The distribution and origin of smooth plains on Mercury
Brett W. Denevi,1 Carolyn M. Ernst,1 Heather M. Meyer,1 Mark S. Robinson,2
Scott L. Murchie,1 Jennifer L. Whitten,3 James W. Head,3 Thomas R. Watters,4
Sean C. Solomon,5,6 Lillian R. Ostrach,2 Clark R. Chapman,7 Paul K. Byrne,5
Christian Klimczak,5 and Patrick N. Peplowski1
Received 31 October 2012; revised 26 March 2013; accepted 29 March 2013; published 6 May 2013.
[1] Orbital images from the MESSENGER spacecraft show that ~27% of Mercury?s
surface is covered by smooth plains, the majority (>65%) of which are interpreted to be
volcanic in origin. Most smooth plains share the spectral characteristics of Mercury?s
northern smooth plains, suggesting they also share their magnesian alkali-basalt-like
composition. A smaller fraction of smooth plains interpreted to be volcanic in nature have a
lower re?ectance and shallower spectral slope, suggesting more ultrama?c compositions, an
inference that implies high temperatures and high degrees of partial melting in magma
source regions persisted through most of the duration of smooth plains formation. The
knobby and hummocky plains surrounding the Caloris basin, known as Odin-type plains,
occupy an additional 2% of Mercury?s surface. The morphology of these plains and their
color and stratigraphic relationships suggest that they formed as Caloris ejecta, although
such an origin is in con?ict with a straightforward interpretation of crater size?frequency
distributions. If some fraction is volcanic, this added area would substantially increase the
abundance of relatively young effusive deposits inferred to have more ma?c compositions.
Smooth plains are widespread on Mercury, but they are more heavily concentrated in the
north and in the hemisphere surrounding Caloris. No simple relationship between plains
distribution and crustal thickness or radioactive element distribution is observed. A likely
volcanic origin for some older terrain on Mercury suggests that the uneven distribution of
smooth plains may indicate differences in the emplacement age of large-scale volcanic
deposits rather than differences in crustal formational process.
Citation: Denevi, B. W., et al. (2013), The distribution and origin of smooth plains on Mercury, J. Geophys. Res. Planets,
118, 891?907, doi:10.1002/jgre.20075.
1. Introduction
[2] Large portions of Mercury?s surface are covered by
smooth plains having densities of impact craters consistent
with ages younger than the cessation of the late heavy
bombardment of the inner solar system [Strom et al., 1975,
2008; Trask and Guest, 1975; Spudis and Guest, 1988;
Denevi et al., 2009; Head et al., 2011]. In Mariner 10
images of the hemisphere of Mercury from ~175
E to
345
E (from the eastern portion of the Caloris basin to just
east of Kuiper crater), smooth plains were observed to
display morphologies consistent with an origin by effusive
volcanism (e.g., ?ooding and embayment relationships and
lunar-mare-type contractional wrinkle ridges) [Murray
et al., 1974; Strom et al., 1975; Trask and Guest, 1975;
Trask and Strom, 1976; Spudis and Guest, 1988]. However,
on the basis of the nearly concurrent ?nding from the Apollo
16 mission that lunar light plains originated as ?uidized
impact ejecta rather than volcanic ?ows [e.g., Eggleton and
Schaber, 1972], an alternative hypothesis for the origin of
smooth plains on Mercury was proposed [Wilhelms, 1976].
Many of the large plains units imaged by Mariner 10 sur-
round the Caloris basin and lack clear albedo contrasts
indicative of lunar-like compositional differences. No
de?nitive volcanic landforms such as vents or domes were
seen in Mariner 10 images, and no craters that are both youn-
ger than the Caloris basin and older than the plains within
and around Caloris were found [Wilhelms, 1976]. On these
grounds, it was suggested that at least some of the smooth
plains on Mercury may have formed as ?uidized ejecta
1The Johns Hopkins University Applied Physics Laboratory, Laurel,
Maryland, USA.
2School of Earth and Space Exploration, Arizona State University,
Tempe, Arizona, USA.
3Department of Geological Sciences, Brown University, Providence,
Rhode Island, USA.
4Center for Earth and Planetary Studies, National Air and Space
Museum, Smithsonian Institution, Washington, D.C., USA.
5Department of Terrestrial Magnetism, Carnegie Institution of
Washington, Washington, D.C., USA.
6Lamont-Doherty Earth Observatory, Columbia University, Palisades,
New York, USA.
7Southwest Research Institute, Boulder, Colorado, USA.
Corresponding author: B. W. Denevi, Space Department, The Johns
Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins
Road, Laurel, MD 20723, USA. (Brett.Denevi@jhuapl.edu)
?2013. American Geophysical Union. All Rights Reserved.
2169-9097/13/10.1002/jgre.20075
891
JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS, VOL. 118, 891?907, doi:10.1002/jgre.20075, 2013
deposits from one or more basin-forming impact events in a
similar manner to the Cayley plains visited by Apollo 16
[Wilhelms, 1976].
[3] Mariner 10 images acquired through two color ?lters
(355 and 575 nm) initially showed an apparent lack of corre-
lation between color units and geologic units [Hapke et al.,
1980; Rava and Hapke, 1987]. When these images were
reprocessed with an improved calibration, however, the
smooth plains near Rudaki crater were found to display a
more steeply sloped spectral re?ectance at near-ultraviolet
to visible wavelengths, a characteristic interpreted to result
from a lower abundance of an opaque mineral component
[Robinson and Lucey, 1997]. This correlation of spectral
contrast with morphology is evidence for a compositional
difference between plains and their surroundings, as would
be expected for volcanic emplacement. Recalibrated Mariner
10 clear ?lter data also show that some plains units, such as
Borealis Planitia, have a higher albedo than average for
Mercury [Denevi and Robinson, 2008].
[4] Data from the MErcury Surface, Space ENvironment,
GEochemistry, and Ranging (MESSENGER) spacecraft
provide the evidence required to better discriminate the
origin of plains units. Volcanic landforms, including a small
shield volcano and other volcanic vents, have been identi?ed
within the Caloris basin and across the planet, and plains
units have been observed far from any large basins [Head
et al., 2008, 2009a, 2009b, 2011; Murchie et al., 2008;
Gillis-Davis et al., 2009; Kerber et al., 2009, 2011; Goudge
et al., 2012]. Color relationships also support a volcanic
origin. Plains units often exhibit contrasts in color from their
surroundings and have a range of spectral properties, from
the high-re?ectance red plains (HRP) to low-re?ectance blue
plains (LBP) [Robinson et al., 2008; Denevi et al., 2009],
where ?red? and ?blue? denote spectral slopes respectively
greater and less than average. The compositional differences
suggested by these color contrasts were con?rmed with
measurements from MESSENGER?s X-Ray Spectrometer
(XRS). HRP-type areas correspond to a low-Fe (no more
than 4wt% Fe) basalt-like composition, and regions with
lower re?ectance are also low in Fe but have higher Mg/Si
and Ca/Si and lower Al/Si and are interpreted to have a more
ultrama?c composition [Nittler et al., 2011; Weider et al.,
2012].
[5] Despite resolution of the volcanic origin of many
plains units, there remain terrains for which no de?nitive
evidence on origin has yet been found. The plains that sur-
round the Caloris basin in particular continue to challenge
simple interpretations. The crater population of these plains
suggests that they are younger than both the Caloris interior
plains and the basin itself [Murray et al., 1975; Strom et al.,
2008; Fassett et al., 2009]. If so, this age relation leaves
volcanism as the only plausible mechanism for the formation
of such relatively young, widespread deposits. However,
lunar studies indicate that there can be an ambiguity in the
ages of plains inferred to be related to major basin-forming
events. Some lunar light plains are estimated on the basis
of crater characteristics to be younger than the basins with
which they are spatially associated [Boyce et al., 1974] or
younger than all large basins [Neukum, 1977], perhaps
calling into question a straightforward interpretation of
the cratering histories of these regions or suggesting an alter-
native origin for some light plains [Neukum, 1977]. The
hummocky textures and kilometer-scale knobs found in some
portions of the circum-Caloris plains (Odin Formation) were
interpreted to have formed as ejecta from the Caloris impact
event, consistent with an impact origin for these plains [Trask
and Guest, 1975; McCauley et al., 1981; Guest and Greeley,
1983]. However, in some regions, the blocks may also have
been embayed and partially buried by later volcanic
resurfacing [Fassett et al., 2009]. The color properties in this
region and the variegated and diffuse boundaries of units also
provide no clear evidence for volcanism [Denevi et al., 2009].
[6] On the basis of Mariner 10 and MESSENGER ?yby
images, smooth plains were estimated to cover 15?40% of
Mercury?s surface [Strom et al., 1975; Spudis and Guest,
1988; Denevi et al., 2009]. Although ?yby images combined
to cover ~98% of the planet, some regions were seen at
illumination and viewing conditions that were not well
suited for observing morphology (i.e., large emission angles
and small solar incidence angles, both measured from the
surface normal, or pixel scales >1 km) and so were not
mapped [Denevi et al., 2009].
[7] Images acquired during MESSENGER?s orbital ope-
rations now provide the means to assess the spatial and
temporal distribution of plains units across Mercury in a
globally consistent manner. From imaging coverage acquired
at optimized geometries and near-complete topographic data,
we created a global map of the distribution of smooth plains
and constrained the range of their ages. From compositional
information obtained with multiple instruments, we examine
here the range of compositions of smooth plains units and we
assess possible variations in composition through time. The
goal of this work is to evaluate the history of plains formation
on Mercury recorded in these younger units, with
implications for older plains units, the history of which has
been partially clouded by the effects of the late heavy
bombardment. What processes have worked to create the sur-
face of Mercury that we see today?
2. Data and Methods
[8] As the basis for mapping the distribution of smooth
plains units onMercury, we used images fromMESSENGER?s
Mercury Dual Imaging System (MDIS), which includes a
narrow-angle camera (NAC; 1.5
 ?eld of view) and a wide-
angle camera (WAC; 10.5
 ?eld of view) [Hawkins et al.,
2007]. Surface morphology was analyzed using images from
the monochrome base map campaign, which covers more
Table 1. Crater Size?Frequency Distributions for Selected Plains
Units on Mercurya
Region
Central
Latitude
Central
Longitude
Area
(km2) N(10) N(20)
Rudaki plains 3
S 305
E 9.8 104 51 23 10 10
South of
Rachmaninoff
6
N 70
E 3.6 105 58 13 17 7
East of Caloris 28
N 194
E 2.1 105 56 16 19 9
West of Caloris 19
N 135
E 4.1 105 91 15 25 8
Caloris interior 31
N 163
E 1.7 106 80 7 26 4
South of Caloris 9
S 169
E 3.7 105 92 16 32 9
Beethoven 19
S 236
E 2.2 105 82 19 32 12
Rembrandt 33
S 87
E 2.9 105 103 19 45 12
Intercrater plains 16
N 23
E 1.0 106 217 14 94 9
aN(10) and N(20) values give the cumulative number of craters per mil-
lion square kilometers larger than 10 km or 20 km in diameter, respectively.
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
892
than 99% of the planet. During its primary orbital mission,
MESSENGER was in a highly eccentric orbit, with an
apoapsis altitude of over 15,000 km in the southern
hemisphere and periapsis altitude in the north as low as
200 km. The monochrome base map includes 25 mrad/pixel
NAC images (largely in the southern hemisphere) and
178 mrad/pixel WAC 750 nm ?lter images (mainly in the
northern hemisphere) acquired at low emission angles and
solar incidence angles targeted to be near 68
 so as to high-
light morphology. The combination of NAC and WAC
images and selective on-chip binning results in an average
pixel scale of 220m (resampled here to 250m/pixel). Aver-
age solar incidence and emission angles are 69
 and 11
,
respectively. At the time of this writing, MDIS is acquiring
a second monochrome base map with a larger average inci-
dence angle (80
), and these images were also consulted
where available. We supplemented the monochrome base
maps with WAC color image sequences (either three or eight
?lters between 430 and 1020 nm) acquired near nadir at small
solar incidence angles and with targeted NAC images at pixel
scales as small as 10m. Topographic information from the
Mercury Laser Altimeter [Zuber et al., 2012] and stereo
imaging [Oberst et al., 2010; Preusker et al., 2011, 2012;
Gaskell et al., 2011; Becker et al., 2012] was used to compare
relative elevations of terrain.
[9] We mapped the distribution of young plains on
Mercury at a scale of 1:1.25M, following unit de?nitions
established by Trask and Guest [1975] from Mariner 10
Figure 1. (top) Map of smooth plains (tan) on Mercury as de?ned in this study. As closely as possible, the
original extent of smooth plains was mapped, regardless of later modi?cation by impact craters. (bottom)
Smooth plains along with the approximate boundaries of Odin-type plains (turquoise). Superposed craters
>20 km in diameter along with crater materials (rims, terraces, peaks) are shown in navy blue and ejecta de-
posits in light blue. Also shown are locations of irregular, rimless depressions interpreted to be volcanic vents
(green circles) or ?probable? volcanic vents (yellow circles), for which our identi?cation of a vent-like feature
was more tentative. The maps are in a simple cylindrical projection; the central longitude is 180
E.
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
893
images. Smooth plains were de?ned as relatively sparsely
cratered, essentially level terrains that typically have distinct
boundaries with adjacent terrain [Trask and Guest, 1975].
Topographic information derived from stereo images and
laser altimetry con?rm the plains to be level over short
distances, but broad areas within Caloris and the north-
ern plains have substantial long-wavelength topographic var-
iations suggestive of post-emplacement deformation [Oberst
et al., 2010; Zuber et al., 2012], a ?nding that warrants some
modi?cation to this de?nition. Thus, we de?ne smooth
plains as smooth, relatively sparsely cratered terrain that
displays sharp boundaries with adjacent regions and is level
to gently sloped over a baseline of ~100?200 km. Trask and
Guest [1975] noted that an abundance of tectonic landforms,
including wrinkle ridges and scarps, often gives smooth
plains a gently rolling appearance, which is also consistent
with observations from MESSENGER data.
[10] Smooth plains are distinguished from older intercrater
plains, which are de?ned as level to gently rolling terrain
with an abundant population of secondary craters 5?10 km
in diameter, more gradational boundaries, and typical loca-
tions between and around large craters [Trask and Guest,
1975]. Later Mariner 10 quadrangle maps also included an
?intermediate plains? unit, with characteristics similar to
those of the smooth plains but with slightly more superposed
craters and more gradational boundaries [e.g., Trask and
Dzurisin, 1984]. On the basis of the Trask and Guest
[1975] de?nitions, however, intermediate plains would be
considered smooth plains, but substantial differences exist
between individual plains exposures [Whitten et al., 2012].
Figure 2. The range of plains morphologies included in our map, each in an equirectangular projection
centered on the indicated latitude and longitude. (a) An example of some of the youngest smooth plains south
of Rachmaninoff basin (5
N, 75
E). (b) Smooth plains within Beethoven basin (20
S, 245
E) that are more
heavily cratered than those in Figure 2a. These plains were originally mapped from Mariner 10 images as
intermediate plains [King and Scott, 1990] (see Figure 3). (c) Dashed white lines indicate the approximate
boundary of the most heavily cratered plains that we still consider to be smooth (35
N, 330
E). The incidence
angles for images in this region of the monochrome mosaic are >80
, making it more dif?cult to compare
directly regions seen at more typical incidence angles (average 69
). (d) Dashed yellow lines indicate the
approximate boundary between knobby Odin-type plains (center of image) and smooth plains found to the
southwest of the Caloris basin (20
N, 140
E). The Odin plains often display gradational boundaries with
the surrounding smooth plains, and nearby smooth plains often contain isolated knobs. The edge of Caloris
rim materials is seen in the northeast corner of this image, and in many places no clear embayment relation-
ships are observed between the circum-Caloris plains and the Caloris rim.
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
894
We assess the utility of dividing young plains into smooth
versus intermediate plains in section 3.1.
[11] In this study, we excluded isolated smooth plains
units (far from any other smooth plains deposits) that are
contained within a single impact crater and are smaller than
~100 km across (i.e., those most likely to have formed from
impact melt). As closely as possible, we mapped the original
extents of smooth plains, regardless of later modi?cation of
boundaries by impact craters (primary and secondary). In
some places, such mapping required estimation of where
the margin would be if not for the presence of a large impact
crater, or the assumption that smooth plains were contiguous
before formation of an impact crater. We then mapped
superposed craters larger than 20 km in diameter as ?crater
materials? (rims, terraces, central peaks, or peak rings), as
well as their ejecta deposits where such material
substantially altered the appearance of the plains. In this
way, we assessed both the original extent of smooth plains
and their present-day exposures. Given the uncertain origin
of knobby, Odin-type plains, we separately mapped their
extent within the circum-Caloris plains. However, as their
boundaries are in places gradational between smooth and
knobby plains, some isolated knobs are included in regions
mapped as smooth plains surrounding Caloris.
[12] In addition to considering the morphology and
qualitatively assessing the relative crater densities of plains
units, we measured the size?frequency distributions of
impact craters on key plains occurrences. We selected nine
representative regions (Table 1) ranging in size from
9.8 104 to 1.7 106 km2 and, from the monochrome base
map, recorded the sizes and locations of superposed craters
larger than 2 km in diameter, making an effort to avoid those
craters in chains or clusters most likely to be secondary in
origin. The interior plains of Caloris were included to
facilitate comparisons between our counts and those of
previous workers [Strom et al., 2008; Fassett et al., 2009].
[13] Finally, to aid in interpreting the origin of smooth
plains units, we mapped the distribution of volcanic vents.
Candidate vents are all irregular, rimless depressions
identi?able in the monochrome base map. As all such
features were identi?ed primarily from this mosaic, no
observational bias due to MESSENGER?s eccentric orbit
affects this data set. Many but not all of these depressions
are surrounded by distinctly colored (higher re?ectance and
steeper spectral slope) halos identi?able in the WAC color
base map. If a clear origin could not be determined from base
map imagery alone, targeted NAC images (if available) were
analyzed or the feature was categorized as a ?probable? vol-
canic vent. The landforms mapped included those identi?ed
previously as the sources of pyroclastic deposits [Kerber
et al., 2009, 2011; Goudge et al., 2012] and irregular
depressions that may have formed through collapse after
the subsurface withdrawal of magma from a magma cham-
ber [Gillis-Davis et al., 2009]. The features are distinguished
from ?hollows? (irregular depressions thought to form
from loss of a volatile material) by their larger size and lack
of shallow (blue) spectral slope [Blewett et al., 2011;
Goudge et al., 2012].
3. Results
3.1. Mapped Smooth Plains
[14] The distribution of smooth plains is shown in
Figure 1. Smooth plains, when mapped as closely to their
original extent as possible, cover 27% of the planet. An
additional 2% of the surface is mapped as Odin-type plains
with kilometer-scale knobs. Although we have excluded
isolated smooth plains units contained entirely within craters
<100 km in diameter, and it is likely that we have missed
small occurrences of smooth plains for which classi?cation
is debatable, the exclusion of such areas would not substan-
tially change our results. The range in appearance of plains
included in our map is shown in Figure 2, from plains with
sharp contacts and the lowest crater densities (e.g., Figure 2a)
through regions with sharp contacts but more superposed
craters (e.g., Figure 2b), to the most cratered regions we still
mapped as smooth plains (Figure 2c). The area shown in
Figure 2c is also an example of a region for which inclusion
Figure 3. Intermediate plains as mapped from Mariner 10
images [Grolier and Boyce, 1984; Spudis and Prosser,
1984; King and Scott, 1990]. (a) Plains within Beethoven
basin (20
S, 245
E) mapped as intermediate plains have
been included as smooth plains in this paper (see Figure 2b).
Several straight edges indicate boundaries between separate
quadrangle maps. (b) An example of intermediate plains
(75
N, 210
E) near Jokai crater that was not included in
our map of smooth plains. The region has a rougher surface
and lacks clear boundaries. Scale same as shown in (a).
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
895
in our map was potentially ambiguous. Whereas the area is
relatively heavily cratered, there are still large portions
where modi?cation by secondary cratering has been minor
and the boundaries with surrounding terrain are clear. This
area of the monochrome mosaic was imaged at larger than
average incidence angles (>80
), resulting in the appearance
of a rougher surface (as even small topographic features cast
long shadows), making a comparison with other smooth
plains imaged at moderate incidence angles dif?cult.
Finally, an example of Odin-type hummocky plains is
shown in Figure 2d. The boundaries of these plains are
often gradational, and their distribution shown in Figure 1
is approximate.
[15] In comparing our mapped plains in regions of imag-
ing overlap with Mariner 10, we ?nd that the distribution
of smooth plains is generally similar to that shown in earlier
maps. Some differences exist for regions mapped as interme-
diate plains on the basis of Mariner 10 data. We compared
our results with large regions of intermediate plains on the
Mariner 10 quadrangle maps, including the plains within
Beethoven basin and those near Jokai crater [Grolier and
Boyce, 1984; Spudis and Prosser, 1984; King and Scott,
1990] (Figure 3). By the classi?cation of Trask and Guest
[1975], the Beethoven plains (Figure 3a) were included in
our map as smooth plains, whereas the plains near Jokai
were not included and may be better classi?ed as intercrater
Figure 4. Locations of count areas for crater size?frequency distributions shown in Figures 5, 6, and 7
and listed in Table 1.
Figure 5. Crater size?frequency distributions for the range of smooth plains areas in Figure 4 show a
relatively narrow range of crater densities. Here and in subsequent ?gures, an unbinned cumulative his-
togram is shown on the left, and on the right is an R plot with root-two binning; errors shown equal
the square root of the number of craters in a given bin [Crater Analysis Techniques Working Group,
1979]. The crater size?frequency distributions of the smooth plains are most similar in shape to the
characteristic ?Population 2? [Strom et al., 2005] of surfaces formed largely after the cessation of
the late heavy bombardment.
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
896
plains, as they have a host of secondary craters and poorly
de?ned boundaries. We note that the variation in illumina-
tion and viewing conditions in Mariner 10 images is likely
to have contributed to the large range of surface roughnesses
in areas previously mapped as intermediate plains (the
Beethoven plains, in particular, were imaged when near the
subsolar point). Future mapping of these older regions may
permit intermediate and intercrater plains to be distin-
guished, but for the work here and the recent study by
Whitten et al. [2012], the de?nitions of smooth and
intercrater plains are suf?cient to describe the range of
morphologies observed.
3.2. Density of Impact Craters on the Plains
[16] As the de?nitions of plains are based in part on the
density of superposed impact craters, we examined whether
a quantitative assessment of crater size?frequency distribu-
tions could aid more qualitative visual measures for discrim-
inating smooth plains. For the representative smooth plains
regions (Table 1, Figure 4), there is a relatively narrow range
of variation in cratering history (Figure 5). The cumulative
number of craters larger than 20 km in diameter per million
square kilometers, N(20), provides a metric for comparing
the crater populations among smooth plains. The youngest
large exposures, a broad expanse of plains to the south of
Rachmaninoff basin and the plains near Rudaki crater, have
N(20) values in the range 10?17. The oldest large exposure
of smooth plains, that within Rembrandt basin, has an N(20)
value of 45 12. When the cumulative size?frequency
distributions are normalized to a power law slope of 3 on
a standard R plot [Crater Analysis Techniques Working
Group, 1979] (Figure 5b), all of the representative smooth
plains units have the characteristic ?Population 2? shape
indicative of an impactor population that largely postdated
late heavy bombardment [Strom et al., 2005]. In comparison,
the representative intercrater plains unit has an N(20)
value of 94 9, twice that of the most cratered smooth
plains. Its R plot indicates cratering by ?Population 1?
impactors during the late heavy bombardment, as found for
more heavily cratered terrain on Mercury (Figure 6) [Strom
et al., 2008, 2011; Fassett et al., 2011; Ostrach et al., 2011].
Thus, we ?nd a clear distinction in crater density between
smooth and intercrater plains. If volcanic in origin, this
population of smooth plains is analogous to the extant lunar
maria, emplaced largely after the cessation of the late heavy
bombardment.
[17] The circum-Caloris plains display a range of mor-
phologies from smooth to knobby. Whereas smooth plains
are de?ned as having sharp boundaries with surrounding
terrain, such a characteristic does not always apply to the
plains surrounding Caloris. Knobby, Odin-type plains are
observed to grade to smooth plains in places, with an associ-
ated decrease in the density of knobs. This gradational
relationship may be because volcanic smooth plains have
embayed the knobs, burying them to varying degrees, or
because the smooth plains in this region share an origin with
the knobs as Caloris ejecta. To help discriminate between
these two scenarios, we selected three representative areas
for comparison of their crater populations (smooth, moder-
ately knobby, and knobby). These regions also show varia-
tions in their style and degree of tectonic deformation. The
?rst region, to the south of the basin, is an extension of Tir
Planitia and shares the typical characteristics of smooth
plains (Figure 7f) with sharp boundaries and only rare posi-
tive relief features that could represent Odin-type knobs. The
area displays color contrasts with some surrounding terrain,
and the presence of wrinkle ridges attests to contractional
deformation within these plains. The second study area is in
the plains to the west of Caloris, which, although generally
smooth, have more common exposures of knobs (Figure 7e).
In contrast to the ?rst region, there is a dearth of wrinkle
ridges and tectonic deformation has instead been accommo-
dated by lobate scarps [Watters et al., 2009b] that are in
many cases radial to the basin. Any contact between these
plains and the more typical smooth plains of the ?rst region
to the southeast is obscured by the 240-km-diameter Mozart
basin (Figure 7). The third region, to the east of Caloris, is in
the hummocky Odin plains (Figure 7d), with the highest
Figure 6. Crater size?frequency distribution for the most sparsely and most heavily cratered smooth
plains units (Table 1) are displayed for comparison with a region of intercrater plains (location shown in
Figure 4). A separation in cratering populations is seen between smooth plains and intercrater plains, with
the intercrater plains more closely following the ?Population 1? shape of surfaces affected by the late
heavy bombardment [Strom et al., 2005].
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
897
density of knobs among the three areas and a distinctly
rougher texture. Contractional deformation here was also
primarily accommodated by lobate scarps.
[18] The plains to the east and west of Caloris have a
similar to slightly lower density of craters than the interior
plains of Caloris, consistent with previous results [Murray
et al., 1974; Strom et al., 2008; Fassett et al., 2009]
(Figures 7a and 7b, Table 1). The N(20) value of the interior
plains is 26 4 (in agreement with the value of 23 4 found
by Fassett et al. [2009]), compared with 25 8 for the
smooth plains with a moderate density of knobs west of
Caloris, and 19 9 for the knobby Odin plains east of
Caloris. The smooth plains to the south of Caloris have a
slightly higher crater density, with N(20) = 32 9, than
those of the interior. Interestingly, despite having been
mapped as Caloris ejecta [Trask and Guest, 1975; McCauley
et al., 1981; Guest and Greeley, 1983] and displaying
stratigraphic relationships that suggest embayment by
Figure 7. (a, b) The range of crater size?frequency distributions for plains associated with the Caloris
basin; the locations of each count area are given in Table 1 and shown in navy blue in the map at bottom
center. (c) Smooth plains within Caloris basin having a high concentration of wrinkle ridges and graben.
(d) Hummocky plains to the east of Caloris. (e) A mixture of smooth plains with more isolated knobs to the
west of Caloris. (f) Smooth plains to the south of Caloris with a high concentration of wrinkle ridges. Any
contact between the plains deposits shown in Figures 7e and 7f is obscured by the Mozart basin, indicated
with an ?M? on the context image.
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
898
Figure 8. Examples of the relationship between smooth and hummocky plains outside of the Caloris
basin. (a) A region to the east of Caloris (30
N, 200
E) where smooth plains appear to embay hummocky
plains (white arrows indicate contact). Within the smooth plains, isolated knobs (yellow arrows) may be
kipukas of a buried hummocky unit. (b) Similar relationships are observed to the southwest of Caloris
(5
N, 147
E), with relatively sharp contacts between smooth and hummocky regions (white arrows) and
isolated knobs or small clusters of knobs within the smooth plains (yellow arrows).
Figure 9. Four orthographic projections of the map shown in Figure 1b (labels give the central latitude and
longitude of each projection). These views highlight the hemispherical differences in the areal abundances
and distributions of smooth plains. Colors and symbols are the same as those shown in Figure 1b.
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
899
smooth plains (Figure 8), the knobby Odin plains have the
lowest crater density of all plains associated with Caloris
and the largest difference from the crater population of the
Caloris rim materials, for which Fassett et al. [2009] found
an N(20) value of 54 12. Although the crater populations
of the circum-Caloris plains are substantially lower in
density than for the Caloris rim materials, we ?nd no clear
stratigraphic relationships between the circum-Caloris plains
and the rim of the basin; the units appear to grade smoothly
from one to the next.
3.3. The Asymmetric Distribution of Smooth Plains
[19] As previously observed [Denevi et al., 2009], smooth
plains are found across all longitudes and at nearly all
latitudes. However, there is an asymmetry in the distribution
of the most areally extensive occurrences. The majority of
smooth plains are contained within three formations: the
Caloris interior plains, described by Murchie et al. [2008]
and Watters et al. [2009b]; the circum-Caloris plains; and
the northern plains, described by Head et al. [2011]. These
three areas together account for over half of the area of
mapped plains and contribute to a large difference in plains
concentrations between northern and southern hemispheres
(Figure 9). Previous maps of smooth plains from only
Mariner 10 images [Spudis and Guest, 1988] or from a
combination of Mariner 10 and MESSENGER ?yby images
[Denevi et al., 2009] did not display as clearly the asymme-
try in smooth plains distribution, and plains were earlier
estimated to cover up to 40% of the surface. The difference
in estimated fractional area is due to two main factors. First,
a broad region centered on 0
E has a dearth of smooth
plains. The majority of this area was not viewed by Mariner
10 and was imaged only at low solar incidence angles (near
noon) and high emission angles (oblique viewing geometry)
by MESSENGER during its Mercury ?ybys and was
therefore not previously mapped. Second, orbital images
show that previous work included some plains that are more
heavily cratered than smooth plains and so are more
appropriately classi?ed as intercrater plains.
[20] The rimless, irregular depressions in our map
interpreted to be volcanic vents [Gillis-Davis et al., 2009;
Kerber et al., 2009, 2011; Goudge et al., 2012] are found
almost entirely outside smooth plains, with some concentrated
along the margins of large smooth plains deposits, consistent
with the ?ndings of Head et al. [2011], or within craters in
isolated smooth plains exposures (Figure 1b). This distribution
is consistent with smooth plains having formed largely by the
emplacement of relatively low-viscosity ?ood basalts, which
buried vents that may have once existed within their bound-
aries. Vents located outside large smooth plains deposits were
preserved. However, we note that few vents are observed at
high southern latitudes or at midlatitudes near 0
E, where there
is also a lack of large-scale smooth plains deposits.
[21] Finally, we compared our map of smooth plains with
that of crustal thickness [Smith et al., 2012] for the northern
hemisphere of Mercury (Figure 10). Considerable variation
in crustal thickness is found for regions of smooth plains.
Some large deposits such as the northern plains and Sobkou
Planitia are underlain by relatively thin crust. Other large
deposits, especially those within and around the Caloris
basin, have a large variation in crustal thickness. Addition-
ally, there are regions of relatively low crustal thickness with
no associated smooth plains, most conspicuously east of
Sobkou near 50
N, 270
E.
4. Discussion
4.1. Plains Composition and Origin
[22] Morphologies and color properties revealed in images
obtained from MESSENGER?s ?ybys of Mercury and
Figure 10. A comparison of the distribution of smooth plains (white outlines) to model crustal thickness
values [Smith et al., 2012]. Some regions of smooth plains, including the northern plains and Sobkou Planitia,
are found in regions of low crustal thickness, whereas considerable variation is seen in other regions,
including the plains associated with Caloris. The shading in the south indicates where crustal thickness is
unconstrained because of the lack of altimetry data as a result of MESSENGER?s highly eccentric orbit
and northern periapsis.
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
900
subsequent orbital observations support a volcanic origin for
the Caloris interior plains and northern plains [Head et al.,
2008, 2011; Murchie et al., 2008; Robinson et al., 2008].
Other smooth plains deposits with clear embayment relations
and color properties distinct from underlying terrain also
provide strong evidence for an origin via effusive volcanism
[e.g., Head et al., 2009a]. The majority of smooth
plains occurrences are relatively high in re?ectance, have
steep spectral slopes, and have been classi?ed as HRP
[Denevi et al., 2009]. Thus far, the most detailed composi-
tional information for HRP is for the northern plains, which
have Mg/Si, Ca/Si, and Al/Si values that are consistent with
a magnesian basalt-like lithology [Nittler et al., 2011;
Stockstill-Cahill et al., 2012; Weider et al., 2012] with a rela-
tively high abundance of K [Peplowski et al., 2012]. Analysis
of XRS data also shows that the Caloris interior plains
have a composition similar to that of the northern plains
[Weider et al., 2012] except for a lower K abundance, perhaps
due to higher maximum surface temperatures at Caloris that
mobilize and redistribute this moderately volatile element
from the Caloris plains [Peplowski et al., 2012]. By extension,
the majority of smooth plains may share a magnesian alkali
basalt composition, as they share HRP spectral properties.
However, the lack of diagnostic absorption bands in re?ec-
tance spectra means that more work is required to con?rm this
suggestion. Plains of intermediate re?ectance and spectral
slope, such as those near Rudaki crater, may be composition-
ally related to the HRP via partial melting. Future analysis of
XRS spectra from areas dominated by smooth plains of inter-
mediate re?ectance and further petrographic experiments
such as those of Charlier et al. [2013] will help to elucidate
the relationships among smooth plains compositions.
[23] But what of the circum-Caloris plains? Determining
their origin has implications both for the distribution of
volcanism on Mercury and for the evolution of lava compo-
sitions over time. The LBP spectral characteristics (low
re?ectance and shallower spectral slope) of these plains
[Robinson et al., 2008; Denevi et al., 2009] are comparable
in color to more heavily cratered terrain that predates the
smooth plains and has been identi?ed as having more ma?c
compositions than the northern plains [Nittler et al., 2011;
Weider et al., 2012]. Direct observations of a portion of the
western plains exterior to Caloris also show that they may
be more ultrama?c than other smooth plains [Weider et al.,
2012]. If these plains are volcanic, their composition
suggests that the thermal history of Mercury was such that
the high source region temperatures required to achieve high
degrees of partial melting of the planet?s mantle (resulting in
the generation of melts of more ultrama?c composition)
were not limited to Mercury?s most ancient terrain but
continued throughout much of the duration of smooth plains
formation.
[24] Of the circum-Caloris plains, the smooth plains to the
south and east of Caloris that make up Tir and Budh planitiae
display the strongest evidence for volcanism. Those plains
contain few knobs or other features thought to be associated
with Caloris ejecta. They also contain a high density of wrin-
kle ridges [Watters et al., 2009a] and several wrinkle-ridge
rings indicative of buried impact craters [Klimczak et al.,
2012]. Although not diagnostic of volcanic plains, wrinkle
ridges are typically associated with such deposits [e.g.,
Watters, 1988].
[25] However, for regions mapped as Odin-type knobby
plains, the morphologic and stratigraphic relationships are
more consistent with emplacement as Caloris impact ejecta,
perhaps in combination with effusive and/or impact melt
deposits. The knobby and hummocky textures observed
within these plains are similar to those of plains associated
with some lunar basins and interpreted to be impact ejecta
(Figure 11) [e.g., Wilhelms, 1987]. Their morphology and
spatial distribution led to the interpretation that the Odin
plains observed by Mariner 10 were emplaced as ejecta from
the Caloris impact event [Murray et al., 1974; Strom et al.,
1975; Trask and Guest, 1975]. The knobby plains to the
southwest and west of Caloris also lack the dense population
of wrinkle ridges seen to the southeast [Watters et al.,
2009b]. In many regions, the Odin-type plains appear to
grade continuously into Caloris rim deposits (Figure 12).
Several locations also show evidence of ?ow from
the lower-re?ectance Odin-type deposits into the basin,
but no low-re?ectance signature is seen within the basin
(Figures 12a?12f). This observation is consistent with the
interior smooth plains having embayed Odin-type deposits,
implying that the interior plains are younger.
[26] The interpretation that Odin-type plains are Caloris
ejecta is also consistent with stratigraphic relations showing
discrete regions of hummocky plains that appear to be
embayed by smooth plains exterior to the basin containing
more isolated exposures of knobs (Figure 8). These relations
may indicate that the hummocky plains once covered a wider
area but were buried in part by later volcanic deposits. The
isolated knobs within the smooth plains could represent
kipukas (preexisting terrain ?ooded by lavas to produce an
island-like exposure) of this underlying hummocky unit, as
suggested by Fassett et al. [2009].
[27] However, regional crater size?frequency distributions
do not support a sequence of events in which Odin-type
plains were emplaced as part of the Caloris impact event
and are thus the oldest plains associated with the basin. Of
the three areas we examined exterior to Caloris, the hum-
mocky plains have the smallest number of superposed cra-
ters, implying the youngest age (Table 1, Figure 7). They
also show the largest difference from the crater size?
frequency distribution measured for Caloris rim materials
by Fassett et al. [2009]. If the formation of these plains
accompanied the formation of the Caloris basin, the plains
and basin should have a similar age. The relative crater
populations contradict the observed stratigraphic relationships.
[28] How can this con?icting evidence be reconciled? One
possible explanation is that the observed differences in the
crater populations (Table 1) are not statistically signi?cant
and the units are similar in age. Such a statement clearly
holds for differences among the Caloris plains, given the
overlap in uncertainties. However, this explanation does
not address the larger difference in crater size?frequency dis-
tribution between hummocky plains and Caloris rim mate-
rials [Fassett et al., 2009], which is more dif?cult to
attribute to overlapping uncertainties, particularly at the
larger crater diameters. Moreover, the Caloris rim size?
frequency distribution displays the characteristic Population
1 shape of the late heavy bombardment [Fassett et al.,
2009, 2011], but that for the hummocky plains does not. If
hummocky plains are Caloris ejecta, they should share this
trait. Perhaps some physical characteristic of the ejected
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
901
material (e.g., physical strength of the material) resulted
in smaller crater sizes for a given projectile size
[Schultz et al., 1977; van der Bogert et al., 2010;
Hiesinger et al., 2012]. Another possibility is that the crater
population of the Caloris rim was substantially affected by
the fallback of Caloris ejecta, such that some signi?cant por-
tion of the craters observed are secondaries from the Caloris
event itself that did not affect ?uidized ejecta and impact melt
emplaced in a different manner at a slightly later time and
greater distance. For smaller craters, impact melt has been
shown to be emplaced late in the crater excavation stage after
the majority of fragmental ejecta [e.g., Howard and Wilshire,
1975; Hawke and Head, 1977; Denevi et al., 2012],
suggesting that any ?uidized material from the Caloris basin
may have been deposited immediately after the majority of
near-?eld secondary cratering. If the crater size?frequency
distributions are representative of real age differences, how-
ever, then all of the circum-Caloris plains have been
resurfaced, no large exposures of Caloris ejecta remain, and
some other process is responsible for the hummocky texture
observed in Figures 7d, 8, and 12.
[29] Another important aspect of the circum-Caloris
plains, including both the Odin-type plains and the majority
of the circum-Caloris smooth plains, is their low re?ectance
and shallow spectral slope, spectral characteristics more
similar to those of low-re?ectance material (LRM) typically
concentrated in basin ejecta, such as that surrounding the
Tolstoj basin [Rava and Hapke, 1987; Robinson et al.,
2008; Denevi et al., 2009]. The circum-Caloris plains are
only ~15% lower in re?ectance than the global average,
compared with up to 30% lower for LRM, but the diffuse
color boundaries between inter?ngered low-re?ectance
plains and nearby higher-re?ectance plains and the lack of
clear color or stratigraphic contacts with the Caloris rim do
not support a volcanic interpretation. The most straightfor-
ward explanation is that the color properties are consistent
with a mixture of materials ejected from depth and deposited
around the basin. An alternative hypothesis for the composi-
tional contrast between the exterior and interior plains is that
the Caloris impact caused regional changes in mantle
dynamics, leading to different depths of partial melting to
produce the magmas that formed the different plains
[Roberts and Barnouin, 2012].
[30] The circum-Caloris plains are not the only low-
re?ectance smooth plains. Another example is found near
15
N, 47
E, a region far from any large basins, unlikely to
have been emplaced as ejecta, and thus interpreted here to
be volcanic in origin. A large region of low-re?ectance
smooth plains is also found ~350 km north of Beethoven
basin and another ~150 km northwest of Rembrandt basin,
but the proximity of these two areas to these large basins
leaves open the possibility that the plains are of impact
origin. An irregular, rimless depression that we interpret to
be volcanic in origin is found within a region of low-
re?ectance plains to the northwest of Caloris (Figure 13).
Although not all volcanic vents are sources of effusive vol-
canism on Mercury [e.g., Head et al., 2011], they are often
found proximal to effusive volcanic deposits, so this feature
lends credence to a volcanic origin for at least some
LBP units.
[31] Altogether, we ?nd that for ~65% of the smooth
plains mapped here, there are multiple lines of evidence for
a volcanic origin (e.g., ?ooding and embayment relation-
ships, color contrasts with surrounding terrain, relationship
with volcanic vents). This fraction encompasses all areally
extensive units apart from the circum-Caloris plains, of
Figure 11. A comparison of knobby terrain associated with
the Caloris basin on Mercury (32.5
N, 197.3
E) and with
the Orientale (6.9
S, 265.1
E) and Imbrium (65.7
N,
4.1
E) basins on the Moon. Lunar examples are from a
Lunar Reconnaissance Orbiter Camera base map with a pixel
scale of 100m. The scale bar applies to all panels.
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
902
Figure 12. The southwestern portion of the Caloris basin. In the context image (top), knobby plains are
observed to grade smoothly to the Caloris rim. (a?f) Three regions at the rim where evidence for channel-
ized ?ow from the Odin-type plains into the basin is observed. Each region is shown from the mono-
chrome base map (a, c, e) and in enhanced color (b, d, f); the second principal component, ?rst
principal component, and 430/1020 nm ratio are shown in red, green, and blue, respectively. A sharp
color contrast is observed between the spectrally distinct interior plains and rim deposits, implying
that the interior plains superpose material that ?owed into the basin. The scale bar in (f) applies
as well to (a?e).
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
903
which only the portions to the south and east with the stron-
gest evidence for volcanism are included in this count. Most
of the uncertainty in the remaining fraction lies in the smaller
patches that often lack color contrasts with their surroundings
and in deposits contained entirely within a crater or basin that
may have formed from impact melt, such as in Raditladi.
[32] Although we ?nd strong evidence in support of a
volcanic origin for the majority of smooth plains deposits,
?nding de?nitive evidence in favor of an origin as ?uidized
ejecta for some fraction of the plains is dif?cult. For lunar
light plains, the primary evidence from imaging for a
different origin from that of the mare deposits is their high
albedo, which is not a useful discriminator for Mercury.
Many examples of smooth plains deposits clustered within
topographic lows around basins, such as Beethoven, have
distributions consistent with ejecta emplacement, but these
units typically have spectral properties that indicate a
compositional difference from adjacent terrain, or they
embay craters superposed on the basin, characteristics that
favor a volcanic origin. We ?nd several regions where
smooth plains without distinct color properties embay rough
terrain covered in secondary craters (Figure 14). One area
where an impact origin for smooth plains is strongly
supported is at Kuiper crater, where ejecta or melt ponded
outside the crater, covering secondaries from the Kuiper
impact (Figure 14a) [Beach et al., 2012; D?Incecco et al.,
2012]. It is likely that some other deposits with similar
morphology (e.g., Figure 14b) also formed from impact
events. However, many of the regions containing such
deposits are located farther from likely source craters or
basins than the examples in Figure 14.
4.2. Ages
[33] Crater size?frequency distributions can be used to
estimate the absolute ages of a surface given a model for
the production of craters on a body. In the case of Mercury,
Figure 13. (a) A color image of a region of LBP northwest of Caloris basin (20
N, 50
E). Images from
the WAC ?lters centered on 1000, 750, and 430 nm wavelength are displayed in red, green, and blue,
respectively. The location of Figure 13b is indicated. (b) A higher-resolution monochrome view of a
possible volcanic vent within this area of LBP, supporting a volcanic origin for at least some LBP.
Figure 14. (a) Ejecta and impact melt (yellow arrows) from Kuiper crater embay a ?eld of secondary cra-
ters created by the impact event (15
S, 330
E). These characteristics might provide an explanation for
other regions where smooth plains deposits are patchy and appear to partially bury a region with a large
population of secondary craters, such as that seen in Figure 14b. (b) A region of smooth plains (outlined
in white) could be impact melt from Rajnis crater (~80 km in diameter), which is just outside the frame to
the east, ~40 km from the edge of the deposit (5
N, 260
E).
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
904
many unknowns remain in estimating absolute age from
crater density, and thus in the preceding sections we
presented our results in terms of N(20) values, which are
not model dependent and can be easily compared. However,
as a ?rst look at absolute ages we applied the crater chronol-
ogy model of Strom and Neukum [1988] by ?tting an isochron
to craters larger than 10 km in diameter. From this model, the
representative large-scale smooth plains units in Table 1 all
have estimated ages between ~3.7 and 3.9Ga. These estimates
are consistent with previous results for the Caloris interior
plains [Strom et al., 2008, 2011; Fassett et al., 2009] and
for the northern plains [Head et al., 2011; Ostrach et al.,
2011], which also have estimated ages of ~3.7?3.8Ga.
Although small patches of smooth plains may have ages that
extend into the Mansurian with ages as young as 1Ga
[Prockter et al., 2010], we ?nd no large expanses with young
ages that would be comparable to the lunar Oceanus
Procellarum, which includes plains with ages estimated to
be 1?2Ga [Hiesinger et al., 2003]. Thus, all major smooth
plains units dated thus far, accounting for ~60% of the
mapped plains, may have formed within ~200 My, a
relatively short interval in Mercury?s geologic history. If this
result holds in spite of the much larger size and implied
longer planetary cooling time of Mercury than of the Moon,
the difference in duration of plains volcanism may indicate
that the compressional stress state of Mercury?s lithosphere
[e.g., Strom et al., 1975; Solomon et al., 2008; Wilson and
Head, 2008; Watters et al., 2009b] worked to inhibit
voluminous volcanic eruptions later in Mercury?s history
[e.g., Solomon, 1978; Chapman, 1988]. However, the
determination of absolute ages can vary substantially (on
the order of 1Gy) with the cratering chronology model used
for Mercury [Ostrach et al., 2011]. Further work on the deter-
mination of ages of surface units would help in understanding
the timing of smooth plains emplacement on Mercury.
4.3. Crustal Asymmetry
[34] As noted above, the distribution of smooth plains
across Mercury is not uniform. A marked lack of plains is
observed south of ~45
S and near 0
E (Figures 1 and 9).
What is the cause of this global asymmetry? One hypothesis
for the nearside?farside asymmetry in smooth plains (maria)
on the Moon is that it resulted from hemispherical differ-
ences in mantle temperature, due in part to differences in
radioactive element abundances in the uppermost mantle that
resulted in generally cooler temperatures in the farside man-
tle and less production of basaltic partial melt [e.g.,
Wieczorek and Phillips, 2000; Wieczorek et al., 2001].
Spatial variations in radioactive elements, if they mirror
similar variations at mantle depths, could indicate a
tendency for increased melting and volcanism in some
regions later in Mercury?s history, but no evident correlation
between young plains and radioactive element concentra-
tions is observed in Mercury?s northern hemisphere
[Peplowski et al., 2012]. However, diurnal heating of the
surface may have resulted in the mobilization and redistribu-
tion of K [Peplowski et al., 2012], which may have obscured
any such relationship. Moreover, radioactive element
abundance information is not available for the southern
hemisphere. Any spatial relationship between radioactive
element abundances and volcanic plains may thus not be
resolvable at this time.
[35] On the Moon, regional variations in the thickness of
the upper highlands crust, lower in density than mare basalt
magma [Wieczorek et al., 2013], also likely played a role in
determining the locations of eruptions, as a thinner crust
increases the likelihood that basaltic magma can ascend to
the surface [e.g., Solomon, 1975; Head and Wilson, 1992;
Shearer et al., 2006; Whitten et al., 2011]. In Mercury?s
northern hemisphere, some smooth plains units are in areas
of lower than average crustal thickness [Smith et al., 2012],
but there is no general correlation between smooth plains lo-
cations and relatively thin crust (Figure 10); the lack of esti-
mates for crustal thickness in the southern hemisphere
prevents a global assessment of this relationship. Moreover,
surface elemental compositions on Mercury lie within a
narrower range than on the Moon, with elemental ratios con-
sistent with magnesian basalt-like to more ultrama?c litholo-
gies [Nittler et al., 2011; Weider et al., 2012]. This narrower
range suggests that magmas on Mercury may generally have
densities similar to or lower than the densities of crustal
rocks, reducing the likelihood that a low-density crust
inhibited eruptions to the surface in regions of increased
thickness. The observed compositional variations are also
consistent with a range of volcanic surface materials formed
through different degrees of partial melting [Weider et al.,
2012]. Previous geologic interpretations of the intercrater
plains [Malin, 1976; Strom, 1977; Spudis and Guest, 1988]
suggest that much of the surface may be volcanic. Thus,
we suggest that Mercury?s asymmetry of smooth plains
distribution may re?ect a different age of the most recent
large-scale volcanism rather than a difference in crustal
formational processes. Future work assessing the distribu-
tion and origin of intercrater plains will be essential to an
evaluation of this suggestion.
5. Conclusions
[36] Mercury?s surface is covered by ~27% smooth plains,
and Odin-type knobby plains comprise an additional 2% of
the surface. The crater size?frequency distributions of these
plains suggest emplacement after the majority of the late
heavy bombardment and possibly within a relatively narrow
range of time for the largest expanses of plains. The majority
(at least 65%) of these plains are volcanic in nature, with
clear ?ooding and embayment relationships and color and
compositional contrasts with underlying terrain.
[37] We ?nd con?icting evidence as to the question of the
origin of the circum-Caloris plains. Regions mapped as
Odin-type plains share strong morphologic similarities with
lunar basin deposits, and they often grade smoothly to the
Caloris rim. Both the Odin-type plains and the majority of
the circum-Caloris smooth plains share a relatively low
re?ectance and shallow spectral slope and display no color
contrasts with rim deposits. Stratigraphic relationships
suggest that Odin-type plains were embayed by local smooth
plains that may have been effusively emplaced, and that the
isolated knobs within smooth plains may be kipukas of the
hummocky unit. Alternatively, both units may be facies of
the Caloris basin ejecta. However, the hummocky plains to
the east of Caloris have slightly lower crater densities than
other plains associated with Caloris, and all circum-Caloris
plains are less cratered than the Caloris rim, suggesting they
could not have formed contemporaneously with the basin.
DENEVI ET AL.: SMOOTH PLAINS ON MERCURY
905
Thus, if any of the circum-Caloris plains are material ejected
by the Caloris impact event, the crater size?frequency distri-
butions in these regions are not meaningful discriminators of
age. The con?icting scenarios presented by the morphologic,
stratigraphic, and color relationships, on one hand, and the
crater size?frequency distributions, on the other, could be
reconciled if either physical properties of the hummocky
plains led to differences in crater sizes for given impactor
dimensions, or some process, such as near-?eld secondary
cratering, resulted in a higher population of craters on the
Caloris basin rim than on the hummocky ejecta. If the crater
populations are representative of age, then neither the
smooth plains around Caloris nor the Odin-type plains are
Caloris ejecta, and some other process is responsible for
the characteristic texture of the hummocky plains.
[38] The majority of the smooth plains are relatively high
in re?ectance, consistent with low-iron basalt-like composi-
tions by inference from XRS elemental compositions of the
northern and Caloris interior plains [Nittler et al., 2011;
Weider et al., 2012]. Several regions of smooth plains with
low re?ectance and relatively shallow spectral slopes (e.g.,
the plains near 15
N, 47
E; plains north of Beethoven; and
plains northwest of Rembrandt) may correspond to more
ma?c lithologies, indicating that high temperatures and
greater degrees of partial melting were possible in mantle
source regions relatively late in Mercury?s history. If of
volcanic rather than impact origin, the circum-Caloris
smooth plains would more than double the area of more
ultrama?c plains emplaced subsequent to the end of late
heavy bombardment.
[39] Although smooth plains are generally globally
distributed, they are more heavily concentrated in northern
latitudes and in the hemisphere surrounding Caloris. The
uneven distribution of smooth plains may be due to differ-
ences in age and thus preservation, rather than formational
processes, of sur?cial material. Much of the older terrain
on Mercury may prove to share a volcanic origin with the
smooth plains.
[40] Acknowledgments. We thank Aileen Yingst and an anonymous
reviewer for constructive reviews. This research has made use of the
Integrated Software for Imagers and Spectometers (ISIS) of the U.S.
Geological Survey. The MESSENGER project is supported by the NASA
Discovery Program under contracts NAS5-97271 to The Johns Hopkins
University Applied Physics Laboratory and NASW-00002 to the Carnegie
Institution of Washington.
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