It’s a common problem in geologic mapping. Scientists from different disciplines, interested in different things, map only the areas they want to study. The result is a patchwork of maps that don’t quite align with each other and therefore provide an incomplete picture of the whole.
In the case of an ambitious mapping project recently completed at the U.S. Geological Survey’s (USGS) Astrogeology Science Center in Flagstaff, Ariz., that whole is the Moon.
Although Earth’s closest celestial neighbor has been mapped in some form since the early 17th century, the new “Unified Geologic Map of the Moon,” published in March, is the first standardized geologic version ever made. As such, it could be an essential tool for future science and exploration.
“We’re all talking and using the same language through this map,” said Jim Skinner, a USGS research geologist in Flagstaff. Skinner coordinates the production of all standardized geologic maps of nonterrestrial solid surface bodies in the solar system on behalf of NASA, which funded the 4-year project. “It kind of sets the baseline for everybody to be able to communicate effectively, which is what maps are anyway.”
Prior to the completion of the unified map, the Moon’s geology was represented on six separate maps, themselves agglomerations of Lunar Orbiter images gathered in preparation for the Apollo missions, as well as photographs taken by the Apollo astronauts themselves.
One drawback of the six previous maps was that they existed only on paper or in two-dimensional scans. In 2013, a team led by USGS geologist Corey Fortezzo completed a project to digitize them for use within GIS software—a step Skinner estimates quadrupled their usefulness. Joined by preeminent lunar geologist Paul Spudis, the team then set to work stitching the six maps together into one covering the entire Moon, creating consistent geological nomenclature and adding much more detailed topography data collected from lunar orbit.
Fitting the Pieces Together
The first task in creating the unified version was to conform the six old maps into a single spatial representation. Because the old maps were based on composites of hundreds of photographs, the scale of any given section varied depending on the orbiting spacecraft’s positions and camera angles. Fortezzo’s team had to essentially bend and warp the maps to fit them into a uniform framework.
Topography data had also been limited in the old maps—scientists had to “eyeball” the relative height of many features on the basis of the shadows they cast. To solve that problem, Fortezzo’s team flooded the new map with a digital elevation model data set that combines stereo images taken from Japan’s Selenological and Engineering Explorer (SELENE) spacecraft, which orbited the Moon in 2007, and laser altimeter data and some imaging from NASA’s Lunar Reconnaissance Orbiter (LRO), launched in 2009. The result was the addition of more than 7 billion points of altimetry data, providing an altitude for every 60 square meters of the Moon’s surface.
The map comes as lunar science is on the verge of a heyday, with new probes returning vastly more detailed observations of the Moon than have been available in the past. “It’s hard to get your head around all these data,” said Kip Hodges, a geologist at Arizona State University whose wide-ranging research includes the ebb and flow of meteoroid impacts on the Moon over time. “What’s novel about this geologic map is putting all of these observations together at a time when the rate of observation is increasing incredibly fast.”
The new map identifies over 12,000 distinct outcrops. At a scale of 1:5,000,000, the smallest feature discernible on it is a crater rim roughly 3 kilometers across, about the width of Manhattan.
The USGS team chose to use what’s called a bridge scale between global, regional, and local units, maximizing the map’s usefulness in identifying areas for more detailed study. Once they’ve used the global map to narrow down a location, researchers can request very high resolution images—as sharp as 1 or 2 meters per pixel—from spacecraft like the LRO. Future lunar landing missions may use the global map to select a few areas with the right kinds of features before using smaller-scale maps to pick a landing site for a sample return mission.
“We know that the Apollo sample collection and the lunar meteorite collection [are] not representative of all the lithologies that are on the surface of the Moon,” said Clive Neal, a lunar geologist who leads a project to put a new geophysical observation network on the Moon. “We can now target those [lithologies] much better with this map.”
By the same token, the global map will help planetary geologists use local observations, say, from a lunar lander, to better understand similar features elsewhere on the Moon. “[The map] allows you to extrapolate out those observations you’re making in one or two or three different locations on the surface of that body…so we can understand the broadest area of [the body] that we can,” Skinner said.
NASA’s Planetary Geologic Mapping Program, which the USGS astrogeology center coordinates, has produced 243 maps of nonterrestrial bodies in the solar system so far, most of which are physical maps that Fortezzo’s team is also digitizing. Although Mars and Jupiter’s moons Io and Ganymede have also been mapped globally using the rigorous cartography standards the USGS is known for (and its global maps of Mercury, Jupiter’s moon Europa, Saturn’s moon Enceladus, and the asteroids Ceres and Vesta are currently in progress), Skinner believes this most recent version of the Moon’s is possibly “the purest form of a planetary geologic map.”
That’s because the map preserves so much of the data from its previous versions at a similar scale, adding without subtracting. “It’s just a good, aesthetically good, technically good map.”
An Evolving Process
When geologists first began mapping the Moon in the 1960s, it was a much less precise process that involved cutting up physical photographs and pasting them together. Cartographers would then copy the photo collage onto a new base map by hand using an airbrush.
With that process, “you’re losing things in the translation into kind of an artistic device,” Skinner said. Fortezzo’s team found that some outcrops shown on the old maps don’t actually exist, having been misinterpreted by an airbusher’s eye. The six separate map sections also suffered from poor translation across their boundaries, simply because they were not all created by the same scientists using the same methods.
For instance, there were about 200 different types of geological features on those maps—but some were duplicates, meaning the same feature may have been labeled differently depending on which map it was on. The new map uses the same data set for the entire lunar surface, eliminating the need to reconcile map sections across their boundaries, and boils the list down to 43 unique global stratigraphic units, each represented by a different color.
No two geologists will interpret every data point the same way, and Spudis and Fortezzo would often disagree about how a given outcrop should be classified. “They would sit there and argue about the different units,” said Skinner. “And it was a perfect way to do this….You need to argue and discuss it and talk about it. But they’d work together to try to dial everything in.” Spudis died in 2018 before the map was completed.
The consolidation and unification of so much of what is known about the Moon’s geology has far-reaching benefits. An express goal of the new map was to make lunar geology accessible to the general public, not just to planetary scientists, said Skinner. “Laymen can approach this map and look at it and start to have an understanding about what’s going on, even if they don’t know what the units mean. And they can start to see patterns.”
According to Skinner, the fractured nature of past maps made seeing those patterns difficult even for scientists. “There was not a whole lot of cross correlation between actual disciplines in planetary science,” he said. “This map enables people to actually talk across disciplines, and that enables discovery and exploration. Because now we have one device that will kind of rule them all, so to speak.”
Broadening the Lunar Science Community
Thanks to the consistency of USGS standards, the new map describes the Moon in the same basic visual language used in geologic maps of Earth. Hodges, whose work spans both planetary bodies, believes that simple fact can allow a wealth of knowledge to flow into lunar science from Earth science. “A geologic map actually puts the geology of the Moon in a context that is accessible to terrestrial geoscientists,” he said. “And that’s going to lead to new collaborative interactions with people who generally don’t study the Moon.”
Skinner added that making a unified, global map of any planetary body is mostly about making something that will be useful to the largest number of people doing the widest range of science. That’s done partly by separating the objective observations made from orbit and astrogeologists’ best interpretation of what features are made of and how they were formed, represented in two separate columns in the map’s key. Without being able to go to the Moon and directly analyze every outcrop, the interpretations will always be limited. Skinner and his colleagues are keenly aware that now that more data are available, many of the classifications made by early geologic maps of the Moon have turned out to be incorrect.
That’s why the new Moon map, although it’s the first standardized geologic representation of the entire lunar surface, won’t be the last. It will continue to be updated and improved as better data come in, a process that will be much easier now that the map is completely digital.
But the map’s usefulness lies in neither perfection nor permanence, according to Hodges. Its synthesis of new data and the way it elegantly presents that data in an accessible form are the map’s real contribution to future research. “These big, gigantic-scale geologic maps are principally useful to titillate your senses and get you working on really interesting problems,” Hodges said.
The “Unified Geologic Map of the Moon” is available for public use in GIS, PDF, and JPG formats.
—Mark Betancourt (@markbetancourt), Science Writer