Meteors and cosmic dust

Meteors and cosmic dust David W. Hughes

There is something rather poignant about seeing a shooting star. In the quiet of the night, under the clear dark sky, one is witnessing the death of a cosmic dust particle, a particle that had its own independent orbit around the Sun, and whose only fault was having an orbit that intersected the Earth’s, and being at the intersection point at the same time as Earth. Every year about 200,000 tonnes of extraterrestrial material hits Earth.

The collisions between solar system cosmic dust particles and planet Earth take place at velocities somewhere between 11 and 74 km/s, the actual value depending on whether the particle simply drops to Earth under the influence of the Earth’s gravitational field, or is going around the Sun in the opposite direction and has a head-on collision. Note that 74 km/s is equivalent to 166,000 mph, so a particle moving at this velocity has a considerable amount of kinetic energy.

The meteor As a dust particle enters the Earth’s atmosphere it is progressively hit by more and more atmospheric molecules. Their impact energy is dissipated in the particle surface and the surface temperature quickly rises. Soon the surface of the particle is boiling vigorously and the particle quickly loses both mass and velocity. Atoms boiling off its surface still have the majority of the extraterrestrial component of their velocity and they collide energetically with the sur- rounding air molecules. Many of these atoms and air molecules become ionized and excited. The ablating particle thus leaves behind it an expanding train of both electrons and positive ions, which will reflect radio waves and can be detected using radar, and a cylinder of excited atoms, the decay of this excitation being exhibited in the form of light. So meteors can be observed with the naked eye, by ground- based television systems and with optical detectors on satellites,

Meteor activity starts at an atmospheric density of about one millionth that of ground level. After moving about 20 km the typical cosmic dust particle has lost all its mass and energy. Unfortunately for meteor scientists only about 1 per cent of the kinetic energy of the incoming particle is

David W. Hughes, BSc., D.Phil, FRAS, F.lnst.P., C.Phys.

Graduated in physics at the University of Birmingham and studied for his doctorate at the University Observatory, Oxford. On leaving Oxford he became a lecturer at the University of Sheffield and is now Reader in Astronomy. He has spent his life studying the minor bodies in the solar system and teaching astronomy.

converted into visible light, and even less, 0.01 per cent, goes into ionization. The vast majority of the kinetic energy simply heats up the air and breaks up the particle. So, when it comes to measuring the mass, velocity, density and orbit of the incoming particle, the scientist on Earth is presented with a rather small signal.

The glowing streak in the sky is pro- saically known as a ‘shooting star’, although it has nothing to do with stars or shooting. The correct term is meteor, from the Greek, p,e~e~pa, for ‘things up in the air’. In fact, it was only at the end of the eighteenth century that scientists were con- vinced that meteors were produced by extraterrestrial impacts as opposed to mere meteorological conditions. The phenom- enon lasts about a second or so and occurs about 95 km above the Earth’s surface. After this time the electrons have attached them- selves to neutral atmospheric molecules or recombined with the ions, the emission of photons has ceased and the vast majority of the incident energy has been dissipated into the atmosphere.

The brightness of a meteor is usually given in terms of its magnitude, this being a much loved, but archaic, astronomical method of recording logarithmic brightness. The faintest star visible to the naked eye on a dark night is of magnitude 6, and the brightest star is of magnitude about -1. A change of unity in magnitude is equivalent to a ratio of 100.4, that is, 2.512, in brightness, so magnitude is a logarithmic unit. The meteor most likely to be observed by the naked eye has a magnitude of about 2. Meteors fainter than +2 are not seen often because the eye is too insensitive to detect them; and brighter ones are not seen as often, simply because fewer occur.

Meteors are not rare. An observer looking skyward at about midnight with dark-adapted eyes on a clear moonless night expects to see about 10 per hour [l]. There will be more in the early morning (around 06.00). when the observer is on the forward side of Earth and is being pushed into dust particles at high speed. And there will be fewer in the early evening (around 18.00) when the observer has been spun round to the rear side of Earth and only sees those meteors that are produced by meteoroids that are

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overtaking at low speed. Ten per hour is the average rate throughout a 24-hour period.

The observer’s naked-eye field of view is about 70” in diameter. Looking up at an altitude of 45” to the horizon this field of view covers about 30,000 km’ of the atmospheric region in which meteors produce their max- mum light, this region being about 95km above the Earth’s surface. Combining these figures means that an observed rate of 10 meteors per hour is equivalent to a total of 4,000,OOO meteoroids hitting the whole Earth each day.

A meteor observer who assesses the brightness of each meteor soon realizes that, for magnitudes brighter than +2, the fainter ones are more common than the highly luminous ones. For a specific area of sky the observed flux increases by a factor of about 3.8 for every unit increase in meteor magnitude. So, for example, if you observe all night and see one meteor brighter than magnitude -1, you will have seen four brighter than zeroth magnitude, 14 brighter than first magnitude, 55 brighter than second magnitude, and so on. To see a meteor of magnitude -3 you will have to observe for about 14 nights. The sensitivity of the eye is a problem when it comes to fainter meteors, but if you help the eye by using telescopes and television systems the ‘factor 3.8’ sequence continues.

A typical zeroth magnitude meteor is produced by a meteoroid dust particle of about one gram. This varies as a function of the velocity of the meteoroid, and an average value of 2Okm/s is assumed here. Because the amount of emitted light is proportional to the kinetic energy of the incident meteoroid the 1Oo.4 factor applies to the mass scal- ing. So, a first-magnitude meteor is produced by a 10-l x’).Jg particle and a second-magnitude meteor by a I O-2 x (I 4 g particle and so on.

There are certain natural mass barriers in this sequence. As one moves to smaller and smaller particles the cross-sectional-area-to- mass ratio becomes progressively larger. (Within an order of magnitude it can be equated to l/R cmVg, where R cm is the dust particle radius.) Soon one reaches a particle size that has such a large cross-sectional-area-to-mass ratio that it can radiate away the heat it receives from the impacting

Endeavour Vol. 21(i) 1997 31

air molecules as fast as it receives it. This is called the micrometeoroid limit and is equivalent to a radius of 1c3crn for a particle impacting at 20 km/s and 1Vcm for one impacting at 5Okm/s. Particles with radii less than these values are retarded gently by the Earth’s atmosphere and then float down to the surface.

At the other end of the mass range one reaches a ‘particle’ where the cross-sectional- area-to-mass ratio is so small that, in comparison with its own mass, the particle hits a negligible amount of the atmosphere as it passes through, and thus hits the Earth’s surface with very nearly its full extraterrestrial velocity and energy. This limit is equivalent to a radius of about 1OOOcm (a mass of around 1000 tonnes) and an impacting particle of this size would produce a crater in the Earth’s surface of about 0.2km in diameter.

Meteor showers On certain nights of the year the meteor rate can leap up to about 100 per hour. These shower meteors do not have random paths

and they all seem to be radiating from a specific point in a specific celestial constel- lation. This radiant effect is shown in Figure 1 for the Perseid meteor shower, a shower that reaches a maximum rate on 12 August each year and provides a free firework show for about five days around that date. The Perseid radiant is above the horizon at sun- set and reaches its maximum altitude at 05.40 when it is close to the zenith. The number of meteors seen per hour reaches a peak in the dark pre-dawn sky and the best places to look for meteors are at an altitude of about 45” above the horizon and about 45” on each side of the radiant. The cosmic dust in the Perseid stream has been produced by the decay of periodic comet Swift-Tuttle, the Great Comet of 1862. This comet was also seen in 1737 and 1992. Every 130 years or so it gets to within 146,000,OOOkm of the Sun, and whilst it is in the inner solar system it loses mass. The dust particles responsible for meteors are pushed away from the cometary nucleus and take up independent orbits around the Sun on which they either gain on or fall

PERSEID RADIANT

Figure 1 At the time of maximum activity of the Perseid shower the meteors appear to diverge from a single region in the sky called the radiant (right ascension 46”, declination +56”). This is a perspective effect. In reality the dust particles and the resulting meteors have parallel trajectories in the upper atmosphere. The intersection geometry is shown in Figure 2.

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behind the parent comet. After a few thousand years a complete annulus of dust is formed around the cometary orbit, this annulus being thin near the cometary peri- helion and wide near the aphelion. The geometry of the Perseid stream is shown in Figure 2. This stream is so old and well developed that the flux of dust particles at the Earth’s orbit remains reasonably constant from year to year.

Table 1 lists the prominent showers that can be seen by northern hemisphere observers. Streams can be divided into two groups, these being regular and periodic. The Perseids are a good example of a regular stream, and they produce a reasonably constant flux every August. The Leonids are periodic, reaching a maximum activity every 33 years or so, at the times when their parent comet, Temple, returns to the Sun. The variations in the peak shower rates given in Table 1 are due to the fact that the parent comets are of differing initial sizes and also have been decaying for differing times. The geometry is also a very impor- tant factor. Many streams have nodes that are nowhere near the Earth’s orbit and thus do not produce meteors in the atmosphere. Gravitational perturbations by the Jovian planets can move certain streams. This hap- pened to the Geminids and the Quadrantids and they were seen from Earth for the first time in the early nineteenth century, at a time when the perturbations pulled the streams into intersecting orbits. In another two hundred years or so these streams will have moved so much that again they will not intersect the Earth’s orbit.

The Leonids have been recorded for about a thousand years and extremely active Leonid displays were witnessed in 1799, 1833,1866 and 1966. In fact the early hours of the morning of 18 November 1999 are already being looked forward to with antici- pation. Figure 3 shows the world’s most famous meteor shower picture [3], this being a record of the Leonid storm of 13 November 1833. On that night shooting stars started to be seen at about 11 o’clock: ’ . . . in a few hours they became a perfect shower. They could no more be counted than one can count the fast falling flakes of snow in a hard storm. They continued to fall without any diminution of numbers until the dawn of the day obscured them’ (see [3]). ‘It seemed as if the whole starry heavens had congregated to a point near the zenith, and were simultaneously shooting forth, with the velocity of lightning, to every part of the horizon; and yet they were not exhausted; thousands swiftly followed in the track of thousands’ [3]. ‘No spectacle so terribly grand and sublime was ever before beheld by man as that of the firmament descending in fiery torrents’ (see [4]). The 1866 storm was not as brilliant a spectacle as the 1833 one but was still very fine. The meteoroid stream is so narrow in physical extent that the activity was confined to observers on the eastern side of the Atlantic. ‘As to the appearance of the meteors, it was noticed

03

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p/gwift-luft'>" ---

Figure 2 (a) The intersection geometry of the orbits of the Perseid dust particles and their parent comet Swift-Tuttle. The descending node of the stream orbit crosses the Earth’s orbit at a solar longitude of about 140”, the intersection occurring about 12 August. (b) The relationship between the orbit of the parent comet and the major planets in the solar system. The orbit is inclined at 113.6” to the plane of the solar system and this high inclination keeps it away from the major Jovian planets, thus ensuring reasonable orbital stability. The comet will return to the Sun in AD 2126, 2261, 2392, etc.

that the majority of them were of a whitish or yellowish colour. Some, however, were reddish or orange-coloured, and one meteor was noticed to be bluish’ (see [S]). Bright meteors left trains behind them that were visible for a few seconds and the visible train of one very bright meteor persisted for some minutes. A less active but still note- worthy shower was observed in November 1867 and 1868.

Even though huge numbers of meteors were seen at the height of the Leonid activity (see Figure 4) we must remember that the visual observer is looking at a very large region of the Earth’s atmosphere. At maximum the flux was equivalent to about one cosmic dust particle hitting every square kilometre every hour. One of the reasons

why the Leonids put on such an impressive show is that the cosmic dust particles are hitting the atmosphere at 72 km/s, and thus have a large amount of kinetic energy. A typical Leonid meteor would be produced by a particle with a mass just larger than 0.005 g. In space, in the very core of the Leonid stream, these particles are about 60 km apart. Just imagine how empty interplanetary space is outside meteoroid streams!

Returning to Table 1 it can be seen that most meteoroid streams have known parent comets, these comets being embedded in the stream and still feeding the stream with new cosmic dust every time the comet passes the Sun. There are, however, some exceptions. The Quadrantid stream has an orbit with an ascending node close to the orbit of Jupiter; in fact a Quadrantid meteor shower also occurs on that planet. This stream suffers considerably from the gravitational influence of Jupiter and the Quadrantid parent comet has been moved away from most of the dust that it has lost. So the Quadrantids are somewhat of an orphan and, even though many suggestions have been made, no one knows for certain which comet is responsible.

The Geminid stream is also unusual, inas- much as the Apollo-type asteroid 3200 Phaethon has an orbit which is very close to the mean stream orbit. This, coupled with the fact that the ‘dust’ in the Geminid stream is found to have a higher density than the more usual cometary stream dust, points to the Geminids being parented by the collision between two asteroids.

Most cosmic dust is ‘sponge-like’. It is very fragile and full of holes. On hitting the Earth’s atmosphere many cosmic dust particles break up, and this can cause the result- ant meteors to flare. The sudden increase in the surface area of the incident mass leads to a jump in the ablation rate and a blaze of light.

When the cosmic dust was locked into the cometary nucleus its interstitial volumes were filled by snow. The vaporization of this snow, when the comet is in the inner solar system, leads to the production of a great deal of gas, and the pressure exerted

TABLE 1 PROMINENT METEOR SHOWERS

Shower Date of Peak Duration of Associated Geocentric maximum rate peak comet

(h-7 velocity

(days) (km/s)

Quadrantids 3 Jan 90 0.5 41 April Lyrids 21 April 12 2 1861 I 48 Eta Aquarids 5 May 35 10 64 Delta Aquarids

Halley 29 July 20 15 41

Perseids 12 Aug 5 Swift-Tuttle 60 Draconids 10 Ott

(2::) 0.1 Giacobini-Zinner 23

Orionids 21 act 30 5 66 Taurids

Halley 1 Nov 40 Encke 30

Leonids 17 Nov (22) 4 Temple 72 Geminids 13 Dee 100 6 asteroid 3200 35 Ursids 23 Dee 10 2 Tuttle 34

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n that took place in the early morning hours of Wednesday- 13 November 1833. The original painting was executed around 1887 by the Swiss painter Karl Jauslin under the instructions of the Seventh-day Adventist minister Joseph Harvey Waggoner, who had witnessed the storm from rural eastern Pennsylvania when he was aged 13. The figure shows the Adolf Vdllmy engraving that was published in April 1888 in The Signs of the Times, a weekly publication of the Seventh-day Adventist Church. The association of the Leonid storm with the apocalyptic opening of the sixth seal (Revelation 6.13 and Matthew 24.3) was stressed.

by this gas not only breaks up the fragile dusty surface of the cometary nucleus but also accelerates these dusty fragments away from the nucleus to form the meteoroid stream.

Cosmic dust flux It is clear that cosmic dust is constantly hitting the Earth. Let us start by being a touch cavalier with our definition of the

word ‘dust’. The influx mass range can be divided into four regions: ‘particles’ with mass, 112 > 109g, form craters; ‘particles’ in the 104 < m < 109g range suffer considerable ablation in the atmosphere and produce a brilliant fireball, but not all the mass is lost and a meteoritic remnant falls to the ground [6]; in the 10-8 < m < 104g range the ‘particles’ evaporate on entry producing meteors of a whole range of brightnesses;

and finally the m < 1Wg particles survive the passage through the atmosphere and drift down to the Earth’s surface.

Let us consider three surface areas, these being the total area of Earth (some 5.3 x 1Ol4 m2), the area of the British Isles, that is, 3.2 X 10” m* and the area of a typical garden, some 200m2. The whole Earth is struck by only one particle bigger than about 5 X 109g every year. In this mass range, for every tenfold increase in the size of the ‘particle’ the numbers in space and hitting the Earth decrease by a factor a thousand, so to witness the impact of a ‘particle’ more massive than 5 X 1012g one has to wait on average a thousand years, and one more massive than 5 X 1015g a million years, and so on. When it comes to the British Isles, over the past two centuries a falling meteorite has been found and col- lected from the surface on average every decade. But the population distribution of the British Isles and the attention of that population is such that meteorites are, in fact, falling to the ground about every three weeks.

The 200m2 garden has about 6000 meteors burning out above it every year (the vast majority of these being very faint radar meteors). It also has about 400,000 micro- meteoroids in the lo-l2 < m < IO-sg range floating down to its surface. Needless to say, this number usually pales into insignifi- cance when compared with the number of industrial dust, aircraft exhaust, volcanic ash and Saharan sand particles that hit the average garden each year.

Every year the Earth travels a distance of 9.4 X 108km at a speed of 3Okm/s and everything that gets in its way is picked up. The total amount adds up to some 2.0 X IOr* g (or 200,000 tonnes) on average (see [7] and [S]). The majority of this mass is in the form of individual objects with masses greater than 106g, ‘particles’ with masses less than 1OOOg making an insignificant contribution. At the top end of the mass range the influx is made up of asteroids and comets. More typical incoming particles are the rocky and metallic asteroidal fragments, and the dust particles given off by decaying comets. Sometimes larger fragments of comets hit Earth and even rarer are the por- tions of the surfaces of objects like the Moon and Mars that escape during crater- forming events.

The influx has to be assessed in two ways. First, by observing the rate of occurrence of meteors, fireballs, meteorite falls and crater- ing events, and estimating the mass of the causative parent. In many of these cases the influx is only distributed locally and not globally. And, secondly, by collecting up the dusty material that actually falls to the surface. This is recognized by the unusual iridium content of the dirt locked up in, for example, pre-industrial Arctic and Antarctic ice, and in the metallic spherules and the manganese and osmium found in the old sediments at the bottoms of some of the world’s oceans. The latter indicate

34 Endeavour Vol. 21(l) 1997

__ _.__ _.. _ _ .-- UVLRAGE

WOBER Bows OF O3scruc~~on/ - 18cic3. No,wmZer 13-X . PER

120 k----b -

-

-

-

1 - 3 4 I

II-----l- ------IA-

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---

Figure 4 The diagram shows the average number of meteors recorded per minute at the Royal Greenwich Observatory, south London, on the night of the Leonid storm of 13-l 4 November 1866. (Reproduced from Dunkin [5].)

that about 3 X 10yg per year of material survives atmospheric entry without vaporization.

Vertical laser beams have been used to measure the light scattered from the dust floating through the mesospheric region of the atmosphere. These give a value similar to that obtained from sedimentation studies. Another approach is to estimate the dust influx just before it gets to the Earth’s atmosphere. The Shuttle-launched Long Duration Exposure Facility (LDEF) orbited at a height of about 400 km for 5.677 years.

On return to Earth, 761 extraterrestrially produced microcraters were found on the 5.6m* surface of the thermal control panel. Needless to say, LDEF did not sample any of the larger Earth-impacting particles. If it had, the collisions would have destroyed it. Many other studies have also been made of the microcratered surface of exposed Moon rocks, these rocks having been returned to Earth by the Apollo missions. The exposure age of the rocks can be estimated from the damage they have received from cosmic rays, but again, these rocks do not give a

measurement of the large body flux. Anything that would smash the rock to pieces is not recorded.

When you see a meteor you are witnessing planet Earth gaining mass. The production of the ‘shooting star’ entails a whole range of complicated atmospheric physical and chemical effects. In collecting cosmic dust particles you are picking up pieces of the failed planetary growth process that took place in the asteroid belt as well as bits of the Moon and Mars. All in all, meteors and their causative cosmic dust particles form a fascinating chapter in the subject of astronomy.

References 111

[21

I31

[41

151

Hughes. D.W. Mon. Not. R. Astronorn. Sot. 166,33943, 1974. Bone, N. Observer’s Handbook of Meteors. George Philips Ltd, 1993. Hughes, D.W. Earth, Moon and Planets 68, 31 l-22, 1995. Armstrong Grondal. F. The Romance of Astronomy. Macmillan, New York, 1942. Dunkin. E. The Midnight Sky: Familiar Notes 011 the Stars and Plunets (revised edition). The Religious Tract Society.

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[71 181

- London, 1891. Hughes, D.W. Spuce Sci. Rw. 61, 275-99. 1992. Hughes, D.W. Phy. Rev. 1,22-26, 1992. Ceplecha, Z. Astrcm. Astrophys. 263. X-66, 1992.

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Meteors and cosmic dust