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: Jboyce@higp.hawaii.edu (J.M. Boyce), thomas.giguere@

intergraph.com (T.A. Giguere), pmm@higp.hawaii.edu (P.J. Mouginis-Mark),

robinson@ser.asu.edu (M.S. Robinson), slawrence@ser.asu.edu (S.J. Lawrence),

rang@higp.hawaii.edu (D. Trang), ryan.n.clegg@gmail.com (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).

i

c

l

n

a

t

c

t

t

c

a

t

b

a

e

h

u

s

a

p

a

a

u

H

t

(

s

n

a

l

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r

M

s

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M

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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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