Sugar molecules discovered in deep space may have seeded life on Earth

A sugar found in raspberries and some sunless tanning products has been detected near the center of the Milky Way, marking the first direct discovery of a true sugar in interstellar space and expanding scientists' understanding of cosmic chemistry.

The molecule, erythrulose, was identified in the molecular cloud G+0.693−0.027, roughly 8.2 kiloparsecs from Earth. Located near the Galactic Center, the cloud is one of the galaxy's richest known reservoirs of complex organic molecules.

An international team led by Izaskun Jiménez-Serra of Spain’s Center for Astrobiology (CAB) confirmed the discovery by matching 12 groups of radio signals from the cloud with erythrulose's laboratory spectrum, accounting for 17 individual molecular transitions.

The finding is significant because sugars are fundamental to life. They form key components of DNA and RNA and play essential roles in metabolism, yet scientists have long struggled to explain how these molecules could have formed naturally under conditions thought to resemble those on early Earth.

Astronomers used highly sensitive surveys from the 40-meter Yebes radio telescope and the IRAM 30-meter telescope.

Finding one molecule in G+0.693 is difficult because the cloud contains signals from more than 180 identified molecular species and their isotopic forms. Many lines overlap.

Six of the erythrulose line sets were mostly unblended, with contamination at or below 25 percent. Other lines overlapped with unidentified or known molecules, but the complete spectral model still closely matched the observations.

The chance that all six cleanest lines aligned randomly was estimated at 0.2 percent. Even using only three or four lines would produce confidence levels of 95.2 percent and 98.3 percent.

Erythrulose is the only possible four-carbon ketose, a sugar containing a ketone group. With 14 atoms, it is also the largest non-cyclic molecule yet identified in interstellar space and the first detected there with four oxygen atoms.

It is also only the second chiral molecule reported in the interstellar medium. Chiral molecules can exist in mirror-image forms, a property central to biological chemistry.

The team expected smaller three-carbon sugars to be easier to find. Instead, glyceraldehyde and dihydroxyacetone were absent.

Erythrulose appeared at least eight to 17 times more abundant than those three-carbon sugars. That runs against a common astrochemical pattern in which each added carbon atom lowers a molecule’s abundance by roughly tenfold.

“This finding was unexpected, as the prevailing view in astrochemistry is that interstellar molecules grow in size through the sequential addition of carbon atoms,” Jiménez-Serra said.

The researchers therefore examined whether erythrulose could form by joining two-carbon ingredients already present in the cloud. Their work focused on glycolaldehyde and ethylene glycol, both detected at comparable or higher abundances.

Chemical calculations found a possible pathway on amorphous water ice coating interstellar dust grains. Hydrogen atoms first remove hydrogen from the two molecules, creating reactive radicals. Those radicals can eventually join to form erythrulose.

The slowest stage involves a change in electron spin before the carbon atoms can combine. At dust temperatures of about 20 to 30 kelvin, the full process remained fast enough to operate under Galactic Center conditions.

Because the pathway has two mirror-image versions with equal energy, it would produce both forms of erythrulose in equal amounts.

The team added the proposed reactions to computer simulations of ice forming inside a collapsing cloud. The model included several three-carbon and four-carbon sugars, related compounds, ultraviolet destruction and different cosmic-ray ionization rates.

Erythrulose became the most efficiently produced four-carbon sugar under most conditions. It often reached slightly greater abundances than the smaller sugars, especially when cosmic-ray activity was high and the chemical evolution time was shorter.

The researchers linked that pattern to faster ultraviolet destruction of the three-carbon sugars. Larger molecules may spread absorbed energy across more internal motions, making them harder to break apart.

Models with cosmic-ray ionization rates 1,000 times the standard Galactic value came closest to the observed amounts. Such intense rates have previously been used to explain other molecules in G+0.693.

The cloud may also be undergoing a collision that drives a shock near 20 kilometers per second. That disturbance could knock molecules from icy grains into the gas, where radio telescopes can detect them.

The simulations did not reproduce every measurement. They overproduced the two undetected three-carbon sugars by factors of about 25 to 70.

Several uncertainties could explain the mismatch. Only part of the ice may leave the grains. Sugars may quickly freeze back onto cold dust, or gas-phase reactions may destroy them after release. Reaction rates and ultraviolet destruction efficiencies also remain uncertain.

Sugars including ribose and glucose have already been found in primitive meteorites and samples from asteroid Bennu. Those discoveries raised the possibility that some of Earth’s early sugar supply came from space.

Erythrulose now pushes that chemistry back into the interstellar medium, before asteroids and planets formed.

Laboratory work has shown that mixtures containing erythrulose can help produce ribonucleotides, the building blocks of RNA. In water, erythrulose can also change into threose.

Threose forms the backbone of threose nucleic acid, a simpler RNA alternative proposed for a possible pre-RNA world.

Using the abundance measured in G+0.693, the team estimated that roughly 0.5 million to 50 million tonnes of erythrulose could have reached Earth during heavy bombardment between about 4.1 and 3.9 billion years ago. The authors noted that the scale and timing of that bombardment remain debated.

Small sugars and related molecules can survive meteorite impacts in experiments, supporting a possible delivery route to planetary surfaces.

“The detection of erythrulose is very exciting because it opens up the possibility of discovering in space other sugars such as ribose, which is part of RNA, and other important molecules for the origin of life,” said co-author Carlos Briones.

The discovery provides a plausible natural source of sugars that origin-of-life experiments often rely on without explaining how they formed.

It also gives astronomers a proven way to search for other sugars in space. Precise laboratory measurements enabled the detection after years of failed attempts because these molecules are too fragile and moisture-sensitive for conventional analysis.

The findings do not show that interstellar sugars sparked life on Earth. Instead, they demonstrate that biologically important sugars can form without life, survive the harsh conditions of space, and become incorporated into young planetary systems.

Researchers can now search for ribose and other sugars to determine how widespread this chemistry is across the Milky Way and in the material that eventually forms comets, asteroids, and planets.

Research findings are available online in the journal Nature Astronomy.

The original story "Sugar molecules discovered in deep space may have seeded life on Earth" is published in The Brighter Side of News.

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