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Cassini A: the nature of the 'Washbowl' described by Wilkins & Moore on 1952 April 3.

Introduction

While observing the Moon on 1952 April 3, H. P. Wilkins & P. A. Moore, using the 83cm refractor of Meudon Observatory, described a feature inside the crater Cassini A. They wrote that they 'discovered ... a white, very shallow crater within which is a minute central pit, like a plughole, the whole strongly resembling a washbowl. (1) The authors did not include the exact time of their observation, making it difficult to repeat this observation for verification. However, after applying the software tools developed by H. D. Jamieson to Wilkins' descriptions, we were able to infer a lunar phase of approximately 60%, with the Moon exceeding the first quarter; a colongitude between 16[degrees]-17[degrees], corresponding to a solar altitude over Cassini A between 16.95[degrees] and 17.80[degrees].

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In a recently published paper, (2) we illustrated some observations of the structure that Wilkins & Moore called the 'washbowl'. In the present article we describe this feature in detail, along with suggesting the mechanism of its origin and its geology. While Wilkins & Moore used the terminology of a 'minute pit,' we avoid this term in order to avoid confusion with a similarly named volcanic feature (the 'pit crater').

A general description of the crater is given in the book by G. North. (11)

Inside Cassini there are Cassini A (the oddly shaped 15km crater near the center of Cassini), Cassini B (to the west of Cassini A) and Cassini M (outside Cassini's ring to the north). No specific references to the crater Cassini A were found in a list of books consulted. (12-20)

Digital images

Examination of Cassini A in Figures 1+4 reveals several interesting details. Cassini A is a double crater, as shown in Figures 1-2. The smaller eastern crater accounts for the structure that Wilkins & Moore called the 'washbowl.' Moreover the wall separating the two craters is straight. This is different from the round rim drawn in the Lunar Astronautical Chart (LAC map 25). (3) We address the origin of this feature below.

As noted above, Wilkins & Moore described this crater as a 'very shallow crater within which is a minute central pit, like a plug-hole, the whole strongly resembling a washbowl.' This description seems to fit the kind of detail shown in Figure 3, though the Sun angle is from the opposite direction. From this, and other imagery, we can find no clear evidence for a 'minute central pit'. However some bright spots are visible in Figures 1 and 3, as well as in the high resolution image from the Clementine data (Figure 4). These bright spots are probably hilly material, comprised of fragmental debris.

Similarly, USGS map I-666 describes this material as fragmental debris and masses of rim material that, likely, have moved down slope. (4)

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Geology of the Cassini A crater

The unusual element of this cratering pair is the straight rim that separates the two (Figures 1-4). While craters normally possess round rims, atypical geometries do exist. The most common rim variant is a straight segment, such as is found in Meton C (Figure 5). Straight rim segments come in several different types. One type occurs when an impact strikes near a fracture in the underlying bedrock. A fracture represents a weakness in the bedrock. In small craters, the shock wave preferentially excavates along such zones, creating straight segments between the fractures. (5) In larger craters, the modification phase includes collapse along the faults (similar to landslides), and this creates straight segments. (5) Crater Meton's straight rim segments were due to this latter mechanism. However a different process has occurred in Cassini A, as the straight part is only along the common rim segment.

As the straight segment occurs between two craters, we might examine rim morphology where craters overlap. Overlapping craters are common features on the Moon. When an older crater is struck at its edge by another impact, the second impact will have a deformed ring. This is because the material that the new impact excavates has already been deeply fragmented (by the older impact). This may result in a short straight segment as part of an elongated rim. Crater Shirakatsi is an example of this process (Figure 6). However, the rim of Cassini is not elongated, but foreshortened, so it is likely that a different process is at work here.

[FIGURE 5 OMITTED]

Another explanation might be a landslip within the crater. There are two possibilities here. If the slide was from a circular rim separating the two craters, then some circular element of the original rim should still be visible (which is not the case). If the slide was from the side of a single crater, so creating a straight segment within the crater, then there should be deformation of the side from which the slide occurred, a decrease in the crest of the slide with distance, and hilly materials. Thus, this mechanism does not fit the geology of the area.

[FIGURE 6 OMITTED]

So how did the straight segment form in Cassini? Historically, a number of double craters, with straight segments separating them, have been found. Examples include Bessarion B (Figure 7) and Segner C (Figure 8). Such segments were difficult to explain, and early explanations tended to emphasise volcanic mechanisms. (6) However, studies of artificial craters, created by using high velocity projectiles, finally revealed the answer. When twin projectiles are simultaneously fired, the impact pattern revealed two craters with a straight segment between them (Figure 9). Here the straight segment represents the interaction of ejecta between two craters that are formed simultaneously. The shape of these craters, reproducible from earlier studies of projectile impacts in the laboratory, provides evidence for the proposed mechanism (cf. Wilhelms, Chapter 3, crater materials, and the corresponding Figure 3.7). (6)

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This suggests that Cassini A was formed by two meteorites striking the Moon at the same time. Our theory about a double crater is confirmed by Dr Charles Wood (personal communication).

But do twin projectiles exist in the solar system? This question was dramatically answered by the Galileo mission to Jupiter in the mid-1990s. In 1993-1994, the spacecraft imaged the asteroid Ida, revealing for the first time an asteroid with a small circling satellite (Dactyl) (Figure 10). Astronomers have proposed several mechanisms for the production of double asteroids. Durda (7) suggests that the parent asteroid was struck by another, and that fragments along a parallel path became gravitationally bound. On the other hand, Britt & Lebofsky (8) suggest that the satellite is a fragment of Ida, created when Ida was struck by a small meteorite. This created a fragment that was gravitationally captured, and continues to move with its parent. A third possible mechanism for a double projectile involves tidal disruption. This occurs when a comet enters inside the Roche limit of a planet. However, the tidal forces of our planet are insufficient to disrupt an asteroid, unless it has already been fractured, in which case it would increase the separation of the fragments. (9) Such a mechanism would be expected to produce a string of fragments, as occurred when Jupiter fragmented comet Shoemaker--Levy 9, and so is not appropriate here.

Thus, the history of the Cassini A crater began when an asteroid was struck by an impact. This, in one of the ways mentioned above, produced a double asteroid. At a later time, this pair struck the Moon at high velocity (most strike the Moon in the 20-30km/sec range). As these impacts were nearly simultaneous, the ejecta curtains interacted, creating the straight rim segment that both craters share.

[FIGURE 11 OMITTED]

Conclusion

Thirty years ago, in the Journal of the Association of Lunar and Planetary Observers, Walter Haas reported on two American observations of this feature, and described it as a shallow depression. (10) Our images show that Cassini A is a double crater: the smaller eastern crater accounts for the structure named the 'washbowl' by Wilkins & Moore. This double crater is the result of an impact by a double meteorite, gravitationally bound so as to strike at the same time.

As a final remark, we cannot be sure what Wilkins & Moore saw and drew for the 'minute central pit'. Figure 11 displays the crater Cassini A under a solar angle and colongitude similar to, though slightly later than, the inferred time for the observation carried out at the Meudon Observatory. This image was taken by C. Fattinnanzi on 2005 March 19 at 18:23 UT (solar colongitude of 20.5[degrees] and solar altitude of 18.56[degrees]). The most likely explanation for Wilkins' observation is that the small hills and their shadows, integrated by an earth-based observer, combined to give the appearance of a 'minute pit'.

Acknowledgments

We wish to thank W. Higgins and C. Fattinnanzi for their contributions to this paper.

Received 2007 October 15; accepted 2008 April 30

References

(1) Wilkins H. P. & Moore P., The Moon, 2nd Ed., Faber & Faber, 1961

(2) Lena R., Bregante M. T., Douglass E. & Mengoli G., 'A study about Cassini A and the washbowl as described by Wilkins & Moore', Selenology 26(2), 2-5 (2007)

(3) LAC 25, Lunar Astronautical Chart (LAC) Series, Aeronautical Chart Information Center, US Air Force, http://www.lpi.usra.edu/resources/mapcatalog/LAC//lac25

(4) Page N. J., Geologic Map Of The Cassini Quadrangle Of The Moon, USGS I-666, 1970

(5) Melosh H., Impact Cratering: A Geologic Process, Oxford University Press, New York, 1989

(6) Wilhelms D., The Geologic History of the Moon, USGS Professional Paper 1348, US-GPO, Washington, 1987

(7) Durda D., 'Two by Two They Came,' Astronomy 23(1), 32-33 (1995 January)

(8) Britt D. & Lebofsky L., 'Asteroids' in Encyclopedia of the Solar System, ed. P. Weissman, L. McFadden & T. Johnson, Academic Press, San Diego, 1999

(9) Bottke W. Jr., 'Making Crater Chains on the Earth and Moon with Planetary Tidal Forces,' LPSC XXVIII, abstract 1062

(10) Haas W., 'Cassini A and the Washbowl', JALPO, 26, nos 3-4, p. 82 (1976)

(11) North G., Observing the Moon, Cambridge University Press, 2000

(12) Taylor S. R., Planetary Science: A Lunar Perspective, Lunar and Planetary Institute, Houston, 1982

(13) Kopal Z., A New Photographic Atlas of the Moon, Taplinger Publishing, New York, 1971

(14) Mutch T., The Geology of the Moon: A Stratigraphic View, Princeton University Press, 1970

(15) Heiken G., Vaniman D., & French B., Lunar Sourcebook, Cambridge University Press, 1991

(16) Melosh H., Impact Cratering: A Geologic Process, Oxford University Press, New York, 1989

(17) Dinsmore A., Lunar Atlas, Dover, New York, 1968

(18) Rukl A., Atlas of the Moon, Kalmbach Publishing, 1990

(19) de Callatay V., Atlas of the Moon, Macmillan, 1964

(20) Guest J., Planetary Geology, Halsted Press, New York, 1979

Raffaello Lena, Eric Douglass, Maria Teresa Bregante & Giorgio Mengoli

Address (RL): Geologic Lunar Research Group [GLR] Coordinator, Via Cartesio 144 D, 00137 Rome, Italy. [r.lena@sanita.it]
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Author:Lena, Raffaello; Douglass, Eric; Bregante, Maria Teresa; Mengoli, Giorgio
Publication:Journal of the British Astronomical Association
Article Type:Report
Geographic Code:4EUUK
Date:Feb 1, 2009
Words:1832
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