Messenger comets might be why Earth has life, asteroid Ryugu samples suggest

(Main) the asteriod Ryugu. (Inset top) A melt splash from the surface of Ryugu (inset bottom) a CT slice of the melt showing voids
(Main) the asteriod Ryugu. (Inset top) A melt splash from the surface of Ryugu (inset bottom) a CT slice of the melt showing voids (Image credit: (Main) JAXA/ (Inse) Megumi Matsumoto et al.)

A detailed investigation of asteroid Ryugu samples has provided further evidence that the organic molecules which gave rise to life to our planet were brought here by ancient comets.

These space rock samples were returned to Earth by Japan's Hayabusa2 mission, which visited the spinning-top-like space rock Ryugu in 2018. Hayabusa2 spent around 18 months studying the asteroid and collecting surface material, which has proved to be a treasure trove of information about our solar system.

Ryugu, formally known as 162173 Ryugu, is a 2850-foot (870-meter) wide near-Earth asteroid that lacks a protective atmosphere. This means its surface is directly exposed to space and can gather interplanetary dust, which changes the asteroid's surface composition when struck.

The team of scientists behind the new developments in the quest to find life's origins found 5 to 20-micrometer-wide "melt splashes" created when cometary dust in particular pelted the surface of Ryugu. And within these melt splashes, the researchers found small carbonaceous materials similar to primitive organic matter.

Related: Life beyond Earth may form in the coldest depths of space, Ryugu asteroid samples reveal

"This organic matter might be the small seeds of life once delivered from space to Earth," team member and Tohoku University Graduate School of Science assistant professor Megumi Matsumoto said in a statement.

Comets tend to exist in wide orbits around the sun, which means they spend most of their time at the frigid outer edges of the solar system. But when they do sweep into the inner solar system, solar radiation heats their inner, icy material. That causes the material to transform straight into a gas, a process called sublimation.

As this gaseous material explodes from the comet, it carries with it some of the object's surface material. Not only does this create the characteristic tails and auras, or "comas," of comets, but it also leaves trails of cometary dust around the sun. When Earth passes through these trails, we witness meteor shows as the dust fragments burn up in our planet's atmosphere.

This cometary material is far more likely to make it to the surface of atmosphere-less bodies like Ryugu, where it can be preserved, however. Thus, studying those dusty remnants on the Hayabusa2 samples could reveal clues about the material in the early solar system.

The melt splashes studied by the team were created when the asteroid's surface material was struck by cometary dust, with the two materials melting and mixing together during impact-caused heating and eventually cooling off.

The spongy carbonaceous materials found in the Ryugu melts differ chemically from organic matter usually found in cometary material because they lack oxygen and nitrogen. This could, however, hint at how the material was formed in the first place.

"We propose that the carbonaceous materials formed from cometary organic matter via the evaporation of volatiles, such as nitrogen and oxygen, during the impact-induced heating," Matsumoto said. "This suggests that cometary matter was transported to the near-Earth region from the outer solar system."

A CT scan shows carbonaceous material found in the melt splash from Ryugu.

A CT scan shows carbonaceous material found in the melt splash from Ryugu. (Image credit: Megumi Matsumoto et al.)

Sealed with the melts as further evidence of their origin are tiny voids caused when the impacts released water vapor from materials on Ryugu's surface; this water was captured by the impact heated matter. 

"Our 3D CT imaging and chemical analyses showed that the melt splashes consist mainly of silicate glasses with voids and small inclusions of spherical iron sulfides," Matsumoto added. "The chemical compositions of the melt splashes suggest that Ryugu's hydrous silicates mixed with cometary dust."

Matsumoto and the team are continuing to examine Ryugu samples collected by Hayabusa2 in the hope of discovering more melts that may hold within them evidence of cometary dust strikes.

The hope is that that will deliver more insight into the transportation of primordial organic material to the space around Earth over 4 billion years ago prior to the emergence of life.

The team's research is published in the journal Science Advances.

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Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

  • rod
    Interesting report by and the paper cited,
    Very detailed study but I did not find one reference in the paper to *life* or Earth getting life kick started for example via abiogenesis because asteroids, comets, or meteorites brought the stuff of life here. 15 references to earth and the paper made it clear this asteroid has been in a short orbital period now in its present location in the solar system.

    "Ryugu should have experienced some degree of regolith resurfacing (33). Okazaki et al. (34) estimated cosmic ray exposure age of Ryugu samples to be ~5 Ma. As the average resurfacing time of the top ~1-m layer of Ryugu has been estimated to be longer at the near-Earth region (~2 to 8 Ma) than at the asteroid belt (<1 Ma), the relatively long sample exposure age suggests residence at the near-Earth region (1 au from the sun) for several Ma (14, 33, 34). This further suggests that the microcrater and the melt splashes reported here probably formed during Ryugu’s residence in a near-Earth orbit within the last ~5 Ma."

    Whatever interpretation of organics said to be found, Ryugu has not existed for billions of years in its present position around the Sun.
  • Manix
    So in a nutshell, without quite saying it, this is mounting evidence for the Panspermia hypothesis
  • Torbjorn Larsson
    Manix said:
    So in a nutshell, without quite saying it, this is mounting evidence for the Panspermia hypothesis
    More likely it is evidence against it since it provides data on how organic material got to Earth where evolution could start. Panspermia is a hypothesis that life evolved elsewhere so is inherently less likely.

    Life evolves and diversify fast, and we have now a trait phylogeny that show biology splitting from geology, so it is an easy process all the way. Thus we can take a recent analysis as suggesting life evolved on Earth (or most habitable planets) by more than 5 sigma compared against panspermia.
    “We conclude that the maximum probability that panspermia sparked life on Earth is on the order of magnitude of 10-5, or 0.001%.