Sampling Gruithuisen

The following is an abstract I wrote for – but never submitted to – the 2011 micro-symposium on The Importance of Solar System Sample Return Missions to the Future of Planetary Science. Because of time constraints I had to limit myself to a single presentation on how and why we sample crater ejecta (PDF of that abstract here).

Introduction:  The case for planetary differentiation has been well established for inner Solar System planets. Samples from meteoritic material and robotic and manned missions have contributed to current models of planetary evolution. A key early finding of the Apollo missions was evidence for a lunar magma ocean (LMO) and the early differentiation of terrestrial planets [1, 2]. This model was later applied to other rocky planets such as the Earth, Mars, and Venus.

Silicic volcanism represents a compositional end-member of planetary differentiation. As late-stage products, these evolved magmas may be used to place constraints on mantle sources and processes. Silicic magma in sufficient quantities may also play a role in early planetary mantle dynamics, magmatism and crustal evolution.

Non-terrestrial silicic volcanism is potentially identified on Venus (e.g., Pancake Volcanoes) and the Moon (e.g., Gruithuisen Domes) [3, 4]. Of these locations, the Moon is both uniquely preserved and accessible to robotic and manned missions. The Gruithuisen Domes were a Constellation region of interest, and much work has been done to characterize the area.

Samples to determine the extent and range of products of differentiation are among the highest lunar science priorities [5, 6]. Silicic volcanic terranes are rare in the current lunar sample collection. Those that have been identified are of uncertain provenance [4]. Origins as the products of silica-liquid immiscibility or basaltic underplating have been proposed for non-mare domes, but it is unclear whether they are volumetrically minor late stage residual melt or form large intrusive (and extrusive) bodies [7, 8].

Gruithuisen Dome region

Image and caption from [NASA/GSFC/Arizona State University]: “All three of the Gruithuisen domes and the surrounding terrain are shown in WAC frame M117752970. Image width is 64 km and illumination is from the left.” Click the photo to visit the post on the LROC website.

Gruithuisen Domes:  Located on the northeast margin of Oceanus Procellarum, the Gruithuisen Domes area contains three dome structures: Gruithuisen Delta (27 km diameter), Gruithuisen Gamma (19 km), and Gruithuisen Northwest (7.5 km) [4]. As nearside non-mare volcanic features, they represent an accessible and valuable scientific site. Spectral observations indicate the domes are low in iron and titanium compared to mare and are also enriched in thorium (~20-40 ppm), similar in nature to rhyolite domes on Earth [7,9]. Emplacement and rheology models also indicate similarities with rhyolite [10]. Elevation profiles of central summit craters are consistent with non-impact origin [6].

The Gruithuisen Domes are located in a geologically complex region proximal to highland and mare units. On the basis of crater counts and geologic mapping, the timing of dome emplacement has been calculated as 3.85-3.72 Ga (Late Imbrian), earlier than the ≤3.55 Ga surrounding mare units [7, 11]. A significant contribution of sample return would be the establishment of an absolute age for these units and the association with the surrounding mare and underlying highlands. [Click here to visit the featured post on the Gruithuisen Domes at the LROC website.]

Sample Missions:  Scientific potential is significant for a stationary lander, and only increases if mobile rovers and manned missions are also considered.

Automated Sample Return.  There are several potentially key areas of focus for a sample return mission from the Gruithuisen Domes. The summits of larger domes are plateaus large enough to target for automated landing. Targeting the plateaus may avoid issues associated with landing on mare or highland terrane. Flank slopes (11-18 degrees [6]) may present problems for a lander, but are manageable by rover. A single sample return from the Gruithuisen Domes would likely yield rock types currently lacking in the lunar sample collection.

Manned Sample Return.  Adding a human element enhances the diversity and quality of collected samples. Mobile missions are currently restricted to a 10km radius around the lunar module on slopes of less than 25 degrees. Within these architectural constraints, a single mission could fully explore one dome or sample the flanks of two domes and the surrounding mare.

References:

  1. J. A. Wood et al. (1970) Lunar anorthosites and a geophysical model of the Moon. Proceedings of the 11th Lunar Science Conference, 965-988.
  2. P. H. Warren (1985) The magma ocean concept and lunar evolution. Annual Reviews of Earth & Planetary Science 13, 201-240.
  3. J. H. Fink et al. (1993) Shapes of Venusian “pancake’ domes imply episodic emplacement and silicic composition. Geophysical Research Letters 20, 261-264.
  4. J. W. Head and T. B. McCord (1978) Imbrian-Age Highland Volcanism on the Moon: The Gruithuisen and Mairan Domes. Science 199, 1433-1436.
  5. NRC (2007) The Scientific Context for Exploration of the Moon, 107p.
  6. S. E. Braden et al. (2010) Morphology of Gruithuisen and Hortensius Domes: Mare vs nonmare volcanism (PDF). Lunar and Planetary Science Conference XXXXI, #2677.
  7. S. D. Chevrel et al. (1999) Gruithuisen domes region: A candidate for an extended nonmare volcanism unit on the Moon. Journal of Geophysical Research 104, 16515-16529.
  8. S. E. Braden et al. (2007) Unexplored Areas of the Moon: Nonmare Domes. Planetary Science Decadal Survey, 2013-2022.
  9. J. J. Hagerty et al. (2006) Refined thorium abundances for lunar red spots: Implications for evolved, nonmare volcanism on the Moon. Journal of Geophysical Research 111, E06002.
  10. L. Wilson and J. W. Head (2003) Deep generation of magmatic gas on the Moon and implications for pyroclastic eruptions. Journal of Geophysical Research 108, 5012.
  11. R. Wagner et al. (2002) Stratigraphic sequence and ages of volcanic units in the Gruithuisen region of the Moon. Journal of Geophysical Research 107(E11), 5104.
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