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Register a new userThe timeline of the Lunar bombardment - revisited
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Added by
Alessandro Morbidelli
Publication date
2018
fields
Physics
and research's language is
English
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The timeline of the lunar bombardment in the first Gy of the Solar System remains unclear. Some basin-forming impacts occurred 3.9-3.7Gy ago. Many other basins formed before, but their exact ages are not precisely known. There are two possible interpretations of the data: in the cataclysm scenario there was a surge in the impact rate approximately 3.9Gy ago, while in the accretion tail scenario the lunar bombardment declined since the era of planet formation and the latest basins formed in its tail-end. Here, we revisit the work of Morbidelli et al.(2012) that examined which scenario could be compatible with both the lunar crater record in the 3-4Gy period and the abundance of highly siderophile elements (HSE) in the lunar mantle. We use updated numerical simulations of the fluxes of impactors. Under the traditional assumption that the HSEs track the total amount of material accreted by the Moon since its formation, we conclude that only the cataclysm scenario can explain the data. The cataclysm should have started ~3.95Gy ago. However we show that HSEs could have been sequestered from the lunar mantle due to iron sulfide exsolution during magma ocean crystallization, followed by mantle overturn. Based on the hypothesis that the lunar magma ocean crystallized about 100-150My after Moon formation, and therefore that HSEs accumulated in the lunar mantle only after this time, we show that the bombardment in the 3-4Gy period can be explained in the accretion tail scenario. This hypothesis would also explain why the Moon appears so depleted in HSEs relative to the Earth. We also extend our analysis of the cataclysm and accretion tail scenarios to the case of Mars. The accretion tail scenario requires a global resurfacing event on Mars ~4.4Gy ago, possibly associated with the formation of the Borealis basin, and it is consistent with the HSE budget of the planet.
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The Lunar University Network for Astrophysics Research (LUNAR) undertakes investigations across the full spectrum of science within the mission of the NASA Lunar Science Institute (NLSI), namely science of, on, and from the Moon. The LUNAR teams work on science of and on the Moon, which is the subject of this white paper, is conducted in the broader context of ascertaining the content, origin, and evolution of the solar system.
The giant planets of our solar system possess envelopes consisting mainly of hydrogen and helium but are also significantly enriched in heavier elements relatively to our Sun. In order to better constrain how these heavy elements have been delivered, we quantify the amount accreted during the so-called late heavy bombardment, at a time when planets were fully formed and planetesimals could not sink deep into the planets. On the basis of the Nice model, we obtain accreted masses (in terrestrial units) equal to $0.15pm0.04 rm,M_oplus$ for Jupiter, and $0.08 pm 0.01 rm,M_oplus$ for Saturn. For the two other giant planets, the results are found to depend mostly on whether they switched position during the instability phase. For Uranus, the accreted mass is $0.051 pm 0.003 rm,M_oplus$ with an inversion and $0.030 pm 0.001 rm,M_oplus$ without an inversion. Neptune accretes $0.048 pm 0.015 rm,M_oplus$ in models in which it is initially closer to the Sun than Uranus, and $0.066 pm 0.006 rm,M_oplus$ otherwise. With well-mixed envelopes, this corresponds to an increase in the enrichment over the solar value of $0.033 pm 0.001$ and $0.074 pm 0.007$ for Jupiter and Saturn, respectively. For the two other planets, we find the enrichments to be $2.1 pm 1.4$ (w/ inversion) or $1.2 pm 0.7$ (w/o inversion) for Uranus, and $2.0 pm 1.2$ (w/ inversion) or $2.7 pm 1.6$ (w/o inversion) for Neptune. This is clearly insufficient to explain the inferred enrichments of $sim 4$ for Jupiter, $sim 7$ for Saturn and $sim 45$ for Uranus and Neptune.
We create the first quantitative model for the early lunar atmosphere, coupled with a magma ocean crystallization model. Immediately after formation, the moons surface was subject to a radiative environment that included contributions from the early Sun, a post-impact Earth that radiated like a mid-type M dwarf star, and a cooling global magma ocean. This radiative environment resulted in a largely Earth-side atmosphere on the Moon, ranging from $sim$10$^4$ to $sim$10$^2$ pascals, composed of heavy volatiles (Na and SiO). This atmosphere persisted through lid formation and was additionally characterized by supersonic winds that transported significant quantities of moderate volatiles and likely generated magma ocean waves. The existence of this atmosphere may have influenced the distribution of some moderate volatiles and created temperature asymmetries which influenced ocean flow and cooling. Such asymmetries may characterize young, tidally locked rocky bodies with global magma oceans and subject to intense irradiation.
Since the Apollo program or earlier it has been widely believed that the lunar regolith was compacted through vibrations including nearby impact events, thermal stress release in the regolith, deep moon quakes, and shallow moon quakes. Experiments have shown that vibrations both compact and re-loosen regolith as a function of depth in the lunar soil column and amplitude of the vibrational acceleration. Experiments have also identified another process that is extremely effective at compacting regolith: the expansion and contraction of individual regolith grains due to thermal cycling in the upper part of the regolith where the diurnal thermal wave exists. Remote sensing data sets from the Moon suggest that the soil is less compacted in regions where there is less thermal cycling, including infrared emissions measured by the Diviner radiometer on the Lunar Reconnaissance Orbiter (LRO). Here, we performed additional experiments in thermal cycling simulated lunar regolith and confirm that it is an effective compaction mechanism and may explain the remote sensing data. This creates a consistent picture that the soil really is looser in the upper layers in polar regions, which may be a challenge for rovers that must drive in the looser soil.
The asteroid belt is an open window on the history of the Solar System, as it preserves records of both its formation process and its secular evolution. The progenitors of the present-day asteroids formed in the Solar Nebula almost contemporary to the giant planets. The actual process producing the first generation of asteroids is uncertain, strongly depending on the physical characteristics of the Solar Nebula, and the different scenarios produce very diverse initial size-frequency distributions. In this work we investigate the implications of the formation of Jupiter, plausibly the first giant planet to form, on the evolution of the primordial asteroid belt. The formation of Jupiter triggered a short but intense period of primordial bombardment, previously unaccounted for, which caused an early phase of enhanced collisional evolution in the asteroid belt. Our results indicate that this Jovian Early Bombardment caused the erosion or the disruption of bodies smaller than a threshold size, which strongly depends on the size-frequency distribution of the primordial planetesimals. If the asteroid belt was dominated by planetesimals less than 100 km in diameter, the primordial bombardment would have caused the erosion of bodies smaller than 200 km in diameter. If the asteroid belt was instead dominated by larger planetesimals, the bombardment would have resulted in the destruction of bodies as big as 500 km.
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