Why scientists are intrigued by the air in NASA’s Mars sample tubes

Atmospheric scientists are getting a little more excited about each rock core that NASA’s Perseverance Mars rover seals in its titanium sample tubes, which are collected for eventual delivery to Earth as part of the Mars Sample Return campaign. Twenty-four have been taken so far.

Most of these samples consist of rock cores or regolith (broken rock and dust) that can reveal important information about the planet’s history and whether microbial life was present billions of years ago. But some scientists are just as excited about the prospect of studying the “headspace,” or the air in the extra space around the rocky material, inside the tubes.

They want to learn more about Mars’ atmosphere, which is mainly carbon dioxide but may also contain traces of other gases that may have existed since the planet’s formation.

“The Martian air samples would tell us not only about the current climate and atmosphere, but also how they have changed over time,” said Brandi Carrier, a planetary scientist at NASA’s Jet Propulsion Laboratory in South America. California. “It will help us understand how climates different from ours evolve.”

The value of headroom

Among the samples that can be brought to Earth is one tube filled exclusively with gas deposited on the surface of Mars as part of a sample deposit. But much more of the gas in the rover’s collection is in the headspace of rock samples. These are unique because the gas will interact with rocky material in the tubes for years before the samples can be opened and analyzed in laboratories on Earth.

What scientists glean from that will provide insight into how much water vapor is floating near Mars’ surface, a factor that determines why ice forms there on the planet and how Mars’ water cycle has evolved over time.

Scientists also want to better understand trace gases in the Martian air. Most scientifically tempting would be the detection of noble gases (such as neon, argon and xenon), which are so non-reactive that they may have existed unchanged in the atmosphere since they emerged billions of years ago.

If collected, these gases could reveal whether Mars started out with an atmosphere. (Ancient Mars had a much thicker atmosphere than today, but scientists aren’t sure if it was always there or developed later). There are also big questions about how the planet’s ancient atmosphere compares to that of early Earth.

The headspace would also provide a chance to assess the size and toxicity of dust particles – information that will help future astronauts on Mars.

“The gas samples have a lot to offer Mars scientists,” said Justin Simon, a geochemist at NASA’s Johnson Space Center in Houston who is part of a group of more than a dozen international experts helping decide which samples the rover should collect . “Even scientists who don’t study Mars would be interested because it will shed light on how planets form and evolve.”

Apollo’s air samples

In 2021, a group of planetary researchers, including scientists from NASA, studied the air brought back from the moon by Apollo 17 astronauts some 50 years earlier in a steel container.

“People think of the moon as airless, but the atmosphere is very thin and interacts with the rocks on the moon’s surface,” says Simon, who studies a variety of planetary samples at Johnson. “That includes noble gases leaking from the moon’s interior and collecting on the moon’s surface.”

The way Simon’s team extracted the gas for research is similar to what could be done with Perseverance’s air samples. First, they placed the previously unopened container into an airtight enclosure. They then pierced the steel with a needle to draw the gas into a cold trap – essentially a U-shaped pipe extending into a liquid, such as nitrogen, with a low freezing point. By changing the temperature of the liquid, scientists captured some of the gases with lower freezing points at the bottom of the cold trap.

“There are maybe 25 laboratories in the world that manipulate gas in this way,” Simon said. This approach is not only used to study the origins of planetary materials, but can also be applied to gases from hot springs and gases emitted from the walls of active volcanoes, he added.

Naturally, these sources provide much more gas than Perseverance has in its sample tubes. But if a single tube doesn’t carry enough gas for a given experiment, Mars scientists can combine gases from multiple tubes to get a larger total sample — another way that free space provides a bonus opportunity for science.