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Icarus 283 (2017) 268–281
Contents lists available at ScienceDirect
Icarus
journal homepage: www.elsevier.com/locate/icarus
Geological mapping of impact melt deposits at lunar complex craters
Jackson and Tycho: Morphologic and topographic diversity and
relation to the cratering process
Deepak Dhingra
a , ∗, James W. Head
b , Carle M. Pieters b
a Deptartment of Physics, University of Idaho, 875 Perimeter Drive MS 0903, Moscow, ID 83843, United States b Deptartment of Earth, Environmental and Planetary Sciences, Brown University, 324 Brook Street, Box 1846, Providence, RI 02912, United States
a r t i c l e i n f o
Article history:
Received 13 July 2015
Revised 24 December 2015
Accepted 1 May 2016
Available online 9 May 2016
Keywords:
Moon surface
Impact processes
Cratering
a b s t r a c t
High resolution geological mapping, aided by imagery and elevation data from the lunar reconnaissance
orbiter (LRO) and Kaguya missions, has revealed the scientifically rich character of impact melt deposits
at two young complex craters: Jackson (71 km) and Tycho (85 km). The morphology and distribution of
mapped impact melt units provide several insights into the cratering process. We report elevation dif-
ferences ( > 200 m) among large, coherent floor sections within a single crater and interpret them to be
caused by crater wall collapse and/or large scale structural failure of the floor region. Clast-poor, smooth
melt deposits are correlated with floor sections at lower elevations and likely represent ponded deposits
sourced from higher elevation regions (viz. crater walls). In addition, these deposits are also located in
the inferred downrange direction of the impact. Melt-coated large blocks spanning several kilometers are
common on the crater floors and may represent collapsed wall sections or in some cases, subdued sec-
tions of the central peaks. Spatial trends in the mapped impact melt units at the two craters provide
clues to decipher the conditions during each impact event and subsequent evolution of the crater floor.
© 2016 Elsevier Inc. All rights reserved.
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1. Introduction
Crater floors are the largest repositories of impact melt de-
posits at impact craters. The floor region also exhibits a chaotic
landscape owing to its continuous evolution during the cratering
process, starting with the compression of the material at the im-
pact point and immediate vicinity, followed by rapid expansion
into a transient cavity aided by the excavation and displacement
of material (along with melting) and finally taking shape during
the modification processes wherein the displaced and melted ma-
terial, that failed to escape, starts to accumulate on the crater floor
as the crater units take form e.g. ( Gault et al., 1968; Grieve et al.,
1977 ). Additional levels of complexity and disturbance are incor-
porated during the formation of complex craters and basins with
the formation of terraces, central peaks, peak rings and rings, e.g.
( Melosh, 1982; Melosh and Ivanov, 1999; Head, 2010; Baker et al.,
2011; Potter, 2015 ). The morphological characteristics of impact
melt and their spatial distribution around craters have been ex-
tensively studied to understand the process of cratering, especially
∗ Corresponding author. Tel.: + 1 401 451 8785.
E-mail addresses: [email protected] , [email protected]
(D. Dhingra).
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http://dx.doi.org/10.1016/j.icarus.2016.05.004
0019-1035/© 2016 Elsevier Inc. All rights reserved.
he relationship of melt movement and emplacement with various
ratering parameters, e.g. ( Howard and Wilshire, 1975; Hawke and
ead, 1977; Krüger et al., 2013 ). Several studies have utilized im-
act melt deposits to obtain estimates of their physical properties,
.g. ( Onorato et al., 1976; Simonds et al., 1976; Bray et al., 2010;
enevi et al., 2012; Xiao et al., 2014 ).
The floors of impact craters present a complex geological set-
ing with wide variations in the morphology and topography which
s, at first glance, difficult to interpret. Geological mapping is a
undamental technique to explore complex terrains by sequentially
rouping together material with similar morphological attributes.
he resulting geological map forms the primary set of information
o understand and interpret the geological history of the region.
mpact craters provide a challenging environment and geological
apping has been extensively used to understand their character
.g. ( Schmitt et al., 1967; Wilhelms and McCauley, 1971; Öhman
nd Kring, 2012; Kramer et al., 2013 ).
Here, we map the complexity of impact melt deposits on the
oor regions of Jackson and Tycho craters ( Fig. 1 ) in great detail
tilizing high spatial resolution datasets in order to understand the
orphological diversity and spatial distribution of the various melt
nits. In addition, we identify interesting trends in the mapped
nits and utilize them to understand the probable impact condi-
ions at each of the mapped craters as well as the subsequent
D. Dhingra et al. / Icarus 283 (2017) 268–281 269
Fig. 1. LRO wide angle camera (WAC) mosaic (centered on the lunar nearside) showing the geologic setting of highland craters Jackson (22.4 °N 163.1 °W; 71 km) and Tycho
(43.29 °S 11.22 °W, 85 km) on the lunar surface. Their numerous similarities (size, age, target lithology) make them interesting candidates for comparison of impact melt
deposits. (Image credit: NASA/GSFC/ASU; Equirectangular projection).
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mplacement dynamics of the impact melt as the crater floor
volved.
. Motivation and major objectives
High spatial resolution datasets from recent missions includ-
ng LRO, Kaguya/SELENE and Chandrayaan-1, e.g. ( Chin et al., 2007;
htake et al., 2008; Goswami and Annadurai, 2009 ) have revealed
rich diversity in the morphological form and wide distribution of
mpact melt deposits on the Moon, e.g. ( Plescia and Cintala, 2012;
topar et al., 2014 ). Apart from panchromatic imagery, near in-
rared and radar datasets have added new dimensions to the study
f impact melt deposits, e.g. ( Carter et al., 2012; Neish et al., 2014;
öhler et al., 2014 ). In certain cases, entirely new perspectives on
he character of impact melt have been presented. The recent dis-
overy of a mineralogically distinct sinuous impact melt deposit
n the floor of Copernicus crater has revealed crater scale miner-
logical heterogeneity in lunar impact melts ( Dhingra et al., 2013 ).
n addition, the sinuous melt feature does not have any detectable
orphological signature, which is in contrast to the conventional
se of morphological criteria for the identification of impact melt
eposits. In this vein, it is an entirely new perspective on the oc-
urrence of impact melt deposits which needs to be included in
he identification criteria. Another example of new insight is the
mpact melt affiliation of a large olivine-bearing deposit that is
pectrally similar to an olivine-bearing lithology of primary origin
Dhingra et al., 2015a ). This finding has highlighted the importance
f geologic context for crystalline lithologies on the Moon that are
ften assumed to be always of primary origin.
Panchromatic imagery provides the highest spatial resolution
iews of the lunar surface and forms the baseline dataset for en-
ancing the interpretation of information from other datasets (viz.
ear-IR, radar datasets). The rich morphological diversity of impact
elt deposits observed in the visible imagery is the product of
he rapid succession of events that took place during the impact
rocess, something that is beyond the current modeling capabili-
ies and that has not yet been replicated in a controlled environ-
ent in the laboratory. The natural exposures of impact melt de-
osits at various craters are, therefore, the most accessible source
f information available to decode the rapid time steps involved
n the cratering process. Besides, the widespread occurrence of
mpact melt deposits makes them an important crater unit that
eeds to be characterized in detail. This forms our fundamental
otivation for the work presented here. Geological mapping pro-
ides a systematic way to document the sequence of events that
ie overprinted in the impact melt deposits. The major objectives
f this study are the following:
(i) Systematically document the various morphological forms of
impact melt on the crater floors of Jackson and Tycho.
(ii) Analyze the spatial distribution of various impact melt-
associated units with respect to each other within a crater
and contrast the characteristics across the craters.
(iii) Explore the possibility of using the mapped crater units
for understanding the cratering process and the parameters
controlling the melt formation, distribution and its morpho-
logical character.
. Data and methods
In this study, we have evaluated impact melt properties at com-
lex craters Jackson and Tycho ( Fig. 1 ). The craters have simi-
ar size (71 km and 85 km, respectively), both are Copernican in
ge (evident from their bright-rayed nature) e.g. ( Wolfe et al.,
975; Arvidson et al., 1976; Neukum and König, 1976 ; König, 1977 ;
ucchitta, 1977; Drozd et al.,1977; van der Bogert et al., 2010;
iesinger et al., 2012 ) and have formed in a predominantly high-
ands terrain. These set of similarities help simplify the interpreta-
ions which may otherwise be complicated by different crater size,
arget lithology and age. These similarities also allow the explo-
ation of other factors/processes that may be playing a role in the
mplacement dynamics of the impact melt. Accordingly, these two
raters make a good pair for comparing and contrasting the nature
f impact melt deposits for highland craters. Both the craters also
ave an extensive ray system, have prominent central peaks, and
how extensive impact melt occurrences.
.1. Geologic setting
.1.1. Jackson
Crater Jackson (22.4 °N 163.1 °W; 71 km) is located on the lu-
ar far side, east of Mare Moscoviense and north of the South
ole Aitken basin ( Fig. 1 ). The crater has a spectacular ray sys-
em, has well-formed terraces as well as a cluster of central peaks
ith varying degrees of morphological freshness. Jackson’s location
n the deep far side highlands in the feldspathic highland terrane
Jolliff et al., 20 0 0 ) suggests its principal geologic setting to be
eldspathic in nature. The crater diameter suggests an excavation
epth of about 6 km (e.g. Cintala and Grieve , 1998 ).
270 D. Dhingra et al. / Icarus 283 (2017) 268–281
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3.1.2. Tycho
Tycho (43.29 °S 11.22 °W, 85 km) is located in the southern high-
lands region on the lunar near side ( Fig. 1 ). However, some large
mare basalt exposures can be observed in the north and west of
the crater about 200 km away. Tycho has an extensive ray sys-
tem that is observable with the naked eye, a prominent central
peak and spectacular impact melt deposits that occur almost ev-
erywhere on the crater including the rim, walls, floor and even the
central peak, e.g. ( Howard and Wilshire, 1975 ; Hawke and Head,
1977; Dhingra and Pieters, 2011; Carter et al., 2012 ). Tycho is also
known for its dark halo observable under high Solar Illumina-
tion which has been suggested to represent quenched glass (e.g.
Smrekar and Pieters, 1985 ). The crater diameter suggests an exca-
vation depth of about 7 km, e.g. ( Cintala and Grieve, 1998 ).
3.2. Geological mapping
Impact melt deposits at both the craters have been mapped
based on their morphological character on a scale of 1:25,0 0 0 us-
ing ArcGIS ® software. The geographical extent of the mapping ef-
fort in this study covers the crater floor. The primary data used to
map the impact melt deposits is Kaguya Terrain Camera (TC) at a
spatial resolution of ∼10 m/pixel ( Haruyama et al., 2008 ). We have
used the ortho-rectified and map projected version (TCO) of the
TC dataset in our study. The data was downloaded as image tiles
from the SELENE data archive ( http://l2db.selene.darts.isas.jaxa.jp/ )
and subsequently imported into ArcGIS ۚfor generating the base im-
age for impact melt mapping. The morphological details mapped
out in this study are dependent on the illumination geometry. We
have therefore taken care that images selected for mapping have
comparable illumination conditions across the two selected craters.
We have also utilized elevation information from LRO lunar orbiter
laser altimeter (LOLA) e.g. Chin et al. (2007 ), Smith et al. (2010) for
defining the floor units, thus increasing the information content of
the geological map. The data was obtained from the LOLA PDS data
node ( http://imbrium.mit.edu/BROWSE/ LOLA_GDR/) in the form of
a digital elevation model (DEM) at a resolution of 512 pixel per
degree. In addition, we have used LRO WAC derived topography
( Robinson et al., 2010 ) through the LROC Quick Map functional-
ity ( http://target.lroc.asu.edu/q3/ ). We also used this online tool for
extracting selected elevation profiles across the crater floor as well
as general crater profiles. We show that integrating elevation in-
formation in the geological mapping provides additional scientific
insights compared to the morphological details alone.
3.2.1. Mapping rules
There is a certain set of rules which were followed while map-
ping the impact melt units to ensure a systematic and effective
approach. In an effort to avoid over-interpretation of the datasets,
ambiguous impact melt regions were not mapped and were in-
cluded in the undivided category. This category comprises of
regions which were too complex to be sub-divided into the ge-
ological mapping framework adopted in this study or regions that
have inadequate morphological detail to be classified meaningfully.
However, it should also be recognized that the mapping effort
is always a mix of objective criteria and subjective intuition that
builds over time. We describe below the general rules that were
followed while mapping:
(i) The unit boundaries are primarily defined based on the dif-
ferences in physical characteristics such as albedo, texture
and structure. In this study, we also used regional elevation
differences as an additional criteria.
(ii) The morphological units should have sufficient spatial coher-
ence in order to be mapped as a unit. Chaotic units that dis-
play a mixed character on small spatial scales are classified
as ‘undivided’. In cases where one unit transitions into the
other, the unit with the larger extent is used for assigning
unit classification.
(iii) The morphological units are broadly classified into floor and
peak sub-units for simplicity and to accommodate variability
that may be specific to these broad units. This convention is
followed even when certain units on the floor and the peaks
share similar morphological character.
.2.2. Geologic units
The various morphological forms of impact melt deposits iden-
ified on the floors of craters Jackson and Tycho are shown in
ig. 2 and explained below. We use an umbrella term ‘megaclasts’
or meter to kilometer-sized boulders which occur along with
usually embedded in) the melt deposits on the crater floor. We
elieve that some of these rocks represent broken-up fragments re-
ulting from fracturing of rocks during the cratering process. Some
f them could also represent displaced sections of crater walls or
he central peaks. Megaclasts occur in variety of settings and at
ifferent spatial scales. Accordingly, they have been sub-divided
nto three units based on their relative sizes as Hummocky Unit ,
solated Mounds and Large Blocks ( Fig. 2 c, d and e respectively).
he megaclast units are embedded in impact melt to different ex-
ents, similar to clasts in the impact melt and hence we use the
erm megaclasts, to emphasize the nature of this relationship be-
ween the boulders and the melt. However, there is a difference
n terms of the spatial scale (mm to cm sized clasts in hand sam-
les of impact melt, versus kilometer-size boulders embedded in
mpact melt here). Accordingly, the megaclasts should not be con-
used with the conventional small-size usage of the term ‘clast’.
e use a ratio of area to the perimeter (each measured in meters)
s a measure of megaclast size in each of the mapped megaclast
nits (i.e. Hummocky Unit, Isolated Mounds and Large Blocks ).
he ratio is calculated by obtaining measurement statistics stored
n ArcGIS ۚfor all of the mapped individual megaclasts in a given
nit. Each mapped unit has a size range as is the case with any
easurement. In addition, due to the variable melt-cover, the dis-
rete nature of megaclasts within a unit is at times, hard to dis-
ern. This factor also expands the size range. The purpose of defin-
ng the area to perimeter ratio is to have a simple metric that
an be used for making broad scale comparisons between differ-
nt megaclast units. Accordingly, for any given megaclast unit, we
ave obtained an average value. However, the units are not mutu-
lly exclusive. There is some overlap in the size range between the
egaclast units since in many cases there are transitions from one
nit to the other. In cases, where a given megaclast has size in the
verlap range between two units, then, it will be grouped in the
nit (amongst the two) which is closest to it. This strategy avoids
reating unnecessary small islands of a given unit, which by itself,
ill not be useful in the scientific analysis.
We use a combination of morphological character, surface
lbedo and elevation information to define the various impact melt
nits on the floors of craters Jackson and Tycho. Although majority
f the units are common to the two craters, there are a few geo-
ogical units that occur at one crater but are not observed in the
ther crater. We describe below the various impact melt morpho-
ogical units:
(i) Smooth Unit : This unit is usually devoid of any mega-
clasts ( Fig. 2 a) and is therefore the smoothest among all the
mapped impact melt deposits with minimal, if any, relief.
We have sub-divided the Smooth Units based on their rel-
ative elevation on the crater floor as well as surface albedo
differences. These are named as (a) Low Elevation Low
Albedo Smooth Unit , (b) High Elevation Low Albedo Smooth
D. Dhingra et al. / Icarus 283 (2017) 268–281 271
(c) Hummocky Unit (a) Smooth Unit (b) IntermediateUnit
(d) Isolated Mound(e) Large Block
(i) Central PeakBoulder Regions
(k) Smooth Pond
(j) Central PeakUnconfined
Perched Deposit (l) Flows
(g) Melt FrontStria�ons
(f) Melt Fronts
(h) Central Peak Low Albedo Coa�ng
Fig 2. Representative impact melt-associated morphologies used for mapping the geologic units on the floors of craters Jackson and Tycho. All the images have been derived
from Kaguya TC data [Image Ids: TCO_MAP_02_S42E345S45E348SC, TCO_MAP_02_ S42E348S45 E351 SC, TCO_Map_02_N24E195N21E198SC]. The scale bar in each of the
figures is 1 km.
Unit and (c) Low Elevation Intermediate Albedo Smooth
Unit .
(ii) Intermediate Unit : This unit has an intermediate morphol-
ogy between smooth and rough ( Fig. 2 b). It is comprised of
a topographically subdued megaclast population where in-
dividual clast boundaries are indistinct and they occur more
as a continuous unit with low but observable relief. We have
divided the Intermediate Unit into two sub-units based on
differences in albedo and surface elevation within the unit:
(a) Low Elevation High Albedo Intermediate Unit ; (b) High
Elevation Low Albedo Intermediate Unit .
(iii) Hummocky Unit : The unit is comprised of abundant mega-
clasts along with a relatively smaller proportion of smooth
material ( Fig. 2 c). There is a high contrast in relief due to
the hummocks and intervening low areas. This unit covers
the smallest size range of megaclasts with an average area
to perimeter ratio of ∼50. The Hummocky Unit usually oc-
curs as a pervasive unit but sometimes is also observed to
be interrupted by, and interspersed among other units.
(iv) Isolated Mounds : This unit is comprised of megaclasts that
stand out distinctively on the crater floor due to their rel-
atively high relief and considerable spacing between in-
dividual megaclasts ( Fig. 2 d). This unit covers the inter-
mediate range of megaclast sizes with an average area
to perimeter ratio of ∼200. This unit displays an identi-
fiable coating of melt on individual megaclasts. In many
cases, cooling cracks are distinctively observable in the melt
layer.
(v) Large Blocks : This unit is comprised of very large blocks (or
likely aggregates in some cases) on the crater floor ( Fig. 2 e)
272 D. Dhingra et al. / Icarus 283 (2017) 268–281
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that attain the size and elevation characteristics comparable
to the central peaks (a few kilometers in size and a few hun-
dred meters in elevation). However, in contrast to the cen-
tral peaks, the individual blocks in this unit are generally
more topographically subdued with gentler slopes and ex-
tensive melt cover. The Large Blocks cover the largest size
range of megaclasts with an average area to perimeter ra-
tio of ∼400. In contrast to other megaclast units, this unit
has the least number of individual megaclasts. At the same
time, each megaclast in this unit has a much larger areal ex-
tent compared to other megaclast classes (as highlighted by
their sizes). Individual megaclasts in this unit are sometimes
located very close to the central peaks, while in other cases
they are located much closer to the crater walls (along the
periphery of the crater floor).
(vi) Melt Fronts : These are large (several kilometers in length
and width), continuous sheet-like structures ( Fig. 2 f) with
clearly identifiable leading edge, generally having a lobate
character. These features recently described by Dhingra et al.
(2015b) generally tend to occur in clusters but occasionally,
isolated Melt Fronts have also been observed. Some mor-
phological features identified by previous workers (e.g. El-
Baz and Roosa, 1972; Howard and Wilshire, 1975; Schultz,
1976 ) bear some similarities (viz. superposition relationship)
with the Melt Front features mapped in this study. This
unit is observed in different parts of the crater floor and
wall-floor interface. The surface elevation characteristics of
this unit highlight its perched nature with some of the melt
fronts located at least a few hundred meters above the lo-
cal floor elevation and emplaced on the slopes of the central
peaks (e.g. Jackson crater). In other cases, the perched occur-
rences have been noticed on the crater wall-floor interface
(e.g. Tycho crater). The Melt Fronts are far fewer in number
and more localized compared to the previously described
units. We define a sub-unit of Melt Fronts and name it as
Melt Front Striations . This sub-unit has a morphology that
is similar to the melt fronts. However, in this case, only the
leading edge of the melt front is identifiable while the trail-
ing side is merged with the background floor melt deposits
( Fig. 2 g).
(vii) Central Peak Low Albedo Coating : This unit is fairly thin
(sub-surface undulations are visible), has a low albedo and
is principally associated with the central peaks although not
all peaks host this impact melt unit ( Fig. 2 h). This unit is rel-
atively smooth in texture with no observable cooling cracks.
It has also been determined to be mineralogically distinct
[e.g. Tompkins, 1997; Ohtake et al., 2009 ]. This unit is only
observed at Jackson crater.
(viii) Central Peak Boulder Regions : This central peak unit is
comprised of boulder fields that are pervasive over the peak
region ( Fig. 2 i). The mapped boulders are at the limit of im-
age resolution and so are expected to be few 10s of meters
in size or less. As a consequence, instead of mapping indi-
vidual boulders, large boulder clusters were mapped.
(ix) Central Peak Unconfined Perched Deposits : This central
peak impact melt unit is principally defined as a smooth,
perched deposit that is not confined by high standing to-
pography on all the sides ( Fig. 2 j). This lack of complete to-
pographic confinement makes these deposits different from
impact melt ponds that otherwise occur in close proximity
to this unit. This unit generally occurs on the slopes of the
peaks but also sometimes occurs in relatively flat regions on
the central peaks.
(x) Smooth Ponds : These are flat deposits with usually smooth
surfaces (negligible clasts if any) that are confined on all
sides ( Fig. 2 k) by high standing topography.
(xi) Flows : These are elongated features with lobate ends, gen-
erally occurring along the crater wall–floor interface in this
study ( Fig. 2 l).
. Impact melt morphological diversity on crater floors
The impact melt deposits on the floors of craters Jackson and
ycho display spectacular diversity in their morphological form,
he relationships among various units and the overall hierarchy
f complexity. The young age of the two craters allows confident
dentification of a variety of impact melt deposits located on the
rater floor. Our detailed mapping of the floor impact melt de-
osits, the single largest impact melt unit in any typical crater,
rovides useful information about the character of the melt and
he processes controlling impact melt formation and emplacement.
e describe here the properties of these impact melt deposits.
.1. Jackson
The floor of Jackson crater provides a diverse variety of im-
act melt units. We have mapped 15 geological units on the crater
oor based on the surface texture, albedo and surface elevation.
he latter was included due to a distinct elevation pattern ob-
erved on the crater floor ( Fig. 3 ). There are five large melt units
n the floor forming the base unit framework onto which the re-
aining units are overprinted ( Fig. 4 ). The three most extensive
nits are Low Elevation High Albedo Intermediate Unit (N and
W crater floor), High Elevation Low Albedo Intermediate Unit (N
nd E crater floor) and Low Elevation Low Albedo Smooth Unit
SW crater floor). Each unit usually occurs as a coherent entity
hen viewed at the crater scale but may be locally interrupted by
maller units. In the case of the Low Elevation Low Albedo Smooth
nit , it appears to extend into the Low Elevation High Albedo In-
ermediate Unit . Relatively smaller-scale continuous units include
he Smooth Unit and High Elevation Low Albedo Smooth Unit .
hese units are more limited in their spatial extent and are princi-
ally located in parts of the southern and eastern crater floor.
The second important set of mapped geological units includes
he sparsely populated megaclasts. The largest megaclast unit rep-
esented by the Large Blocks (average area/perimeter ratio ∼400)
as a size range comparable to the central peaks. There are two
arge Block units mapped at Jackson crater, one located in the
orthern crater floor and the other is close to the crater center. The
atter is far away from the crater walls but is in close proximity to
he central peaks. We interpret this spatial relationship to mean
hat the Large Block located near the crater center could poten-
ially be a subdued central peak section. Both the Large Block units
re extensively draped with impact melt with the latter showing
n extensively fractured melt layer ( Fig. 5 a, b).
The next megaclast unit in terms of size is comprised of Iso-
ated Boulders (average area/perimeter ratio ∼200). This unit is
argely located in the western part of the crater floor at Jackson.
s reported for the Large Blocks , the Isolated Mounds also appear
o be covered with impact melt with some showing an overlap-
ing Melt Front ( Fig. 5 c). It is interesting to note that a similar re-
ationship has also been observed earlier using lunar orbiter data
e.g. Schultz, 1976 ; plate 33b] where it was inferred that either the
ounds protruded through the cooling melt or the melt had con-
racted and slipped off from the boulder due to gradual cooling.
e have added a third possibility in this context based on the oc-
urrence of these Melt Fronts elsewhere in the crater as well as at
ycho ( Dhingra et al., 2015b ). The lobate margins of these features
ould be indicating an advancing sheet of impact melt that was
talled due to higher elevation of the boulder in its path. How-
ver, further studies are required to develop a more robust in-
erpretation of these morphologic features. Many of the mapped
D. Dhingra et al. / Icarus 283 (2017) 268–281 273
Fig 3. Elevation differences observed among large floor sections within craters Jackson and Tycho. (a) Kaguya TC albedo image of the floor region of Jackson crater (b) LRO
LOLA derived DEM overlain on Kaguya TC dataset showing relatively higher elevation of the eastern crater floor compared to the western section. (c) W −E topographic
profile derived from LROC WAC on northern floor of Jackson crater showing steep variations in topography. (d) W −E topographic profile on crater southern floor highlighting
the same trend. (e) Kaguya TC albedo image of the floor region of Tycho crater. (f) LRO LOLA derived DEM overlain on Kaguya TC dataset showing a higher elevation of the
western crater floor in contrast to the eastern crater floor. (g) W −E topographic profile derived from LROC WAC on northern floor of Tycho crater showing steep variations
in topography. (h) W −E topographic profile on the southern crater floor highlighting the same trend.
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oulder units are comprised of high-standing topography with
oorly-defined boundaries ( Fig. 5 d).
The unit comprising the smallest size megaclasts (average
rea/perimeter ratio ∼50) is represented by the Hummocky Unit
n the crater floor. It is spatially less extensive at Jackson crater
ompared to the other megaclast units, and tends to occur in clus-
ers. A major cluster is located on the NW crater floor (observ-
ble in Fig. 4 b and c) surrounded by Isolated Mounds . Relatively
maller clusters of the Hummocky Unit are located in the remain-
ng part of the crater floor and can be found associated with all
he major extensive units discussed above.
The central peaks on the crater floor also contain small impact
elt occurrences mainly as Unconfined Perched Deposits . Major
ccurrences are on the southern central peaks. The melt deposits
ithin this geologic unit show textural diversity. Another promi-
ent melt occurrence is on the apex of the southernmost peak.
wing to the different morphological character of this latter occur-
ence, namely a smooth low-albedo coating, it has been classified
eparately as Central Peak Low Albedo Coating . In addition, there
re numerous boulder clusters, Central Peak Boulder Regions , dis-
ributed throughout the central peak on flat as well as on sloping
urfaces.
.2. Tycho
The impact melt on the floor of Tycho occurs at two different
levations with the western crater floor at a relatively higher el-
vation ( > 200 m difference) than the eastern crater floor ( Fig.
f, g, h). This difference in elevation has also been noted ear-
ier by Margot et al. (1999 ) using Earth-based radar observations.
his two floor-level setting is similar to Jackson which contains a
elatively elevated eastern crater floor as compared to the west-
rn floor ( Fig. 3 b–d). It is also interesting to note that the spa-
ial extent of the two floor sections at Tycho (at different eleva-
ions) correlates roughly (but not exactly) with the spatial extent
f the floor as divided by surface roughness (e.g. Pohn, 1972 ) (also
ee Fig. 3 e and f). Based on the broad scale surface roughness of
he crater floor, the western crater floor is hummocky while the
astern crater floor is smooth which is consistent with the geo-
ogic map of Pohn (1972) based on lunar orbiter IV and V images.
owever, we have been able to subdivide the floor into additional
eologic units. We have mapped 15 geological units on the crater
oor of Tycho with four extensive units and 11 units of limited
xtent or discontinuous nature ( Fig. 6 ). Among the four continu-
us units, two units ( High Elevation Low Albedo Intermediate Unit
nd Low Elevation Intermediate Albedo Smooth Unit ) have protru-
ions into nearby units ( High Elevation High Albedo Intermediate
nit and Low Elevation Intermediate Albedo Intermediate Unit ).
even units amongst the 11 discontinuous or limited extent units
re interspersed between the extensive units stated above. The re-
aining four units are located on the central peaks. We describe
elow several interesting trends in the distribution of the mapped
nits. While some units have similar trends as at Jackson crater,
ertain other units display a contrasting trend.
Among the megaclasts (viz. Hummocky Unit , Isolated Mounds
nd Large Blocks ), the Isolated Mounds Unit is largely concen-
rated in the northern crater floor at Tycho while the Large Blocks
nit is mostly located in the western part of the crater floor
Fig. 6 ). The Hummocky unit is observed to be quite extensive on
he crater floor ( ∼7% of the floor area) and this trend is in contrast
o the relatively limited extent of this unit ( ∼3% of the floor area)
t Jackson crater ( Fig. 7 ). In addition, the low elevation region on
he eastern crater floor of Tycho, with the exception of the Low
levation Intermediate Albedo Smooth Unit ( Fig. 6 ; brown color),
as a notably higher concentration of the Hummocky Unit ( Figs. 6
nd 7 ; magenta color) as compared to the high elevation region
n the west. The High Elevation Low Albedo Smooth Unit ( Fig. 6 ;
ight pink) , located in the southwestern crater floor is almost de-
oid of the Hummocky Unit and Isolated Mounds Units .
The observed contrast in the density of the Hummocky Unit be-
ween the eastern and western crater floor sections at Tycho, if
aken by itself, will make the eastern crater floor rougher com-
ared to the western crater floor. This is in stark contrast to our
arlier observations and crater floor unit assignments in geologic
ap of Tycho by Pohn (1972) where on broad scale, the western
rater floor is hummocky while the eastern crater floor is smooth.
274 D. Dhingra et al. / Icarus 283 (2017) 268–281
Fig 4. Impact melt units at Jackson crater. (a) Albedo image from Kaguya Terrain Camera (TC) [Image Id: TCO_Map_02_N24E195N21E198SC]. (b) Geological map of impact
melt units on the crater floor. (c) Geological map draped over the albedo image.
D. Dhingra et al. / Icarus 283 (2017) 268–281 275
Fig 5. Morphological details of selected megaclast units at Jackson crater. (a) Large Block unit located close to the central peaks shows extensive melt cover on its surface.
(b) Large block unit (likely an aggregate) located in the northern part of crater floor shows thick melt deposits covering all over the region. Very few clear exposures of
the underlying rock unit are otherwise visible. (c) An Isolated Mound (marked with solid black arrow) located in the northwestern crater floor partially covered by a Melt
Front. (d) Isolated Mounds located in the southern crater floor showing indistinct boundaries due to extensive melt cover which is also heavily fractured due to subsequent
cooling. Also note the sinuous feature in the center-bottom of the image (marked with a hollow black arrow). It could represent a flow channel that is now covered by the
impact melt. All images have been taken from Kaguya TC [Image id: TCO_Map_02_N24E195N21E198SC].
T
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his observation highlights the scale dependent interpretation of
urface roughness which may help identify multiple factors and
rocesses at work.
The crater floor has some Large Blocks located in the western
art that are draped completely by the impact melt. Although they
re large and coherent in nature, these Large Blocks appear rel-
tively subdued as compared to the observations of this unit at
ackson crater. Tycho also displays Flows and Melt Fronts (and
elt Front Striations ), mostly along the periphery of the crater
oor.
The central peaks of Tycho display notable melt cover. Central
eak Unconfined Perched Deposit is the most common unit on the
eak region. There is also a large melt pond in the northern sec-
ion of the main central peak unit. Many impact melt locations on
he peak display an extensive set of cooling cracks. The Central
eak Boulder Regions are mostly located at the distal ends of the
apped melt occurrences at Tycho.
. Insights into the cratering process
The diverse array of relationships observed between the
apped impact melt units and their contrast at the two craters
ackson and Tycho provides us the opportunity to utilize this
nowledge in interpreting some of the details of the cratering pro-
ess. Here, we assimilate all the observations to understand the
arious aspects of the cratering process.
.1. Structural evolution of the crater floor
The crater floors of both Jackson and Tycho have been shown
o be composed of coherent sections that lie at different eleva-
ions ( > 200 m difference in elevation). We have previously also
ocumented a topographic low on the floor of Copernicus crater
Dhingra et al., 2013 ). In view of these observations from craters
ocated in widely different regions of the Moon, we suggest that
levation differences on the crater floors may be a relatively com-
on phenomenon. There could be multiple causes for the ob-
erved differences in floor elevation within a crater: (a) differential
ubsidence of the floor during cooling of the melt column ( e.g .
ilson and Head , 2011 ), (b) collapse of a wall section that could
ile up material on the adjacent crater floor (e.g. Hawke and Head,
977; Margot et al., 1999 ), (c) impact conditions including impact
irection, impact angle, pre-impact topography (e.g. Hawke and
ead, 1977 ) and (d) structural failure of the floor section along a
ajor plane of weakness. We provide here some observations that
ould help in evaluating the dominant cause of the elevation dif-
erences on the crater floors of Jackson and Tycho.
276 D. Dhingra et al. / Icarus 283 (2017) 268–281
Fig 6. Impact melt units at crater Tycho. (a) Albedo image from Kaguya Terrain Camera (TC) [Image ids: TCO_MAP_02_S42E345S45E348SC, TCO_MAP_02_S42E348S45E351SC].
(b) Geological map of impact melt units on the crater floor. (c) Geological map draped over the albedo image.
D. Dhingra et al. / Icarus 283 (2017) 268–281 277
Fig 7. Contrasting trend in the spatial distribution of the hummocky unit on the
crater floors. Mapped distribution of hummocky unit overlain on Kaguya TC image
for (a) Jackson (3% of the floor area) and (b) Tycho (7% of the floor area).
c
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Scenario 1. Subsidence due to melt cooling: Cooling of the melt
olumn causes contraction in lateral and vertical directions lead-
ng to subsidence (e.g. Bratt et al., 1985; Wilson and Head, 2011 ).
n view of the highly energetic nature of the impact cratering pro-
ess, large scale slumping of material occurs along with melting
nd the floor is expected to be a dynamically created mixture of
elt and rock lacking spatial coherence. As a consequence, sub-
idence of crater floor due to the cooling of melt column should
ead to elevation differences on the local scale (irregular topog-
aphy) due to varying amounts of impact melt and rock mix-
ures at different locations in the crater. However, such small scale
opographic variations do not dominate at Jackson or Tycho at
he available spatial resolution. Instead, we observe that the floor
s divided into two very large sections (several hundred square
ilometers), each occurring at a different elevation ( > 200 m; also
ee selected topographic profiles shown in 3c, 3d and 3 g, 3 h).
n such a scenario, subsidence due to melt cooling seems to
e an unlikely dominant cause for the crater scale elevation
ifferences.
However, a special case where cooling-induced subsidence
ould be important is where the impact melt column has substan-
ial differences in the melt-clast ratio in different geographical sec-
ions of the crater floor. Such a setting may lead to a significant
ubsidence in clast-poor section of the floor (due to higher volume
f molten material and therefore greater contraction) compared to
he clast-rich section. One such scenario could involve late stage
rater wall collapse which would contribute significant clast vol-
me to the melt-column in the immediate vicinity but not much
o the melt-column located further away. It is further discussed
s a separate scenario in the next section. In principle, we could
lways invoke a hybrid scenario where subsidence due to cooling
omplemented by crater wall collapse led to the observed eleva-
ion differences.
Scenario 2. Crater wall collapse: If crater wall collapse played a
ominant role in causing the observed elevation differences on the
rater floor, then such an event would contribute abundant rock
ragments on the adjacent floor section raising its elevation e.g.
Margot et al., 1999 ). The distribution of megaclasts ( Hummocky
nit, Isolated Mounds and the Large Blocks ) at the two craters
an be used as a measure of the rock fragment abundance in dif-
erent floor sections. In the case of Jackson, the higher elevation
astern crater floor section has a distinctively lower proportion of
egaclasts when compared to the low elevation western floor sec-
ion which is contrary to the expectation that wall collapse should
dd rock fragments to the nearby floor region. The wall collapse
cenario is thus not favored at Jackson to explain the floor eleva-
ion differences.
The trends at Tycho seem to support crater wall collapse as
probable cause. The higher elevation western floor section has
n abundance of megaclasts with respect to the lower elevation
astern floor section. The western floor section has distinctively
igher proportion of intermediate size megaclasts, namely Isolated
ounds Unit . In addition, all the large size megaclasts, namely
arge Blocks Unit , are also concentrated mostly within the west-
rn floor section. These collective insights prompt us to suggest
hat wall collapse could have possibly played an important role in
ausing the observed floor elevation differences at Tycho. This in-
erence is supported by independent set of observations by Hawke
nd Head (1977 ), Margot et al. (1999 ) and more recently by Krüger
t al. (2013 ) who suggested wall collapse in the SW crater wall of
ycho based on parameters such as the distinctively gentler slope
f the SW crater wall compared to the other sides, lower rim crest
levation and wider terraced walls in the western and south west-
rn parts of the crater.
Scenario 3. Impact conditions: Several parameters collectively de-
ne the impact conditions. These include direction and angle of
mpact, impactor velocity, pre-impact target topography and target
aterial properties. While a detailed assessment of all the param-
ters is beyond the scope of this study, we present an evaluation
f some important parameters. We infer the direction of impact
ased on either the observed zone of avoidance or other informa-
ion such as spatial distribution of impact melt ponds on the crater
im (as reported in the literature). In the case of crater Jackson, we
sed the observed zone of avoidance which refers to a ‘no crater
jecta zone’ and is generally observed in the uprange direction of
mpact when the impact angle of incidence is less than 45 ° (but
ould also occur downrange at increasingly smaller impact angles)
.g. Gault and Wedekind (1978 ).
We compare the location of zone of avoidance (that provides
he direction of impact) and the elevation characteristics of the
oor units to explore possible correlations. At Jackson, the inferred
mpactor direction is from NW based on the observed zone of
voidance ( Fig. 8 a). The lower elevation floor section is located in
roximity to the uprange direction ( Fig. 8 b, c). The impact direc-
ion for Tycho is from SW ( Fig. 8 d) e.g. ( Hawke and Head, 1977;
argot et al., 1999; Krüger et al., 2013 ). The lower elevation floor
ection is located in the downrange direction in this case ( Fig. 8 e,
). There are therefore, no consistent trends observed with respect
o the impact direction and so it is unclear in the present context
278 D. Dhingra et al. / Icarus 283 (2017) 268–281
Fig 8. Regional distribution of crater rays and topographic relationships at craters Jackson and Tycho. (a) High Sun LROC WAC global mosaic derived image of crater Jackson
showing the distribution of its extensive rays. Note the zone of avoidance on the NW rim which is almost devoid of rays and indicates the direction of the impactor.
(b) LROC WAC derived topography (overlaid on WAC imagery) around the Jackson crater shows that the crater’s western edge lies at regional topographic boundary. (c) WAC
derived regional topographic profile across crater Jackson confirms the W-E asymmetry. (d) High Sun LROC WAC image of crater Tycho showing the distribution of crater
rays. (e) LROC WAC derived topography for the region around crater Tycho does not show large scale topographic differences as observed in the case of Jackson. (f) WAC
derived regional topographic profile across crater Tycho. The black arrows in images (a) and (d) indicate the likely direction of impact based on the zone of avoidance (in
the case of Jackson) and distribution of impact melt ponds (in the case of Tycho. Sourced from Krüger et al. (2013) . The white dashed lines in (b) and (e) represent the
geographic location of the profiles shown in (c) and (f).
D. Dhingra et al. / Icarus 283 (2017) 268–281 279
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hether impact direction alone could have caused the observed el-
vation differences on the crater floors.
Pre-impact topography has been shown to be an important pa-
ameter affecting the crater morphology and melt movement e.g.
Hawke and Head, 1977; Gulick et al., 2008; Neish et al., 2014 ).
he regional topography at Jackson crater ( Fig. 8 b, c) indicates a
W −SE asymmetry with the NW section at the edge of a topo-
raphic low. The floor sections exhibit an E −W asymmetry in ele-
ation with the western section at a lower elevation. These two ob-
ervations seem to be correlated to certain extent. We make some
dditional observations. The higher elevation SE crater wall at Jack-
on is much wider than other wall sections of the crater, possibly
inting at a slumped wall section e.g. ( Hawke and Head, 1977 ). As
xpected, the adjacent floor section also has a higher elevation but
urprisingly there is a dearth of rock fragments that should have
een produced by the wall slumping. Based on all these observa-
ions, pre-impact topography may have affected the crater floor el-
vation differences at Jackson but there is ambiguity about the sur-
ace expression of the slumped material.
The available topography data for Tycho does not indicate a to-
ographical asymmetry ( Fig. 8 e, f) on a regional scale (as observed
n the case of Jackson) so any direct correlations of the observed
oor elevation differences with the pre-impact regional relief is
uled out at this scale. However, it is known that pre-existing lo-
al topography did affect the distribution of exterior impact melt
eposits around Tycho, e.g. ( Margot et al., 1999 ).
Scenario 4. Structural failure along a major plane of weakness : The
ailure of coherent floor sections along substantially weak zones is
onceivable in view of the massive structural damage taking place
uring the cratering process. This mechanism could be favorable in
iew of the observed trends in the spatial distribution of the Hum-
ocky Unit at Tycho. If floor elevation differences were produced
uring the early stages of the crater formation, then melt will be
xpected to move towards the lower elevation floor unit thereby
nundating the region and submerging any small-scale topography.
ontrary to this expectation, the low elevation floor units at crater
ycho (i.e. the eastern crater floor) have an observationally high
ensity of the Hummocky Unit ( Fig. 7 ). We interpret this obser-
ation to suggest that floor elevation differences likely developed
n the later part of the cratering process, probably during the fi-
al adjustment stages or even at a much later stage when the
ajority of the crater formation process came to rest. Structural
ailure could be one of the mechanisms still possible in the very
ate stages and may have caused the observed elevation differ-
nces. In such a scenario, the impact melt deposits would have ac-
uired sufficient viscosity to not drain out from the high elevation
ection of the floor to the low elevation section, avoiding flooding
f the low-lying units. Accordingly, the observed high density of
he Hummocky Unit on the lower elevation floor section of Tycho
ould be maintained even after the development of floor elevation
ifferences.
.2. Possible origin of the smooth melt deposits
The smooth melt deposits ( Smooth Unit and sub-units) pro-
ide an interesting contrast to the otherwise rough crater floor.
e make a couple of interesting observations about the smooth
elt deposits that may be related to the evolution of the crater
oors. The smooth melt deposits have been observed to occur both
t high elevations and low elevations on the floors of craters Jack-
on and Tycho. However, the Smooth Unit , the largest amongst the
mooth melt deposits (light brown unit in the geological maps),
as consistently been observed to be located within the floor
ections at the lowest elevation in the case of both Tycho and
ackson craters. We interpret these units as the likely locations
here the final dregs of impact melt ponded, flowing down from
earby regions at higher elevations, either on the floor or from the
alls.
Another associated observation is the approximate correlation
etween the geographic locations of the Smooth Units and the in-
erred downrange impact direction. It is known that at impactor
ncidence angles less than 45 °, preferentially larger volume of melt
s focused in the downrange direction e.g. ( Gault and Wedekind,
978 ). This relationship has been utilized at crater Tycho to in-
er the impactor direction (from SW) based on higher abundance
f mapped melt ponds on the NE crater rim e.g. ( Krüger et al.,
013 ). We think that a larger melt volume focused in the down-
ange direction will also lead to the downrange region receiving
elatively large volume of melt-fall back into the crater. As a re-
ult, melt may preferentially accumulate in the region that lies in
he downrange impact direction, consistent with the observations.
n addition, since draining of melt back into the crater, is a rela-
ively late stage activity, significant melt movement after ponding
ay be limited, which may have helped in preserving this relation-
hip. However, we would like to acknowledge that this is only one
f the possibilities which seem to satisfy all the constraints. There
ay well be other aspects which could have also contributed to
hese observations.
.3. Central peak–impact melt relationship
The central peaks represent a unique geological unit on the
oor of impact craters. In contrast to their spatial proximity to the
rater floor, they are believed to represent material from a differ-
nt part of the crustal column and have therefore been extensively
sed in determining the mineralogy of deeper parts of the lunar
rust e.g. ( Tompkins and Pieters, 1999; Cahill et al., 2009; Song et
l., 2013; Lemelin et al., 2015 ). Impact melt, on the other hand,
epresents a mixture of lithologies present in the crustal column
hich may not share any direct mineralogical link with the cen-
ral peak lithologies. The two units represent different sampling
epths: the impact melt is produced from an upper shallower part
f the lithological column while central peaks have been suggested
o form from material below the melted lithologic horizon e.g.
Cintala and Grieve, 1998 ).
Our mapping of impact melt deposits on the central peaks
f Tycho and Jackson therefore documents a hybrid geological
etting where material from different depths and physical form
melt vs un-melted peak material), that could conceivably be dif-
erent in composition, is juxtaposed in various forms. It includes
elt-draped surfaces, melt ponds and flows. This geologic setting
herefore has obvious implications for the use of central peaks to
ecipher the sub-surface mineralogy. Similar observations of im-
act melt deposits on central peaks have also been made earlier
.g. ( Ohtake et al., 2009; Dhingra et al., 2014 ) although systematic
nd detailed mapping of various impact melt deposits associated
ith the central peaks has been lacking so far. The mapping of
mpact melt exposures on the central peaks of Tycho and Jackson
n this study highlights the fact that direct interpretations of the
ubsurface mineralogy based on central peaks could be mislead-
ng unless melt-free regions on the peaks are specifically chosen
or determining the mineralogy. Systematic geological mapping of
mpact melt deposits of the central peaks could help identify such
elt free regions for compositional analysis.
. Summary and major conclusions
The detailed investigations presented in this study document
he morphological properties of impact melt at two young, com-
lex craters, Jackson and Tycho. The salient findings from this
tudy are as follows:
280 D. Dhingra et al. / Icarus 283 (2017) 268–281
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(i) There is a rich morphological diversity in the impact melt
deposits on the crater floors of Jackson and Tycho that hold
clues to the understanding of the cratering process. The de-
tailed mapping carried out in this study highlights the fact
that there is a certain degree of order (or systematics) asso-
ciated with the impact melt deposits which otherwise ap-
pear quite chaotic in the first look. Accordingly, scientifi-
cally significant trends could be inferred based on impact
melt mapping studies to understand the cratering process
in general and impact melt generation and emplacement in
particular.
(ii) We observe elevation differences between coherent floor
sections within both Jackson and Tycho and have also re-
ported such differences in the case of Copernicus crater
( Dhingra et al., 2013 ). These observations suggest the pos-
sibility that elevation differences between coherent floor
sections may be a common phenomenon at many of the
impact craters. We have evaluated several possibilities to
understand the potential cause(s) for the observed eleva-
tion differences on the crater floors. We propose that late-
stage wall collapse, structural failure of the floor along major
weak zones and pre-impact topography could be the pos-
sible causes of observed elevation differences on the floor.
However, additional observations at other large craters (e.g.
Dhingra et al., 2016 ) could help validate these interpreta-
tions.
(iii) This study documents the occurrence of very large sized
rock fragments ( Large Blocks Unit ) on the crater floor that
have the size and elevation characteristics similar to the cen-
tral peaks. Some of the mapped large blocks are located
in close proximity to the central peak complex and could
therefore be subdued sections of the central peak unit.
(iv) Our detailed mapping of impact melt deposits on the central
peaks highlights the often overlooked fact that the central
peaks may not always represent pristine exposures of the
sub-surface crustal material and could be contaminated with
impact melt deposits. As a consequence, the prevalent use of
central peak units in deciphering mineralogy of deeper part
of the crust needs to take this factor into consideration. This
has implications not only for the Moon but for other plan-
etary bodies as well where central peak mineralogical sur-
veys have been carried out. Melt-free regions should be first
identified through systematic geological mapping before de-
ciphering the mineralogy of the peaks.
Acknowledgments
This research was supported by SSERVI grant no. NNA14AB01A .
We thank the LRO and Kaguya mission teams for making high
quality datasets available in the public domain. We would also like
to thank the two anonymous reviewers and the editors for their
thoughtful comments and suggestions.
References
Arvidson, R. , Drozd, R. , Guinness, E. , Hohenberg, C. , Morgan, C. , Morrison, R. , Ober-beck, V. , 1976. Cosmic ray exposure ages of Apollo 17 samples and the age of
Tycho. In: 7th Proc. Lunar Sci. Conf., pp. 2817–2832 . Baker, D.M.H., Head III, J.W., Fassett, C.I., Kadish, S.J., Smith, D.E., Zuber, M.T., Neu-
mann, G.A., 2011. The transition from complex crater to peak-ring basin on theMoon: New observations from the lunar orbiter laser altimeter (LOLA) instru-
ment. Icarus 214, 377–393 http://dx.doi.org/10.1016/j.icarus.2011.05.030 .
Bratt, S.R. , Solomon, S.C. , Head, J.W. , 1985. The evolution of impact basins: Cooling,subsidence and thermal stress. J. Geophys. Res. 90 (B14), 12415–12433 .
Bray, V.J., et al., 2010. New insight into lunar impact melt mobility from LRO camera.Geophys. Res. Lett. 37, L21202. doi: 10.1029/2010GL044666 .
Cahill, J.T.S., Lucey, P.G., Wieczorek, M.A., 2009. Compositional variations of the lu-nar crust: results from radiative transfer modeling of central peak spectra. J.
Geophys. Res. 114, E09001 http://dx.doi.org/10.1029/2008JE003282 .
arter, L.M., Neish, C.D., Bussey, D.B.J., Spudis, P.D., Patterson, G.W., Cahill, J.T.,Raney, R.K., 2012. Initial observations of lunar impact melts and ejecta flows
with the Mini-RF radar. J. Geophys. Res. 117, E00H09. doi: 10.1029/2011JE003911 .hin , et al. , 2007. Lunar reconnaissance orbiter overview: The instrument suite and
mission. Space Sci. Rev. 129, 391–419 . intala, M.J., Grieve, R.A.F., 1998. Scaling impact melting and crater dimensions: Im-
plications for the lunar cratering record. Meteorit. Planet. Sci. 33 (4), 889–912.doi: 10.1111/j.1945-5100.1998.tb01695.x .
Denevi, B.W. , Koeber, S.D. , Robinson, M.S. , Garry, W.B , Hawke, B.R , Tran, T.N. ,
Lawrence, S.J. , Keszthelyi, L.P. , Barnouin, O.S. , Ernst, C.M. , Tornabene, L.L. , 2012.Physical constraints on impact melt properties from lunar reconnaissance or-
biter camera images. Icarus 219 (2), 665–675 . hingra, D. , Pieters, C.M. , Head, J.W. , Isaacsson, P.J. , 2013. Large mineralogically dis-
tinct impact melt feature at Copernicus crater – Evidence for retention of com-positional heterogeneity. Geophys. Res. Lett. 10, 1–6 .
hingra, R.D. , Dhingra, D. , Carlson, L. , 2014. Evaluating the extent of impact melt on
central peaks of lunar complex impact craters. In: 45th Lunar and Planet. Sci.Conf .
hingra, D. , Pieters, C.M. , Head, J.W. , 2015a. Multiple origins of olivine at Coperni-cus. Earth Planet. Sci. Lett. 420, 95–101 .
hingra, D. , Head, J.W. , Pieters, C.M. , 2015b. Impact melt wavefronts at lunar com-plex craters: morphological characteristics and insights into impact melt em-
placement dynamics. In: 46th Lunar and Planet. Sci. Conf. Abstract# 1645 .
hingra, D. , Head, J.W. , Pieters, C.M. , 2016. Elevation differences on the floors ofcomplex craters: Insights into the crater evolution processes. In: 47th Lunar and
Planet. Sci. Conf. Abstract# 2718 . Dhingra, D. , Pieters, C.M. , 2011. Mineralogical diversity of impact melts on central
peak of Tycho and its vicinity. In: Lunar Exploration and Analysis Group Meet-ing, Houston Abstract# 2025 .
Drozd, R.J. , Hohenberg, C.M. , Morgan, C.J. , Podosek, F.A. , Wroge, M.L. , 1977. Cos-
mic ray exposure history at Taurus-Littrow. In: 8th Proc. Lunar Sci. Conf.,pp. 3027–3043 .
El-Baz, Farouk , Roosa, S.A. , 1972. Significant results from Apollo 14 lunar orbitalphotography. Proc. Lunar Sci. Conf. 3, 63–83 .
Gault, D.E. , et al. , 1968. Impact cratering mechanics and structures. In: French, B.,Short, N.M. (Eds.), Shock Metamorphism of Natural Materials. Mono Book Corp.,
USA, pp. 87–99 .
Gault, D.E. , Wedekind, J.A. , 1978. Experimental studies of oblique impact. In: 9thProc. Lunar Planet. Sci. Conf., pp. 3843–3875 .
Goswami, J.N. , Annadurai, M. , 2009. Chandrayaan-1: India’s first planetary sciencemission to the Moon. Curr. Sci. 96 (4), 4 86–4 91 .
rieve, R.A.F. , et al. , 1977. Cratering processes: As interpreted from the occurrence ofimpact melts. In: Roddy, D.J, Pepin, R.O, Merrill, R.B (Eds.), Impact and Explosion
Cratering. Pergamon Press, NY, pp. 791–814 .
Gulick, S.P.S. , et al. , 2008. Importance of pre-impact crustal structure for the asym-metry of the Chicxulub impact crater. Nat. Geosci. 1, 131–135 .
aruyama, J. , et al. , 2008. Global lunar-surface mapping experiment using the lunarimager/Spectrometer on SELENE. Earth Planets Space 60, 243–256 .
Hawke, B.R. , Head, J.W. , 1977. Impact melt on lunar crater rims. In: Roddy, D.J.,Pepin, R.O., Merrill, R.B. (Eds.), Impact and Explosion Cratering. Pergamon Press,
NY, pp. 815–841 . ead, J.W., 2010. Transition from complex craters to multi-ringed basins on ter-
restrial planetary bodies: Scale-dependent role of expanding melt cavity and
progressive interaction with the displaced zone. Geophys. Res. Lett. 37, L02203http://dx.doi.org/10.1029/2009GL041790 .
iesinger, H., van der Bogert, C.H., Pasckert, J.H., Funcke, L., Giacomini, L., Os-trach, L.R., Robinson, M.S., 2012. How old are young lunar craters? J. Geophys.
Res. 117, E00H10. doi: 10.1029/2011JE003935 . oward, K.A. , Wilshire, H.G. , 1975. Flows of impact melt at lunar craters. J. Res. US
Geol. Surv. 3 (2), 237–251 .
olliff, B.L., Gillis, J.J., Haskin, L.A., Korotev, R.L., Wieczorek, M.A., 20 0 0. Major lunarcrustal terranes: Surface expression and crust-mantle origins. J. Geophys. Res.
105 (E2), 4197–4216. doi: 10.1029/1999JE001103 . rüger, T. , van der Bogert, C.H. , Hiesinger, H. , 2013. New high-resolution melt distri-
bution map and topographic analysis of Tycho crater. In: 44th Lunar and Plane-tary Science Conference .
önig B. (1977) Untersuchungen von primären und sekundären Einschlagsstruk-
turen auf dem Mond und Laborexperimente zum Studium des Auswurfs vonSekundärteilchen, Ph.D. thesis, Univ. Heidelberg, Heidelberg, Germany.
ramer, G.Y. , Kring, D.A. , Nahm, A.L. , Pieters, C.M. , 2013. Spectral and photogeologicmapping of Schrödinger Basin and implications for post-South \ sPole-Aitken im-
pact deep subsurface stratigraphy. Icarus 223 (1), 131–148 . emelin, M., Lucey, P.G., Song, E., Taylor, G.J., 2015. Lunar central peak mineral-
ogy and iron content using the Kaguya Multiband Imager: Reassessment of
the compositional structure of the lunar crust. J. Geophys. Res. 120, 869–887.doi: 10.10 02/2014JE0 04778 .
ucchitta, B.K., 1977. Crater clusters and light mantle at the Apollo 17 site: A resultof secondary impact from Tycho. Icarus 30, 80–96. doi: 10.1016/0019-1035(77)
90123-3 . argot, J. , Campbell, D.B. , Jurgens, R.F. , Slade, M.A. , 1999. The topography of Tycho
crater. J. Geophys. Res. 104 (E5), 11875–11882 .
elosh, H.J. , 1982. A schematic model of crater modification by gravity. J. Geophys.Res. 87 (B1), 371–380 .
Melosh, H.J. , Ivanov, B.A. , 1999. Impact crater collapse. Ann. Rev. Earth & Planet. Sci.27, 385–415 .
D. Dhingra et al. / Icarus 283 (2017) 268–281 281
N
N
O
O
Ö
O
P
P
P
R
S
S
S
S
S
S
S
T
T
v
W
W
W
W
X
eish, C.D., Madden, J., Carter, L.M., Hawke, B.R., Giguere, T., Bray, V.J., Osinski, G.R.,Cahill, J.T.S., 2014. Global distribution of lunar impact melt flows. Icarus 239,
105–117. doi: 10.1016/j.icarus.2014.05.049 . eukum, G. , König, B , 1976. Dating of individual lunar craters. In: 7th Proc. Lunar
Sci. Conf., pp. 2867–2881 . htake, M. , et al. , 2008. Performance and scientific objectives of the SELENE
(KAGUYA) Multiband Imager. Earth Planets Space 60, 257–264 . htake, M., et al., 2009. The global distribution of pure anorthosite on the Moon.
Nature 461, 236–240. doi: 10.1038/nature08317 .
hman, T., Kring, D.A., 2012. Photogeologic analysis of impact melt-rich lithologiesin Kepler crater that could be sampled by future missions. J. Geophys. Res. 117,
E00H08. doi: 10.1029/2011JE003918 . norato, P.I.K. , Yhlmann, D.R. , Simonds, C.H. , 1976. Heat flow in impact melts:
Apollo 17 Station 6 Boulder and some applications to other breccia and xenolithladen melts. In: Proc . 7th Lunar. Sci. Conf. , pp. 2449–2467 .
ohn, H.A. , 1972. Geologic Map of the Tycho Quadrangle of the Moon. IMAP .
otter, R.W.K. , 2015. Investigating the onset of multi-ring impact basin formation.Icarus 261, 91–99 .
lescia, J.B., Cintala, M.J., 2012. Impact melt in small lunar highland craters. J. Geo-phys. Res. 117, E00H12. doi: 10.1029/2011JE003941 .
obinson, M.S. , et al. , 2010. Lunar reconnaissance orbite camera (LROC) instrumentoverview. Space Sci. Rev. 150 (1-4), 81–124 .
chmitt H.H., et al., 1967. Geologic map of the Copernicus quadrangle of the Moon.
USGS Map I-515 (LAC-58). chultz, P.H. , 1976. Moon Morphology: Interpretations based on Lunar Orbiter Pho-
tography. University of Texas Press, p. 626 . imonds, C.H. , Warner, J.L. , Phinney, W.C. , 1976. Thermal regimes in cratered terrain
with emphasis on the role of impact melt. Am. Min. 61, 569–577 . mith, D.E., et al., 2010. The lunar orbiter laser altimeter investigation on the lu-
nar reconnaissance orbiter Mission. Space Sci. Rev. 150, 209–241. doi: 10.1007/
s11214- 009- 9512- y .
mrekar, S. , Pieters, C.M. , 1985. Near-Infrared spectroscopy of probable impact meltfrom three large lunar highland craters. Icarus 63, 442–452 .
ong, E., Bandfield, J.L., Lucey, P.G., Greenhagen, B.T., Paige, D.A., 2013. Bulk mineral-ogy of lunar crater central peaks via thermal infrared spectra from the diviner
lunar radiometer: A study of the Moon’s crustal composition at depth. J. Geo-phys. Res. 118 (4), 689–707. doi: 10.10 02/jgre.20 065 .
topar Julie, D. , Hawke, B.R , Robinson, M.S. , Denevi, B.W. , Giguere, T.A. , Koeber, S.D. ,2014. Occurrence and mechanisms of impact melt emplacement at small lunar
craters. Icarus 243, 337–357 .
ompkins S. (1997) Central peaks of impact craters as probes of lunar crustal com-position: Results from laboratory and remote spectral data analysis, Ph.D. The-
sis, Brown University, 213.pp. ompkins, S. , Pieters, C.M. , 1999. Mineralogy of the lunar crust: results from clemen-
tine. Meteorit. Planet. Sci. 34, 25–41 . an der Bogert, C.H. , Hiesinger, H. , McEwen, A.S. , Dundas, C. , Bray, V. , Robinson, M.S. ,
Plescia, J.B. , Reiss, D. , Klemm, K. , 2010. Discrepancies between crater size-fre-
quency distributions on ejecta and impact melt pools at lunar craters: An effectof differing target properties? In: 41st Proc. Lunar Planet. Sci. Conf . .
ilhelms, D.E. , McCauley, J.F , 1971. Geologic Map of the Near Side of the Moon,IMAP 703. United States Geological Survey .
ilson, L. , Head, J.W. , 2011. Impact melt sheets in lunar basins: Estimating thicknessfrom cooling behavior. In: 42nd Lunar and Planet. Sci. Conf. .
öhler, C. , Grumpe, A . , Berezhnoy, A . , Bhatt, M.U. , Mall, U. , 2014. Integrated topo-
graphic, photometric and spectral analysis of the lunar surface: Application toimpact melt flows and ponds. Icarus 235, 86–122 .
olfe, E.W. , Lucchitta, B.K. , Reed, V.S. , Ulrich, G.E. , Sanchez, A.G. , 1975. Geology ofthe Taurus-Littrow valley floor. In: 6th Proc. Lunar Sci. Conf., pp. 2463–2482 .
iao, Z., Zeng, Z., Li, Z., Blair, D.M., Xiao, L., 2014. Cooling fractures in impact meltdeposits on the Moon and Mercury: Implications for cooling solely by thermal
radiation. J. Geophys. Res. Planets 119, 1496–1515. doi: 10.10 02/2013JE0 04560 .