Ionizing radiation exposure on Arrokoth shapes a sugar world

Abstract

The Kuiper Belt object (KBO) Arrokoth, the farthest object in the Solar System ever visited by a spacecraft, possesses a distinctive reddish surface and is characterized by pronounced spectroscopic features associated with methanol. However, the fundamental processes by which methanol ices are converted into reddish, complex organic molecules on Arrokoth's surface have remained elusive. Here, we combine laboratory simulation experiments with a spectroscopic characterization of methanol ices exposed to proxies of galactic cosmic rays (GCRs). Our findings reveal that the surface exposure of methanol ices at 40 K can replicate the color slopes of Arrokoth. Sugars and their derivatives (acids, alcohols) with up to six carbon atoms, including glucose and ribose-fundamental building block of RNA-were ubiquitously identified. In addition, polycyclic aromatic hydrocarbons (PAHs) with up to six ring units (¹³C₂₂H₁₂) were also observed. These sugars and their derivatives along with PAHs connected by unsaturated linkers represent key molecules rationalizing the reddish appearance of Arrokoth. The formation of abundant sugar-related molecules dubs Arrokoth as a sugar world and provides a plausible abiotic preparation route for a key class of biorelevant molecules on the surface of KBOs prior to their delivery to prebiotic Earth.

Keywords: Arrokoth; colors; sugars; ultrared matter.

Fig. 1.

Visible reflectance spectra normalized at 550 nm collected during the irradiation of methanol (¹³CH₃OH) and carbon monoxide (¹³C¹⁸O) ices. (A) Spectra of methanol ice irradiated at 10 K, and (B) 40 K. (C) Spectra of carbon monoxide ice irradiated at 10 K, and (D) 20 K.

Fig. 2.

Comparison of the color between irradiated methanol and carbon monoxide ices with Kuiper Belt objects (KBOs). (A) Color slope evolution of irradiated methanol ice and comparison with cold classical Kuiper Belt objects (CCKBOs) and Arrokoth. The gray zone defines the range of CCKBOs’ color slopes. The color slope position of Arrokoth indicates its surface has been processed by galactic cosmic rays (GCRs) at a dose of about 57 eV amu⁻¹. Error bars shown are from linear fitting of spectra. (B) Comparison of the color between irradiated methanol ice with KBOs; methanol ices irradiated at 10 K are shown with circles and at 40 K coded with squares, dose sequence is coded with different colors. Arrokoth position is marked with an orange star. Error bars shown are 1σ values. (C) Color slope evolution of irradiated carbon monoxide ice and comparison with CCKBOs. The gray zone represents the range of CCKBOs’ color slope. Error bars shown are from linear fitting of spectra. (D) Comparison of the color between irradiated carbon monoxide ice with KBOs; carbon monoxide ices irradiated at 10 K are shown with circles and at 20 K coded with squares, dose sequence is coded with different colors. Arrokoth color is marked with an orange star. Error bars shown are 1σ values.

Fig. 3.

Deconvoluted FTIR spectra of methanol ice irradiated at 40 K. (A) Pristine ices at 40 K. (B) After irradiation at 40 K. (C) The magnified view of the region 1,800 to 620 cm^–1 of methanol ice after irradiation at 40 K. (D) The residue at 320 K. The experimental spectrum is plotted in black, while the deconvoluted peaks are blue and their sum is shown with the dashed red line. For clarity, only noticeable peaks are labeled, and detailed peak assignments are listed in SI Appendix, Table S5.

Fig. 4.

The evolution and kinetic fits of key species and functional groups during the irradiation of methanol (¹³CH₃OH) ice at 40 K. (A) Column densities of methanol (¹³CH₃OH, average of absorptions at 1,124 cm⁻¹ and 1,005 cm⁻¹). (B) Column densities of carbon dioxide (¹³CO₂, 2,275 cm⁻¹). (C) Column densities of carbon monoxide (¹³CO, 2,091 cm⁻¹). (D) Column densities of formyl radical (H¹³ĊO, 1,805 cm⁻¹). (E) Column densities of methane (¹³CH₄, 1,294 cm⁻¹). (F) Area of ¹³C═O and ¹³C═¹³C stretching modes (integrated between 1,520 and 1,790 cm⁻¹). Note that, both ¹³CO and ¹³CH₄ are volatile at 40 K and they are likely trapped within the polymer matrix formed from processed methanol ice. All the error bars are ±10% of the corresponding column densities and areas. Rate constants derived from the kinetic fitting and corresponding chemical reaction scheme are compiled in SI Appendix, Table S8.

Fig. 5.

Sugars and related compounds detected in the residue of methanol ices irradiated at 40 K by two-dimensional gas chromatography along with their detailed structure. (A) Sugar and related compounds detected as BSTFA derivatives in the irradiated methanol residue resolved on a Chirasil-Dex column in the first dimension coupled to a DB Wax in the second dimension. Mass-to-charge ratios (m/z) 104, 193, 206, 215, 220, 294, and 323 are displayed. ¹D and ²D represent the first-dimension and second-dimension times of the columns, respectively. (B) Detailed structures are identified in the residues by means of two-dimensional gas chromatography. Structures of the chiral molecules represent the d-enantiomer. Names of molecules are color-coded according to the number of carbon atoms: two carbon atoms (C2) orange, C3 blue, C4 black, C5 green, and C6 red. The quantities of 23 molecules are given in SI Appendix, Table S10.

Fig. 6.

Secondary ion mass spectra from residues of methanol (¹³CH₃OH) ices irradiated at 40 K with a radiation dose of 8.8 eV amu⁻¹. The mass spectra were recorded in (A) positive and (B) negative ion detection modes. For clarity, only sugars and sugar-related fragments are labeled. Detailed mass spectra obtained from irradiated methanol ices by higher doses can be found in SI Appendix, Figs. S16 and S18.