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ISOCHRONS FOR MARTIAN CRATER POPULATIONS
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Application to Selected Surfaces on Mars
Here we will present a survey of our crater count data for several sample surfaces on Mars, plotted against a background of our new isochrons. Some of the data have been published in reports using earlier isochron iterations, and are presented here for comparison; other data are newly published. Because of the important geophysical implications of young volcanism on Mars, we start with an effort to test the shape of the crater production function predicted above against the shape actually observed in reasonably homogenous young areas, where craters are best preserved, and to affirm young ages of lava flows.
Unfortunately, areas of perfectly homogeneous age are not available on Mars. Broad Martian lava plains show mixes of lava flows with much greater variation in crater density and age than found in the lunar maria, not to mention that dunes and dust drifts cover some localized patches of these units. Thin, young lava flows can be detectable in MGS images but not in Viking images, since they may not be thick enough to cover pre-existing > km-scale craters and rampart ejecta blankets (Hartmann, 1999). Also, as discussed in Part 3, geographic distribution of local secondaries may be patchy (McEwen, 2003), and we try to avoid “contaminated” regions.
A. Lava Flows in Amazonis Planitia
Viking imagery shows identifiable flow fronts in Amazonis Planitia, and MGS image MO2-04131, which crossed one of the young flow fronts (Fig. 7a), is one of many MGS images revealing typical lava morphologies. The youngest flow, with a prominent flow front, is rough-textured, very sparsely cratered, and darker than the surroundings.
[Figure 7] (a)
Young, rough-textured lava flow with extremely few impact craters,
overlapping
sparsely cratered lava plains in Amazonis
Planitia.
26N, 167W, MGS MO2-04131.
In at least some cases, the intricate ridge systems (right upper
center), similar to examples in Elysium Planitia,
appear to be termini of flows.
(b) Crater count data for Amazonis
Planitia, plotted on background of new 2004
isochrons. Solid data points give
an
average of counts for older regions, using Viking (larger D) and
selected MGS (smaller D) frames.
Open data points give crater counts for the unusually young,
uppermost
overlapping flows, such as the example in Fig. 7a. Isochron fits suggest older lavas date
back
to a few hundred Ma, and younger lavas date from a few tens of Ma.
Figure 7b gives crater count results for Amazonis Planitia. The solid data points give an average of a number of Viking (D / 400 m) and MGS (D . 400 m) crater counts on various selected areas of Amazonis Planitia (counted by WKH and Space Grant student Julie Batten), but not including MGS MO2-04131. The open data points give data on the youngest flow on M02–4131 (counted by WKH and D.C. Berman) and very young flows on two other MGS frames. The background surface on MGS 02-04131 was considered “intermediate” in age and is shown by a grey symbol. The newly revised isochrons fit the data better than the isochrons presented by Hartmann (1999). The “older” lava flows of Amazonis Planitia appear to fit well the isochrons giving a model age of about 100 Ma to 1000 Ma, interpreted as the true age of the flows, and the younger flows give a model age of about 9 Ma to 90 Ma for the youngest flows.
B. Olympus
Figure 8 gives a preliminary overview of data we have obtained. Some of these data were obtained in a workshop at the International Space Science Institute (Bern, Switzerland) with students from Universidad Complutense of Madrid, Spain, and complemented independently with our own data generated at PSI on Mariner 9 and Viking frames, as well as a very sparsely cratered area of fresh flows in SPO2-41105. Following the concept used above, we divided these data into those from “older flows” (Fig. 8a), which had higher crater densities, and dramatic examples of “younger flows,” such as shown in Fig. 8b (MGS SP2-41105), which have very well-defined flow fronts and few visible craters, and which give notably younger ages (Fig. 8c). The data for “older flows,” averaging over much of the slopes and summit, fit an isochron giving an characteristic model age of about 200 to 800 Ma. The “younger flows,” while giving poorer statistics because of the sparse cratering, give a model age of about 10 to 200 Ma. Consistent with the good state of preservation of these flows, the model ages are interpreted as the characteristic age of formation of the lavas in the layers of mean depth roughly equal to the depth of the craters involved, of order 100 m for the “older flows” and somewhat less for the “younger flows.” The results are very similar to the age results for Amazonis Planitia, not far to the northwest.
[Figure 8] (a) Crater counts averaging over older
background
surface of Olympus Mons, using Mariner and Viking
data and an average of several MGS and THEMIS frames, suggesting that the
upper
few hundred meters of lavas average no more than a few hundred Ma in age.
(b) Unusually
young lava flows on Olympus Mons, with tongue-like
flow fronts emphasized by low light at
From these data we conclude that Olympus Mons has built up over a long time period. The exposed surface flows appear to range from no more than a few tens of Ma to 1000 Ma, and so much of the bulk of the mountain must have built up prior to 1000 Ma, consistent with the reconstruction of the Tharsis volcanism by Dohm et al.(2001).
C. Arsia Mons
These data (Fig. 9), with counts by various workers in our group, averaging over the whole surface, suggest a surface model age of about 200 to 1000 Ma, similar to the results for the older lavas of Olympus Mons. The results are interpreted as the formation ages of the lavas in the upper 100 m or so of the surface of Arsia Mons.
[Figure 9] (a) Floor
of summit caldera of Arsia Mons. MGS M14-01151, 9S,
121W. (b) Summary of crater counts
on
caldera and slopes of Arsia Mons,
suggesting lava ages of some hundreds of Ma.
D. Ares Vallis
Figure 10a shows a typical portion of the floor of Ares Vallis, which is heavily cratered at MGS resolutions. The heavy cratering might be taken a priori as a sign of great age, but we must remember that craters of a few hundred meters diameter can go into saturation in less than the age of Mars. The crater counts in D ranges free of saturation effects, as shown in Fig. 10b, give a model age of perhaps 500 to 3000 Ma, probably representing late Hesperian or Amazonian time. This is interpreted as the time of the last water flow down the channel. Consistent with this, Scott and Tanaka (1986) map the floor of Ares Vallis as late Hesperian in age.
[Figure 10] (a) Floor (upper right), banks (lower left), and
surrounding upland (LL corner) of Ares Valles. MGS frame M20-01813, 9N, 23W. (b) Crater counts on the different
parts of the floor of Ares Vallis confirm the high
densities of small scale craters glimpsed on (a), and fit isochrons,
consistent
with mapping of the feature as late Hesperian in age.
Although the counts in Ares Vallis clearly indicate an older relative age than in any of the previous lava flow examples, this is an example of a case where the crater counts do not give very useful absolute “geophysical” information, because they fall in the mid-range of Martian history, so that the factor 2+ error bars cover much of Martian time and do not constrain the feature incontestably to either the first, middle, or last third of Martian history.
E. Marte Vallis
In contrast to Ares Vallis and most outflow channels, Marte Vallis is geologically young. Figure 11a shows a streamlined “island” and part of the floor of the Marte/Athabasca Vallis system, showing that the channel floor is relatively devoid of craters formed since flow-sculpted streamlines. As noted by Burr et al. (2002a) and Berman and Hartmann (2002), this establishes that this channel system is much younger than Ares Vallis. As shown in Fig. 11b, counts by Berman and by the author, plotted against the new isochrons, give a fairly good model age of ~ 30 Ma to ~ 300 Ma, which is interpreted as the age of the last major water flow in the system. This is within the range of earlier results, and indicates that the Marte Vallis system is much younger than most classic Martian outflow channels, possibly being produced by melting of massive ground ice during the emplacement of the very recent surrounding lavas, which also date from the last few hundred Ma.
[Figure 11] (a)
Marte Vallis, a fluvial system
cut into recent lava plains, is distinguished in this region by a dark-toned
lava flow that entered the channel upstream (south) from this view, and has
flowed down the channel, solidifying into well-formed flow fronts in the
riverbed (top right).
(b) Crater
counts in and around various parts of Marte Vallis fit an isochron with model age around 30-300 Ma,
indicating that this system is much younger than most Martian outflow
channel
systems.
F. Exhumation in Crommelin Crater in Western Arabia Terra
Figure 12a shows Crommelin crater, in western Arabia Terra that was chosen by Malin and Edgett as an excellent example of exhumed sedimentary layers. Remarkably, there are essentially no visible impact craters superimposed on the exhumed terrain. The fact that strata edges are well exposed indicates that superposed, fresh-looking impact craters would date the time since exhumation, not the time since the last mantling deposition (because recently deposited mantles would be draped over all the terrain and blanket strata edges). While the lack of craters demolishes the chance for a statistically meaningful fit of crater counts to an isochron, it puts an upper limit on the age – not of the bedrock layers (which may be much earlier) but of the of the exhumation process. As shown in Fig. 12b, this upper limit is in the range of 0.1 to a few Ma, depending on how much we trust the isochrons for the smallest craters. This result gives a good counter-example to Malin and Edgett (2003), who assert that the exhumation process makes crater-count chronologic interpretation impossible. On the contrary, a careful examination of exhumed surfaces should give information about the chronology of the exhumation process itself.
[Figure 12]
(a) Dramatic stratified terrain in crater Crommelin, showing a virtual absence of visible craters
post-dating the exhumed surface.
8N, 7W, M14-01647.
(b)
Crater counts on the exhumed terrain give only upper limits, suggesting very
young ages within the last few Ma for the
exhumation
process.
As discussed by Berman et al. (submitted), a characteristic age of a few Ma to a few tens Ma is also found for possible ice flow features and gullied hillsides at moderate-to-high latitudes. These results, taken together, suggest the important result that recent exhumation (and ice deposition) processes may operate on a timescale comparable to the last few high obliquity excursions (5 Ma, 20 Ma), and may be associated with subaerial deposition and later wind-removal of the ice-rich dust mantles proposed by Mustard et al. (2001)and Costard et al. (2002). This raises the important question, for future investigation, of the extent to which axial tilt, in the last 4 to 30 Ma, has affected decameter topography and crater preservation at low latitudes, as well as moderate and high latitudes.
G. Terra Meridiani Landing Site as a Region of Complex Geologic History
The Terra Meridiani hematite-rich region, as discussed by Hartmann et al. (2001) shows a complex pattern of history hinting at a two-stage geologic history. First, a population of very ancient, shallow, degraded, faint white rings exists, sometimes with central patches of dunes (Fig. 13a). These appear to be very degraded ancient impact craters, termed “fossil craters” by Hartmann et al. (2001), and as shown by Fig. 13b they show a size distribution close to saturation equilibrium for impact craters. The larger (unsaturated?) craters suggest a very ancient Noachian age, 3.5 Ga, for the surface in which they are formed. In addition is seen a distinct population of small, sharply defined, bowl shaped craters (visible in Fig. 13a; there may be some transitional examples in some areas). As shown in Fig. 13b, these fit a much lower model age of the order of 10 Ma, as counted by different workers in different areas, at least for craters of D < 250 m. The suggested interpretation is that the present surface (or top 80 m) has been exposed for #~ 10 Ma. This in turn indicates an exhumation episode at around that time. The ancient surface itself, exposed by this exhumation, appears to date back to the first 20% of Martian history, and the hematite deposits in this area may relate to very early Noachian fluvial processes.
[Figure 13]
(a) Portion
of Meridiani Terra showing faintly defined,
“fossil
crater” rings and sharply, recent, small craters. (b)
Crater counts in the same area suggest that the “fossil craters” date
back to very early Martian history, but the surface has been accumulated
fresh
craters only for the last ~ 10 Ma, suggesting a recent exhumation event (see
text). 1S, 7W,
M00-01661.
CRITIQUES OF CRATER COUNT TECHNIQUES
Note that we do not attempt to reject secondary impact craters from primaries, except to avoid areas obviously Acontaminated@ by swarms of locally derived secondaries from specific primary craters. Of course, secondaries are created by primary impact events in space and in time, but there is no denying that over extended periods of time, a globally averaged background of secondaries accumulates. To the extent that we can avoid patches of Alocal secondaries@ and deal with the global Abackground secondaries,@ we can tie the small-crater populations (D < ~0.5 km, many secondaries) to the large-crater populations (D > ~2 km, mostly primaries) and get good results. In any case, to the extent that we occasionally pick up anomalous clumpings or concentrations of local secondaries, our derived ages can only be reduced (because the true crater numbers at small sizes should be reduced). This is contrary to a critique by McEwen (2003), who asserts that derived ages could be increased by as much as a factor 20 due to uncertainties introduced by secondaries.
Having worked with Mariner 9, Viking, Mars Global Surveyor, and Mars Odyssey imagery for many years, we have been able to define the isochrons over more than four orders of magnitude in size, from D = 11m to D = hundreds of km. The wider the D range measured, the more confidence one can have in fitting crater counts to isochrons and deriving age information.
The best dating comes from paying attention to all factors mentioned above B picking homogenous stratigraphic units; counting a wide D-range of craters; avoiding areas Acontaminated@ by locally derived secondaries, crater rays, or clusters of secondaries; paying attention to crater morphology and making separate counts of fresh craters and degraded craters if necessary,
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