As an Earth scientist fascinated by the processes at work on this planet, I have regularly been drawn to studies investigating the asteroids and terrestrial planets of our Solar System, particularly those which hint at past or present geologic activity. For me, these bodies are exciting: they expand my field from a sample size of one – the Earth – and highlight both the similarities and differences of each individual world. The dwarf planet Ceres, the largest object in the asteroid belt, has been the subject of a surge in investigation thanks to NASA’s Dawn mission. One recent paper on organic material on the Cerean surface particularly caught my eye.
Organic material has been detected in localised surface regions on Ceres, but where it came from is unclear. The most convincing observations of this material come from within and immediately adjacent to Ernutet impact crater. This association raises the possibility that these organic molecules were delivered to the surface by an asteroid or comet. Impacts have been widely interpreted as the delivery mechanism of elements and material to many other terrestrial bodies in the Solar System.
However, as it turns out, aliphatic organic molecules (such as those observed at Ernutet crater) may not survive the heat and pressure of a high-velocity astronomical collision: simulations run by T. J. Bowling and colleagues (Earth and Planetary Science Letters https://doi.org/10.1016/j.epsl.2020.116069) indicate that aliphatic organics within large or high velocity impactors, such as the one responsible for Ernutet crater, are almost certain to be destroyed through thermal degradation during the course of the impact. Indeed only dust sized objects are likely to impart low-enough energy to preserve their organic load. But if the organic material wasn’t delivered to Ceres by impact, its origin and its distribution in such heterogeneous concentrations is all the more puzzling.
In the same study, the researchers go on to simulate the effects of a collision on the Cerean subsurface. In these scenarios, high temperatures destroy most aliphatic organics immediately below the impact; but in a significant portion of the ejected material, aliphatic organics survive. What’s more, during the crater forming event, material from the impactor is diluted by abundant material from Ceres, to the extent that the observed concentrations of organic molecules must trace their origin to Ceres itself, not the impactor. That would imply that the collision excavated material from an organic rich reservoir inside the dwarf planet, and exposed it on the surface.
Based on the observed distribution of the organic deposits in and around the crater, it seems the reservoir was present within the lower part of the excavated region, possibly at a depth of 3 to 6 km. If so, then Ceres – already known to contain water as ice, gas hydrates and possibly briny fluids – may be (or have been) home to hydrothermal systems or other geological processes capable of forming simple organic molecules.
And the mechanism may be relevant beyond Ceres, too: impact-induced excavation of subsurface reservoirs has the potential to explain similar, localised deposits of unusual material on other asteroids and terrestrial bodies.
Ceres is much closer to the Earth than other organics-bearing terrestrial objects, such as the moons of the gas giants. As such, it is an exciting potential target for future astrobiological and planetary science missions. It appears even the small rocky worlds of the asteroid belt are not as geologically desolate as at first they might seem.
Poster image is from NASA and shows Ernutet crater with the reddish areas of enhanced colour highlighting organic-rich regions.
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