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ASTEROIDS

ASTEROIDS. Objects orbiting the Sun – Not volatile enough to be classed as comet Too small to be planets (< few 1000 km) Too large to be meteroids (> few 10 ’ s km) e.g. fragmented and disrupted moons and planets + relics of the early stages of accretion of the solar nebula. Gaspra (16 km).

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ASTEROIDS

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  1. ASTEROIDS • Objects orbiting the Sun – • Not volatile enough to be classed as comet • Too small to be planets (< few 1000 km) • Too large to be meteroids (> few 10’s km) e.g. fragmented and disrupted moons and planets + relics of the early stages of accretion of the solar nebula Gaspra (16 km)

  2. Asteroid belt A “gap” in the known planetary system between Mars (1.5 AU) and Jupiter (5.2 AU) – `predicted’ by the Titius-Bode rule! (see ealier notes) • Discoveries: Ceres 1801, Pallas 1802, Juno 1804, Vesta 1807 • Largest is Ceres (940 km diameter) • …several million with diameters of 1 km or more • Total mass ~1/20 mass of Moon

  3. Belt Asteroids… • Semimajor axes 2.2–3.3 AU • Periods 3.3–6 years • Sub-groups of asteroids (similar orbits, surface appearance)  may be fragments of a single asteroid produced by collisions • Some gaps in belt caused by resonances with Jupiter: …ratio of orbital period at that distance to Jupiter’s orbital period

  4. Classification based on visual appearance – i.e. albedo, spectral characteristics S-type – High albedo (A > 0.15) Silicon rich  similar spectral characteristics to stony meteorites Peak in main belt between 2 – 2.5 AU C-type – Low albedo (A < 0.03) Carbonaceous in nature (like chondrite meteorites) Peak further out at ~ 3 AU ~75% of asteroids are C-type M-type – Metal-rich (above all Iron and Nickel)

  5. Other asteroid groups…TROJANS • 60° ahead and behind Jupiter • Stable orbits (see Lagrange pts.)  the asteroids will not be swept up by Jupiter • May be several thousand in number • Size: Most are a few km, some are >100 km Earth Approaching – May be several thousand >1 km diameter Radar images are available for several that approached Earth Amor -- Have orbits crossing Mars’s orbit Perihelion distances between 1.017 and 1.4 AU (ie between Earth and Mars)

  6. Methods of study to derive properties of asteroids: • Occultation with stars – rare occurrence  radii • ii) Polarimetric data – light becomes polarised depending on angle of incidence with surface  surface structure • iii) IR and visible radiometry – IR brightness due to re-emission of solar radiation  albedo • iv) Earth-based radar  surface size, roughness • v) Since 1991, direct imaging (Galileo, NEAR,…)

  7. (25143) Itokawa • Target of the Japanese Hayabusa mission • Best-fit ellipsoid: 535294209 m • S asteroid with ordinary spectrum • Density = 1.9 g/cm3 • About 40% porosity; rubble pile structure • Smooth terrain  Potential: mobility of fine material

  8. Asteroid spin rates • Very rapid spins, i.e., periods shorter than 1 hour, only exist for very small asteroids • Interpretation: all the larger asteroids have a rubble-pile structure that does not survive a rapid spin

  9. Asteroid Lutetia has been revealed as a battered world of many craters. ESA’s Rosetta mission has returned the first close-up images of the asteroid showing it is most probably a primitive survivor from the violent birth of the Solar System

  10. Lutetia asteroids seen by ESA-Rosetta

  11. Flyby Lutetia

  12. Comets

  13. Comet - Structure

  14. Comets • Comets are icy objects that release gas and dust as they orbit the Sun. The solid part of a comet is called the nucleus and is mainly made of frozen water, dust and sometimes other frozen substances such as ammonia.Solar radiation heats the nucleus and gives it an atmosphere of gas and dust called the coma. A comet's distinctive tail is caused by solar radiation and a stream of charged particles that constantly jets away from the Sun called the solar wind.It is thought that comets are material leftover from the formation of the outer planets, although another theory is that many formed outside our solar system.

  15. Comets • The comets are ice-rich bodies that become prominent when heat from the sun causes their trapped volatiles to sublimate. The most visible and distinctive features of comets are the coma and tail. However, most of the mass of a comet is contained within a comparatively tiny central nucleus, and it is this body that is of the highest scientific interest because of its likely identity as a planetesimal from the outer regions of the solar nebula.

  16. Nucleus • The solid, centrally located part of the comet is known as the "nucleus". The nucleus is a repository of dust and frozen gases. When heated by the sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the • Size
The sizes of cometary nuclei are mostly unknown because the measurement is a difficult one. We have reliable measurements of the sizes of about 10 nuclei. Most of them have diameters from a few km to 10 or 20 km. The nucleus of comet Schwassmann-Wachmann 1 is probably one of the largest (perhaps 20 km), as is the nucleus of comet Hale-Bopp (perhaps 40 km). Except in the special cases of comets Halley and Borrelly, the sizes are inferred.

  17. Nucleus • Composition
The composition of the nucleus is determined by measuring the composition of the coma. We know nothing directly about the interior The dominant volatile is water, followed by CO, CO2 and a host of minor species present at the <1% level. There is some evidence for abundance variations among comets. The CO/H2O ratio reached 0.2 to 0.3 in Hale-Bopp but is typically 4 or 5 times smaller. • The refractory (non-volatile) dust consists of some silicate minerals and carbon rich CHON (Carbon-Hydrogen-Oxygen-Nitrogen) grains. The upper layers of the nucleus are volatile depleted, consisting of a refractory "mantle". • The ratio of volatile mass to refractory mass is probably near 1.

  18. Nucleus • Lifetime
The lifetimes of active comets are limited for at least two reasons: • the nuclei are losing mass at rates that cannot be sustained for very long. For example, a 5 km radius spherical nucleus would have a mass about 4x10^15 kg. When near the sun, this nucleus might sublimate at 10^4 kg/s (10 tonnes per second), so the sublimation lifetime is 4x10^11 s = 1000 years. True, the comet might spend only part of each orbit near the sun, and so might be able to keep going for more than 1000 years, but it is simply unable to sustain mass-loss for the 4.5x10^9 year age of the solar system.

  19. Nucleus • Lifetime
The lifetimes of active comets are limited for at least two reasons: • the active comets are under the gravitational control of the planets. There is a finite chance that a comet will be either ejected from the solar system, injected to the sun, or absorbed by an impact with one of the planets (as happened in the famous case of Shoemaker-Levy 9). The "dynamical" lifetime of a typical comet is about 1/2 million years.

  20. Nucleus structure

  21. COMA The gas coma consists of molecules liberated from the nucleus by solar heating and sublimation. Once clear of the nucleus, molecules in the coma are exposed to direct solar radiation and can be damaged in various ways. Most molecules are broken apart ("dissociated") within a day of leaving the nucleus. For example, and are both reactions in which a molecule released from the nucleus absorbs a photon and breaks into two pieces. By convention, the initial molecule is often referred to as the "parent molecule" while the fragments produced by the absorption of a solar photon are known as "daughters" (mysteriously, there are no "sons" in coma physics, only daughters).

  22. COMA It turns out that the daughters are quite easy to observe because they have strong spectral lines at optical wavelengths. In fact, most of the light scattered from a comet at optical wavelengths is scattered by daughters. For this reason, the daughters have received a lot of observational attention and many are well understood. What we would prefer to understand is the origin and abundance of the parent molecules, but the parents generally lack useful optical spectral features. In addition to being photo-dissociated, gas species in comets can also be ionised, as in . The ions are susceptible to a magnetic force due to the solar magnetic field carried by the solar wind. Consequently, the ions are swept almost radially away from the sun, into a long, distinctive tail.

  23. TAIL The neutral gas species in cometary comae can be ionised by solar UV photons, as in The ions are susceptible to a magnetic force due to the solar magnetic field carried by the solar wind. Consequently, the ions are swept out of the coma into a long, distinctive ion tail. Because the most common ion, CO+, scatters blue light better than red, the ion tail often appears to the human eye as blue. Also, the magnetic force is very strong and produces ropes, knots and streamers that distinguish the ion tail from the dust tail. The solar wind sweeps past the comet at about 500 km/s, causing the ion tail to be swept almost exactly in the anti-solar direction.

  24. Hunt for molecules in comets (spectroscopy) A typical optical/near-IR comet spectrum 109P/Swift-Tuttle

  25. UV cometary spectra HST spectra of C/1996 B2 (Hyakutake) FUSE spectrum of C/2001 A2 (LINEAR)

  26. Comet Holmes taken by the Spitzer Space Telescope (NASA)

  27. Structure of a Comet • Solar heat vaporizes the nucleus to produce • Coma - Hydrogen gas Envelope • Dust tail • Ion tail

  28. Cometary nucleus

  29. Comets • Icy leftover planetesimals of the outer solar system. • Today comets exist mainly in the Kuiper belt and the Oort Cloud. • The strong gravity of the Jovian planets cleared most of the comets in between Jupiter and Neptune  • sent to a collision course with other planets, • or ejected to the Kuiper Belt and the Oort Cloud. • Comets beyond the orbit of Neptune have time to grow bigger and stay in stable orbit. • Pluto may be (the biggest) one of them??!

  30. Nucleus ~ 10—20 km dia. - `dirty snowball’ conglomerate of silicate rock/dust + ice • Coma ~ 105 km  outgassing of water, CO2 etc. from surface in jets • H corona – very extended (~107 km)…sparse envelope of neutral H • Ion tail – outgassed ions driven away from Sun by solar wind e.g. CO+ flourescence and emission lines --- up to ~ 1 AU in length • Dust tail – shorter (<0.1 AU) – micron sized dust particles – driven by photon pressure …fan shaped due to different mass distribution of grain and ejection velocities

  31. IR spectroscopy IKS/VEGA Combes et al. (1986) Simple species: H2O, CO, CO2, H2CO, CH3OH 3.3-3.5 mm band: CH-bearing species in gas phase unidentified compounds at 3.42mm 3.28 mm band: Hydrocarbons

  32. Radio spectroscopy • OH 18cm lines (1973, comet Kohoutek) • HCN 89 GHz (1985, comet Halley) • 19 molecules now detected • Isotopes: HDO, DCN, H13CN, HC15N, C34S, H234S • Radicals and ions: NS, CS, SO, CN, H3O+ ,CO+

  33. Deuterium in comets In H2O: D/H = 3 10-4 In HCN: D/H = 2.3 10-3 Atomic D detected (HST) In CH3OH, H2CO, NH3, CH4: upper limits of 10-2 to a few 10-2 C/1996B2 Hyakutake CSO Bockelée-Morvan et al. (1998) JCMT Meier et al. (1998)

  34. Tempe 1

  35. Tempe 1- Deep Impact

  36. What’s new from Deep Impact ? • 9P/Tempel 1, July 4, 2005 • 4.9 x 7.6 km dark nucleus with low thermal inertia, low density, negligible strength • smooth and rough terrains, natural impact craters + escarpments… • DI impact: fine dust ejected, no dramatic increase in gas production

  37. What’s new from Deep Impact ? Deep Impact spectra : large increase in the amount of organics compared to water

  38. Comet Wild 2 (from Star Dust) Image, taken using an electron microscope, of a portion of a comet particle from Comet Wild 2. The light gray doughnut surrounding the X consists of carbon-rich organic material. The scale bar at the bottom of the figure is 0.1 microns long. Looks like the organic material produced when we expose water-rich ices to ultraviolet radiation under space-like conditions in the laboratory. It suggests the possibility that comets contain some of the original organic products churned out by dense interstellar nebulae when their particle contents were bombarded by cosmic rays and ultraviolet radiation

  39. Open questions in comet chemistry • a lot of lines still unidentified • origin of HNC : coma or nucleus product • origin of CN ? • nature of dust ? • How abundances in the coma are related to abundances in the nucleus ? (chemical differentiation in the nucleus) • degree of compositional uniformity in comet nuclei

  40. What does composition tell us about the origin of comet material? • molecular composition present analogies with composition of star forming regions and interstellar ices • D/H ratios kept interstellar signatures low-T formation (grain surface, ion-molecule processes) • highly processed material is present (cristalline silicates) mixed with nebular products Origin of chemical diversity in comets?

  41. Comet origins • Two kinds of comets • Short period (<200 yrs) and long period (>200 yrs) • Different orbital characteristics: ecliptic Short period: prograde, low inclination Long period: isotropic orbital distribution • This distribution allows us to infer the orbital characteristics of the source bodies: • S.P. – relatively close (~50 AU), low inclination (Kuiper Belt) • L.P. – further away (~104 AU), isotropic (Oort Cloud)

  42. Short-period comets • Period < 200 yrs. Mostly close to the ecliptic plane (Jupiter-family or ecliptic, e.g. Encke); some much higher inclinations (e.g. Halley) • Most are thought to come from the Kuiper Belt, due to collisions or planetary perturbations • Form the dominant source of impacts in the outer solar system Long-period comets • Periods > 200 yrs (most only seen once) e.g. Hale-Bopp • Source is the Oort Cloud, perturbations due to nearby stars (one star passes within 3 L.Y. every ~105 years). •  Such passages also randomize the inclination/eccentricity • Distances are >>100 A.U. • Maybe 10-100 Earth masses in total • Sourced from originally scattered planetesimals • Source of periodic bio. extinctions(?)

  43. Kuiper Belt • The Kuiper belt is a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun. • It is similar to the asteroid belt, although it is far larger—20 times as wide and 20–200 times as massive. • Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. While the asteroid belt is composed primarily of rock and metal, the Kuiper objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water. • The belt is home to at least three dwarf planets – Pluto, Haumea, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, are also believed to have originated in the region

  44. Kuiper Belt • ~800 objects known so far, occupying space between Neptune (30 AU) & ~50 AU • Largest objects incl. Quaoar (1250km diameter), + Pluto and Charon??? • Two populations – low eccentricity, low inclination (“cold”) and high eccentricity, high inclination (“hot”) “hot” ECCENTRICITY “cold” • Total mass small, ~0.1 Earth masses • Difficult to form bodies as large as 1000 km – too little total mass is available • A large number (few percent) are in binaries

  45. KBO

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