**GEOLOGIC MAP**

**OF THE**

**DISCOVERY (H-11)**

**QUADRANGLE**

**OF MERCURY**

By

Newell J. Trask and Daniel Dzurisin

1984

Prepared for the

National Aeronautics and Space Administration

by

U.S. Department of the Interior, U.S. Geological Survey

(Published in hardcopy as USGS Miscellaneous Investigations Series Map
I--1658, as part of the Atlas of Mercury, 1:5,000,000 Geologic Series.
Hardcopy is available for sale from U.S. Geological Survey, Information
Services, Box 25286, Federal Center, Denver, CO 80225)

Please direct questions or comments about the digital version to:

Richard Kozak

U.S. Geological Survey

2255 N. Gemini Drive

Flagstaff, AZ 86001

e-mail: rkozak@flagmail.wr.usgs.gov

> **DESCRIPTION OF MAP UNITS**
>
> **PLAINS MATERIALS**

pvs VERY SMOOTH PLAINS MATERIAL---Occurs only on floors of c5 craters
(FDS 27382). Moderate to low (0.13--0.16) uniform albedo; surface smooth
at all resolutions; very low density of small superposed craters. Sharp
contacts with central peaks and walls of craters. *Interpretation:*
Mostly impact melt pooled by fallback in crater floors; in large
craters, such as Equiano (lat 39° S., long 31° W.), may be volcanic,
induced by impact

ps SMOOTH PLAINS MATERIAL---Occurs throughout quadrangle in relatively
small patches within and between craters (FDS 166668). Crater density
1.3 x 102 (\>10 km)/106 km2; moderate albedo, but includes circular
patches and irregular blotches of brighter material; small tracts and
tracts near terminator do not show mottling (FDS 166619). Very gentle
rolling relief interrupted by rare scarps 100 to 600 m high (FDS 27424).
Sharp contacts with adjoining high ground. *Interpretation:* Mostly
volcanic; unequivocal volcanic vents and lava flow fronts not observed;
smaller patches (\<50 km) may be melted fallback or accumulations of
loose debris pooled in lows by secondary-impact events

psi INTERMEDIATE PLAINS MATERIAL---Occurs within cl to c3 craters and
basins (FDS 166675) where its crater density is higher than that of
nearby smooth plains unit and lower than that of nearby intercrater
plains unit (FDS 27428); also mapped as patches within intercrater
plains unit on basis of crater density; moderate albedo and some patches
of low albedo. Small pits and hills rare to common. Gradational contacts
with units of adjoining high ground. Mapped as cratered plains material
in Kuiper quadrangle (De Hon and others, 1981). *Interpretation:* Mostly
volcanic; smaller patches (\<50 km) may be melted fallback or
accumulations of loose debris pooled in lows by secondary-impact events.
Most occurrences within craters are clearly younger than intercrater
plains unit; tracts shown enclosed by intercrater unit may be
contemporaneous with it

pi INTERCRATER PLAINS MATERIAL---Forms widespread, gently rolling ground
between large craters and basins; especially extensive in northwestern
part of quadrangle where large cl and c2 craters are rare (FDS 166653).
Albedo medium to low (0.13--0.17); crater density 3.0 to 6.0 x 102 (\>10
km)/106 km2. Overlapped by c3 craters and basins. Rare subsequent scarps
and ridges. Most superposed craters less than 10 km in diameter are
shallow and irregular in shape, and many occur in strings and clusters,
suggesting secondary-crater morphology (FDS 27420). Mapped in part as
rough terra material in Kuiper quadrangle (De Hon and others, 1981) in
area of overlap of northern part of Discovery quadrangle.
*Interpretation:* Volcanic materials of regional extent; probably
includes unmapped tracts of intermediate plains material that may be
younger than cl and c2 craters

> **RELIEF-FORMING MATERIALS**

hl HILLY AND LINEATED MATERIAL---Limited to area near northeast corner
of quadrangle. Forms hills and intervening hollows 5 to 10 km wide;
hills are 0.1 to 1.8 km high (FDS 27422: 27463). Area cut by numerous
lineaments trending generally N. 50°--60° W., some lineaments consist of
open-ended, coalescing scalloped depressions. Preexisting crater rims
broken into hills and depressions like those on adjacent ground; no
undeformed c3 or older craters. *Interpretation:* Material subjected to
intense deformation and erosion,

perhaps by focusing of seismic energy at the antipode to the Caloris
Basin

ph HUMMOCKY PLAINS MATERIAL---Occurs only as patches within modified
craters on or near hilly and lineated material (FDS 27423). Forms level
areas studded with hummocks averaging 2 km in width; hummocks smaller
and farther apart than those of adjacent hilly and lineated unit.
*Interpretation:* Formed by same processes that formed hilly and
lineated unit but initial material different, resulting in smaller and
sparser hummocks; initial material may have been intermediate plains
unit

> **CRATER AND BASIN MATERIALS.**
>
> The morphology of virtually all mercurian craters and basins is
> comparable to that of lunar craters and basins, when allowance is made
> for differences in gravity on the two bodies, indicating that
> mercurian craters and basins almost certainly formed by impact. Crater
> and basin materials are mapped according to their relative ages as
> determined by apparent degree of degradation and local overlap
> relation. The age classification is that of N. J. Trask in McCauley
> and others (1981). Craters smaller than 30 km in diameter are not
> mapped

c5 Material of rims, walls, and floors of very sharp rimmed craters (FDS
27382)---Flat floors, well-defined wall terraces, sharp break between
wall and floor. Larger craters surrounded by well-defined continuous
field of relatively crisp satellitic craters. Some c5 craters can be
seen under high solar-illumination angles to have bright halos (albedo
0.20--0.27), rays, and areas of anomalously low albedo (FDS 166653)

cp5 Material of single or multiple rugged peaks near center of c5 crater

cr5 Material of radial-rim facies of very sharp rimmed
craters---Anastomosing radial ridges grade outward to irregular hills
and valleys that grade to individual satellitic craters (FDS 27382).
Around craters \<70 km in diameter, individual ridges and valleys cannot
be discerned, but unit appears to mantle surrounding topography

cs5 Material of small, crisp, elongate, and coalescing craters clustered
around or radial to large c5 craters

c4 Material of rims, walls, and floors of slightly subdued craters and
basins (FDS 166669)---Terraces and wall structure present; sharp break
at base of wall; more superposed craters than c5 class. Larger craters
and basins have flat floors, prominent, continuous rims, slightly
modified terraces or other wall features, and well-preserved, extensive
satellitic crater fields; many filled with plains, so central peaks and
inner rings, if present, are not evident

cp4 Material of single or multiple rugged peaks near center of c4 crater

cr4 Material of radial-rim facies of slightly subdued craters and
basins---Irregular ridges and valleys as much as 5 km across that are
roughly radial to c4 craters and grade outward to irregular, elongate,
subdued satellitic craters (FDS 27416)

cs4 Material of small, subdued, elongate, and coalescing craters roughly
radial to large c4 craters and basins

c3 Material of rims, walls, and floors of subdued craters and basins
(FDS 166650)---Low, rounded rims and some remnants of wall structures;
flat floors, many filled with plains materials; floor-wall boundary
indistinct in many craters. Larger craters and basins have modified but
intact rims, walls with discontinuous terraces, and discontinuous fields
of satellitic craters (FDS 166671)

cp3 Material of single or multiple subdued peaks near center of c3
crater

cr3 Material of radial-rim facies of subdued basins (FDS
27415)---Irregular hummocks and depressions with vague radial texture

cs3 Material of small, highly subdued, elongate, and coalescing---
craters roughly radial to large c3 craters and basins (FDS 166646)

c2 Material of highly degraded craters and basins (FDS 27418)---
Shallow, pan-shaped; no wall structure. Larger craters and basins have
flat floors and low but continuous rims bearing many smaller superposed
primary and satellitic craters; a few have low central peaks; one has
inner concentric ring; all flooded by plains materials

cp2 Material of isolated low peak near center of c2 craters and basins
and material of peak ring in crater Chekhov (lat 36° S., long 62° W.)

rrl Material of Raphael Basin---Lineated terrain with strings of
coalescing craters and a few elongate hummocks, radial to Raphael Basin,
which is located to north in Beethoven quadrangle (FDS 166640). Unit
poorly defined because of high sun angle

cs2 Material of small, highly subdued, elongate, and coalescing craters
roughly radial to large c2 basin (FDS 166685)

c1 Material of nearly destroyed craters and basins (FDS
274l9)---Relatively large, flat-floored; extremely low or discontinuous
rims rising only slightly above surrounding plains; all flooded by
plains units

> CONTACT
>
> FAULT---Bar and ball on downthrown side
>
> RIDGE---Barbs point downslope
>
> SCARP---Line at base of slope; barb points downslope
>
> FISSURE OR DEPRESSION OF STRUCTURAL ORIGIN---Found only in hilly and
> lineated material (unit hl)
>
> LINEAMENT---Trough, ridge, or scarp with low relative relief
>
> CRATER RIM CREST
>
> CRATER RIM CREST, GREATLY SUBDUED OR
>
> BURIED
>
> AREA OF BRIGHT CRATER-RAY MATERIAL---Shown around some craters too
> small to map; shown primarily in northwest quarter of map where
> illumination angle allows discrimination
>
> AREA OF ANOMALOUSLY LOW ALBEDO---Shown only in northwest quarter of
> map where illumination angle allows discrimination

**INTRODUCTION**

The Discovery quadrangle lies within the heavily cratered part of
Mercury in a region roughly antipodal to the 1300-km-wide Caloris Basin.
Like the rest of the heavily cratered part of the planet, the quadrangle
contains a spectrum of craters and basins ranging in size from those at
the limit of resolution of the best photographs (200 m) to those as much
as 350 km across, and ranging in degree of freshness from pristine to
severely degraded. Interspersed with the craters and basins both in
space and time are plains deposits that are probably of several
different origins. Because of its small size and very early segregation
into core and crust, Mercury seemingly has been a dead planet for a long
time---possibly longer than the Moon (Murray and others, 1975; Trask and
Guest, 1975). Its geologic history, therefore, records with considerable
clarity some of the earliest and most violent events that took place in
the inner Solar System.

**STRATIGRAPHY**

**CRATER AND BASIN MATERIALS**

As on the Moon and Mars, sequences of craters and basins of differing
relative ages provide the best means of establishing stratigraphic order
on Mercury (Pohn and Offield, 1970; Stuart- Alexander and Wilhelms,
1975). Overlap relations among many large mercurian craters and basins
are clearer than those on the Moon. Therefore, as this map shows, we can
build up many local stratigraphic columns involving both crater or basin
materials and nearby plains materials.

Over all of Mercury, the crispness of crater rims and the morphology of
their walls, central peaks, ejecta deposits, and secondary-crater fields
have undergone systematic changes with time. The youngest craters or
basins in a local stratigraphic sequence have the sharpest, crispest
appearance. The oldest craters consist only of shallow depressions with
slightly raised, rounded rims, some incomplete. On this basis, five age
categories of craters and basins have been mapped; the characteristics
of each are listed in the explanation. In addition, secondary crater
fields are preserved around proportionally far more craters and basins
on Mercury than on the Moon or Mars, and are particularly useful in
determining overlap relations and degree of modification.

**PLAINS MATERIALS**

All low-lying areas and the areas between craters and basins in the
Discovery quadrangle are covered by broadly level, plains-forming
material, except for small areas covered by the hilly and lineated
material and hummocky plains material described below. Tracts of plains
materials range in size from a few kilometers across to intercrater
areas several hundred kilometers in width. This material is probably not
all of the same origin. Strom and others (1975) and Trask and Strom
(1976) cited evidence that many large areas of plains are of volcanic
origin. Smaller tracts are more apt to be impact melt, loose debris
pooled in low spots by seismic shaking (Schultz and Gault, 1975), or
ejecta from secondary impacts (Oberbeck and others, 1977). The origin of
many individual tracts must necessarily remain uncertain without
additional information.

Plains materials have been grouped into four units on the basis of both
the density of super- posed craters and the relation of each unit to
adjacent crater and basin materials. These units are listed as follows
from oldest to youngest. (1) Intercrater plains material (unit pi) is
widespread, has a high density of small craters (5 to 15 km in
diameter), and appears to predate most of the relatively old and

degraded craters and basins, although some tracts of intercrater plains
material may be younger than some old craters. (2) Intermediate plains
material (unit psi) is less abundant than the intercrater plains unit
and has a density of superposed small craters that is intermediate
between those of the intercrater plains and smooth plains units. The
intermediate plains material is most readily mapped on the floors of
those c1, c2, and c3 craters and basins that are surrounded by
intercrater plains material with a distinctly higher crater density (FDS
27428). Contacts between intercrater plains and intermediate plains
units that occur outside mapped craters and basins are gradational and
uncertain. In parts of the quadrangle, photographic resolution and
lighting do not permit the intermediate plains unit to be separated from
the intercrater plains or smooth plains units with a high level of
confidence. (3) Smooth plains material (unit ps) occurs in relatively
small patches throughout the quadrangle on the floors of c4 and older
craters and basins and in tracts between craters. More bright-halo
craters occur on this unit than on either the inter- crater plains or
intermediate plains units. (4) Very smooth plains material (unit pvs)
occurs on the floors of some of the youngest craters. In summary, a
complex history of contemporaneous formation of craters, basins, and
plains is thus indicated by the mapping.

**RELIEF-FORMING MATERIALS**

The Discovery quadrangle includes some of the most distinctive
relief-forming material on the planet, the hilly and lineated terrain
unit mapped by Trask and Guest (1975). The unit consists of a jumble of
evenly spaced hills and valleys about equal in size. Most craters within
this material appear to predate its formation, and their ages cannot be
estimated: their rims have been disrupted into hills and valleys
identical to those of the hilly and lineated unit; the floors of some of
these degraded craters contain hummocky plains material (unit ph) that
resembles the hilly and lineated unit, except that the hills are fewer
and lower.

The hilly and lineated unit and the enclosed hummocky plains unit appear
to be relatively young; they may be the same age as the Caloris Basin.
In addition, they lie almost directly opposite that basin on the planet.
Both observations strengthen the suggestion that the hilly and lineated
unit and the hummocky plains unit are directly related to the formation
of Caloris (Schultz and Gault, 1975), possibly through the focusing of
seismic waves at the antipodal point.

**STRUCTURE**

Morphologically diverse scarps, ridges, troughs, and other structural
lineaments are relatively common in the Discovery quadrangle. Dzurisin
(1978) documented a well-developed pattern of linear lithospheric
fractures in the quadrangle that predate the period of heavy
bombardment. A dominant structural trend is recognized at N. 50°--45°
W., and subsidiary trends occur at N. 50°--70° E. and roughly due north.
Joint-controlled mass movements were most likely responsible for the
fact that many craters of all ages have polygonal outlines, and some
linear joints may have provided surface access for lavas that formed the
intercrater plains. Evidence of the latter may be recorded by several
linear ridges that may have been formed by lava accretion along linear
volcanic vents (for example, Mirni Rupes at lat 37° S., long 40° W., FDS
27420).

Planimetrically arcuate escarpments in the Discovery quadrangle cut
intercrater plains and crater materials as young as c4. These scarps are
typically 100 to 400 km long and 0.5 to 1.0 km high, and they have
convex-upward slopes in cross section that steepen from brink to base.
More trend closer to north-south than to east-west. Discovery (lat 55°
S., long 38° W.), Vostok (lat 38° S., long 20° W.), Adventure (lat 64°
S., long 63° W.), and Resolution (lat 63° S., long 52° W.) Rupes are the

most prominent examples in the quadrangle. Vostok transects and
foreshortens the crater Guido d'Arezzo, which suggests that arcuate
scarps are compressional tectonic features (thrust or high-angle reverse
faults). Melosh and Dzurisin (1978) have speculated that both arcuate
scarps and the global mercurian lineament pattern may have formed as a
result of simultaneous despinning and thermal contraction of Mercury.

Planimetrically irregular scarps on the floors of many plains-filled
craters and basins are the youngest recognized structural features in
the quadrangle, as they cut both the smooth plains and intermediate
plains materials. Their occurrence inside only smooth-floored craters
and basins suggests that the stresses responsible for their formation
were local in extent, perhaps induced by magma intrusion or withdrawal
beneath volcanically flooded craters.

**GEOLOGIC HISTORY**

Any reconstruction of mercurian geologic history must include the
inference that at an early time the planet was differentiated into a
core and crust. Mercury has a weak magnetic field (Ness and others,
1976) coupled with high density. Both facts can most easily be accounted
for by the presence of an iron core, possibly liquid, roughly 4,200 km
in diameter, overlain by a silicate crust a few hundred kilometers
thick. The postulated volcanic origin of a substantial fraction of the
Mercurian plains also implies a thick silicate crust, and thereby
supports the existence of a large iron core (Murray and others, 1975).

Early, rather than late, differentiation of Mercury is attested to by
the compressional scarps that are so clearly seen in the Discovery
quadrangle. Segregation of the core must have released large amounts of
heat, which would have resulted in significant expansion of the crust
(Solomon, 1976; Solomon and Chaiken, 1976). However, unambiguous
extensional features (very rare on the planet as a whole) are not seen
in the Discovery quadrangle; only compressional scarps occur. Thus, core
segregation occurred relatively early (before formation of a solid
lithosphere) and was followed by cooling and contraction, the last
phases of which probably contributed to the formation of arcuate scarps
that predated the end of heavy bombardment (Dzurisin, 1978).

Rotational breaking by solar torques is another process likely to have
occurred early in Mercurian history (Goldreich and Soter, 1966). With
the formation of a solid lithosphere, stresses induced by tidal
despinning most likely were sufficient to cause widespread fracturing.
Melosh (1977) has shown analytically that the expected pattern of
fracturing includes linear strike-slip faults oriented roughly N. 60° W.
and N. 60° E., and a younger set of thrust faults with east-west throw
and rough north-south trends. Melosh and Dzurisin (1978) have pointed
out the similarity between this predicted tectonic pattern and that
observed on Mercury, and they have proposed that the global system of
lineaments and arcuate scarps, which is well developed in the Discovery
quadrangle, formed in response to early, simultaneous planetary
contraction and tidal despinning.

The observable stratigraphic record in the Discovery quadrangle starts
with formation of the intercrater plains, parts of which may have been
coeval with the oldest observable craters. During this period, rates of
volcanism were probably high as heat from core formation was being
dissipated. If the crust was in a state of extension, there would have
been easy pathways for large volumes of magma to reach the surface. The
resulting plasticity of the crust probably caused large numbers of c1
and c2 craters to be destroyed by isostatic adjustment (Malin and
Dzurisin, 1977; Schaber and others, 1977), so the present inventory of
c1 and c2 craters may not be complete.

By c3 time, the rate of volcanism had declined although the impact rate
was still high. The preservation of many secondaries 1 to 5 km across
around c3 basins indicates that surface flows that would have
obliterated them were highly restricted. However, some degradation of c3
basins occurred by isostatic adjustment. Most of the intermediate plains
material formed at this time. Smooth plains material appears to be
largely coeval with c4 craters and basins. The crust was under
compression during c3 and c4 time, inasmuch as the compressional scarps
and ridges post- date some c3 and c4 craters, and are cut by some c4
craters and by c5 craters. Formation of intermediate and smooth plains
materials may have been abetted by the c3 and c4 crater- and
basin-forming events that opened up temporary magma conduits. One of the
latest large impacts was the Caloris event, which occurred on the other
side of the planet from the Discovery quadrangle and which may have
initiated formation of the hilly and lineated material within it.

Subsequent to formation of the smooth plains material, the Discovery
quadrangle underwent minor tectonic adjustments that formed scarps on
plains within craters. The very smooth plains unit was formed in some
young craters. The only other activity was a steady rain of relatively
small impacts, apparently at about the same rate as on the Moon.

**REFERENCES CITED**

De Hon R. A., Scott, D. H., and Underwood, J. R., Jr., 1981, Geologic
map of the Kuiper quadrangle of Mercury: U.S. Geological Survey
Miscellaneous Investigations Series Map I-1233, scale 1:5,000,000.

Dzurisin, Daniel, 1978, The tectonic and volcanic history of Mercury as
inferred from studies of scarps, ridges, troughs and other lineaments:
Journal of Geophysical Research, v. 83, no. B10, p. 4883--4906.

Goldreich, Peter, and Soter, Steven, 1966, Q in the Solar System:
Icarus, v. 5, p. 375--389.

Malin, M. C., and Dzurisin, Daniel, 1977, Landform degradation on
Mercury, the Moon, and Mars: Evidence from crater depth/diameter
relationships: Journal of Geophysical Research, v. 82, no. 2, p.
376--388.

McCauley, J. F., Guest, J. E., Schaber, G. G., Trask, N. J., and
Greeley, Ronald, 1981, Stratigraphy of the Caloris Basin, Mercury:
Icarus, v. 47, no. 2, p. 184--202.

Melosh, H. J., 1977, Global tectonics of a despun planet: Icarus, v. 31,
no. 2, p. 221--243.

Melosh, H. J., and Dzurisin, Daniel, 1978, Mercurian global tectonics: A
consequence of tidal despinning?: Icarus, v. 35, no. 2, p. 227--236.

Murray, B. C., Strom, R. G., Trask, N. J., and Gault, D. E., 1975,
Surface history of Mercury: Implications for terrestrial planets:
Journal of Geophysical Research, v. 80, no. 17, p. 2508--2514.

Ness, N. F., Behannon, K. W., Lepping, R. P., and Whang, Y. C., 1976,
Observations of Mercury's magnetic field: Icarus, v.28, p. 479--488.

Oberbeck, V. R., Quaide, W. L., Arvidson, K. E., and Aggarwal, H. R.,
1977, Comparative studies of lunar, martian, and mercurian craters and
plains: Journal of Geophysical Research, v. 82, no. 11, p. 1681--1698.

Pohn, H. A., and Offield, T. W., 1970, Lunar crater morphology and
relative-age determination of lunar geologic units---Part 1.
Classification: *in* Geological Survey research 1970, U.S. Geological
Survey Professional Paper 700-C, p. C153--C162.

Schaber, G. G., Boyce, J. M., and Trask, N. J., 1977, Moon-Mercury:
Large impact structures, isostasy and average crustal viscosity: Physics
of the Earth and Planetary Interiors, v. 15, nos. 2--3, p.189--201.

Schultz, P. H., and Gault, D. E., 1975, Seismic effects from major basin
formation on the Moon and Mercury: The Moon, v. 12, p. 159--177.

Solomon, S. C., 1976, Some aspects of core formation in Mercury: Icarus,
v. 28, p. 509--521.

Solomon. S. C., and Chaiken, John, 1976, Thermal expansion and thermal
stress in the Moon and terrestrial planets: *in* Lunar Science
Conference, 7th, Proceedings, Geochimica et Cosmochimica Acta,
Supplement 7, v. 3, p. 3229--3244.

Strom, R. G., Trask, N. J., and Guest, J. E., 1975, Tectonism and
volcanism on Mercury: Journal of Geophysical Research, v. 80, no. 17, p.
2478--2507.

Stuart-Alexander, D. E., and Wilhelms, D. E., 1975, The Nectarian
System, a new lunar time-stratigraphic unit: U.S. Geological Survey
Journal of Research, v. 3, no. l, p. 53--58.

Trask, N. J., and Guest, J. E., 1975, Preliminary geologic terrain map
of Mercury: Journal of Geophysical Research, v. 80, no. 17, p.
2461--2477.

Trask, N. J., and Strom, R. G., 1976, Additional evidence of mercurian
volcanism: Icarus, v. 28, no. 4, p. 559--563.

**NOTES ON BASE**

This map sheet is one of a series covering that part of the surface of
Mercury that was illuminated during the Mariner 10 encounters (Davies
and Batson, 1975). The source of map data was the Mariner 10 television
experiment (Murray, 1975).

ADOPTED FIGURE

The map projections are based on a sphere with a radius of 2439 km.

PROJECTION

The Lambert conformal conic projection is used for this sheet, with a
scale of 1:4,623,000 at lat 22.5° S. Latitudes are based on the
assumption that the spin axis of Mercury is perpendicular to the plane
of the orbit. Longitudes are positive westward in accordance with the
usage of the International Astronomical Union (IAU, 1971). Meridians are
numbered so that a reference crater named Hun Kal (lat 0.6° S) is
centered on long 20° (Murray and others, 1974; Davies and Batson, 1975).

CONTROL

Planimetric control is provided by photogrammetric triangulation using
Mariner 10 pictures (Davies and Batson, 1975). Discrepancies between
images in the base mosaic and computed control---point positions appear
to be less than 5 km. No attempt was made to resolve discrepancies in
feature positions on this sheet and those on the Kuiper (H-6) quadrangle
to the north and the Bach (H-l 5) quadrangle to the south. The latter
sheets were controlled by an earlier, more preliminary net.

MAPPING TECHNIQUES

Mapping techniques are similar to those described by Batson (1973a,
1973b). A mosaic was made with pictures that had been digitally
transformed to the Lambert conformal conic projection. Shaded relief was
copied from the mosaics and portrayed with uniform illumination with the
Sun to the west. Many Mariner 10 pictures besides those in the base
mosaic were examined to improve the portrayal. The shading is not
generalized and may be interpreted with nearly photographic reliability
(Inge, 1972; Inge and Bridges, 1976).

Shaded relief analysis and representation were made by Susan L. Davis.

NOMENCLATURE

All names on this sheet are approved by the International Astronomical
Union (IAU, 1977).

H-11 Abbreviation for Mercury (Hermes) sheet number 11.

H 5M --45/45 G: Abbreviation for Mercury (Hermes) 1:5,000,000 series;
center of sheet, lat --45°; long 45°; geologic map, G.

REFERENCES CITED

Batson, R. M., 1973a, Cartographic products from the Mariner 9 mission
Journal of Geophysical Research, v. 78, no. 20, p. 4424--4435.

\_\_\_\_\_1973b, Television cartography: U.S. Geological Survey
Open-File Report, Astrogeology 58, 35 p.

Davies, M. E., and Batson, R. M., 1975, Surface coordinates and
cartography of Mercury: Journal of Geophysical Research, v. 80, no. 17,
p. 2417--2430.

Inge, J. L., 1972, Principles of lunar illustration: Aeronautical Chart
and Information Center Reference Publication RP-72-1, 60 p.

Inge, J. L., and Bridges, P. M., 1976, Applied photointerpretation for
airbrush cartography: Photogrammetric Engineering and Remote Sensing, v.
42, no. 6, p. 749--760.

International Astronomical Union, 1971, Commission 16: Physical study of
planets and satellites, *in* 14th General Assembly, Brighton, 1970,
Proceedings: International Astronomical Union Transactions, v. 14B, p.
105--108.

\_\_\_\_\_1977, Working Group for Planetary System Nomenclature, *in*
16th General Assembly, Grenoble, 1976, Proceedings: International
Astronomical Union Transactions, v. 16B, p. 321--325, 330--332,
351--355.

Murray, B. C., 1975, The Mariner 10 pictures of Mercury---an overview:
Journal of Geophysical Research, v. 80, no. 17, p. 2342--2344.

Murray, B. C., Belton, M. J. S., Danielson, G. E., Davies, M. E., Gault,
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