[1] We have studied the effects of 4 keV He + ion irradiation on olivine while measuring, in situ, changes in the near-infrared (NIR) reflectance and in the chemical composition of the surface. The observed changes in reflectance are reddening and the attenuation of the Fe-3d absorption bands in the NIR. Spectral reddening of irradiated olivine powder (<45 mm) correlates with the amount of metallic iron formed by ion impact, consistent with the idea that space-weathering effects in the reflectance of olivine-bearing S type asteroids are due to the formation of metallic iron. The metallization rate of the powder is about half that of a sectioned rock of olivine, which we propose is a consequence of redeposition of sputtered material. The NIR spectral changes observed in ion irradiation experiments are similar to those observed in our previous experiments on vapor redeposition, indicating that different space weathering mechanisms can lead to similar final effects on reflectance. Finally, we estimate that at 1 AU the spectral reddening caused by the solar wind saturates approximately 2 orders of magnitude faster than comparable reddening caused by micrometeorite impacts.

Abstract.We have studied the synthesis of carbon dioxide from solid carbon monoxide at 16 K induced by photolysis with Lyman-α photons and by irradiation with 200 keV protons to quantitatively compare the effects of photolysis and ion irradiation on CO ice and to determine the importance of these processes in interstellar ice grains. The CO and CO 2 concentrations during irradiation of an initially pure CO film evolve with fluence to a saturation value, a behaviour that is explained by a two-state model. Our results indicate that the initial CO 2 production rates for both radiation processes are similar when normalized to the absorbed energy and that the solid CO 2 abundance observed in the interstellar ices cannot be explained only by radiolysis and photolysis of pure solid CO.

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The recently discovered minor body 1I/2017 U1 ('Oumuamua) is the first known object in our Solar System that is not bound by the Sun's gravity. Its hyperbolic orbit (eccentricity greater than unity) strongly suggests that it originated outside our Solar System; its red color is consistent with substantial space weathering experienced over a long interstellar journey. We carry out an simple calculation of the probability of detecting such an object. We find that the observed detection rate of 1I-like objects can be satisfied if the average mass of ejected material from nearby stars during the process of planetary formation is ∼20 Earth masses, similar to the expected value for our Solar System. The current detection rate of such interstellar interlopers is estimated to be 0.2/year, and the expected number of detections over the past few years is almost exactly one. When the Large Synoptic Survey Telescope begins its wide, fast, deep all-sky survey the detection rate will increase to 1/year. Those expected detections will provide further constraints on nearby planetary system formation through a better estimate of the number and properties of interstellar objects.

We have studied the compaction of vapor-deposited amorphous solid water by energetic ions at 40 K. The porosity was characterized by ultraviolet-visible spectroscopy, infrared spectroscopy, and methane adsorption/desorption. These three techniques provide different and complementary views of the structural changes in ice resulting from irradiation. We find that the decrease in internal surface area of the pores, signaled by infrared absorption by dangling bonds, precedes the decrease in the pore volume during irradiation. Our results imply that impacts from cosmic rays can cause compaction in the icy mantles of the interstellar grains, which can explain the absence of dangling bond features in the infrared spectrum of molecular clouds.