Apollo Next Generation Sample Analysis

The goal of the Apollo Next Generation Sample Analysis (ANGSA) Program is to maximize the science derived from samples returned by the Apollo Program in preparation for future lunar missions anticipated in the 2020s and beyond. To achieve this, ANGSA has selected 9 proposals that focus on specially curated materials from the Apollo 15, 16, and 17 sample collections, which were returned to Earth in 1971-72 and haven’t been touched since then. One of the selected proposals is the “Study on lunar regolith mixing using cosmogenic radionuclide measurements”, led by PI Kees Welten and co-I Kunihiko Nishiizumi, both senior fellows at UC Berkeley’s Space Sciences Laboratory (SSL) and co-I Marc Caffee, director of Purdue University’s PRIME Laboratory.

The goal of our project is to understand the mixing history of the lunar surface utilizing radionuclides (¹⁰Be, ²⁶Al, ³⁶Cl, ⁴¹Ca and ⁵³Mn) produced by cosmic rays, both galactic (GCR) and solar (SCR). With half-lives ranging from 0.1 to 3.7 million years (Myr) these nuclides provide information on lunar surface processes on a timescale of 0.1 to 10 Myr. While ¹⁰Be, ³⁶Cl, ⁴¹Ca are mostly produced by high-energy GCR, the production rates of ²⁶Al and ⁵³Mn have large contributions from low-energy SCR. These SCR produced nuclides show steeply declining depth profiles in the top few cm of the lunar surface if the surface was undisturbed for the last few million years.  The GCR produced nuclides show relatively flat depth profiles near the surface (top ~50 cm) and a more gradual decline with greater depth. The measurement of multiple nuclides with different half-lives and different production mechanisms provides an excellent framework to address the mixing rate of the lunar regolith by impact processes as well as possible disturbances by man-made processes during collection on the Moon and/or during transfer to Earth. Our work includes: (1) measurements of cosmogenic radionuclide depth profiles in bulk samples of lunar core 73002/1 with a resolution of ~1 mm at the top decreasing to ~5 cm at the bottom of the core, and (2) measurements of cosmogenic radionuclides in several grain size fractions of shadowed soil samples, 72320 and 76240, to verify if these were shielded from the Sun and from SCR for the last few Myr.

Experimental Methods

To isolate the cosmogenic radionuclides from the lunar soils, we will dissolve each sample (typically 50-100 mg) in an HF/HNO₃mixture in the presence of Be, Cl. After dissolution, the various elemental fractions (Be, Al, Cl, Ca, Mn) will be separated using a combination of anion and cation exchange chromatography and solvent extraction techniques and will then be purified for isotopic analysis by accelerator mass spectrometry (AMS). In addition, small aliquots of the dissolved samples will be taken for chemical analysis by inductively coupled plasma optical emission spectroscopy (ICP-OES).

Radionuclide analysis. The concentrations of cosmogenic radionuclides in lunar samples are determined by measuring the isotopic ratios (¹⁰Be/Be, ²⁶Al/Al, ³⁶Cl/Cl,⁴¹Ca/Ca) of the purified fractions and combining this with the amount of the stable nuclide that was either added as carrier (Be, Cl) or was already present in the sample (Al, Ca, Mn). The ¹⁰Be, ²⁶Al, ³⁶Cl and ⁴¹Ca AMS measurements will be performed at PRIME Lab, Purdue University. The ¹⁰Be, ²⁶Al and ³⁶Cl measurements have been routine for many years, while recent upgrades at PRIME Lab have also made the measurement of ⁴¹Ca possible. The Mn fraction will be saved for future analysis of ⁵³Mn by AMS. All measured isotope ratios are normalized to internationally recognized AMS standards.

Chemical analysis. Precise chemical analysis of the samples is needed to obtain Al, Ca and Mn concentrations that are required to calculate the concentrations of ²⁶Al,⁴¹Ca, and ⁵³Mn from each isotopic ratio (²⁶Al/Al,⁴¹Ca/Ca and ⁵³Mn/Mn) obtained by the AMS measurements. In addition, the concentrations of the main target elements that produce each cosmogenic nuclide have to be known to normalize the measured cosmogenic nuclide concentration to that of a “standard” lunar sample. For lunar samples, the measured radionuclide concentrations are usually normalized to those of the Apollo 15 drill core, since this core is the least disturbed core in the past ~10 Myr. We will measure concentrations of Mg, Al, K, Ca, Ti, Mn and Fe, in an aliquot of each of the dissolved lunar samples by ICP-OES. Small corrections of the measured cosmogenic radionuclide concentrations due to variations in chemical composition are then made based on existing production rate models.

Team members :

Kees Welten

Kunihiko Nishiizumi

Marc Caffee