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Swirl distribution on the lunar surface: locations antipodal to the young
basins?
V.V.SHEVCHENKO
STERNBERG STATE ASTRONOMICAL INSTITUTE, MOSCOW UNIVERSITY, MOSCOW, RUSSIA

THE NATURE OF DIFFUSE ALBEDO ANOMALIES ON THE LUNAR SURFACE THAT LOOK
LIKE SWIRLS IS ONE OF MOST INTERESTING MYSTERY IN CURRENT LUNAR STUDIES.
THERE ARE TWO MAIN CLASSES OF HYPOTHESISES OF THE SWIRL ORIGIN: FORMATION
OF THE SWIRLS IN THE REGIONS ANTIPODAL TO LARGE IMPACT BASINS (1), AND
FORMATION OF THE SWIRLS IN RESULT OF COMETARY IMPACTS (2).
UNUSUAL, LIGHT-COLORED SURFACE MARKINGS WERE OBSERVED IN MARE MARGINIS
IN RESULTS OF THE ANALYSIS OF APOLLO-8 PHOTOGRAPHY AND VISUAL OBSERVATION
(WHITAKER, 1969). IDENTICAL MARKINGS WERE DISTINGUISHED BY EL-BAZ (1971)
IN MARE INGENII ON THE LUNAR FAR SIDE AND WERE ASSOCIATED WITH THOSE IN
MARE MARGINIS. IT WAS NOTED THAT THE POSITIONING OF THESE TWO REGIONS ON
THE LUNAR GLOBE WAS STRIKING: THE SWIRLS OF MARE MARGINIS LOCATION WAS
DIAMETRICALLY OPPOSED TO THE MARE ORIENTALE, AND THE SWIRLS OF MARE INGENII
LIED ON OPPOSITE SIDE OF THE MARE IMBRIUM (EL-BAZ AND WORDEN, 1972).
ACCORDING TO THE OPINION OF THE AUTHORS THESE MARKINGS ARE PERHAPS DUE TO
CHEMICAL ALTERATION OF THE SURFACE MATERIAL, AND THE ALTERATION MAY HAVE
BEEN CAUSED BY THE RELEASE OF GASES FROM THE LUNAR INTERIOR. THE RELEASE OF
GASES, IN TURN, MAY HAVE BEEN TRIGGERED BY SEISMIC-WAVE ATTENUATIONS AT THE
ANTIPODAL AREAS OF THE IMPACTS THAT CREATED BASIN IMBRIUM AND BASIN
ORIENTALE.
LATER, HOOD AND SCHUBERT (1980) PROPOSED THAT SWIRLS REPRESENT REGIONS
WHOSE HIGHER ALBEDO HAVE BEEN PRESERVED DUE TO DEFLECTION OF THE SOLAR WIND
ION BOMBARDMENT BY STRONG CRUSTAL FIELDS. THE MOST LIKELY MAGNETIZATION
MECHANISM WAS PROPOSED BY HOOD (1987), IN WHICH THE IONIZED VAPOR CLOUD
PRODUCED IN A HYPERVELOCITY BASIN-FORMING IMPACT EXPANDS AROUND THE MOON
AND CONCENTRATES THE PRE-EXISTING AMBIENT MAGNETIC FIELD AT THE BASIN
ANTIPODE. THE ANALYSIS OF THE SWIRL DISTRIBUTION CARRIED OUT BY LIN ET AL.
(1988) CONFIRMS THAT THE SWIRL-LIKE ALBEDO MARKINGS OF THE REINER GAMMA
CLASS ON THE FAR SIDE OF THE MOON ARE LOCATED NEAR THE ANTIPODES OF THE
YOUNG IMPACT BASINS, IMBRIUM, ORIENTALE, SERENITATIS, AND CRISIUM. THE MOST
RECENT DATA OBTAINED BY THE LUNAR PROSPECTOR SHOW THAT SWIRL FEATURES ARE
ASSOCIATED WITH MAGNETIC ANOMALIES AND THEY LIE ON REGIONS ANTIPODAL TO THE
IMBRIUM, SERENITATIS, AND CRISIUM BASINS (HOOD ET AL., 1999).
GOLD AND SOTER (1976) SUGGESTED A MECHANISM OF A LOCAL MAGNETIC FIELD
ORIGIN ON THE MOON IN RESULT OF COMETARY IMPACT. THE LOCAL SHOCK PRODUCED
BY COLLISION OF THE MAIN MASS OF A COMET NUCLEUS WITH THE MOON WILL INDEED
OCCUR JUST WHEN THE AMBIENT SOLAR WIND FIELDS HAVE BEEN STRONGLY ENHANCED,
AS THE LARGE PARTIALLY IONIZED COMETARY COMA IS COMPRESSED AGAINST THE
LUNAR SURFACE. SCHULTZ AND SRNKA (1980), BELL AND HAWKE (1987), SHEVCHENKO
(1993) CONSIDERED THAT SWIRL PATTERNS ON THE LUNAR SURFACE COULD BE RELATED
TO THE IMPRINT OF RECENT COMETARY IMPACTS. THIS HYPOTHESIS DOES NOT SUGGEST
CORRELATION BETWEEN THE SWIRL LOCATIONS AND THE REGIONS ANTIPODAL TO
BASINS.
IN ORDER TO INVESTIGATE THE FEATURES OF THE SWIRL DISTRIBUTION ALONG
LUNAR SURFACE THERE WERE IDENTIFIED AND MAPPED SWIRL LOCATIONS WITHIN
REGIONS WHERE THEY WERE OBSERVED. THE AREAS OF ALL IDENTIFIED SWIRL
FRAGMENTS WERE MEASURED AND STATISTICAL ANALYSIS OF THE DISTRIBUTION WAS
PERFORMED (SHEVCHENKO, 1994, 1996). IN FIGURE 1 MODIFIED SKETCH MAP OF THE
DISTRIBUTION OF SWIRL MARKINGS ON THE LUNAR SURFACE IS SHOWN. CONTOURS
INDICATE THE TOTAL AREA OCCUPIED BY SWIRL FRAGMENTS INSIDE EACH FRAME
10OX10O. CONTOUR INTERVAL IS 2.5X103 KM2.
ALL SWIRL MARKINGS (WITH THE ONLY ONE EXCEPTION - REINER GAMMA
FORMATION) LIE ON THE FAR SIDE OF THE MOON. REINER GAMMA SWIRL IS
RELATIVELY SMALL FORMATION. ITS AREA IS 3.3 X103 KM2. SWIRL REGION IN MARE
INGENII EXTENDS ON THE AREA EQUAL TO 50.2 X103 KM2. THE SWIRL AREA IN MARE
MARGINIS ACHIEVES 73.9 X103 KM2.
LOCATIONS OF THE REGIONS ANTIPODAL TO YOUNG AND LARGE (DIAMETER OF THE
MAIN RIM IS MORE THAN 500 KM) BASINS ARE SHOWN IN THE SKETCH MAP TOO
(LETTER "A" MEANS "ANTIPODE" OF THE CORRESPONDING BASIN). RELATIVE AGES AND
DIAMETERS OF THE BASINS WERE IDENTIFIED BY WILHELMS (1984). THE ORIENTALE
BASIN IS YOUNGEST. IT IS RELATED TO THE YOUNGER IMBRIAN PERIOD (3.2 - 3.8
B.Y.). THE FORMATION OF THE IMBRIUM BASIN IS RELATED TO THE IMBRIAN PERIOD
TOO. ACCORDING TO WILHELMS' CLASSIFICATION THE IMBRIUM BASIN IS RELATIVELY
OLDER THEN ORIENTALE ONE. OTHERS BASINS SHOWN IN THE SKETCH MAP ARE RELATED
TO THE YOUNGER NECTARIAN PERIOD (3.8 - 3.9 B.Y.). THEY HAVE NEARLY THE SAME
AGE.


[pic]FIG. 1


The more strong correlation is observed for youngest large basins:
Orientale and Imbrium. The swirls are absent in region antipodal to
youngest (second age group, more younger than Imbrium basin) but small (320
km) Schrodinger basin. This antipodal region locates on the near side of
the Moon. The swirl area is observed on region antipodal to Serenitatis
basin, but any swirl markings are absent on regions antipodal to Humorum,
Hertzprung, and Humboldtianum basins in spate of similar age and diameter
of them.
On the other hand, there are two cases of absence of correlation
between swirl areas and regions antipodal to impact basins (for small swirl
formations). The Reiner Gamma formation is most obvious example of that
the correlation mentioned above is not statistically strong and do not
exclude the swirl origin associated with external reason, such as cometary
impact (Pinet et al., in press). Of great interest is to compare the
typical morphological features of the different swirl markings. The
available Clementine images (Clementine DIM, 1997) allows one to compare,
for example, albedo features of the Reiner Gamma formation with albedo
features of the swirl markings located in the area antipodal to the Crisium
basin. The area includes a fragment that is located in the crater
Gerasimovich itself and some small albedo formation to the North of the
main structure.
Fig. 2a shows a bright "loop" - typical detail of the Reiner Gamma. In
Fig. 2b it is shown the analogous detail of swirl located into crater
Gerasimovich. The linear scale of the both images is the same. Other type
of characteristic detail - "close loop" is shown in Fig. 3a (Reiner Gamma
swirl) and in Fig. 3b (crater Gerasimovich swirl). The linear scale of the
both images is the same.

[pic] [pic]

Fig. 2a
Fig. 2b

[pic] [pic]


Fig. 3a
Fig. 3b


The presence of the characteristic details in the form of complex
loops inside crater Gerasimovich swirl allows unambiguous classification of
this albedo anomaly as a swirl of the same type as Reiner Gamma.
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