DESCRIPTION OF MAP UNITS UNIT LABEL AND UNIT NAME AND DESCRIPTION ADDITIONAL INTERPRETATION [Note: Unit groups and labels are discussed in pamphlet. Unit definitions include morphologic character, infrared brightness or albedo (where diagnostic), nature IMAGE OF TYPE (coordinates of center(s) of type area(s) CHARACTERISTICS of stratification (if observed), and typical unit thickness (where top and bottom of unit identified and assuming flat-lying materials; measured or estimated using AREA and, where available, counting localities MOLA elevation data). Additional characteristics include location, complete record of observed superposition relations with other map units (except with unit AHi, (fig. 1 and table 2)) which displays complex age relations with other units), and other secondary and (or) local characteristics and associations including morphologies, spectrally VOLCANIC UNITS—Continued based compositional information, and radar-sounding properties. See Geologic Summary in pamphlet for further discussion of map units, including references; [Lava flows and pyroclastic deposits forming volcanoes. Flow fields erupted from fissures and centralized vents; may include local tables 2 (locality numbers) and 3 and fig. 1 for crater-density data; and tables 6 and 7 for comparison with Viking-based, global mapping units] pyroclastic deposits. Most occurrences are likely basaltic, based on rheologic properties estimated from morphologic measurements. Lava plains display low kilometer-scale surface roughness] UNIT LABEL AND UNIT NAME AND DESCRIPTION ADDITIONAL INTERPRETATION Hve Hesperian volcanic edifice unit— Occurs at Tharsis and Elysium rises, Volcanic edifices composed of different com- IMAGE OF TYPE (coordinates of center(s) of type area(s) CHARACTERISTICS Shield-like edifices several tens Hadriacus Mons, and Apollinaris Mons binations of lava flows and pyroclastic and AREA and, where available, counting localities to hundreds of kilometers across; south flank. Superposes unit eNhm; volcaniclastic deposits. Modified by summit (fig. 1 and table 2)) made up of lobate flows meters gradational with units lHv, lHvf, lHt, lHl, collapse from magma withdrawal and fluvial LOWLAND UNITS to tens of meters thick and tens eHv, and HNt (Hesperian part); over- dissection in places [Materials occupying northern lowlands (mostly –5,000 to about –4,000 m in surface elevation; low kilometer-scale surface roughness)] to hundreds of kilometers across, lain by unit AHv. Some outcrops have as well as dissected layers tens summit calderas tens of kilometers mAl Middle Amazonian lowland unit— Distributed within Vastitas Borealis Ice-rich loess. Periglacial modification of meters thick. As much as few across; some radiating valleys Hummocky to undulating; grades and other northern plains; makes up formed thumbprint terrain kilometers thick. (lat 24.65° N., into fields of knobs. Internally the platforms of nearby pedestal-crater long 146.80° E.) stratified. Tens of meters thick. forms and perhaps whorled, low-re- (lat 51.43° N., long 118.45° E.) lief ridge systems (thumbprint terrain, Nve Noachian volcanic edifice unit— Occurrences include Amphitrites and Volcanic edifices composed of different com- unmapped). Superposes units Av, AHv, Shield-like edifices several tens Peneus Paterae, Tyrrhenus Mons, binations of lava flows, pyroclastic deposits, eAb, lHl, lHt, Hpe, Hve, HNt, eHt, lNh, to hundreds of kilometers across; northern Apollinaris Mons, and southern and volcaniclastic deposits sourced from and mNh; underlies unit lApd; relation lobate flow morphologies indis- Tharsis rise. Gradational with units lNv, degraded shields. Domed and fractured by with unit Apu unclear tinct or absent. As much as few mNh, eNh, and HNt (Noachian part); local intrusions. Tectonically contracted kilometers thick. (lat –8.11° N., overlain by units HNt (Hesperian part), lHl Late Hesperian lowland unit— Continuous across most of the northern Fluvial/lacustrine/marine and colluvial sedi- long 174.50° E.) eHv, lHv, Hve, Htu, AHtu, and AHv. Planar to undulating; lobate plains. Embays units lHt, Hto, eHt, eHv, ments sourced from circum-lowland outflow Heavily cratered; some outcrops have and troughed marginal areas in HNt, lNh, mNhm, and mNh; superposed channels and bounding highland terrains; summit depressions tens of kilometers places. Hundreds of meters to by units AHv, Av, eAb, Apu, mAl, lAv, likely intercalated with and underlain by lava across; marked by irregular scarps and kilometers thick. (lat 21.40° N., and lApd; temporal relation to units Hpe and volcaniclastic rocks. Pervasively modi- ridges; some radiating valleys long 118.20° E.; localities 1, 6, and Hpu unclear. Contains hundreds fied and obscured by periglaciation, sedi- APRON UNITS 10, 11) of superposed pedestal-crater forms, mentary diapirism, and particulate mantling thumbprint terrain, topographically sub- lAa Late Amazonian apron unit— Covers northwest peripheries of Tharsis Drop moraines left by cold-based glaciers dued wrinkle ridges, and narrow grabens Concentrically ribbed, knobby and Olympus Montes. Superposes northeast of Alba Mons lobes extending as much as 500 units Aa, Ave, and AHv. Craters sparse; IMPACT UNIT km from shield-like edifices. underlying lobate flows only moderately Meters (or less) to tens-of-meters subdued by distal parts of unit AHi Amazonian and Hesperian impact Global occurrence. Superposes Upturned, ejected, and brecciated target thick. (lat 4.33° N., long 243.07° unit—Craters with rims and sur- Noachian units; other unit superposition rocks and sediments, with local areas of E.) rounding blankets; some include relations diverse. High kilometer-scale impact melt. Post-impact mass-wasting and single to multi-lobed blanket surface roughness; crater floors may be fluvial-lacustrine and eolian infill of craters Aa Amazonian apron unit—Lobes Surrounds Olympus Mons and covers Landslide and gravity-spreading deposits forms, dense secondary crater smooth to rough common bounded by scarps tens to hun- parts of walls and floors of Valles derived from flanks of Olympus Mons and Av lHt lHv chains, and (or) central peak or pit. dreds of kilometers across, marked Marineris. Superposes units , , , walls of Valles Marineris eHh Ht Htu lNh lNv Nhu Blanket thicknesses of meters to by transverse ridges few kilome- , , , , , and ; overlain lAv lAa lAvf AHtu a few hundred meters. (lat 23.17° ters wide and tens of kilometers by units , , , , and young- Ave N., long 207.77° E.) long; linear grooves in places. As est part of much as 1,000 m thick. (lat –8.04° POLAR UNITS N., long 281.80° E. and lat 30.17° [Mostly ice, dust, and other fines at polar latitudes; north polar outcrops occur within the northern lowlands and south polar outcrops are within N., long 214.65° E.) the southern highlands. Some of the units have a relatively low dielectric constant consistent with water ice and water ice and dust mixtures] lApc Late Amazonian polar cap unit— Thinly caps parts of Planum Boreum Residual ice (H2O in northern cap and mainly ANa Amazonian and Noachian apron Occurs in Deuteronilus and Protonilus Ice-rich Amazonian materials derived from Hummocky and pitted at meter and Planum Australe. Superposes units CO2 in southern cap) with dust lags trapped unit—Irregular knobs and mesas Mensae and east of Hellas Planitia. air-fall and mass-wasting erosion and trans- (northern cap) to hundreds of Apu and Hpu. Location of unit margin in north polar pits. Margins and surface of tens of kilometers across and Noachian part gradational with units ported by mass flow and underlying, relict meter (southern cap) scales. Very varies and south polar surface pits unit actively changing due to ice accumula- aprons extending tens of kilome- HNt, lNh, mNh, and eNhm and overlain Noachian highland materials. Modified by high albedo except on pit floors. enlarge seasonally and annually tion and ablation ters from them. Aprons hundreds by unit AHi. Amazonian part embays thermokarst processes Mostly <2 m (northern cap) to <10 of meters thick. (lat –43.22° N., units AHv, lHv, lHvf, lHt, eHv, eHt, m (southern cap) thick. (lat 83.00° long 26.66° E.) HNhu, lNh, mNh, and eNhm; gradational N., long 317.85° E. and lat –86.04° with HNt (Hesperian part). Aprons N., long 291.81° E.) sparsely cratered, locally grooved and pitted, and in places display low-ra- lApd Late Amazonian polar dunes unit— Surrounds Planum Boreum. Superposes Wind-blown sand organized into variety of dar-dielectric constant Mounds with barchan-like, linear, units mAl, lHl, Hpe, and most of Apu. dune forms owing to variable wind activity TRANSITION UNITS and other dune morphologies. Low Includes spectral detections of sulfates, and permafrost development [Materials occupying highland/lowland marginal zones and dissected areas of highland terrain (mostly just above –4,000 to –1,000 m albedo. Marked by meter-scale including gypsum surface elevation). Mass-wasting, sedimentary, and possibly volcanic origins likely. Units may include knobs and mesas of highland ripples. Mounds typically tens to materials too small to map separately] hundreds of meters across and tens of meters high. (lat 81.21° N., long AHtu Amazonian and Hesperian transi- Occurs along highland/lowland bound- Fine-grained eolian sediments and (or) pyro- 217.34° E.) tion undivided unit—Irregularly ary from Olympus Mons to south- clastic air-fall deposits. Linear ridges and shaped plateaus hundreds to more western Elysium Planitia and as one grooves form yardangs Apu Amazonian polar undivided unit— Forms