The Diversity of Interplanetary Dust
Walter van Dijk
September 2020
Abstract
The space between planets in the solar system is all but empty. All solid bodies in the solar system produce dust particles, which populate the interplanetary space. These particles can provide opportunities for research in many fields including the mechanics of the solar system and Earth’s history. They can also pose collision risks for spacecraft and influence the Earth’s climate by blocking solar radiation. Due to their small size the orbits of dust particles are influenced by solar radiation, which makes it possible for the material to travel throughout all of the solar system. Dust particles can be small enough to pass through the Earth’s atmosphere without being vaporised as a result of friction, which makes it possible to collect samples on the Earth itself. The main goal of this review is to provide an introduction to interplanetary dust research and give some insight in the wide variety of disciplines that are involved. To achieve this, I summarise and describe the research on distribution, abundance, collection methods and modelling of interplanetary dust, and discuss potential future developments in the field. The review is focused around investigating the physical characteristics of dust particles and the methods to study them, along with the associated motivations. Studying dust can include a wide variety research areas. Remote sensing of dust can provide insights in their distribution and composition and samples can provide valuable insights properties of their origin object, but also modelling and in-situ experiments are used for studying dust in space. Extraterrestrial dust can provide unique insights in the development of their host object and subsequently provide clues for how our solar system works and other topics considering origin and development of the universe. Interplanetary dust occurs all over the Earth and there is a constant influx of material. However, although it is abundant, it can be difficult to find extraterrestrial dust on Earth because terrestrial materials can be similar. Therefore, terrestrial research usually takes place on locations with limited anthropogenic influence, weathering and erosion rates, such as Antarctica, the deep sea and deserts. Besides analysis on Earth, dust particles are also studied in the upper atmosphere and in space. Dust research has expanded to involve various fields, from paleogeology through astrobiology, which highlights the interdisciplinary nature of dust research. Similarly, there are many different methods for collecting cosmic dust on and around Earth, the results of which complement each other because the obtained samples differ in size distribution and composition. Interplanetary dust research appears to be highly interdisciplinary, and some research questions might only be answered by combining disciplines. One example is the urgent need for the development of methods to reduce the concentration of orbital debris around Earth. This problem is technically complex, but also involves economic and geopolitical aspects that requires collaboration among all these fields.
1
Introduction
Extraterrestrial materials have been of interest to humanity for a long time. They occur throughout history as sacred objects and were often deemed to have religious significance. It wasn’t until the early 19th century however, that scientists
started to accept the hypothesis that rocks falling from the sky had extraterrestrial origins (Mar-vin, 2006). The report on meteorites falling near L’Aigle in 1803 is considered a turning point in the recognition of the extraterrestrial origin of meteorites (Gounelle, 2006). Since then, a great effort has been made studying the composition, origin and implications of extraterrestrial matter. It is not surprising that many scientists are interested in (micro)meteorites. Extraterrestrial matter can provide insight in many questions re-garding the origins and development of the uni-verse, because it resembles the composition of materials that formed planets billions of years ago. This allows for unique insights in big topics such as the formation of stars or the origin of life. The vast majority of extraterrestrial matter that arrives at the Earth is in the form of dust. An estimated 40,000 metric tonnes of dust parti-cles arrive on Earth per day (Love & Brownlee, 1993). This implies that, although meteorites are quite rare, everyone who sets a foot outside will probably come in contact with a fragment of ex-traterrestrial material in the form of cosmic dust. The abundance of dust particles is almost never directly visible to the untrained eye, but the particles can have major implications for our understanding of cosmological and terres-trial phenomena. They are involved in the de-velopment of solar systems and how light travels through space. As a result, the particles can in-fluence climate and interfere with cosmological observations. Besides these natural phenomena dust particles can also pose a danger to space-craft due to their high speed. Even some extinc-tion events might be related to changes in the amount of cosmic dust particles Earth encoun-ters (Kataoka et al., 2013). Consequently, it is of interest to develop methods to obtain and iden-tify these dust particles and improve our insight in their behaviour.
Dust in the interplanetary medium is pro-duced from a wide variety of host objects. All solid bodies in our solar system can inject dust into space (Carrillo-S´anchez et al., 2016), and some of the dust in our solar system even has interstellar origins (Talbot Jr & Newman, 1977), however the most common sources of interplane-tary dust are asteroids and comets.
The aim of this literature review is to sum-marise the current knowledge about dust in the interplanetary medium and provide an overview of the various ways it is studied. This includes the abundance and distribution of interplanetary dust, physical collection, in-situ measurements, modelling and a discussion of the most impor-tant associated phenomena such as the Zodiac Light and the interstellar component. The de-velopment of methods to collect, analyse and ob-serve interplanetary dust particles (IDPs), as well as some promising new technologies that can aid this field of study are discussed as well.
The research questions to be investigated in this review are:
1. What does the life cycle of IDPs look like?
2. How do IDPs populate our solar system?
3. In which ways are IDPs studied?
4. What are the reasons to study IDPs?
I aim to provide a comprehensive overview of the field of dust research, understandable even without prior knowledge of cosmic dust. This pa-per is based on three key review articles (Koschny et al., 2019; Gr¨un et al., 2019; Nesvorn`y et al., 2010) and references therein. Some words and terms essential to the field might be unfamiliar to readers without background knowledge, therefore a glossary is provided, and the included words and terms are printed bold in the text.
2
What are IDPs?
Extraterrestrial matter occurs in a variety of shapes, sizes and compositions. The size can range from electrons (Grimani et al., 2009) to
gas-giants like Jupiter, with everything in be-tween. The IAU has classified extraterrestrial ob-jects according to their size. The smallest obob-jects are classified as dust, having a size smaller than 30µm. Objects measuring between 30 µm and 1m are classified as meteoroids. Although this scale is somewhat arbitrary, as there is a continu-ous population of bodies with sizes from <30µm to >1m. Confusingly, the zodiacal dust cloud (Zodiacal Light) and cometary dust trails con-tain particles >30µm which would not be clas-sified as dust, but as meteoroids, though they are discussed in the context of dust populations. This results in some discrepancies in the litera-ture regarding the exact definition of dust. In this review paper dust particles (sizes smaller than 30µm) are discussed, but there is an inevitable overlap with meteoroids. Further information re-garding the current classification of particles can be found on the IAU web site1.
Dust that occurs in space can be classified according to its location: there is intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust. On the largest spa-tial scale there is intergalactic dust, which occu-pies the space between galaxies (Wszolek et al., 1988). Within galaxies interstellar dust can be found in the space between solar systems (Draine, 2003). Within solar systems there is interplan-etary dust, a part of which is circumplaninterplan-etary dust. Interplanetary dust is the dust occupy-ing or travelloccupy-ing through the space between plan-ets, and circumplanetary dust is dust orbiting a planet, which could form a ring system. This review is focused on interplanetary dust par-ticles (IDPs), although most of the collection and analysis methods discussed here can apply to research considering other types of dust as well. Additionally, the solar system has an influx of in-terstellar dust (Gr¨un et al., 1994), which can have a significant influence on dust research within the solar system.
Beside location dust particles can further be classified according to their composition (figure 1). They can either be rich or poor in metal con-tent, when a dust particle has a low metal content they are classified as chondrites, while metallic
particles are achondrites. By far most particles fall in the chondrite category. Chondrites can be further classified in three groups: carbona-ceous, ordinary or enstatite. These groups divide the chondrites in classes based on their degree of oxidation, carbonaceous being the least and en-statite being the most oxidised.
To describe dust in the interplanetary medium, currently the term ’interplanetary dust particles’ or ’IDPs’ is used most often, but in the past the term ’Zodiac Light’, ’Zodiacal dust cloud’ or similar wording was used. The Zodiacal Light is a glowing along the zodiac in the night sky, on some occasions visible with the naked eye, produced by the reflection of sunlight off dust particles. In this review ’IDPs’ will be used to describe the dust particles, as is common in other recent literature, but research about the Zodiacal Light is also included when it is relevant.
Some aspects of the life cycle of IDPs make them especially interesting. Generally, the dust particles in our solar system have a lifetime of 105
years. Because dust has existed in the interplane-tary medium for billions of years (D. Brownlee & Rajan, 1973), this implies that dust is constantly being created and destroyed at an approximately equal rate (Leinert et al., 1983). Lunar micro-crater studies, i.e. studies of impact micro-craters on lu-nar rocks, indicate that this process has had pro-found effects on shaping planetary surfaces since the formation of the solar system (H¨orz et al., 1975). Studying extraterrestrial dust can thus contribute to our understanding of how objects in space are formed and modified.
IDPs are mainly created from asteroids and comets (Carrillo-S´anchez et al., 2016). These are small bodies with low surface gravities, no permanent atmospheres, and they are by far the most abundant bodies in the solar system. How-ever, Earth-like planets can also produce IDPs as a result of major impact events and even Io’s (a moon of Jupiter) extreme volcanism is known to eject dust into space (Graps et al., 2000). Much research has focused on investigating the rela-tive contribution of IDPs from various sources in the solar system (Gr¨un et al., 1985; Nesvorn`y et al., 2010; Poppe, 2016; Carrillo-S´anchez et al.,
Figure 1: A classification based on composition. This is a simplified representation of the various types of particles explained by D. E. Brownlee (1985).
2016). A comparison of modelling and obser-vations (IRAS data) showed that outgassing of Jupiter-family comets are the main contributor of IDPs in the inner solar system (Nesvorn`y et al., 2010), while in the outer solar system mutual col-lisions between Edgeworth-Kuiper Belt objects is the dominant contributor of IDPs (Poppe, 2016). When IDPs are created there is a continuous dis-tribution of particle sizes. The size of a particle has a significant effect on the probabilities of its fate.
Non-gravitational forces such as radiation pressure, Poynting-Robertson drag and Lorentz forces move the orbits of newly cre-ated dust particles away from the parent object, especially for smaller dust particles. Collisions and sublimation further shape the distribution of IDPs until they are incorporated in the back-ground interplanetary dust cloud. If the parti-cles lose enough mass, they can become what is known as β-meteoroids. This happens when particles become so small that the relative influ-ence of radiation pressure becomes larger than the effect of gravity, effectively causing solar ra-diation to push them out of the solar system into interstellar space. However, most IDPs reach the end of their life as a result of Poynting-Robertson drag (Gor’kavyi et al., 1997). This force reduces the velocity of dust particles due to interaction with solar radiation, which eventually results in the particles spiralling down into the sun.
Col-lisions with other bodies in the interplanetary medium can result in the destruction of IDPs as well, however collisions don’t always destroy particles, and can also create new dust parti-cles in the process. Most IDPs will not be com-pletely destroyed by passing through the Earth’s atmosphere because their size is too small to pro-duce enough heat tp vaporise the particle. Con-sequently, physical collection in sediments, the stratosphere, and on the surface of the Earth is possible (Laevastu & Mellis, 1961).
3
How are IDPs studied?
Dust particles can be analysed in three distinc-tive ways. Firstly, the way dust reflects light can be sensed remotely, which can indicate how dust behaves and give an idea of its abundance and characteristics (Davies et al., 2016). Secondly, physically collected particles can be analysed to study morphology and composition. And thirdly, particles can be modelled to either explain their distribution phenomenologically or explain the dynamic processes that underlie dust creation. It depends on the research question which method is preferred. In general, these methods are inter-twined with each other because they provide dif-ferent insights in the behaviour of dust in the in-terplanetary medium that can complement each other in various ways.
3.1
The Zodiacal Light
The scientific investigation of IDPs began with observations of the Zodiacal Light. This phe-nomenon occurs as a result of the scattering of sunlight from IDP’s causing a glowing along the zodiac in the night sky (Leinert, 1975). It can appear just before sunrise or after sunset but can only be seen in an extremely dark sky. Addition-ally, to observe the Zodiacal Light it is required that the Sun rises and sets close to the East and West, respectively. Because it can be observed with the naked eye at the right location and time, the Zodiacal Light has been studied for centuries. The 17th century French astronomer Cassini
was the first to hypothesise dust to cause the Zo-diacal Light. He began a 10-year study on the subject in 1683, in which he observed the phe-nomenon from different locations near the equa-tor. It wasn’t until the 1970s however that the Pioneer 10 mission confirmed that the Zodiacal Light was indeed caused by IDPs (Hanner et al., 1976). Nevertheless, prior to this time investiga-tion of the Zodiacal Light provided a foundainvestiga-tion for research on IDPs. Later observations of the Zodiacal Light also provided valuable insights in the behaviour and distribution of IDPs (Black-well, 1960; Dumont & Sanchez, 1975; May, 2008).
3.2
Abundance of Dust in the
So-lar System
In parallel to the investigation of the Zodiacal Light, the abundance of dust particles has been investigated empirically since early space explo-ration. The first motivations to do so originate from the danger dust particles in the interplane-tary medium can impose to instruments in space. Dust can cause significant damage when travel-ling at a speed in the order of tens of kilome-tres per second (Whipple, 1958), therefore instru-ments in space need to be adequately protected. However, improving protection also means in-creasing weight, which can be expensive. This makes an accurate estimation of the danger valu-able. As a result, the chance of impact with dust particles of various sizes is studied extensively, which provides insight in the size distribution and
abundance of these particles.
As early as the late 1960’s empirical obser-vations from Explorer XIV, Explorer XXIII and Pegasus I, II and III led to the formulation of a differential equation explaining the probabil-ity of impact as a function of particle size (Nau-mann, 1966). This equation indicated that there is an exponential relation between the size of dust particles and its occurrence in the interplanetary medium: smaller particles are exceedingly more abundant. However, as Giese (1961) showed, par-ticles smaller than 0.3µm would be ejected from the solar system by light pressure from the sun, making them significantly less abundant.
IDPs are constantly being destroyed as a result of collisions and Poynting-Robertson drag, making the particles spiral into the Sun (Gor’kavyi et al., 1997). This makes it necessary for the particles to be replenished at an approx-imately equal rate, if an equilibrium is assumed. In an early estimation by Whipple (1967) a cre-ation rate of 10 to 20 tons/sec was quoted to sus-tain the current concentration of IDPs in the so-lar system. Later research by Gr¨un et al. (1985) indicated that approximately 10 tons/sec of par-ticles are destroyed within 1 AU (Astronomical Unit = distance from Earth to Sun) of the Sun, implying a similar creation rate.
3.3
Distribution of Dust in the
So-lar System
The abundance of dust is however only one as-pect of describing the interplanetary dust envi-ronment, since IDPs are not homogeneously dis-tributed through the solar system. The abun-dance is increased at some places and reduced in others. This makes the distribution of IDPs another key aspect of dust research. Insight in where IDPs are located aids the development of models that simulate development of solar sys-tems, but it is also relevant for studies concerning background radiation (Schlegel et al., 1998), and it’s essential for assessing the safety of spacecraft. In the early developments of cosmic dust re-search, the possibility of an enhanced concentra-tion of dust near Earth was hypothesised. This would have enormous implications for the
devel-Observing the Zodiacal Light
The Zodiacal Light can be observed with the naked eye, albeit under certain conditions. The opportunities to observe the Zodiacal Light are geographically limited by man-made factors, and temporally limited by astronomical factors. It occurs as a result of the reflection of sun-light from the particulate matter in the interplanetary medium. Therefore, it is brightest where the concentration of IDPs is the highest. Since IDPs are mainly concentrated in the ecliptic plane, the Zodiacal Light is located in the sky near the path of the Sun. This also means that the observer should be aligned with the ecliptic plane and the Sun to find it. As a result, the Zodiacal Light can be observed best when the path of the Sun passes through exactly west and east during sunset and sunrise respectively. In both hemispheres this makes the spring equinox the best time to observe the Zodiacal Light in the west after dusk, and the autumn equinox the best time to find it in the east before dawn. Of course, these pe-riods fall on different dates depending on the whether you are in the Northern or Southern hemisphere. Additionally, since the path of the Sun is less variable near the equator, the periods for observing the Zodiacal Light are extended with closer proximity to the equator. When visible, the Zodiacal Light appears as a diffuse cloud shaped like a triangle that is wide near the horizon and narrowing with elevation. It can have a similar brightness as the Milky Way, but due to the diffusivity, even small amounts of light pollution can make it hard to distinguish it from the background. This makes a nearly moonless sky and a remote area additional prerequisites.
oping field of space exploration and navigation. However, research of Shapiro et al. (1966) and Colombo et al. (1966) quickly determined that more than an increase of 10 times the interplane-tary background concentration would not be pos-sible. Indeed, data from NASA’s Long Duration Exposure Facility indicated that the dust con-centration near Earth is only approximately dou-bled due to gravitational focusing (McDonnell et al., 1993). Because this observation provided an upper limit to the danger of dust to spacecraft, it also changed the focus of IDP research from an engineering to an astronomical point of view, with the related consequences to funding.
The infrared astronomical satellite (IRAS) provided major new insights in the distribution of dust in the interplanetary medium (Low et al., 1984). Launched in 1983, it was the first mission to put a telescope in space to observe the sky in infrared. By observing from space, terrestrial interference was eliminated. The resulting ob-servations changed our image of the distribution of IDPs from a diffuse cloud to a complex, but structured body of dust. It is considered a turn-ing point in the history of dust research.
The most accurate measurements concern dust at 1 AU since this location is the closest to Earth and most important for satellites and spacecraft orbiting Earth, however considerable research has been done to map the dust concen-trations throughout the solar system. The major-ity of dust in the solar system forms a flat cloud concentrated near the ecliptic plane. The den-sity of this cloud increases with proximity to the Sun, roughly proportional to r−1, where r = dis-tance from the Sun (Mann, 1998). However, close to the Sun the various components of IDPs start to sublimate resulting in a deprivation of dust particles. Over (1958) estimated sublimation of SiO2 particles within 4 solar radii distance from
the Sun, and Mukai et al. (1974) has shown that carbonaceous grains start to sublimate at 4 solar radii, while Mann et al. (1994) calculated that pure silicate particles could survive up to 2 solar radii.
In various places in the solar system the con-centration of IDPs is amplified. Near Earth the natural dust concentration is approximately dou-bled due to gravitational focusing (McDonnell et al., 1993), this phenomenon is also present at
Runaway dust in Earth’s orbit
Perhaps the most peculiar dust environment is the area directly surrounding the Earth. Here the objects orbiting Earth can be found. Artificial satellites in orbit of the Earth can provide technologies such as GPS, communication and environmental monitoring. However, they also pollute the near-Earth environment. Especially the low Earth orbit area (less than 2000km above the Earth’s surface) is heavily populated with anthropogenic objects. Collisions between spacecraft and particulate matter, the breakup of rocket boosters and the harsh environment all have the potential to produce dust and debris in Earth’s orbit. The rate at which dust particles are produced is proportional to the number of objects subject to weathering. Con-sequently, there is a positive feedback mechanism resulting from the increase of debris due to collisions of objects. When collisions increase the number of objects, the chances of collisions are increased as well. This can result in exponential growth of orbital debris, hindering future space missions. This is known as the Kessler-syndrome. In 1978, Kessler & Cour-Palais did a conservative estimation that the first collision between trackable debris would occur before 2005, with an exponential growth of collisions following. Indeed, in 1996 the first accidental collision between a functioning satellite and a piece of orbital debris took place, severely dam-aging the 50 kg French satellite Cerise. The orbital debris causing this collision was produced 10 years earlier by the breakup of a booster rocket. In 2009 a far more dramatic collision occurred when the 556 kg Iridium satellite accidentally collided with the 900 kg Kosmos satellite with 42,000 km h−1. This event signalled the beginning of the Kessler-syndrome, since it produced more than 1300 pieces of trackable debris, thereby increasing the chance of subsequent collisions (Kelso, 2009). This event also stressed the need for international collab-oration to limit collisions by increasing the sharing of data as well as implementing universal guidelines to safely dispose of decommissioned satellites. Both are significant aspects of the reduction of the debris production of satellites. However, the amount of objects in orbit is still growing. Analysis of impacts on the solar arrays on the Hubble space telescope show that the concentration of dust from anthropogenic sources are currently already in the same order of magnitude as the natural background concentration (Kearsley et al., 2005). Moreover, the artificial dust concentration will keep increasing as satellites decay and more satellites are put into orbit. It is essential to prevent the amount of debris from becoming too large for future generations to also benefit from satellite technology. This makes removal of debris from orbit increasingly important, which is a major challenge since it is much harder to remove debris from orbit than it is to create debris. Additionally, there are already derelict objects in orbit which could cause catastrophic damage if they were to collide with a debris. One such object is the 8,000 kg Envisat satellite. Its danger was stressed in 2010 by an avoidance manoeuvre to reduce the chance of hitting a Chinese rocket stage. In 2012 communications with Envisat was lost, and since then it poses a significant risk.
other planets (Singer & Stanley, 1976). Res-onance with the orbits of planets is another way the concentration of dust may be increased (Klaˇcka et al., 2008). Dust in Earth’s orbit is also increased due to the exceptional case of man-made debris. About 95% of all trackable debris is man-made. As the number of man-made ob-jects in the Earth’s orbit increases, the chances
of collision increase as well. These collisions pro-duce fragments that increase the probability of further collisions (Kessler & Cour-Palais, 1978). Since the 1960’s the total mass in orbit has been steadily accumulating, and with that the amount of debris from decommissioned objects has been increasing as well (Johnson, 2010). Especially the Chinese anti-satellite test in January 2007 and
the collision of two spacecraft in February 2009 contributed to the amount of fragments and dust orbiting Earth. Additionally, in January 2020 the decommissioned IRAS and GGSE-4 spacecraft nearly collided, their closest approach was esti-mated to be within 47 metres. If they would have collided the amount of debris in orbit would have increased severely. Currently, the main concern consists of the risk untraceable fragments larger than 1mm pose to spacecraft. However, the in-creasing dust concentration in orbit of Earth also produces scientific challenges such as obscuring vision and limiting research on the natural dust concentration near Earth.
3.4
The
Proportion
Interstellar
Dust
Not all IDPs originate from the solar system. In 1992 the Ulysses spacecraft detected high veloc-ity dust grains moving in orbits opposite to the motion of the planets (Gr¨un et al., 1993). This was the first time interstellar dust particles were unambiguously detected within the solar system. In subsequent research, dust detectors on the Galileo (Baguhl et al., 1996) and Cassini (Alto-belli et al., 2003) spacecraft have observed in-terstellar dust as well. Furthermore, interstel-lar dust is found as a foreground radiation com-ponent in infrared emission (Rowan-Robinson & May, 2013), which complicate remote sensing. Additionally, Amari et al. (2001) describe the ob-servation of interstellar material found within a meteorite, which provides interesting opportuni-ties for research investigating the origin of the solar system.
The amount of interstellar dust encountered is not constant through time. Research by Tal-bot Jr & Newman (1977) indicated that our solar system has encountered approximately 135 inter-stellar dust clouds with a hydrogen atom density higher than 100 cm-3 and 16 clouds with a hy-drogen atom density higher than 1000 cm-3. Fur-ther research by Pavlov et al. (2005) showed that moving through one of the 16 larger interstel-lar dust clouds could cause severe cooling within our solar system, which could potentially induce a snowball Earth climate. Whether such
cli-mate change manifests depends on the state of the Earth’s climate and the density of the en-countered interstellar dust cloud. If the Earth’s climate is already heading towards an ice age, the chances are increased.
Detection of interstellar material on Earth provides a novel opportunity to study the Earth’s climatic past. If interstellar material is found in a substrate, our solar system has either moved through an interstellar dust cloud or has encoun-tered remnants of a supernova explosion, both of which might have influenced the Earth’s cli-mate. One isotope that is of interest regarding this field of study is 60Fe, which is a long-lived
isotope of iron with a half-life of 2.6 million years (Kutschera et al., 1984). This isotope is created by supernovae and can as a result occur in inter-stellar dust clouds. The background60Fe
concen-trations on Earth are nearly non-existent because there have been more than 1500 half-lives since the formation of Earth. Consequently, 60Fe can
only be found on Earth when it is deposited from the interstellar environment.
The first evidence of the occurrence of 60Fe on Earth was discovered by Knie et al. in 1999. And more recently,60Fe was found by accelerator mass spectrometry of 500 kg of Antarctic snow (Koll et al., 2019). This was the first time re-cently deposited60Fe was discovered, as the snow
was only 20 years old. There are two hypothe-sis that might explain the origin of this recently deposited interstellar material. Firstly, the dust could be the remnants of a supernova, where the material was directly ejected into space on a tra-jectory towards Earth. As a second option, the observed60Fe could originate from the local
in-terstellar dust cloud our solar system is currently moving through (Draine, 2003). Our solar system entered this interstellar dust cloud up to 40,000 years ago. If material from ice cores over 40,000 years old don’t contain60Fe this provides a ma-jor verification of the cloud hypothesis and fur-ther analysis can provide profound insights in the local interstellar environment. However, by the time of writing this is still ongoing research, and in the coming years there will surely be more dis-coveries made regarding the interstellar compo-nent of interplanetary dust (Koll et al., 2019).
Dust and the Earth’s climate
The climate on Earth is influenced by many processes. Variation in the Earth’s orbit around the Sun, variation in the amount of solar radiation, volcanic eruptions, and more recently anthropogenic emissions of greenhouse gasses are all contributors to how the Earth’s climate behaves. A common element is that all these phenomena influence how solar radiation reaches the Earth. If the amount of dust between the Earth and the Sun changes, this also influences how solar radiation can travel from the Sun to the Earth, which can manifest in a change of the climate on Earth. Dust can block solar radiation, effectively reducing the amount of energy received by the Earth. A similar process happens after a major volcanic eruption. Throughout the Earth’s climatic past there have been various periods of cooling and warm-ing. These periods have been attributed to a combination of driving forces. However, not all variation is explained yet. For example, the transition between the Eocene and the Oligocene (roughly 34 million years ago) is characterised by major cooling not clearly attributed to a single event. This cooling caused a large-scale extinction amongst flora and fauna (Keigwin, 1980). If a correlation with the amount of dust particles in substrates of the same age can be found, this can help to explain that climatic anomaly. Variation in climate is most often a result of the interaction of many processes. Therefore, attributing a climatic period only to a change of the dust concentration in space is often insufficient to explain the full phenomenon. However, the history of dust in the interplanetary medium can aid analyses of the Earth’s cli-matic past by providing an explanation for discrepancies or currently unidentified anomalies. Especially research on climatic boundaries in Earth’s history can benefit from knowledge on the history of interplanetary dust, because encountering or leaving a dust cloud can induce rapid change.
3.5
Physical Collection on Earth
Modern technologies have enabled us to locate IDPs and collect and analyse them physically. This can be done with various techniques, which are usually complementary. Due to the aerody-namic qualities of IDPs they can be gradually de-celerated in the Earth’s atmosphere. This makes it possible to collect particles on Earth. However, once the particles mix with the terrestrial envi-ronment it can become extremely complicated to distinguish between extraterrestrial and terres-trial material. In the stratosphere particles have already lost most of their velocity and have not significantly mixed with terrestrial material yet. This makes the stratosphere an ideal location to collect IDPs. Stratosphere collection started with balloon-borne collection (D. E. Brownlee et al., 1971), moving on to routine collection us-ing aircraft (D. Brownlee et al., 1977). Recently, a novel method of stratosphere collection using ’dry collectors’, which does not use oil and sol-vents is developed (Messenger et al., 2015).
Beside the stratosphere, IDPs can be col-lected from deep-sea sediments and pristine ter-restrial environments as well. As is the case with debris in space, many terrestrial environ-ments experience contamination with anthro-pogenic particles such as soot, but also volcan-ism and weathering influence the preservation of IDPs on Earth. Therefore, most collection efforts are made in places such as the ocean floor, Antarctica, deserts and other ’clean’ ar-eas. In these environments there is little in-flux of terrestrial material, resulting in a surpris-ingly high concentration of extraterrestrial mate-rial (Duprat et al., 2007). Especially larger par-ticles (>100µm) are more conveniently collected at the surface or sea floor than in the strato-sphere due to their low influx and high fall speed (D. E. Brownlee, 1985). The first evidence of ex-traterrestrial material in deep-sea sediments was found by analysis of the relative concentration of elements in sediment cores (Pittersson & Rotschi, 1952). Currently, research on IDPs in deep-sea
sediments has branched out to various fields such as paleogeology (Onoue et al., 2011), where the influx of IDPs is related to periods in the history of the Earth.
Collection on land is often more complicated than deep-sea collection due to a generally lower concentration of IDPs and a higher level of con-tamination with terrestrial magnetic particles. Most collection on land is done at locations with as little anthropogenic contamination as possi-ble. One of such exceptionally clean terrestrial environments is Antarctica. Besides being clean, the low temperatures severely reduce weathering, and the icy surface limits sedimentation. As a result, various collection efforts have been made there Harvey (2003). Collection on land also al-lows for the correlation of found particles to cer-tain events. An example is the Tungsuka event in central Russia, where in 1908 a large explosion generally attributed to the disintegration of an object measuring roughly 100m in size caused the flattening of approximately 2000km2 of forest.
˙
Zbik (1984) analysed a sample of 100 spherules found in the area relating to this event, to inves-tigate its origins.
3.6
In-situ measurements
In-situ collection and analysis can provide valu-able information about IDPs as well. The Galileo, Ulysses, Helios, Pioneer 8-11, Cassini-Huygens, Rosetta and New Horizons spacecraft all carried devices to measure dust particles while navigating the solar system and have provided IDP measurements throughout the solar system. Additionally, various spacecraft travelling near Earth have assessed the dust environment near 1 AU from the Sun.
As a special case, the Stardust spacecraft has successfully completed a sample return mission where the vehicle collected dust particles from Comet 81P/Wild, along with interstellar parti-cles, and returned them to Earth (D. Brownlee et al., 2006). This sample return mission resulted in the discovery of cometary glycine, one of the building blocks of life. Since the Earth has been bombarded with comets before life evolved (Mor-bidelli et al., 2000), this supports the idea that
life in the universe might be common.
Similarly, the collection of materials that have been subject to bombardment of IDPs also pro-vide valuable insights. One such case is the anal-ysis of Lunar rock material (H¨orz et al., 1975), which showed the effect of dust on the develop-ment of planetary surfaces. Anthropogenic ob-jects that have been exposed to interplanetary space can yield insights in the distribution and composition of IDPs as well (Kearsley et al., 2005).
3.7
Modelling
Computer simulations have been applied to gain insight in IDP’s ever since they became avail-able. As early as 1961, Giese used a computer model to indicate that particles smaller than ap-proximately 0.3µm would be ejected from the so-lar system by light pressure originating from the Sun.
Currently, the main models used for space ap-plications in the inner solar system are the Mete-oroid Engineering Model by NASA (McNamara et al., 2005), and ESA’s Interplanetary Meteoroid Environment Model (Dikarev et al., 2004). The main purpose of both models is to estimate me-teoroid fluxes on spacecraft to justify adequate protection.
The current increase in computing power pro-vides various opportunities for studying the be-haviour of dust quantitatively. For example, the DustPedia project by Davies et al. (2017), which uses legacy data from the Herschel and Planck missions to relate the observed dust emission from 4231 local galaxies to its physical proper-ties and processes that create and destroy it.
Advanced computer models are also used to track orbital debris around Earth. Their accu-racy is essential to protect functional satellites because they can provide a warning when there is a high chance of collision. The functional satellite can then perform an avoidance manoeuvre. How-ever, in 2009 the paths of the Kosmos and Iridium satellites were not predicted accurately enough, which resulted in a catastrophic collision (Kelso, 2009). This shows that, although very accurate predictions are already being made, research to
improve these models can still improve the safety of the near-Earth environment.
4
The future of IDP research
IDPs can be studied in various ways. Remote sensing from terrestrial or extraterrestrial envi-ronments can provide insight in the distribution, abundance and chemical composition of the dust particles. Experiments in space and on Earth, for example subjecting materials to high-velocity particles, can provide information on the physi-cal properties. Particles can be collected directly in space, within the atmosphere or at the Earth’s surface to study the composition. Modelling and simulation are used to improve the understand-ing of the lifecycle and dynamics.
There is not a single best method to study IDPs. Each collection and analysis method has benefits and drawbacks and can answer different questions. Furthermore, the different collection methods provide particles with a different com-position and size distribution, and the various experiments and analyses investigate various as-pects of the behaviour of dust in interplanetary space. Scientists with a wide variety of exper-tise are involved with the study of IDPs, from remote sensing to engineering space instruments and from modelling to mineralogy. Even astro-biology can involve analysis of IDPs. Combin-ing the knowledge of these fields can produce a new understanding of IDPs and provide in-sight in major scientific questions such as how our solar system was formed and where life orig-inated (and how rare life is in the universe), and aid the remediation of urgent problems such as the contamination of the near-Earth dust envi-ronment. Therefore, collaboration between fields can be very beneficial, especially in the current time where a wealth of knowledge is available and
many methods for interaction are possible. During the start of space exploration scien-tists with various backgrounds unified in assess-ing the danger of IDPs to spacecraft. Such re-search questions related to IDPs can have many facets, which make them unsuitable to be stud-ied from a single discipline. The increase in man-made debris is another one of these topics. Not a single debris has been removed from space yet, while there are plans to put 1000s of new satel-lites in orbit in the near future (for example Star-link, OneWeb and Boeing’s satellite internet). Although newer satellites must adhere to guide-lines that aim to limit the production of orbital debris, it is of high urgency to develop methods to reduce the amount of debris currently orbiting Earth (Shan et al., 2016). The lunar environment similarly contains man-made debris (Johnson & McKay, 1999). However, due to a lack of atmo-sphere around the moon, disposing of this debris poses its own unique challenges. The most cost-effective dust removal methods will focus of re-moving large fragments from the Earth’s orbit, so they are no longer at risk of collision and frag-mentation.
Remediation of the near-Earth environment requires mathematical and physical coordination, but also rocket science and meteorology. Addi-tionally, it is not only a scientific project, but also a geopolitical problem. Since currently the sin-gle most polluting event was an anti-satellite test, global agreements are essential for the limitation of orbital debris. Similarly, questions regarding the building blocks of life, space exploration and origins of the solar system are highly interdisci-plinary. From a single discipline these topics can only be investigated to a limited extent. New or deeper insight often requires the connection of multiple views to construct more fundamental theories, this is especially the case with some-thing as diverse as dust.
Glossary
β-meteoroid
A particle that is primarily affected by radiation pressure, which pushes them out of the solar system into interstellar space. These are the smallest of dust particles.
Asteroid
Relatively small rocky object with a diameter >1m orbiting the Sun.
AU (Astronomical Unit)
The distance between the Earth and the Sun, roughly 150 million kilometres.
Circumplantery
Around planets. An example is the rings of Saturn.
Comet
Relatively small icy object that releases gasses when passing close to the sun.
Extraterrestrial
From outside the Earth or its atmosphere.
Half-life
The time it takes for half of a substance to decay.
IDP (Interplanetary Dust Particle)
A particle with a diameter ¡30µm travelling through, or coming from, interplanetary space. Intergalactic
Between galaxies, this is the space that fills the rest of the universe.
Interplanetary
Between planets, this is the space that fills solar systems.
Interstellar
Between solar systems, this is the space that fills galaxies.
Isotope
Variant of a chemical element determined by the total number of protons and neutrons in it.
Lorentz force
Photopolaritmic
Measured with a device to identify the polarization of light from space photographically.
Poynting-Robertson drag
The reduction of the speed of dust particles orbiting a star, caused by solar radiation. This process causes most dust particles in the solar system to spiral into the Sun.
Radiation pressure
The mechanical pressure inflicted by the momentum of light, or electromagnetic radiation. This process causes the smallest dust particles to be ejected from the solar system.
Snowball Earth
Hypothetical historic climatic condition where the Earth’s surface became entirely or nearly entirely frozen.
Solar system
Gravitationally bound system of the Sun including all objects that orbit it, either directly or indirectly.
Spectrogram
Visual representation of a range of frequencies.
Supernova
Powerful stellar explosion where a star ejects most of its mass into space. This phenomenon can create unique materials.
The ecliptic plane
The plane through which the Earth orbits the Sun. Due to the way our solar system was formed, all major bodies in the solar system have an orbit near this plane.
Zodiac
An area in the sky extending approximately 8deg north or south of the apparent path of the Sun.
Zodiacal Light
The glowing in the night sky along the zodiac visible just before sunrise and just after sunset when the Sun’s path is at a high angle to the horizon.
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