5.1 Interpretation of the Results
5.1.1 Ridge Area
The flexure signals give us a hint that the elastic thickness is about 5-10 km. The modeling results also show that the maximum value of deflection with a 5-10 km thickness varies from 3-7 km. This situation gives us an implication: the original equatorial ridge would be higher than the present ridge. The average height of the present ridge is ~7 km, so the original height of the ridge may reach over 10-14 km.
using a wing width of 37.4 km (profile r1) yields the average slope of the original ridge of 15-20 degrees. It becomes a hint on determining the proper model of the origin of the equatorial ridge, and will be discussed in section 5.2.2.
5.1.2 Ultrahigh Bulge
The existence of the bulge is the powerful evidence for flexural signals. Most of the profiles have its bulges that represent for the flexure with an elastic thickness of 5-10 km. (See section 4.1 and 4.1.1.) But the height of bulge ranges a lot, from ~1 km to the maximum of 7 km. No evidence for the other reason causes the ultrahigh bulge, except flexure. However, Flexure contributed only by vertical loading is not sufficient to make the high bulge. As proved in Chapter 4, the height of the bulge caused by the flexure of vertical loading is limited to ~1 km. So, we must consider the other forces that generated deflection. There are still 2 possibilities: one is the tectonic horizontal force, another is the horizontal force induced by cratering. Due
64
to the lack of tectonic linear structure and the non-renewing surface, it’s less possible that the tectonic force buckled the high bulge. On the other hand, cratering may play a significant role on the formation of the ridge since Iapetus’
surface is mostly occupied by numerous craters.
R2 profile is just an example for the bulge’s origin. From Fig. 2-2, large multiple craters (No.1 and No.4 crater in Fig. 2-2) lie on the northeast of the r2 profile. In general, the crater rim is higher than the original surface since the rim may be pushed by the impact pressure and be stuffed by impact ejecta (Melosh, 1996). And then, the northern side of the profile r2 is just located on the crater rim of crater No.
1. Therefore, the ultrahigh bulge may be caused by the large impact event, not the loading of the ridge. This interpretation is also useful to r3, r4 and r5 profile because they are in the same relative position of crater No. 1.
5.1.3 Craters
Impact crater is a powerful material for determining the age of the equatorial ridge. From Fig. 2-2, Crater No. 2 obviously truncated the ridge and totally
devastated it. The other example is between profile r5 and r6 (right side of profile A-A’ in Fig. 2-2) although this crater is smaller. The truth tells us that some impact events happened after the ridge formed. However, we don’t find any craters which seem to be truncated by the ridge. Crater No. 5 is the potential one since its
northern rim, which adjoins the ridge, is steeper than the other direction; but it is not sufficient to be evidence for the truncated crater. So, if we assume the age of Iapetus’ surface is 4400 Myr which has been discussed in section 2.1.2, the ridge may be little younger than this age but not too much since a young ridge has a larger
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possibility of truncating a crater. This idea will be a hint on the chronology of Iapetus; we are going to discuss more in section 5.3.
5.2 Formation Model of the Equatorial Ridge
Until now, we have proposed that the depression and the bulge of Iapetus’
surface might be caused by ridge flexure on a thin elastic shell. This thin-shell model also offers constraints on the formation theory of the equatorial ridge. In this section, we will discuss these constraints and infer the proper model for ridge origin.
5.2.1 Possibility of a Thin Elastic Shell
This study suggests that the elastic lithospheric thickness of Iapetus is only 5-10 km when the ridge formed; the value is about one twentieth to the previous study (Giese, Denk et al., 2008; Sandwell & Schubert, 2010). Is it possible that there was a thin elastic shell on Iapetus? To solve the question, we start from the thermal history of Iapetus. If the thermal flux was high, the surface of Iapetus would be more plastic, and then the elastic shell was thinner. In the early stage of Iapetus, the abundance of SLRI (short-lived radioactive isotopes, see section 2.1.4) was possibly high. Therefore, if the ridge formed in this period (200-1000 Myr after Iapetus formed, and before the synchronizing of Iapetus), the thermal flux generated by radioactive decay of SLRI was enough to maintain such a thin elastic layer. This idea is correlated with the observations of the ancient surface and the truncation
relationship between craters and the ridge. All evidence implies that the ridge is an old structure formed in the very beginning stage of Iapetus.
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In a nutshell, there are 2 factors supporting the hypothesis of the thin elastic layer. One is the depressed surface and the bulge areas on Iapetus, and another is the correlation between the formation time and the thermal flux.
5.2.2 Endogenic or Exogenic?
We are about to reexamine the origin model of the equatorial ridge. From the discussion of the previous section, the ridge was possibly formed when the thermal flux was high. Thus, the endogenic buckling model doesn’t adapt to this condition since it needs a thick elastic layer. However, flexure model has less relationship with the other endogenic model such as the convection model. Therefore, Flexure model would not determine whether the ridge formation is endogenic or not.
On the other hand, flexure model has enhanced the possibility of the exogenic hypotheses that the ridge is supposed to be constructed by the ring remnant. This is because the result of this study tells us that the original slope of the ridge may be steeper than present. When the ring particles deposited on the surface of Iapetus, the slope of the pile would reach the angle of repose. Recent study suggested that when the gravity is reduced, the dynamic angle of repose decreases (Kleinhans et al., 2011). According to this study, the dynamic angle of repose on Iapetus is only 20-30 degrees. If we assume the original slope of the ridge is 15-20 degrees as discussed in section 5.1.1, it would be more possible that the ridge was constructed by the ring particles. So, the result of the study shows the tendency of an exogenic origin of the equatorial model. But the mechanics of the ring formation (accretion disk or impact splash) remains unknown.
67 5.3 Chronology of Iapetus
Iapetus has 2 significant geological events: 1) the formation of the equatorial ridge; 2) the formation of the fossil 16-h shape. We have discussed the time constraints on these 2 events. These implications conclude to a time series of the geological events on Iapetus. The chronology is listed below:
1) Iapetus formed from the solar nebula 4567 Myr ago, accompanying Saturn and the other planets in the Solar System.
2) Ridge has formed several hundred million years after the formation of Iapetus. In this time, the SLRI inside Iapetus generated sufficient heat to maintain a thin elastic outer shell.
3) Iapetus was despinning because of the tidal locking of the Saturn. In the same time, the heat from SLRI was reducing. After the ridge formed, the 16-h shape of oblate spheroid was fixed due to the cooling down of the Iapetus’ inner core.
However, we cannot sure the time scale between the formation of the ridge and the formation of fossil shape.
4) Iapetus was totally synchronized with Saturn ~1000 Myr after the formation of Iapetus.
5) No obvious geological events except cratering happened on Iapetus after synchronizing.
68 5.4 Conclusion
Iapetus is the only object that has the equatorial ridge in the Solar System. In this study, we use the DTM data and the assumptions of the early-time Iapetus to construct the elastic lithospheric flexure model. Both analytical and numerical models are proposed in this study, giving similar results. The research results show that the elastic thickness of Iapetus’ outer shell is 5-10 km when the ridge formed. It seems to contradict the earlier studies since these studies suggested that the elastic thickness must be over 100 km, but our study fits more observational data:
1) Depression and bulge areas around the ridge are interpreted as a deflection caused by the loading of the ridge acted on the thin elastic layer.
2) Studies on the thermal history of Iapetus also suggested that Iapetus once had the sufficient heat to maintain a thin elastic shell and a big portion of plastic inner core.
This result also gives an inference on the exogenic origin of the equatorial ridge.
The ridge may be a rubble pile deposited by an ancient ring (Dombard et al., 2012; Ip, 2006; Levison et al., 2011), which is similar to the scene happened on another
Saturnian satellite, Rhea (Schenk et al., 2011). And the last, we give a chronology of important geological events on Iapetus. Our research supports that ridge formation is earlier than the fixing of fossil 16-h shape. All of these constraints will help us understanding the most peculiar mountain in our Solar System.
69
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