Searching For Martian Life with X-Ray Crystallography

Where there is water, there is life.

NASA’s Mars Science Laboratory spacecraft approaching Mars.


Even a simple life form like mold would not grow in old bread that is dry as a rock. Moisture needs to be present for any lifeform, at least as we know on Earth, to thrive. Water is so fundamentally connected to life that scientists have long agreed the liquid is one of its important prerequisites, and thus the search for life beyond Earth often begins with the search for water.

For a long time, we have looked to our planetary next-door neighbor and asked: is there – or was there ever – water on Mars? Surprisingly, the crystal structure of minerals in Martian soil helps to answer that question.

X-ray crystallography is an experimental method used to determine the atomic and molecular structure of crystals.

Ice crystals forming on a window

Ice crystals forming on a window

Using this method, scientists can identify materials that are known to form crystalline structures, such as salts, metals, minerals, and many other inorganic, organic and biological molecules. It works by sending a beam of X-rays into the crystal, and analyzing how its structure diffracts these rays.

The crystallographer uses the measured angles and intensities of these diffracted rays to form a 3D picture of the electron density within the crystal.  By examining the electrical and chemical properties of the electrons, scientists can infer much information about the crystal, such as the mean positions of the atoms and their chemical bonds. This data combines to help identify new materials, and distinguish between materials that appear otherwise very similar.

This method has been fundamental in the development of many scientific fields, and has revealed the structure and function of many biological molecules, such as vitamins, proteins and even DNA.

The potential of X-ray crystallography to determine the structure of molecules and minerals has been clear since the beginning of the twentieth century, when Max von Laue’s pioneering work on x-ray scattering angles to determine the size and orientation of crystal cells earned him the 1914 Nobel prize in Physics . Curiously enough, although we now use more sophisticated calculations and hardware, the technique used for space exploration is basically the same as the one used by the creators of crystallography one hundred years ago.

Max von der Laue (on the right) in conversation with Albert Einstein, Max Planck and others (Berlin, 1931)

Max von der Laue (on the right) in conversation with Albert Einstein, Max Planck and others (Berlin, 1931)

In August 2012, the Mars Science Laboratory landed in the Gale crater, on Mars.

The Mars Science Laboratory is a NASA space probe launched in November 2011, carrying  the Curiosity rover, a robot vehicle much like the Spirit and Opportunity that was previously used on the Mars Exploration Rover mission., the Curiosity rover is equipped with a miniaturized X-ray diffraction/X-ray fluorescence (XRD/XRF) instrument called the Chemistry and Mineralogy Instrument, or CheMin. It was thanks to CheMin that we’ve uncovered one of the most important mysteries of our red neighbour.

Scientists have been searching for bacterial life on Mars. “One of the bigger questions is whether the environment on Mars was ever conducive [to life], was ever clement for life. We need to look at the rocks and see under what processes they formed. And to do that, we use tricks like X-ray diffractions”, says astrophysicist Dr. Lewis Dartnell.

The CheMin is one of ten instruments on or inside the Curiosity rover, all of which are designed to analyze ancient sediments and provide detailed information on the rocks, soils and atmosphere of this fascinating planet. In a paper published in 2014, Dr. David Bish and colleagues discussed the findings from CheMin’s analyses. From four samples – a soil sample, two samples drilled from mudstones, and a sample drilled from a sandstone – they found a complex mineralogy. But the most outstanding finding was in the mudstone samples: they all contained one or more phyllosilicates that appeared to have been altered by liquid water.

“Phyllosilicates are abundant minerals in Earth and other planets. They have a nanometric interlayer space accessible to water molecules and other compounds. This interlayer space represents about 90% of the mineral total surface yielding a high adsorption capacity”

Curiosity rover takes a selfie on Mars

Curiosity rover takes a selfie on Mars

In other words, scientists had found clay. “To find this on Mars is absolutely mind-blowing. This is clay that was formed in an aqueous environment. So, what Curiosity had actually found was river mud, the mud on the bottom of what would have been a babbling brook billions of years ago, on an ancient Martian surface. And the conditions needed to make this clay would not have been too acidic, not too alkaline, not too salty. In fact, if you could have jumped into a spaceship and a time machine and gone back to that place on the surface of Mars billions of years ago (…), you could have drunk that Martian river water”, says Dr. Dartnell.

“It’s only really because of Curiosity that we know we’ve had these long-standing bodies of water, lakes which lasted for thousands of years”, says Dr. John Bridges, a participating scientist at NASA Mars Science Laboratory. “So, if you’d been a microbe on Mars four billion years ago, yeah, it would have been habitable for you. We don’t yet know if there were any. Whether it did support life is the next stage”.

As of August 2022, the Curiosity rover is still operational, and it has been active on Mars for more than 3600 Sols, or Martian days, which means almost ten years after its landing. Since then, other rovers have also landed and started collecting data on Mars. The NASA Perseverance rover, which arrived in 2021, carries among other instruments a “marscopter”, a helicopter to test powered flight in the Martian atmosphere. In the same year the Zhurong rover, China’s first successful mission on Mars, also landed.

While Curiosity’s main goal is to find evidence of microbial life on Mars, Perseverance’s mission is to scan the red planet for signs of past life, as well as help determine the potential for human habitability – for instance, whether we can produce oxygen, and locate water below the surface . The Zhurong rover is designed to study Mars’ surface composition, its magnetic field, geological features, and ice water distribution, to answer the question of what Mars looked like in the past.

The Curiosity rover, equipped with its CheMin instrument, has made history by revealing the existence of huge bodies of water on Martian surface millions of years ago. Dr. Dartnell’s long-term hope is to find life in another world: “I think that would be one of the most fundamental and revolutionary discoveries that we could make as a species. We’d know that we’re not alone in this universe”.

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