How do you build a planet, let alone one capable of sustaining and evolving life? The clues to the “recipe” can perhaps be found in the leftovers scattered around our solar system. Things like asteroids or Ceres, a dwarf planet like Pluto which orbits in the asteroid belt between Mars and Jupiter.
A new study in Nature Communications says carbon appears to be blown into space when small rocks collide with each other, which has implications for understanding how planets like Earth "grew up" early in their history. Most scientists believe planets gradually got larger as pebbles and asteroids collided with each other; while a lot of material was lost in each crash, over time enough stuck around to make a small rock grow to planetary scale.
Ceres is an intriguing in-between world, when talking about planets and asteroids: it's a quarter of the size of Earth's moon, but has slightly more gravity than the average space rock. As such, worlds like Ceres can keep more carbon on their surface than asteroids. The study team argues that searching for this carbon means we should put a sample return mission together as soon as we can.
"I think that detailed analysis of return samples with microscopes [on Earth] — or something like that — is required," lead author Kosuke Kurosawa, who has affiliations both at Japan's Kobe University and Chiba Institute of Technology's planetary exploration research center, told Salon.
"In simple terms, this is an explosion."
The aim of the study was to discuss "shock metamorphism," which sounds like a band name but in reality describes what happens to a space rock after a cosmic crash. For decades, carbon has been quite the puzzle. It appears higher-speed impacts left behind less carbon than lower-speed impacts. In other words, one plausible explanation appeared to be that carbon only formed if the rocks crashed into each other relatively slowly — which always seemed strange to scientists.
Kurosawa's team recalled another study at Kobe 20 years ago, this one talking about another important building block of life in meteorites: water. That investigation suggested that as space rocks crash into each other, the impact threw water-enriched minerals into space — and therefore removed much of the evidence of water itself. Could the same be true for carbon?
The older study had limitations that Kurosawa felt needed to be addressed. For one thing, the authors didn't consider if water vapor would be produced in the collision. For another, some meteorites that include carbon — but don't include water-enriched minerals — still appear to be less shocked. Something more had to be responsible.
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Kurosawa's team created a two-stage light gas gun that was connected with a sample chamber. They fired pellets from the gun into different samples meant to represent meteorites. Afterwards, the researchers examined what gases were produced by the collision. Simply put, two types of meteorites were considered: those with carbon, and those without carbon.
The novel setup allowed the team to look at the gases produced, without contaminating the samples being blasted. This allowed them to spot something: as the pellets crashed into the sample meteorites, chemical reactions occurred that produced both hot carbon monoxide and hot carbon dioxide gases.
"In simple terms, this is an explosion," Kurosawa said. Such a chemical reaction would wipe away the impact records on carbonaceous — or carbon-rich — asteroids as the explosion of organic materials took place, he added. It would therefore be difficult to find evidence of an ancient collision, because there would be little stuff left to examine.
That was likely true of the Ryugu asteroid sample return mission undertaken by the Japanese mission Hayabusa2, noted Kurosawa, who was also a team member of a Ryugu sample analysis group led by Tomoki Nakamura. "The Ryugu rocks are virtually unshocked, as well as carbonaceous chondrites fallen on Earth."
Ceres is different, however: "shocked material on Ceres would re-accrete and accumulate on the surface of Ceres due to its strong gravity" compared to smaller space rocks, he said. But the evidence would still be hard to spot even from orbit. So NASA's Dawn mission, which studied Ceres from orbit between 2015 and 2018, couldn't see as much carbon as some future spacecraft on the surface.
So how soon could such a spacecraft arrive? In 2022, the National Academies Planetary Science Decadal Survey — essentially, a report representing the consensus of the planetary science community in the United States — recommended a Ceres sample return mission to search for organic materials. Giving urgency was Dawn's finding that Ceres likely has an ocean beneath the surface; that doesn't mean life on this airless world, but it has implications for finding life elsewhere.
Here is the rub, however — it depends on funding. In 2023, the space community learned the New Frontiers NASA program under which this (pretty expensive) spacecraft would run delayed their next proposal period by three years, to 2026. Budget issues were responsible for the wait, and these days NASA's funding is even more difficult to predict.
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Earlier this month, the Trump administration's "skinny budget" proposal for NASA's fiscal 2026 suggested slashing agency programs by $6 billion, or roughly 25 percent, focusing especially on science work. The White House's proposal is just the opening volley in what will be months of budget negotiations with stakeholders and Congress, however.
Assuming funding and selection goes forward, the Ceres sample return mission focuses on the youngest region on the dwarf planet — Occator crater. The team is unclear whether the carbon-shocked material would be present there, said sample return proposal participant Julie Castillo-Rodriguez, a senior research scientist at NASA's Jet Propulsion Laboratory. (She also was deputy principal investigator for the Dawn mission.)
Castillo-Rodriguez said she hopes to connect with Kurosawa's team about Ceres, as she liked the approach of their study. "It's really cool. It's inclusive. And because they work with very tiny samples, it must have taken them a very long time to get all that material and all these results."
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