The Geology
The 10 colors of Phase 2 stand for rock units, mapped from orbit and simplified from the 44 on the published map.
Point at the map to read a unit.
Wait a second...
Are there really only 10 units on Mars?
No! Not at all! The published map we built from is the work of the United States Geological Survey, the government agency that maps the rocks of this planet and, as it turns out, several others. Their map of Mars has 44 units, spread across about 1,300 separate patches of ground.
We squashed those 44 down to 10 before we started, because 44 colors of brick would be too cluttered and prone to errors. We also wanted to focus on the large-scale behaviors and units, which give a better sense of Mars' key regions. The colors below are therefore groups of units. Everything we call "Volcanic" is really several volcanic units of different ages piled into one color, and "Noachian Highland" covers several kinds of old highland.
The colors are not what Mars looks like either. Because the real planet is mostly shades of rust and dust, a true color map of it would be close to unreadable. The colors we chose are only reminiscent of what each unit is. For example, blue stands for water that (maybe) once pooled there or ice that still does, brown for the old highlands, and dark red for the great basins.
The published sheet uses a Robinson projection and the board has been redrawn into that frame for the comparison, the same way it was for the elevation maps. What separates the two pictures is then resolution and legend.
The upper picture shows more than a thousand mapped craters in yellow, the dashed contacts that divide one unit from another, and a range of purples, teals and pinks for units that our 10 colors combine into a single group. At 96 by 48 squares the board keeps the dichotomy, Hellas and Tharsis, and loses most of the rest.
The method
How does anyone know what the rocks are?
Composition comes from orbit, using spectroscopy. Much of the surface is basalt, the same rock as many of Earth's lava flows. Subdividing that into units takes more than composition alone.
Nobody (so far!) has walked over Mars with a hammer. The mapping is field geology done at a distance, using the methods a geologist would use on Earth.
What it looks like. The common base map is the THEMIS global mosaic at 100 m per pixel, taken in the infrared, which records the temperature of the ground instead of its brightness. Rock stays warm into the small hours while dust and sand cool down by dawn, which separates a lava plain from a dune field even where the two look alike in daylight. THEMIS is the Thermal Emission Imaging System, on NASA's Mars Odyssey.
What is on top of what. Ordinary stratigraphy, the same reasoning applied to a road cut in Nevada. The principle of superposition puts a layer later than whatever it lies on and cross-cutting puts a fault or channel later than whatever it interrupts.
Read across enough unit boundaries, and as a result these relationships give a relative order for the whole planet, even in the absence of absolute dating. The unit descriptions on the sheet are written in those terms, which is why they keep using superposes and embays.
Counting craters. Impacts arrive at a near enough constant rate, which results in an older surface having more craters than a young one. Counting them by diameter and comparing the result against a production function gives us an absolute age. These ages are calibrated against the Apollo samples, whose ground could be both dated in a laboratory and crater-counted. The mappers did this for 23 of the 44 units and the ages they report carry stated uncertainties.
Fassett, C. I. (2016) Analysis of impact crater populations and the geochronology of planetary surfaces in the inner solar system. Journal of Geophysical Research: Planets 121, 1900–1926.
The result is 3 periods, oldest to youngest, Noachian then Hesperian then Amazonian, split into 8 finer epochs. Both methods together produce the chart the map sheet calls the correlation of map units:
The legend
The ten units
The units are listed in map order, with the number of squares each one takes on the board.
1. Noachian Highland1,714 squares
The oldest ground on Mars and the biggest color on the board. Densely cratered, rugged, and hundreds of meters to several kilometers thick. The USGS map splits it into early, middle, and late Noachian highland units, each a mix of impact debris, volcanic rock, and river sediment, heavily degraded by 4 billion years of weather and bombardment. This is the surface that was around when Mars still had running water.
2. Hesperian Highland44 squares
Smooth high plains that stretch for hundreds of kilometers, overlying the older Noachian ground around the Tharsis rise and in scattered highland lows. A mix of impact, volcanic, wind-blown, and river material, knobby and wrinkle-ridged in places. Only 44 squares on the whole board, which makes it the hardest color to find.
3. Transition296 squares
The dichotomy boundary, where the old high south falls away into the young low north. The USGS map has 7 separate transition units here, full of knobs, mesas, aprons, and flat-floored plains. Highland material coming apart by mass-wasting, rivers, and wind, with outflow channel deposits cutting through it. Mars Pathfinder landed on one of these units, in Ares Vallis.
4. Lowlands499 squares
The northern plains, planar, low, and continuous across most of the northern third of the planet. Interpreted as river, lake and possibly marine sediment washed in from the outflow channels and the highlands, mixed with lava, then reworked by ice and kilometers thick in places. Any ocean Mars may have had would have covered this unit.
5. Impact Basin141 squares
The floors of the great basins, Hellas, Argyre, and Utopia. Low-lying plains of wind-blown, lake, and volcanic fill, squeezed into wrinkle ridges by later contraction. Argyre carries sinuous ridges interpreted as inverted river channels or eskers. Hellas is the deepest place on the planet, deep enough that it is the only part of our board where the stacks come down to a single plate.
6. Volcanic811 squares
Lava. This one color covers the bulk of the Tharsis and Elysium rises together with the giant shields, Olympus, Alba, Ascraeus, Pavonis, and Arsia Montes. Stacked lobate flows, meters to tens of meters thick and hundreds of kilometers long, with a combined thickness of several kilometers. The rock is mostly basalt, judging by how the flows behaved.
7. North Polar Deposits24 squares
The layered plateau and dune fields around the northern cap. Meters-thick beds of ice and dust with unconformities inside them, over 1,000 m thick in places and ringed by dark gypsum-bearing dunes, with the layering recording swings in the planet's climate.
8. North Polar Cap15 squares
The residual northern ice cap, mostly water ice, hummocky and pitted at meter scale and less than about 2 m thick. Its edges move with the seasons. Only 15 squares, right at the top of the board.
9. South Polar Deposits80 squares
The southern equivalent. Layered ice and lithic fines forming Planum Australe, cut by spiral troughs, with older ice sheets underneath that show up in radar.
10. South Polar Cap4 squares
The residual southern cap, mainly frozen carbon dioxide where the northern one is water ice. Pitted at the scale of hundreds of meters, with the pits growing visibly from one year to the next. It covers 4 squares, which makes it the rarest color in the build.
The full legend
What do the 44 units look like?
The published map comes with a Description of Map Units, 44 entries carrying a photograph each of the unit's type area. A type area is the patch of ground the mappers picked to define what a unit looks like, and the reference specimen for the rest of the map.
Pointing at any of the 44 opens its entry.
Where it is
What it is read as
The descriptions are quoted from the published sheet, which is why they are terse and use IR for infrared, the heat picture described above. Bright in IR is ground that holds its warmth, usually rock instead of dust.
The small map marks where that type area falls on our board. The projection uses no geological information, yet the lowland units still land in the lowlands and the polar units on the poles.
The source
Where the map came from
The United States Geological Survey's geologic map of Mars, the standard modern map of what the surface is made of and how old it is. Assembled from the Viking missions onwards and published at a scale of 1 to 20 million, where 1 cm on the printed sheet stands for 200 km of Mars.
Tanaka, K. L. et al. (2014) Geologic map of Mars. U.S. Geological Survey Scientific Investigations Map 3292.
Simplifying the 44 units into our 10 was done by hand in QGIS, a free program for drawing and editing maps, before any of the LEGO work started. GDAL and GMT then do the same job here as on the topography.
GDAL, the Geospatial Data Abstraction Library, and GMT, the Generic Mapping Tools, do the same work here as they do on the topography page. What differs is the resampling rule. Each square takes the most common unit inside it instead of an average, since a category cannot be averaged.
-ts 96 48 -r mode geo_simp.tif coarse.tif
-r mode sets the resampling rule. Each square gets whichever unit covers most of it, keeping every square a real unit instead
of an average of two. The other
flags, including the ESRI:104971 code that names the target, are the same as
on the topography page.