24 March 2021
Recordings of the presentations are now available – see the table below:
Whether Mars could have supported life has driven intensive exploration of the planet’s surface through satellite and robotic missions. Complementary research has focused on identifying and understanding meteorites from Mars, which offer the only direct samples of the crust available to science. These studies have sought not only signs of extra-terrestrial life and habitable environments, but also to understand how the planet has changed over time; from an ancient world of oceans and landforms with striking similarities to Earth, to the cold, barren planet we observe today. Why Mars has followed a dramatically different path to Earth is one of the key questions in the understanding of terrestrial planet evolution.
Determining the absolute ages of samples from Mars is key to addressing these issues and a principal objective for current and future Mars exploration. Recent work on martian meteorites has pushed the analytical envelope in Mars sample geochronology, overcoming challenges that include pervasive shock metamorphism during impact ejection from Mars, microscale heterogeneity and very low concentrations of radiogenic isotopes. This work informs future plans for the geochronology of samples returned to Earth through Mars Sample Return, an effort that starts with the collection and caching of samples by the NASA Mars 2020 Perseverance Rover Mission. A virtual workshop, co-sponsored by The Mineralogical Society of Great Britain and Ireland and The Meteoritical Society was held on March 24, 2021 via Zoom.
Over 60 people from 13 countries registered for the workshop; online attendance peaked at 30 attendees midway through the 4-hour session. The workshop started with opening remarks by co-convener James Darling (University of Portsmouth). Fellow co-convener Chris Herd (University of Alberta) then gave an overview of progress made in the geochronology of martian meteorites, beginning with the seminal 1986 U-Pb study by Chen and Wasserburg – done at a time when only a handful of martian meteorites were known – progressing through the Rb-Sr and Sm-Nd isotopic studies of Borg, Nyquist, and others, and finishing with recent advances in ion microprobe analyses of U-Pb isotopes in accessory minerals such as baddeleyite.
The first few talks summarized recent geochronological studies of martian meteorites: Leanne Staddon (University of Portsmouth) described the correlation – or lack thereof – between degree of shock metamorphism and the amount of Pb loss in baddeleyite, demonstrating that microstructural studies of baddeleyite are important to carry out, but that this mineral lives up to its reputation to retain Pb through shock events. Stephanie Suarez (University of Houston) summarized work done on the famed Tissint meteorite from Mars (fell 18 July 2011), including new Rb-Sr isotopic results that indicate labile Sr is present in the rock, perhaps mobilized at the time of impact ejection from Mars. Minako Righter (University of Houston) summarized the wide range of studies done in the University of Houston lab in recent years, highlighting results from some meteorites that have ages distinct from others; significantly, these “oddballs” also have igneous textures that differ from other martian meteorites. The takeaway from this is that it may be worthwhile concentrating geochronological efforts on those meteorites with distinct textures – a useful criterion given the plethora of martian meteorites that now exist in the world’s collections.
The toolkit for geochronology of planetary materials – whether meteorites or returned samples – is increasingly broad. James Darling provided an overview of some of these tools, highlighting new approaches to grain extraction for high-precision mass spectrometry using plasma source focused ion beam (FIB) technology, as well as the utility of methods for microstructural analysis (e.g., electron backscatter diffraction; EBSD) to provide geological context for microbeam analyses and atom probe tomography (APT). The latter has provided impressive trace element and preliminary isotopic results from nanoscale sample volumes. This was no more evident than in a presentation by Gabe Arcuri (University of Western Ontario) who showed that a remarkably detailed history can be obtained from APT analysis of baddeleyite or zircon, including the effects of thermal metamorphism (or lack thereof) to low-temperature alteration events.
The limitations of current methods were also a common theme in the workshop, including in a talk by Alex Sheen (University of Alberta) whose recent experience using microdrilling for extraction of mineral powders informed a study of the minimum amount of analyte (e.g., Sr) required to determine an isochron using current mass spectrometry techniques. The answer, unfortunately, is that microdrilling provides no real advantages over mineral separation, at least with current technology. However, the study demonstrates the directions in which the technology needs to advance.
Noting that we cannot always bring back samples from other planets – even if we might want to – Barbara Cohen (NASA Goddard Spaceflight Center) provided a comprehensive overview of the state-of-the-art in instruments designed for in situ geochronology. Multiple techniques are in development, including those that utilize the K-Ar, Rb-Sr and U-Th-Pb systems. Advances in laser induced breakdown spectroscopy (LIBS) coupled with mass spectrometry are particularly intriguing, as this technique would obviate the need for samples to be collected and manipulated onboard the spacecraft – an advance that parallels those made in ion microprobe mass spectrometry for lab-based analyses.
One of the main goals of Mars Sample Return is to assist in establishing a timescale for Mars. Currently, this timescale is based on the cratering chronology of the Moon, extended (with assumptions) to Mars. Fred Calef (NASA JPL) provided an invited overview of the challenges involved in this endeavor, highlighting the assumptions that are intrinsic to crater frequency-based chronology. An absolute age derived from a returned Mars sample that represents a specific event on Mars would provide a “golden spike” for a Mars chronology and a test of the assumptions involved in the extrapolation of the lunar record. But how to choose the right rock for this? Studies of the floor of Jezero Crater (Perseverance’s landing site) have already demonstrated the complex exhumation history of the surface, and the challenges involved in choosing a sample that would be suitable. Perhaps more useful would be impactites that can be tied to the impact event that formed the Isidis basin; alternatively, at least two samples from a unit that extends across the area within and near Jezero Crater would be a better choice.
In spite of the numerous challenges involved in Mars sample chronology – from the complex histories of martian meteorites to finding the samples that will establish an absolute martian timescale – the toolkit for the geochronology of martian samples continues to grow. In this way, the frontiers continue to advance at a remarkable pace. The organizers hope to hold follow-on workshops and track these advances into the future.
Christopher Herd, University of Alberta
James Darling, University of Portsmouth
Presentations hosted on the Mineralogical Society YouTube channel