka & Hartmann 2012

In her talk, Carolyn Ernst answered the obvious follow-on question to Ariel Deutsch's talk: if Mercury's water ice arrived only 50 million years ago, could it all have been delivered by one cometary impact, and if so, can we link that event to a specific impact crater? Hokusai is the clear candidate; it's an impressively rayed crater (so is quite young, maybe tens of millions of years old) and is good-sized at 97 kilometers. If Hokusai couldn't have delivered all of Mercury's polar water, then no single impact could have. Ernst deduced its impact angle from various geomorphic observations: it has asymmetric ejecta and a horseshoe-shaped central peak ring, so is an oblique impact, but has a circular cavity and no uprange "zone of avoidance" that you get from very highly oblique impacts; combining these gives you an impact angle estimate around 30 to 40 degrees. It's hard to know what its impact velocity was, because things can hit Mercury with velocities ranging from 10 to 80 kilometers per second, and Hokusai could've resulted from asteroids ranging from 31 down to 6 kilometers across for that range of speeds. How big the impactor was clearly affects the amount of water the impactor delivered -- at the bigger end of the size range, either an asteroid or comet could've delivered enough water, but at the smaller end, one impact isn't enough. Is there a way to estimate the impact velocity? Ernst said "there are suggestions" that its impact generated more impact melt than the average crater. Counterintuitively, you get more melt in craters where impactors came in with lower velocity. (Holding impact energy/crater diameter constant, a faster impact has a smaller projectile depositing its energy in a smaller region, which generates less melt; slower impacts of larger projectiles distribute impact energy over a larger area, generating more melt). Ernst found that an impact velocity of about 25 kilometers per second would produce the largest quantity of melt for Hokusai's diameter, and that indeed, it's entirely possible that the body that slammed into Mercury to make Hokusai and its rays also brought enough water to make Mercury's fresh, crisp polar water deposits. It's not proven, but it's a physically plausible story. At the end of the talk, I asked how BepiColombo could follow up this hypothesis. Ernst suggested that Bepi's mapping of ice deposits in the southern hemisphere would be important (are they fresh like in the north?)

Mercury in color from MESSENGER
Mercury in color from MESSENGER As it departed from its second flyby of Mercury, MESSENGER snapped a color mosaic of the planet using its wide-angle camera; this version is a four-frame (2x2) mosaic of images captured through the red, green, and blue filters. Mercury's color variations are very subtle. This view consisted almost entirely of territory not previously seen from a spacecraft and included a spectacular set of rays radial to a small impact crater located near Mercury's north pole that was later named Hokusai.Image: NASA / JHUAPL / CIW / color mosaic by Jason Perry

Hannah Susorney followed with an amusing talk working through attempts to estimate the thickness of the ice deposits in the polar craters. Her work showed them to be 24 plus or minus 27 meters thick. Oops. The conclusion is that the deposits are thin enough that MESSENGER's laser altimeter doesn't have the ability to resolve their thickness. The deposits do have to be at least 7 meters thick for them to be detectable to Arecibo's radar.

Exosphere and Magnetosphere potpourri

I can report that there was a session, but I didn't understand much of it, sorry. The one thing I took away from it was a cool item from Ron Vervack's talk: MESSENGER observed X-ray emissions from Mercury's surface on its nightside. They have the same cause as aurorae on Earth and Jupiter and other planets with atmospheres. Mercury doesn't have an atmosphere, so it has an X-ray aurora playing about its surface. That's pretty neat.

Mercury's crustal geophysics

Fortunately, Wednesday afternoon got back into material I was better equipped to understand. Mazarico Erwan kicked it off by summarizing MESSENGER findings on Mercury's crust. MESSENGER's gravity maps were much better than initially expected because of its lengthy extended mission, but they have little ability to resolve features in the southern hemisphere. The best estimate for Mercury's crustal thickness is about 35 kilometers, but there's a lot of variability in thickness. The thinnest spot is under the crater Rachmaninoff. Crustal density is low, 2800 kilograms per cubic meter. The gravity data contains several "mascons" in the northern hemisphere, which are places where there's extra dense mass hidden beneath the surface, usually a sign of an unrecognized impact basin. There are hints of several other unrecognized basins in the south, and geophysicists are looking forward to BepiColombo's gravity maps revealing them.

Mercury's gravity field and crustal thickness from MESSENGER
Mercury's gravity field and crustal thickness from MESSENGER Mollweide (left, 0°E in center) and north polar (right) projections of free-air gravity anomalies (top, in mGal) and crustal thickness (bottom, in km) derived from the most recent gravity field solution, HgM008. From an abstract (#6027) presented to the 2018 "Mercury: Current and Future Science of the Innermost Planet" meeting.Image: Figure from Genova A, S. Goossens, E. Mazarico, F. G. Lemoine, G. A. Neumann, W. Kuang, T. J. Sabaka, S. A. Hauck, II, D. E. Smith, S. C. Solomon, M. T. Zuber (2018). Geodetic Evidence of Solid Inner Core at Mercury. Submitted.

Remember how I mentioned that I think the discovery of the northern volcanic plains was one of the coolest things to come out of the MESSENGER mission? MESSENGER topography data revealed a mystery within it: it harbors a broad topographic bulge. Peter James looked into it for this Ph.D. thesis work. Mercury's north polar topography became so central to his life that his wife Hannah cross-stitched the topography for him (picture below; the northern rise is the yellow and green blob to the lower right of the blue and black blob representing the low-elevation northern volcanic plains). After all James' research, its nature is still mysterious. It has no correlation to any geology visible at the surface (composition, wrinkle ridges and other tectonic features). It's best explained by some kind of low-density material located beneath the crust, but that's about all he can say. He finished his talk by saying: “It’ll be great when BepiColombo gets gravity over the southern hemisphere. With any luck, we’ll find even more features that we can’t explain.”

Then came two talks, one by Nadine Barlow and one by Lorenza Giacomini, that came to opposite conclusions about the age of the most recent activity on Mercury's large-scale faults. Barlow's group found recent activity (that is, within the Kuiperian) on some faults; Giacomini's found that it ended a very long time ago, 3.6 to 3.7 billion years ago. More work is needed. Then Robbie Herrick gave a talk suggesting that the distribution of peak-ring basins on Mercury is clustered, not random, and suggested that peak-ring formation was more likely on some kinds of crust than others.

Frank Preusker talked about progress on developing topographic maps of the southern hemisphere based on camera images. The laser altimeter didn't work much south of the equator because MESSENGER's elliptical orbit took it too far above the surface, so stereo imaging is the only way to get topography there. This is tedious, exacting work, combining the overlapping parts of 80,000 distinct images. He's working on a quad-by-quad basis (there are 15 "quads" covering Mercury for mapping purposes -- two at the poles, six around the equator, and four for each of the north and south midlatitudes). The north pole and four northern midlatitude quads are already complete, sped along by the availability of laser altimeter data. He hopes to complete the development of all the topographic maps for all the southern hemisphere quads this year, and the equatorial quads early next year. Then will come the gargantuan task of merging it all into one global map, which he hopes to complete in 2020. This work is all the more amazing because Preusker is doing it in his "spare time". Eventually Preusker's map will be superseded by BepiColombo's laser altimeter map.

Finely-textured slope patches on Mercury
Finely-textured slope patches on Mercury During its final months in orbit, MESSENGER took some very close-up views of Mercury's surface. Among the interesting textures it observed at close range was a type of terrain Misha Kreslavsky and coworkers have termed "finely-textured slope patches", which have no equivalent on the Moon.Image: NASA / JHUAPL / CIW / Misha Kreslavsky et al.

Misha Kreslavsky looked at MESSENGER's very highest-resolution images of Mercury's surface, taken late in the mission as its orbit was decaying, revealing surface textures at resolutions better than a meter per pixel. It's a difficult data set, because the camera system wasn't designed for imaging at such close range or high speed, so the exposures are short (therefore, the images don't have a lot of signal) and were downsampled to just 512 by 512 pixels square to boost signal and avoid motion blur. All that being said, it's still a cool set of images. He asked whether Mercury still looked like the Moon even at such close range. He developed a comparison set of Lunar Reconnaissance Orbiter images, selected randomly, and degraded to the same resolution as the MESSENGER images. He found that, superficially, Mercury and the Moon remain similar at close range. There is regolith, and most surfaces are covered by small craters with rounded crests; they age and disappear by impact gardening on both worlds. Both worlds feature patches of "elephant hide" texture on slopes, but it's more common on the Moon than on Mercury. There are a few notable differences. Mercury has hollows down to very small scales, with the smallest being on the order of 10 meters across. Hollows are thought to result from places where volatile-rich minerals decompose, causing the ground to collapse. The Moon doesn't have them. Mercury also has scattered spots of "finely-textured slope patches." These patches form only on crater slopes, have very sharp boundaries, and avoid high latitudes and pole-facing slopes. Their origin is unknown. Mercury also has very few boulders compared to the Moon, but this difference Misha could explain: Mercury's much wider temperature swings and higher meteorite fluxes are expected to destroy boulders faster than happens on the Moon.

Future Mercury exploration

Several talks concerned open questions that will drive the next several decades of Mercury exploration. Some of them might be answered by BepiColombo, others won't. How did Mercury form with so much sulfur and carbon? In what form do they exist in the crust? Why is its core so big?

Christian Klimczak talked about what we don't know about Mercury's global contraction. The planet has shrunk as it's cooled, producing large fault scarps. How much contraction exactly? The current estimate is 7 kilometers, but we don't know for sure. Is there current tectonic activity at Mercury? It's still cooling, so there should be. If so, is it detectable? Is it aseismic, or does it happen in major faulting events? Did the lithosphere and the mantle have the same contraction history, or did one contract faster than the other, and can we find tectonic evidence for that? GRAIL detected at the Moon some interesting gigantic structures related to global expansion early in its history; do the same things exist at Mercury? How did Mercury's tidal despinning (the slowing of its rotation to lock into spin-orbit resonance) affect its global tectonic history?

Paul Byrne brought up questions about volcanism. Most smooth plains are lava. We think probably the intercrater plains are also probably lavas. All the big lava effusions seemed to end by 3.5 billion years ago, but explosive volcanism has occurred much more recently. Are all smooth plains volcanic? Are all intercrater plains volcanic? Are there subunits? How old are the other smooth plains?

Carolyn Ernst brought up Mercury's low-reflectance material, dark stuff that is generally associated with Mercury's oldest surfaces, often brought up from depth in impact craters. What is its composition? Could it be an ancient, carbon-rich crust? How much can we learn about it without a lander sampling it directly to measure composition, chemistry, and age?

Beyond BepiColombo

There were two main ideas that were discussed for missions beyond BepiColombo. Erwan Mazarico suggested we send a GRAIL-like mission to Mercury in which two satellites range to each other to acquire a data set usable for detailed gravity mapping. He argued that it could be done with SmallSats using laser ranging to each other, rather than small but still regular-sized satellites and radio ranging as GRAIL did. I'm sold; ever since GRAIL I've thought we ought to send a GRAIL-like mission to Mercury, Venus, and Mars. Results will depend on having a good topographic data set, which we now have from MESSENGER and will improve on with BepiColombo. (Sadly, we do not have a good topographic data set for Venus yet.)

Steve Hauck and Doug Eng discussed a future lander mission concept. Hauck spoke more about the major obstacle to a Mercury lander: lack of advocacy for Mercury exploration. He pointed out that in the last Decadal Survey, out of 199 white papers submitted, not a single one specifically promoted Mercury science. Mercury alone among the planets does not have a NASA-funded "assessment group" to develop consensus on exploration goals and desired future missions. (The Moon has its own LEAG, Mars has MEPAG, Venus has VEXAG, asteroids and comets and trans-neptunian objects have SBAG, and the giant planets have OPAG. Mercury occasionally gets mentioned at SBAG because a lot of the same scientists study asteroids and Mercury, but it's otherwise without a home.) Several people in the audience argued for the creation of a Mercury AG, but Sean Solomon pointed out that despite the efforts of VEXAG, no Venus mission has been selected for a long time; would we expect any different for Mercury?

The gathered scientists discussed how to keep the community alive between now and when BepiColombo finally starts its science mission, and how to develop effective advocacy for future Mercury exploration. Dave Blewett and Brett Denevi run a Mercury listserv for scientists to discuss their work with each other. Nancy Chabot After the meeting, Nancy Chabot emailed the listserv to state that "Discussions since the Mercury 2018 meeting have revealed that NASA's Planetary Science Advisory Committee (PAC) discussed the need for a Mercury AG at their February 2018 meeting and drafted a finding to support the formation of such an AG." So stay tuned. Other scientists planned special Mercury sessions at upcoming professional meetings. There was feeling that another Mercury science meeting that gathered people working with MESSENGER data together with BepiColombo scientists would be beneficial in 2 or 3 years; plans are now in the works for a Mercury 2020 meeting.

There was a third day of the meeting that I did not attend, but that's probably good because I've written more than 5000 words about just two days' worth! It was a great way to get rapidly up to speed on current Mercury science, and a good reminder that we're far from done learning from MESSENGER. It was also a terrific opportunity for me to meet people working on BepiColombo and to get excited about the mission.

Launch is coming in October. There are seven years until Mercury orbit insertion. That's a lot of time to keep doing MESSENGER science, and consider what comes after BepiColombo.

BepiColombo approaching Mercury (5)
BepiColombo approaching Mercury (5) Image: ESA/ATG medialab; Mercury: NASA/JPL

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