Lunar and Planetary Science XXXII (2001) 1510.pdf

CRATER CHAINS ON . V. V. Shevchenko and T. P. Skobeleva, Sternberg State Astronomical In- stitute, Moscow University, Universitetsky 13, Moscow 119899, Russia, [email protected]

Introduction: After discovery of disruption comet It was examined many images from the Shoemaker-Levy 9 into fragment train before it’s colli- set and there were identified a total 15 crater chains sion with Jupiter there was proposed that linear crater and were unable to link any of these directly chains on the large satellites of Jupiter and on the to any specific large crater associated with ejecta de- Moon are impact scars of past tidally disrupted comets posits. Chain craters are remarkably aligned. All dis- [1 - 4]. tinguished crater chains are superposed on preexisting It’s known that radar images have revealed the pos- formations. sible presence of water ice deposits in polar regions of Chain Morphology: A total 127 craters were Mercury. Impacts by a few large comets seem to pro- identified in the 15 recognized crater chains. The vide the best explanation for both the amount and number of craters per chain ranges from 4 to 11. cleanliness of the ice deposits on Mercury because they Fig. 2 shows that crater diameters range from 3 to have a larger volatile content that others external 13 km, with an average of 7 km. Crater statistics dem- sources, for example, [5]. onstrates remarkable uniformity of the crater popula- A number of crater chains on the surface of Mer- tion of the chains. May be the feature indicates the cury are most likely the impact tracks of “fragment similar type of origin of these crater chains. trains” of comets tidally disrupted by Sun or by Mer- The other confirmation of the thesis is result of cury and are not secondary craters. Crater Chains Identification: Mariner 10 image Crater statistics set (the three Mariner 10 flybys in 1974-1975) was used to recognize the crater chains which did not asso- 50 ciate with secondary crater ejecta from observed im-

s 40 pact structures. As example, in Fig. 1 is shown such crater chain located near crater Imhotep and crater Ib- 30 sen (The of Mercury). 20

Number of crater 10

0 2,0-4,0 4,0-6,0 6,0-8,0 8,0-10,0 10,0-12,0 12,0-14,0 Crater diameter, km

Fig. 2

Chain morphometry

12 10 8 6 km 4 2 Fig.1 0 Average diameter of crater, 0 20406080100120140 Resolution of the Mariner 10 image is about 0.54 km/pixel. The crater chain is about 50 km long. Length of chain, km Image resolution limits possibility to examine the form of craters strongly. It seems the craters in chains Fig. 3 have roughly flat floor and smooth form. Most chain craters are approximately circular. Lunar and Planetary Science XXXII (2001) 1510.pdf

CRATER CHAINS: V.V.Shevchenko and T.P.Skobeleva

the crater chain morphometry. Fig. 4 shows the close comets are particularly promising candidates for deliv- correlation (r = 0.72) the average diameter of chain ering water to Mercury. The density of nucleus for crater with the length of the chain (with two excep- large young comets (like as comet Hale-Bopp, for ex- tions). So, it’s possible to conclude that ratio between ample) may be decrease to 0.1 g/cm3 [7]. The Roche the crater chain length and average diameter observed distance for a body with such small density passing chain crater ranges from 4.5 to about 11, with an aver- near to Mercury is 7.6 Rm. This distance is too short age value of about 8.4. for “fragment train” formation. The distance would be It’s needed to note that range of the value of crater pass by the large cometary nucleus during about 5 chain length is relatively narrow: from about 30 to minutes. about 100 km. Crater chain with length of 128 km is There are other characteristics of the possible im- possibly exception (may be its identification is not cor- pactors – orientations of their orbits in space. Fig. 4 rected and it is a crater chain of a secondary, ejecta shows distribution of the measured angles between type). chain direction and the equatorial plane of Mercury. Origin of Primary Impact Crater Chains on Since the equatorial plane of Mercury is close to Mercury: It’s possible to consider two external type of the ecliptic the most orbit inclinations are strongly impactors which be able to form the mentioned crater typical for the long-period comets. These data confirm chains: and comets. that impact of tidally disrupted comets is the most The total mass influx of asteroids to Mercury re- likely explanation for distinguished crater chains. mains uncertain. Since a small part of observed now The following is an example of numerical model. A asteroids have perihelion distances less than 0.3 AU comet is moving with an orbital velocity of 60 km/s the asteroid impact probability with Mercury theoreti- near perihelion at 0.3 AU from the Sun. It’s a moment cally exist. However, a mechanism of the asteroidal of the beginning of the cometary nucleus disruption 6 body disruption by Mercury or by Sun near Mercury is during next 1.6x10 s up to mercurian surface contact unknown. (through 0.66 AU). In the case of crater chain shown in

The Roche distance dr for body with density ρb Fig. 1 (about 50 km long) the length of “fragment passing near to the Mercury is possible to estimate train” must be 972000 km with time of impact of all from dependence, offered in [6]: fragments equal 4.5 h (velocity of rotation a spot on the 1/3 dr=2.0(ρm/ ρb) Rm, mercurian surface is about 11 km per hour). In this where ρm , Rm - density and radius of Mercury. scenario, the fragments drift apart with velocity of For a body with density about 3g/cm3 (stone aster- about 0.6 km/s. There is considered usually that such

oid) the expression gives value of dr = 2.4 Rm. A simple velocity is about 15 m/s. simulation of the process of disintegration under effect Discussion: As it was mentioned above the length of tidal forces of stone body shows, that “fragment of the crater chains distinguished in this work as comet train” can not be formed during such short time be- tracks ranges values from 30 to about 100 km. Schenk tween the asteroid disruption and the collision with et al. [3] predicted a comet chain length of about 1500 Mercury. km at Mercury for a 2-km diameter comet passing For cometary nucleus with density about 0.5 g/cm3 the within 1.5 solar radius of the Sun. This discrepancy Roche distance is 4.4 Rm . Recent study of the mercu- remains unexplained. rian ice problem by Moses et al. [5] results that a large Acknowledgments: The authors thank M. Davies, Rand Corp., for placing at our disposal the copies of Crater chain orientation the Mariner 10 image set with documentation. This work was supported by State Program “Astronomia” and partially by the grant of INTAS-ESA 99-00403. 140

m References: [1] Melosh, H., and P. Schenk (1993) 120 100 Nature, 365, 731-733. [2] Melosh, H.J., and 80 E.A.Whitaker (1994) Nature, 369, 713-714. [3] 60 Schenk P.M. et al. (1996) Icarus, 121, 249-274. [4] 40 Shevchenko V.V. (1997) LPS XXVIII, 1305–1306. [5]

Chain length, k 20 Moses J.I. et al. (1999) Icarus, 137, 197-221. [6] Rahe 0 J. et al.(1994) In: Hazards due to Comets and Aster- 0 30 60 90 120 150 180 oids. Ed. T.Gehrels. Tucson & London, 623. [7] Angle of chain direction, deg Shevchenko V.V. (1999) Solar System Research, 33, Fig. 4 400-408.