Interplanetary Dust Particles
Summary and Keywords
Scattered sunlight from interplanetary dust particles, mostly produced by comets and asteroids, orbiting the Sun are visible at dusk or dawn as the Zodiacal Cloud. Impacts onto the space-exposed surfaces of Earth-orbiting satellites indicate that, in the current era, thousands of tons of interplanetary dust enters the Earth’s atmosphere every year. Some particles vaporize forming meteors while others survive atmospheric deceleration and settle to the surface of the Earth. NASA has collected interplanetary dust particles from the Earth’s stratosphere using high-altitude aircraft since the mid-1970s. Detailed characterization of these particles shows that some are unique samples of Solar System and presolar material, never affected by the aqueous and thermal processing that overprints the record of formation from the Solar Protoplanetary Disk in the meteorites. These particles preserve the record of grain and dust formation from the disk. This record suggests that many of the crystalline minerals, dominated by crystalline silicates (olivine and pyroxene) and Fe-sulfides, condensed from gas in the inner Solar System and were then transported outward to the colder outer Solar System where carbon-bearing ices condensed on the surfaces of the grains. Irradiation by solar ultraviolet light and cosmic rays produced thin organic coatings on the grain surfaces that likely aided in grain sticking, forming the first dust particles of the Solar System. This continuous, planet-wide rain of interplanetary dust particles can be monitored by the accumulation of 3He, implanted into the interplanetary dust particles by the Solar Wind while they were in space, in oceanic sediments. The interplanetary dust, which is rich in organic carbon, may have contributed important pre-biotic organic matter important to the development of life to the surface of the early Earth.
The Zodiacal Cloud is seen from dark areas of the Earth as a roughly triangular glow on the horizon just before sunrise or right after sunset (Figure 1). This glow results from sunlight scattered by interplanetary dust particles (IDPs) that orbit the Sun near the plane of the ecliptic. These particles range from a fraction of a micrometer up to many hundreds of millimeters in diameter. Astronomical measurements of the intensity, spectrum, and polarization of this scattered sunlight indicate that the size-frequency distribution of this interplanetary dust peaks in size at about 60 μm in diameter.
Infrared astronomical measurements are very sensitive to the properties of the dust particles The temperature of the dust particles, determined from the continuum infrared emission spectrum, is consistent with the Zodiacal Cloud being dominated by large (>10 μm radius), low-albedo (reflectivity), rapidly rotating particles. A weak excess in infrared emission in the 9 to 11 μm range, where silicate grains have emission features, indicate the dust contains a mixture of silicates—including amorphous silicate, crystalline olivine, and a hydrous silicate (clay). While large particles dominate the size distribution some small particles (radii ~1 μm) are also needed to match the silicate emission feature. The present mass of the interplanetary dust making up the Zodiacal Cloud is estimated to be about 1019 grams, which is roughly equivalent to the mass of a single 25 km diameter asteroid.
Interplanetary dust particles do not survive in space for very long compared to the 4.5 billion year age of our Solar System. They can be ejected by gravitational encounters with Jupiter, destroyed by evaporation if the pass near the Sun, or physically shattered by collisions with other dust particles. But the most significant limit on their lifetimes results from their interaction with solar radiation. The smallest particles, generally less than 1 μm in diameter, are blown out of the Solar System by solar radiation pressure. Larger particles spiral inwards, towards the Sun, as a result of their interaction with solar radiation, which causes a drag force, called Poynting-Robertson radiation drag (Wyatt & Whipple, 1950). Their lifetimes in the inner Solar System, inside of about five astronomical units, are estimated to be only 10,000 to a few 100,000 years, so these particles must have resided in a much larger parent body for most of the 4.5 billion year age of the Solar System. Modeling of the loss rate indicates that more than 10,000 kg of dust must be added every second to maintain the Zodiacal Cloud in a steady state (Nesvorny et al., 2011).
Sources of the Interplanetary Dust
Comets are a well-established source of dust emission, since their dust trails are frequently visible to the eye. Since dust is most easily detected by its infrared emission, infrared space observatories are used to detect and characterize other sources of interplanetary dust. The Infrared Astronomical Satellite (IRAS), the first space telescope to perform a survey of the entire night sky at infrared wavelengths, detected the dust trails emitted by comets, but the IRAS also found bands of dust associated with families of asteroids located in the asteroid belt between Mars and Jupiter. This asteroidal dust was produced by the collisional fragmentation of larger asteroids, and smaller cratering impacts also generate dust from asteroid surfaces.
Dust detectors on spacecraft have been used to determine the impact rate of interplanetary dust as these spacecraft pass through the Solar System. Six spacecraft have carried dust detectors beyond the asteroid belt—Pioneers 10 and 11, Galileo, Ulysses, Cassini, and New Horizons. In each case there was no significant drop in the dust impact rate after the spacecraft passed through the asteroid belt. Since the dust would spiral inward, the lack of dropoff indicates that asteroids are not the major contributor to the Zodiacal Cloud in the present era. The Galileo and Ulysses spacecraft never went beyond the orbit of Jupiter and Cassini never went beyond Saturn. However, the Pioneers and New Horizons spacecraft probed far into the outer Solar System, out to 18 astronomical units for the Pioneer spacecraft and past the orbit of Pluto for New Horizons. The dust counters on the Pioneers and New Horizons each detected significant dust fluxes far into the outer Solar System, consistent with about 900 kg/s of IDPs being produced by activity in the Kuiper Belt, the region beyond Neptune that is the source of short period comets. However, much of the dust produced in the Kuiper Belt is ejected from the Solar System by gravitational encounters with the gas giant planets, so it never reaches the Earth.
The Galileo, Ulysses, and Cassini spacecraft also detected a significant flux of small (about 0.1 to 0.5 μm in diameter) interstellar particles, grains moving through the Solar System as a result of the Sun’s motion through the local interstellar medium.
Combining infrared and spacecraft measurements provides as assessment of the dust production rate and the relative proportions of the contributions from various sources. Recent attempts to fit to all this data to a self-consistent model have been presented by Nesvorney et al. (2011), Poppe (2016), and Carrillo-Sanchez, Nesvorný, Pokorný, Janches, and Plane (2016), who each show that more than 85% of the dust originates from Jupiter Family Comets, short period comets believed to originate in the Kuiper Belt. Their modeling of the orbital evolution of the IDPs produced by the Jupiter Family Comets indicates that the majority of these particles that encounter the Earth enter the atmosphere at speeds less than 15 km/s, which should allow many of them to survive atmospheric deceleration intact. These models indicate that less than 15% of the dust is from asteroids and from dust produced by collisions in the Edgeworth-Kuiper Belt of objects orbiting beyond Neptune. The contributions from long period comets, like Comet Halley, and interstellar grains are also small compared to the mass of dust contributed by the Jupiter Family Comets and the asteroids.
A class of “primitive” meteorites, called the carbonaceous chondrites, have compositions very similar to the solar composition for the non-volatile, rock-forming elements (Anders & Grevesse, 1989; Lodders, 2003). This composition is called “chondritic.” Comets were expected to contain unprocessed material preserved since Solar System formation, thus having a chondritic composition for the non-volatile elements. The record of Solar System formation has been erased in the Earth, due to planetary differentiation, and modified by parent body aqueous and thermal processing in most or all meteorites. So comets are believed to be a treasure of unprocessed material, preserving in cold storage the record of Solar System formation.
In situ analyses of cometary dust were performed at Comet 1P/Halley by the European Space Agency’s Giotto spacecraft, and the VEGA-1 and VEGA-2 spacecraft flown by the USSR. Each carried a time-of-flight mass spectrometer, which analyzed the elemental composition of the impacting dust. The bulk composition of the refractory, rock-forming elements in Comet 1P/Halley dust was, within the experimental uncertainty of factor 2, consistent with the chondrite composition (e.g., Jessberger, Christoforidis, & Kissel, 1988). The major components of this dust were Mg-rich silicates, Fe-sulfides, and cartbonaceous matter. Some particles, called CHON particles, showed high levels of the elements C, H, O, and N, indicative of organic compounds,
The European Space Agency’s Rosetta spacecraft orbited Comet 67P/Churyumov–Gerasimenko, collecting dust that was also analyzed using a time-of-flight mass spectrometer. Bardyn et al. (2017) reported that the elemental abundances was generally within a factor of 3 of chondritic for those elements analyzed, with a significant overabundance of carbon being an exception. They concluded the 67P/Churyumov–Gerasimenko dust consisted of mostly anhydrous silicates and almost 50% macromolecular cartbonaceous matter.
Only NASA’s Stardust spacecraft has collected and delivered to the Earth samples of a comet, the Jupiter Family Comet 81P/Wild 2. This dust was analyzed in terrestrial laboratories using sensitive, high-precision instruments, providing more comprehensive analyses than were possible with instruments that can be carried on spacecraft. The mean elemental composition of these particles was found to be within 35% of chondritic for the major rock-forming elements Mg, Si, Mn, Fe, and Ni (Flynn et al., 2006), but carbon could not be quantified because of significant carbon contamination in the collection material. In addition, heating caused by the 6 km/s collection speed, as the Stardust spacecraft traversed the coma of the comet, mobilized the more volatile elements and destroyed many of the grains smaller than 1 μm. The mineralogy of the 81P/Wild 2 particles was dominated by olivine and low-Ca pyroxene, both of which spanned a wide range of Fe-contents and Fe-sulfides (Zolensky et al., 2006).
Collection of interplanetary dust incident on the Earth provides an important opportunity to investigate the refractory component of the comets using high-precision laboratory instruments without the complications introduced by the high collection speed employed by Stardust.
Collection of Interplanetary Dust at Earth
The Earth orbits through the Zodiacal Cloud, sweeping up the IDPs in its path. These particles are accelerated by Earth’s gravity, entering the atmosphere at speeds greater than 10 km/s. Most particles larger than a grain of sand are so severely heated as they decelerate by impacting gas molecules in the atmosphere that they vaporize, producing streaks of light, or meteors, visible in the night sky. But some small IDPs do not completely vaporize on atmospheric entry but rather melt, producing spherical droplets that cool and settle to Earth. These particles, called cosmic spherules, were first collected from sea sediments during an oceanographic expedition of the HMS Challenger conducted from 1873 to 1876.
More recently, similar particles called micrometeorites have been collected from the Polar regions, where terrestrial contamination is low. These micrometeorites generally range from 50 μm to 2 millimeters in size. Like the sphereules recovered on the HMS Challenger expedition, most of these micrometeorites are either partially melted or completely melted and partially vaporized by frictional heating during atmospheric deceleration (Taylor et al., 2017). The abundances of the major, rock-forming elements are present in chondritic proportions in many of these micrometeorites, but more volatile elements, including Na, S, and C, are present in lower abundances, suggesting loss by entry heating. One group, called ultracarbonaceous micrometeorites, is composed mostly of organic matter, possibly similar to the CHON particles identified in comets. These Polar collections, as well as a renewed collection of micrometeorites from the seafloor, constitute the largest mass of interplanetary dust collected annually. However, the severe heating experienced by most of the micrometeorites overprints the record of early Solar System processes.
By 1937 Ernst Opik had recognized that the ratio of surface area, which controls the rate of radiational cooling, to cross-sectional area, which controls the rate at which heat is added by collisions, increases as particle size decreases, allowing particles in the micrometer size range to survive atmospheric entry without significant heating. These particles decelerate at altitudes of 70 to 100 km above the surface, experiencing a heating pulse that lasts a few seconds.
Unlike the Stardust spacecraft, which collected comet dust at a speed of 6 km/s, stopping most particles in a distance of less than one centimeter, resulting in the destruction of most grains smaller than a micrometer in diameter, Earth’s atmosphere decelerates incoming interplanetary dust over distances of 10 kilometers or more. Each IDP is heated during its deceleration in the Earth’s atmosphere. The peak temperature reached by any individual particle depends on the size, density, entry speed, and entry angle of the particle. The detailed model of particle heating during atmospheric deceleration was developed by Fred Whipple (1950) in the early 1950s. Whipple’s modeling indicated that most IDPs around 10 micrometers in diameter would survive entry, some having minimal heating above their in-space temperature. More recent computer simulations of atmospheric deceleration of IDPs indicate that about half of all particles larger than 70 μm are melted and less than 1% of the particles larger than 300 μm survive entry (Love & Brownlee, 1991).
Once it was recognized that the small dust particles would survive atmospheric entry, the search was on. Since the Earth’s surface is covered with terrestrial dust, these efforts focused on collection of dust high in the atmosphere, using rockets, balloons, and aircraft. In the early 1960s investigators at the Dudley Observatory in Albany, New York, flew the Sesame series of collectors, each consisting of eight 1” × 2” slides designed to collect particles dropping onto the slide surfaces, placed on top of balloons, to minimize contamination by terrestrial dust carried aloft by the balloon. They also launched collectors carried aloft by sounding rockets. These collections were called the Venus Flytrap because the collectors opened briefly near the high point of the flight, where the amount of terrestrial dust was minimized. But there is terrestrial dust even at high altitudes, produced by solid fueled rocket exhaust, or transported upwards by volcanic eruptions, severe thunderstorms, and above-ground nuclear explosions. So, even though the Sesame and Venus Flytrap experiments collected dust, the collections were plagued by significant contamination of terrestrial particles, and there was not widespread acceptance that any specific particle in these early collections was extraterrestrial.
The initial identification of IDPs relied on differences between the chondritic composition of unprocessed extraterrestrial material and the surface dust on Earth. The Earth is believed to have formed from material having the same composition, for the refractory elements, as the Sun. However, Earth is a differentiated planet, having melted and segregated much of its iron and other elements soluble in liquid iron in its core. Thus chondritic composition of primitive materials is easily distinguishable from that of Earth’s surface dust, which is depleted in Ni, Ir, and several other elements compared to chondritic. For example, the chondritic abundance of Ni is about 1%, while the mean abundance of Ni is two orders of magnitiude lower in terrestrial crustal rocks, since much of the Ni is sequestered in the Earth’s core.
The introduction of Scanning Electron Microscopes with energy dispersive x-ray analysis, a technique that identifies the atoms making up a sample by exciting the electrons in the sample and detecting the element-specific x-rays emitted as the electrons return to the ground state, provided the opportunity to determine the major element contents of particles only a few micrometers in size. This technique allowed the elemental characterization of the small IDPs that were modeled to survive atmospheric entry, allowing the first compelling identification of dust particles having a chondritic composition.
The routine collection of IDPs from the Earth’s stratosphere began in the early 1970s. The first successful collection from the stratosphere used a high-altitude balloon, flying about 35 km above the Earth’s surface, where the abundance of terrestrial dust is low. The balloon carried a large vacuum used to suck in air and deposit it on a sample collector. A few of the particles collected by balloon matched the chondritic composition.
In the mid-1970s NASA began an interplanetary dust collection program using a U-2 high altitude research aircraft flying from the NASA Ames Research Center. The collection effort was transferred to the NASA Johnson Space Center in 1981 and has expanded to include U-2, ER-2, and WB-57 aircraft flying at altitudes of 18 to 20 km. Flat plastic collector surfaces covered with silicone grease, which remains fluid at the cold temperature of the collection altitude, are deployed into the airstream once the aircraft reaches the collection altitude, allowing particles to impact and stick to the collector surface. After tens of hours of aircraft flight a typical collector is removed from the aircraft and curated (Brownlee, 1985). Even at the 20 km collection altitude there are still terrestrial particles. Experience indicates that the chondritic particles are generally opaque, so the collector is viewed under a microscope in transmitted light and dark particles are picked from the collector surface using a glass needle. The particles are transferred to a mount suitable for examination in a Scanning Electron Microscope, washed with hexanes to remove the silicone grease, then imaged and analyzed in the electron beam.
Based on their composition, the particles are classified as cosmic (or extraterrestrial), natural terrestrial contaminants (typically dust from the Earth’s surface or injected into the atmosphere by volcanic eruptions), or manmade terrestrial contaminants (typically exhaust from solid fuel rocket engines). Element to Si ratios are generally used to determine the composition of rocks. In the case of the IDPs, the possibility of residual silicone oil on their surfaces requires classification based on element to Fe ratios instead. Catalogs of the particles are prepared at the Johnson Space Center, allowing researchers around the world to request specific particles for more detailed analyses.
While the chondritic composition made it likely the particles classified as cosmic were interplanetary dust from the Zodiacal Cloud, compelling evidence required a clear demonstration that these particles had been in space. The first definitive evidence that some particles were the long-sought IDPs was provided by measurement of the amount of deuterium, a heavy isotope of hydrogen having a neutron as well as a proton in its nucleus. Deuterium is very rare, compared to the abundance of normal hydrogen, in natural terrestrial materials. Measurements showed much higher levels of deuterium in several particles, identifying them as extraterrestrial particles (Zinner, McKeegan, & Walker, 1983).
Other techniques provided further evidence of the extraterrestrial origin of the chondritic particles. The Sun is active, constantly emitting low-energy charged particles as the Solar Wind and sporadically emitting higher energy, charged particles from Solar Flares. These charged particles are deflected by the Earth’s magnetic field, so they are not seen in terrestrial dust. Researchers sought evidence that the chondritic particles from the collectors had been exposed to the Solar Wind or to the Solar Flares, using techniques developed to study samples from the Lunar surface collected by the Apollo missions to the Moon. Detection of implanted atoms He, Ne, and Ar in similar proportions to those in the Solar Wind, released upon heating of chondritic particles, provided further evidence that the chondritic particles were samples of the Zodiacal Cloud. This was followed by the identification of radiation damage, called nuclear particle tracks, in crystalline minerals in the chondritic particles caused by higher energy Solar Flares. By the early 1980s it was clear that the NASA stratospheric collections were yielding the long-sought samples of the interplanetary dust.
In the past decade NASA has conducted short-duration stratospheric collections specifically timed to coincide with the Earth’s passage through the intense dust stream produced by a cometary outburst or disruption. The first collection, in 2003, targeted dust from comet Grigg-Skjellerup. A second collection targeted dust from comet Giacobini-Zinner following an intense meteor storm that was detected by radar. Although particles from the target source are mixed on the collector with the background of interplanetary dust from other sources, these targeted collections serve as relatively inexpensive sampling missions compared to the cost and complexity of spacecraft sample collection.
Characterization of the Interplanetary Dust Particles
The elemental, mineralogical, and isotopic compositions of interplanetary dust collected from the Earth’s stratosphere and larger particles collected from the polar ices have been measured since the 1970s. In most cases these particles are quite different from the meteorites, which had previously been the only samples, other than Lunar rocks and soils collected by NASA’s Apollo missions and the Luna spacecraft flown by the USSR, available for laboratory analysis.
The smallest particles, ranging from a few micrometers up to 25 to 50 μm in diameter, collected from the stratosphere and those particles in the smallest size fraction of the Polar micrometeorites, are the least severely heated IDPs. Since the typical IDP is only about 10 μm in size and weighs only a few nanograms, several of them would fit across the width of a single human hair, and their analyses required development of new microanalysis techniques.
Two types of chondritic IDPs were easily distinguished. One group consists of highly porous, fluffy particles consisting of mostly anhydrous mineral grains. These particles are called chondritic porous IDPs. The second group consists of more compact particles, called chondritic smooth IDPs, and are dominated by hydrous, or clay, minerals. An intermediate type of particle, with some hydrous minerals in an otherwise anhydrous particle, occurs less frequently. Examination at high spatial resolution, using Transmission Electron Microscopes, showed that neither group was identical in structure or mineralogy to any type of meteorite. In addition to these chondritic particles, larger crystalline grains, from a few micrometers up to 20 μm, are found on the collectors. Many of these larger crystalline grains have small amounts of fine-grained, anhydrous chondritic material adhering to their surfaces, suggesting they sample the same parent body as the chondritic porous IDPs.
A single ~10 μm chondritic porous IDP (Figure 2) is typically an aggregate of tens of thousands of individual grains that are weakly held together. This aggregate structure is very similar to that seen in particles emitted by comet 67P/Churyumov–Gerasimenko (Bentley et al., 2016), a Jupiter Family Comet examined in detail by the Rosetta spacecraft. This similarity is consistent with the modeling that indicates most of the IDPs are emitted by Jupiter Family Comets.
The individual mineral grains in the chondritic porous IDPs are mostly silicates, generally olivine and pyroxene, and Fe-sulfides, each of which is commonly found in meteorites. However, the olivine and pyroxene grains found in meteorites generally have a narrow range of compositions, typically being very Mg-rich. Olivine can have a range of compositions from pure Mg2SiO4, called forsterite, to pure Fe2SiO4, called fayallite. Similarly, pyroxene can range from MgSiO3 to pure FeSiO3.The silicates in most meteorites are equilibrated; that is, in any specific meteorite the proportions of Mg and Fe in all of the olivines or all pyroxenes are quite similar (Figure 3). Unlike the meteorites, the olivines and the pyroxenes in most chondritic porous IDPs are unequilibrated, spanning a wide range of Mg to Fe ratios (Figure 3). The unequilibrated nature of these particles indicates that they have never been exposed to the high parent body temperatures experienced by most meteorites.
The isotopic ratios of H, N, and O have proven diagnostic of the origins and thermal evolution of geological materials. Each element has a relatively narrow range of isotopic ratios in Solar System materials. However, grains that formed in different environments, such as novae, supernova, asymptotic giant branch stars, and red giants, have distinctly different isotopic ratios characteristic of their specific source (Nittler, 2003). Because O is the most abundant element in most planetary materials, its isotopic composition has proven useful in distinguishing different types of meteorites and identifying individual grains in the meteorites that are of extrasolar origin.
The chondritic porous IDPs contain two phases not generally found in meteorites: amorphous (noncrystalline) silicate grains, typically a few hundred nanometers in diameter, having small inclusions of metal and sulfide, called GEMS (Glass with Embedded Metal and Sulfide), and rare, whisker-like crystals of enstatite that have characteristics indicative of direct condensation from a gas.
The GEMS are the dominant silicate in some chondritic porous IDPs. The crystalline silicates observed in chondritic porous IDPs also show marked similarities in terms of mineralogy, size, composition, and abundance to those observed forming around young stars and in comets through astronomical infrared spectroscopic measurements, but similar processes of grain formation likely occurred early in the history of our Solar System.
The GEMS have been extensively studied over the past two decades. Infrared spectroscopy showed a good match between the 10 micron silicate stretching feature of GEMS and a feature attributed to a phase called “astronomical silicate.” This astronomical silicate feature had not previously been matched by any naturally occurring material, but GEMS matched this feature in two molecular clouds as well as in the regions surrounding a T-Tauri star and a post-main sequence M star (Bradley et al., 1999). In addition, oxygen isotopic measurements established that a few percent of the GEMS grains exhibit isotopic anomalies inconsistent with Solar Nebula material and consistent with origins from AGB stars, supernova, and low metallicity stars (Keller & Messenger, 2011). However, Keller and Messenger found that most GEMS have element/Si ratios too low to match the interstellar grain composition, indicating that most GEMS are condensates from our own Solar Nebula. The incomplete removal of silicone oil by the hexane wash used to clean IDPs has long been recognized as a problem, resulting in most IDP elemental analyses being reported as element/Fe ratios rather than element/Si ratios. Bradley (2013) suggested that about 30% of Si in GEMS might be from silicone oil and, and once the silicone oil contamination is accounted for, GEMS would match the interstellar silicate abundance pattern. Thus, the origin of the GEMS remains unresolved.
A few rare grains of crystalline silicate (olivine or pyroxene) as well as a few of the amorphous GEMS in the chondritic porous IDPs have oxygen isotopic ratios well outside the solar range, identifying them as grains that formed around other stars that survived incorporation into the Solar System without equilibration (Messenger, Keller, Stadermann, Walker, & Zinner, 2003). Comparison of the specific isotopic ratios with modeling of the conditions in various types of stars can identify the specific type of source for each isotopically anomalous grain. Messenger et al. identified three 17O-rich grains that appear to originate from red giant or asymptotic giant branch stars, while one 16O-rich grain may be from a metal-poor star. These few grains demonstrate that some of the grains of the Solar Protoplanetary Disk, presumably in the outer, cooler region of the disk, where the grains were never homogenized, allowed these rare grains to preserve isotopic signatures of their sources.
Scanning Electron Microscope energy dispersive x-ray analysis of more than 100 chondritic porous IDPs demonstrated that their average composition was within 10% of the chondritic values for the major, rock-forming elements, although C was found to be enriched by a factor of 3 over the chondritic abundance (Schramm, Brownlee, & Wheelock, 1989). Analysis of minor and trace elements in these nanogram-mass particles required more sensitive techniques including neutron activation analysis, proton induced x-ray emission, and synchrotron x-ray fluorescence. Analyses by these techniques showed chondritic abundances of the refractory minor elements, but moderately volatile minor elements including P, K, Na, Cu, Zn, Ga, Ge, and Se were each enriched over chondritic by about the same factor 3 as previously reported C (Flynn et al., 1996). However, the moderately volatile major element S showed no enrichment over chondritic. A variety of suggestions were advanced to explain the enrichment of moderately volatile elements over the Solar System abundances. At one extreme it was suggested that these particles were “late stage nebular condensates,” having formed after some more refractory, high-temperature phases had condensed and been removed from the region where the chondritic porous IDPs aggregated into dust. At the other extreme it was suggested that moderately volatile elements volatilized from meteors that had recondensed in the atmosphere onto the surfaces of the chondritic porous IDPs.
The meteorites, which consist of large crystalline mineral grains embedded in a porous, fine-grained matrix, provide a clue to the reason for the enrichment of moderately volatile elements in the chondritic porous IDPs. The matrix of the meteorites is volatile rich while the crystalline minerals are volatile poor, with the two adding together to give the chondritic composition. This suggested that the elemental composition of the parent body of the IDPs would be better determined by sampling the parent body at a larger size scale.
Larger particles, too weak to survive impact with the aircraft collectors, are found on the collectors as an isolated cluster of fragments. These “cluster interplanetary dust particles,” which would be up to about 100 μm in size if all the fragments were reassembled into a single particle, were first characterized in a consortium study by Thomas et al. (1995). Because these cluster particles included both fine-grained material and larger individual mineral grains, they cautioned against interpreting the properties of individual 10 to15 μm chondritic porous IDPs as representative of their parent bodies.
When examined by the same techniques employed to characterize the 10 μm IDPs, most cluster IDPs are generally highly porous, consisting of fine-grained material like the chondritic porous IDPs and larger crystalline mineral grains. The inclusion of larger crystals of volatile-poor olivine and pyroxene as well as larger sulfides results in an elemental composition of these cluster IDPs that is very similar to the chondritic composition, deviating by less than 30% from chondritic in an analysis of five large cluster IDPs. This is consistent with the chondritic porous IDP parent body having formed from a reservoir of solar composition. The overall chondritic composition of the large cluster IDPs indicates that they consist of a mixture of about 25% by mass fine-grained, volatile-rich chondritic porous interplanetary dust matrix mixed with about 75% larger, volatile-poor larger mineral grains. Since the mineral grains are significantly denser than the matrix, the volume fraction of matrix is much higher.
The heating pulse experienced during atmospheric deceleration can erase the solar flare tracks in silicate minerals and cause the loss of the Solar Wind noble gases and some volatile elements such as Zn and S. The Solar Wind implants a significant amount of He in the surface of each particle. Step-heating a particle and determining the temperature at which He is first released provides an indication of the peak temperature of outgassing reached during atmospheric deceleration (Nier & Schlutter, 1993). In addition, the mineral magnetite forms on the surface of an IDP that experienced significant heating. By measuring one or more of these properties, a subset of the chondritic porous IDPs that experienced little thermal alteration on atmospheric deceleration has been identified. This group of minimally altered particles has been well studied to characterize their properties and infer the conditions under which they formed.
Our Solar System is believed to have formed from a rotating disk of gas surrounding the forming Sun. Solid grains are believed to have formed from this Solar Protoplanetary Disk by direct condensation from the gas cloud. The identification of the enstatite whiskers in chondritic porous IDPs indicates that this process of direct condensation from the gas phase occurred and that the chondritic porous IDPs sample the products of this condensation.
The composition of this gas disk is expected to have been the same as that of the Sun, since the Sun contains more than 99% of the mass of the Solar System. The composition of the Sun has been measured by spectroscopy of the solar photosphere, the Sun’s outer shell from which light is radiated. After the Sun formed, this gas disk cooled allowing mineral grains to condense. Modeling of the formation products from a cooling of a gas of solar composition at a total pressure of 10-4 bar, taken to represent the gas pressure in the disk, has been used to predict the temperatures at which specific minerals condense (Lodders, 2003). The first minerals to form are Ca-, Al-, and Ti-oxides, which condense at around 1800 K, with Mg-silicate minerals condensing at around 1350 K, and sulfides forming around 700 K as S in the gas reacts with the previously condensed Fe-metal. Other minor minerals are also formed.
Modeling of the Solar Protoplanetary Disk indicates that the gas was hot in the inner Solar System, but too cold outside of Jupiter’s orbit, where the comets formed, to allow the formation of olivine and pyroxene. If the majority of the interplanetary dust comes from Jupiter Family Comets, these crystalline silicates must have been transported outward from the inner solar system to the region where the comets formed, a region cold enough to permit condensation of water and carbon dioxide ices, which are the most abundant ices detected in these comets. Confirmation of this transport comes from the dust collected directly from the coma of comet 81P/Wild 2 by NASA’s Stardust spacecraft. These Wild 2 particles contained Ca-Al-rich minerals, as well as olivine and pyroxene crystals, demonstrating grain transport from the inner Solar System to the region where the comets were assembled (Brownlee et al., 2006).
The individual grains in the chondritic porous IDPs are a mixture, on the submicrometer scale of a diverse variety of minerals. Since the chondritic porous IDPs appear to have experienced minimal processing after formation (Ishii et al., 2008), the presence of Fe-sulfides intermixed with the silicates demonstrates that grain aggregation only took place after the Solar Protoplanetary Disk had cooled below the Fe-sulfide formation temperature, since aggregation at an earlier time would have resulted in clusters of silicates without the Fe-sulfides. This suggests that grain aggregation was inhibited in the time when the silicate grains formed but became possible much later, after the grains of the disk had cooled below the Fe-sulfide formation temperature.
The chondritic porous IDPs themselves provide evidence for grain transport from their formation region in the inner solar system to the location where grain aggregation into these dust particles occurred. These particles vary from very fine grained to coarse grained. Since silicates and Fe-sulfides differ in density, aerodynamic transport would result in size sorting of the grains. Examination of several chondritic porous IDPs spanning the range of grain sizes showed that the relative sizes of silicates and Fe-sulfides are consistent with aerodynamic transport (Wozniakiewicz, Bradley, Ishii, Price, & Brownlee, 2013).
In many chondritic porous IDPs the grains are not in direct contact with one another. The individual grains are coated with a very thin layer, only 50 to 200 nanometers thick, of organic matter (Figure 4; Flynn, Wirick, & Keller, 2013). Modeling shows that in the cold outer solar system, the irradiation by solar ultraviolet light or cosmic rays of carbon-bearing ices that condense onto the grain surfaces can leave residues of organic matter when these grains warm to the ice vaporization temperature (Ciesla and Sandford, 2012). These organic coatings likely cushioned the collisions between grains and served as a glue to aid in grain aggregation in the outer Solar Protoplanetary Disk (Flynn et al., 2013).
Since the chondritic porous IDPs are believed to sample the direct condensates from the Solar Protoplanetary Disk, conditions very different from the formation conditions of terrestrial minerals, they would be expected to yield some minerals that do not occur naturally on Earth. A 4 μm diameter particle from the Grigg-Skjellerup timed collection contained the first three small grains, the largest only 600 nanometers across, of a new Mn-silicide mineral, identified by a group at NASA’s Johnson Space Center (Nakamura-Messenger et al., 2010). This mineral was named Brownleeite, after the University of Washington astronomer Donald Brownlee who pioneered interplanetary dust collection from the Earth’s stratosphere in the 1970s and who was the principal investigator on the Stardust mission that delivered the first cometary dust samples to Earth. Like the enstatite whiskers, Brownleeite is believed to have condensed directly from the Solar Protoplanetary Disk. Particles from the Grigg-Skjellerup timed collection also show unusually high abundances of presolar silicates (~1%) identified by high 17O isotopic enrichments (Busemann et al., 2009).
The hydrous IDPs are generally more compact with relatively smooth surfaces. These particles are dominated by hydrous silicates, carbonates, and Fe-sulfides, similar to the phases found in the hydrous meteorites. The hydrous IDPs likely originated by the aqueous parent body processing of the anhydrous chondritic porous IDPs. Evidence for this comes from the identification of intermediate particles looking like the chondritic porous particles but containing a few hydrous grains. In those hydrous particles where the aqueous alteration was not complete, some olivine and pyroxene crystals remain. Like the anhydrous IDPs, these olivines and pyroxenes are unequilibrated, spanning a wide range of compositions.
Scanning Electron Microscopic energy dispersive x-ray analysis of about 100 chondritic smooth IDPs showed their average elemental composition to be approximately chondritic for all the major rock-forming elements except Ca. The Ca depletion suggests Ca leaching by the fluid during parent body aqueous alteration (Schramm et al., 1989). The average C content is about twice that of the most carbon-rich chondritic meteorites. Trace elements in the chondritic smooth particles are also present at approximately the chondritic abundance levels, rather than showing the enrichments found in the chondritic porous particles.
Like the hydrous carbonaceous chondrite meteorites, the chondritic smooth IDPs are dominated by hydrous minerals, but they differ in significant ways from hydrous meteorites. Most striking is the type of hydrous minerals that are found in the particles. The hydrous IDPs are dominated by smectite clay, rather than the saponite and montmorillinite clays that dominate in the hydrous carbonaceous chondrite meteorites. Generally, the chondritic smooth IDPs are not fully hydrated. Rather they include silicates and sulfides, many similar to the ones in the chondritic porous IDPs, consistent with the formation of the chondritic smooth IDPs from chondritic porous IDPs by aqueous processing on a parent body. A few, rare chondritic smooth IDPs do have the same clay minerals found in the hydrous carbonaceous chondrite meteorites, suggesting these might be fragments of the meteorite parent bodies (Bradley & Brownlee, 1991; Keller, Thomas, & McKay, 1992).
The differences in mineralogy between the carbonaceous chondrite meteorites and most hydrous IDPs indicates that these dust particles provide the opportunity to probe aqueous processing under different conditions (e.g., fluid composition or temperature) than was experienced by the hydrous meteorites. If, as expected from modeling of the sources of interplanetary dust, many of these hydrous particles are from Jupiter Family Comets, the presence of hydrous minerals indicates that some comets experienced significant aqueous processing, consistent with the observation of some hydrous grains in the dust generated in the cratering of Comet 9P/Tempel 1 (Lisse et al., 2006). Thus, they provide an opportunity to study aqueous processing on cometary parent bodies, while the meteorites are generally accepted to be fragments of asteroids. However, the hydrous IDPs have been less well-studied than their anhydrous counterparts.
The organic rims of the grains of the chondritic porous IDPs are not the only organic matter in these particles. Both the chondritic porous and the chondritic smooth IDPs have higher contents of organic matter than the most carbon-rich meteorites. The amount of carbon varies dramatically from one to another 10 μm particle, with a few, rare particles consisting of as much as 90%, by volume, carbonaceous material (Flynn, Keller, Jacobsen, & Wirick, 2000).
For larger samples like the meteorites, organic matter is generally analyzed by extracting and concentrating specific classes of compounds and analyzing them by techniques such as gas chromatography, which identify the molecules that are present. Because of their small masses, characterization of this organic matter cannot be done with these traditional methods. Instead, techniques that extract the organic matter by heating, which can break up molecules, or techniques that identify only specific “functional groups,” a portion of a molecule that consists of a recognizable group of bound atoms, within the molecule have been employed.
The first compelling identification of organic matter in the IDPs came by heating the particles with a laser, ionizing the vaporized organic matter using a second laser, and analyzing the resulting ionized molecules with a mass spectrometer (Clemett, Maechling, Zare, Swan, & Walker, 1993). Comparison of the resulting mass spectra to those obtained on meteorites revealed two major differences between the interplanetary dust and the meteorites. The IDPs released ions having a higher mean mass distribution, indicating the presence of larger organic molecules in the interplanetary dust than in the meteorites. In addition, the organic matter released by the IDPs produced strong peaks at odd masses, indicating that the organic matter in the interplanetary dust contained more nitrogen than that found in meteorites (Clemett et al., 1993).
Infrared spectroscopy, used to characterize organic matter extracted from meteorites, has identified C-H functional groups. However, the minimum sample size that can be analyzed by infrared spectroscopy is limited by diffraction to a few micrometers, so, except in rare cases, only whole particles have been analyzed. Even for the C-rich IDPs, the infrared spectrum is typically dominated by the silicate absorption, which is near 10 µm. Aliphatic C-H2 and C-H3 stretching absorptions, which generally indicate the presence of chains of C-H2 terminated by a C-H3, are detected, using highly sensitive synchrotron-based infrared micro-spectrometers, in almost all IDPs. These aliphatic features are commonly seen in meteoritic organic matter. However, aromatic C-H absorption features, indicative of carbon rings with a H atom attached to each C, which are commonly seen in the insoluble organic matter extracted from carbonaceous meteorites, are below the detection limit in most IDPs (Flynn et al., 2003).
Scanning Transmission X-ray Microscopes, developed to visualize the structure of individual cells, focus a low-energy x-ray beam to a spot around 20 nm in size and use this to image the carbon distribution and characterize the carbon, nitrogen, and oxygen functional groups at the nanoscale (Flynn et al., 2003). The carbon in the IDPs shows a wide range of morphologies: the 100 nm thick coatings on the individual grains in the chondritic porous IDPs, as well as larger distributed regions of organic matter up to a few micrometers in size in both types of particles, and discrete 15N-rich nanoglobules in the hydrous IDPs.
Taken together, the functional group analyses are best explained if the organic carbon in the IDPs consists of relatively large aromatic units (linked carbon rings), at least five or six rings in diameter, that are linked by aliphatic chains. The organic matter in the IDPs differs from the interstellar and circumstellar organic matter that has been characterized by astronomical infrared spectroscopy in the relative strengths of the aliphatic CH2 and CH3 absorptions. The organic matter in the IDPs has a higher ration of CH2 to CH3, indicating a longer mean length of the aliphatic chains, which consist of a chain of C-H2 molecules terminated by a CH3 group (Flynn et al., 2003; Flynn, Keller, Wirick, & Jacobsen, 2008). If both types of organic matter originated by the same process, this could indicate the interstellar organic matter has experienced more severe radiation processing, reducing the lengths of the aliphatic chains, than the organic matter in the IDPs.
The Rosetta spacecraft obtained infrared spectra of comet 67P/Churyumov–Gerasimenko and the Dawn spacecraft obtained infrared spectra of the asteroid Ceres, providing opportunities for comparison of the organic spectral features of a Jupiter Family Comet and a large, dark asteroid with the organic matter identified in the IDPs. The spectra of the surface of 67P/Churyumov–Gerasimenko differ from the typical IDP spectra in that the 67P/Churyumov–Gerasimenko spectra have easily detectable aromatic CH, which is below the detection limit in most IDPs. If Jupiter Family Comets like 67P/Churyumov–Gerasimenko are the dominant source of the Zodiacal Cloud, this result likely indicates that the dust on cometary surfaces has experienced significant thermal and radiation processing, altering it from its pristine state in the cometary interior. The IDP spectra are similar to those of organic-rich spots on the asteroid Ceres in that Ceres does not show a detectable aromatic CH signature, but Ceres has a higher ratio of aliphatic CH3 to CH2 absorption than is typical of the IDPs. If the current IDP population is responsible for the aliphatic organic matter detected on absorption in the spectra of Ceres, then some modification to the IDP organic matter either during accretion onto or residence on Ceres would be required.
Effects on Planetary Atmospheres and Surfaces
The flux of interplanetary dust incident on the Earth has been determined by measuring impact craters on the space exposed surfaces of orbiting satellites. The best measurement comes from the Long Duration Exposure Facility (LDEF), a school-bus-sized cylindrical satellite that evaluated the long-term effects of exposure to the space environment. LDEF was placed in orbit in April 1984 and returned to Earth in January 1990. Evaluation of the impacts indicated that, in the current era, about 30,000 tons of interplanetary dust over the size range from a micrometer up to several centimeters in diameter is incident on the top of the Earth’s atmosphere every year (Love & Brownlee, 1993). This flux corresponds to the impact of one 10 μm diameter particle per square meter per day and one 100 μm diameter particle per square meter per year. Cremonese, Borin, Martellato, Marzari, & Bruno (2012) reanalyzed the LDEF impact data using lower speed distributions in accord with the Nesvorny et al., 2011) model having the interplanetary dust dominated by releases from Jupiter Family Comets and found about 4,000 tons/year of interplanetary dust incident on the top of the atmosphere. Even this lower value exceeds the mass of material contributed to Earth by meteorites by more than a factor of 50.
Combining the LDEF flux with estimates of the flux of larger particles, based on the recovered meteorites, the number of impact craters on Earth and the Moon, and measurements of meteors by both Earth-based and space-based observatories, results in a bimodal mass-frequency distribution, dominated by the continuous, planet-wide rain of interplanetary dust and the infrequent impacts of kilometer-sized bodies (Figure 5). This continuous, planet-wide input of interplanetary dust is sufficient to have small but measurable effects on the composition of the Earth’s atmosphere and surface (Plane et al., 2018).
The peak temperature experienced by an IDP during atmospheric deceleration depends on the mass, density, entry speed, and angle of each particle. In general, the smallest IDPs, those less than 25 micrometers in diameter, are not heated to their melting temperature. Most of the larger particles, particularly those larger than a few hundred micrometers in diameter, are melted or vaporized during deceleration in the atmosphere.
Layers of metallic ions, including ionized iron, magnesium, sodium, and silicon, produced by vaporized IDPs were detected by rocket-borne mass spectrometers at altitudes between 75 and 100 km beginning in the 1960s. The sporadic E layer of metallic ions and electrons, occurring above 90 km, plays an important role in radio communications. Neutral sodium, potassium, iron, and calcium layers were detected later by laser-induced resonance fluorescence. The relative abundances of these metal atoms deposited in the upper atmosphere are quite different from their relative abundances in chondritic meteorites, mostly because elements ablate with widely different efficiencies.
The neutral species contributed by meteor ablation recondense into meteoric smoke particles, which slowly settle to the Earth’s surface. These meteoric smoke particles may serve as condensation nuclei for the Polar Mesospheric Clouds, water-ice clouds that form at about 85 km at high latitudes during the summer months, that are observed in the atmospheres of Earth. At a lower altitude, the meteoric smoke particles may serve as sinks to remove acidic vapors, including sulfuric acid and nitric acid, from the Earth’s atmosphere above about 40 km. NASA’s Maven spacecraft detected a similar layer of metallic ions, specifically Mg, Fe, and Na ions, in the ionosphere of Mars (Grebowsky et al., 2017).
The continuous, planet-wide rain of interplanetary dust alters the surface composition of elements or isotopes that are abundant in interplanetary dust but depleted in Earth surface materials. The most striking effect is for iridium, whose concentration is used to monitor the rate deposition of ocean sediments and polar ices. However, the largest enrichments of iridium correspond to the infrequent impacts of large bodies, for example the iridium spike at the Cretaceous-Tertiary boundary, 65 million years ago, that is implicated in the extinction of the dinosaurs (Alverez, Alvarez, Asaro, & Michel, 1980). The most striking effect of the rain of IDPs, which accumulate Solar Wind He while in space, is to perturb the 3He/4He ratio in oceanic sediments (Farley & Patterson, 1995). Farley and Patterson found evidence for a periodicity in the terrestrial interplanetary dust flux of about 100,000 years and suggest that minima in the flux occurred about 100,000; 190,000; 280,000; and 385,000 years ago. The 100,000-year periodicity would suggest that the LDEF measurements were made near another minimum in the interplanetary dust flux.
Planetary differentiation, which melted silicate rock allowing the iron core to settle to the core of the Earth, destroyed any organic matter that accreted with the Earth. Since primitive meteorites contain percent-level organic carbon and the accretion rate of interplanetary dust significantly exceeds that of meteorites, even before the organic content of the IDPs was measured, Anders (1989) suggested that the planet-wide rain of these particles supplied a surface layer of organic matter important for the origin of life to the early Earth. The high concentration of organic matter, present at the several percent level, in the IDPs suggests that in the current era IDPs that survive atmospheric entry without severe heating contribute a significant amount of unpyrolized organic matter to the surface of the Earth (Anders, 1989; Flynn, Keller, Jacobsen, & Wirick, 2004). During the first 0.6 billion years of Earth’s history, when the Lunar cratering record shows a much higher impact rate for larger objects, this contribution is likely to have been much greater.
The contribution of unpyrolized organic matter to the surface of Mars is much greater because the lower surface gravity results in particles entering the atmosphere at slower speeds, allowing larger particles to survive without severe heating (Flynn, 1996). Because the mass-frequency distribution of the IDPs increases steeply over the mass range from 10-9 to 10-4 grams (Figure 5), a small increase in the maximum size of the particles not heated above the pyrolysis temperature results in a large increase in the mass of the IDPs accreted with their organic matter intact.
Interplanetary dust particles impact the surfaces of airless bodies, for example Mercury or the Moon, at speeds of several kilometers per second. These impacts result in the slow erosion of exposed rock surfaces. In addition, these impacts vaporize volatile elements in both the impacting particle and the target rock, giving rise to a transient “atmosphere” of ions, typically the volatile species potassium and sodium (Colaprete et al., 2016).
Interplanetary dust particles are challenging to collect and to characterize, but the chondritic porous IDPs are the rare surviving material produced from the Solar Protoplanetary Disk. Detailed characterization of these particles indicates that many of them never experienced significant parent body thermal or aqueous processing, gravitational compaction, or impact shock, processes that overprint the record of formation from the Solar Protoplanetary Disk in meteorites and terrestrial rocks. The chondritic porous IDPs are the best preserved samples of the original solid products that formed at the beginning of our Solar System (Ishii et al., 2008), retaining much of the record of grain formation and dust aggregation. The structure of these chondritic porous IDPs indicates that crystalline minerals condensed from the Solar Protoplanetary Disk in the inner Solar System, a region where grain aggregation was inhibited, the grains were transported to the cooler outer region of the disk where GEMS and rare surviving pre-solar grains were added to the mix, carbon-bearing ices condensed on the grains and was irradiated, producing an organic coating allowing the grains to aggregate into the first dust of the Solar System, and these grains were incorporated, along with ices, into the Jupiter Family Comets.
Most of the hydrous IDPs differ in mineralogy from the hydrous meteorites, providing an opportunity to study parent body aqueous processing that occurred under different conditions, possibly on cometary parent bodies rather than on the asteroids sampled by the hydrous meteorites. However, the hydrous IDPs have been less well-studied than the anhydrous ones.
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