**GEOLOGIC MAP**

**OF THE**

**BOREALIS REGION**

**(H-1)**

**OF MERCURY**

By

Maurice J. Grolier and Joseph M. Boyce

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--1660, 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**

ps SMOOTH PLAINS MATERIAL---Forms flat to gently rolling plains at
resolution of Mariner 10; commonly occurs in low depressions such as
Borealis and Suisei Planitiae, in basins such as Goethe (FDS 160), and
in large cl to c4 craters (FDS 156). Intermediate albedo. Characterized
by sharp or gradational overlapping contacts with older terrain and by
sparsity of c1 to c3 craters, except for subjacent ghost craters.
Sinuous ridges and scarps common. *Interpretation:* Probably lava; no
volcanic vents observed. Two very long but discontinuous scarps
concentric to Goethe Basin and one circular scarp bounding shieldlike
platform around crater Depréz may represent fronts of viscous volcanic
lava flows that crossed Borealis Planitia. Alternatively, may consist of
fluidized shock-melted material

psi INTERMEDIATE PLAINS MATERIAL---Forms flat to gently rolling plains
north and northeast of Suisei Planitia (FDS 157, 161). Albedo similar to
that of smooth plains material. Contains fewer superposed craters than
intercrater plains material and more than smooth plains material.
Embayment relations with older and younger plains materials unclear,
except along western border of Borealis Planitia. *Interpretation:*
Uncertain: may consist of impact materials ranging from basin and crater
ejecta, similar to some facies in Caloris Basin ejecta, to shock melt
and breccia; may also consist in part of volcanic materials.
Intermediate crater density may indicate age transitional between
intercrater plains and smooth plains materials

pi INTERCRATER PLAINS MATERIAL---Forms hummocky, pitted, locally grooved
and lineated plains mostly west of the 100° meridian (FDS 152, 165).
Albedo similar to that of smooth plains and intermediate plains
materials. High density of superposed, overlapping craters 5 to 10 km in
diameter; most craters elliptical, shallow, and open on one side,
superposed on degraded remnants of larger craters. Unit overlaps or
overlies rounded rims of ghost craters in Suisei Planitia and north of
Goethe Basin. *Interpretation:* May be mixture of basin and crater
ejecta and volcanic deposits emplaced during terminal phase of heavy
bombardment

**BASIN AND CRATER MATERIALS**

CRATER MATERIALS---Craters \>30 km in diameter showing various degrees
of degradation and burial. Subscripts refer to degradation sequence from
cl (most degraded) to c4 (least degraded). *Interpretation:* Formed by
impact. Degree of degradation is assumed to represent relative age (c1
oldest to c4 youngest) of craters of closely similar diameters (McCauley
and others, 1981)

c4 Material of craters with scalloped or slightly subdued rims, terraced
walls, and terraces disrupted by slumping---Central peak or peaks
usually present; sharp contact of wall with floor.

*Interpretation:* Rim material consists of overturned flap and
fall-back; upper part of wall is bedrock; slump and colluvium along
footwall and base of terraces. Floor consists of brecciated bedrock and
fallback ejecta; floors partly flooded with smooth plains material
representing impact melt or post-impact lava flows

cp4 Central peak material of c4 craters---Peaks either single or more
rarely in clusters of multiple peaks that rise above crater floors near
their centers (Verdi, FDS 166; Jókai, FDS 161). *Interpretation:* Formed
by rebound after impact

cf4 Crater floor material of c4 craters---Material of some crater floors
is hilly, hummocky, or rolling; other crater floors surfaced by smooth
plains material. *Interpretation:* Hummocks are blocks of fallback
ejecta; smooth surface represents resurfacing by shock melt or lava
flows

cr4 Crater ejecta material, radial---Forms well-developed, radially
lineated, distinctive annulus adjacent to rims of large c4 craters.
*Interpretation:* Ballistically emplaced ejecta, mixed with precrater
material: the small-crater equivalent of lineated facies of Van Eyck
Formation (McCauley and others, 1981) surrounding Caloris Basin

cs4 Secondary crater material---Forms crater chains or clusters of
small, fresh craters oriented radially to and generally related to
primary craters Verdi and Jókai. Only a few major chains of secondary
craters around Verdi and Jókai mapped. *Interpretation:* Nearby and
farflung secondary ejecta from primary impacts

c3 Material of craters with low, rounded, but continuous rims (FDS
156)---Floors flat, partly filled with smooth plains or intermediate
plains materials. Wall terraces rare, but radial channels on wall common
at Mansart; floor-wall boundary indistinct in many craters.
Discontinuous field of secondary craters common. Moderate number of
superposed craters

cp3 Central peak material of c3 craters---Same as cp4 except occurs
within c3 craters. Peaks either single or multiple

cf3 Crater floor material of c3 craters---Same as cf4 except occurs
within c3 craters

c2 Material of pan-shaped craters with low rims covered with superposed
primary and secondary craters (FDS 165)---Degraded wall terraces. Crater
floor flooded with intercrater plains and smooth plains materials. No
visible secondary craters

cp2 Central peak material of c2 craters---Same as cp3 and cp4 except
occurs in c2 crater Turgenev

c1 Material of flat-floored craters with no central peak and with low,
discontinuous rims (FDS 164)---No wall structure; no field of secondary
craters. Crater floor flooded with smooth plains and intercrater plains
materials. *Interpretation of units c3 through cl1* Materials of impact
craters displaying various stages of degradation and infilling. Bimodal
morphologic classification of craters \>100 km and \<100 km after
McCauley and others (1981)

cg Material of ghost craters---Cannot be dated by usual sequence of
degradation; rounded rims of intercrater plains material

embayed by smooth plains material (FDS 162). Age may range from c1 to
late c3. *Interpretation:* Material of crater rims, including overturned
layers, primary and secondary crater ejecta, and superposed plains
material

> CONTACT---Dashed where approximately located; dotted where
> obliterated; queried where probable
>
> FAULT---Bar and ball on downthrown side
>
> LINEAMENT---Probably a fault with no vertical relief or a fault of
> uncertain displacement. May include unresolved crater chains of
> secondary impact origin
>
> NARROW TO BROAD RIDGE---Symbol on ridge crest. Interpreted within
> smooth plains material mare-type wrinkle ridge
>
> RIDGE SCARP, LINEAR TO ARCUATE---Line marks base of slope. Barb points
> downslope; dotted where concealed. Interpreted within smooth plains
> material as distal edge of lava flow
>
> FISSURE OR TROUGH OF STRUCTURAL ORIGIN, LINEAR TO ARCUATE---Barbs
> point downslope
>
> CRATER RIM CREST
>
> CRATER RIM CREST, GREATLY SUBDUED OR BURIED---In Borealis Planitia and
> Goethe Basin, symbol represents ghost craters
>
> CRESTLINE OF GOETHE BASIN---Much subdued, only approximately located
>
> AREA OF BRIGHT CRATER-RAY MATERIAL---Interpreted as ejecta from very
> fresh impacts
>
> AREA OF ANOMALOUSLY LOW ALBEDO

**INTRODUCTION**

The Borealis region surrounding the north pole of Mercury contains the
Goethe Basin, whose diameter of at least 400 km makes it the sixth
largest impact basin observed on Mariner 10 images (Murray and others,
1974; Strom and others, 1975; Trask and Guest, 1975; Boyce and Grolier,
1977; Strom, 1977, 1979). The west half of the mapped area (between long
100° and 190° W.) is dominated by older craters of c1, c2, and c3 ages
and by intercrater plains material that lies between and within them.
Younger crater materials of c4 age, intermediate plains material, and
small patches of smooth plains material are superposed on all other
units. The c4 crater Verdi, 122 km in diameter, is the largest of the
younger craters. Its extensive ejecta blanket and secondary crater field
are superposed on plains materials and older craters.

The east half of the mapped area (between long 0° and 100° W.) is
characterized by smooth plains material (Murray and others, 1974; Trask
and Strom, 1976). This unit covers vast expanses of Borealis Planitia, a
depression about 1,000 km in diameter that has an irregular arcuate west
boundary. This depression is located over the site(s) of one or several
old impact structures (Trask and Strom, 1976; Boyce and Grolier, 1977;
Strom, 1979).

In the Borealis region, Mariner 10 images are available for only the
western hemisphere, from long 0° to about long 190° W. Mercury was in
darkness beyond long 190° W. on March 29, 1974, when the first Mariner
10 flyby acquired the most useful photographs of the region.

Most of the photographs used for geologic mapping were acquired by the
departing spacecraft during the first pass (Mercury I). The Mercury II
encounter provided no usable images of the map area; two low-oblique
photographs suitable for geologic mapping were acquired during the third
flyby on March 17, 1975 (Davies and others, 1978, p. 31). No
stereoscopic photographic pairs are available for the Borealis region.

Because the terminator was a few degrees away from the 0°-180° meridian
at the time of the first encounter, photographs of the region were
acquired under a wide range of lighting conditions. These conditions and
the large obliquity of the photographs hampered geologic interpretation
of surface materials in the map area, as they did in the Kuiper (De Hon
and others, 1981), Victoria (McGill and King, 1983), and Shakespeare
(Guest and Greeley, 1983) quadrangles to the south.

Mercury's Equatorial plane is inclined less than 2° to its orbital plane
(Klaasen, 1976; Murray and others, 1981, p. 28); its rotation period of
58.64 terrestrial days is in two- thirds resonance with its orbital
period of 87.97 terrestrial days (Colombo, 1965; Colombo and Shapiro,
1966; Davies and others, 1978, p. 31). The resulting lag and orbital
eccen- tricity create a variation of mean temperature not only with
latitude, as on the Earth, but also with longitude. However, because of
Mercury\'s relatively slow rotational period, diurnal variations in
temperature probably greatly exceed mean-temperature variations along
latitude and longitude, even in the high latitudes. Its pronounced
orbital eccentricity (0.2563) causes the apparent solar intensity at
Mercury to vary by more than a factor of 2 throughout a mercurian year
(Davies and others, 1978, p. 2), corresponding to about a 20 percent
change in equilibrium temperature. Further, conservation of orbital
angular momentum and spin-orbit coupling cause considerable variation in
the length of daylight. Dawns and sunsets are prolonged by the long
transit time of the Mercurian horizon across the solar

disk, so that daylight is lengthened and nightime reduced by several
terrestrial days at sunset and vice versa at sunrise (Robert Wildey,
U.S. Geological Survey, oral commun., 1982). Despite these
considerations and despite the daily range in surface temperatures of
several hundred degrees Kelvin, the subsurface temperature in the polar
regions always remains well below freezing (Murray, 1975).

**STRATIGRAPHY**

Within the Borealis region, three widespread plains units are recognized
largely by their obvious differences in crater density, which is closely
related to relative age (Soderblom and Boyce, 1972). From most heavily
cratered (oldest) to least cratered (youngest), these units are
intercrater plains material (unit pi), intermediate plains material
(unit psi), and smooth plains material (unit ps). Visual identification
is confirmed and refined by actual crater counts (table 1). If we use
the lunar surface as a frame of reference, the crater density of
Mercurian plains in the Borealis region is bracketed by that of the
lunar uplands, the most heavily cratered lunar surface, and that of
Oceanus Procellarum, a moderately cratered mare surface (figs. 1, 2, and
3). The curve for the lunar uplands was derived from crater counts in
the region northwest of crater Tsiolkovskiy, between crater Mendeleev
and Mare Smithii. The curve for the southeastern part of Oceanus
Procellarum was obtained in an area centered near lat 2°00\' N. and long
31°00\'W., south of the crater Kunowsky. Ocean Procellarum has long been
considered close to the "average lunar mare" (Hartmann, 1966, 1967); its
crater density is intermediate between those of the heavily cratered
Mare Tranquillitatis and the lightly cratered Mare Serenitatis.

TABLE 1.---*Crater counts in the Borealis region*

Stratigraphic Counting Cumulative Number of Probable

unit area (A) number (N) craters error

(in km2) of craters (diameter √[N]{.underline}

(diameter ≥10 km) A

≥10 km) per unit area

counted (106 km2)

Smooth

plains

material 128,900 12 93 ±27

Intermediate

plains

material 136,800 26 190 ±37

Intercrater

plains

material 236,000 61 258 ±30

Material of Borealis Planitia was not included in the smooth plains
count because images of the area were blurred by spacecraft motion, and
so reliable crater counts could not be obtained. However, smooth plains
south

of lat 65° N. in the Shakespeare quadrangle, in the crater Strindberg
and in Suisei Planitia, are included in these counts. Craters ≥10 km in
diameter that are superposed on the material within the floors of
craters Strindberg and Turgenev are included in the plots shown in
figure 3.

The plains materials that lie outside Borealis Planitia are distributed
in irregular belts, which are subparallel to the terminator and to one
another. Eastward from long 190° W., the following belt pattern is
observed: intercrater plains material, intermediate plains material, and
again intercrater plains material. All three belts extend southward into
the Shakespeare quadrangle (Guest and Greeley, 1983).

Distinguishing one type of plains material from another by variations in
roughness and crater density is highly dependent on the resolution and
lighting conditions of individual Mariner frames (Schaber and McCauley,
1980). This constraint is well documented for the Moon (Masursky and
others, 1978, p. 80--81) and for Mars (Boyce and others, 1976). In the
Borealis region, where intercrater and intermediate plains materials
were imaged at an increasingly low sun angle close to the terminator,
the number of observable small craters increase with decreasing distance
from the terminator and concomitantly decreasing sun angle. This
discrepancy in the apparent abundance of craters occurs only for craters
that have small diameters and can be obviated by counting only craters
larger than 3 km in diameter.

**OLDER PLAINS MATERIALS**

The intercrater plains material (unit pi) is the oldest recognizable map
unit in the Borealis region. It lies between large craters from about
long 155° to long 190° W., and it also occurs between clusters of
closely packed and overlapping large craters west of crater Gauguin and
south and southeast of crater Mansart. The unit was described originally
by Trask and Guest (1975), who considered it to be the most widespread
unit on Mercury; Strom (1979) reported that this material covers
one-third of the surface viewed by Mariner 10. The principal morphologic
characteristic of the intercrater plains material is the high density of
superposed craters 5 to 10 km in diameter, which are commonly shallow
and elongate; probably they are secondary craters derived from nearby
large primary craters that are superposed on the unit. As one group, the
large craters and associated intercrater plains form some of the heavily
cratered terrain defined by Trask and Guest (1975).

The relative age and nature of intercrater plains material are as
uncertain in the Borealis region as they are elsewhere on Mercury. Strom
(1979) noted the similarity in surficial morphologies between mercurian
intercrater plains and pre-Imbrian pitted plains south-southwest of Mare
Nectaris on the Moon (Wilhelms and McCauley, 1971; Scott, 1972). The
pits in the lunar pre-Imbrian pitted plains are similar to the small
secondaries that pepper the surface of Mercurian intercrater plains
material. On the Moon, pre-Imbrian pitted plains material embays the
Janssen Formation (Scott, 1972), whose base is defined as the base of
the Nectarian System (Stuart-Alexander and Wilhelms, 1975). However, the
crater density of the intercrater plains material in the Borealis region
(fig. 1) matches that of an area on the far side of the Moon, in the
region northwest of crater Tsiolkovskiy bounded by crater Mendeleev and
Mare Smithii. This area is dominated by pre-Nectarian

unmantled terra and pre-Nectarian and Nectarian craters (Wilhelms and
El-Baz, 1977). The similarity in crater density of intercrater plains
material on Mercury and of pre-Nectarian terrain on the Moon is
geologically significant, inasmuch as it shows that the oldest
recognizable surfaces on both Mercury and the Moon went through similar
stages of crustal cratering, but not necessarily at the same absolute
geologic time. Differences in crater density as well as embayment
relations in the Borealis region show that the intercrater plains
material and the smoother intermediate plains material are younger than
many cl craters in the area northeast of crater Turgenev, and older than
smooth plains material in Borealis Planitia.

The relative age of intercrater plains material has a bearing on its
origin (Strom, 1979). If very old, intercrater plains material may
consist of anorthosite derived from a magma ocean such as may have
existed on the Moon (Wood and others, 1970). If emplaced during later
stages of mercurian evolution, it may consist of basin ejecta or lava
flows. However, planetwide, the morphologic evidence for an impact
origin rather than a volcanic one is not compelling (Strom, 1979).
Whether or not either hypothesis is eventually substantiated, the
emplacement of intercrater plains material likely began during an early
stage of intense accretionary bombardment (Malin, 1976; Guest and
O'Donnell, 1977) and lasted until the time of formation of intermediate
plains material.

This general conclusion seems to be supported in the Borealis region by
the relative scarcity of craters between 30 km and 60 km in diameter
(fig. 1). This scarcity may indicate resurfacing by crater overlap and
blanketing by crater ejecta or resurfacing by lava flows. Craters ≥60 km
in diameter on Mercury also are relatively scarce compared to similar
craters in the lunar uplands northwest of crater Tsiolkovskiy (fig. 1).
The reduced density of large craters and basins on Mercury relative to
the Moon could be either a function of different crater-population rates
on these bodies or an effect of different crustal histories (Schaber and
others, 1977).

Intermediate plains material (unit psi) has a roughness and crater
density transitional between intercrater plains material and smooth
plains material (fig. 2). In the Borealis region, the unit occurs in a
rather extensive belt that extends from the Shakespeare quadrangle into
Borealis north and northeast of Suisei Planitia. Intermediate plains
material was first recognized and mapped in the Tolstoj quadrangle
(Schaber and McCauley, 1980), where it primarily occurs on the floors of
cl, c2, and c3 craters. It was identified there by a lower crater
density than that of intercrater plains material and by "a lower
incidence of small bright-halo craters than are found on the smooth
plains material" (Schaber and McCauley, 1980). Both characteristics are
also typical of intermediate plains material in the Borealis region.

**BASIN MATERIALS**

Goethe Basin is a large circular depression that measures approximately
400 km in diameter from rim crest to rim crest. It was not listed as an
impact basin by Wood and Head (1976) because they considered the Mariner
10 photography too poor to confirm basin structures. However, most
workers, beginning with Murray and others (1974, p. 174), have
identified it as a basin. Goethe is bounded on its north and east sides
by a gently sloping wall and discontinuous, low, hummocky rim material
that may consist of ejecta deposits. These materials are similar to
those occurring around the Caloris Basin in the Tolstoj quadrangle
(Schaber and McCauley, 1980). On its west side, Goethe is bounded by at
least three subparallel ridges or tilted

blocks, which are separated by narrow troughs partly filled with smooth
plains material. If an inner concentric ring ever existed (Trask and
Strom, 1976), it is buried under the smooth plains material that now
extends across the basin. A narrow, concentric structural bench, in part
resurfaced by smooth plains material, is recognizable at the base of a
gently sloping and much degraded basin wall. Although rectilinear
mountain massifs and the radially lineated facies of basin ejecta of the
Caloris Croup surround the Caloris Basin (McCauley and others, 1981),
similar units cannot be unambiguously recognized around the Goethe Basin
(FDS 164). However, hilly and hummocky remnants resembling basin
deposits and ejecta protrude above the gently sloping basin wall. They
extend southwest and north of the basin beyond a much subdued, low,
barely perceptible rim crest for a distance of one-half to one-third of
the basin radius.

Goethe is older than the smooth plains material by which its wall, rim
crest, and most of its ejecta were partly buried. The outlines of the
rim crests of several adjacent craters are recognizable through the
smooth plains material that partly fills the Goethe Basin (H-1
photomosaic, Davies and others, 1978, p. 28). These buried craters
probably were formed on the basin floor after excavation of the basin
and were subsequently flooded to their rims by smooth plains material
(Strom and others, 1975). The terrain northwest of crater Depréz (FDS
160 and 164) is more hummocky than that farther south, suggesting that
smooth plains material northwest of Depréz is so thin that the older and
rougher topography of buried intercrater plains material protrudes
through it. The density and size of ghost craters within the Goethe
Basin are similar to those of the c2 craters superposed over intercrater
plains material near the terminator. These ghost (c2) craters and the
original intercrater plains material they characterize are younger than
the Goethe Basin, as they were not obliterated by the impact that formed
it. Therefore, Goethe may have impacted onto a surface older than
intercrater material and been partly filled by this material at a later
date. If so, the Goethe impact basin may be older than some intercrater
plains material and large c1 craters nearby. It is also much older than
the Caloris Basin, which was created during the time of formation of c3
craters (McCauley and others, 1981).

Several additional impact structures within and to the south of the
Borealis region display sufficient structural detail to be called
basins, even though their diameters are less than the arbitrarily chosen
200-km lower limit adopted by Murray and others (1974) for mercurian
basins. The largest and oldest of these is Botticelli, a crater 140 km
in diameter centered at lat 64°N., long 110°W. Only the northernmost
parts of the crater's rim and interior lie within the mapped area, but
the ghost remnant of an inner ring now flooded by smooth plains material
is recognized (FDS 148) farther south in the Shakespeare quadrangle.
Turgenev, 110 km in diameter, is large enough to be a central-peak basin
(Wood and Head, 1976), even though the peak ring probably has been
concealed under smooth plains material. The rims of both Botticelli and
Turgenev are covered with densely packed craters, most of which resemble
the secondary craters that typically occur on intercrater plains
material. Therefore, Botticelli and Turgenev are at least as old as
intercrater plains material and may be equivalent in age to the Goethe
impact basin. A similar argument can be advanced for the age of the
Monteverdi Basin, 130 km in diameter, centered at lat 64° N., long 77°
W. in the Victoria quadrangle. The younger craters Jókai and Verdi,
which have prominent central peaks and ghostlike discontinuous inner
rings, probably

qualify as central-peak basins (Wood and Head, 1976). Both structures
are considerably younger than the Caloris Basin.

No material similar to either the lineated or the secondary-crater
facies of the Van Eyck Formation, the most distinctive and distant unit
of the Caloris Group (McCauley and others, 1981), can be unambiguously
identified in the Borealis region. A few rounded hills or knobs, too
small to be mapped, are present; they are morphologically similar to
blocks of the Odin Formation surrounding the Caloris Basin in the
Shakespeare quadrangle (Guest and Greeley, 1983), and to features of the
Alpes Formation around the Imbrium Basin on the Moon. Two of the most
striking of these knobs are possibly 2 km long and 0.2 km across; they
rise above smooth plains material that fills a much degraded, unmapped,
irregular crater at 69° N., 157° W. (FDS 088). These knobs are about
1,100 km northeast of Caloris Montes and may represent Caloris Basin
ejecta. Alternatively, they may be associated with crater Verdi ejecta
or with lineated and secondary-crater ejecta that flare southeastward
from an unnamed crater north of and adjacent to the crater
Niz\\O(a,-)m\\O(i,-). Another morphologic feature that may be related to
the Caloris Basin event consists of grooves on intercrater plains
material and on the southwest-facing walls of craters such as Mansart.
These grooves are as much as several kilometers long and several hundred
meters wide. The direction of elongation of many small secondary craters
also suggests an origin related to the Caloris event.

**YOUNGER PLAINS MATERIAL**

Smooth plains material (unit ps) forms the vast expanses of Borealis and
Suisei Planitiae, as well as most basin and crater floors. It is the
most extensive stratigraphic unit in the Borealis region, covering 30
percent of the mapped area. The surface of the smooth plains material is
rather sparsely cratered compared to that of the intercrater plains
material (figs. 3, 1). Wrinkle ridges are common. Both the floor of the
Goethe Basin and the younger craters (now observed as buried craters)
superposed on it are mantled by smooth plains material; the unit also
fills ghost and flooded craters that are common on both Borealis and
Suisei Planitiae and resemble the lunar crater Archimedes. The enormous
volume of smooth plains material that must underlie Borealis Planitia in
order to bury pre-existing topography, as well as the presence of the
material in basin and crater floors, suggest that the smooth plains
material was emplaced in a fluidized state as volcanic lava flows
(Murray and others, 1974; Strom and others, 1975). Even though flow
fronts cannot be unambiguously mapped on Borealis Planitia, further
evidence of the units's volcanic origin is supplied by its overlap onto
intercrater plains material, best observed along the west edge of
Borealis Planitia (FDS 85, 152, 153, 156, and 160).

The various types of plains material recognized on Mercury exhibit
little tonal contrast. The albedo of smooth plains material is higher
than that of lunar mare material (Hapke and others, 1975). The
similarity in albedo between mercurian smooth plains material and lunar
light plains material led Wilhelms (1976) to extend the analogy to
composition: he suggested that both units consist of impact ejecta
similar to the lunar Cayley Formation sampled by Apollo 16. Wilhelms
(1976, p. 556) even hypothesized that the source basin for material of
the extensive plains of Borealis Planitia "could well be lurking in the
darkness beyond the terminator." A fuller discussion of the problem is
given by Strom (1979).

**CRATER MATERIALS**

In the Borealis region, craters are mapped according to the fivefold
classification proposed by McCauley and others (1981), which determines
Mercurian crater ages on the basis of crater diameter and morphologic
degradation. Craters less than about 30 km in diameter are not mapped.
All basins between 100 and 200 km in diameter (including those that have
central peaks and peak rings) are mapped as craters. Criteria used to
determine impact structures are morphologic crater components such as
rays, secondary rays, hummocky rims, various facies of crater ejecta,
crater geometry and structure, or a combination of these. The Verdi
crater is defined as the type example of a young, large, c5 crater by
McCauley and others (1981), but was mapped as a c4 crater in the
Shakespeare quadrangle (Guest and Greeley, 1983) prior to the release of
McCauley and others' crater classification. For this reason, the Verdi
crater is also mapped as a c4 crater in the Borealis region.

No rayed craters ≥30 km in diameter were observed in the mapped area,
but many moderately bright and diffuse rays extend across smooth plains
material or occur as halos around very small craters in Borealis
Planitia. The most conspicuous of these are indicated on the map. A
train of northeast-trending discontinuous rays, which extends across
Borealis Planitia as far as the Goethe Basin, may radiate from small
unnamed and unmapped rayed craters near the south edge of the map area.
The relative scarcity of small bright-halo craters on intermediate
plains material, perhaps due to unique physical properties of this
material, was first noted in the Tolstoj quadrangle (Schaber and
McCauley, 1980); this scarcity is also characteristic of the unit in the
Borealis region.

The reduced ballistic range on Mercury compared to the Moon is caused by
Mercury's stronger gravitational field (Gault and others, 1975; Strom,
1979; McCauley and others, 1981). This phenomenon, which results in a
reduced dispersion of ejecta and secondary craters, is best observed
within the Borealis region around craters Verdi (Gault and others, 1975;
Trask and Guest, 1975) and Depréz. Slight differences between mercurian
and lunar crater morphologies are unrelated to differences in the
Mercurian and lunar gravitational fields (Cintala and others, 1977;
Malin and Dzurisin, 1977, 1978; Strom, 1979). Instead, the morphologic
components of crater interiors and the abundance of central peaks and
terraces on both bodies seem to be related to the physical properties of
the target material (Cintala and others, 1977; Smith and Hartnell, 1978;
Strom, 1979).

The clusters of closely packed and overlapping large c2 and c3 craters
west of crater Gauguin and east of crater Mansart, together with nearby
isolated craters and surrounding material, were mapped by Trask and
Guest (1975) as heavily cratered terrain. According to them, many of the
small craters superposed on the intercrater areas may be secondaries
from the large c2 and c3 craters. They also noted that the interiors of
these large craters are filled with material that is less cratered,
smoother, and therefore younger than the intercrater plains material. We
have mapped these craters individually and have mapped the material
between them as intercrater plains or intermediate plains materials.

Two types of ghost craters occur in the Borealis region; both are nearly
obliterated by smooth plains material. In one type (unit cg) found along
the northwest border of Suisei Planitia (Guest and Greeley, 1983), only
the uppermost parts of walls and rims protrude above smooth plains
material. Ghost craters of this type display rounded rim crests that are
densely

cratered with secondaries, a feature typical of the rough surface of
intercrater plains material. These craters are floored by smooth plains
material and are therefore older than it; a similar relation occurs on
the Moon, where the crater Archimedes is seen to be older than the mare
material it contains. Another type of ghost crater, common in Borealis
Planitia, is recognized only by an irregular or thin outline of a rim
crest under a thin mantle of smooth plains material; the buried rim
crest is shown on the map. The polygonal ghost crater centered at lat
82.5° N., long 100° W., northwest of Depréz, is a transitional form
between these two types. Ghost craters in the Borealis region are
estimated to be of c1 to c3 age on the basis of degradation of their rim
crests, relative density of superposed craters on their rims, and
embayment by smooth plains material.

Polar darkening is generally lacking on Mercury (Hapke, 1977), but
darkening in restricted areas may be due to vapor-phase deposition
accompanying micrometeorite impacts (Hapke, 1977; Strom, 1979). In the
Borealis region, surface darkening affects some crater floors, and
low-albedo areas are mapped in both intermediate plains and smooth
plains materials. The low-albedo plains are marginal to the borders of
Borealis and Suisei Planitiae, which suggests that darkening may be due
to internal volatile materials escaping along the fractured margins of
unrecognized buried or very degraded basins.

**STRUCTURE**

One of the major differences between the mercurian and lunar surfaces is
"the widespread distribution \[on Mercury\] of lobate scarps which
appear to be thrust or reverse faults resulting from a period of crustal
compression\..." (Strom, 1979, p. 10--11). These scarps are unique
structural landforms that were noted soon after the acquisition of
Mariner 10 photographs. Murray and others (1974) described them as
having a sinuous outline, a slightly lobate front, and a length of more
than 500 km. A more detailed description is given by Strom and others
(1975). Dzurisin (1978) classified these scarps, differentiating between
intercrater and intracrater scarps (a scheme adopted in mapping the
Borealis region) in an attempt to understand the tectonic and volcanic
history of Mercury. Melosh (1977) and Melosh and Dzurisin (1978)
proposed a planetary grid composed of conjugate northeast- and
northwest-trending shear fractures formed by the stresses of tidal
despinning early in mercurian history. They thought that these fractures
were later modified, and predicted that east-trending normal faults
caused by tensional stresses would be found in the polar regions. In a
later report, Pechmann and Melosh (1979, p. 243) stated that "the NE and
NW trends become nearly N-S in the polar regions."

The northwest-trending component of the postulated global grid of
fractures is markedly absent in the Borealis region. Northeast-trending
scarps and troughs are conspicuous, however, across intercrater plains
material and in crater fill (smooth plains material) between the 155°
and 185° meridians, and from crater Van Dijck northward to crater
Purcell and beyond. The scarps tend to be straight in intercrater plains
material, but become notably lobate in crater fill (for example, within
Saikaku). This set of northeast-trending scarps and troughs, and another
set of north-trending scarps and troughs within and north of crater Van
Dijck, probably follow zones of structural weakness in the mercurian
crust. Ancient fractures that were reactivated by later impacts may have
first provided the conduits for crater fill (smooth plains material) and
later been propagated upward

through the fill. That these ridges, scarps, and troughs are parts of a
global grid of fractures cannot be stated conclusively because of their
proximity to the terminator and the lack of photographic coverage beyond
the 190° meridian. Some scarps probably were formed by normal faulting
of the smooth plains material that covers some crater floors, as in the
Kuiper quadrangle (Scott and others, 1980). We cannot, however,
determine whether most lineaments are internal or are parts of a faulted
and lineated facies associated with a nearby but unphotographed impact
basin. Melosh (1977) predicted that normal east-trending faults would
form in high Mercurian latitudes as a result of slight crustal
shortening. His predicted faults may be represented by a generally
east-northeast-trending scarp and a lineament that cut across
intermediate plains material and the crater Jókai between the 125° and
155° meridians. The north pole is too close to the terminator to detect
the presence or absence of a "polygonal arrangement without preferred
orientation," as predicted by Melosh and Dzurisin (1978, p. 233).

Arcuate and radial lineaments that might result from tectonic
adjustments of the Mercurian crust, following excavation of very large
multiring impact basins such as the one postulated under Borealis
Planitia (Trask and Strom, 1976; Boyce and Grolier, 1977; Strom, 1979),
were not unambiguously identified in the Borealis region. On one hand,
some ridges on the surface of the smooth plains material in Borealis
Planitia may be of structural (internal) origin; this type of ridge
elsewhere on Mercury has been ascribed to compression and a slight
shortening of the crust (Strom and others, 1975; Melosh, 1977; Melosh
and Dzurisin, 1978). On the other hand, the wrinklelike sinuous ridge
along the northeast border of the Goethe Basin, together with the
outward-facing concentric scarps along its periphery, may represent the
fronts of lava flows that are associated with the development of a
structural moat between the basin fill and the wall. The latter
interpretation supports the view that impact craters and basins on
Mercury, as on the Moon (Schultz, 1977) and Mars, "have played a
dominant role in controlling the surface expression of igneous activity"
(Schultz and Glicken, 1979, p. 8033). Slow, long-lasting isostatic
adjustment of the basin floor may have continued well after the
emplacement of the basin fill, a structural situation similar to that of
crater Posidonius on the Moon (Schaber and others, 1977, Schultz, 1977).

In Borealis Planitia, however, most of the ridges are of external
origin. They appear either to outline the rim crests of subjacent ghost
craters that are lightly mantled by smooth plains material or to be lava
flow fronts. The map shows the rim crests of 20 ghost craters, ranging
in diameter from 40 to 160 km, that are buried under the smooth plains
material of Borealis Planitia, which material is coextensive with the
fill covering the floor of the Goethe Basin. In addition, ejecta from
the crater Depréz extend more than 40 km eastward beyond a circular
scarp that may represent the rim crest of a buried crater 170 km in
diameter (FDS 156, 160) or, more likely, the front of lava flows. The
size and density of these ghost craters suggest that, prior to
emplacement of smooth plains material, the original heavily cratered
surface of Borealis Planitia---which may have been the cratered floor of
a very large multiring impact basin---and the cratered floor of the
Goethe Basin were similar in composition and age to the intercrater
plains material of the highlands to the west. Many scarps in Borealis
Planitia are subconcentric to the rim of the Goethe Basin and have
steeper slopes that face away from it, suggesting that they represent
the fronts of lava flows that resurfaced

extensive areas of heavily cratered terrain (intercrater or older plains
material).

**GEOLOGIC HISTORY**

Five periods were postulated by Murray and others (1975) to constitute
the history of Mercury's surface: (1) accretion and differentiation; (2)
terminal bombardment; (3) formation of the Caloris Basin; (4) flooding
of that basin and other areas; and (5) light cratering on the smooth
plains. Only the periods following accretion are directly interpretable
within the Borealis region.

Intercrater plains material, which may be a reworked and mixed aggregate
of impact and volcanic deposits, was emplaced over a long period that
extended past the creation of the Goethe Basin and many smaller c1 and
c2 basins and craters. The scarps and troughs that trend across
intercrater plains material may indicate an early compressional episode
that followed even earlier expansion and differentiation of the crust.

The size and density of ghost craters that are detectable under the
smooth plains material in the interior of the Goethe Basin are
indicative of an original basin floor much modified by cratering and
emplacement of intercrater materials prior to the emplacement of
intermediate and smooth plains materials. This interpretation implies,
therefore, that the formation of the Goethe Basin predated or occurred
soon after the emplacement of intercrater plains material had begun. The
relative similarity in albedo of the Mercurian plains, whether formed of
intercrater, intermediate, or smooth plains materials, also suggests a
similarity in chemical composition and possibly in mode of emplacement
of plains materials. The high crater density of intercrater and
intermediate plains materials makes it likely, however, that the
original rock types of these two units (whether basalt, impact melt, or
impact breccia) were modified considerably by further brecciation
following emplacement.

Morphologic and stratigraphic evidence from the Shakespeare and Tolstoj
quadrangles shows that the Caloris event occurred late in the time span
during which c3 craters were formed. As indicated above, our
interpretation is that the Goethe Basin is older than some intercrater
material and large c1 craters and, therefore, is considerably older than
the Caloris Basin. Emplacement of the smooth plains material of Borealis
Planitia during several or many episodes resulted in resurfacing and
smoothing of the original material of the Goethe Basin and its
surroundings for hundreds of kilometers.

The mercurian surface reached its present configuration several billion
years ago (Solomon, 1978). It has been only slightly altered since by
the impacts that created c4 craters, which are ubiquitously superposed
on all other deposits. Generalized summaries of the history of Mercury
have been given by Guest and O'Donnell (1977), Davies and others (1978),
and Strom (1979).

**REFERENCES**

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Boyce, J. M., and Grolier, M. J., 1977, The geology of the Goethe (H-l)
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237.

Cintala, M. J., Wood, C. A., and Head, J. W., 1977, The effects of
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Davies, M. E., Dwornik, S. E., Gault, D. E., and Strom, R. G., 1978,
Atlas of Mercury: National Aeronautics and Space Administration Special
Publication SP-423, 128 p.

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.

Gault, D. E., Guest, J. E., Murray, J. B., Dzurisin, Daniel, and Malin,
M. C., 1975, Some comparisons of impact craters on Mercury and the Moon:
Journal of Geophysical Research, v. 80. no. 17. p. 2444--2460.

Guest, J. E., and Greeley, Ronald, 1983, Geologic map of the Shakespeare
quadrangle of Mercury: U.S. Geological Survey Miscellaneous
Investigations Series Map I-1408, scale 1:5,000,000.

Guest, J. E., and O'Donnell, W. P., 1977, Surface history of Mercury: A
review: Vistas in Astronomy, v. 20, p. 273--300.

Hapke, Bruce, 1977, Interpretations of optical observations of Mercury
and the Moon: Physics of the Earth and Planetary Interiors, v. 15, p.
264--274.

Hapke, Bruce, Danielson, G. E., Jr., Klaasen, Kenneth, and Wilson,
Lionel, 1975, Photometric observations of Mercury from Mariner 10:
Journal of Geophysical Research, v. 80, no. 17, p. 2431--2443

Hartmann, W. K., 1966, Early lunar cratering: Icarus, v. 5, no.4, p.
406--418.

\_\_\_\_\_\_1967, Lunar crater counts, III: Post mare and "Archimedian"
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pt. 3, p. 125--129.

Klaasen, K. P., 1976, Mercury's rotation axis and period: Icarus, v. 28,
no. 4, p. 469--478.

Malin, M. C., 1976, Observations of intercrater plains on Mercury:
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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, 7 figs., 7 tables.

\_\_\_\_\_\_1978, Modification of fresh crater landforms: Evidence from
the Moon and Mercury: Journal of Geophysical Research, v. 83, no. Bl, p.
233--243.

Masursky, Harold, Colton, G. W., and El-Baz, Farouk, eds., 1978, Apollo
over the Moon: A view from orbit: National Aeronautics and Space
Administration Special Publication 362, 255 p.

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.

McGill, G. E., and King, E. A., 1983, Geologic map of the Victoria
quadrangle of Mercury: U.S. Geological Survey Miscellaneous
Investigations Series Map I-1409, scale 1:5,000,000.

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 despining?: Icarus, v. 35, no. 2, p. 227--236.

Murray, B. C., Belton, J. J. S., Danielson, G. E., Davies, M. E., Gault,
D. E., Hapke, Bruce, O'Leary, Brian, Strom, R. G., Suomi, Verner, and
Trask, Newell, 1974, Mercury's surface: Preliminary deseription and
interpretation from Mariner 10 pictures: Science, v. 185, no. 4146, p.
169--179.

Murray, B. C., Malin, M. C., and Greeley, Ronald, 1981, Earthlike
planets: San Francisco, W. H. Freeman and Co., 387p.

Murray, B. C., Strom, R. G., Trask, N. J., and Gault, D. E., 1975,
Surface history of Mercury: Implications for terrestrial planets:
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Schaber. G. G., Boyce, J. M., Trask, N.J., 1977, Moon-Mercury: Large
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the Earth and Planetary Interiors, v. 15, nos. 2--3, p. 189--201.

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quadrangle of Mercury: U.S. Geological Survey Miscellaneous
Investigations Series Map I-1199, scale 1:5,000,000.

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Wilhelms, D. E., 1976, Mercurian volcanism questioned: Icarus, v. 28,
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**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 polar stereographic projection is used for this sheet, with a scale
of 1:4,290,000 at lat 65°. 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 2 km, except for the area east of about long 90°. Pictures
of this area are so foreshortened that accurate map transformations were
not possible. Since the base mosaic was controlled by a later iteration
of the control net, discrepancies as large as 20 km exist between this
sheet and the Victoria (H-2) and Shakespeare (H-3) sheets to the south.
These discrepancies were adjusted within the area north of lat 70°.

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 polar stereographic 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 J. L. Inge.

NOMENCLATURE

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

H-l: Abbreviation for Mercury (Hermes) sheet number 1.

H 5M 90/0 G: Abbreviation for Mercury (Hermes) 1:5,000,000 series;
center of sheet, lat 90° N., long 0°; geologic map, G.

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