Supplementary material to “Issues in Planetary Chronology”

Nadine G. Barlow, Department of Physics and Astronomy, Northern Arizona University, Flagstaff; Paul Schenk, Lunar and Planetary Institute, Houston, Texas; E. Beau Bierhaus, Lockheed-Martin, Denver, Colorado

Citation:
Barlow, N. G., P. Schenk, and E. B. Bierhaus (2007), Issues in planetary chronology, Eos Trans. AGU, 88(18), 199. [Full Article (pdf)]

Ages of surface units constrain a body’s geologic and thermal evolution. Linking the histories of individual planets and moons provides the framework to understanding the chronology of the entire solar system. The last major effort discussing planetary chronology issues was the Basaltic Volcanism Study Project between 1976 and 1981. Major advances in data analysis, laboratory measurements, and numerical modeling have occurred in the intervening 25 years. The goal of the Workshop on Surface Ages and Histories: Issues in Planetary Chronology was to bring together researchers from the impact cratering, radiometric analysis, and numerical modeling communities to discuss the current status of chronologic techniques and identify directions of future research. The workshop was held May 21–23, 2006, at the Lunar and Planetary Institute in Houston, TX. The following three topics were the focus of discussion at the workshop.

Late Heavy Bombardment

Heavily cratered terrains on the Moon, Mercury, and Mars suggest that the inner solar system impact rate was much higher 3.8–4.4 Ga ago (109 yr = 1 Ga). After this Late Heavy Bombardment (LHB) period, the impact rate declined to a level that has been approximately constant for the last 3.8 Ga. Two major unanswered questions are

  1. Did the LHB decline from an early peak or was there a “spike” in the impact rate at 3.8–4.0 Ga, and
  2. How did the LHB affect the entire solar system?

William Bottke (Southwest Research Institute, Boulder, CO) presented models of asteroid flux throughout the inner solar system’s history. If LHB was simply a steady decline, he estimates that the primordial asteroid belt must have contained >10 Earth masses in order to have enough remaining material to form the observed large lunar impact basins ~3.8 Ga ago. This is a much higher mass than expected, leading Bottke to conclude that LHB was a spike. Hal Levison (Southwest Research Institute, Boulder, CO) showed that outer solar system dynamics affected the inner solar system impact flux 3.8–4.0 Ga ago, a likely explanation for the LHB. Levison’s models place the initial orbits of the giant planets closer to the Sun and embed the planets in a disk of planetesimals (left-over material from planet formation). Dynamical interactions between the outer planets and the planetesimal disk caused the planets to migrate outward. Eventually Saturn’s orbital period reached twice Jupiter’s period, increasing the orbital eccentricities of Jupiter and Saturn and causing Uranus and Neptune to rapidly move outward. These events perturbed material from the planetesimal disk into the inner solar system, significantly increasing the impact flux for a short period of time. Thus, numerical modeling suggests that the LHB was a solar system-wide cataclysmic event 3.8–4.0 Ga ago.

Impact Crater Size-Frequency Distributions (SFDs)

Impact crater analysis is the major technique for estimating ages of solid surface bodies in the solar system. SFDs are plots of crater frequency per unit area (“crater density”) versus crater size and provide relative ages (“older” or “younger”) of different surface units. One can compare the radiometrically-derived absolute ages from the lunar Apollo and Luna rock samples with crater densities of the region surrounding the sample location. The resulting lunar chronology curve (LCC) can be used to date lunar surface units from which samples have not yet been returned. The LCC can be extrapolated to another solar system body by making three assumptions:

  1. Impactor SFD between the Moon and other body is identical,
  2. Timing of major events such as the end of LHB is identical between the two bodies, and
  3. The ratio of impact fluxes on the two bodies is known.

However, these assumptions may not always be valid.

The greatest uncertainty in extrapolating LCC to Mars is in the impact flux ratio. Boris Ivanov (Russian Academy of Sciences, Moscow) determined the impact flux ratio relative to the Moon (R) is 0.92 for Mercury, 1.12 for Venus, and 1.68 for Earth. Mars’ current R value is 2.93, but, because of orbital eccentricity variations, martian R-values range between 2.58 and 3.10. Using the current martian R-value, William Hartmann (Planetary Science Institute, Tucson, AZ) has developed “isochron” charts to allow determination of surface age from crater density. Many martian surfaces recently dated by Hartmann and Gerhard Neukum (Freie Univ., Berlin, Germany) have utilized craters < 3-km-diameter in order to provide statistically reliable results for small surface areas.

The validity of obtaining chronologic information from small craters was a major discussion topic at the workshop. Alfred McEwen (Univ. AZ, Tucson) argued that most small martian craters are secondary craters, produced by ejection of material from a larger (primary) crater. Secondaries provide no information about the original impact population and their spatial density is uncorrelated with relative age. However, Neukum and Stephanie Werner (Freie Univ., Berlin) presented models suggesting secondaries are not a major contributor to SFDs. Hartmann argued that by avoiding clustered secondaries the counts reflect primary craters plus “background” secondaries. Over time, primaries plus background secondaries should accumulate in a manner similar to primaries alone. Further complexities were reported by Nadine Barlow (Northern AZ Univ, Flagstaff), who compared obvious secondaries close to the parent primary crater on the Moon and Mars. She finds that the secondary crater production rate is approximately twice as high on Mars. Ivanov’s simulations show that the secondary SFD differs from that of primary craters and might be identifiable in the analysis. Beau Bierhaus (Lockheed Martin, Denver, CO) used clustering analysis to show that craters <1-km-diameter on Jupiter’s moon Europa are dominated by secondaries and suggested this is likely the case on other solar system bodies. The potential that small crater analysis is contaminated by secondaries led Kenneth Tanaka (US Geological Survey, Flagstaff, AZ) to argue that relative ages are best determined using only the freshest craters >5-km-diameter on surfaces which have undergone little modification since their formation.

Radiometric Ages

Tim Swindle (Univ. AZ, Tucson) reviewed geochronologic evidences from the Moon, Earth, and asteroids which support a cataclysmic LHB. Radiometric analysis of lunar meteorites, lunar impact melts, and meteorites all display major impact events between 3.8 and 4.0 Ga. Few samples remain from the pre-4.0 Ga period on the Moon and meteorite parent bodies. Suggestions that the 3.8-4.0 Ga ages of the lunar samples result from the 3.8-Ga Imbrium basin event are contraindicated by lunar meteorite information (some probably sample the uncontaminated lunar farside) and particularly by non-lunar meteorite results. Hartmann noted that there likely were pre-cataclysm impacts, but these materials were destroyed by cataclysm-related events. Numerous degraded impact basins exist on the Moon which pre-date Imbrium, but there are few constraints on their absolute ages from sample analysis. Don Bogard (NASA Johnson Space Center, Houston, TX) argued that future lunar exploration needs to collect samples associated with specific craters and from lunar farside basins to better constrain the timing of impact events. He noted that lunar sample studies and the terrestrial crater record suggest an increased impact flux in the last 0.5 Ga, which also needs further investigation.

Martian meteorite analyses combined with insights from the cratering record constrain the martian chronology. Larry Nyquist (NASA Johnson Space Center, Houston, TX) reviewed age information of 37 identified martian meteorites. Only ALH84001 displays an ancient (4.5 Ga) age while the nakhlite-chassignite meteorites display a 1.36 Ga formation age. The largest group of martian meteorites, the shergottites, show a range of formation ages (162 to 174 million years (Ma)) and more complex histories. Nyquist argued that the clustering of ages and compositional variations indicate perhaps as few as 5 separate ejection events for the 37 meteorites. Francis Albarede (Ecole Normale Supérieure, Lyon, France), however, noted that most of the radioisotopes used to date martian meteorites are contained within phosphates, which are leachable in acid solutions. He argued that shergottite ages simply date the last fluid interaction and that the formation ages are actually much older. Hartmann suggested that this could be tested by determining ages of terrestrial rocks in various weathering states.

The Next Step

The final 2 hours of the workshop were devoted to a discussion of current uncertainties in planetary chronology techniques and identification of future missions and/or research that will help resolve these issues. A white paper summarizing this discussion is currently in production with additional input from many of the workshop participants. The white paper will be published as a full-length manuscript in a planetary science journal and its recommendations will be presented to the lunar, Mars, Venus, and Outer Planets Advisory Groups (LEAG, MEPAG, VEXAG, and OPAG, respectively) for inclusion in the reports produced by these groups for NASA and the planetary community.