新着論文• 2009年に発表された「衝突クレーター」

新着論文の紹介に関する論文• 火星の衝突クレーター• 火星以外（水星、小惑星、氷衛星）の衝

名古屋大学大学院環境学研究科

突クレーター

保井みなみ（D2）1

1. 火星のクレーター2

1. 火星のクレーター• Degradation of mid-latitude craters on Mars

• Geomorphic analysis of small rayed craters on Mars: Examining

Daniel C. Berman, David A. Crown and Leslie F. Bleamaster III
Icarus, 200, 77-95, 2009

primary versus secondary impacts
Fred J. Calef III, Robert R. Herrick, and Virgil L. Sharpton
JGR, in press, doi:10.1029/2008JE003238, 2009!"#$%&'()(*+,-./0.12%3456789:;<=>?=
1/%@A.BCDEFGH?%IJ1&'()(%/%K#L/%@A.M
NGOPQR%ST.BGU%VWGLTXYH}[~•Z€•‚yƒTD9:%„{F&'()(yw…k>†a[‡baf_`ab]caLˆc`^\‰baf_`ab]caLyŠ‹NGŒ•yŽ•k|>LTXYH• Sinton crater, Mars: Evidence for impact into a plateau icefield andConcentric crater fill: Glacial and periglacial change• Latitude dependence of Martian pedestal
craters: Evidence for amelting to produce valley networks at the Hesperian-Amazonianboundary

sublimation-driven formation mechanism
Seth J. Kadish, Nadine G. Barlow, and James W. Head
JGR, in press, doi:10.1029/2008JE003318, 2009

Gareth A. Morgan and James W. Head
Icarus, 202, 39-59, 2009•‘%†c‰cˆ]be_`ab]ca1u+’7“"””#•B–—"˜™#Bš›œ.ijk>
l=<12•žŸ ¡.[‡†b`]^a7mnk>?%¢£¤¥¦K7§:FCD¨©
ªt«7¬-k|lLV6zk|&'()(VWGLTX®kT¯°y±²|YH

Z[\]^\_`ab]ca*+.1dbeecf_\c]g^ah7ijk>l=1mn.BCD2.opq
rst7uR.vwk>xy6z{=|LTXYH

3

4

468

1. 火星のクレーター

J.S. Levy etHiRISE
al. / Icarus
202 locations(2009) 462–476
Fig. 1. Context map of study area in Utopia Planitia. Individual
image
are shown. Base map is MOLA shaded relief topography.

Levy et al. (2009)• concentric crater fillにはbrain terrainと呼ばれる構造がある。ま!Concentric crater fill in Utopia Planitia: History and interaction

たbrain terrainの周囲にはpolygonal構造を持ったmantle material

between glacial “brain terrain” and periglacial mantle processesが存在する。どうやって出来たのか？形成・進化について

Joseph S. Levy, James W. Head III, and David R. MarchantIcarus, 202, 462-476, 2009

Fig. 11. Low-center mantle polygons (LC-MP) present in a scalloped depression in PSP_002782_2230. High-center mantle polygons (HC-MP) surround the low-center mpolygons, both within, and outside the depression. Illumination is from the lower left. North to image top.

• 火星のUtopia Planitia（北緯40度付近）のconcentric crater fillの•‘!•³"#.;<=G`^\`c\]a[`_`ab]ca_´[ee*.1µab[\_]caab[\L¶·=G5
67Wš>l%¸¹7ºX6z{=|%»LTXYHHiRISE画像を解析• North-south asymmetry in Martian crater slopes

R. A. Parsons and F. Nimmo
JGR, 114, E02002, doi:10.1029/2007JE003006, 2009•‘!•³"#%&'()(1>'¼½Ÿ¾.WG2¡%¿ª&½(À.BšÁ6
NGH8ÂÃÄÅ'(ÃÆÇV1•‘%£¤¥¦LPQR%ÈÉ»<>&'()(/%¥¦7›@ALÊ¤@AVEFGlL7ËÌ{=DTG|Í>ÎÏÐÑÒÓ•‚yw…kDÔÌÕ.UÎÖ{=G»ºX»y×Ø|LTXYH

12)
5

6

Fig. 2. Image
maps for
subsequent
Concentric
fill andmantlematerial
from
PSP_002782_2230
over CTX
image and
P05_002782_2232.
Nor
Fig. location
12. Low-center
mantlepolygonsfigures.
(LC-MP)(a)present
alongcratera gently
sloping
contact
betweenhigh-center
mantle polygons(HC-MP)“brain terrain” (CC-B
PSP_002782_2230.
Illuminationfrom(b)theConcentric
lower left. craterNorth totop.
top and illumination
is from
the lowerisleft.
fillimageand mantle
material from PSP_002175_2210 over CTX image P03_002175_2211. North to ima
illumination is from the lower left.

14 and 15). Open-cell “brain terrain” is rarely observed without
near-by and/or overlying mantle material, and low-center mantle
polygons are not observed in isolation from extensive regions patterned by high-center mantle polygons.
Complete transitions between surfaces dominated by highcenter mantle polygons, low-center mantle polygons, open-cell“brain terrain,” and closed-cell “brain terrain” occur on lengthraphy of “brain terrain” and polygonally patterned mantle sur
textures is shown in Fig. 16.
3. Surface ages

Crater counting on concentric crater fill and mantle terr
is complicated by the obscuration and disruption of crater

1I. 火星以外のクレーター

Levy et al. (2009)• 形成メカニズムについては両構造ともに似ている。

• Effects of ejecta accumulation on the crater population of asteroid• 熱収縮によるcracking、氷の昇華、氷河流動

433 Eros
C. Blitz, P. Lognonne, D. Komatitsch, and D. Baratoux
JGR, 114, E06006, doi:10.1029/2008JE003229, 2009

• ただ、polygonal構造を持ったmantle materialは上記の前2つのメカニズムで形成したと考えられる。
472€a^ˆÙÚ.„{T&'()(7ÛLpº;<=FTÜÝ1>mn.BCDÞß|àáâ&).BCDãä=|%Låæ.5çè4.BCDÙÚ'¼½Ÿ7é4k>&'()(yãC|LTXêë%ìíîïð.BGU%>LTXYH

J.S. Levy et al. / Icarus 202 (2009) 462–476• クレーター年代から予測された形成期間• brain terrain：約10~100Myr前• Evolution of the Rembrandt Impact Basin on Mercury• mantle polygon：約1.5Myr前Thomas R. Watters, James W. Head, Sean C. Solomon, Mark S. Robinson,
Clark R. Chapman, Brett W. Denevi, Caleb I. Fassett, Scott L. Murchie, andRobert G. Strom
Science, 324, 618-621, 2009• brain terrainが形成したのはAmazonian中期の氷河期から間氷期への移り変わり時？
• mantle polygonが形成したのはAmazonian後ìñòÇáó(%ôõàöà.BCDŽŠ{=|÷øùúûüh‡%~c‡µab\‰]_`ab]ca
yýþ.×Ø>ÿ!%•"#4$%&Õ'(%Ö)7;ë»C|LTXY

期の氷河期？
7

8

Fig. 18. Crater counts for fresh craters on mantle material (open circles) and all
craters on “brain terrain” surfaces (filled circles) using Hartmann (2005) isochrons.
Counts combine data from four concentric crater fill surfaces in Utopia Planitia, andstrongly indicate a surface at of ∼1.5 My for mantle polygons. Brain terrain counts
show a roll-off at small crater sizes typical of lobate debris apron and lineated valley
fill surfaces. Craters in the 50–250 m diameter range suggest an age of ∼10–100 My
for brain terrain.

1I. 火星以外のクレーター

Krichoff and Schenk (2009)• Enceladusのcratered plainのクレーター密度分布とMimas, Tethys,! Crater modification and geologic activity in Enceladus’ heavily
Fig. 17. (a) Fresh crater in LDM with high-center mantle polygons. Portion of
PSP_002782_2230. Illumination is from the left. North to image top. (b) “Ringmold”-like crater (Kress and Head, 2008) in closed-cell “brain terrain.” Portion of
PSP_002175_2210. Illumination is from the left. North to image top. (c) Elongate
crater in closed-cell “brain terrain.” If the crater was deformed by glacier-like flow,
linear strain of ∼60% is documented. Portion of PSP_002175_2210. Illumination isfrom the left. North to image top.

Fig. 19. MOLA shot-point profile across a concentric crater fill-filled crater (present
in PSP_002175_2210). Inset. The MOLA shot-point profile reprojected to show theactual profile (near-horizontal line) and the modeled depth of the crater, assuming
a parabolic profile, and depth-diameter ratios documented by Garvin et al. (2002).
Up to ∼80% of the crater has been filled by concentric crater fill material.

Dione, Rheaのクレーター密度分布を比較した。

cratered plains: Evidence from the impact crater distribution
Michelle R. Kirchoff and Paul Schenk
Icarus, 202, 656-668, 2009

(e.g., Marchant et al., 2002; Schorghofer and Aharonson, 2005;
Schorghofer, 2007). Fractures would initially represent sites of depressed surface troughs (Fig. 20c) (Marchant et al., 2002). Thisis because enhanced contact with the dry atmosphere at polygon fractures would result in greater sublimation of subsurface
ice along polygon cracks (Marchant et al., 2002; Kowalewski etal., 2006, 2007), generating widened, deepened troughs analogous to those outlining terrestrial sublimation polygons (Marchant
et al., 2002). Thermal fractures would accumulate sediment derived from winnowing of overlying lag deposits (e.g., Marchant etal., 2002) and infiltration of aeolian sediments, forming wedges• Enceladusは他の衛星と比べて0.6!D!2kmとD"6kmのクレー

analogous to sand-wedge structures common in cold and arid terrestrial environments (Fig. 20d) (Péwé, 1959; Berg and Black, 1966;
Marchant et al., 2002; Marchant and Head, 2007). Lateral transport of surface materials into deepening troughs would result inever-thicker accumulations of sediment relative to “stable” polygon interiors (e.g., Marchant et al., 2002).
Inversion of sublimation-polygon-like topography (polygons
with convex-up centers) could occur if sublimation continued toremove near-surface ice at polygon centers, but slowed at polygon troughs due to the presence of thickened accumulations of
sediment along polygon troughs (Fig. 20e) (e.g., Marchant et al.,2002). Cementation of wedge sediments at polygon margins by
ice deposited during periods of reversed vapor flux would enhance the protection of ice located beneath the sediment wedges

ター数が極端に少ないことが分かった。なぜか？

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ë&'()(7›4.5FT0y6CD•š>{<."#7@V&'()(
87EFGlL7’»C|LTXYH

Enceladus’ impact crater distributions

663

Fig. 7. Relative (R) size-frequency distributions.
The relative plot (R-plot) shows the ratio of the actual distribution to a distribution with a differential slope of −3 in log–log√
format. Diameters are √binned by ( 2) D. The dashed gray line indicates possible equilibrium saturation (e.g., Melosh, 1989). (A) Raw data for the Mimas, Enceladus, Tethys,
Dione and Rhea with N error bars. cp — cratered plains; sp — smooth plains; rp — ridged plains. (B) Averages of the raw data for the satellites to facilitate comparisons.
This plot illustrates how similar the distributions are for Mimas, Dione, Tethys and Rhea, implying their distributions are representative of a single, heliocentric impactor
population. The deviation of Enceladus’ cratered plains distribution is also illustrated, implying that its distribution has likely been affected by geological processes.

9butions. Meanwhile, the data presented here, while not complete,does give an indication of the population for these small diameters, which was not available with Voyager imaging.
2.5. Results — Ages

10extent evenly distributed across the satellites (Alvarellos et al.,2005) and their terrains, so that the effect is uniform. This would
result in all terrains appearing equally older; therefore the ages
calculated here could be regarded as upper limits. Finally, for Enceladus only, some mechanism (as discussed below) appears to be“removing” craters with D # 6 km and D " 2 km from the record.
This would likely alter our age calculations for these diameters on
Enceladus and the best recourse would be to not use densities forthese crater diameters for age calculations. Because the younger
calculated ages (for Case A) appear to be anomalous for Enceladus (and the other terrains), however, we propose that overall
Enceladus’ cratered plains date from early bombardment
are / Icarus 202 (2009) 656–
M.R. Kirchoff,andP. Schenksimilar in age to the other heavily cratered plains discussed here.
The calculated ages for the Enceladus’ ridged plains (Table 3)
sulating layer
indicate that these are young surfaces. The oldest (rp6, Fig. 4) islikely "2 Gyr old and the youngest (rp1, Fig. 4) could be only
active and com
10 Myr old. The results also imply that the resurfacing generatlikely as we hing the ridged plains has progressed southward and has occurred
muted topogr
on a regular interval through time. This agrees with the findings of
atively ancien
Kargel and Pozio (1996). In addition, we find that the calculations
burial is occur
for densities at different diameters are more consistent for theas we need aridged plains. This is most likely a result of these being younger
layer as it collterrains with less time to modify with the cratering record, andDespite ththat we understand the impact rate better as the age gets closer toplains, the str
the present.

Before discussing the results of computing absolute ages, theauthors would like to note that while the ages are informative forvery broad comparisons, they are not very accurate. Estimations of
the current impact rate are uncertain within at least a factor of 4,
especially for the small impactors that form most of the craters on
Saturn’s icy satellites (Zahnle et al., 2003). Furthermore, the histor-666
ical cratering rate is not well constrained. Here we have assumed
a rate that decays as t −1 over time to the current rate. Dynamical evolution calculations of the likely outer solar system impact
Enceladus’ impact crater distributions
665
source region indicate, however, that the rate could decay alongother functions (e.g., Levison et al., 2008). Intended future work by
the authors will recalculate ages using these other estimations of
the decay of the cratering rate to determine what affect this assumption has on the results. Overall these uncertainties imply thatyoung surfaces (!2 Gyr) can be dated with large error bars, but
the age of older surfaces are not really resolved. Therefore, agespresented here should be used and interpreted very carefully, andnot taken as absolute values.
Given the caveats above, the densities and estimated ages forthe heavily cratered terrains on all the bodies included in this
3. Crater modification and geologic activity in Enceladus’study, even Enceladus, generally indicate very old surfaces, >4 Gyrcratered plains(Case A, Table 3). Enceladus’ cratered plains, however, could be
as young as ∼1 Gyr according to some calculations in Table 3.
The data
presented above for the cratered plains on EnceladusThethe different
666 discrepancy between the calculated ages using
M.R. Kirchoff,
P. Schenk / Icarus 202 (2009)656–668
indicate that its impact crater distribution is different from heavily
minimum diameters for Enceladus’ cratered plains (and, indeed,cratered terrains on the other satellites considered here. We haveother terrains) could be due to several reasons. First, as discussed
sulating
layer
thatcauses anincreasein roll-off
the heatforflow
found that the R-values
havea steep
0.6 andabove, the rates used to calculate the ages for each diameter" D more" 2 kmactive and complete relaxation. The first possibility could be very
could be wrong. Second, a population of secondaries could possiand D # 6 km (Fig. 7) and the changes in cumulative slopes withmage from the global mosaic of terrain centered at ∼8.5◦ S, 179◦ W. This image shows examples of “muted”, softened impact craters (black circles) in thelikely as we have observed large craters viscously relaxed to verygion on Enceladus, which also may be partially buried. (b) In contrast, this image from the global mosaic around 41◦ N, 214◦ W illustrates that craters in the
bly be affecting the records for D " 2 km (Bierhaus et al., 2001;
diameters (Table 2) are different for Enceladus. We now explore
muted topography within the mid-latitude regions, which are rels have much sharper rims and little apparent in-filling. Both images are 9◦ by 9◦ wide.
the hypotheses that these characteristics of Enceladus’ cratered
Chapman et al., 2002; McEwen and Bierhaus, 2006). Third, theatively ancient (Fig. 10), and we reason from small craters thatplains distribution could be due to a change in the heliocentric,
planetocentric impactor population, if present, could be affecting
burial is occurring. The second possibility is harder to substantiate,primary impactor population for these diameter ranges, anotherthe record. Craters formed by planetocentric debris resulting fromas we need a better idea of the physical properties of this surface
impactor population (e.g., secondaries, planetocentric), a mixing ofsatellite disruptions or escaped ejecta of large basins could speculayer as it collects, and then what effect it has on the heat flow.
the primary and another impactor population, and/or geologic prolatively create a good number of craters with D " 5 km (Alvarellos
662
M.R. Kirchoff, P. Schenk / Icarus 202 (2009) 656–668
Despite the partial destruction of cratered plains by ridged
cesses altering the cratering record at these diameters.
et al., 2005), which would be indistinguishable from the heliocenplains, the strongly symmetric pattern of increasing crater density
A change in the primary population is not a likely candidate, as
tric population. Therefore, contamination of the heliocentric crater
within this ancient terrain toward the poles places additional imthis change is not indicated on the other satellites in this diamedistribution is likely and not easily sorted out. One optimistic posportant constraints on parts of the satellites history. Asymmetricter range (Fig. 7), even for Dione’s smooth plains, which is similar
sibility, however, is that the planetocentric population is to somecrater distributions are predicted for synchronously rotating satellites, although only a weak asymmetry has been detected thus far
in bright terrains on Ganymede (Zahnle et al., 2001). This pattern
has a strong sinusoidal shape, with a peak crater density at onepole (the apex of orbital motion) and a low density at the opposite
pole (the antapex). There is no possible reconfiguration of Enceladus to rotate the observed pattern (Fig. 5), which is essentially abipolar pattern with peaks at opposite poles, to fit the predicted
pattern from apex asymmetry. We thus rule out orbital cratering
asymmetries as a cause of the observed spatial distribution.
If burial and/or viscous relaxation as described above created
11created with a counting circle
12
Fig. 5. Impact crater density map showing the number of craters !2 km per unit area in the cp unit (Fig. 4) for Enceladus. The map was
the observed pole-centered symmetric pattern, then the current

Krichoff and Schenk (2009)• 0.6!D!2kmが少ない理由：

Krichoff and Schenk (2009)• D"6kmが少ない理由：• 南極のplumeから飛び出た物質や土星のEリングから降ってきた物

質がクレーターを埋めてしまった• 氷物質（地殻）の粘性緩和＋

南極plumeやEリング物質が埋めた• 粘性緩和だけではクレーターを消すことは不可能なので（Dombard• クレーター密度が緯度によって異なる。

and McKinnon, 2006）、恐らく粘性緩和と埋める効果両方が働いたと• 赤道域が少なく、中緯度域が多い。

•

思われる。

• Eリングから降ってくる物質が赤道域付近で集中した？
•（Michaud et al., 2008）

urements of Enceladus’ craters visually identified as “pristine” or “fresh” (solid circles)
are comparable
similarcraters
on Mimas(solidindicateline). Most
othercraters while cool ones indicate lower densities. The map indicates that the mid-latitude regions◦ radius).
analysisdescribed in tothetext (10Hot colors
higherdensities,s, classified as softened or muted (open circles), fall well below this trend, indicating
manydensityof Enceladus’
craters
arethanrelativelyThis
may White
imply designates
thathavethata higherof impact
craters
in the shallow.
equatorial
region.
regions where no data is available or terrains other than cratered plains.
m the south polar plumes or E-ring has been filling in the craters, possibly enough to bury them. Data compiled from digital elevation map of area in Fig. 3a.

n where the equatorial region appears to have less craters
mid-latitudes.
r evidence for crater burial comes from the craters’ toWhen Cassini ISS images of Enceladus are examined,
all craters appear to have crater rims that look moreout and muted, and many crater floors appear to be
ome are indeed barely recognizable; Fig. 8). These obare corroborated by topographic profiles of numerous
erived from stereo-photoclinometric analysis techniquewith increasing latitude into the southern hemisphere. Our topographic results, however, are not yet complete and might in theend agree with results of Bray et al. (2007). If this is the case, thenthe south polar plumes may have a greater contribution to burial
in the southern hemisphere than indicated here.
Observation and analyses of linear depressions, referred to as
pit chains, also supports the hypothesis that a layer of fine material blankets the terrain around Enceladus’ equator within thecratered plains unit. Michaud et al. (2008) observed these features

Fig. 10. Images from Enceladus’ global mosaic illustrating the occurrence and characteristics
of relaxed
impactsuggests
craters. Blackare pointing
to thedensity
distributionthat arrows
polar wanderof theicyremnants
outer ofsomelayers
relaxedCraters
thisstate as
imply
that thebyheat
flow isor Paponce wasof craters.
Enceladus
didinnotoccur
proposedNimmo
andhighpalardo
enough tosoften They
the icepropose
crust, allowing
it to flow viscously.
(2006).
that Enceladus’
ice shell (a)hasCenteredbeen at∼43◦ N, 253◦ W and is 18◦ wide by 13◦ tall. (b) Centered at ∼40◦ N, 294◦ W andreorientedinorder to place negative gravity anomaly caused by◦◦is 36 wide by 19 tall.

the formation of a less dense diapir under the plume outlets at
one of the poles. If we assume that the observed crater distribu-

andtion
McKinnon,2006).
Generally,
the wander
rim andcentralwas
strucformed prior
to proposed
polar
andanyEnceladus
byalthough45◦ or 90
turerotatedremain,it◦ canbe very
muted
(Fig.
10).
aroundthe tidal
axis(i.e.,
theTherefore,
sub- and weantisaturn
Nimmo
Pappalardo
shouldstill beaxis)
ableastosuggested
recognizebyandcountandlarge
relaxed (2006),
craters. In
the patternincreasing
density
towardthetopoles
formed refact,then
we have
many ofalreadyin thedatabase.
Thus
completely
in some90◦ rotation,theeitherpolar alone
terrains
move
cratersotherfromconfiguration.
the surface,For
another
process,
or inwithin this anportant constr
crater distribu
lites, althoughin bright terra
has a strong
pole (the apex
pole (the anta
ladus to rotate
bipolar patterpattern fromasymmetries a
If burial an
the observed
density distrib
layers of Ence
palardo (2006
reoriented in
the formation
one of the potion formed p
rotated by 45
antisaturn axi
then the pattin some othewould have otapex. As disccratering asym
bution. For th
distributions w
mid-latitudes,
seems highly
lar crater distrstate. Finally,a concentric z
radially fromlarge basins w
to form the bi