P53B-2117

Latitudinal Variation in the Color of the Lunar Maria• The color of the Moon varies systematically with latitude
and Implications for Space Weathering• Latitudinal color trend matches the signature observed at lunar swirls• Reduced solar wind flux could account for both color variations• Helps distinguish between effects of solar wind and micrometeoroids

1

1

Department of Earth & Planetary Sciences, University of California Santa Cruz
2
Now at Miller Institute for Basic Research in Science, University of California Berkeley
12 Color Signature of Lunar Swirls

The color of the surface is mainly controlled by mineralogy—this is most obvious in the contrast between thebright, anorthositic highlands and the dark, basaltic maria. The global latitudinal color variation we report here is
therefore not apparent until compositional differences are taken into account. Once a specific composition isisolated (e.g., Fig. 3a), it becomes clear that equatorial regions are darker and higher in the 950 nm/750 nm band ratio
than high latitude regions of the same composition [1].
Similar trends occur over a range of distinct compositions and are especially consistent across the lunar maria(Fig. 3b). In addition to controlling for composition, we verified that the latitudinal color trends we observe are not
artifacts of phase angle biases in the Clementine data (Fig. 4), nor can they be explained by contamination fromhighlands material [1].
The latitudinal color variation is unlike the color trends associated with impacts but statistically equivalent tothose associated with swirls (Fig. 3b), suggesting a common mechanism. Solar wind flux is a good candidate
because it is reduced both with increasing latitude (due to increasing incidence angle) and at swirls, where strong
magnetic fields may be partially shielding the surface from the impinging solar wind [11-13] (Fig. 5). In contrast,
micrometeoroid flux is reduced at high latitudes, but not at magnetic anomalies [14].

a0.12 0.13 0.14 0.15
750nm Reflectance

0.16

0.17

0.18

950 nm/750nm reflectance ratio) of pixels sampled from the five colored boxes in (a). The red point cloud represents abackground region whereas the dark blue point cloud corresponds to the brightest part of the swirl. Each of the point clouds
displays a steep trend that follows the progression from fresh impact craters (lower right part of each point cloud) to darker
background soils (upper left part of each point cloud) [10]; the dashed black line is a best fit through the red point cloud andhas a slope of m2 = -5.7 ± 0.5. The black circles represent the steady state (equilibrium) colors reached with maximum opticalmaturity in each rectangle. The solid black line is a best fit through the medians (black circles) of each cluster and has a slopeof m1 = -1.6 ± 0.2. Reported slopes are averaged over three separate mare swirl areas [1], and are used to obtain the constantsin the equations below.

αR950= R750 + 0.175
R750

90.76

β0.3

90.295

0.74

7.5

0.73

70.72

6.5

0.71

0.285

8

0.28
7.5

0.275
0.27

70.265
0.26

6.5

0.255
6300

300.5

301

301.5

302

Longitude (degrees E)

302.5

303

0.7

6300

0.14

0.16

750nm Reflectance

0.18

0.2

0.22

0.5

nds

0.8
0.75
0.08

0.09

0.1

90

a90

60

60

30

30

0−30b300.5

301

301.5

302

Longitude (degrees E)

302.53030.25

Figure 2: Maps of the α and β parameters (defined above) derived from the swirl- and impact-related trend slopes shownin Fig. 1b. Maps of α show impact craters but not swirl features, whereas maps of β highlight swirl features while impact
craters appear muted.

bluex)

strongalud
gra eringath ters
ew crails
ofos
into ow β)l(at(transition from
fresh craters tomature soils)

alud
gra eringath ters
ew crails
ofos
into igh β)h(atrapid decreas

hypothetical“hyperfresh”
surfacee in β750nm Reflectance

Figure 6: Inferred optical evolution process. For a given composition, all impact craters

0.110.120.130.14

750nm Reflectance

0.150.160.170.18

low solar wind flux athigh incidence angles

large enough to excavate fresh material should begin with the same characteristic color(the hypothetical ‘‘hyperfresh” point at right). In this model, following the impact event,the freshly exposed material experiences rapid weathering until it reaches anequilibrium value of β, controlled by solar wind flux. This rapid color change occurs
primarily in albedo, but is accompanied by a small change in the 950 nm/750 nm band
ratio. The crater material then continues to mature gradually (increases in α) until itreaches the local saturation color of the surrounding, well-developed soils. This gradualmaturation involves additional darkening, but is also accompanied by a proportionallygreater band ratio increase than during the initial mode of rapid weathering.

Based on these observations, and on comparisons with M3 results [22], we
propose that solar wind weathering takes place relatively rapidly and involves
significant darkening and reddening, due perhaps to the accumulation of
nanophase iron [4,23]. The gradual transition from bright craters to darker
background soils, on the other hand, mainly involves the weakening of spectral
absorption features along with some additional darkening (Fig. 6). Since it
involves no further change in the solar wind-flux dependent β, the latter
progression (increasing α) may be driven mainly by impact vitrification [24] andthe soil’s disintegration into increasingly finer grains [25].

0−300.29

Beta88.5

Latitude (degrees N)

0.75

Alpha

Latitude (degrees N)

8.5

950nm−to−750nm Reflectance Ratio

950nm−to−750nm Reflectance Ratio

Clementine 750nm Reflectance

Latitude (degrees N)

Figure 1: (a) 750 nm reflectance at the Reiner Gamma swirl. (b) Spectral characteristics (750 nm reflectance versus

R950= R750 + 0.625
R7500.12

Latitude (degrees N)

0.11

0.1

0.85l tre

FeO and between 1 and 2 wt% TiO2 [15,16]. Points are color-coded by cosine latitude, illustrating that the optical characteristics of thesurface vary systematically with latitude. Equatorial regions (red points) tend to have lower albedo and higher 950 nm/750 nm reflectanceratios than regions farther from the equator (blue points). The black trend line represents a least squares best fit through the data points.
The solid black circle corresponds to the color predicted by the best-fit line at the equator and the white circle represents the color
predicted at latitudes of ±70°. (b) Compilation of the 28 latitudinal trends we examined across the maria (gray lines), one for eachcompositional bin, compared with the trends found at swirls: the solid red, green and blue lines represent the swirl-related color trends
observed at Reiner Gamma, Ingenii, and Marginis, respectively; the dashed lines represent the impact-related color trends observed at eachof those swirls. The latitudinal trends have a best-fit slope of m3 = -1.5 ± 0.3, statistically indistinguishable from the swirl-related trend slopeof m1 = -1.6 ± 0.2.

Latitude (degrees N)

0.1

0.08

0.55

swirincrea(contrsing βolledby solar wind fluslope Fe band

α

0.7
0.09

0.09

15−17 wt% FeO
1.0−2.0 wt% TiO20.9

weaking303

0.60.95

redsrea

Longitude (degrees E)

302.5

0.8

3s

302

0.65

m

nd

2

301.5

0.85

slopetreem

301

0.7

1Solar wind saturation takes place so rapidly that, even with a 90% reduction influx, the surface becomes saturated in just ~1 Myrs [4]. This suggests that β is
controlled not by total accumulation of solar wind ions, but instead by some
flux-dependent equilibrium—e.g., between regolith gardening and solar
wind-induced alteration of exposed grain surfaces.
Over length scales of a few kilometers, the surface exhibits considerable
variability in α, whereas variability in β is low (Fig. 1b). This suggests that theequilibrium β is reached so rapidly that we do not observe variability in its
value over short distances (except perhaps at very young craters), whereas theevolution of α occurs gradually, as impact craters transition into mature soils
over longer timescales (Fig. 6).

ctlop

300.5

0.9

inal trends,

pa,snd10 km
63000.75

latitud

Figure 3: Color variation as a function of latitude. (a) Spectral characteristics of pixels sampled from regions with between 15 and 17 wt%

tre

0.75

0.95

0.7

ct

pa

0.1

0.8

0.8

10.75

1

im

6.5

0.85

gα

0.11

pe m

sin

7

rl tr
end
, slo

0.9

rea

0.12

swirl
equilibrium
color

swi

inc

7.5

β0.85

im

0.13

ing

1.05

1.05

gα

80.95

0.9

sing β

1.1

sin

0.14

incr
eas

1.1

increaare8.5bbackgroundequilbirium color

10.15

0.95

inc

a91.154 What is this telling us?

inc

Lunar swirls are optical anomalies that accompany many of the Moon’s crustal magnetic anomalies[7]. Swirls are bright, superficially resembling immature surfaces such as young impact craters [8-9].
However, swirls exhibit spectral characteristics that are distinct from those associated with impacts[10] (Fig. 1). We quantify the two distinct color trends with parameters α and β, defined below (Fig. 2) [1].

b1.15

1

950nm−to−750nm Reflectance Ratio

Space weathering alters the optical properties of exposed surfaces over time, complicating theinterpretation of spectroscopic observations of airless bodies like asteroids, Mercury, and the Moon. Solarwind and micrometeoroids are thought to be the dominant agents of space weathering, causing
exposed surfaces to darken and redden with time, and weakening spectral absorption features, but therelative contributions of these agents are not yet well understood.
Using both Clementine and LOLA reflectance data, we report a previously unrecognized systematic
latitudinal variation in the near-infrared spectral properties of the lunar maria [1]. We argue that this colortrend, which matches the color signature observed at lunar swirls, is the result of reduced solar wind flux athigh latitudes. This result could have implications for the interpretation of spectral observations fromdifferent latitudes and could help to distinguish between the effects of solar wind andmicrometeoroid bombardment [2-6].

3 Latitudinal Color Variation

Cosine Latitude

1 Introduction and Summary

950nm-to-750nm Reflectance Ratio

Doug Hemingway (djheming@berkeley.edu), Ian Garrick-Bethell , Misha Kreslavsky
1,2−60−60−90−900.1
0.150.2Clementine 750nm Reflectancelow solar wind flux in partsof magnetic anomalies
0.150.20.25
LOLA 1064nm Reflectance

Figure 4: Profiles of (a) Clementine 750 nm reflectance[17,18] and (b) LOLA 1064 nm reflectance [19] across thelunar maria (as identified according to surface roughness[1,20]). Reflectance tends to be lowest near the equatorand higher toward higher northern and southernlatitudes, with the curve being approximately symmetric
about the equator. The black line and gray band
represent the mean and standard deviation at each
latitude.

Figure 5: Since the Moon’s equator is nearly aligned with the ecliptic
plane, the maximum surface flux of solar wind occurs at the equator. At
higher latitudes, where the incidence angle is greater, solar wind flux isreduced. Likewise, solar wind flux is very likely reduced at strong
magnetic anomalies, due to magnetic deflection of the incoming
charged particles and the associated changes in electrostatic potential[12,13,21].

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