Five spacecraft are currently investigating Mars, and a number of new missions to the Red Planet either have been launched or are in development. They are designed to probe the surface, subsurface, and atmosphere with a host of scientific instruments. Where will they make new discoveries? Clues to where they should focus investigations can be gleaned from a new geologic map of the planet (Figure 1) [Tanaka et al., 2014].
The map records the materials, terrains, and geologic features of Mars at 1:20,000,000 scale, assembling them in a detailed geospatial and temporal context. It thus provides a comprehensive digital geologic database of Mars, useful for guiding exploration and research into its geological history, resources, astrobiology potential, and geophysical and climatological information.
Furthermore, the map is useful as a training tool for students of planetary science, helping them learn how to approach mapping a planetary surface by using the latest digital and analytical techniques. In addition, the map represents a synoptic product for educating the interested public about recent achievements resulting from spacecraft exploration of Mars.
The map, released on 14 July 2014 at the Eighth International Conference on Mars at the California Institute of Technology (Caltech) in Pasadena, Calif., was 7 years in the making. The mapping team, supported by NASA’s Planetary Geology and Geophysics Program, consists of researchers from the U.S. Geological Survey, the Tokyo Institute of Technology, the Smithsonian Institution, and Google Inc. The map also incorporates results from crater statistics experts at Freie Universität, who were sponsored by the Helmholtz Association and the Deutsches Zentrum für Luft- und Raumfahrt.
The map primarily incorporates data supplied by NASA’s Mars Global Surveyor (MGS), Mars Odyssey (ODY), and Mars Reconnaissance Orbiter spacecraft and by the European Space Agency’s Mars Express orbiter. This global map of Mars is the first to be drafted in a fully digital environment using geographic information system software tools, which will facilitate application and incorporation of the map into other research.
Mapping of Mars Matures
Two previous global geologic maps of Mars have been published. Each map represents a culmination of geomorphologic mapping and surface dating results following a major phase of spacecraft exploration. The first map was based on relatively low resolution Mariner 9 image data in which geologically diagnostic landforms are discernible but to a limited degree [Scott and Carr, 1978]. A highlight of the map was the establishment of the first global chronologic units as exemplified in terrains of progressively lower crater density (and thus younger relative age)—the Noachian, Hesperian, and Amazonian Periods.
The next map incorporated Viking Orbiter image data about an order of magnitude higher in spatial resolution, which provided for much improved landform recognition and crater counting [Scott and Tanaka, 1986; Greeley and Guest, 1987; Tanaka and Scott, 1987]. Also, the larger scale of the map allowed for greater detail in map unit subdivision and in outcrop mapping. A more detailed chronology was determined, leading to subdivision of the three periods into eight epochs. Initially hand drafted on an airbrushed, shaded-relief base (as was the preceding map), the map was later digitized and eventually spatially corrected to the topographic surface produced by the Mars Orbiter Laser Altimeter (MOLA) on board MGS.
The current map incorporates an unprecedented wealth of spatially registered data, not only at higher resolution but also with greater diversity in type. Along with MOLA data, the map utilizes global, 100 meters per pixel image mosaics derived from ODY’s Thermal Emission Imaging System infrared images. These data sets permitted consistent landform visualization at generally higher resolution and accuracy and with a great reduction in detrimental atmospheric haze and illumination effects compared with previous mapping data sets.
This visualization capability is critical for defining and distinguishing map units and features defined primarily by geomorphologic characteristics and for determining relative-age relationships among the units and features. Locally, much higher resolution (5–6 meters per pixel) images provided by the Context Camera were available for more accurate mapping, as needed.
In addition, results of other spacecraft-based investigations—including studies of diagnostic landforms, surface mineralogy and elemental composition, and radar sounding of ice-rich substrates—were incorporated for improved characterizations of map units and structures. Geographic names portrayed on the new map follow the latest definitions established by the International Astronomical Union.
The approach for relative dating of units was multifold, yielding the most comprehensive, integrated surface dating of the planet thus far achieved. First, map unit and feature overlap and crosscutting relationships were compiled. Second, detailed crater dating was performed for 48 sites that represent crater-age type localities for many of the map units. Third, the global crater database of Robbins and Hynek  was used to determine cumulative crater densities at definitive diameters greater than 1 kilometer for all outcrops. Fourth, applicable results in other published studies of Martian chronology, including a host of post–Mariner 9 geologic maps, were incorporated.
Getting to Know Mars Better
In detail, the new map portrays Mars much differently than its predecessors and with improved understanding. For example, the Early Noachian highland unit in the new map comprises most of the oldest materials exposed on Mars and covers 11% of the planet yet was mapped previously by Scott and Tanaka , Greeley and Guest , and Tanaka and Scott  as mostly Middle and Late Noachian units. All well-preserved impact craters and basins greater than 150 kilometers in diameter are now dated, and they document a rapid decline in basin formation rate following the Early Noachian Epoch. Noachian units as a whole are now understood to make up about 45% of the Martian surface (versus 40% previously), whereas Hesperian units are correspondingly less extensive. Some surfaces are now known to be highly time transgressive, such as the Amazonian and Hesperian lava flow fields of the expansive Tharsis rise. Further advances will be gained as the map is used as a backdrop for continued analysis of the planet’s makeup and history.