COMPARISON AND ANALYSIS OF YOUNG AND OLD AREAS ON MARS

We have been able to characterize same representative surfaces of various ages, using images from the MOC MGS camera. The MOC images allow a tremendous advance over previously published data, because we can examine local and regional erosional effects down to scales of 20 m, far smaller than previously analyzed.

FIGURE 1

To study a very young area, we chose MOC frame 3308, which crosses a strip of the floor of the summit caldera of the very young Tharsis volcano, Arsia Mons. Figure 1 shows some of this surface together with crater counts obtained on Arsia Mons and its summit caldera. The largest crater in the summit caldera is only not quite 1 km across, and so the caldera counts cut off at that size. However, we added with additional counts from the Arsia Mons flanks (using open symbols). Using the analogy of terrestrial shield volcanoes, such as Mauna Loa and Kilauea in Hawaii, we'd expect the caldera floor, where eruptions continue, to be younger than the flanks. (Most flows on in the Kilauea caldera are less than 100 years old, while most of the flanks are many centuries old.) The two sets of counts (caldera and whole volcano) earth match the shape of the lunar diameter distribution. At the upper left, the curve runs into the solid saturation equilibrium line at D ~ 60 m, analogous to the behavior in the lunar maria, where the secondary curve hits the saturation line at D ~ 300 m. The interpretation is that the caldera lavas are so young that no substantial obliterative losses have occurred even for craters as small as 60 m, or depths as shallow as about 10 m (13). The isochrons suggest that average surface age of major last flows covering the caldera is around 100-300 My old, while the characteristic mean surface age of flows constituting the larger Arsia Mons structure is around 250-750 My. The uncertainty is estimated at a factor three. This suggests that Arsia Mons has been active within the last 2-7% of the history of Mars, and hence that volcanism is not a primordial, but essentially a contemporary process, on Mars -- a striking finding, if true!

Our finding of a lunar-like size distribution on the summit of Arsia Mons is not to say that there is no dust deposition or obliteration there. Surprisingly, although the altitude is 24 km above the mean surface of Mars, we see direct evidence for dust deposition. A portion of MOC frame 3308 (not yet released) covers a region of horst-graben structure just outside the north caldera rim. Rilles and other textures are sharply seen on the horst surfaces, but are muted or covered entirely by dust deposits on the graben floors, especially in the drifts banked against the edges of the grabens. This appears to prove that dust is collecting in low spots, even at this altitude. It is surprising that dust deposits are as prominent at such high altitude, but the dust source may be fallout from global dust storms that inject dust into the very high martian atmosphere. Prominent dust layers were seen at 30-40 km altitude by Viking (13).

FIGURE 2 FIGURE 3 FIGURE 4

The comparison between this young area and ancient upland areas on Mars is striking. Figure 2 shows part of the moderately heavily cratered upland area near Nirgal Vallis in MOC frame 605, and the crater counts are shown in Figure 3. Figure 4 shows a quite heavily cratered terrain on the floor of the large crater Schiaparelli, which in turn is situated in one of the most heavily cratered Martian terrains. Characteristic of all heavily cratered areas, both of these show a pronounced "Opik" flattening of the primary crater branch from 1km < D < 45 km. The larger craters appear to date back to an early era of 3 to 4 Gy, suggesting that the underlying upland surface strata in these upland areas is that old. Figure 4 shows not only the crater counts in the large crater, Schiaparelli (solid symbols), but also counts for craters in the extremely heavily cratered adjacent surroundings of Arabia Terra, on which Schiaparelli is superposed. As expected, the background gives even higher crater density, near saturation at large diameters, and probably dating to the end of the intense early bombardment, around 4 Gy ago. Furthermore, the secondary branch appears distinctly flatter than on the moon. The MOC frames reveal a whole range of degradation states among 100-m-scale craters, from fresh craters to craters with dune deposits on the floor, to craters whose floors are filled and whose rims barely protrude above the dust. The observed crater counts in both old areas, as in the younger area, approach the saturation line at small sizes, around 60 to 200 m.

The heavy, bent solid line in Figures 3 and 4 is a predicted steady state line for the infilling of craters at a net, average deposition rate of around 10-6 m/year on crater floors. This curve is discussed earlier on this web site. The predicted curve is a good fit for the data in most highland regions. The conclusion is that on the oldest Martian uplands, smaller craters are probably in a rough equilibrium with local obliteration processes, at least if we average over large enough areas. This statement applies especially at D < 2 km, since we see the range of degradation morphologies appropriate to a steady state. While the curve fits at D < 45 km, it is uncertain whether the same type of obliteration applies 2 to 45 km, because some observers have asserted that the large craters do not show the expected range of morphologies, and that some of the flattening might be due to the hypothetical ancient impactor population with fewer small impactors than exist today. An interesting result of these studies is that the smallest craters tend to be in saturation equilibrium unless there have been recent dust drifts or dune activity.

According to our isochrons, 60-m craters will reach saturation equilibrium if a surface is exposed for about 100 My. On older surfaces, age discrimination properties are lost at D < 60 m because the surface saturates with small craters. Local dune activity may sporadically reset the cratering clock. On old cratered surfaces, it seems likely that craters may be exhumed occasionally, as winds remove sediments and dunes. Just as on Earth, when we are dealing with features of characteristic dimensions around 20-60 m, we are often not seeing the origins and histories of the underlying geologic unit, but rather the more recent history of superficial processes. For this reason, the ages determined from craters have been termed "crater retention ages" to distinguish from ages of the underlying rock units (Hartmann, 1966, Icarus).

Based on our crater data and the isochrons presented here, we estimate that in the most heavily cratered Martian uplands, craters of D > 45 km date back more than 3 By. Craters of D = 1 km in these two regions appear to date back to about 3 My, but craters of 100 m date mostly back not more than a few hundred My. In most regions of Mars, we estimate that craters of D = 20 m date back less than 50 My.

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