Can an asteroid smaller than New Jersey have life-supporting conditions?

One of the surprises of the Dawn mission to the protoplanet (1) Ceres was the presence of salt deposits in craters, suggesting recent hot spring activity on a protoplanet the size of the island of Great Britain (diameter of Ceres = 940 km or 590 miles). This shows the importance initial composition for a how the evolution of a planetary body will progress. The 500 km (or ~300 mi) diameter icy moon Enceladus has a subsurface ocean from which water ice plumes are actively erupting. The dry, rocky asteroid (4) Vesta has the roughly the same diameter (520 km or 330 miles) as Enceladus but is geologically dead as Earth’s Moon. Composition can be the difference between an potentially habitable ocean world and a lifeless rock, but how small can a world be and still be potentially habitable instead of a lifeless rock?

(Left) Saturn's moon Enceladus showing its smooth, geologically young icy surface. (Middle) Artist's conception of the plumes of water ice from geyser eruptions on Enceladus from its subsurface ocean. (Right) In contrast, the dry, rocky protoplanet (4) Vesta is crater scarred showing a lack of geologically recent activity. Images credit: NASA

In 2018, the OSIRIS REx spacecraft visited the 500 m (~1600 foot) diameter asteroid (101955) Bennu and returned samples to Earth on September 24, 2023. Chemical analysis of the sample from Bennu provides evidence of hydrothermal deposits from the interaction between silicate minerals and flowing hot liquid water.

To be clear, this does not mean that there was ever hot liquid water within the ~1600 foot diameter asteroid Bennu. With a diameter a little greater than the height of the Empire State Building, Bennu itself is much too small to have ever sustained high enough internal temperatures to support liquid water in its interior. On the other hand, Bennu is likely a fragment of a larger protoplanet about 100-160 km (~60-90 miles) in diameter that was destroyed in a catastrophic collision. Conditions could have been right for hydrothermal activity when the Bennu parent body had recently formed and still had a relatively warm interior from the decay of radioactive isotopes.

For the sake of clarity, I use the term protoplanet to refer to planetary bodies that formed from the direct accretion of material within the primordial accretion disk that existed around the infant sun when the solar system was still forming. “Asteroid” refers to any rocky body that is above a certain size threshold that is not large enough to be a planet.

The boulder-covered surface of the 500 m (~1600 ft.) diameter asteroid (101955) Bennu as imaged by the spacecraft OSIRIS-REx. Bennu is a fragment of a larger protoplanet that was destroyed in a catastrophic collision. An artist's conception of the OSIRIS-REx spacecraft sampling the surface of the asteroid Bennu. Both images credit: NASA

Could the Bennu parent body have had the right thermal and chemical conditions for liquid water in its interior? A recent paper analyzed boulders on the surface of Bennu, showing evidence from rock texture the the boulder material was originally deposited in running water in the interior of the parent body. Rocks made from sediment that is deposited by flowing water before being lithified are called sedimentary rocks. They are very common on Earth, including sandstone and shale. Sedimentary rock has also been found on Mars.

The possibility that sedimentary rock formed in an environment with running water within a 100-150 km protoplanet is surprising because it suggests that you can get hydrothermal activity in very small protoplanets. Beyond minerals associated with wet environments, evidence of actual surface hydrothermal activity has only been found on one asteroid, the protoplanet Ceres, which is not directly related to the asteroid Bennu.

(Left) The protoplanet (1) Ceres as imaged by the Dawn spacecraft. Note the bright area in prominent crater in the middle right of the image. (Right) A close up of Occator Crater showing the bright spots believed to be salt deposits from near-surface hydrothermal activity in the geologically recent past. Image credit: NASA-JPL

On the other hand, there are other asteroids of comparable size to that predicted for the Bennu parent body, but a little larger (diameter = ~300-350 km), that have a Ceres-like reflectance spectrum. This means the colors absorbed or reflected from these asteroids indicate that they have minerals that were chemically altered by liquid water on their surfaces, a tall-tale sign of hydrothermal activity.

If a 150 km diameter protoplanet could have had near-surface hydrothermal activity, why not 300 diameter protoplanet? Widespread hydrothermal activity on early protoplanets has implications the abundance of environments where life could have formed in the early solar system.

A common hypothesis is that life formed on Earth in oceanic hydrothermal vents. This is why the possibility of hydrothermal vents on the ocean floors of the subsurface oceans of icy moons are so important in the search for life beyond Earth. If hydrothermal conditions were also present in 100 km scale asteroids, that means there were thousands of worlds in the early solar system with the potential for life to form.

Is there any way to test this possibility? There are computer models that can simulate the internal temperature the be expected for a planetary body, based on assumptions about the size and thermal properties of internal layers within the body. Also, there are computer models, like PHREEQC, that can simulate what minerals would be stable within a given environment based on temperature and pressure ranges.

For example, water ice (a mineral) only exists within a certain range of temperatures on Earth’s surface, specifically when the temperature is below 0 degrees Celsius, otherwise it melts or sublimates. The stability range for liquid water by contrast is 0-100 degrees Celsius at an atmospheric pressure of about 1 bar.

One way to predict whether Bennu’s parent body could support hot liquid water with dissolved salts (a hydrothermal brine solution) despite its small size would be to generate models predicting the internal temperature of asteroids or protoplanets the size of the Bennu parent body (D =100-160 km) up to Ceres-sized (D = 1000 km). Once these thermal models have been made, geochemical models can be made predicting the temperature and pressure range within which hydrothermal brine solutions are stable.

Once we have made both the thermal models predicting internal temperatures of asteroids and protoplanets 100-1000 km in diameter and geochemical models predicting the pressure and temperature range within which hydrothermal solutions are stable, we can map the geochemical models showing where hydrothermal solutions are stable onto models of internal asteroid temperature to see if there is an overlap between the temperature and pressure range for stability of hydrothermal solutions and the predicted internal temperature asteroids or protoplanets of the required size range. If it can be shown that hydrothermal solutions are stable at the expected temperatures in the near-surface of a 100 km scale asteroid, this would not prove that there was near-surface hydrothermal activity on 100 km diameter asteroids, but it would support that possibility.

Whether this modeling exercise supports hydrothermal activity on the Bennu parent body or not, the results would very significant. They would tell us more about how planets work and also how common habitable pockets in protoplanets were in the primordial solar system and in other recently formed solar systems across the galaxy. If life could form in Bennu’s parent body, it could form anywhere.

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