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 1 Introduction Dust in the interplanetary medium is produced from a wide variety of host objects. All Extraterrestrial materials have been of interest to solid bodies in our solar system can inject dust humanity for a long time. They occur throughout into space (Carrillo-Sánchez et al., 2016), and history as sacred objects and were often deemed some of the dust in our solar system even has to have religious significance. It wasn’t until interstellar origins (Talbot Jr & Newman, 1977), the early 19th century however, that scientists however the most common sources of interplane- started to accept the hypothesis that rocks falling tary dust are asteroids and comets. from the sky had extraterrestrial origins (Mar- The aim of this literature review is to sum- vin, 2006). The report on meteorites falling near marise the current knowledge about dust in the L’Aigle in 1803 is considered a turning point in interplanetary medium and provide an overview the recognition of the extraterrestrial origin of of the various ways it is studied. This includes meteorites (Gounelle, 2006). Since then, a great the abundance and distribution of interplanetary effort has been made studying the composition, dust, physical collection, in-situ measurements, origin and implications of extraterrestrial matter. modelling and a discussion of the most impor- It is not surprising that many scientists are tant associated phenomena such as the Zodiac interested in (micro)meteorites. Extraterrestrial Light and the interstellar component. The de- matter can provide insight in many questions re- velopment of methods to collect, analyse and ob- garding the origins and development of the uni- serve interplanetary dust particles (IDPs), as well verse, because it resembles the composition of as some promising new technologies that can aid materials that formed planets billions of years this field of study are discussed as well. ago. This allows for unique insights in big topics The research questions to be investigated in such as the formation of stars or the origin of life. this review are: The vast majority of extraterrestrial matter that arrives at the Earth is in the form of dust. 1. What does the life cycle of IDPs look like? An estimated 40,000 metric tonnes of dust parti- 2. How do IDPs populate our solar system? cles arrive on Earth per day (Love & Brownlee, 1993). This implies that, although meteorites are 3. In which ways are IDPs studied? quite rare, everyone who sets a foot outside will probably come in contact with a fragment of ex- 4. What are the reasons to study IDPs? traterrestrial material in the form of cosmic dust. The abundance of dust particles is almost I aim to provide a comprehensive overview never directly visible to the untrained eye, but of the field of dust research, understandable even the particles can have major implications for without prior knowledge of cosmic dust. This pa- our understanding of cosmological and terres- per is based on three key review articles (Koschny trial phenomena. They are involved in the de- et al., 2019; Grünet al., 2019; Nesvorn`yet al., velopment of solar systems and how light travels 2010) and references therein. Some words and through space. As a result, the particles can in- terms essential to the field might be unfamiliar to fluence climate and interfere with cosmological readers without background knowledge, therefore observations. Besides these natural phenomena a glossary is provided, and the included words dust particles can also pose a danger to space- and terms are printed bold in the text. craft due to their high speed. Even some extinction events might be related to changes in the amount of cosmic dust particles Earth encoun- 2 What are IDPs? ters (Kataoka et al., 2013). Consequently, it is of interest to develop methods to obtain and iden- Extraterrestrial matter occurs in a variety of tify these dust particles and improve our insight shapes, sizes and compositions. The size can in their behaviour. range from electrons (Grimani et al., 2009) to

2 gas-giants like Jupiter, with everything in be- particles are achondrites. By far most particles tween. The IAU has classified extraterrestrial ob- fall in the chondrite category. Chondrites can jects according to their size. The smallest objects be further classified in three groups: carbona- are classified as dust, having a size smaller than ceous, ordinary or enstatite. These groups divide 30 µm. Objects measuring between 30 µm and the chondrites in classes based on their degree of 1m are classified as meteoroids. Although this oxidation, carbonaceous being the least and en- scale is somewhat arbitrary, as there is a continu- statite being the most oxidised. ous population of bodies with sizes from <30 µm To describe dust in the interplanetary to >1m. Confusingly, the zodiacal dust cloud medium, currently the term ’interplanetary dust (Zodiacal Light) and cometary dust trails con- particles’ or ’IDPs’ is used most often, but in tain particles >30 µm which would not be clas- the past the term ’Zodiac Light’, ’Zodiacal dust sified as dust, but as meteoroids, though they cloud’ or similar wording was used. The Zodiacal are discussed in the context of dust populations. Light is a glowing along the zodiac in the night This results in some discrepancies in the litera- sky, on some occasions visible with the naked eye, ture regarding the exact definition of dust. In this produced by the reflection of sunlight off dust review paper dust particles (sizes smaller than particles. In this review ’IDPs’ will be used to 30 µm) are discussed, but there is an inevitable describe the dust particles, as is common in other overlap with meteoroids. Further information re- recent literature, but research about the Zodiacal garding the current classification of particles can Light is also included when it is relevant. be found on the IAU web site1. Some aspects of the life cycle of IDPs make Dust that occurs in space can be classified them especially interesting. Generally, the dust according to its location: there is intergalactic particles in our solar system have a lifetime of 105 dust, interstellar dust, interplanetary dust years. Because dust has existed in the interplane- and circumplanetary dust. On the largest spa- tary medium for billions of years (D. Brownlee & tial scale there is intergalactic dust, which occu- Rajan, 1973), this implies that dust is constantly pies the space between galaxies (Wszolek et al., being created and destroyed at an approximately 1988). Within galaxies interstellar dust can be equal rate (Leinert et al., 1983). Lunar micro- found in the space between solar systems (Draine, crater studies, i.e. studies of impact craters on lu- 2003). Within solar systems there is interplan- nar rocks, indicate that this process has had pro- etary dust, a part of which is circumplanetary found effects on shaping planetary surfaces since dust. Interplanetary dust is the dust occupy- the formation of the solar system (Hörzet al., ing or travelling through the space between plan- 1975). Studying extraterrestrial dust can thus ets, and circumplanetary dust is dust orbiting a contribute to our understanding of how objects planet, which could form a ring system. This in space are formed and modified. review is focused on interplanetary dust par- IDPs are mainly created from asteroids and ticles (IDPs), although most of the collection comets (Carrillo-Sánchez et al., 2016). These and analysis methods discussed here can apply to are small bodies with low surface gravities, no research considering other types of dust as well. permanent atmospheres, and they are by far the Additionally, the solar system has an influx of in- most abundant bodies in the solar system. How- terstellar dust (Grünet al., 1994), which can have ever, Earth-like planets can also produce IDPs as a significant influence on dust research within the a result of major impact events and even Io’s (a solar system. moon of Jupiter) extreme volcanism is known to Beside location dust particles can further be eject dust into space (Graps et al., 2000). Much classified according to their composition (figure research has focused on investigating the rela- 1). They can either be rich or poor in metal con- tive contribution of IDPs from various sources tent, when a dust particle has a low metal content in the solar system (Grünet al., 1985; Nesvorn`y they are classified as chondrites, while metallic et al., 2010; Poppe, 2016; Carrillo-Sánchez et al., 1iau.org/science/scientific bodies/commissions/F 1

3 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- lisions with other bodies in the interplanetary vations (IRAS data) showed that outgassing of medium can result in the destruction of IDPs Jupiter-family comets are the main contributor of as well, however collisions don’t always destroy IDPs in the inner solar system (Nesvorn`yet al., particles, and can also create new dust parti- 2010), while in the outer solar system mutual col- cles in the process. Most IDPs will not be com- lisions between Edgeworth-Kuiper Belt objects is pletely destroyed by passing through the Earth’s the dominant contributor of IDPs (Poppe, 2016). atmosphere because their size is too small to pro- When IDPs are created there is a continuous dis- duce enough heat tp vaporise the particle. Con- tribution of particle sizes. The size of a particle sequently, physical collection in sediments, the has a significant effect on the probabilities of its stratosphere, and on the surface of the Earth is fate. possible (Laevastu & Mellis, 1961). Non-gravitational forces such as radiation pressure, Poynting-Robertson drag and Lorentz forces move the orbits of newly cre- 3 How are IDPs studied? ated dust particles away from the parent object, especially for smaller dust particles. Collisions Dust particles can be analysed in three distinc- and sublimation further shape the distribution tive ways. Firstly, the way dust reflects light can of IDPs until they are incorporated in the back- be sensed remotely, which can indicate how dust ground interplanetary dust cloud. If the parti- behaves and give an idea of its abundance and cles lose enough mass, they can become what is characteristics (Davies et al., 2016). Secondly, known as β-meteoroids. This happens when physically collected particles can be analysed to particles become so small that the relative influ- study morphology and composition. And thirdly, ence of radiation pressure becomes larger than particles can be modelled to either explain their the effect of gravity, effectively causing solar ra- distribution phenomenologically or explain the diation to push them out of the solar system into dynamic processes that underlie dust creation. It interstellar space. However, most IDPs reach the depends on the research question which method end of their life as a result of Poynting-Robertson is preferred. In general, these methods are inter- drag (Gor’kavyi et al., 1997). This force reduces twined with each other because they provide dif- the velocity of dust particles due to interaction ferent insights in the behaviour of dust in the in- with solar radiation, which eventually results in terplanetary medium that can complement each the particles spiralling down into the sun. Col- other in various ways.

4 3.1 The Zodiacal Light abundance of these particles. As early as the late 1960’s empirical obser- The scientific investigation of IDPs began with vations from Explorer XIV, Explorer XXIII and observations of the Zodiacal Light. This phe- Pegasus I, II and III led to the formulation of nomenon occurs as a result of the scattering of a differential equation explaining the probabil- sunlight from IDP’s causing a glowing along the ity of impact as a function of particle size (Nau- zodiac in the night sky (Leinert, 1975). It can mann, 1966). This equation indicated that there appear just before sunrise or after sunset but can is an exponential relation between the size of dust only be seen in an extremely dark sky. Addition- particles and its occurrence in the interplanetary ally, to observe the Zodiacal Light it is required medium: smaller particles are exceedingly more that the Sun rises and sets close to the East and abundant. However, as Giese (1961) showed, par- West, respectively. Because it can be observed ticles smaller than 0.3 µm would be ejected from with the naked eye at the right location and time, the solar system by light pressure from the sun, the Zodiacal Light has been studied for centuries. making them significantly less abundant. th The 17 century French astronomer Cassini IDPs are constantly being destroyed as a was the first to hypothesise dust to cause the Zo- result of collisions and Poynting-Robertson diacal Light. He began a 10-year study on the drag, making the particles spiral into the Sun subject in 1683, in which he observed the phe- (Gor’kavyi et al., 1997). This makes it necessary nomenon from different locations near the equa- for the particles to be replenished at an approx- tor. It wasn’t until the 1970s however that the imately equal rate, if an equilibrium is assumed. Pioneer 10 mission confirmed that the Zodiacal In an early estimation by Whipple (1967) a cre- Light was indeed caused by IDPs (Hanner et al., ation rate of 10 to 20 tons/sec was quoted to sus- 1976). Nevertheless, prior to this time investiga- tain the current concentration of IDPs in the so- tion of the Zodiacal Light provided a foundation lar system. Later research by Grünet al. (1985) for research on IDPs. Later observations of the indicated that approximately 10 tons/sec of par- Zodiacal Light also provided valuable insights in ticles are destroyed within 1 AU (Astronomical the behaviour and distribution of IDPs (Black- Unit = distance from Earth to Sun) of the Sun, well, 1960; Dumont & Sanchez, 1975; May, 2008). implying a similar creation rate.

3.2 Abundance of Dust in the So- 3.3 Distribution of Dust in the So- lar System lar System In parallel to the investigation of the Zodiacal The abundance of dust is however only one as- Light, the abundance of dust particles has been pect of describing the interplanetary dust envi- investigated empirically since early space explo- ronment, since IDPs are not homogeneously dis- ration. The ﬁrst motivations to do so originate tributed through the solar system. The abun- from the danger dust particles in the interplane- dance is increased at some places and reduced tary medium can impose to instruments in space. in others. This makes the distribution of IDPs Dust can cause signiﬁcant damage when travel- another key aspect of dust research. Insight in ling at a speed in the order of tens of kilome- where IDPs are located aids the development of tres per second (Whipple, 1958), therefore instru- models that simulate development of solar sys- ments in space need to be adequately protected. tems, but it is also relevant for studies concerning However, improving protection also means in- background radiation (Schlegel et al., 1998), and creasing weight, which can be expensive. This it’s essential for assessing the safety of spacecraft. makes an accurate estimation of the danger valu- In the early developments of cosmic dust re- able. As a result, the chance of impact with dust search, the possibility of an enhanced concentra- particles of various sizes is studied extensively, tion of dust near Earth was hypothesised. This which provides insight in the size distribution and would have enormous implications for the devel-

5 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 sunlight 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 periods 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. The most accurate measurements concern However, research of Shapiro et al. (1966) and dust at 1 AU since this location is the closest Colombo et al. (1966) quickly determined that to Earth and most important for satellites and more than an increase of 10 times the interplane- spacecraft orbiting Earth, however considerable tary background concentration would not be pos- research has been done to map the dust concen- sible. Indeed, data from NASA’s Long Duration trations throughout the solar system. The major- Exposure Facility indicated that the dust con- ity of dust in the solar system forms a flat cloud centration near Earth is only approximately dou- concentrated near the ecliptic plane. The den- bled due to gravitational focusing (McDonnell et sity of this cloud increases with proximity to the al., 1993). Because this observation provided an Sun, roughly proportional to r−1, where r = dis- upper limit to the danger of dust to spacecraft, tance from the Sun (Mann, 1998). However, close it also changed the focus of IDP research from to the Sun the various components of IDPs start an engineering to an astronomical point of view, to sublimate resulting in a deprivation of dust with the related consequences to funding. particles. Over (1958) estimated sublimation of SiO2 particles within 4 solar radii distance from The infrared astronomical satellite (IRAS) the Sun, and Mukai et al. (1974) has shown that provided major new insights in the distribution carbonaceous grains start to sublimate at 4 solar of dust in the interplanetary medium (Low et al., radii, while Mann et al. (1994) calculated that 1984). Launched in 1983, it was the first mission pure silicate particles could survive up to 2 solar to put a telescope in space to observe the sky radii. in infrared. By observing from space, terrestrial interference was eliminated. The resulting ob- In various places in the solar system the con- servations changed our image of the distribution centration of IDPs is amplified. Near Earth the of IDPs from a diffuse cloud to a complex, but natural dust concentration is approximately dou- structured body of dust. It is considered a turn- bled due to gravitational focusing (McDonnell et ing point in the history of dust research. al., 1993), this phenomenon is also present at

6 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 collaboration 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- of collision increase as well. These collisions pro- onance with the orbits of planets is another duce fragments that increase the probability of way the concentration of dust may be increased further collisions (Kessler & Cour-Palais, 1978). (Klaˇcka et al., 2008). Dust in Earth’s orbit is Since the 1960’s the total mass in orbit has been also increased due to the exceptional case of man- steadily accumulating, and with that the amount made debris. About 95% of all trackable debris of debris from decommissioned objects has been is man-made. As the number of man-made ob- increasing as well (Johnson, 2010). Especially the jects in the Earth’s orbit increases, the chances Chinese anti-satellite test in January 2007 and

7 the collision of two spacecraft in February 2009 mate change manifests depends on the state of contributed to the amount of fragments and dust the Earth’s climate and the density of the en- orbiting Earth. Additionally, in January 2020 the countered interstellar dust cloud. If the Earth’s decommissioned IRAS and GGSE-4 spacecraft climate is already heading towards an ice age, the nearly collided, their closest approach was esti- chances are increased. mated to be within 47 metres. If they would have Detection of interstellar material on Earth collided the amount of debris in orbit would have provides a novel opportunity to study the Earth’s increased severely. Currently, the main concern climatic past. If interstellar material is found in consists of the risk untraceable fragments larger a substrate, our solar system has either moved than 1mm pose to spacecraft. However, the in- through an interstellar dust cloud or has encoun- creasing dust concentration in orbit of Earth also tered remnants of a supernova explosion, both produces scientific challenges such as obscuring of which might have influenced the Earth’s cli- vision and limiting research on the natural dust mate. One isotope that is of interest regarding concentration near Earth. this field of study is 60Fe, which is a long-lived isotope of iron with a half-life of 2.6 million years 3.4 The Proportion Interstellar (Kutschera et al., 1984). This isotope is created Dust by supernovae and can as a result occur in interstellar dust clouds. The background 60Fe concen- Not all IDPs originate from the solar system. In trations on Earth are nearly non-existent because 1992 the Ulysses spacecraft detected high veloc- there have been more than 1500 half-lives since ity dust grains moving in orbits opposite to the the formation of Earth. Consequently, 60Fe can motion of the planets (Grünet al., 1993). This only be found on Earth when it is deposited from was the first time interstellar dust particles were the interstellar environment. unambiguously detected within the solar system. The first evidence of the occurrence of 60Fe In subsequent research, dust detectors on the on Earth was discovered by Knie et al. in 1999. Galileo (Baguhl et al., 1996) and Cassini (Alto- And more recently, 60Fe was found by accelerator belli et al., 2003) spacecraft have observed in- mass spectrometry of 500 kg of Antarctic snow terstellar dust as well. Furthermore, interstel- (Koll et al., 2019). This was the first time re- lar dust is found as a foreground radiation com- cently deposited 60Fe was discovered, as the snow ponent in infrared emission (Rowan-Robinson & was only 20 years old. There are two hypothe- May, 2013), which complicate remote sensing. sis that might explain the origin of this recently Additionally, Amari et al. (2001) describe the ob- deposited interstellar material. Firstly, the dust servation of interstellar material found within a could be the remnants of a supernova, where the meteorite, which provides interesting opportuni- material was directly ejected into space on a tra- ties for research investigating the origin of the jectory towards Earth. As a second option, the solar system. observed 60Fe could originate from the local in- The amount of interstellar dust encountered terstellar dust cloud our solar system is currently is not constant through time. Research by Tal- moving through (Draine, 2003). Our solar system bot Jr & Newman (1977) indicated that our solar entered this interstellar dust cloud up to 40,000 system has encountered approximately 135 inter- years ago. If material from ice cores over 40,000 stellar dust clouds with a hydrogen atom density years old don’t contain 60Fe this provides a ma- higher than 100 cm-3 and 16 clouds with a hy- jor verification of the cloud hypothesis and fur- drogen atom density higher than 1000 cm-3. Fur- ther analysis can provide profound insights in the ther research by Pavlov et al. (2005) showed that local interstellar environment. However, by the moving through one of the 16 larger interstel- time of writing this is still ongoing research, and lar dust clouds could cause severe cooling within in the coming years there will surely be more dis- our solar system, which could potentially induce coveries made regarding the interstellar compo- a snowball Earth climate. Whether such cli- nent of interplanetary dust (Koll et al., 2019).

8 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 climatic 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 Beside the stratosphere, IDPs can be collected from deep-sea sediments and pristine ter- Modern technologies have enabled us to locate restrial environments as well. As is the case IDPs and collect and analyse them physically. with debris in space, many terrestrial environ- This can be done with various techniques, which ments experience contamination with anthro- are usually complementary. Due to the aerody- pogenic particles such as soot, but also volcan- namic qualities of IDPs they can be gradually de- ism and weathering influence the preservation celerated in the Earth’s atmosphere. This makes of IDPs on Earth. Therefore, most collection it possible to collect particles on Earth. However, efforts are made in places such as the ocean once the particles mix with the terrestrial envi- floor, Antarctica, deserts and other ’clean’ ar- ronment it can become extremely complicated to eas. In these environments there is little in- distinguish between extraterrestrial and terres- flux of terrestrial material, resulting in a surpris- trial material. In the stratosphere particles have ingly high concentration of extraterrestrial mate- already lost most of their velocity and have not rial (Duprat et al., 2007). Especially larger par- significantly mixed with terrestrial material yet. ticles (>100 µm) are more conveniently collected This makes the stratosphere an ideal location at the surface or sea floor than in the strato- to collect IDPs. Stratosphere collection started sphere due to their low influx and high fall speed with balloon-borne collection (D. E. Brownlee et (D. E. Brownlee, 1985). The first evidence of ex- al., 1971), moving on to routine collection us- traterrestrial material in deep-sea sediments was ing aircraft (D. Brownlee et al., 1977). Recently, found by analysis of the relative concentration of a novel method of stratosphere collection using elements in sediment cores (Pittersson & Rotschi, ’dry collectors’, which does not use oil and sol- 1952). Currently, research on IDPs in deep-sea vents is developed (Messenger et al., 2015).

9 sediments has branched out to various fields such life in the universe might be common. as paleogeology (Onoue et al., 2011), where the Similarly, the collection of materials that have influx of IDPs is related to periods in the history been subject to bombardment of IDPs also pro- of the Earth. vide valuable insights. One such case is the anal- Collection on land is often more complicated ysis of Lunar rock material (Hörzet al., 1975), than deep-sea collection due to a generally lower which showed the effect of dust on the develop- concentration of IDPs and a higher level of con- ment of planetary surfaces. Anthropogenic ob- tamination with terrestrial magnetic particles. jects that have been exposed to interplanetary Most collection on land is done at locations with space can yield insights in the distribution and as little anthropogenic contamination as possi- composition of IDPs as well (Kearsley et al., ble. One of such exceptionally clean terrestrial 2005). environments is Antarctica. Besides being clean, the low temperatures severely reduce weathering, 3.7 Modelling and the icy surface limits sedimentation. As a result, various collection efforts have been made Computer simulations have been applied to gain there Harvey (2003). Collection on land also al- insight in IDP’s ever since they became avail- lows for the correlation of found particles to cer- able. As early as 1961, Giese used a computer tain events. An example is the Tungsuka event model to indicate that particles smaller than ap- in central Russia, where in 1908 a large explosion proximately 0.3 µm would be ejected from the so- generally attributed to the disintegration of an lar system by light pressure originating from the object measuring roughly 100m in size caused the Sun. 2 flattening of approximately 2000km of forest. Currently, the main models used for space ap- ˙ Zbik (1984) analysed a sample of 100 spherules plications in the inner solar system are the Mete- found in the area relating to this event, to inves- oroid Engineering Model by NASA (McNamara tigate its origins. et al., 2005), and ESA’s Interplanetary Meteoroid Environment Model (Dikarev et al., 2004). The 3.6 In-situ measurements main purpose of both models is to estimate meteoroid fluxes on spacecraft to justify adequate In-situ collection and analysis can provide valu- protection. able information about IDPs as well. The The current increase in computing power pro- Galileo, Ulysses, Helios, Pioneer 8-11, Cassini- vides various opportunities for studying the be- Huygens, Rosetta and New Horizons spacecraft haviour of dust quantitatively. For example, the all carried devices to measure dust particles while DustPedia project by Davies et al. (2017), which navigating the solar system and have provided uses legacy data from the Herschel and Planck IDP measurements throughout the solar system. missions to relate the observed dust emission Additionally, various spacecraft travelling near from 4231 local galaxies to its physical proper- Earth have assessed the dust environment near ties and processes that create and destroy it. 1 AU from the Sun. Advanced computer models are also used to As a special case, the Stardust spacecraft has track orbital debris around Earth. Their accu- successfully completed a sample return mission racy is essential to protect functional satellites where the vehicle collected dust particles from because they can provide a warning when there is Comet 81P/Wild, along with interstellar parti- a high chance of collision. The functional satellite cles, and returned them to Earth (D. Brownlee can then perform an avoidance manoeuvre. How- et al., 2006). This sample return mission resulted ever, in 2009 the paths of the Kosmos and Iridium in the discovery of cometary glycine, one of the satellites were not predicted accurately enough, building blocks of life. Since the Earth has been which resulted in a catastrophic collision (Kelso, bombarded with comets before life evolved (Mor- 2009). This shows that, although very accurate bidelli et al., 2000), this supports the idea that predictions are already being made, research to

10 improve these models can still improve the safety many methods for interaction are possible. of the near-Earth environment. During the start of space exploration scientists with various backgrounds unified in assessing the danger of IDPs to spacecraft. Such re- 4 The future of IDP research search questions related to IDPs can have many facets, which make them unsuitable to be stud- IDPs can be studied in various ways. Remote ied from a single discipline. The increase in man- sensing from terrestrial or extraterrestrial envi- made debris is another one of these topics. Not ronments can provide insight in the distribution, a single debris has been removed from space yet, abundance and chemical composition of the dust while there are plans to put 1000s of new satel- particles. Experiments in space and on Earth, lites in orbit in the near future (for example Star- for example subjecting materials to high-velocity link, OneWeb and Boeing’s satellite internet). particles, can provide information on the physi- Although newer satellites must adhere to guide- cal properties. Particles can be collected directly lines that aim to limit the production of orbital in space, within the atmosphere or at the Earth’s debris, it is of high urgency to develop methods surface to study the composition. Modelling and to reduce the amount of debris currently orbiting simulation are used to improve the understand- Earth (Shan et al., 2016). The lunar environment ing of the lifecycle and dynamics. similarly contains man-made debris (Johnson & There is not a single best method to study McKay, 1999). However, due to a lack of atmo- IDPs. Each collection and analysis method has sphere around the moon, disposing of this debris benefits and drawbacks and can answer different poses its own unique challenges. The most cost- questions. Furthermore, the different collection effective dust removal methods will focus of re- methods provide particles with a different com- moving large fragments from the Earth’s orbit, position and size distribution, and the various so they are no longer at risk of collision and frag- experiments and analyses investigate various as- mentation. pects of the behaviour of dust in interplanetary Remediation of the near-Earth environment space. Scientists with a wide variety of exper- requires mathematical and physical coordination, tise are involved with the study of IDPs, from but also rocket science and meteorology. Addi- remote sensing to engineering space instruments tionally, it is not only a scientific project, but also and from modelling to mineralogy. Even astro- a geopolitical problem. Since currently the sin- biology can involve analysis of IDPs. Combin- gle most polluting event was an anti-satellite test, ing the knowledge of these fields can produce global agreements are essential for the limitation a new understanding of IDPs and provide in- of orbital debris. Similarly, questions regarding sight in major scientific questions such as how the building blocks of life, space exploration and our solar system was formed and where life orig- origins of the solar system are highly interdisci- inated (and how rare life is in the universe), and plinary. From a single discipline these topics can aid the remediation of urgent problems such as only be investigated to a limited extent. New the contamination of the near-Earth dust envi- or deeper insight often requires the connection ronment. Therefore, collaboration between fields of multiple views to construct more fundamental can be very beneficial, especially in the current theories, this is especially the case with some- time where a wealth of knowledge is available and thing as diverse as dust.

11 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 ¡30 µm 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 The force electric and magnetic ﬁelds exercise on charged particles.

12 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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