The Great Mars Rock Mystery: How Scientists Determine the Age of Mars Rocks

Elle
Sep 13
6 min read

When NASA announced that the Cheyava Falls rock on Mars was 3.5 billion years old, you might have wondered: How on Earth (or Mars) do scientists figure that out? It's not like rocks come with expiration dates stamped on them. The answer lies in one of science's most ingenious detective techniques: using the universe's own natural clocks hidden inside every rock.

The Cosmic Clock Inside Every Rock

Imagine you have a bag of 1,000 coins, and every hour, exactly half of them magically disappear. After one hour, you'd have 500 coins. After two hours, 250. After three hours, 125. This predictable pattern is exactly how radioactive elements behave inside rocks, except instead of coins disappearing, we have atoms transforming into different atoms over incredibly long periods.

Geologists commonly use radiometric dating methods, based on the natural radioactive decay of certain elements such as potassium and carbon, as reliable clocks to date ancient events. This process is called radiometric dating, and it works because radioactive elements have something called a "half-life" – the time it takes for exactly half of a sample to decay into a different element.

The brilliant part is that this decay rate never changes. Whether a rock is sitting on Mars, floating in space, or buried deep in Earth's crust, the radioactive elements inside it tick away at exactly the same rate they have for billions of years.

The Mars Rock Dating Toolkit

Dating rocks on Mars presents unique challenges. Scientists can't just pick up samples and take them to a laboratory (though NASA's Mars Sample Return mission hopes to change that). Instead, they rely on sophisticated instruments aboard rovers, like Perseverance, to act as mobile laboratories.

SuperCam: The Laser Detective

SuperCam identifies minerals and rock compositions, and it seeks organic compounds that could be related to past life on Mars. It has a laser that can zap and study areas on a rock as small as the period at the end of this sentence. All from about 20 feet, or 7 meters away.

Think of SuperCam as a combination of X-ray vision and a chemistry lab. It fires a laser at rocks, which creates a tiny plasma explosion. By analyzing the light from this explosion, scientists can determine what elements are present in the rock. This information helps them understand the rock's composition and provides clues about its age.

PIXL: The X-Ray Microscope

PIXL (Planetary Instrument for X-ray Lithochemistry) works like a super-powered magnifying glass that can see individual elements. It uses X-rays to map the chemical makeup of rocks in incredible detail, helping scientists identify the specific minerals that formed at different times in Mars' history.

The Dating Game: Multiple Methods for Maximum Accuracy

Just like detectives use multiple pieces of evidence to solve a case, scientists use several dating methods to determine a rock's age. Each method works best for different types of rocks and different time periods.

Potassium-Argon Dating

One technique, potassium-argon dating, determines the age of a rock sample by measuring how much argon gas it contains. Over time, atoms of the radioactive form of potassium (an isotope called potassium-40) decay within a rock to form stable atoms of argon-40 spontaneously.

This method is particularly useful for volcanic rocks and works like this: When lava cools and forms rock, any argon gas that was present escapes. The rock then starts with a clean slate – zero argon. As time passes, potassium-40 in the rock decays to argon-40, which gets trapped inside. By measuring how much argon has accumulated, scientists can calculate how long it's been since the rock formed.

Uranium-Lead Dating

Uranium–lead radiometric dating involves using uranium-235 or uranium-238 to date a substance's absolute age. This scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years.

This is like having an extremely precise stopwatch that's been running for billions of years. Uranium slowly decays to lead through a series of steps, and by measuring the ratio of uranium to lead, scientists can determine the rock's age with remarkable accuracy.

Reading Mars' Ancient Story

When scientists analyzed the Cheyava Falls rock, they didn't just look at one measurement. The rock's age (2-3 billion years old) expands the timeframe for potential Martian habitability, potentially extending beyond previously considered periods. They used multiple instruments and techniques to piece together the rock's story.

Here's how they do it:

Step 1: Remote Analysis Remote (Mastcam-Z, SuperCam) and proximity (PIXL, SHERLOC, WATSON) payload science instruments provided structural, textural, and chemical evidence for both the aqueous deposition and aqueous alteration of the rocks. First, the rover takes detailed photos and uses its laser instruments to study the rock from a distance.

Step 2: Close-Up Investigation NASA's Perseverance Mars rover ground down a portion of a rock surface, blew away the resulting debris, and then went to work studying its pristine interior with a suite of instruments designed to determine its mineralogic makeup and geologic origin.

Step 3: Cross-Referencing Scientists compare their findings with what they know about Mars' geological history. Different types of rocks form during different periods of Mars' past, so identifying the rock type helps narrow down when it formed.

The Challenge of Long-Distance Dating

Dating rocks on Mars is like trying to perform surgery while wearing thick gloves and looking through a telescope. Scientists must work with limited information and can't repeat experiments easily. This is why they use multiple methods and instruments to confirm their findings.

By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable daughter atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed. The key is finding rocks that have remained relatively unchanged since they formed, which is challenging on a planet that has experienced billions of years of weather, impacts, and geological activity.

Why Age Matters for the Search for Life

Understanding when Mars rocks formed is crucial for the search for ancient life. Different periods in Mars' history had different conditions. Early Mars (3.5-4 billion years ago) was warmer and wetter, with conditions that might have supported life. By dating rocks to this period and finding signs of organic compounds or unusual chemical patterns, scientists can identify the most promising places to search for evidence of ancient Martian life.

The 3.5-billion-year age of the Cheyava Falls rock is particularly exciting because it places the rock in Mars' early history when liquid water likely flowed on the surface and conditions might have been suitable for life.

From Mars to Earth: The Future of Rock Dating

Currently, Mars rock dating relies on remote analysis, but this is about to change. Mars Sample Return would be NASA's most ambitious, multi-mission campaign that would bring carefully selected Martian samples to Earth. When Mars samples finally arrive on Earth, scientists will be able to use the full power of Earth-based laboratories to determine their ages with unprecedented precision.

These future analyses will help answer fundamental questions about Mars' history: When did it have liquid water? How long were conditions suitable for life? What caused Mars to transform from a potentially habitable world to the cold, dry planet we see today?

The Bigger Picture

Rock dating isn't just about satisfying scientific curiosity. It's about understanding the history of worlds and the potential for life beyond Earth. Every age determination helps scientists reconstruct the story of how planets evolve and what conditions might support life.

The next time you pick up a rock, remember that you're holding a time capsule containing billions of years of history. And thanks to the ingenuity of scientists and the precision of radiometric dating, we can now read those stories written in stone, whether that stone is in your backyard or sitting on the surface of Mars, 140 million miles away.

The 3.5-billion-year-old Cheyava Falls rock represents more than just an ancient piece of Mars. It's a window into a time when Mars might have been teeming with microbial life, making it one of the most important rocks humans have ever studied – even from a distance of 140 million miles.