Results

Morphology and Distribution

The vast majority of polygonal craters in greater Hellas region is, or has a tendency towards being hexagonal. In addition, pentagons and also parts of octagons occur relatively often, but clearly square-shaped craters tend to be rare. Square-shaped craters are usually small complex craters, but larger complex craters exhibit almost exclusively hexagonal or pentagonal shapes.

Polygonal craters are most common in the northern part of the study area close to the Isidis impact basin. Near Isidis there are regions where more craters are polygonal than circular. In the volcanic plains of Hesperia Planum and Malea Planum the number of polygonal craters is low. Also worth noting is that the polygonal craters in Malea Planum are generally not as clearly polygonal as in other parts of the study area, and they are also typically smaller. The concentration of polygonal craters in certain areas becomes evident in Fig. 1, which displays the distribution of polygonal craters plotted on MGS-MOLA (Mars Global Surveyor - Mars Orbiter Laser Altimeter) topographic data.

Fig. 5. (a) (left) An eroded polygonal crater northwest from Hellas (23°S/324°W) in Viking Orbiter MDIM. The white box indicates the area shown in (b). (b) (right) Southern part of the crater in (a) as seen in Mars Odyssey THEMIS nighttime infrared image. Polygonal appearance of the southern part is markedly enhanced in the infrared image. A subframe of THEMIS image I02140006.

Fig. 5. (a) (left) An eroded polygonal crater northwest from Hellas (23°S/324°W) in Viking Orbiter MDIM. The white box indicates the area shown in (b). (b) (right) Southern part of the crater in (a) as seen in Mars Odyssey THEMIS nighttime infrared image. Polygonal appearance of the southern part is markedly enhanced in the infrared image. A subframe of THEMIS image I02140006.

Rim Strikes

In Hellespontus Montes (block centered at 39°S/312°W), on the western rim of the Hellas basin, the rims of polygonal craters display a prominent easterly direction (Fig. 7b, number of measurements n=118). The strikes are reported here by one direction only, i.e., a crater rim striking east-west is usually reported as "east", especially in the case of intercardinal directions, and for instance a strike 040°-220° is reported as 040°. The rose diagrams, however, are plotted as bidirectional because we feel that strikes are easier to grasp from bidirectional rose diagrams compared to unidirectional diagrams. The azimuth interval 080°-100° gathers about 21% of all the rim direction measurements. Another major rim direction in the area is north-northeast. A number of rims are oriented in a wide azimuth range around 280°-340°, whereas in the direction 040°-080° only very few rims exist.

Fig. 6. The same polygonal crater northwest from Hellas (27°S/324°W) in Viking Orbiter MDIM (a) (left) and Mars Odyssey THEMIS nighttime infrared image (b) (right). Note that the actual rim is rather vaguely discernible in infrared. A subframe of THEMIS image I02140006.

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Fig. 7. (a) (left, block 39°S/328°W, number of measurements n=76) and (b) (right, block 39°S/312°W, n=118). Bidirectional rose diagrams with 10° azimuth intervals, depicting the percentage of polygonal crater rim strikes west from Hellespontus Montes (7a, see Fig. 2 for location) and from Hellespontus Montes itself (7b). In the rose diagrams the outermost circle represents 20% and the darker circles inside mark every 2% except where otherwise stated. Note that the strong E-W peak radial to Hellas is present in both diagrams, whereas the NNE-SSW peak concentric to Hellas and parallel to grabens is predominate in Hellespontus Montes (7b) but diminishes further west (7a).

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Fig. 7. (a) (left, block 39°S/328°W, number of measurements n=76) and (b) (right, block 39°S/312°W, n=118). Bidirectional rose diagrams with 10° azimuth intervals, depicting the percentage of polygonal crater rim strikes west from Hellespontus Montes (7a, see Fig. 2 for location) and from Hellespontus Montes itself (7b). In the rose diagrams the outermost circle represents 20% and the darker circles inside mark every 2% except where otherwise stated. Note that the strong E-W peak radial to Hellas is present in both diagrams, whereas the NNE-SSW peak concentric to Hellas and parallel to grabens is predominate in Hellespontus Montes (7b) but diminishes further west (7a).

The block 39°S/328°W around crater Rabe immediately west from Hellespontus Montes (Fig. 7a, n=76) displays a pattern very similar to that of Hellespontus Montes. Rims striking east towards Hellas predominate with 18% of all the measurements. The northern trend is significantly weaker compared to the prominent north-northeast peak in Hellespontus Montes. The approximately northwest striking component is much more clearly clustered than in Hellespontus Montes. Also in this block crater rims oriented 050°-080° are rare.

The blocks situated between Hellas and Isidis at latitude 6°S display a very fascinating pattern (Fig. 8-10). Block 6°S/296°W (Fig. 8a, n=129) just on the southern edge of Syrtis Major Planitia shows a strong peak (12%) in azimuth interval 020°-030°(-050°). In the azimuth range 270°-360° rims seem to be rather randomly oriented. It is worth noting that there are gaps in the diagram on both sides of the largest peak at azimuths 000°-020° and 050°-070°.

Figure 8b (n=173) depicting the block 6°S/280°W east from crater Fournier and immediately southwest from Isidis shows a beautifully symmetric rim orientation distribution. The rose diagram seems to be a "refined" version of Fig. 8a. The dominant rim strike is 020°-030° (radial to Isidis) with 16% of all the measurements. Other peaks are at 280°-300°

and 330°-350° with about the same percentage spread in a twice as wide azimuth interval. The peaks have obvious gaps between them, and the same gaps can also be seen in Fig. 8a. The peaks evidently reflect the hexagonal outlines of the craters in the area.

Fig. 8. (a) (left, block 6°S/296°W, n=129) and (b) (right, block 6°S/280°W, n=173). Rose diagrams of the percentage of polygonal crater rim strikes southwest from Isidis basin (see Fig. 2 for location). Note the strong 030° trending peak present in both diagrams. This direction is radial to Isidis. Compare to Fig. 9a and 9b.

Fig. 8. (a) (left, block 6°S/296°W, n=129) and (b) (right, block 6°S/280°W, n=173). Rose diagrams of the percentage of polygonal crater rim strikes southwest from Isidis basin (see Fig. 2 for location). Note the strong 030° trending peak present in both diagrams. This direction is radial to Isidis. Compare to Fig. 9a and 9b.

Fig. 9. (a) (left, block 6°S/264°W, n=84) and (b) (right, block 6°S/248°W, n=74). Rose diagrams of the percentage of polygonal crater rim strikes southeast from Isidis basin (see Fig. 2 for location). Peaks at about 330° radial to Isidis are noteworthy, but also other major peaks are present. Compare to Fig. 8a and 8b.

Fig. 9. (a) (left, block 6°S/264°W, n=84) and (b) (right, block 6°S/248°W, n=74). Rose diagrams of the percentage of polygonal crater rim strikes southeast from Isidis basin (see Fig. 2 for location). Peaks at about 330° radial to Isidis are noteworthy, but also other major peaks are present. Compare to Fig. 8a and 8b.

Block 6°S/264°W is situated southeast from Isidis. The corresponding rose diagram in Fig. 9a (n=84) has a lot more scatter than its counterpart on the other side of Isidis (Fig. 8b). The strongest peak (only 10%) is in the direction 330°-340° (radial to Isidis). Despite the fact that this appears as the strongest peak in the diagram, the wide petals of the rose in 290°-320° and 010°-050° gather more measurements. Roughly east-west trending polygonal crater rims are almost completely lacking.

The rose diagram in Fig. 9b (n=74) depicts block 6°S/248°W hosting the crater Escalante on its northern margin. The diagram is again beautifully symmetric with three prominent peaks separated by roughly 60°. The strongest peaks each with about 22-25% of measurements point towards 330°-350° (radial to Isidis), 270°-300° and 010°-040°. Rims oriented in other directions are scarce. Despite some differences, the similarity between Fig. 9a and 9b is evident.

In Fig. 10a (n=65), depicting the block 6°S/232°W mainly west from crater Knobel, the strongest peak (12%) emanates from rims oriented 030°-040°. However, there is plenty of scatter and lots of measurements gather on both sides of 320°-330°. Very few rims have a roughly east-west orientation. An important point to notice is that blocks 6°S/232°W and especially 6°S/214°W differ from previous blocks in that they are partly on the volcanic plains of Elysium Planitia. This also means that they are on the global dichotomy boundary.

In contrast to block 6°S/232°W, block 6°S/214°W in the northeastern corner of our study area hosts plenty (14%) of polygonal craters with rim segments trending 270°-280° (Fig. 10b, n=52). Another prominent peak is at 030°-040°. The major directional gap is at 050°-080°.

In order to see if the directions that are predominate in Fig. 9a and 9b are also present further south from Isidis, we measured rim orientations in blocks 22°S/264°W (Fig. 11a, n=51) and 22°S/248°W (Fig. 11b, n=38). These blocks are actually closer to Hellas than to Isidis, and are partly on Hesperia Planum. Both diagrams are quite symmetrical with three peaks. In block 22°S/264°W, just north from Hadriaca Patera and west from Tyrrhena Patera, the peaks in 330°-340° and 020°-040° correspond to peaks in the blocks closer to Isidis. However, the wide east-west peak (260°-290°) in 22°S/264°W has no clear match in the northern blocks. Almost the same applies for the block 22°S/248°W, which hosts Tyrrhena Patera: the rose diagram has a clearly defined peak (13%) at 340°-350°(-360°) and wider peaks at 020°-050° and 070°-100°.

The rim strike measurements from Malea Planum (Fig. 12a-12d) are scarce and thus discussed below in chapter 7.2.

Fig. 10. (a) (left, block 6°S/232°W, n=65) and (b) (right, block 6°S/214°W, n=52). Rose diagrams of the percentage of polygonal crater rim strikes from the southern margin of Elysium Planitia (see Fig. 2 for location). Note that the strong E-W peak in Fig. 10b is conspicuously missing in Fig. 10a. This might be related to the global dichotomy boundary trending roughly E-W being more dominating in the eastern block (Fig. 10b). The 040° peaks may be due to fracturing induced by Elysium Mons just north of our study area.

Fig. 10. (a) (left, block 6°S/232°W, n=65) and (b) (right, block 6°S/214°W, n=52). Rose diagrams of the percentage of polygonal crater rim strikes from the southern margin of Elysium Planitia (see Fig. 2 for location). Note that the strong E-W peak in Fig. 10b is conspicuously missing in Fig. 10a. This might be related to the global dichotomy boundary trending roughly E-W being more dominating in the eastern block (Fig. 10b). The 040° peaks may be due to fracturing induced by Elysium Mons just north of our study area.

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Fig. 11. (a) (left, block 22°S/264°W, n=51) and (b) (right, block 22°S/248°W, n=38). Rose diagrams of the percentage of polygonal crater rim strikes northeast from Hellas (Hesperia Planum, see Fig. 2 for location). The roughly E-W and NNW-SSE trending crater rims have their parallel counterparts in the wrinkle ridges of Hesperia Planum (Raitala 1988). The NE-SW peaks may be an indication of volcano-tectonism or Hellas centered radial fracturing.

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Fig. 11. (a) (left, block 22°S/264°W, n=51) and (b) (right, block 22°S/248°W, n=38). Rose diagrams of the percentage of polygonal crater rim strikes northeast from Hellas (Hesperia Planum, see Fig. 2 for location). The roughly E-W and NNW-SSE trending crater rims have their parallel counterparts in the wrinkle ridges of Hesperia Planum (Raitala 1988). The NE-SW peaks may be an indication of volcano-tectonism or Hellas centered radial fracturing.

Fig. 12. (a) (upper left, block 57°s/328°W, n=22), (b) (upper right, block 57°S/312°W, n=18), (c) (lower left, block 57°S/296°W, n=16) and (d) (lower right, 57°S/280°W, n=16). Rose diagrams of the percentage of polygonal crater rim strikes from Malea Planum (see Fig. 2 for location). In Fig. 12a-12c the outermost circle represents 20% and darker inner circles mark every 2% as in the other rose diagrams. In Fig. 12d, however, the outermost circle represents 30% and the darker inner circles mark every 3%. Only craters formed in the plains material were included. Note the low number of measurements, which prohibits reliable conclusions. Similar trends (e.g., NW-SE), however, can be seen.

Fig. 12. (a) (upper left, block 57°s/328°W, n=22), (b) (upper right, block 57°S/312°W, n=18), (c) (lower left, block 57°S/296°W, n=16) and (d) (lower right, 57°S/280°W, n=16). Rose diagrams of the percentage of polygonal crater rim strikes from Malea Planum (see Fig. 2 for location). In Fig. 12a-12c the outermost circle represents 20% and darker inner circles mark every 2% as in the other rose diagrams. In Fig. 12d, however, the outermost circle represents 30% and the darker inner circles mark every 3%. Only craters formed in the plains material were included. Note the low number of measurements, which prohibits reliable conclusions. Similar trends (e.g., NW-SE), however, can be seen.

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