The Geology
Phase 2 lays one colored plate on top of each finished stack, turning the terrain into a geological map. The colors stand for 10 groups of rock unit, mapped from orbit and simplified from the 44 the published map carries.
Point at the map to read a unit.
Pointing at the map lights the whole unit, on a phone by tapping. The colors are the actual LEGO bricks, so this is the finished Phase 2.
First, a warning
Are there only 10 units on Mars?
No! Nothing like. 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 impossible to tell apart across a table and no builder would keep that legend in their head. So the colors below are groups, not 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, and the colors we chose are only reminiscent of what each unit is. Blue stands for water that once sat there or ice that still does, brown for the old highlands, and dark red for the great basins.
The published map, and the same planet at the resolution of the board.
The published sheet is drawn in a Robinson projection and our board in a Hammer, so setting the two beside each other would introduce a difference of projection on top of the difference in resolution. The board has therefore been redrawn into the frame of the published sheet, so that the legend is the only remaining difference between the two pictures.
The upper picture carries more than a thousand mapped craters in yellow, together with 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?
Nobody has walked over Mars with a hammer, so the mapping is field geology done at a distance, using the methods a geologist would use on Earth.
What it looks like. The base map is a THEMIS mosaic at 100 m per pixel, taken in the infrared, which records the temperature of the ground instead of its brightness. Rock holds the day's heat into the small hours while dust and sand lose theirs by dawn, and that difference 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 sits 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, these give a relative order for the whole planet with no dates in it. The unit descriptions in the panel above are written in those terms, which is why they keep using superposes and embays.
Counting craters. Impacts arrive at a near enough constant rate, so an old surface carries more craters than a young one. Counting them by diameter and comparing the result against a production function gives an age, 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 carry real error bars.
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 review to read if you want the machinery.
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
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, sitting on top of 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 and 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 sat on this unit.
5. Impact Basin141 squares
The floors of the great basins, Hellas and 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, and 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, and a great deal of it. This one color covers the bulk of the Tharsis and Elysium rises together with the giant shields, Olympus and Alba and Ascraeus and Pavonis and Arsia Montes. Stacked lobate flows, meters to tens of meters thick and hundreds of kilometers long, piling up to several kilometers, and 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, and the pits grow visibly from one year to the next. It covers 4 squares, which makes it the rarest color in the build.
The full legend
All 44, straight from the map sheet
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 was given no geological information, and the lowland units still come down in the lowlands, 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, is what nearly all mapping
software uses underneath to read, write and reproject map files, and its
gdalwarp command puts the map onto Mars and shrinks it to our grid.
GMT, the Generic Mapping Tools, does the arithmetic on the result. Both are
taken apart flag by flag on
the topography page.
The resampling rule differs. 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
ESRI:104971 names the planet and projection to come out in. ESRI is the
company whose catalog of those codes everyone borrows, and 104971 is their number for
Mars measured as a sphere.
-r mode sets the resampling rule. Each square gets whichever
unit covers most of it, so a square is always a real unit and never an average of two.