Icarus 283 (2017) 268–281

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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: deepdpes@gmail.com , deepak_dhingra@alumni.brown.edu

(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].

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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).

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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.

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