Icarus 283 (2017) 254–267
Contents lists available at ScienceDirect
Icarus
journal homepage: www.elsevier.com/locate/icarus
Hansteen Mons: An LROC geological perspective
Joseph M. Boyce
a , ∗, Thomas A. Giguere
a , B. Ray Hawke
a , 1 , Peter J. Mouginis-Mark
a , Mark S. Robinson
b , Samuel J. Lawrence
b , David Trang
a , Ryan N. Clegg-Watkins c , d
a Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI, 96822, USA b School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281, USA c Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA d Planetary Science Institute, Tucson, AZ, 85719, USA
a r t i c l e i n f o
Article history:
Received 1 July 2015
Revised 17 February 2016
Accepted 11 August 2016
Available online 18 August 2016
Keywords:
Moon, surface
Silicic
Geological processes
Geologic mapping
a b s t r a c t
Mons Hansteen is a relatively high-albedo, well-known lunar “red spot” located on the southern mar-
gin of Oceanus Procellarum (2.3 °S, 50.2 °W). It is an arrowhead-shaped ( ∼ 25 km on a side), two-layer
mesa with a small cone-shaped massif on its north edge formed by three morphologically and com-
positionally distinct geologic units. These units were emplaced in three phases over nearly 200 million
years. The oldest ( ∼3.74 Ga), Hilly–Dissected unit, composed of high-silica, and low-FeO content materials
formed a low, steep sided mesa. The materials of this unit erupted mainly from vents along northeast-
and northwest-trending sets of fractures. The Pitted unit, which comprises the upper-tier mesa, is com-
posed of high-silica and even lower-FeO content materials. This material was erupted at ∼ 3.5 Ga from
numerous closely spaced vents (i.e., pits) formed along closely spaced northeast-southwest-trending sets
of fractures. At nearly the same time, eruptions of lower silica and higher FeO materials occurred on the
north flank of Mons Hansteen at the intersection of two major fractures to produce the North Massif unit.
The eruptions that created the North Massif units also produced materials that thinly blanketed small ar-
eas of the Hilly-Dissected and Pitted units on the north flank of Mons Hansteen. Also at nearly the same
time (i.e., ∼ 3.5 Ga), basalt flows formed the surrounding mare. Each unit of Mons Hansteen appears to
be mantled by locally derived ash, which only modestly contaminated the other units. The morphology
of Mons Hansteen (especially the Pitted unit) suggests a style of volcanism where only a relatively small
amount of material is explosively erupted from numerous, closely spaced vents.
© 2016 Elsevier Inc. All rights reserved.
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1. Introduction
Unraveling the nature of lunar “red spots”, such as Mon
Hansteen (MH), has major implications for lunar thermal history
and crustal evolution ( Hagerty, 2006; Jolliff et al., 2011 ), thus
providing crucial information for understanding the early Moon.
Lunar red spots are characterized by a relatively high albedo and
a strong absorption in the UV ( Wood and Head, 1975; Head and
McCord, 1978 ). Some early workers presented evidence that at
least some red spots were produced by non-mare or highlands
volcanism and suggested a connection with KREEP basalts or even
∗ Corresponding author.
E-mail addresses: [email protected] (J.M. Boyce), thomas.giguere@
intergraph.com (T.A. Giguere), [email protected] (P.J. Mouginis-Mark),
[email protected] (M.S. Robinson), [email protected] (S.J. Lawrence),
[email protected] (D. Trang), [email protected] (R.N. Clegg-Watkins). 1 Deceased.
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http://dx.doi.org/10.1016/j.icarus.2016.08.013
0019-1035/© 2016 Elsevier Inc. All rights reserved.
ore evolved highlands compositions such as dacite or rhyolite
Malin, 1974; Wood and Head, 1975; Head and McCord, 1978 ).
One of these red spots, Mons Hansteen (IAU, 1976, see website
t planetarynames.wr.usgs.gov) also known as Hansteen α (e.g.,
ee Wagner et al., 2010 ) or Hansteen Alpha (e.g., see Hawke et al.,
003 ) are hereafter referred to as Mons Hansteen is a relatively
igh-albedo, polygonal, arrowhead-shaped, mesa that measures
25 km on a side. It is located on the southern margin of Oceanus
rocellarum adjacent to the craters Billy and Hansteen at 12.3 °S,
0.2 °W ( Fig. 1 ).
New high-resolution observations from the Lunar Reconnais-
ance Orbiter (LRO) mission Lunar Reconnaissance Orbiter Camera
LROC), Wide Angle Camera (WAC) and Narrow Angle Camera
NAC) images and image mosaics provide the means to signifi-
antly advance our understanding of the geology and morphology
f this volcanic center. The objective of this study is to characterize
eologic units of MH, and to determine their morphology, extent,
istribution, age, composition, and geologic history. Recently
J.M. Boyce et al. / Icarus 283 (2017) 254–267 255
Fig. 1. Mons Hansteen is located at 12.3 °S, 50.2 °W on the southern margin of Oceanus Procellarum near the craters Hansteen and Billy. Base image on the left is a full Moon
telescopic view, and the one on at right is a mosaic of LROC WAC images (from LROC Quickmap). North is at the top in both images. Location of Fig. 4 indicated by white
box.
Fig. 2. Oblique view from the east looking west of Mons Hansteen. The dotted lines trace the centers of the broad valleys developed in the Hilly-Dissected unit. North is to
the right. LROC image M1154506530.
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cquired meter scale images from the LROC cameras ( Robinson
t al., 2010 ) ( Fig. 2 ) combined with datasets from previous mis-
ions (e.g., Clementine, Kaguya) enable new detailed mapping and
iscovery of three geologic units in MH, revealing its history and
ow it compares with other lunar red spot volcanic centers.
. Background
McCauley (1973) described MH as being a steep-sided, bulbous,
ery bright dome of material exhibiting a hackly surface. He also
dentified several small, linear, smooth-walled depressions at the
rests of gentle individual highs and interpreted these depressions
s probable volcanic vents. Wood and Head (1975) noted that MH
as a distinctive surface texture, color, and albedo compared to
he nearby highlands and adjacent mare units.
Wagner et al., (2010) mapped MH from Lunar Orbiter IV images
nd described a flat summit region reminiscent of a mesa. They
oted that the summit area, as well as the flanks, appears much
ore rugged than the Gruithuisen domes ( Head and McCord, 1978;
hevrel et al., 1999 ) which are characteristic of level summits.
urther, Wagner et al., (2010) identified two small, distinct areas
n the summit region of MH, and measured superimposed crater
requency, but the low-resolution images they used prevented
hem from detailed geologic mapping of this feature. From two
istinct areas of the summit they found two statistically significant
rater distributions, with cratering model ages of 3.74 and 3.55 Ga
256 J.M. Boyce et al. / Icarus 283 (2017) 254–267
Fig. 3. LROC WAC images mosaic with the LRO Diviner Standard Christiansen Fea-
ture Value (silica) map superposed. The white indicates areas of relatively high-
silica content. (Image from LROC Quickmap mosaic).
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(a model age of 3.67 for the sum of the two areas). Based on these
measurements, Wagner et al., (2010) suggested that MH is Upper
Imbrian age, clearly postdating the highlands materials, and pre-
dating the surrounding mare materials, confirming earlier results
by Wood and Head [1975]. The younger age of 3.55 Ga measured
on its summit could be connected to active mare volcanism in the
area between 3.5 and 3.6 Ga., but Wagner et al., (2010) did not
map these two count areas as separate geologic units.
Remote sensing estimates and geomorphic analysis suggest
that MH is composed of low-iron and silica-rich rock, and likely
represents an evolved lunar lithology presently thought to be anal-
ogous to terrestrial granites and felsites, although the origin and
emplacement of evolved silicic lithologies on the Moon remains
unknown ( Hawke et al., 2003; Glotch et al., 2010; Greenhagen
et al., 2010; Paige et al., 2010; Glotch et al., 2011; Haggerty,
2006; Jolliff et al., 2011 ). Hawke et al., (2003) and Wagner et al.,
(2010) noted that if Mons Hansteen was present prior to the for-
mation of Billy and Hansteen craters, it should have been covered
with FeO- and TiO 2 -rich ejecta since it is within one crater diam-
eter of the rim crest of each crater. Since it is not, they concluded
that MH was emplaced on top of the FeO-rich ejecta deposits,
consistent with the model crater age of Wagner et al., (2010) .
Recent research using Clementine, Lunar Prospector (LP), and
Lunar Reconnaissance Orbiter (LRO) data have provided strong evi-
dence that some red spots, including the MH, are dominated by Th
and silica-rich, highly evolved highlands lithologies ( Hawke et al.,
20 03; Lawrence et al., 20 05; Hagerty et al., 20 06; Glotch et al.,
2010; Greenhagen et al., 2010; Glotch et al., 2011; Hawke et al.,
2011 ; 2012; Ashley et al., 2016 ). For example, Clementine UV-VIS
images were used to produce FeO, TiO 2 , and optical maturity maps
of the MH region utilizing the algorithms of Lucey et al. (20 0 0a ; b) .
Mare units in this region exhibit FeO abundances > 16 wt%, and
TiO 2 values range between 4 wt% and 8 wt%. In sharp contrast,
much lower FeO and TiO 2 values are exhibited by Mons Hansteen
where FeO values range from 5 wt% to 9 wt% and TiO 2 of < 1 wt%.
In the central portion of MH, the surface materials have an av-
erage FeO value of 6.9 wt% and an average TiO 2 value of 0.5 wt%
( Hawke et al., 2003 ). Hawke et al. (2003) suggested that since
this central region would be less contaminated by debris from the
surrounding mare units thrown there by impacts, its composition
may most closely approximate that of the underlying lithology;
i.e., ejecta from the Imbrian-aged craters Billy and Hansteen.
However, this explanation is unlikely because their map does not
show evidence of contamination from MH on the surrounding
mare surface. Hence, the contamination would have to be unidi-
rectional (from mare to MH and to higher elevations), and would
be substantially greater than noted as occurring elsewhere along
the maria/highlands boundary ( Logan et al., 1972 ).
Lawrence et al., (2005) and Hagerty et al., (2006) used forward
modeling of LP Gamma Ray Spectrometer data to show that the
Th abundance at MH is not 6 ppm, but could possibly range
from ∼20 to ∼25 ppm. This is consistent with Th abundances
measured in evolved lunar lithologies such as granites and felsites.
Subsequently, Glotch et al., (2010) based on thermal emission
signatures measured by the LRO Diviner Lunar Radiometer Exper-
iment (Diviner) found that Si-rich materials are more abundant
near the center of MH and in the high terrain SW of the center
( Fig. 3 ). Glotch et al., (2010) suggest that the lower values on the
margins of the feature may be the result of contamination by
mare debris transported to the slopes of the dome by impacts in
the surrounding mare.
Recently, Kiefer et al., (2016) used gravity modeling based on
high-resolution Gravity Recovery and Interior Laboratory (GRAIL)
mission observations to suggest that MH is composed of relatively
low-density, felsic materials (bulk density of the crust beneath
MH of 150 0–20 0 0 kg −3 ). This is similar to their findings for the
ruithuisen domes. They note that to be consistent with their
ensity observations silica-rich magmas required, can be produced
ither by 1) silicate liquid immiscibility ( Hagerty et al., 2006 ), or
) crustal melting induced by basaltic underplating. They favored
he basaltic underplating mechanism because it can produce a
rustal rhyolitic composition magma that is consistent with the
eO content of MH materials as well as their inferred bulk density
f MH. Kiefer et al., (2016) also suggest that basaltic underplating
s supported by model melting calculations that indicate that
artial melting of KREEP basalt (driven by the heat from the
ntrinsic radioactivity of KREEP and from mare basaltic intrusions)
hould produce significant volumes of rhyolitic magma with the
ight range in FeO, as well as a high thorium abundance like that
bserved at MH and other felsic domes ( Hagerty et al. 2006 ).
Each red spot volcanic complex appears to have its own unique
hape. For example, Mons Hansteen is a two-layer mesa with mul-
iple vents and one satellite cinder cone. The Compton–Belkovich
olcanic Complex ( Jolliff et al., 2011; Chauhan et al., 2015 ), which
s approximately the same size as MH, is a broad area of elevated
opography with a range of volcanic features (e.g., irregular col-
apse depressions, and a variety of size domes). The Lassell Massif
omplex may also be a layered volcanic complex of about the
ame size ( Ashley et al., 2013 ; 2016 ). In contrast, the Gruithuisen
omes include two relatively large elongate domes and a small
ome ( Chavrel et al., 1999 ). The detailed geologic history like that
resented here for MH has yet to be completed for each of these
ed spot volcanic centers, but would help us to understand why
hese volcanic centers are so different.
. Regional context
The Mons Hansteen is located on the southern margin of
ceanus Procellarum centered at ∼ 12.5 °S and 50W ( Fig. 1 ).
his complex formed in highlands materials and is Imbrian age
McCauley, 1973; Wagner et al., 2010 ). It began to form after the
earby impact craters Hansteen and Billy ( ∼ 3.9 Ga), but before
he mare flooded its flanks at ∼ 3.5 Ga.
J.M. Boyce et al. / Icarus 283 (2017) 254–267 257
Fig. 4. Cumulative size-frequency distribution (CSFD) curves of impact craters, and
subdued circular and quasi-circular pits on the geologic units of the MH as well as
the adjacent mare to the east. The Hilly-Dissected unit crater counts are in closed
squares (390 crater between 100 m and 1.48 km dia., in 153 km
2 area); the Pitted
unit are in closed circles (186 craters between 100 m and 900 m dia., in 126 km
2
area); the North Massif unit craters are in inverted closed triangle (40 craters be-
tween 100 m and 500 m dia., in 24.6 km
2 area); the mare east of MH are in open
circles (97 craters between 143 m and 510 m dia., in 134 km
2 area), subdued pits on
the Pitted units are crosses (48 pits between 267 m and 1.3 km, in 126 km
2 area);
and subdued pits on the Hilly-Dissected unit are x s (24 pits between 240 m and
750 m dia., in 153 km
2 area). The best fit model age production functions are plot-
ted for 3.5, 3.74 and 4.0 Ga and the theoretical crater equilibrium curve (see from
Michael and Nuekum, 2010 ). Note the divergence of the CSFD curves on all the
units of MH from lunar impact crater production functions < ∼0.5 km diameter.
The dashed line is the average for all these units. The linear nature of these curves
below ∼0.5 km crater diameter suggest a process that degrades and erases smaller
crater faster than larger ones such as would occur in areas mantled by particulate
material. Also note that the CSFD of the subdued pits on both units also do not
follow the lunar impact crater production function.
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Fig. 5. Oblique view, looking west, of the wrinkle ridge (arrowed) extending into
the southeastern side of the Mons Hansteen from the mare to the southeast. Width
of image in foreground is ∼6.6 km. North is on the right in this image. The image
is a portion of LROC image M1154506530, its location is shown in Fig. 2.
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.1. Highlands
Highlands in the MH region are composed of high albedo
aterial characterized by rugged, hilly or hummocky, furrowed
orphologies ( McCauley, 1973; Wagner et al., 2010 ). The highland
nits in the MH region were mapped as undivided terra material
y Wilhelms and McCauley (1971) . Later McCauley (1973) mapped
he highlands south of Hansteen crater near MH as principally
adial outer rim material of crater Hansteen. McCauley (1973) and
ilshire (1973) assigned the highlands material in the Hansteen
rea as belonging to the Imbrian system, and Wagner et al.,
2010) used crater counts to derive a model age for it of � 3.91 Ga.
The closest large, old craters to MH, Billy and Hansteen (both
45 km diameter), were assigned by McCauley (1973) an Imbrian
ge. This is consistent with the crater model ages of 3.88 Ga, for
illy crater and 3.87 Ga for Hansteen crater measured by Wagner
t al., (2010) .
Imbrian age and possibly older grabens form sets of structures
hat trend northeast-southwest (i.e., concentric with the Humorum
asin in the area of MH), as well as sets of northwest-southeast
rending structures that formed parallel with the edge of Oceanus
rocellarum. The northwest-southeast trending structures most
ikely were created by tectonic stresses following the formation of
he Procellarum or Imbrium basin ( Wilhelms, 1987; Solomon and
ead, 1979 ; 1980 ), while the northeast-southwest trending struc-
ures of MH are probably related to the formation of Humorum
asin, Billy crater, and Hansteen crater.
.2. Maria
Hiesinger et al., (2003) identified several distinct basalt types
n the Hansteen region based on multispectral analyses of mare
sing color data of the Clementine UVVIS-camera. The basalts
urrounding MH have FeO values in the 14 to 18 wt.% range, and
iO 2 values fall in the 2 to 6 wt.% range. These mare materials
re the youngest volcanic deposits in the area evidenced by their
mbayment of all other materials, i.e., all highlands units, older
rater materials, Hansteen or Billy, ( Whitford-Stark and Head,
980 ) as well as the units of MH.
Wilhelms and McCauley (1971) assigned Eratosthenian ages
o most of the mare materials in the MH area. Later, Wagner
t al., (2010) found that mare materials in the Hansteen region
ould be classified into three time-stratigraphic units based on
rater counts, with most being late Imbrian age. They found that
ndividual areas in Oceanus Procellarum near MH range in model
ges from 3.67 Ga to 3.35 Ga. We also counted impact craters on
he mare east of MH using LROC NAC images as a base and found
n impact crater model age of ∼ 3.5 Ga ( Fig. 4 ), consistent with
he ages found by Wagner et al., (2010) and that found by Boyce
1976) based on crater degradation.
Northeast and northwest trending wrinkle ridges have devel-
ped in the mare around MH. LROC images indicate that two
f these structures extend northwest across the mare south of
H discernable on the flank of MH for a short distance ( Fig. 5 ),
ndicating that the stress field that formed these structures ex-
sted after the formation of the mare and MH. Another set of
258 J.M. Boyce et al. / Icarus 283 (2017) 254–267
Fig. 6. Topographic contour map of Mons Hansteen and the surrounding area su-
perposed on a LROC WAC image mosaic (from LROC Quickmap). The contour inter-
val is 100 m and the numbers (black with white backgrounds) are in kilometers.
The data for this map is from Kaguya Terrain Mapper Camera images.
Fig. 7. Geologic units and structures of Mons Hansteen. The map on the left shows
the geologic units of Mons Hansteen. The map on the right shows the locations
of examples of structures (black lines) that cut Mons Hansteen (white) and mare
(wrinkle) ridges in the surrounding mare (short lines with small closed circles).
North is at the top in both figures.
Fig. 8. LROC WAC low-sun angle mosaic form LROC Quickmap showing MH (top),
and the locations of three cross sections (A-A’, B-B’ and C-C’) across it (bottom).
This mosaic (north at the top) and topographic profiles shows that MH is composed
of two progressively smaller, mesa-like layers stacked one on the other. The bottom
mesa is a rugged layer that includes small hills, valleys, and circular to elongate pits
and whose top is at ∼ 500 m (i.e., −1350 m) above the surrounding mare. The top
mesa has an oval shape with a peak just left of its center at an elevation of −924 m.
Topographic data are derived from a combination of LROC high-resolution digital
terrain model (DTM) at 100 m/pixel (LROC WAC Global Lunar DTM 100 m) with a
vertical accuracy of 10 m, 2.0 m post spacing and an RMS error relative to two LOLA
tracks of 4.73 m (data are available at the LROC website at lroc.sese.asu.edu) and
from topographic derived from Kaguya images. Vertical exaggeration 7x.
wrinkle ridges intersects MH on its northwest side ( Figs. 2 and
6 ). These wrinkle ridges are on a line that connects with the
wrinkle ridges in the south. Both sets likely formed by the same
stress system because they are the same trend. In addition, a
northeast-southwest trending wrinkle ridge intersects with Mons
Hansteen on its northeast side ( Figs. 2 and 6 ).
3.3. Mons Hansteen
Mons Hansteen is a polygonal, arrowhead-shaped, mesa that
measures ∼25 km on a side ( Fig. 2 ). Its maximum relief above
the surrounding mare is over 900 m with its base at ∼1900 m
below the global mean surface elevation ( Fig. 6 ). Our mapping
has found that it is composed of three major geologic units: the
Hilly–Dissected unit, the Pitted unit, and the North Massif unit
( Fig. 7 ), described in Section 4 .
Although previously called a volcanic dome ( McCauley, 1973 )
or a mountain of smoothly increasing elevation, LROC NAC-derived
topographic data shows that MH is actually a two-layer mesa
( Figs. 6 and 8 ). This shape is unique among red spot volcanic
complexes on the Moon. The bottom layer of the MH mesa rises
sharply (in places, with slopes ∼35 °) from the surrounding mare
surface (at ∼ –1900 m) to an average height of ∼ 525 m (i.e.,
∼ –1375 m) above the surrounding mare surface. The top layer
of MH is an elongate mesa located in the north-central portion,
and has an average height of ∼ 650 m (i.e., – 1250) above the
surface of the surrounding mare. It is capped by a small hill whose
summit is at –924 m. Obvious impact craters are found on these
mesas. But in addition to these impact craters, both mesas contain
subdued circular to quasi-circular pits ( Fig. 2 ) whose cumulative
size frequency distributions differ (i.e ., higher negative slopes)
substantially from that of the crater production function of lunar
impact craters. The population density of these subdued pits is
greatest on the top mesa ( Fig. 4 ). The subdued pits are typically
J.M. Boyce et al. / Icarus 283 (2017) 254–267 259
Fig. 9. High-resolution NAC image of a small area in the Pitted unit showing that
the ridges between the pits are relatively smooth and rounded, and the floors are
typically broad, gently rounded to nearly flat. Topographic profiles (A – A’, and B
– B’) have also been constructed using NAC DEM data across a portion of the Pit-
ted Unit (see insert for location). These profiles show that most pits are shallow
saucer shaped (i.e., relatively steep slopes on the sides and gently downward curv-
ing, bowl-shaped floors). The image is a portion of the LROC NAC M1127462982LC.
North is at the top of the image.
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few meters to tens of meters in depth and range from a few
undred meters to over a kilometer diameter ( Fig. 9 ).
In addition to the two mesa structure of MH, a small, rounded,
elatively low-albedo massif ∼ 150 m high and ∼ 5 km across is
ocated on the north edge of MH ( Fig. 10 ). A shallow northeast
rending trough ∼ 1 km wide separates this massif from the rest
f MH to the south. To the north, this massif is superposed on
ighlands materials. Two elongate pits sit atop this massif.
Mons Hansteen is cut by crossing sets of northeast-southwest
rending and southeast-northeast trending structures ( Fig. 7 ).
articularly on the lower mesa, these structures commonly form
hallow, graben-like troughs, none of which show evidence of
ateral displacement along their lengths ( Figs. 2 and 10 ). Where
he northeast-southwest trending structures cross the upper mesa,
hey commonly show coincidental alignment with chains of the
argest of the subdued pits. The cumulative size frequency distri-
ution (CSFD) ( Fig. 4 ) and their alignment along structures suggest
hat the subdued pits are most likely to be volcanic vents instead
f impact craters. Two of the major northeast-southwest trending
tructures intersect with the large elongate, cleft-like pits found
n the southwest flank of MH suggesting they also may be vents.
he most northern of these northeast-southwest structures also
ntersect with a major northeast-southwest trending structure at
he pits on the summit of the massif located on the north flank
f MH. In addition, none of the structures on MH can be traced
nto the surrounding mare or highlands nor does any of their
irections match any trends of the wrinkle ridges developed in the
are around MH. This suggests that the stress field that produced
he structures found on MH were produced by a different stress
eld than the stress field that produced the wrinkle ridges, and
as likely a result of deformation associated with MH volcanism.
Hawke et al., (2003) provided an overview of remote-sensing
nd morphology derived geochemistry of MH to suggest that it is a
elatively high-silica, low-FeO and low-TiO 2 mountain interpreted
s volcanic. They used the steep flank morphology of MH to infer
hat the lavas must have been of high viscosity (SiO 2 -rich) in order
o form it’s nearly 35 ° slopes. This is also consistent with LRO
iviner estimates of silica content suggest that MH is composed
f relatively high silica materials ( Fig. 3 ). Hawke et al., (2003) sug-
ested that a high SiO 2 content made more geochemical sense
han an exceptionally aluminous magma that would also produce
igh viscosity and steep slopes. Based on this inference, they spec-
lated that MH may be composed of dacitic or rhyolitic formed by
xtrusions of relatively viscous lavas at low rates. In this overview
hey also mapped the distribution of FeO and TiO 2 based on
lementine UVVIS and Earth-based near - IR reflectance spectra.
hey reported that the average FeO value of MH is 6.9 ± 0.5 wt.%,
ut that it might be composed of more than one geochemical unit
ith a possible unit in the central MH composed of low-FeO ma-
erials surrounded by higher-FeO material. The boundary between
hese possible geochemical units are similar to the boundaries of
he heavily pitted area on the upper mesa and the less pitted,
ummocky terrain of the lower mesa (see Section 4 ).
Hawke et al., (2003) commented that material ejected from
mpact craters on the mare may land on MH and mix with the
aterial on MH to be an important contaminant and that it could
otentially affect the ability to measure accurate compositional
alues on MH. We suggest that the effects of such contamination
re not severe enough to prevent compositional mapping and
dentification of units of different composition on MH. The effects
f contamination from impact transport on remote sensing mea-
urements was evaluated by Logan et al., (1972) at the Apollo 14
nd 15 landing sites and found to be minimal at distances of a
ilometer or two from a contamination source.
To evaluate the finding of Logan et al., (1972) , and to test this
t MH. We turn to the FeO map ( Fig. 11 ) produced by Hawke et al.,
2003) based on Clementine data as a base for this evaluation. This
as approached by mapping the change in FeO wt.% of the mare
rom the edges of MH progressively outward to where FeO content
ecomes relatively uniform, and unchanging. We assume that the
ffects of contamination from low-FeO materials blasted off MH
y small impact craters are minimal from this point outward.
ikewise, the effects of contamination from small impacts on the
are onto MH from the mare/MH border inward should be similar
ith distance to contamination in the opposite direction or even
ossibly less because MH is at higher elevation than the mare. We
onstructed profiles of FeO wt.% ( Fig. 12 ) with distance along the
ines (lines 1 – 2 and 3 – 4) shown in Fig. 11 . The locations of
hese profiles were chosen because the geology is simplest (e.g.,
o nearby highlands), the relief between MH and the mare is at
minimum, and relatively large craters are far enough away to
ontribute little materials (i.e., thickness of ejecta declines roughly
s a power of –3 relative to crater radii, Melosh, 1989 ). These
rofiles ( Fig. 12 ) show a zone of continuously increasing value of
eO wt.% from about 1 km in the interior of MH outward onto
he mare where at a distance of ∼ 1 to 2 km from the mare/MH
oundary become nearly constant. We suggest that this indicates
hat significant contamination extends away from the mare/MH
oundary in both directions for approximately 1 – 2 km.
In order to add more detail and confidence to this assessment,
e have also produced histograms of FeO wt.% values in sample
reas (each with ∼ 13 km
2 area) on the mare (D though G) along
hose profiles, as well as for sample areas containing the Pitted
nit, Hilly and Dissected unit, and North Massif (sample areas
, B and C respectively). These are plotted in Fig. 13 , and show
1) an increase in FeO wt.% of the mare surface along profile
– 2 eastward from the mare/MH boundary from area E to area
, with only a small increase in FeO from F to G (outward of G
ll mare has a FeO content of > ∼ 17.7 wt.%, similar to F and G),
nd (2) a similar trend along profile 3 – 4 where the FeO content
rom sample areas B to D significantly increases from ∼ 8.5 wt.%
260 J.M. Boyce et al. / Icarus 283 (2017) 254–267
Fig. 10. These images show Mons Hansteen under different lighting conditions. These images were used to identify structures and their trends as well as the units shown
in Fig. 7 . Top left is a high-sun angle LROC WAC image mosaic from LROC Quickmap (note low-albedo surface around North Massif outlined in white dots), and at top right
is a low-sun angle LROC WAC image mosaic (sun on the right). The lower two images have been constructed from DEM data (from Kaguya Terrain Mapper Camera data) in
order to assess illumination effects on identification of structures (white arrows show solar illumination direction). The lower left is a shaded relief image, illumination from
top, Sun elevation 15 ° above horizon. Lower right is a shaded relief image, illumination from right, Sun elevation 15 ° above horizon. North is at the top of all images.
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to nearly the average mare values. Based on these sample areas
and the distribution of FeO values on the FeO map of Hawke
et al., (2003) we suggest that a zone of significant contamination
extends outward in both directions approximately 1 – 2 km from
the mare/MH boundary. It should also be noted that if pyroclastic
material was erupted from MH these results suggest that it would
have no more of a contamination effect than did impact transport.
In an effort to identify and define major composition units on
MH, we have used this assessment of the extent of contamination
as a guide to analyze the compositional data produced by Hawke
et al., (2003) . As a starting point, we looked at FeO wt.% values in
a sample area within each of the two compositional units on the
main massif of MH (i.e., sample areas A, and B in Fig. 11 ) shown
in the maps of Hawke et al., (2003) . The locations of these sample
areas were chosen far enough away from the mare/MH boundary
to reduce, as much as possible, contamination from the mare
and/or the other MH units. The histograms of FeO wt.% values
within these two sample areas ( Fig. 13 ) show that sample area A
has FeO content of ∼6.7 wt.% and sample area B has FeO content
f 8.5 wt.%. The North Massif unit (sample area C) is also evaluated
ere, and has the FeO content of ∼ 10.4 wt.% FeO). The surface
aterials of North Massif unit are distinctly different from those
n the other two MH units (i.e., A and B) and from materials of
urrounding mare of ∼ 17.7 wt.% FeO. The North Massif unit was
ot recognized by Hawke et al., (2003) to be part of MH, likely
ecause of its lower albedo and higher FeO contents as well as its
ocation on the edge of the main edifice of MH.
The histograms of the individual sample areas in MH ( Fig. 13 )
re leptokurtic and show little to no overlap suggesting that
ach contains materials of different and distinct compositions. In
ddition, the histograms of FeO content in the sample areas in
he eastern mare (i.e., areas D, E, F, and G in Fig. 11 ) are clearly
ifferent from the sample areas on MH. Consequently, we suggest
here are at least three distinct compositional units on MH, and
hat the map distribution of these units shows a reasonable corre-
ation with the three units defined on the basis of geomorphology.
his correlation suggests the Pitted unit is composed of low-FeO
aterial, the Hilly-Dissected unit is composed of comparatively
J.M. Boyce et al. / Icarus 283 (2017) 254–267 261
Fig. 11. A map of the FeO distribution for MH and the surrounding area. The black
line on the map is the approximate boundary of MH with the other units in the
area. The white boxes are locations of sample areas A, B, C, D, E, F, and G; which
are equal in area (13 km sq.). Sample areas A, B, and C are all on MH. Sample area
A has a mean FeO value of ∼ 6.7 wt.%, with σ = 0.41; sample area B has a mean
value of ∼ 8.5 wt.% FeO, with σ = 0.54; sample Area C has a ∼ 10.4 wt.% FeO, with
σ = 0.45. Sample areas D, E, F, G, and F are all on the mare on the eastern side
of MH. Sample area G (the mare on line 1 – 2) has a mean values of ∼ 17.7 wt.%
FeO, with σ = 0.13; sample area F has a mean value of 16.9 wt.% FeO with σ + 0.15;
sample area E has a mean value of ∼ 15.6 wt.% FeO, with σ + 0.49. Sample area
D (the mare on line 3 – 4), which is about a kilometer from the edge of Mons
Hansteen has a mean value of ∼ 16.6 wt.% FeO, with σ = 0.20 (data from Hawke
et al., 2003 ). Lines 1 -2 and 3 -4 are the locations of profiles plotted in Fig. 12. .
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Fig. 12. Profiles (top profile is lines 1 – 2, and bottom is 3 – 4) of FeO values from
the map of Hawke et al. (2003) shown in Fig. 11 . The FeO values on this figure are
ranges. The closed triangles are the high values of the range and gray circle are the
low values of the range. A dotted line is drawn along the top value. The location
of the mare/MH boundary is marked with a vertical solid line. The top profile (i.e.,
1 – 2) starts in the Pitted unit, extends through a narrow band containing the Hilly-
Dissected units (the two are separated by a dashed line), and for 12 km onto the
mare east of MH. This profile suggests that contamination may be important out
to 2 km from the boundary. The bottom profile (i.e., 3 – 4) crosses a portion of the
southeastern part of MH that only includes the Hilly-Dissected unit and extends for
6 km onto the mare. This profile shows a transition zone between the two units
that suggests that contamination is important only ∼ 1 km on either side of the
boundary.
Fig. 13. Histograms of FeO values (binned in 0.25 wt.% increments) in areas sam-
pled in Fig. 8 and the entire surface of MH. Note that the units on Mons Hansteen
(A, B, and C) are distinct from one another and from those on the mare (D, E, F,
and G). We also suggest that the reason sample area E shows the lowest values
of FeO compared to the other mare sample areas is because of greater effects of
contamination of material ejected by small impact craters from MH.
igher FeO materials, and the North Massif unit is composed of
n even higher FeO content material, but still substantially lower
han the surrounding mare.
. Geologic Units; and chronology
Recently acquired high-resolution, high-quality data from mis-
ions like LRO (principally LROC) provide superb quality imaging,
opographic, and remote sensing data that support new detailed
eologic mapping of MH. Based on these new data, our mapping
as found that MHVC is comprised of three major geologic units
Fig. 7 ) associated with volcanism; (1) the Hilly-Dissected unit,
2) the Pitted unit, and (3) the North Massif unit. These will be
iscussed below in order of their age and stratigraphic position.
.1. Hilly–Dissected unit
The Hilly–Dissected unit is characterized by low hills, scarps,
esas, valleys, troughs, and various shaped depressions that are
ikely to be volcanic vents. It is composed of relatively low-iron (B
n Fig. 11 ), and relatively high-silica content material ( Hawke et al.,
003; Hawke et al., 2011 ). It comprises most of the lower mesa of
he main edifice and surrounds the Pitted unit. This unit butts up
gainst highlands materials on the southwestern and northeastern
ides of Mons Hansteen, while on its southern, northwestern, and
astern sides it is embayed by mare materials. The Hilly-Dissected
262 J.M. Boyce et al. / Icarus 283 (2017) 254–267
Fig. 14. Oblique view (looking southeastward) of the Pitted unit on Mons Hansteen.
Note the dichotomy in morphology of the relatively large pits (P) and the small
impact craters (IC). Image of LROC NAC DTM HANSTEENAL mosaic. The scene
is ∼ 9 km across.
Fig. 15. The interior slopes of most pits (“P”), especially those in the Pitted unit,
change abruptly at the pit floors, and then form a gently curving bowl shape. LROC
image number M1127462982LC. North is at the top.
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unit is the oldest stratigraphic unit of MH based on an impact
crater model age suggesting it is the oldest unit, and the cross
cutting relationship of structures.
Several types on pits and depressions are found on this unit
distinguished by the different morphology and size frequency
distributions. For example, subdued circular to quasi-circular pits
(similar to those in the Pitted unit, see below) are common in
this unit. Most of these pits are located in the southern part of
the unit. None show evidence of ejecta deposits, or have obvious
associated lava flows (these pits will be discussed in great detail
below). As well as the subdued pits, the Hilly-Dissected unit
contains several steep-sided ( ∼ 30 ° based on LROC NAC digital
terrain model [DTM]) cleft-shaped, elongate depressions ( Fig. 2 ).
These elongate depressions are unique to this unit, and unlike
secondary impact crater chains, they are each one long continuous
pit, instead of a series of connected pits. Their shape and occur-
rence in a volcanic complex make the probable volcanic vents.
Though lava flows are not observed emanating from any of them
the surface in the immediate vicinity of the two on the southwest
flank of MH ( Fig. 6 ) is smoother than elsewhere on MH and may
be blanketed by ash. The largest of these steep-sided depressions
is located on the southwestern edge of MH. Its long axis is in a
line with a lineament that cuts across the northwest flank of MH
and intersects with the two elongate subdued pits on top of the
North Massif ( Fig. 7 ).
Impact craters were also identified based on their morphology
(see next section) and counted to derive a model crater age for the
Hill-Dissected unit ( Fig. 4 ). The subdued pits were excluded from
these counts. These counts suggest an impact crater model age of
∼ 3.74 Ga (error of + 75 million years, - 150 million years) for this
unit, consistent with the model age determined by Wagner et al.,
(2010) using Lunar Orbiter IV images. In addition, the age reported
here is based on the population of craters > ∼ 0.5 km diameter
where the CSFD of these craters follows the lunar impact crater
production function. However, the CSFD of craters below that size
forms a curve with a substantially different slope (lower negative)
suggesting that these craters may have been affected by a process
that degrades small craters at a greater rate than larger ones. More
will be discussed about the possible cause of this below.
Relatively broad valleys and ridges, presumably controlled
by faulting, cut the Hilly-Dissected Unit with the broadest ones
commonly terminating at the boundary with the Pitted unit
(see Figs. 2 and 7 ). This suggests that these features predate the
Pitted unit. These are most common along the southern part of
MH in this unit with most striking in a northwest-southeast,
or northeast-southwest direction. However, there are lineaments
that strike northwestward along these large broad valleys on the
southeast side of the Hilly–Dissected unit and continue on a line
with ridges that cross the Pitted unit ( Fig. 7 ) and are on a line
hat intersects the two pits on the North Massif Unit. These two
ineaments also appear to be members of a set of closely-spaced
∼ 1–2 km apart) parallel structures that trend ∼ N 340 ° W, S 160 °) that cross MH. Consequently, there are structures that appear
o cross the Hilly-Dissected unit and extend across the Pitted
nit, suggesting that these structures were active even after the
illy-Dissected unit formed ( Fig. 7 ).
In addition, a line of low ridges extends northwestward across
he southeastern part of the Hilly–Dissected unit, and terminate at
he southern edge of the Pitted unit ( Fig. 2 ). The most northerly of
hese ridge segments cuts across the northeastern major trending
alley on the southern edge of the Pitted unit, but terminates at
hat unit suggesting that the ridge developed after the valley, but
ost likely before the Pitted unit.
.2. Pitted unit
The Pitted Unit is a high-albedo, low-iron ( ∼ 6.7 wt.%, A in
ig. 11 ), relatively high-silica area located in the north central
ortion of MH. This unit contains an area of ∼110 km
2 located
ainly above an elevation of ∼ 500 m (relative to datum). It
s characterized by closely-spaced, commonly overlapping and
ested, subdued circular to irregular-shaped depressions ( Fig. 14 )
imilar to those found in the Hilly-Dissected unit. Similar to the
illy-Dissected unit, none of the pits in the Pitted unit exhibit
jecta deposits, nor have associated lava flows. The average diam-
ter of the pits is ∼ 0.6 km, but they range from ∼0.14 km to > ∼.4 km in diameter. Fig. 9 shows topographic profiles plotted on a
igh-resolution NAC image of a small area in the Pitted unit. The
idges between the pits are relatively smooth and rounded, and the
oors are typically broad, gently rounded to nearly flat ( Fig. 15 ).
owever, the interior slopes of the pits can be relatively steep
20 ° to 25 ° based on the NAC DTM). These characteristics suggest
hat the unit may be blanketed by particulate material, such as a
olcanic ash, that drapes over ridges and pools in low places.
McCauley (1973) also suggested that these pits are of volcanic
rigin because some of them have elongate or irregular shapes.
his interpretation is consistent with their lack of ejecta deposits,
rregular outlines, subdued topography, and their association
ith the small domical mounds on this unit reported by Hawke
t al., (2014) . Mantling of the pits by fine material (discussed in
J.M. Boyce et al. / Icarus 283 (2017) 254–267 263
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Fig. 16. Elongate possible volcanic vents (arrows) at the summit of the North Mas-
sif. Pitted unit is toward the bottom of the image (portion of LROC NAC image
M1127462982LC). The area at lower left is thinly mantled by relatively low-albedo
materials likely from North Massif (see high sun-angle image in Fig. 10 ).
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etail later), such as ash, also suggests that volcanism may have
roduced these pits, although an impact or collapse origin cannot
e completely ruled out.
The cumulative size-frequency distribution of the pits in the
itted unit, and those in the Hilly–Dissected unit, are plotted in
ig 4 . This figure shows that the CSFDs of the pits exhibits marked
ifferences from the impact crater production function with the
SFD of subdued pits > ∼0.5 km in diameter (on both units)
howing steeper distribution curves than those of impact crater
roduction functions at similar diameters. These data suggest that
he pits are not impact craters, but more likely to be related to
olcanism. But, if these closely-spaced pits are volcanic vents, then
hey represent a new style of volcanic eruption.
Some pits on MH are circular, have relatively steep interior
lopes, raised rims, and exhibit ejecta blankets, and hence, appear
o be impact craters ( Fig. 14 ). Their CSFD is shown in Fig. 4 and
uggests a model age for the Pitted unit of ∼ 3.50 Ga (with an
rror of + 0.150 million years and – 1.0 billion years). This model
ge is also consistent with the model age for MH found by Wagner
t al., (2010) using Lunar Orbiter IV images. It should be noted
hat considering the error in crater density age measurements
which was used here to estimate age), this unit could have been
mplaced only a few tens of million years after the Hilly-Dissected
nit, but also could have had been emplaced as long as a billion
ears later, hence, while shorter gaps between episodes would be
ore geologically reasonable, longer gaps cannot be ruled out.
.3. North massif unit
The North Massif units is a small, dome-shape massif
∼ 6 km × 4 km, with ∼ 150 m relief) separated from the main
difice of MH on its northeast flank by a shallow trough ( Figs. 7
nd 10 ). Like the other two units, the surface of the North Massif
nit appears to be mantled, but, in contrast, has a lower albedo.
pair of shallow, elongate pits, also likely to be volcanic vents,
as formed atop this massif ( Fig. 16 ). They have formed at the
ntersection of two major lineaments on the MH. The long axis of
hese pits is on a line with the long axis of the northern most of
he elongate depressions (another vent) on the western edge of
H. These pits are also on lines connecting ridges in the Pitted
nits and the faults in the largest valley in the southern part of
he Hilly-Dissected unit.
The massif is surfaced by lower-albedo, relatively higher-iron
∼10.4% FeO, C in Fig. 11 ), higher TiO 2 ( ∼1–3 wt.%), and lower silica
based on albedo and diviner data, see Fig. 3 ) material compared
ith the other two units of MH ( Hawke et al., 2003; Glotch et al.,
010 ). The low-albedo material from this unit blankets the surface
f the Hilly–Dissected and Pitted units in the near vicinity of the
assif ( Figs. 7 and 10 ). This material is likely to be the result of
sh erupted from the pits on the North Massif and suggests that
he North Massif unit is younger than the other two units.
The model crater age of North Massif unit is difficult to con-
train because its surface area is too small to contain enough
easonable size craters (i.e., > ∼ 0.5 km diameter required to obtain
crater production function) for an accurate model age estimate
sing crater counts. However, we have counted impact craters
n this unit to assess shape of the CSFD and what it may reveal
bout processes that may have affected the crater population. The
esultant CSFD is shown in Fig. 4 and is strikingly similar to those
f the other MH units for crater diameters < ∼ 0.5 km. This sug-
ests that the craters on this unit are not a production function,
ut are also likely being affecting by the same process that affects
he small craters on other units. The density of the craters on
orth Massif, although they are not a production function they
re still similar to those on the other units at those sizes. We
uggest that likely means that the North Massif unit is likely to
e similar in age to those units. Considering that material from
orth Massif unit appears to be thinly blanketing a small area of
illy–Dissected and Pitted units on the northeast flank of MH, it
an be inferred that North Massif unit is most probably slightly
ounger than the Pitted unit but still ∼ 3.5 Ga.
.4. Pyroclastic mantle
There is evidence that the surface of the MH is mantled by a
ayer of particulate material that may have been produced by py-
oclastic volcanism. This is suggested by the subdued topography
f MH, the nature of the CSFD of its superposed impact craters
s well as their detailed morphology, weathering of blocks from
eneath a layer of smooth material at the tops of some slopes,
nd the nature of reflected light from MH.
The morphology of the small relatively fresh impact craters on
H provides evidence that MH is covered by a layer of particulate
aterial similar to regolith. These craters on MH commonly exhibit
nterior benches ( Fig. 17 ), as do those on the surrounding younger
are. Interior benches are rings of material on the interior slopes
f small impact crater that Oberbeck and Quaide (1967) suggest
re caused by the effects of different strength of materials in
ayered targets. The geometry of the benches in the craters on
he surface of MH is shown in Fig. 17 and suggests a low-strength
ayer overlying a stronger substrate. Depth estimates based on
xpected crater shapes suggests that the low-strength surface layer
hat produced these benches is ∼ 9–11 m thick. This thickness is
264 J.M. Boyce et al. / Icarus 283 (2017) 254–267
Fig. 17. Two relatively fresh, small impact craters on Hilly-Dissected unit of Mons
Hansteen that exhibit interior benches. Such benches are indications of a target that
includes a surface layer of low-strength material ∼ 9–11 m thick. Image is portion
of the LRO LROC NAC image M1127462982LC.
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∼ 3 times thicker than expected for the regolith produced by the
flux of impacts on the surface of MH alone ( Moore et al., 1980 ),
and 3 to 4 times that of the regolith on the mare just east of MH
measured using the same technique.
The shapes of impact crater size-frequency distributions (CSFD)
can provide information about surface processes. In the case of the
geologic units of MH, their CSFD ( Fig. 4 ) show that craters with
diameters > ∼ 0.5 km follow the lunar crater impact production
function, but at ∼ 0.5 km diameter (down to 0. 1 km) craters of
these units do not. Instead, these small craters decrease in abun-
dance continuously and more rapidly with decreasing crater size
relative to the production function. This particular shape of the
small crater CSFD curve can be caused by either a constant, slow
deposition of material in crater bottoms or a mantle of particulate
materials, both of which will obliterate small craters rapidly and
larger one slowly ( Hartmann, et al., 1981 , p. 1052). We suggest
that the latter is more likely on the moon.
An additional argument for pyroclastics is the surface bright-
ness of MH. Along with the high reflectance observed in all
wavelengths, the MH region has been observed to have relatively
high reflectance in the visible wavelengths ( Whitaker, 1972; Wood
and Head, 1975; Hawke et al., 2003 ). Clegg et al., (2014 , 2015 )
conducted a photometric analysis of several silicic regions of the
moon and derived a single-scattering albedo ( w ) which is depen-
dent on grain size and composition, for each region, allowing for
the direct comparison of each region corrected for the effects of
viewing geometry and phase angle. Although Compton-Belkovich
had the highest single scattering albedo values (0.59 + /–1.0 w ),
MH also exhibited very high values (0.47 + /–1.0 w ) compared to
other silicic areas and the Apollo landing sites. Apollo and Luna
soil compositions correlate with reflectance and w values such
that more reflective soils (and therefore soils with a higher w )
have higher plagioclase contents and lower mafic mineral content
and Clegg et al., (2015) found that silicic regions plot along the
extrapolation of landing site data to low mafic contents. Elevated
w values for MH indicate a lower mafic component and a higher
plagioclase or silica-rich component on the surface, which is what
would be expected for a silicic pyroclastic.
The pits and the other topography at the meter to tens of
meters scale on all geologic units of MH are subdued (low slopes
and smooth, rounded topography), in contrast to the superposed
impact craters that exhibit much crisper topography ( Fig. 15 ). This
orphology is unlike topography whose subdued morphology
s caused by age where the superposed impact craters show a
pectrum of maturity, with the morphology of oldest of these
onsistent with the degree of terrain softening of the underlying
errain. This morphology is most characteristic of terrain that has
een mantled soon after its formation with subsequent exposure
o background impact bombardment. This results in subdued ter-
ain with superposed impact craters that exhibit morphologically
resher shapes like that of the surface of MH.
Hawke et al., (2011) noted numerous areas on MH with high
lock (i.e., rocks, and boulders) densities associated with steep
lopes and impact craters, although some blocks are also found
n flat terrain. The locations of areas of relatively high rock
bundance on steep slopes on the MHVC generally correlates with
he concentrations of ≥ 1 m rocks shown in the LRO Diviner rock
bundance map ( Fig. 18 ). The areas around these patches of rocks
ppear to be relatively rock-free. A visual examination of these
ocky areas on MH using LROC NAC images shows that the places
long slopes where these blocks are concentrated are also places
here blocks appear to be weathering from under a layer of
mooth material ( Fig. 19 ). Judging by the size of the largest blocks
ompared with the thickness of the smooth material this layer of
mooth material is > ∼ 8–10 m thick along its edges. We suggest
hat this smooth material is volcanic ash.
Based on the observations discussed above, MH is most likely
antled by at least 8–10 m of particulate materials. In addition,
onsidering the results of our contamination assessment (in
ection 3.3 ), the degree of contamination of the surrounding mare
y material from MH is consistent with transport by impacts,
lthough emplacement due to explosive eruption cannot be ruled
ut. Hence, we suggest that the mantle was likely produced by
he volcanism associated with development of the MH before
mplacement of the surrounding mare.
. Geologic history of the Mons Hansteen
After the formation of Hansteen and Billy craters at around
.9 Ga, volcanism at MH began at ∼3.74 Ga (error of + 75 million
ears, - 150 million years) with eruption of relatively high-silica,
ow-iron (mean of ∼8.5% wt. FeO) materials. The materials erupted
t this time may have been vented from elongate, cleft-shaped pits
s well as nearly circular pits. This phase of volcanism produced
he Hilly–Dissected Unit and although we can find no evidence of
ava flow lobes associated with formation of this unit it is possible
hat this early volcanism was effusive in style comparable to the
iscous, high-silica lavas proposed for the Gruithuisen domes, and
assell Massif ( Chevrel et al., 1999; Wilson and Head, 2003; Ivanov
nd Head, 2015; Ashley et al., 2016 ). These eruptions appear to
ave occurred along intersecting sets of northwest-southeast and
ortheast-southwest trending faults and grabens that acted as
onduits for the magma to reach the surface. These structures
ere probably formed as a result of doming caused by intrusion of
agma beneath this center. None of these structures can be traced
nto the surrounding mare suggesting that activity along them
eased before the mare was emplaced. In addition, extension along
ome of these structures produced relatively wide (a few hundred
eter to a few kilometers) fault-controlled valley and troughs.
hese terminate against the younger Pitted unit suggesting that
hey formed during this first episode of MH formation.
The emplacement of the Hilly-Dissected unit was followed by
ruption of relatively high-silica and an even lower iron (mean of
6.7% wt. FeO) material to form the Pitted unit. These materials
over the Hilly-Dissected unit just northeast of the center of
he present MH forming the top mesa and peak of the edifice.
mplacement of the Pitted unit occurred at ∼ 3.50 Ga (error of
150 million years, − 1.0 billion years), but considering error
J.M. Boyce et al. / Icarus 283 (2017) 254–267 265
Fig. 18. Outlined in white is Mons Hansteen with an LROC WAC Quickmap mosaic image on the left and a Diviner surface rock abundance map on the right. The high
concentration of ≥ 1 m blocks occur on the slopes and fresh craters (arrows). North is at the top of both images.
Fig. 19. Top: Oblique view looking west across the Pitted unit (North is on the right
and area is ∼ 3 km across). Arrows indicate areas where rocks are prominent on
steep slopes. Bottom: LROC NAC image showing blocks on the slopes of a ridge
in the Hilly-Dissected unit. Blocks, dominantly in the size range of ∼8 m to ∼2 m,
appear to be bleeding out onto the surface. We suggest that the top of the ridge is
surfaced by a nearly 8 m thick layer of smooth material that is likely to be ash. Top
image is a portion of a LROC image M1154506530LR. The bottom image is a portion
of LROC M166182355LC of MH (north is at the top of this image).
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n the model age estimates it could have been emplaced nearly
ontinuously with the Hill-Dissected unit or over a billion years
ater. Eruption of these materials appears to have been through
umerous closely-spaced, overlapping, nearly circular pits, mainly
long closely-spaced, northwest-southeast trending fractures. Al-
hough the period of time over which the individual pits formed
annot be resolved, it is likely that they were active at different
imes suggesting a magma source that migrated from one locality
o another, not unlike the mode of formation for a field of cinder
ones on Earth (e.g., the San Francisco field; Settle, 1979 ). This
lso raises the question about the total volume of pyroclastics
hat comprise the Pitted unit. There is ∼150 m to 200 m of relief
etween this unit and the Hilly-Dissected Unit, so at one extreme
ll of this elevation might be associated with late-stage pyroclastic
ruptions, while at the other extreme, pyroclastic volcanism may
ave only produced the 9–11 m mantle that presently covers both
nits.
Soon after the emplacement of the Pitted units, relatively low-
ilica and high-iron (mean of ∼10.4 wt.% FeO) material produced
small cone shaped edifice on the north east flank of MH. This
roduced the North Massif unit. The material from these eruptions
lso thinly blankets the area around North Massif including a small
rea of the northernmost side of the Hilly–Dissected and Pitted
nits. This suggests that the North Massif unit is younger than the
ill–Dissected unit and the Pitted unit, but may approximately be
he same age the Pitted unit, or a model crater age of ∼ 3.5 Ga
but this age is only loosely constrained). The North Massif unit
its at the intersection of major faults that cut MH. These faults do
ot extend into the mare suggesting that if there was movement
long them associated with volcanism at North Massif, which is
ikely, then these structures and the North Massif unit are older
han the mare.
At about the same time as the eruption of North Massif mate-
ials and the Pitted unit, mare basalts flooded the vicinity around
H. The model age for this emplacement is ∼ 3.5 Ga and may
uggest a genetic relationship between these units. Following mare
mplacement, northeast and northwest sets of wrinkle ridges
eveloped. The strikes of these structures are also different than
hose of the sets of structures produced by extension that cut
H. These two types of structure are likely not related if wrinkle
idges are, indeed, compressional structures produced by regional
266 J.M. Boyce et al. / Icarus 283 (2017) 254–267
A
B
C
C
C
C
G
G
G
H
H
H
H
H
H
J
K
L
L
L
L
M
M
stresses much later than the structures of MH, which appear to be
extensional features produced by stress associated with volcanism.
Each geologic unit of MH is mantled, probably by volcanic
ash. This mantling likely occurred soon after their emplacement
because each unit remains geochemically distinct based on remote
sensing measurements. In addition, if the mantling material was
distributed widely, then cross-contamination would be greater
than we have observed on the units of MH as well as on the
surrounding mare. This suggests that the mantle on each unit is
composed of materials distributed only short distances from their
source vents.
6. Summary and conclusions
The volcanism at MH produced three major geologic units dur-
ing three episodes of volcanism. This volcanism began at ∼ 3.74 Ga,
relatively soon after the formation of Hansteen and Billy impact
craters, and produced the Hilly–Dissected unit. This unit makes up
the low, steep-sided lower mesa of the edifice. It is composed of
high-silica, low-FeO content materials that are mainly from vents
along northeast, and intersecting northwest trending sets of frac-
tures. This fracture system was likely produced by doming caused
by the volcanic activity under MH. None of these fractures (or
later ones) extend into the surrounding mare. The emplacement
of the Hilly-Dissected unit was followed at ∼ 3.5 Ga by volcanism
in the north central part of the mesa. These eruptions produced a
smaller mesa mapped as the Pitted unit on top the older, larger,
lower mesa. The magma that formed the Pitted unit was high in
silica and even lower in FeO than the Hilly–Dissected unit. This
material was mainly erupted from numerous vents (i.e., pits) along
closely spaced northeast-southwest trending sets of fractures. The
close spacing of the vents in the Pitted unit may represent a new
style of low-volume eruptions. Shortly afterward, lower silica and
higher FeO materials were erupted on the north flank of MH at the
intersection of two major fractures, to produce the North Massif
unit. These eruptions produced a small cone, and thinly mantled
the north flank of Hilly–Dissected and Pitted units. At about the
same time, ∼ 3.5 Ga, the surrounding mare was emplaced, flooding
the base of MH. Wrinkle ridges were subsequently formed in the
mare, but their strikes are different than the structures in MH.
This suggests that the structures of MH and the wrinkle ridges
were produced by different stress fields and at different times,
hence likely they had different origins. In addition, each unit of
MH appears to be mantled by volcanic ash. These ash deposits
also appear to have only modestly contaminated the other units
suggesting that although explosive volcanism occurred at MH it
was likely not particularly violent.
Acknowledgements
We dedicate this manuscript to our late friend and colleague
Dr. B. Ray Hawke, who had a love for all things lunar for his entire
career, and a specific long-term interest in Hansteen Alpha dating
back many decades. B. Ray was instrumental in the targeting of
many of the LROC data sets used here and we will sorely miss his
encyclopedic knowledge of lunar geology and his willingness to
share it with others. We would like to thank James Ashley, and
an anonymous reviewer for their thoughtful comments and help
to make this a much better contribution. We would also like to
acknowledge NASA’s support for coauthors MSR and SJL of ASU
through a LRO/LROC contract.
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