Space-Weathering of Solar System Bodies: A Laboratory Perspective

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Space-Weathering of Solar System Bodies: A Laboratory Perspective Chris J. Bennett, Claire Pirim, and Thomas M. Orlando* Department of Chemistry & Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, United States 5.7. Neptunian and Uranian Systems 5.8. Trans-Neptunian Objects and Comets 6. Experimental Methods 6.1. Specific Examples of Laboratory Instrumentation 6.1.1. Electron Interactions 6.1.2. Ion-Beam Studies 6.1.3. Photon Interactions 6.1.4. Hypervelocity Impact Studies 6.2. Laboratory Results on Radiation-Induced Chemical and Physical Sputtering 6.2.1. Water Ice 6.2.2. Organic and Mixed Ices 6.2.3. Planetary Surface Analogues 6.3. Laboratory Results on Radiation-Induced Surface and Bulk Chemistry 6.3.1. Water Ice 6.3.2. Organic and Mixed Ice 6.3.3. Planetary Surface Analogues 6.4. Laboratory Simulations of Micrometeoroid Impacts 7. Summary and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Setting the Scene 2.1. What is Space-Weathering? 2.2. Early Evidence for Space-Weathering 2.2.1. Alteration of Surface Features 2.2.2. Exosphere Production 2.3. Airless Rocky and Icy Bodies throughout the Solar System 3. Distribution of Space-Weathering Agents throughout the Solar System 3.1. Energy and Variation of Photon Fluxes 3.2. Energy Distribution, Compositions, and Fluxes of Charged Particles 3.2.1. Energetic Solar Particles 3.2.2. Energetic Galactic Particles 3.2.3. Magnetospheres 3.3. Physical Distribution of Dust and Meteoroids 4. Mechanisms of Space-Weathering Agents with Surfaces of Airless Bodies 4.1. General Principles of Radiation Chemistry 4.2. Desorption Induced by Electronic Transitions 4.2.1. Perturbation of Surface Charge Carriers 4.2.2. Formation of a Transient Negative Ion 4.2.3. Decay Channels 4.2.4. Desorption of Neutrals and Ions 4.3. Desorption Induced by Momentum Transfer 4.4. Surface and Bulk Chemistry 4.5. Heavily Charged Particles 4.6. Micrometeoroid Impacts 5. Space-Weathering of Solar System Bodies 5.1. The Moon 5.2. Mercury 5.3. Phobos and Deimos 5.4. Asteroid Belt and Near-Earth Objects 5.5. Jovian System 5.6. Saturnian System © 2013 American Chemical Society

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1. INTRODUCTION In the past few years, several major space missions have been completed (e.g., Deep Impact, LCROSS, Chandrayaan-1, Stardust, Hayabusa), and a number of spacecraft are currently exploring both the inner (e.g., MESSENGER, DAWN, Rosetta) and outer (Cassini, New Horizons) solar system. There has been an accompanying flood of recent data from these and other space missions which have had a large impact on our knowledge of space-weathering processes as they occur throughout the solar system. It is thus topical to present a review of many of the recent observations and laboratory results occurring within such a highly evolving and dynamic field. Due to the vast amount of research that encompasses these research areas, a comprehensive review of all the relevant material is unfortunately beyond the scope of this article. The present article provides an overview of laboratory studies which are relevant to space-weathering in the inner and outer solar

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Special Issue: 2013 Astrochemistry Received: March 8, 2013 Published: November 25, 2013 9086

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from molten magma thus explaining why some were often associated with one another, while others were typically spatially distinct. Thus, there is a gradual transition through minerals that crystallize at high temperatures such as olivine, (Mg,Fe)2SiO4, and anorthite, CaAl2Si2O8, to those that crystallize at lower temperatures such as quartz, SiO2. Goldich2 found that the same general trend held regarding how susceptible minerals are to degradation when exposed to the variety of chemical (e.g., reaction with oxygen or water) and physical (mechanical; e.g., thermal stress or cryofracturing) processes acting at Earth’s surface. Accordingly, the term weathering refers to the effects that these processes would have on surfaces exposed to the Earth environment. By analogy, space-weathering refers to the alteration of exposed surfaces via their interaction with the space environment.3 The latter encompasses numerous sources of energetic radiation (e.g., solar, galactic, magnetospheric) and meteoroids objects which can impinge surfaces (a meteoroid is a 10 μm to 1 m size natural solid object moving in interplanetary space while a meteorite is defined as a meteoroid that has fallen to the surface of Earth).4

system. Though the emphasis here is on presenting the results of laboratory experiments in terms of the underlying chemical and physical processes, accompanying details of the astronomical objects and environments are provided. This allows the reader to understand the implications of the laboratory results within the general context of the solar system. Many of the references provided in each theme are books or comprehensive reviews, covering particular aspects in considerably more detail. The reader is encouraged to refer to these articles to gain additional insights. The majority of work presented in this article has been published within the past 5 years. However, since this review encompasses such a wide field of topics, a number of older works are cited where they were deemed pivotal to the topic since they provide a good basis for the reader to work from. The paper is outlined as follows. In section 2, the general context of what constitutes space-weathering is introduced to the reader, along with some of the early evidence for such processes occurring on the Moon. Brief evidence from groundbased spectral observations as well as laboratory analysis of returned samples is presented. A general overview of the solar system is given to demonstrate that although the solar system is generally divided into the inner (mostly rocky) and outer (mostly icy) regions, however, recent dynamical modeling suggests that these bodies have been substantially redistributed throughout the solar system over time. Once the reader is familiarized with how rocky and icy bodies are dispersed throughout the solar system, section 3 details the distribution of the space-weathering agents (electrons, ions, photons, and meteoroids) throughout the solar system. Each of the individual space-weathering agents is further discussed in section 4 in terms of the chemical and physical processes that are occurring. In section 5, individual solar system bodies are revisited in order to describe some of the space-weathering processes that are thought to be occurring. This section is accompanied by photographs from space missions as well as actual spectra in order to relate to the reader how observations can be related to space-weathering processes. Since each topic covered is in fact a field in its own right, our discussion is again not comprehensive, but focused on some of the more relevant space-weathering papers and recent results coming from mission-related data. In section 6, the laboratory apparatus used to investigate spaceweathering phenomena are introduced. A particular focus is on instrumentation that the authors are most familiar with, although many different variations of such apparatus exist throughout the world. Laboratory results are presented and discussed within the context of the appropriate spaceweathering environments. Lastly, in section 7, we present a summary and outlook of how some of the laboratory experiments contribute to the understanding of some recent topics currently under debate; such as the relative involvement of each process, and highlight the advantages of a synergistic approach.

2.2. Early Evidence for Space-Weathering

2.2.1. Alteration of Surface Features. The easiest body available to study space-weathering effects happens to also be the closest, the Moon, on which large craters can clearly be seen even with the naked eye. As far back as 1955, Gold3 recognized that the brighter impact craters as well as impact ejecta strewn across the surface in radial streaks (known as rays) during a large impact were consistent with the excavation of a brighter material buried below a thin surface layer of darker material. Gold initially suggested that weathering agents such as meteoroids or solar X-rays, as well as transported dust could be responsible for gradually darkening or covering the bright ejecta material over time. An example of the bright radial streaks, visible only around the younger craters, on the surface of the Moon is depicted in Figure 1a. The very successful NASA Apollo and Soviet Luna programs furnished the scientific community with approximately 382 kg of lunar rock and soil samples for analysis. Lunar soils showed three general characteristic spectral differences in comparison to pristine rocks/minerals of the same composition from a terrestrial origin when pulverized to a similar size.5,6 The differences observed at this time were summarized as follows: (i) darkening (overall reduction in reflectance, or albedo), (ii) weaker characteristic absorption bands from mineral absorption features, and (iii) spectrally redder slopes (reflectance increases with increasing wavelength). Although the lunar regolith has a wide distribution of particle sizes, it has been noted by Hapke7 that the smaller particle sizes dominate the optical properties (Figure 1b)8 and thus are primarily responsible for the spectral reflectance that was observed. Finer grained soils exhibited the flattest spectral features with the lowest albedo and therefore were assessed to be the most space-weathered. Upon close examination of the Apollo samples, a high proportion were found to be agglutinates, i.e., minerals and rock fragments imbedded within a dark glass, that were thought to be formed during the shock-heating and fast cooling of a rock during the impact of a meteoroid (vitrification). Vitrification experiments on lunar soils performed in the 1970s5,6,9−11 showed conflicting results dependent upon the environment in which the studies were conducted. Accordingly, the impact melts exhibited deepening and broadening of their

2. SETTING THE SCENE 2.1. What is Space-Weathering?

Shortly after the turn of the 20th century, the petrologist Bowen1 carried out pioneering melting experiments on volcanic magma that paved the way toward our current understanding of how minerals are altered through geochemical processes. His studies demonstrated that there was a temperature-based sequential reaction pathway through which minerals crystallized 9087

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Figure 2. (a) Digital X-ray Fe Kα image of diverse grains from the 10− 20 μm size fraction of soil sample 79221. Iron bearing minerals are bright because plagioclase (Plag) contains very little Fe as FeO; the enrichment of Fe (as np-Fe0) on the rims of plagioclase grains is readily apparent (arrows). Scale bar is 10 μm. (b) Transmission electron microscope images through a plagioclase anorthosite grain (An) from solid sample 79221. Small spheres of metallic Fe (np-Fe0) appear dark. Scale bar is 10 nm. (c) Transmission electron microscope images through a lunar agglutinate. Multiple layers of np-Fe0 along the rim of the grain are indicated with arrows. The size of metal particles in the interior is a factor of 3 larger than those deposited on the rim. Scale bar is 50 nm. Reprinted with permission from ref 13. Copyright 2000 Meteoritical Society.

Figure 1. (a) The Moon. The bright crater to the bottom-right of the image is the young Tycho crater (est age ∼100 Myr) with extended bright rays covering much of the southern hemisphere. In time these rays will darken due to space-weathering processes (image credit: NASA). (b) reflectance spectra of rock and soil fragments taken from Apollo 11 return samples. Reprinted from ref 8, with permission from Elsevier.

absorption bands when formed under vacuum or a reducing atmosphere which mimicked only partially the optical properties of lunar regolith. Further disparities were found when the experiments were carried out under oxidizing conditions owing to the production of large amounts of ferric iron (Fe3+).7 In addition, the optical properties of glasses produced from vitrification were distinctly different from lunar samples at sizes 2.7 AU) is populated mostly by C-type (carbonaceous) asteroids (waterbearing with high contents of carbonaceous material). It is now thought that the C-type asteroids originated in the outer solar system, and were consequently scattered into the inner solar system as the giant planets migrated.35,36 It is known that a vast reservoir of icy bodies exists in the outer solar system. These are generally referred to as TransNeptunian Objects (or TNOs). TNOs are further classified as Kuiper Belt Objects (KBOs; 30−50 AU), Scattered-Disk Objects (SDOs; 30−100 AU; orbits influenced by Neptune with high eccentricities and inclinations), and Oort cloud Objects (OCOs; 2000−50 000 AU; mostly scattered objects with a roughly spherical distribution). These regions are

3. DISTRIBUTION OF SPACE-WEATHERING AGENTS THROUGHOUT THE SOLAR SYSTEM For each of the space-weathering agents considered (photons, charged particles, and meteoroids), both intrinsic (solar system) as well as extrinsic (galactic) sources contribute to their distributions throughout the solar system. Though the key objective in an overview such as this is to provide accessible information (i.e., fluxes listed in tables), in order to do so, each of these elements must be portrayed as quite static or predictable which is unfortunately far from being realistic, particularly on short time scales. The magnitude of variations of solar photons and charged particles is somewhat dependent on 9090

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the solar cycle. Since the solar cycle also affects the degree to which extrasolar particles (dust, ions, and electrons) are able to penetrate into the solar system, a brief discussion is presented here. Solar activity has been shown to be correlated to the number of observed sunspots (darker spots that appear visibly on the Sun) for which there are reliable telescopic records dating back to at least the early 17th century.51 The observed higher levels of solar activity are additionally associated with increased numbers of very energetic processes emanating from the Sun’s surface such as solar flares and interplanetary coronal mass ejections (ICMEs), which are not necessarily closely associated with one another.52 Both of these events occur on short time scales (hours to days) and typically carry large numbers of highly energetic photons and charged particles into the interplanetary space. Solar activity cycles, occurring approximately every 11 years (Schwabe cycle), are intimately correlated with the number of sunspots which gradually comes to a maximum as the magnetic pole of the Sun flips then declines again. The current cycle, number 24, began in January 2008 during the solar minimum. It has been observed that the current cycle, in particular the 2008−2009 minimum, has been rather weak in comparison to previous cycles.53

Figure 5. Class X17 solar flare observed on 28 October 2003. Top shows the large sunspot cluster (in visible, left, and extreme UV, right) that produced the flare and associated CME (seen in the lower two coronograph images). Image credits: SOHO/EIT, SOHO/LASCO, SOHO/MDI (ESA and NASA).

3.1. Energy and Variation of Photon Fluxes

The energy distribution of photons leaving the photosphere can be approximated to that emitted by a blackbody of temperature 5777 K, although it deviates from this due to increased levels of short-wavelength emissions. The total spectral irradiance (TSI) is often referred to as the amount of power received per unit area at the top of the Earth’s atmosphere (often referred to as zero air mass) and has an accepted value of 1366.1 W m−2 (the solar constant) which varies by ∼0.1% over a solar cycle.54 Tabulated values of the solar irradiance as a function of photon energy over the range of hard X-rays (0.1 nm or 12.4 keV) to the ultraviolet-B (UVB; 318 nm, or 3.9 eV) during a solar minimum as well as their variation during a solar maximum can be found elsewhere.55 Previously, Madey et al.56 compiled photon fluxes (cm−2 s−1) and energy fluxes (eV cm−2 s−1) into convenient wavelength ranges through the infrared (700− 10 000 nm or 0.001−1.8 eV) to the EUV (10−121 nm or 12.4−124 eV) during solar minimum conditions at 1 AU. As they discuss the photon and energy fluxes can be determined as a function of radial distance from the sun, based on a 1/r2 relationship (where r is the distance of the object from the Sun, in AU). Similar approaches have been reported within the literature on the basis of different assumptions or wavelength ranges.57−60 For example, Baratta et al.60 presented the fluxes in terms of visible (∼2 eV), near-UV (∼ 4 eV), and far-UV (∼6 eV) regions, where photon fluxes were presented as 2 × 1017, 1.5 × 1016, and 3.0 × 1013 cm−2 s−1, respectively. Since the flux of X-rays occurring at solar minimum is orders of magnitude lower, they are often ignored. However, at solar maximum the photon flux can be significant since solar flares release large quantities of these energetic photons. The irradiance within the EUV and UVC regions is also typically enhanced during a solar maximum, although to a lesser extent.54,55,61−65 Geostationary operational satellites (GOES) have been observing solar flares since 1974 by monitoring the energy flux of soft X-rays in the range 0.1−0.8 nm (1.5−12 keV). Solar flare events are classed according to the intensity observed in this region according to an order of magnitude scale based on the magnitude of the peak flux (A = 10−8, B = 10−7, C = 10−6, M = 10−5, X = 10−4 W m−2), and are subdivided with an additional digit. As an

example, during the solar maximum of 2003, a solar flare of intensity X17.2 was recorded on October 28th (Figure 5).66 This correlates to an observed peak power of 1.7 × 10−3 W m−2 which corresponds to a flux of 8.6 × 107 to 6.9 × 108 photons cm−2 s−1 in the soft X-ray energy range. Aschwandron et al.66 demonstrate that over the period of 1975−2011 (∼31/4 solar cycles) the GOES instrument observed approximately 3 × 105 solar flares per solar cycle, of which approximately 104 were C-class, 103 were M-class, and about 75 were X-class. The time between flares averaged out to be approximately 2 days during solar minimum conditions, but could be as short as 30 min during solar maxima. In Table 1, the photon fluxes and energy fluxes are presented for both solar minimum and solar maximum conditions at a distance of 1 AU. Here, the hard X-ray (0.1 nm or 12.4 keV) to the ultraviolet-B (UVB; 318 nm, or 3.9 eV) are again taken from Jursa.55 For comparison, the FUV (122−200 nm or 6.2− 10.2 eV) and Lyman-α (121.6 or 10.2 eV) photon fluxes and energy fluxes are also provided. The UV-A (318−400 nm or 3.9−4.4 eV), visible (400−700 nm or 3.1−3.9), and infrared (700−10 000 nm or 0.001−1.8 eV) photon fluxes were taken from the 2000 ASTM standard extraterrestrial spectrum reference E-490-00 (air mass zero) over the wavelength range 119.5−10 000 nm which is normalized to the solar constant of 1366.1 W m−2 (obtained from http://rredc.nrel.gov/solar/ spectra/am0/). The values listed for the fluxes and energy fluxes in Table 1 agree well with those previously published by Madey et al.56 Ratios of the spectral irradiance observed for the 2003 solar maximum values compared to those for the 2008− 2009 solar minimum over the UV-C (100−280 nm or 4.4 to 12.4 eV) to hard X-ray regions were determined from the Solar Radiation and Climate Experiment (SORCE; http://lasp. colorado.edu/lisird/sorce/sorce_ssi/ts.html). These ratios were then applied to the previously determined fluxes for the solar minimum. However, the overall increase observed during a solar maximum compared to a solar minimum is perhaps 9091

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Table 1. Solar Photon Fluxes and Energy Fluxes for Solar Minimum and Solar Maximum Conditions at 1 AU from the Infrared to X-ray Regions along with the Solar Irradiance and Average Photon Energiesa flux (photons cm−2 s−1)

type

wavelength range (nm)

energy range (eV)

average energy (eV)

infrared visible UV-A UV-B UV-C [FUV]f [Lyman-α]f EUV soft X-rays hard X-rays total

700−10 000 400−700 318−400 280−318 100−280 122−200 121−122 10−121 1−10 0.1−1 0.1−10 000

0.001−1.8 1.8−3.1 3.1−3.9 3.9−4.4 4.4−12.4 [6.2−10.2] [10.2] 12.4−124 0.124−1.24 × 103 1.24−12.4 × 103 0.001−12.4 × 103

0.9 2.3 3.4 4.1 4.9 6.6 10.2 28.2 219.4 6819 1.3

Lyman-αe

121−122

10.2

10.2

solar minb

solar maxc

Solar Photons 4.9 × 1017 4.9 × 1017 17 1.4 × 10 1.5 × 1017 1.5 × 1016 1.5 × 1016 2.5 × 1015 2.5 × 1015 14 8.6 × 10 9.1 × 1014 12 [6.9 × 10 ] [7.3 × 1012] [2.6 × 1011] [3.4 × 1011] 4.0 × 1010 7.2 × 1010 8 5.9 × 10 1.4 × 109 5 5.0 × 10 1.5 × 107 6.6 × 1017 6.6 × 1017 Interstellar UV Photons 2.1 × 108 2.1 × 108

energy flux (eV cm−2 s−1)

solar irradianced

solar min

solar max

(W m−2)

(% total)

4.6 × 1017 3.3 × 1017 5.2 × 1016 1.0 × 1016 4.2 × 1015 [4.5 × 1013] [2.6 × 1012] 1.1 × 1012 1.3 × 1011 3.4 × 109 8.5 × 1017

4.6 × 1017 3.3 × 1017 5.2 × 1016 1.0 × 1016 4.4× 1015 [4.8 × 1013] [3.4 × 1012] 2.0 × 1012 3.7 × 1011 1.0 × 1011 8.5 × 1017

730.79 529.53 82.51 16.57 6.68 [0.07] [0.004] 0.002 2 × 10−4 5 × 10−6 1366.1

53.5 38.8 6.0 1.2 0.5 [0.005] [0.0003] 0.0001 2 × 10−5 4 × 10−7 100

2.1 × 109

2.1 × 109

n/a

n/a

Interstellar photon fluxes which will be invariant across the solar system are also provided. Please refer to the text for additional details. bSolar minimum values adopted from Jursa55 and the 2000 ASTM standard extraterrestrial spectrum reference E-490-00. cEnhancements during solar maximum were derived from SORCE data. dPercentages are given for solar minimum conditions. eTaken from Gladstone et al.69 FUV values outside the ecliptic plane more appropriate for permanently shadowed regions are provided in Gladstone et al., ref 69. fValues listed do not contribute to the total since their wavelengths are covered under the UV-C range, and are listed here for convenience. a

summary of results for the solar and galactic sources is presented in Table 2. 3.2.1. Energetic Solar Particles. The stream of particles emanating from the Sun, known collectively as the solar wind, is a result of the continuous (∼adiabatic) expansion of the hot solar corona (∼106 K) into the vacuum of space.70 Although the solar wind properties can be approximated as a bulk distribution, it is important to appreciate the properties of the underlying contributions. The approach taken here will first detail the bulk conditions of the solar wind, and then build additional concepts in order to demonstrate how the underlying contributors vary with solar activity. This, in turn, will influence the degree of space-weathering. The bulk properties of the solar wind under solar minimum conditions provided by Jursa55 at 1 AU are given as a velocity (vsw) of 468 km s−1 and a proton density (np) of 8.7 cm−3 leading to a flux (Φsw) of 4.1 × 108 cm−2 s−1. Reisenfeld et al.71 recently presented bulk values for the 1996−2006 solar cycle as vsw = 470 km s−1, np = 6.7 cm−3, giving Φsw = 2.9 × 108 cm−2 s−1. The average contribution of the solar wind to the total heat given out by the Sun has been determined from observations with the Ulysses, Helios, and Wind spacecraft to be 1.5 ± 0.4 × 10−3 W m−2,72 only a small fraction of the amount of heat that is emitted through solar photons (1366.1 W m−2). The solar wind, however, is nonuniform; variations in the wind speed, density, and chemical composition are associated with features identified on the solar corona which themselves vary throughout a solar cycle. The variation of wind speed as a function of latitude during solar minimum and solar maximum conditions as observed with the Ulysses spacecraft is shown in Figure 6, along with the sunspot number.73 During the solar minima, the wind velocity is found to be uniformly fast (∼ 750 km s−1) and steady at high latitudes where large coronal holes are found (openings in the magnetic field lines that allow ions to pass easier through the upper layers74). The midlatitudes (±15°) feature a slower (down to 250 km s−1), more dense wind identified as a streamer belt which is more variable.73,75

closer to 50% in the UV-C region and around 100% for Lymanα photons and X-rays;55,61,64,65,67 thus, the values presented for the solar maximum in Table 1 are considered conservative. It is also worth noting that the solar activity has not been constant throughout the lifetime of our Sun. Güdel68 has calculated the enhancement factors of X-ray, EUV, and FUV flares throughout the solar history finding mild enhancements through the past 3 Gyr. Estimates included at the end of the T-Tauri stage indicate that the enhancement for hard X-rays was 1600 the present day values, with similar enhancements of 100 times for the soft X-ray and EUV photons, and 25 times for the FUV photons. For completion, Table 1 also lists the averaged interplanetary medium Lyman-α flux of 1.8−2.3 × 108 cm−2 s−1, which accounts for interstellar photons as reported by Gladstone et al.69 These photons can be important at large heliocentric distances. Additionally, since photons from extrasolar sources are able to access regions that solar photons cannot (i.e., permanently shadowed regions), these photons are able to cause space-weathering on otherwise hidden regions. Gladstone et al. also list an estimation of the fluxes arriving at permanently shadowed regions. Here, FUV fluxes were recorded using the Lyman alpha mapping project (LAMP) instrument onboard NASA’s lunar reconnaissance orbiter (LRO) to be 3.1 × 107 cm−2 s−1 at the south pole of the moon, and 4.2 × 106 cm−2 s−1 at the north pole. As noted by the authors, since the galactic center is in the southern ecliptic hemisphere, most UV-bright stars are also located here, accounting for the observed difference. 3.2. Energy Distribution, Compositions, and Fluxes of Charged Particles

The surfaces of airless bodies are bombarded by energetic charged particles. However, the relative contributions of these charged particles born from the Sun or extragalactic sources or trapped in magnetospheres vary throughout the solar system. Here, each of these will briefly be discussed in turn while the 9092

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Figure 6. Velocity of the solar wind as measured by the Ulysses spacecraft during solar minimum periods (a and c), and during a solar maximum (b). The sunspot number which correlates with the solar maximum (d). Notice that that magnetic field (blue/red) is reversed in the left and right frames. Reprinted with permission from ref 73. Copyright 2008 by the American Geophysical Union.

interaction regions (CIRs, which arise when fast solar wind overtakes slower winds which causes shocks and accelerates ions85,86), which were split into equal contributions of the fast and slow winds they originate from, the majority of their categories were treated as transient (ICME) events. Here, at solar minimum the percentage contributions are found to be 45% slow, 30% fast, and 25% transient. At solar maximum these contributions become 38% slow, 19% fast, and 43% transient. The velocity of ICMEs ranges from approximately 300 to 3000 km s−1 with the mean velocity reported in the range 449− 476 km s−1.71,80,81,87 Since these velocities are typically higher than that of the solar wind, they can often overtake the slow (and fast) wind generating regions of higher density and shocks that can lead to geomagnetic storms on the Earth.84,88 Here, we take the averaged properties measured from the Ulysses spacecraft as measured by Ebert et al.78 normalized to the values at 1 AU. The slow, fast, and transient solar winds are reported to have average velocities of 392, 745, and 449 km s−1 and np = 5.6, 2.1, and 5.9 cm−3, respectively. The propagation of an ICME through the interplanetary medium is demonstrated in the MHD model at the bottom of Figure 7.86 In addition to the fast, slow, and transient components, however, a large number of charged particles released during an ICME event can be highly accelerated. McCracken et al.89 were able to correlate the abundances of nitrate found in thin layers within ice cores extracted from the Arctic and Antarctic poles to proton fluence of >30 MeV originating from the Sun, dating back to 1561. Although approximately 70 events were identified over the 450 year sample, the largest fluence (18.8 × 109 protons with >30 MeV energy) was found associated with the first of September, 1859. On this day, Richard Carrington famously observed the largest solar flare on record.87,90 The first particles from the associated ICME arrived to Earth within the hour, although the peak intensity arrived some 18 h later bringing with it one of the largest magnetic storms in history, as well as spectacular views of the aurora reported in midlatitudes. So how do these particles contribute to the overall composition of the solar wind? The fluence of O atoms as a function of their energy within the solar wind during the period of 10/1997−06/ 2000 (midcycle) was measured with the advanced composition explorer (ACE) spacecraft, shown in Figure 8a.91,92 The trend for other species in the solar wind is well-represented by that of

During the solar maximum on the other hand, the solar wind is found to be more complex, and highly variable in both the highlatitudes and midlatitudes with greater contribution of slower wind to the high-latitudes and faster winds (again, attributable to coronal holes appearing) in the midlatitudes, or to put it another way, “gustier”.76 Over a solar cycle, the heat flux as measured by both the Wind and Ulysses spacecraft was found to be approximately 31% higher during a solar maximum as compared to solar minimum values.72 Also, note in Figure 6 that the interplanetary magnetic field (IMF) of the Sun is reversed between the solar minimum on the left and right panels. As the solar wind propagates through the interplanetary medium it takes on a spiral appearance as is shown from magnetohydrodynamic (MHD) models of the solar wind as shown in Figure 7 (top).77 The solar wind can broadly be divided into slow (250−400 km s−1) and fast (400−800 km s−1) components as defined by Schwenn.75 At a distance of 1 AU, the slow wind is reported here as having a proton density (np) of 10.7 cm−3 compared to only 3.0 cm−3 for the fast wind. The average flux is thus given as 3.7 × 108 cm−2 s−1 for the slow wind and 2.0 × 108 cm−2 s−1 for the fast wind. The helium (He2+) content is also shown to contrast from 2.5% (and highly variable) in the slow wind to 3.6% in the fast wind. Similar values are reported by Ebert et al.78 and Echim et al.79 Solar flares and interplanetary coronal mass ejections (ICMEs) are transient in their nature, but deserve special consideration due to the large numbers of highly energetic particles that can be released during these events (solar energetic particles; SEPs). They are typically associated with reconnection events of magnetic field lines. The ICME associated with the solar flare of October 28th, 2003, can be seen in Figure 5. The rate of these events is low during periods of solar minimum where only 4 or so occur each year, but rises with solar activity reaching more than 50 per year during the solar maximum.80 To what extent, however, are the slow, fast, and transient (ICME) solar winds contributing at different stages of the solar cycle? Estimates of the contribution of these transient events to the solar wind as a whole vary from as low as 6% during the solar minimum to 25−40% during a solar maximum.81−84 Yermolaev et al.84 produced distributions for the contributors to solar wind from 1977 to 2001 categorizing them into eight different types, based on the percentage of time they were observed. With the exception of the corotating 9093

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Figure 7. Models of solar wind propagating through the interplanetary medium showing the parker spiral as well as the inverse relationship between (a) velocity and (b) density. Reprinted with permission from ref 77. Copyright 2012 by the American Geophysical Union. (c) Propagation of a coronal mass ejection into the interplanetary medium. Note the rotation period of the Sun relative to the Earth (Carrington rotation) is approximately 27 days. Reprinted with permission from ref 86. Copyright Springer Science + Business Media B.V. 2012.

and fast winds under solar minimum and maximum conditions are provided by Lepri et al.93 and for the transient wind by Wiedenbeck et al.94 with the exception of helium, which was taken from Jursa.55 For consistency, only helium, carbon, oxygen, silicon, and iron “heavies” were considered, since data were available under each wind type for these species. Finally, total heat flux for the solar minimum and maximum were constrained to agree with the values published by Le Chat et al.72 The total flux during solar minimum conditions agrees well with the bulk properties given by Jursa.55 The heavy ions as well as suprathermal components contribute higher proportions to the heat flux than their flux contribution to the solar wind. Indeed, the higher percentage of transient wind during the solar maximum as well as the larger percentage of heavies explains the 31% increase in the heat flux over a solar maximum.72

O atoms. The individual components that make up the suprathermal tail, extending into the MeV/nucleon range, are labeled in Figure 8a. The contributions arising from the ICMEs are listed as gradual and impulsive SEPs. Contributions from CIRs are also listed, as well as anomalous cosmic rays (ACRs) and galactic cosmic rays (GCRs) which will be covered in the next section. Table 2 shows details of the solar wind flux and energy flux as a function of each of the components described. The averaged properties of the slow, fast, and transient winds are taken from Ulysses data as presented in Ebert et al.78 The relative contributions of each during solar minimum and maximum conditions are taken from Yermolaev et al.84 Here, 5.4% of the solar wind is assumed to have a suprathermal distribution as described by the average solar wind conditions of Mewaldt et al.91 and Mason and Gloecker.92 The compositions of slow 9094

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Figure 8. (a) Fluence of energetic oxygen nuclei measured by the ACE spacecraft between October 1997 and June 2000. The approximate velocities corresponding to the slow and fast solar winds are indicated. Contributions to the suprathermal tail are shown arising from both solar componensts (SEPs and CIRs) as well as ACR and GCRs. Reprinted with permission from ref 91. Copyright Springer Science + Business Media B.V. 2007. (b) Fluence of energetic electrons measured by WIND (black) and STEREO A (green) and B (red) on December 6th, 2007. Solid lines are produced from Maxwellian, Kappa, and power-law fits to the solar wind core, halo, and superhalo distributions, respectively. Interplanetary and galactic cosmic electrons are also shown in blue diamonds. Reprinted with permission from ref 97. Copyright 2012, The American Astronomical Society.

are found to have one electron attached.100 The abundances and fluxes listed in Table 2 are derived from Jursa55 and George et al.100 Although the flux of these particles is much lower than that of the solar particles, since they are very energetic they are able to provide a comparable energy flux, particular in regions of the outer solar system or at depth. Note that the flux of approximately 4 protons cm−2 s−1 is reported under solar minimum conditions, whereas this falls to approximately 2 protons cm−2 s−1 during solar maxima. Therefore, the penetration of the GCR into the solar system is hampered by stronger solar wind conditions. A rapid decrease in the levels of galactic cosmic rays is observed after an ICME event, known as a Forbush decrease. In the outer solar system, it is therefore expected that the GCR will have a more pronounced effect and become the dominant charged particle radiation source at some distance from the Sun. This “modulation” of the GCR flux by the solar wind is described in detail, and several approaches to more accurately describe its function with radial distance can be found elsewhere.55,104−107 Here, it is sufficient to say that the flux of GCR in the outer solar system (∼80 AU) reaches approximately an order of magnitude higher than what reaches the Earth at 1 AU. Kudela108 gives a review of the characteristics of the cosmic energetic particles in wide energy range related to space physics. In addition to the galactic cosmic rays, neutral atoms and gas (mostly H, He, but also a small fraction of Ne, O, N, C, and Ar) flow into the heliosphere from the interstellar medium at approximately 25 km s−1. Since they carry no charge, they are able to penetrate deeper into the magnetic field than fully ionized cosmic rays, which are modulated by the solar wind. The neutrals can become ionized by either photoionization with the solar photons, or charge-transfer processes with highly ionized species within the solar wind. Once these species are ionized, they become “picked-up” and carried by the solar wind.99 These pick-up ions can be carried to the boundary between the solar wind and interstellar space where shock events can accelerate the ions and return them back toward the inner solar system. These particles are described as anomalous

The electron population is approximately balanced by the total charge flux of ions in each solar wind regime.55,71 Thus, in Table 2, the electron fluxes reported correspond to the totals for the solar minimum and maximum conditions and are derived directly from the ion fluxes. In comparison to ions, the bulk velocity of electrons within the solar wind shows less drastic variation between the fast, slow, and transient winds. Louarn et al.95 divide the solar electrons into four general populations: (i) the “core” (bulk) electrons of energy below 60 eV (averaging approximately 10−12 eV), (ii) the “halo” which is a suprathermal distribution from 60 to 1 keV (very isotropic, average is approximately 70 eV), (iii) the “super halo” which extends beyond keV energies, and (iv) the “Strahl” which are up to several hundred eV (i.e., also suprathermal) and aligned strongly to the magnetic field of the Sun. The latter of these will not be considered further here, but details can be found elsewhere.96 Wang et al.97 recently compiled data on the flux and energy distribution of interplanetary electrons including measured data from the WIND and STEREO (A and B) spacecraft, which is shown in Figure 8b. The approximate distributions of electrons into these populations are given as 95% core, 4% halo, and 1% super halo in Table 2.95,97,98 In addition, approximately 0.001% of the electrons will carry in excess of 1 keV in energy. 3.2.2. Energetic Galactic Particles. Beside the radiations emitted from the Sun, galactic cosmic rays (GCRs) emanating from outside the solar system penetrate inside the heliospheric cavity and follow complex trajectories under the effect of the IMF. GCRs possess particularly high energies, with energies reaching up to 1011 GeV.99 The energy maximum for those reaching a distance of 1 AU is approximately 300 MeV amu−1.55,100 The isotopic abundances indicate that the source of these particles is consistent with a mix of ∼20% massive star ejecta (stellar wind and core-collapse supernova ejecta) and ∼80% normal interstellar material with solar system abundances.101−103 The chemical composition is approximately 87% H+, 12% He2+, 1% “heavies”, and 3% electrons.55,100 Most nuclei are fully stripped of electrons, although a small fraction 9095

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Table 2. Distributions of Charged Particles throughout the Solar System During Solar Max and Solar Min at 1 AUa,b flux (cm−2 s−1)

radiation

velocity range (km s )

energy range (eV amu−1)

slow wind ISS fast wind CH transient ICME suprathermal tail

250−400 400−800 300−3000 727−982

326−835 835−3341 470−46 979 2763−5034

982−1492 1492−5050 5501−14 255 >14 255

5034−11 627 11 627−133 080 133 080−1.06M >1.06 MeV

1.9 × 104

1000 eV

−1

total [H+] [He2+] [C4−6+] [O5−8+] [Si6−12+] [Fe6−20+] core halo super halo total total [H+] [He2+] [C5/6+] [O7/8+] [Si13/14+] [Fe25/26+] electrons a

mean velocity (km s−1)

mean energy (eV amu−1)

Solar Wind Ions 392 802 745 2897 449 1052 760 3.0 × 103 5.6 × 103 3.0 × 104 2.9 × 105 3.5 × 106 ∼1.3 × 103

1032 2390 7432 25712 ∼500

Solar Wind Electrons 2.1 × 103 12 5.0 × 103 70 8.4 × 103 200 5.9 × 105 ∼104 3 2.4 × 10 16 Galactic Cosmic Rays ∼3 × 108 ∼3 × 108 ∼3 × 108 ∼3 × 108 ∼3 × 108 ∼3 × 108 ∼3 × 108 ∼3 × 108

solar min 1.7 1.1 1.1 2.3

× × × ×

108 108 108 107

energy flux (eV cm−2 s−1)

solar max 1.7 8.6 3.4 3.6

× × × ×

108 107 108 107

solar min 1.4 3.3 1.1 7.6

× × × ×

1011 1011 1011 1010

1.4 2.5 3.6 1.2

× × × ×

1011 1011 1011 1011

9.1 8.4 1.4 2.3 4.1 [4.0 [1.1 [5.5 [1.0 [1.9 [4.6

× 104 × 103 × 103 × 102 × 108 × 108] × 107] × 104] × 105] × 104] × 104]

1.4 1.3 2.1 3.3 6.4 [6.1 [3.0 [9.5 [2.2 [5.0 [1.4

× 105 × 104 × 103 × 102 × 108 × 108] × 107] × 104] × 105] × 104] × 105]

3.9 1.7 4.1 4.1 4.1

× 108 × 107 × 106 × 103 × 108

6.1 2.6 6.4 6.4 6.4

× 108 × 107 × 106 × 103 × 108

4.7 1.2 8.3 4.1 6.7

× 109 × 109 × 108 × 107 × 109

7.3 × 109 1.8 × 109 1.3 × 109 6.7 × 107 1.0 × 1010

2.0 × 10° 2.0 × 10° 3.6 × 10−1 7.3 × 10−3 7.3 × 10−3 1.5 × 10−2 1.5 × 10−3 6.0 × 10−2

1.2 1.2 8.7 5.2 7.0 2.1 2.9 3.6

× 109 × 109 × 108 × 107 × 107 × 107 × 107 × 107

6.0 6.0 4.4 2.6 3.5 1.0 1.5 1.8

4.0 × 10° 4.0 × 10° 7.3 × 10−1 1.5 × 10−2 1.5 × 10−2 2.5 × 10−3 1.7 × 10−3 1.2 × 10−1

5.5 × 108 2.7 × 108 4.3 × 108 8.1 × 108 6.7 × 1011 [6.1 × 1011] [6.1 × 1010] [1.1 × 109] [2.4 × 109] [7.7 × 108] [2.6 × 109]

solar max

8.9 × 108 4.4 × 108 7.0 × 108 1.3 × 109 8.7 × 1011 [7.4 × 1011] [1.1 × 1011] [1.6 × 109] [4.3 × 109] [1.6 × 109] [7.7 × 109]

× 108 × 108 × 108 × 107 × 107 × 107 × 107 × 107

Refer to text for details. bValues in parentheses indicate atomic contributions to the total flux and energy fluxes and their common ionic charge states.

As the solar wind encounters the magnetic field of the Earth, a bow shock is produced as the supersonic wind is slowed at approximately 13 Earth radii (RE) upstream. The solar wind then enters the magnetosheath. The edge of Earth’s magnetosphere lays only approximately 10 RE upstream; however, the magnetotail on the night side extends to more than 200 RE. Thus, the Moon passes into this region for approximately 6 days each month where it is partially shielded from the direct solar wind; however, complex interactions of the Moon surface with plasma caught in the magnetotail and magnetosheath are just beginning to be investigated.122−125 It is also interesting to note that recently discovered magnetic anomalies on the Moon are able to act like mini-magnetospheres deflecting the solar wind and protecting regions of the surface.126,127 Mercury has an intrinsic magnetic field, although it is approximately 1% as strong as the Earth’s. Similar to the Earth there are reconnection regions in the northern and southern cusps where the solar wind can directly access the surface (see Winslow et al.128 for recently measured solar wind fluxes at these regions). In addition, calculations demonstrate that strong solar winds are able to push the magnetic field below the surface leaving the entire day-side surface accessible to the solar wind for short periods.129 Detailed summaries of the solar−wind

cosmic rays (ACRs), and can bear energies up to 20 MeV nucleon−1.55 Kallenbach et al.109 give the composition and spatial distribution of pick-up ions in the heliosphere. Additional details leading to the production of pick-up ions and ACRs are given by Schwandron et al.,110 Scherer et al.,111 Giacalone et al.,112 and Lee et al.113 3.2.3. Magnetospheres. The magnetic field lines and the plasma’s flow dynamic pressure from the solar wind are crucial parameters that together drive the interaction with intrinsic magnetic fields and potentially induce the creation of a magnetosphere around planetary objects. For general reviews on solar system magnetospheres, refer to Johnson,114 Blanc et al.,115 and Banegal.116 Russell reviews the magnetospheres of the terrestrial planets,117 and the magnetospheres of outer planets.118,119 Induced magnetospheres are reviewed by Luhmann et al.,120 and the specific case of cometary magnetospheres is reviewed by Cravens.121 The intrinsic or induced magnetospheres are additional sources of fresh charged particles whose chemical composition reflects that of the parent bodies, via ionized atmospheric, sputtered neutrals, evaporation, photodetachment, etc. The energy regime of these magnetospheric ions can range from eV to MeV containing both ions and electrons. 9096

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the increased sputtering of sodium from the moon during the Leonid155 and Geminid156 showers. The influx of interplanetary dust to the early Earth could well have been a major source of organic carbon, and therefore has been implicated to have played a major role in how life may have originated on this planet (e.g., Brack157). The in-falling cometary and meteoritic dust is estimated to constitute approximately 1−4% of the Moon’s surface, and could be up to 15−20% for Mercury.158 Many other bodies in the inner solar system (such as the asteroid 4-Vesta159) and outer solar system (such as the Galilean satellites Ganymede and Callisto,160 as well as the Saturnian satellite Iapetus161) appear to be coated with dark materials thought to be transported from elsewhere in the planetary or solar system. Dust can be generated through collisions which occur frequently in the asteroid and Kuiper belts, but also through space-weathering processes.162,163 These processes are thought to contribute to the production of the observed rings of Jupiter,164 Saturn,165−168 and Uranus.169 In addition, there are also interstellar sources of dust, traveling at approximately 26 km s−1;170,171 however, these are only briefly worth mentioning since they contribute a minor proportion of the inventory of interplanetary dust, according to the review of Mann,172 as well as the models of zodiacal dust from Rowan-Robertson.173 The dynamics of small particles (r < 1 cm) are driven by the Poynting-Robertson effect whereby solar radiation pressure causes them to slowly spiral toward the sun, whereas larger particles of >1 cm are driven by gravity only (e.g., Borin et al.174). Poppe and Horányi175 discuss the amount of dust coming from the outer solar system into the Saturnian planetary system. In general, meteoritic and dust (and meteoroid) sources in the outer solar system are more likely to contain contributions from the Kuiper belt than asteroid belt.176 Gravitational focusing will cause both the flux and velocity of dust (and meteoroids) to increase closer toward objects of larger mass such as the Sun and large planetary bodies. The relative velocities encountered for satellites and planets in close orbits additionally increase the impact velocities.165,177,178 The relative velocity of a satellite can additionally affect whether the majority of impacts occur on the trailing (if they are being overtaken by the dust/meteoroid) or leading (if they are overtaking the dust/meteoroid) hemispheres. The impact probability distribution as a function of impactor velocity and diameter are compared for the Earth, Moon, and members of the Asteroid Belt in Figure 9.179,180 Information on the anticipated mass flux as well as impact velocities for bodies throughout the solar system are presented in Table 3. Since the probability of large impactors hitting a surface is small, statistical analysis of the frequency and size distributions of craters that are present on the surface of a body can be used to provide estimations of its age.181 Estimates for the cratering rates are provided for the inner solar system182,183 and outer solar system.178,184,185 Additionally, more specific cratering rates (and occasionally mass fluxes and gardening rates) are presented for the Moon,179,183,186,187 Mercury,186,188,189,190 Mars system,191,192 the asteroid belt,180,193,194,195 Jupiter system,137,196 and the Saturnian system.176

interactions with Mercury’s magnetosphere prior to the MESSENGER mission are given by Milillo et al.130 One of the recent unexpected discoveries of the MESSENGER mission is that the magnetic equator is found to be offset 484 km from the geographic equator.131 The electron precipitation flux and energy at Mercury is given in Schriver et al.,132 and the presence of suprathermal electrons in Mercury’s magnetosphere has been observed during MESSENGER flybys.133 Asteroids are typically thought to be too small to hold sufficiently strong magnetic fields that would be impenetrable to the solar wind; however, several have been identified which contain small magnetic fields such as 951-Gaspra.134,135 Meteoroids thought to have originated from the asteroid 4-Vesta indicate that this body may have previously held a magnetic field strong enough to prevent the solar wind from weathering its surface.136 Jupiter’s magnetic field is an order of magnitude larger than the Earth’s, with many of its moons falling inside regions where they are exposed to high fluxes of very energetic particles (20 keV to 100 MeV). So extensive is the magnetic field that high-energy electrons from the Jovian magnetosphere of energy of a few MeV are detected at Earth, for details refer to Jursa.55 Overviews of the magnetosphere and its interaction with the satellites can be found in Cooper et al.137 and Bagenal et al.138 Additional details on the irradiation environment of Europa can be found in Paranicas and co-workers,139,140 Carlson et al.,141 and Johnson et al.142 Ganymede is thought to have more complex interactions with the magnetosphere and solar wind due to the presence of its magnetosphere, which was detected during the Galileo mission.143 Details of the plasma interactions with the surface of Ganymede are given in Khurana et al.144 and Allioux et al.145 Saturn has the second largest magnetosphere in the solar system next to Jupiter. Prior to the Cassini mission, our knowledge of the plasma flux impacting the satellites was derived from data collected with the Pioneer 11 and Voyager 1 spacecraft, surmised by Delitsky and Lane.146 The Cassini spacecraft has recently greatly improved our knowledge of the magnetosphere of Saturn, as reviewed by Gombosi.147 Paranicas et al.148 give values of the energy dependent fluxes of electrons for the satellites of Saturn and show that 10 keV to 10 MeV electrons primarily hit on the trailing hemisphere of Enceladus. Most of the information derived on the magnetospheres of Uranus and Neptune has been derived from the single passage of the Voyager 2 spacecraft.149−152 The magnetic fields of Uranus and Neptune are tilted by 59° and 47° with respect to their planet spin axis. Their magnetospheres are thought to be quite similar to the Earth’s. Although they are larger, extending upstream to approximately 25 planetary radii in both cases, they are thought to be more quiescent.118,119 3.3. Physical Distribution of Dust and Meteoroids

The solar system is a very dusty place, particularly within the inner solar system. This can be appreciated by those whom have seen the zodiacal light, scattered sunlight in the ecliptic plane best observed close to sunrise or sunset where it can be seen near the horizon. Nesvorný et al.153 recently attributed approximately 85% of this dust as originating from Jupiter family comets (JFCs) with