Planetary Time Machine

An SwRI-developed geochronometer measures the age of rocks on the Moon and other planets without need to return samples to Earth

F. Scott Anderson, Ph.D.     image of PDF button

photo of F. Scott Anderson, Ph.D.

Dr. F. Scott Anderson is a principal scientist in the Planetary Science Directorate of SwRI’s Space Science and Engineering Division. He is a planetary geologist interested in the geology, geophysics and chronology of planetary bodies, including Mars, the Moon and Venus. The main focus of his research is understanding the isotopic abundance of minerals and rocks using laser desorption resonance ionization mass spectrometry.


image of a schematic chart

In this schematic chart, a laser vaporizes a small sample of the target rock, and wavelength-tuned lasers excite electronic states of neutral strontium, which are then extracted into the mass spectrometer. Microseconds later, a corresponding process occurs for rubidium, which follows behind the strontium in time, allowing it to be measured independently.


image of a first-generation bench-top version
of the laser desorption resonance ionization mass spectrometer (LDRIMS)

A first-generation bench-top version of the laser desorption resonance ionization mass spectrometer (LDRIMS) was developed in an SwRI laboratory in Boulder, Colo. It comprises a desorption laser (1), resonance ionization lasers (2) and a mass spectrometer (3).


image of isochron of a granite sample

This isochron of a granite sample measured with LDRIMS-1 compares aging data derived by LDRIMS (red line) with traditional aging methods (blue line).


image of a a sample from the LDRIMS-1

The LDRIMS-1 measured hundreds of spots on a granite sample, shown here by a group of colored dots, spaced 300 microns apart and producing microscopic pits about 75 microns wide and 0.5 micron deep.

The two highest science priorities for planetary exploration are searching for life and understanding our place in the history of planetary evolution. Straightforward as these goals may appear, however, they are anything but simple. Identifying life has proven to be complicated, requiring a wide range of laboratory techniques to identify the signatures of living or fossilized organisms. Furthermore, understanding the history of the scarred and cratered surfaces of other planets has been hindered by the difficult and timeconsuming laboratory measurements required for radiometric dating. Worse, surface samples from bodies other than the Moon have yet to be brought home to Earth for study, due to difficulty and cost. In fact, the analytical challenges alone were considered so daunting that it was commonly assumed that samples must be returned to Earth for analysis. Besides costing much more than in-situ measurement, there is also the risk that some samples may prove to have been of little value once they have been analyzed. After all, how does one choose which few, thimble-sized samples to bring back to Earth to represent a whole planet?

All of this is about to change. A team of scientists from Southwest Research Institute (SwRI) is nearing completion of an instrument that can fit onboard a spacecraft rover to triage rock samples with a quick search for organic molecules — the building blocks of life — and also provide radiometric dates of samples before they are selected for return to Earth for detailed study. This development comes in the nick of time because NASA is expected to solicit instrument ideas for a 2020 rover mission to Mars sometime this fall, with additional calls for dating the oldest impact basin on the Moon in approximately 2015.

The SwRI-developed instrument uses laser desorption resonance ionization mass spectrometry (LDRIMS) to date rock samples by measuring the relationship between certain isotopes of the elements rubidium (Rb) and strontium (Sr), plus it has a laser desorption-secondary ionization mass spectrometry (L2MS) capability for measuring elemental and organic chemistry. The SwRI team built a bench-top prototype that has produced dates for terrestrial samples in as little as eight hours compared to many months required for traditional laboratory techniques. A second-generation, portable instrument for field use is testing organic analysis protocols using the secondary L2MS capability. The goal is to provide organic and dating measurements in a fast, portable package for use by both robotic and human missions to the Moon and Mars.

An opportune time

Every 10 years, the National Academy of Science solicits ideas from the planetary science community for a planning document for NASA called the Decadal Survey. The most recent National Research Council Decadal Survey for the Moon proposed a sample-return mission to improve our knowledge of the age of the lunar surface.

For Mars, the survey proposed a three-mission plan to cache and return samples from the surface, known as Mars Sample Return (MSR), at a total cost of approximately $11 billion, extending into 2020 and beyond. Primary science goals are the search for organics and life, and as with the Moon, constraining the surface age of Mars, while collaterally (but not insignificantly) refining the evolutionary history of every rocky planet in the solar system. The Decadal Survey concluded that these goals would best be achieved by instrumentation and analysis techniques available only on Earth.

Amid shrinking federal budgets, NASA last December made a surprising announcement of a new Mars flight opportunity for 2020. A Mars 2020 Rover Science Definition Team was formed, with a charter that supports caching samples in the search for organics and life, to be placed in geologic context according to both age and chemistry.

Organics on Mars

Two decades of research on terrestrial life, plus largely discredited bio-signatures in the Martian meteorite ALH84001, have shown that a single, easily measured definition for life is elusive. However, there is consensus that organic compounds are the building blocks of living organisms. If found in a sample from the surface of another planet they would represent a highpriority target for return and additional analysis.

Unfortunately, the solar system is filled with abiotic sources of organic materials, including meteorites that rain down on Mars and other planets. A key question is how to distinguish potential native Martian organics from the abundant abiotic organics found in meteorites throughout the solar system. Despite the prediction of abundant organic infall from meteors and the observed presence of meteorites on the surface of Mars by the Mars rovers, no native organic signatures have yet been found, despite the presence of instruments like the gas chromatography mass spectrometers aboard the Viking and Mars Science Laboratory spacecraft. One hypothesized explanation is that organic compounds are rapidly broken down on the surface of Mars by the highly oxidizing surface environment. Nonetheless, given that organic signatures on Mars are hard to find, thus hindering in-situ analysis and identification, it is very likely that the mere detection of an organic compound in a Mars sample would justify its inclusion in a sample-return cache.

Surface dating on the Moon and Mars

In-situ geochronology measurements are important because current estimates of surface age, and hence surface history, are derived from crater counting, in which one assumes that older surfaces have both bigger and more craters while younger surfaces have relatively smaller and fewer craters. For the Moon, these estimates have been calibrated by the radiometric dating of rocks brought back to Earth by the Apollo astronauts. No samples have yet been returned from a known region of Mars that can be used to directly calibrate the ages derived from crater counts, so we are forced to extrapolate estimates of the impact rate for the Moon to Mars. However, even for the Moon there are uncertainties in the duration and timing of the period of heaviest bombardment of asteroids and comets, while no timing constraints exist for the period from 1 billion to 3.5 billion years ago. Worse yet, cratering rate estimates from the most recent era are non-unique. Improving our knowledge of the age of the lunar surfaces has proven to be so pressing a goal that the Decadal Survey lists missions to return lunar samples as a top priority.

For Mars studies, the ratio of impact rates between the Red Planet and the Moon is uncertain, leading to wide variances in determining the age of surface features. This is complicated further by the Moon’s own impact modeling uncertainties, leading the Decadal Survey to support the development of future instruments with a focus on the most important in-situ measurements, including in-situ geochronology experiments [National Research Council, 2012].

Rocky bodies throughout the solar system, including Mars, the Moon, Venus, Mercury and Earth itself, would similarly benefit from new measurements. These constraints could dramatically improve our understanding of solar-system-wide bombardment history, and thus of the relationship between the end of the heaviest bombardment and the rise of life.

Radiometric dating of rocks

Radiometric dating is based on the observation that radioactive isotopes decay into a different isotope over fixed periods of time, so that the age of a rock sample can be measured by comparing the abundance of each isotope. For example, half of the 87Rb in a mineral will decay into 87Sr over 48.8 billion years. Since the solar system itself is only about 4.5 billion years old, one can calculate that only about 6 percent of 87Rb will have decayed to 87Sr in even the oldest rocks. Modern mass spectrometers are very sensitive and can easily measure the abundance of Rb and Sr to better than 0.002 percent, producing dates as young as about 1 million years. The LDRIMS measurement focuses on the Rb-Sr system because it has been used for reliably dating a wide range of terrestrial and lunar samples, as well as Martian meteorites, and Rb and Sr are present in relatively high abundances in most rocks, making them easy to measure [Faure, 1986].

What makes Rb-Sr dating difficult is that 87Rb and 87Sr have very nearly the same mass, so a mass spectrometer alone cannot tell them apart. The traditional technique is to crush the sample, separate the minerals by hand under a microscope, leach the Rb and Sr from each mineral powder in acid, and use liquid chromatography to separate the Rb and Sr. The separates are then plated onto filaments for each element and mineral, and only then are they measured in a mass spectrometer. Obviously, chemical separation techniques for 87Rb-87Sr are unsuitable for in-situ planetary exploration due to this complexity, as well as its need for large mass, volume and power to perform the analysis.

Technology of tomorrow, today

The SwRI geochronometer uses a technique for dating called resonance ionization, which eliminates the need for chemical separation of 87Rb from 87Sr. In this method, a laser vaporizes a small sample of the target rock, generating greater than 99.9 percent neutral atoms; then wavelength-tuned lasers are used to resonantly excite electronic states of neutral strontium, and the resulting excited atoms are photoionized. The strontium is then extracted into the mass spectrometer. This process is followed a couple of microseconds later by a similar process for rubidium, which, although it has isotopes of the same mass, is separated from the strontium in time. The mass spectrometer thus measures the rubidium independently, which eliminates mass interferences and ensures that the measured atoms came from the same ablation event and hence the same mineral. This method enables good estimates of Rb and Sr, and hence of the time that has passed since the rock’s formation.

For organics, the SwRI instrument employs a subset of the lasers used for dating to ablate neutral atoms from the sample and then ionizes them with a deep-ultraviolet laser.

Results

The first-generation, bench-top LDRIMS-1 system has demonstrated a sensitivity to 300 parts per trillion, which is more than sufficient for dating. It typically obtains isotope ratio precisions of ±0.3 to ±0.1 percent in 3,000 ablations of one spot on a sample in about three minutes. It measures 100 to 300 spots in a raster pattern, sampling a range of different minerals, and thus their rubidium-strontium ratios. The LDRIMS-1 has been tested on samples of Boulder Creek Granite from Elephant Butte, Colo. Traditional measurements, and the SwRI team’s own preliminary micro-drill thermal ionization mass spectrometry (TIMS) measurements of individual minerals, are consistent with an age of 1.7 billion years ±40 million years.

To obtain an LDRIMS-1 date of the sample, the team measured hundreds of spots with approximately 300 micron (μm) spacing, producing microscopic pits about 75 μm wide and 0.5 μm deep. Traditional analyses can take one to six months to measure enough spots to generate an age estimate, compared to the LDRIMS-1 data for which hundreds of points were collected in less than 4.5 hours with no sample preparation. Assuming 300 spot measurements and 3,000 shots, approximately 1 million shots are required per date; the LDRIMS diode laser design typically produces billions of shots, allowing for 1,000 or more dating measurements.

Repeat measurement runs were carried out over six months to address subtle issues in software automation and laser reliability. The results were well within the age measured using TIMS techniques and had a precision and accuracy exceeding that called for by NASA (±200 Ma).

The second-generation LDRIMS-2 system is under development. The control electronics, mass spectrometer, miniature desorption laser and software are functioning, while the miniature strontium resonance laser system is undergoing final tuning and the rubidium laser system has been selected for rapid development by NASA.

The SwRI team has already been able to demonstrate laser ablation mass spectrometry (LAMS) and laser desorption secondary ionization mass spectrometry (L2MS) using LDRIMS-2. LAMS uses high-power laser ablation to directly create ions to characterize a sample, while L2MS uses a second, high-intensity laser beam to ionize neutral atoms removed from the sample by laser desorption. Both techniques are subsets of the full LDRIMS capability, requiring software timing control similar to LDRIMS. Advantages of LAMS include the measurement of a wide array of elements, while L2MS is one of the most sensitive organic detection methods available. Demonstrating these techniques is an important part of the LDRIMS science strategy, as measurements of geochemistry and organics can provide insight into habitability and identify potential biomarkers.

The future

The SwRI team is working to prepare the instrument and techniques described here for a Mars 2020 Rover proposal. To that end, they are dating more samples, expanding the suite of organics measurements, preparing to take data in the field, and working with development partners to miniaturize the instrument to a 1 cubicfoot box. The team seeks to enhance the characterization of landing sites on Mars by providing in-situ triage of potential samples for Earth return, improving the odds of returning relevant samples, and enhancing near-term science return.

Questions about this article? Contact Anderson at (303) 546-9670 or anderson@boulder.swri.org.

Sources cited

Conrad, P. G., F. S. Anderson, R. C. Anderson, W. J. Brinckerhoff, P. Doran, V. E. Hamilton, J. A. Hurowitz, A. S. McEwan, D. W. Ming, and D. A. Papanastassiou (2009), Geochronology and Mars exploration: Critical measurements for 21st century planetary science, 8 p, Decadal Survey white paper posted September.

Faure, G. (1986), Principles of Isotope Geology, 2nd ed., 589 pp., John Wiley and Sons, New York.

Hand, E. (2012), "Planetary science: The time machine," Nature, 487, 422-425.

National Research Council (2012), Vision and Voyages for Planetary Science in the Decade 2013-2022, National Academies Press.

Robbins, S. (2013), "Revised Lunar Cratering Chronology for Planetary Geological Histories," LPI Contributions, 1719, 1619.

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04/15/14