Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989) 8047.pdf
GEOCHRONOLOGY AS A FRAMEWORK FOR PLANETARY HISTORY THROUGH 2050. B. A. Cohen1, R. Arevalo Jr.2, W. F. Bottke, Jr.3, P. G. Conrad2, K. A. Farley4, C. I. Fassett1, B. L. Jolliff5, S. J. Lawrence6, P. R. Mahaffy2, C. Malespin2, T. D. Swindle7, M. Wadhwa8, 1NASA MSFC ([email protected]), 2NASA GSFC, 3Southwest Research Institute, 4California Institute of Technology, 5Washington University, 6NASA JSC, 7University of Arizona, 8Arizona State University.
Introduction: In the decades of planetary explora- tem and obtaining more precise ages of the oldest and tion since the 1970’s, the science community has made youngest magmatic products will provide a way to un- great progress characterizing the contemporary state derstand the dynamics of magma oceans and crust for- and relative geologic histories of the terrestrial and outer mation, and the longevity and evolution of interior heat planets, satellites, and primitive bodies. In parallel, we engines and distinct mantle/crustal source regions. have significantly advanced the state-of-the-art in labor- Bombardment History: Determining the flux of im- atory-based absolute geochronology techniques and ex- pactors on all bodies, and whether it was constant across posure age determinations applied to planetary samples. the inner and outer solar system, is a primary goal of the Despite this progress, little headway has been made im- planetary science community. The energetic nature of proving our knowledge of absolute ages for common impact cratering can have wide-ranging consequences events, such as the Late Heavy Bombardment, planetary extending to a planet’s subsurface and atmosphere, per- volcanism, and the establishment of astrobiologically- haps destroying life or creating transient abodes for it. relevant environments. In the next 40 years, we advo- One of the biggest questions is whether there was a lu- cate constructing a common framework of geologic nar cataclysm, or late heavy bombardment, defined as time across our solar system, linking individual plane- the creation of multiple lunar nearside basins within a tary evolution to solar system history. Accomplishing short period [7, 8]. This event potentially relates the im- this theme requires the integration of geochronology pact bombardment history of the inner solar system to with in situ investigations, targeted sample return mis- the time when life began on Earth . Yet, the crater- sions, and continued advancements in laboratory analy- based age estimates of the Rheasilvia basin on Vesta sis and modeling. range from 1 Ga to 3.5 Ga [10, 11] and the epoch of Absolute Geochronology: Our knowledge of abso- large-basin formation on Mercury and Mars is uncertain lute surface ages on other bodies, including Mars, Mer- by hundreds of Myr . It is crucial to determine the cury, asteroids, and outer planet satellites, relies primar- time interval for the creation of large basins on the ter- ily on the crater calibration record for the Moon. While restrial planets and establish how the flux delivered to lunar cratering history is bounded between ~1 and ~4 inner and outer planets reflects the dynamical evolution Ga by isotopic ages of the Copernicus and Imbrium im- of the solar system . pacts and multiple volcanic units, the impact rates be- Astrobiology: An incomplete knowledge of absolute fore 4 Ga and after 1 Ga are more poorly constrained Martian geochronology limits our understanding of the . Absolute ages of Martian surface units can be un- timing of the planet’s evolutionary milestones – for ex- certain by a factor of two on older (Hesperian) surfaces, ample, whether the Noachian-Hesperian boundary oc- and by an order of magnitude on younger, lightly-cra- curred before, after or concurrent with the late heavy tered surfaces [2, 3]. This uncertainty encompasses ma- bombardment on the Moon , or when Mars’ surface jor events on the terrestrial planets, including thermal environment transitioned from wetter and more chemi- evolution, impact bombardment, and climate change. cally neutral conditions to volcanically dominated, Planetary Origin: Chemical evolution of planetary acidic, oxidizing, and dry surface conditions . Ab- bodies, ranging from asteroids to the large rocky plan- solute dating also will be required to relate habitability ets, is thought to begin with differentiation through so- markers to the timescale of evolution of life on Earth lidification of magma oceans. Rocks from the crust and . Moreover, measurements of exposure ages are mantle date the processes of silicate (and metal) segre- proxies of biosignature preservation potential, enabling gation of planetary formation and magmatic evolution – the prioritization of samples to be returned to Earth yet ancient lunar crustal rocks have ages that range to and/or analyzed by life-detecting techniques in situ. much younger than magma-ocean models would predict Strategies through 2050: Through the next several [4, 5]. The most ancient Martian meteorite, ALH84001, decades, a sustained effort will be required to create a crystallized much later than predictions of crustal for- framework that relates planetary geologic events to each mation on Mars . Some worlds, such as Europa and other. In this decade, investment is needed to increase Venus, have evidence of extremely recent activity, indi- the technology readiness levels to TRL 6 for in situ ge- cating long-lived heat sources driving crustal processes. ochronology instruments using complementary radio- Identifying the most ancient crust across the solar sys- genic isotopic systems. Sample collection and handling
Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989) 8047.pdf
systems are required to ingest samples for all in situ da- site and architecture discussions. We urge the commu- ting methods; these systems need to be matured, along nity to make a geochronology anchor sample a critical with operating scenario for their use, such that the oper- sample in MSR, or to consider groundbreaking MSR to ational burden for sample collection and analysis is re- a suitable surface for this purpose. Such a sample would duced. Further improvements to spacecraft mobility and be able to be studied using multiple geochronological dexterity will enable more geologic units to be interro- systems in state-of-the-art laboratories on Earth, as well gated during each mission. In the 2020’s, these technol- as other techniques (such as isotopic and trace element ogies will be ready to be included in developing mis- analysis) that provide additional constraints on under- sions to key stratigraphic targets on terrestrial planets, standing the history of the planet. alongside planning sample-return efforts for the Moon Laboratory Facilities: Missions such as Genesis and and Mars. In the 2030’s, an in situ geochronology com- Stardust drove the advancement of laboratory capabili- ponent should be considered as an augmentation to hu- ties for the analysis of smaller and smaller samples  man exploration of the Moon and Mars and for robotic and the streamlining of analytical protocols (e.g., begin- missions to targets beyond our current capabilities for ning with non-destructive techniques). For sample geo- sample return in the inner and outer solar system (in- chronology, the primary instruments are high precision cluding Mercury, Venus, Europa, and Io). By the mass spectrometers, equipped with thermal or plasma 2040’s, we should expect in situ geochronology to be a ionization sources, secondary ion and noble gas mass standard capability on planetary landers. In parallel with spectrometers, and accelerator mass spectrometers. Sus- these developments, Earth-based laboratory capabilities tained investment in laboratory upgrades and advance- for returned samples must continue to advance in sensi- ments, as well as in training future generations of re- tivity, accuracy and precision, as well as efficiency in search analysts, will be needed to extract maximum sci- the handling and processing of diverse samples. entific return from geochronological investigations of In situ Dating: The capability of flight instruments existing and future samples from planetary targets. to conduct in situ geochronology is specified in the References:  Stöffler (2001) Space Sci Rev 96, 9-54. 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