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Barr A.C.,Southwest Research Institute | Barr A.C.,Center for Lunar Origin and Evolution | Citron R.I.,Center for Lunar Origin and Evolution | Citron R.I.,Laboratory for Atmospheric and Space Physics | And 2 more authors.
Icarus | Year: 2010

Accretional temperature profiles for Saturn's large moon Titan are used to determine the conditions needed for accretion to avoid global melting as a function of the timing, duration, and nebular conditions of Titan's accretion. We find that Titan can accrete undifferentiated in a " gas-starved" disk even with modest quantities of ammonia mixed in with its ices. Simulations of impact-induced core formation are used to show that Titan can remain only partially differentiated after an outer Solar System late heavy bombardment capable of melting its outer layers, permitting some of its rock to consolidate into a core. © 2010 Elsevier Inc.

Morbidelli A.,French National Center for Scientific Research | Marchi S.,French National Center for Scientific Research | Marchi S.,Center for Lunar Origin and Evolution | Marchi S.,Lunar and Planetary Institute | And 2 more authors.
Earth and Planetary Science Letters | Year: 2012

We revisit the early evolution of the Moon's bombardment. Our work combines modeling (based on plausible projectile sources and their dynamical decay rates) with constraints from the lunar crater record, radiometric ages of the youngest lunar basins, and the abundance of highly siderophile elements in the lunar crust and mantle. We deduce that the evolution of the impact flux did not decline exponentially over the first billion years of lunar history, but also there was no prominent and narrow impact spike ~3.9. Gy ago, unlike that typically envisioned in the lunar cataclysm scenario. Instead, we show the timeline of the lunar bombardment has a sawtooth-like profile, with an uptick in the impact flux near ~4.1. Gy ago. The impact flux at the beginning of this weaker cataclysm was 5-10 times higher than the immediately preceding period. The Nectaris basin should have been one of the first basins formed at the sawtooth. We predict the bombardment rate since ~4.1. Gy ago declined slowly and adhered relatively close to classic crater chronology models (Neukum and Ivanov, 1994). Overall we expect that the sawtooth event accounted for about one-fourth of the total bombardment suffered by the Moon since its formation. Consequently, considering that ~12. -. 14 basins formed during the sawtooth event, we expect that the net number of basins formed on the Moon was ~45. -. 50. From our expected bombardment timeline, we derived a new and improved lunar chronology suitable for use on pre-Nectarian surface units. According to this chronology, a significant portion of the oldest lunar cratered terrains has an age of 4.38-4.42. Gyr. Moreover, the largest lunar basin, South Pole Aitken, is older than 4.3. Gy, and therefore was not produced during the lunar cataclysm. © 2012 Elsevier B.V.

Marchi S.,University of Nice Sophia Antipolis | Marchi S.,Center for Lunar Origin and Evolution | Marchi S.,USRA Lunar and Planetary Institute | Bottke W.F.,Center for Lunar Origin and Evolution | And 2 more authors.
Earth and Planetary Science Letters | Year: 2012

The earliest bombardment history of the Moon potentially provides powerful constraints for solar system evolution models. A major uncertainty, however, is how much of this history is actually recorded in lunar craters. For example, some argue that most ancient lunar craters and basins were produced by a declining bombardment of leftover planetesimals produced by terrestrial planet formation processes. Others believe that most lunar craters and large basins were formed in a narrow time interval between 3.8 and 4.0. Ga, the so-called lunar cataclysm. In the light of recent improvements in our understanding of early solar system evolution, it is possible that the contributions from both scenarios could be represented in the lunar crater record. If so, when did the declining bombardment end and the lunar cataclysm begin?Here we show, using new counts of 15-150. km diameter craters on the most ancient lunar terrains, that the craters found on or near Nectaris basin appear to have been created by projectiles hitting twice as fast as those that made the oldest craters on various Pre-Nectarian-era terrains. This dramatic velocity increase is consistent with the existence of a lunar cataclysm and potentially with a late reconfiguration of giant planet orbits, which may have strongly modified the source of lunar impactors. This work also suggests that the lunar cataclysm may have started near the formation time of Nectaris basin. This possibility implies that South Pole-Aitken basin (SPA), the largest lunar basin and one of the oldest by superposition, was not created during the cataclysm. This view is strengthened by our interpretation that a substantial fraction of ancient craters on SPA were made by low velocity impactors. Finally, we believe these results shed new light on the impact history of the primordial Earth. © 2012 Elsevier B.V..

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