Featured Image: Zircon grain under the Scanning Electron Microscope (SEM). Image used with permission from Wikipedia (Emmanuel Roquette).
Article: The internal structure and geodynamics of Mars inferred from a 4.2-Gyr zircon record.
Authors: Maria M. Costa, Ninna K. Jensen, Laura C. Bouvier, James N. Connelly, Takashi Mikouchi, Matthew S. A. Horstwood, Jussi-Petteri Suuronen, Frédéric Moynier, Zhengbin Deng, Arnaud Agranier, Laure A. J. Martin, Tim E. Johnson, Alexander A. Nemchin, and Martin Bizzarro
While sitting in Geology 101 studying the geological time scale, most of us have gone through this experience where we imagined ourselves going back in time; visualizing mammoths passing by, dinosaurs hunting and fighting. But all these pictures start to become hazy and unclear when we reach close to 4 billion years. It is the time for which we have no rock records, and this is where zircons or what I would like to call “tiny survivors” comes in.
Zircons (chemical formula, ZrSiO4)- the birthstone of the December born- are tiny crystals usually found in crustal rocks of the terrestrial planets, i.e. the stones that form our continents. This is a hard mineral, and their incredible durability makes them resilient to any kind of erosional processes, even melting to some extent. After their formation, zircons can last through several geological events (billions of years) and capture these time periods within rings that grow around the original crystal, analogous to the tree rings (see figure 1). Zircons recovered from a rock at Jack Hills in Australia is reported to be the oldest material (~ 4.4 billion-year-old) on the Earth to date. Most of our understanding of the first ~500 million years (for which there is no rock record) of our planet comes from these zircons. Like Earth, zircons can be useful to understand the past of other terrestrial planets such as Mars.
In contrast to our home planet, Martian crust has been resurfaced by prolonged basaltic volcanism, and this makes zircon an unlikely constituent of the red world. However, recent research led by Costa M.M. et al. extracted a significant number (~72) of zircon crystals from the Martian meteorites, creating the largest available zircon archive of Mars. These tiny zircons span over entire geological time (~4.4 billion years to 300 million years) and offer critical insight into the geologic history of Mars.
Zircons hosts the radioactive element uranium. Uranium (U) decays to lead (Pb) at a known rate over geological timescale and thus act as a clock. Measuring the amount of U and Pb in the zircons, authors calculated the absolute ages of the crystals around ~ 4485 to 4331 million years. The ages of zircons in this study falls into two distinct populations with mean ages around 4474 and 4442 million years. Researchers inferred these ages to be within the main episodes of bombardment via meteorites. Initially, impacts were commonplace on the terrestrial planets and it is evident from countless craters present on their surface; progressively, the number of impacts declined. The authors related the unusual second surge of zircons population, at ~4331 million years, due to sudden increase in impacts by migration of giant gas planets to the outer solar system. These migrations would have disturbed meteorites and sent their orbits on a collision course with the inner rocky planets. Impacts would have destroyed the primary Martian crust, burying it under a pool of lava thus creating the perfect conditions for zircon to crystallize.
To further understand the formation conditions, Costa and their team also measured the decay path of lutetium to hafnium within these zircons. This path often gets disturbed by volcanism and energetic impacts. Evaluating these isotopes, researchers realized that most of the primary crust was modified by later impacts forming zircons just after the solidification of the planet. This interpretation is in line with the previous reports and thermal models, suggesting the formation of Mars within ~10 million years of the solar system formation.
Anomalously, some zircons in the study were found to have formation ages of ~1548 to 300 million years. These younger zircons tell a different story and are thought to represent a deep and isolated part of the early Martian mantle. As volcanic rocks, in principle, lack zircon crystals, the authors’ model suggests that under Martian conditions, with the absence of plate tectonics, the continuous evolution of Martian magma might favour zircon crystallization. Elysium and Tharsis volcanism might have given birth to these young zircons and later, strong Martian winds dispersed these zircons throughout mars. This theory also explains the observed round and abraded nature of these zircons.
Although mafic rocks on Earth lack zircons, it seems like they are widespread on the Martian surface. This study presents an unmatched magmatic record of the Martian magmatism covering a span of 4.2 billion years. It tells us of a time on Mars rife with impacts and volcanism. Zircons are hardy crystals that, whilst not directly conducive to humanity’s deep-seated desire to discover life on Mars, allow us a window in the earliest history of our planetary neighbour. Thus, targeting zircon-bearing samples through future sample return missions will be crucial to understand the earliest history of terrestrial planets.
Tiny Crystals, Big Story: Time capsules from the Early Mars by Yash Srivastava is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.