Ceres Aliphatic Organics from Large Subsurface Reservoir

Introduction

Ceres, the largest object in the Solar System asteroid belt, has  a complex geological and chemical history  and  experienced extensive water-related processes (1). Its surface is globally characterized by dark materials, phyllosilicates, ammonium-bearing minerals, carbonates, water ice, and salts (2-5) and the presence of carbon on a global scale, up to 20 wt.%, has also been inferred (6). Organic matter was unambiguously identified in the region of Ernutet crater (3) in the form of long-chain aliphatic organics (Aliphatic Organics).  The origins and persistence of these organics are under debate due to the intense aliphatic signature and radiation levels in Ceres' orbit, which could lead to their destruction, hindering detection. To investigate this, we conducted  laboratory experiments to replicate how the signature of the organic-rich regions would degrade  by fast ions, ultraviolet radiation, and neutral  atoms in conditions that simulate the environment of Ceres. Our experiments give hints on the lifetime of aliphatics on the Cerean surface, allowing us to constrain the age and  mechanisms promoting the preservation of aliphatic material following exposure.

Experiments- For simulants, we used a mixture of minerals, reproducing the average Ceres’  surface mineralogy and undecanoic acid (C₁₀H₂₁COOH) that shows a strong 3.4 μm feature and that is thus spectrally representative of the Aliphatics  compounds on the Ceres surface. The organic was then added to the mineral mixture. The experimental results were then scaled to the actual radiation environment estimated at the orbit of Ceres.

UV irradiation experiments were performed at INAF-Osservatorio Astrofisico di Arcetri (Firenze, Italy). The Ceres mixture was exposed to a UV-enhanced Xe-lamp for about 7  hours, and the organic degradation process was monitored in real time. The 3.4 μm band area decreased as the irradiation fluence increased, resulting in a very short lifetime. However, only the uppermost layers of the surface are affected by the destruction induced by UV photons.

Exposure of Ceres mixture sample to hydrogen atoms was performed at INAF-Osservatorio Astronomico di Capodimonte (Napoli, Italy), resulting in a decrease of the organic band, suggesting  that hydrogen atoms may also contribute to the degradation of organics on Ceres.

Energetic ion irradiation experiments were performed at INAF – Osservatorio Astrofisico  di Catania (Italy). Three analogs in the form of compact pellets were irradiated under high vacuum at room temperature by 200 keV He⁺, N⁺, and H⁺ ions respectively. The results demonstrate a fast decrease in intensity of the 3.4 μm feature in all irradiated samples.

Results- Our experiments show that the radiation environment on the Cerean surface can destroy the 3.4 μm band of Aliphatic Organics even if mixed with clays and carbonates, commonly assumed to preserve organic from degradation. Moreover, UV processing and H atom cause the alteration of Aliphatic Organics on very short timescales (from few days to 10⁵ years), but the damage is limited to  a few hundreds of nanometers to a few  micrometers of the surface. Fast ions penetrate surfaces much deeper than UV photons and H atoms, and thus induce the destruction of the 3.4 μm band to much larger depths.  From our results, the presence of the strong aliphatic feature at Ernutet crater implies exposure on the surface < 10 Ma.

The strong sign observed at Ernutet combined with our results on the fast degradation of Aliphatic Organics, suggests the presence of a considerable quantity of aliphatic organics and/or a process for their continuous formation and replenishment. In fact, given the estimated fast degradation rate, we can argue that Aliphatic Organics were much higher in the past and the detected Aliphatic Organics are yet partially degraded.  This last hypothesis implies an original very high amount of Aliphatic Organics, given the present inferred quantity (> 22% in the mixtures (7,8)).

 

REFERENCES

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