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.