F. Scott Anderson

Tectonic effects of climate change on Venus

Venusian plains regions are commonly crossed by small-strain deformation features such as wrinkle ridges, polygonal terrains, and gridded terrains. Of these, polygonal terrains are observed to have a relatively uniform spacing and are widely distributed on plains that formed during global resurfacing. Models of Venusian climate that assume resurfacing occurred through massive volcanic events suggest that surface temperatures could have dramatically changed over nearly a billion years, propagating thermal stresses into the surface and potentially causing small-strain features. We investigate this hypothesis by approximating the temperatures predicted by climate models as a step function and employ a fixed plate and strength envelope model to predict the resulting depth of failure and amount of strain. Our calculations indicate that strains due to temperature changes of 50–100 K, which are favored for volcanic resurfacing events of 1–10 km thickness, are consistent with the observed spacing of polygonal and gridded terrains as well as some wrinkle ridges. The global nature of the climate change event is consistent with the global distribution and uniformity of such features and implies that such terrains may be a global stratigraphic marker.

Admittance survey of type 1 coronae on Venus

Coronae are volcano-tectonic features on Venus which range from 60km to 2600km and are defined by their nearly circular patterns of fractures. Type 1 (regular) coronae are classified as having >50% complete fracture annuli. Previous work has examined the factors controlling the morphology, size, and fracture pattern of coronae, using lithospheric properties, loading signature and geologic characteristics. However, these studies have been limited to Type 2 (topographic) coronae (e.g. coronaes with <50% fracture annuli), and the factors controlling the formation of Type 1 coronae remain poorly understood. In this study, we apply the methodology of to survey the admittance signature for Type 1 coronae to determine the controlling parameters which govern Type 1 coronae formation.

Dating the Martian meteorite Zagami by the 87Rb‐87Sr isochron method with a prototype in situ resonance ionization mass spectrometer

RATIONALE The geologic history of the Solar System builds on an extensive record of impact flux models, crater counts, and ∼270 kg of lunar samples analyzed in terrestrial laboratories. However, estimates of impactor flux may be biased by the fact that most of the dated Apollo samples were only tenuously connected to an assumed geologic context. Moreover, uncertainties in the modeled cratering rates are significant enough to lead to estimated errors for dates on Mars and the Moon of ∼1 Ga. Given the great cost of sample return missions, combined with the need to sample multiple terrains on multiple planets, we have developed a prototype instrument that can be used for in situ dating to better constrain the age of planetary samples. METHODS We demonstrate the first use of laser ablation resonance ionization mass spectrometry for 87Rb-87Sr isochron dating of geological specimens. The demands of accuracy and precision have required us to meet challenges including regulation of the ambient temperature, measurement of appropriate backgrounds, sufficient ablation laser intensity, avoidance of the defocusing effect of the plasma created by ablation pulses, and shielding of our detector from atoms and ions of other elements. RESULTS To test whether we could meaningfully date planetary materials, we have analyzed a piece of the Martian meteorite Zagami. In each of four separate measurements we obtained 87Rb-87Sr isochron ages for Zagami consistent with its published age, and, in both of two measurements that reached completion, we obtained better than 200 Ma precision. Combining all our data into a single isochron with 581 spot analyses gives an 87Rb-87Sr age for this specimen of 360 ±90 Ma. CONCLUSIONS Our analyses of the Zagami meteorite represent the first successful application of resonance ionization mass spectrometry to isochron geochronology. Furthermore, the technique is miniaturizable for spaceflight and in situ dating on other planetary bodies.

Rb‐Sr resonance ionization geochronology of the Duluth Gabbro: A proof of concept for in situ dating on the Moon

Rationale We report new 87Rb‐87Sr isochron data for the Duluth Gabbro, obtained with a laser ablation resonance ionization mass spectrometer that is a prototype spaceflight instrument. The gabbro has a Rb abundance and a range of Rb/Sr ratios that are similar to those of KREEP‐rich basalts found on the nearside of the Moon. Dating of previously un‐sampled young lunar basalts, which generally have a KREEP‐rich composition, is critical for understanding the bombardment history of the Moon since 3.5 Ga, which in turn informs the chronology of the solar system. Measurements of lunar analogs like the Duluth Gabbro are a proof of concept for in situ dating of rocks on the Moon to constrain lunar history. Methods Using the laser ablation resonance ionization mass spectrometer we ablated hundreds of locations on a sample, and at each one measured the relative abundances of the isotopes of Rb and Sr. A delay between the resonant photoionization processes separates the elements in time, eliminating the potential interference between 87Rb and 87Sr. This enables the determination of 87Rb‐87Sr isochron ages without sophisticated sample preparation that would be impractical in a spaceflight context. Results We successfully dated the Duluth Gabbro to 800 ± 300 Ma using traditional isochron methods like those used in our earlier analysis of the Martian meteorite Zagami. However, we were able to improve this to 1100 ± 200 Ma, an accuracy of <1σ, using a novel normalization approach. Both these results agree with the age determined by Faure et al. in 1969, but our novel normalization improves our precision. Conclusions Demonstrating that this technique can be used for measurements at this level of difficulty makes ~32% of the lunar nearside amenable to in situ dating, which can complement or supplement a sample return program. Given these results and the scientific value of dating young lunar basalts, we have recently proposed a spaceflight mission called the Moon Age and Regolith Explorer (MARE).

A laser desorption resonance ionization mass spectrometer for Rb-Sr geochronology: Sr isotope results

We are developing a portable laser desorption resonance ionization mass spectrometer (LDRIMS) for determining the radiometric age of rocks using the 87Rb-87Sr isotope system, as well as constraining lithologic evolution and measuring chemical compositions. The bench-top prototype has been used to assess the capability of LDRIMS to measure 87Sr/86Sr. In this paper, we demonstrate that LDRIMS can measure the isotope ratios of a glass standard with 10 ppm net Sr to a precision of ±0.5% (1σ), with a sensitivity of 1:1010 in less than 1 minute. Increasing the measurement time to 15 minutes improves the precision to 0.1% (1σ). The speed of the LDRIMS measurement allows samples to be measured in significantly shorter periods of time than traditional methods, with little or no sample preparation. Models of the age error derived from isochron dating the SNC meteorites that would be obtained using 100-1000 LDRIMS measurements at ±0.1% (1σ) accuracy show that for ALH84001 and Zagami, which have ages ranging from 4.09 Ga to 165 Ma, dates with analytical uncertainties less than ±50 Ma are possible. These results were obtained using low laser powers (~10 μJ for resonance, <;1 mJ for photoionization), consistent with the potential for space flight to Mars.

In-situ Rb-Sr geochronology

This paper reports on the first rubidium-strontium (Rb-Sr) radiometric dates using a Laser Desorption Resonance Ionization Mass Spectrometry (LDRIMS) instrument capable of being miniaturized for flight to another planet. The LDRIMS instrument produces dates in under 24 hours, requires minimal sample preparation, and avoids the interference and mass resolution issues associated with other geochronology measurements. We have begun testing the bench-top prototype on the Boulder Creek Granite (BCG), from Colorado, comprised primarily of a gneissic quartz monzonite and granodiorite; whole rock Rb-Sr TIMS measurements result in dates of 1700±40 Ma [1]. Data reduction of the LDRIMS Rb-Sr measurements on calibrated repeat runs result in a date for the BCG of 1.727±0.087 Ga (n=288, MSWD=1). Most geochronology applications are willing to accept an MSWD up to ~2.7; at MSWD=2, the precision improves to ±0.062 Ga. This technology is moving from lab prototype to field deployable instrument, and provides an opportunity to directly address the science goals of Mars Sample Return (MSR) within the bounds posed by current scientific, fiscal, and political pressures on the Mars program. Additionally, LDRIMS could potentially be flown to the Moon under the Discovery or New Frontiers program. We posit that in-situ geochronology missions to Mars to triage and validate samples for Mars Sample Return (MSR) are technically feasible in the 2018-2022 time frame.

Generating multiple wavelengths, simultaneously, in a Ti:sapphire ring laser with a ramp-hold-fire seeding technique

The ability to simultaneously produce pulsed laser output over multiple discrete wavelengths can mitigate many of the timing and jitter issues associated with the use of multiple laser systems. In addition, Fourier-transform limited laser output on every pulse is required for many applications such as with pump-probe detection, non-linear frequency mixing, differential absorption lidar (DIAL), and resonance ionization. As a matter of practice, such lasers need to be capable of operating within uncontrolled or noisy environments. We report on a novel Ti:sapphire ring laser that has been developed to produce Fourier-transform limited 20-ns laser pulses at multiple discrete wavelengths, simultaneously, utilizing a Ramp-Hold-Fire (RHF) seeding technique. Resonance of the seed light is achieved by using a KD*P crystal to modify the phase of the light circulating within the slave oscillator cavity where the fast response of the crystal results in a seeding technique that is immune to noise throughout the acoustic regime.

A Dual Wavelength Ti:sapphire Laser Using A Ramp-Hold-Fire Seeding Technique for Resonance Ionization of Rb87

We present a novel dual wavelength ring laser for selective ionization of Rb87 atoms. The design incorporates a Ramp-Hold-Fire technique for generating 15ns Fourier transform limited pulses immune to noise within the acoustic range.