THE INTERCRATER PLAINS OF MERCURY. M. A. Leake, Dept, of Planetary Sciences, U. of Arizona , Tucson, AZ. 85721 Following the Mariner 10 flybys of Mercury, a study was undertaken to de- fine and resolve or constrain the nature and origin of the intercrater plains. This study consisted of geologically mapping Mercury's incoming side and of determining crater statistics of heavily cratered terrain and sur- rounding intercrater regions. The smooth plains of the incoming side and the lunar intercrater plains south and east of Maurolycus in the southern high- lands were studied similarly for comparative purposes. Geological Map. The geological map of the incoming side of Mercury dif- fers in important ways from the earlier general terrain map of Trask and Guest (1 ) . The new map is more detailed, incorporating a1 1 craters 3 40 km in diameter as well as some smaller features. Map units are extended into the hilly and lineated terrain establishing an overall map continuity. Final- ly, plains units are assigned an age based on the relative ages of craters which they embay or which are superposed upon them: a technique currently being used for the Mercury quadrangle maps. Relative crater ages were de- rived from the degradational state of craters patterned after that system used by the Lunar and Planetary Lab for lunar craters; Class 1 is freshest and Class 5 is most degraded (2). For example, plains within Class 1 craters or superposed on their ejecta blankets are designated Pi plains. Older plains are determined in a similar manner. However the oldest plains (P5) both overlie and underlie the oldest craters (Cg). P5 through P3 plains are equivalent to the intercrater plains as mapped by Trask and Guest (I), while Pi and P2 plains are the textural equivalent of the smooth plains occupying large areas of the outgoing side. Table 1 1ists the crater classes and plains units, their descriptions and the percentage of the map area covered by each. Hilly and Lineated Terrain. Rather than designating the hilly and lin- eated terrain as a separate unit, crater and plains ages were continued into that region from the surrounding area. The hilly and lineated zone is not a separate deposit, but instead a region which has been disturbed and modified by some distinct event. Sections of rims of some Class 3 craters which ex- tend into the hilly and lineated terrain have been modified by it, while the part of the rims exterior to this terrain have remained intact. Class 1 and 2 craters superposed on this terrain are unmodified. Furthermore, smooth P2 pl ains in this zone are undisturbed and fill 01 der, heavily disrupted craters. Hummocky P3 plains material fills some craters of Classes 3 and 4. Therefore, the event which caused this terrain occurred sometime between the formation of Class 2 and 3craters. Based on a somewhat similar dating technique, McCauley et al. (3) found that the Caloris impact also took place between the formation of Class 2 and 3 craters. This agreement lends support to the pro- posal by Schul tz and Gaul t (4) that the hilly and 1ineated terrain is the result of focused seismic waves generated by the Caloris impact. It is now possible to establish a time sequence for the formation of craters and plains relative to the Caloris event on the incoming side of Mercury. Paleogeologic Maps. Using the Caloris event at the end of the Class 3 period as a 'time horizon,' the geologic map was broadly grouped into Post- Cal oris features (units of Classes 1 and 2) , "Caloris" features (Class 3 units) and Pre-Caloris features (units of Classes 4, 5, and ancient circular depressions (c~'). The Pre-Caloris surface is dominated by large basins and by ubiquitous plains. The "Caloris" surface also consists of mostly Pre- Caloris material, but it serves to emphasize the continuing plains emplace- ment and fairly heavy cratering that preceded the Caloris impact. By con-

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Leake, M. A. trast, the Post-Caloris surface is sparsely populated with both craters and plains. Flany of these smooth plains are remote from younger craters, are in- terspersed with older plains deposits and lie in Pre-Caloris craters and basins; plains formation by deposition of ballistic ejecta is unlikely. Use of the paleogeologic maps and measurements of the existing plains area (Table 1) confirm that plains formation on the incoming side generally decreases with time, falling off extensively after the C3 cratering period. The intercrater plains are pre-Caloris units; the smooth plains are post- Caloris units. The percentage of the mapped area (1.0284.1 07 km2) covered by different ages of plains, both exterior and interior to craters, was compared to the percentage area covered by the craters. This latter value is an upper 1imit to crater area determined by summing the values of p = (~~2~*100)/(4A) over all diameters >40 km for each class of crater. D is the geometric mean of the diameter bin size and N is the number of craters in that diameter bin. In general, the percentage of area covered by craters also declines with de- creasing age, although not in a manner to suggest plains formation strictly through ejecta deposition. Plains Origins and Ages. Three competing theories of the origin of Mer- cury's plains are: (1) a primordial surface remaining after global me1 ting and solidification, (2) basin ejecta and melt deposits, and (3) volcanic de- posits. It is unlikely that the intercrater plains represent a primordial surface because their formation spans a range of time overlapping Mercury's differentiation and bombardment history. Furthermore, the oldest plains unit (P5) partially buries craters even 01 der than Class 5. The two 01 dest and most extensive intercrater plains units (P4 and P5) were emplaced at least throughout the period of heavy bombardment represented by Class 4 and 5 cra- ters; these crater densities are twice those of Class 1 and 2 craters. Exten- sive areas of even younger intercrater plains (p3) were apparently emplaced near the end of heavy bombardment. The continual resurfacing processes noted above, although eliminating the primordial origin hypothesis, still permits both volcanic and ballistic depositional origins. This ambiguity is ill ustrated by basins 1i ke Checkhov which are embayed with Pq plains while in other regions Cq basin secondaries are preserved on adjacent Pg plains. In addition, the youngest plains have two distinct textures which may depend on their mode of origin: moderately rough where they over1 ie or embay basin ejecta, and relatively smooth where they are either thicker or cover older plains deposits. "Smooth plains'' em- placed by either volcanic or ballistic means in the Pre-Caloris Era may later become knobby and pitted intercrater plains. Cratering processes also subdue features with time, depending on the size and frequency of impacts. One ex- ample of possible ball istic deposition is found in the plains between the heavily cratered complexes around Kuiper and Imhotep. Successively thicker, younger and smoother materi a1 has been deposited, with the youngest plains (p2) lying between the youngest craters. However, some of these P2 units ap- pear to embay the southern C2 crater and thus may not be directly related to it. Trask and Strom (5) have noted that preexisting craters are relatively well preserved near basin-sized craters, indicating that the resurfacing po- tential of Mercury craters is not very extensive. If ballistic deposition were the single mechanism for plains formation, the ratio of percentage area covered by craters to that covered by plains might be expected to remain con- stant with time, if ejection mechanisms do not change. Yet the ratios of ex- terior plains area to crater area increase roughly with increasing age, sug- gesting a contribution from volcanic deposition to plains material . A ball is- tic deposition mechanism is unlikely to change in time, although volcanic

0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System THE INTERCRATER PLAINS OF MERCURY Leake, M. A. deposition probably changed throughout the planet's thermal history. Arguments for plains formation by volcanism include the extensive nature of the intercrater plains, the lack of source basins for smooth plains filling older craters, and tentative volcanic features. Intercrater plains P3-P5 cover 37.4% of the mapped surface compared to 30.1% covered by craters. If the re- surfacing potential of craters is small, many more basins than are present are required to create this amount of intercrater material . In another example pointed out by Dzurisin (6) and others, young plains in a degraded C3 crater next to Renoir basin (C3) extend over the smaller crater's rim downward into the inner basin, ending in a rough lobate extension. Other young P2 plains fill Renoir's outer ring and bury two craters in the innermost depression. There are no nearby young basins to contribute ejecta to this formation. A volcanic origin of the P2 plains is therefore suggested. Domical structures, large sinuous valleys, rim1 ess depressions, and flat, inundated craters with scarp-1 i ke rims and radiating tectonic features are examples of possible vol- canic activity. Dzurisin (6) has argued that ridges like Mirni Rupes may be Pre-Caloris fissures which formed a ridge while extruding large volurnes of pl ai ns deposits in Mercury's expansion period. In summary, complex and varied origins are indicated for the plains of Mercury's incoming side. A primordial surface is unlikely; instead there ap- pears to be a subtle interplay of volcanic and ball istic depositional origins for the intercrater pl ains . Table 1 : Description of the Geological Map of Mercury's incoming side. / Era Plains/Crater Brief Descriotion of Materials % of mapped class area covereda p1 Very smooth plains materi a1 0.9 (.01 ,.86)b ~reshand/or' rayed craters 1.3C Post c1 Smooth plains material where thick, 7.8 (2.7,5.1) Cal ori s p2 rougher where thin, burying rough toooaraohv C? oder rat el^" fresh craters 5.4 "3 Moderately smooth to hummocky plains 12.1 (9.6,2.5) Caloris C2 Moderate1 v subdued craters ,rounded rims 10.0 pi' oder rat el; rough to hilly intercrater 9.6 (8.3,1 .3) plains Pre c4 Subdued, dissected craters 12.5 Cal ori s P5 Veryrough,knobbyandpittedplains 15.7(15.6,0.1) Highly subdued craters 7.6 C51 c5 Ancient ci rcul ar depressions ;vague rims 10.5 Hilly & Hilly and 1ineated material , large mas- 0.06 Li neated sive hills and trouqhs a. Total mapped area = 1.0284-1 01 kmz. b. In parentheses are % area of exterior and interior plains,respectively. c. Crater area percentage = C(TD~N-~OO)/(~A)for each class crater over all diameter bins of geometric bin size D km >40 km. N craters per bin. References: (1) Trask N.J. and Guest J.E.(1975) J.G.R. 80, p. 2461-2477. (2) Arthur D.W.G. et a1 . (1 963) Comm. Lunar Planet.Lab 2, p.71-78. (3)McCauley, J.F. et a1 .(I9781 Icarus, in press. (4) Schultz P.H. and Gault D.E. (1976) Geol. Rom. 15, p.479-480. (5) Trask N.J. and Strom R.G. (1976) Icarus 28, p. 559-563. (6) Dzurisin D. (1978) J.G.R. 83, p.4883-4906.

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