A chemical tour of the solar system highlights Earth’s uniqueness

Earth – The solar system’s most lively planet

Discovering our history far from home

Humans love an origin story. And in space science, there is no greater question than why Earth is the vibrant, living planet we see today. As humans return to the heavens, understanding our planetary origins and evolution can help us imagine the possibilities held by other planets.

Earth alone can tell us only so much of its history. The planet actively recycles its rock when crustal plates creep, joltingly, back into the planet’s hot mantle. That material is lost to time, as is the mineral history it preserved. And further, it’s difficult to find clues to the origins of life on Earth when life itself is inescapable.

It will take more than looking here on Earth to understand what makes our planet unique.

So, for decades, scientists have turned to other locales in the solar system to understand our home. Samples returned from the moon and nearby asteroids provide researchers with invaluable access to those extraterrestrial environments. Martian rovers send home geochemical data that scientists interpret through an Earthly lens. And farther afield, the icy moons of gas giants hint at the diverse oceanography that could be present on exoplanets light-years away from our solar system.

Together, these places help researchers fill in the blanks of our own history. That information doesn’t just help us understand our origin story; it can also help us plan for a future where humanity has expanded its footprint in the solar system and search for life beyond.

Moon: Space weathering and surprising waterin lunar dust

In the years after the Apollo 17 astronauts returned the last set of lunar samples to Earth in 1972, there was one thing researchers felt sure of. “We used to think the moon was dry, dry, dry, no water at all,” says Michelle Thompson, a planetary scientist at Purdue University.

But through the 2000s and 2010s, evidence from multiple lunar missions and laboratory work suggested that lunar rocks might be more hydrated than expected. The theory was confirmed when researchers found water persisting in dust on the sunlit side of the moon. It’s not a lot of moisture—the mission scientists think the surface is about 100 times as dry as the Sahara—but it’s there.

“One of the things we’ve been looking at is how space weathering can actually contribute to producing water on the surface of the moon,” Thompson says.

Space weathering encompasses all the interplanetary processes that change the composition and material characteristics of a place. “If you think about Earth, we are very fortunate to be protected from this really harsh interplanetary space environment,” Thompson says. “We’ve got an atmosphere that’s preventing high-velocity dust particles from hitting the surface. We have a magnetic field which is preventing us from getting irradiated with particles that are coming off the sun.”

The lunar surface is not so fortunate. So Thompson and her collaborators mimic space weathering in lab to understand how it might transform moon rocks.

When it comes to finding samples to experiment with, many researchers turn to analogous materials. Such specimens are either synthesized to resemble the extraterrestrial environment of interest or collected from a place on Earth thought to be similar. In some cases, Thompson does use analogs to study space weathering. But for the moon, she has the real deal.

Using lunar regolith (moon dirt) returned from the Apollo 17 mission, Thompson has shown that when hydrogen atoms in solar wind bombard the moon, they implant into oxygen-rich minerals and form water (Earth Planet. Sci. Lett. 2025, DOI: 10.1016/j.epsl.2024.119178). Now it’s a question of how future lunar missions might use that water, perhaps in fuel production or even directly in day-to-day operations.

“In situ resource utilization is a big thing right now,” Thompson says. “So, if we were going to set up a moon base . . . how can we get that water out?”

Near-Earth asteroids: Returned samples reveal asteroids’ geochemical secrets

Space weathering doesn’t just affect the local resources available to future astronauts. It also changes how scientists interpret observational data. The asteroids Ryugu and Bennu are prime examples.

Both near-Earth asteroids are classified as carbonaceous because of their high carbon content. Both dark, rocky bodies are also considered rubble pile asteroids. They likely formed from the remnants of larger rocky bodies that broke apart around 4.5 billion years ago, when the solar system was young.

So, in theory, both asteroids should reflect the same wavelengths of light because they are made of the same stuff. And yet, observationally, Ryugu is spectroscopically redder than Bennu. Why doesn’t Ryugu look more similar to Bennu?

With samples from both asteroids in hand on Earth, Thompson and others had a way to answer that question.

They found that both contain iron sulfide nanoparticles that form and grow as a surface weathers in space (Nat. Astron. 2022, DOI: 10.1038/s41550-022-01841-6). “On average, it looks like the nanoparticles in Ryugu are smaller, which might be causing that reddening,” she says (Nat. Geosci. 2025, DOI: 10.1038/s41561-025-01745-w). It’s possible that this means the surface of Ryugu is far less weathered than that of Bennu, but it will take more research to confirm the theory, she says.

Beyond providing a detailed look at the asteroids’ current geomorphology, the returned samples also give scientists a peek into the chemistry present in the early solar system.

The returned Bennu sample, for example, contains unique salt crystals that are markedly similar to those left behind in evaporated lakes on Earth. The scientists used what they knew about the analogous sites to paint a picture of the asteroid’s parent body. Tepid, sodium-rich brine likely flowed through the cracks and crevices of the larger body, which suggests that environments friendly to prebiotic life might have existed during the early days of the solar system.

Scientists haven’t found any direct evidence that life evolved on asteroids. But they have found the fundamental ingredients for life on Bennu and on Ryugu. By using advanced instrumental techniques, one team found multiple amino acids and all five nucleobases in samples from Bennu. And another team recently found all five nucleobases in dust grains from Ryugu.

Exactly how space weathering affects the organics in asteroids is an open question, Thompson says. Her team used meteorite samples as analogs to the asteroids and showed that their organic geochemistry may change unpredictably as they weather.

Such work is “a next frontier that’s really going to help [us understand] the origin story of not just how the planets got here, but also how life got here,” Thompson says.

Mars: Earthly analogs help interpret rover data from the red planet

The samples returned from asteroids and the moon have provided scientists with unprecedented access to extraterrestrial environments; for now, at least, it’s impossible to equip a remote lab with the same powerful instrumentation researchers have on Earth. But despite the data windfall extraterrestrial samples provide, sample return missions are few and far between.

In 2020, NASA launched the Perseverance Rover mission in part to bring Martian samples home to Earth. The rover landed in the Jezero crater near an ancient river delta the next year and began drilling. Scientists carefully chose each sample to answer key questions about the area’s current geochemistry and search for signs of past life. Unfortunately, the 33 samples collected so far may remain grounded on Mars indefinitely due to fluctuating administrative priorities.

Without the Martian samples in hand to study, scientists have turned to the next best thing: terrestrial analogs.

Of course, Earthly analogs of the Martian environment can’t reveal whether life flourished on Mars. The samples do, however, offer a detailed geochemical baseline that scientists can use to better interpret the data streaming to Earth from the laboratories on Mars rovers.

“I’ve been kind of scouring the earth for clues to help us unravel what [data] comes back from the rovers and what we see on Mars,” says Michael T. Thorpe, a sedimentary geochemist at the University of Maryland at College Park and NASA Goddard Space Flight Center. The trick is finding somewhere on Earth that looks a lot like Mars.

Mars and Earth are cousins in our solar system. They formed around the same time, but the two planets have markedly different compositions today. Earth is largely made of granite, while Mars is mostly basalt, a black rock that also spews from cracks in Earth’s crust. So to find Mars analogs on Earth, Thorpe travels to lands shaped by volcanoes.

“I’ve been to places like Lanzarote in the Canary Islands,” Thorpe says. He’s also traveled to Hawaii and Idaho to study the basalt there. The chemistry of the rock in each location has been altered by its unique local environmental conditions. Weathering in a warm, dry place produces mineral deposits different from those produced in areas that are warm and wet.

One place stands out when Thorpe compares its basalt deposits to Martian minerals: Iceland.

Chemically, the sedimentary rocks—those formed from other rocks—in Iceland and on Mars both look surprisingly similar to the unadulterated basalt from which they originate (J. Geophys. Res.:Planets 2022, DOI: 10.1029/2021JE007099). This finding suggests that billions of years ago, when water flowed freely on the Martian surface, Mars’s environment likely looked more like chilly Iceland than tropical Hawaii, Thorpe says.

Studying Earthly analogs for Mars can also help scientists interpret potential biosignatures. In 2024, Perseverance captured images of curious spotted rocks. Then, the rover’s scientific instruments confirmed that the samples were sedimentary rocks rich in organic carbon, oxidized iron, and sulfur.

“These could be biosignatures,” Thorpe says, “but to be honest, we need a better understanding of potential biosignatures here on Earth.” So this summer, he and others are heading to Scotland, where similar stones can be found, to focus on how organics evolve in sedimentary environments.

“It’s going to be really cool to understand what’s going on with the sedimentary dynamics,” he says.

Icy moons: What might we learn from these watery worlds?

Clues to the evolution of the solar system and Earth’s uniquely habitable environment are contained not only within the inner solar system. Icy moons orbiting Jupiter and Saturn may also hold clues in their watery interiors.

“We’ve learned that a lot of these icy bodies are different than the terrestrial planets in the sense that their building blocks are different,” says Christopher Glein, a planetary geochemist at the Southwest Research Institute. Unlike in the inner solar system, those bodies contain a lot of carbon in complex organic molecules, Glein says.

Importantly, that organic carbon is probably not biological in origin; organics do not necessarily equal life. “It’s just organic carbon that was available as a building block to make planetary bodies,” Glein says. Closer to the sun, the tar-like organics that formed as the planets were born likely burned away.

The persistent organics in the outer solar system complicate the search for life. “It’s this alien chemistry, this foreign chemistry, and it overlaps with biological chemistry,” Glein says. And we don’t have a good sense of how this organic material evolves chemically over geologic timescales, he adds

But the moons’ inorganic chemistry is a bit more straightforward to parse.

Just over a decade ago, NASA’s Cassini spacecraft discovered hydrogen gas in the watery geysers erupting from Enceladus, one of Saturn’s moons. This discovery was strange. The gas is light and rarely sticks around planets without immense gravitational pulls.

“Enceladus is tiny; it’s like the size of the state of Arizona,” Glein says. The only way hydrogen could be present in the quantities observed is through active production. But no spacecraft has ever landed on Enceladus, or on any of the icy moons in the outer solar system, to get a good look at what’s going on there. Analogs on Earth, however, have provided some insight.

“It’s very similar, we think, to how hydrogen is produced in hydrothermal vents on the Earth,” Glein explains. The Lost City Hydrothermal Field at the bottom of the Atlantic Ocean is one classic analog. There, reactions between alkaline ocean water and hot rocks produce large amounts of hydrogen gas. A similar process is likely happening on Enceladus today (Science 2017, DOI: 10.1126/science.aai8703).

Other places on Earth have also helped researchers conceptualize Enceladus’s ocean.

The scientists found abundant amounts of phosphate in Enceladus’s plume, Glein says. This chemistry is in stark contrast to that of Earth’s ocean, where phosphate is biologically limiting—it constrains population sizes. But carbonate-rich soda lakes elsewhere on Earth mirror the data coming back from Cassini (Geophys. Res. Lett. 2020, DOI: 10.1029/2019GL085885). Together with the evidence for hydrothermal activity, these data paint the moon as very geochemically active.

Beyond our solar system: The diversity of our planetary neighbors helps scientists imagine worlds light-years away

Through lab work, sample return, and analogs of extraterrestrial environments here on Earth, researchers are writing the history of our solar system. Each chapter will help clarify our understanding of Earth’s history and life’s place in the universe. As technologies advance and scientists begin the search for habitable worlds light-years away that we may never be able to physically reach, the question of habitability has never been more prescient.

“In the solar system, we have a certain number of test subjects,” Glein says. With each observation and experiment, the basis set for understanding exoplanets expands.

“People might think, ‘Well, if you can’t ever go and dive into an exoplanet ocean, we’re hopeless,’ ” Glein says. But he doesn’t think that’s true. “Astronomers are clever; chemists are clever. If we put our forces together, I think we’re going to be able to learn some incredible things.”