A mineral found in Kachchh could help date events on Mars

The Perseverance rover is exploring Jezero Crater on Mars, searching for signs of ancient life and collecting samples that might one day be brought back to Earth. One of the big questions scientists are trying to answer is: when did key events happen on Mars? When was there water? When did volcanoes erupt or wind blow sediment around? Right now, dating events on Mars often rely on counting impact craters, which is like guessing someone's age just by looking at how many freckles they have – it gives you a general idea, but it's not very precise and needs a lot of calibration. Scientists are looking for more accurate ways to put a timeline on Martian history. A mineral called jarosite, found both on Earth and Mars, might hold the key, acting like tiny clocks recording the passage of time.

A group of researchers from the Physical Research Laboratory, Ahmedabad, Indian Institute of Technology (IIT) Gandhinagar, Indian Institute of Technology (IIT) Kharagpur, Presidency University, Kolkata, and the CSIR-National Geophysical Research Institute, Hyderabad has been studying jarosite samples from Kachchh, Gujarat. Because of its similar geological features, the location is considered a good stand-in, or Martian analogue.

They wanted to see if they could use luminescence dating on the jarosite. Luminescence dating works because minerals like jarosite can trap energy from natural radiation, like cosmic rays from space or radiation from elements like uranium, thorium, and potassium in the rock. Exposing the mineral to light or heat then releases this stored energy as light, a phenomenon called luminescence. The brighter the light, the more radiation the mineral has absorbed, and the longer it's been since that energy was dissipated. The energy is generally released when the mineral is exposed to sunlight or heat. By measuring the luminescence and knowing how much radiation the mineral receives each year, scientists can calculate how long ago the mineral was buried or shielded from light.

To determine if jarosite could work as a dating tool for Mars, the researchers put the Earth samples through a series of tests. They used different methods to measure the luminescence, including Thermoluminescence (TL), which uses heat to release the stored energy, and Optically Stimulated luminescence (OSL), which uses blue or infrared light.

They found that jarosite does indeed produce luminescence signals. They also checked if heating the jarosite, even up to 450°C, above its decomposition temperature, destroyed the luminescence properties. Surprisingly, while heating did change the sensitivity of some samples (how brightly they glowed for a given dose), the basic luminescence signals remained, suggesting the parts of the mineral responsible for trapping energy weren't completely altered. This is important because geological processes on Mars might involve some heating.

They also looked at how reproducible the luminescence signals were after repeated cycles of radiation and measurement, finding they were quite consistent. A crucial test was seeing how well the signals bleached or reset when exposed to light, like sunlight on the Martian surface. They used a solar lamp and found that different luminescence signals faded at different rates, with some peaks in the TL signal taking about 100 minutes of exposure to be significantly reduced. On Mars, UV radiation is more intense due to its thinner temperature, making the bleaching process likely more effective, ensuring that surface samples are well-reset by sunlight.

Another key property they studied was fading, when the trapped energy leaks out over time, even without light or heat exposure. Some luminescence signals in jarosite showed significant fading, which would make them unreliable for dating unless corrected. However, they found that specific signals, specifically a high-temperature peak in the TL signal (around 350°C) and a type of infrared-stimulated luminescence (called pIRIR225), showed near-zero fading. These stable signals are excellent candidates for dating because the stored energy stays put over long periods.

The researchers also determined the saturation dose for different signals – essentially, how much radiation the mineral can absorb before the traps are full and it can't store any more energy. These saturation doses ranged from about 590 Gy to 1600 Gy depending on the signal. By combining the saturation dose with an estimate of the annual radiation dose rate on Mars (which is mainly from cosmic rays, estimated at about 65 mGy/year), they calculated a potential dating range for jarosite. They estimated that jarosite could be used to date events up to about 25,000 years ago.

This work improves on previous methods like crater counting by offering the potential for absolute dating – giving a specific age in years, rather than just saying one surface is older or younger than another. It also builds on previous luminescence studies of other Martian analogue materials like basalts, some of which suffered from problematic fading. The identification of stable luminescence signals in jarosite is a significant step forward.

However, while Kachchh is a good analogue, Martian jarosite might have slightly different properties due to the differences between the two planets. The study was also published on the pre-print server EarthArxiv and hasn't gone through the full peer-review process yet. More studies are needed, ideally on actual Martian jarosite samples if they are returned to Earth or with instruments sent to Mars, to confirm these findings and refine the dating parameters.

Accurately dating surface processes on Mars could improve our understanding of the planet's recent geological and climate history. It could help scientists figure out when water was present, how long ago wind shaped the landscape, and even map how cosmic radiation has varied over time. The research brings us closer to building a detailed timeline for the Red Planet.

This research article was written with the help of generative AI and edited by an editor at Research Matters.