1. The solar system • Earth and Moon • Telluric planets • Jovian planets…
• … and their moons
• Small bodies

2. Earth and Moon A unique couple RMoon = 0.27 REarth MMoon = 1/81 MEarth
Special case in the solar system
RTitan = RSaturn
MTitan = 1/4400 MSaturn
RTriton = RNeptune
MTriton = 1/4700 MNeptune
Le couple Terre – Lune vu par Galileo

3. gravitational attraction
Earth and Moon - 2
Tides
Gravitational attraction of Moon FA > FC > FB → bulges on Moon side and opposite side (same for Sun with a 46% strength)
• ocean tides (up to 15 m) and continental tides (30 cm)
Earth
Moon
gravitational attraction
centrifugal force A C B

4. Effects of tides on the Earth – Moon system (1)
Earth and Moon - 3
Effects of tides on the Earth – Moon system (1)
Earth rotation carries the tidal bulges
Moon attraction on the bulges slows down Earth rotation
Conversely, the orbital motion of the Moon is accelerated
Earth
Moon A C B
rotation

5. Effects of tides on the Earth – Moon system(2)
Earth and Moon - 4
Effects of tides on the Earth – Moon system(2)
Lunar tides caused by Earth attraction (stronger)
→ slowing down of Moon rotation, until synchronisation with orbital motion → always the same side towards Earth
Visible side and hidden side of the Moon

6. Effects of tides on the Earth – Moon system (3)
Earth and Moon - 5
Effects of tides on the Earth – Moon system (3)
Now:
• day’s length increased by 1 minute every 4 millions years
• Moon gets 3.7 cm further each year
400 millions years ago, length of day was 20 h
When rotation of Earth will be synchrone (in a few dozen billion years) it will rotate in 47 present days

7. Peculiarities of the Earth – Moon system
Earth and Moon - 6
Peculiarities of the Earth – Moon system
Earth is the only telluric planet with a genuine satellite
Among all solar system satellites, our Moon is unique because:
• its orbit does not coincide with the planet’s equatorial plane
• its large size compared to the planet
Moreover, the Moon was closer to the Earth in the past
→ suggests a formation scenario different from the other satellites
Neptune Earth
Triton Moon

8. Formation scenario for the Earth – Moon system
Earth and Moon - 7
Formation scenario for the Earth – Moon system
100 million yeras after its formation, the proto-Earth would have collided with another proto-planet, of size similar to Mars
→ debris ring around proto-Earth
→ debris start to stick together
→ formation of a big moon close to the planet
Then, gradually, the two bodies move further away

9. Internal structure of the Earth (1)
Earth and Moon - 8
Internal structure of the Earth (1)
Mean density ≈ 5.5 (5500 kg/m3) – Density earth crust ≈ 3
→ cannot be made of the same rocks in its whole volume
Earthquakes
→ seismic waves
Propagation: depends on the medium crossed
→ allow to model the Earth interior

10. Internal structure of the Earth (2)
Earth and Moon - 9
Internal structure of the Earth (2)
Continental crust (granite) – oceanic crust (basalt)
Mantle (olivine = heavy silicate)
• rigid in upper layers
• viscous below
Metallic core (iron, nickel,…)
• external (liquid, T ≈ 3800 – 4200 K)
• internal (solid, T ≈ 4200 – 4300 K)

11. Plate tectonics Convection in mantle → displacement of the crust
Earth and Moon - 10
Plate tectonics
Convection in mantle → displacement of the crust
→ continental drift
→ volcanism

12. Dating rocks Time elapsed since rock solidification:
Earth and Moon - 11
Dating rocks
Time elapsed since rock solidification:
Measured by radioactive clocks
Half-life: T½ = time for half of the nuclei to disintegrate
The proportion of children / parents nuclei increases with time
Parent Child T½ (109 yrs)
40K Ar
238U Pb
232Th Pb
87Rb Sr

13. Earth’s age Age of oldest • terrestrial rocks: 4.0 billion years
Earth and Moon - 12
Earth’s age
Age of oldest • terrestrial rocks: 4.0 billion years
• lunar rocks:
• meteorites:
Most solar system bodies probably formed at the same time
→ Earth’s age = meteorites age
Terrestrial rocks: appear younger because they experienced fusion phases during the first hundred billion years

14. Terrestrial magnetism
Earth and Moon - 13
Terrestrial magnetism
North Magnetic Pole (NMP) 20° from North Geographic Pole
Continuously moves, on average 40 m per day
Analysis of different age rocks → displacement of NMP during geological eras (polarity inversions)
Cause of magnetism:
Rotation of the outer metallic core (partly ionized) faster than crust
→ `dynamo effect´

15. Magnetosphere Earth’s magnetic field extends through space
Earth and Moon - 14
Magnetosphere
Earth’s magnetic field extends through space
`Shield´ which deflects solar wind charged particles
→ essential for life on Earth
Capture of charged particles
→ Van Hallen belts
Particle overflow
→ enter the atmosphere close to the magnetic poles
→ polar auroras
vent solaire

16. Earth and Moon - 15
Polar auroras (1)
Collision of charged particles with atoms of upper atmosphere
→ excitation of atoms
The excited e– falls back to the fundamental level while emitting a photon E e– hν

17. Earth and Moon - 16
Polar auroras(2)
Green color: atomic oxygen (577.7 nm) (altitude ~100 km)
Purple-red color: nitrogen molecules N2

18. Earth’s atmosphere (1) 75 % of its mass in a 10 km layer Composition:
Earth and Moon - 17
Earth’s atmosphere (1)
75 % of its mass in a 10 km layer
Composition:
N %
O %
Ar %
CO %
H2O 0 – 4 %

19. Earth and Moon - 18
Earth’s atmosphere (2)
Compared to those of Venus (96% CO2) and Mars (95% CO2) Earth’s atmosphere has a very peculiar composition
That peculiarity is related to:
• oceans (dissolve CO2)
• life:
Plants photosynthesis converts CO2 in O2
→ close link between life and atmospheric composition

20. Earth and Moon - 19
Earth’s orbit
Sidereal period: days Angle equator – orbit: 23.5°
Mean orbital radius: ×106 km = 1 astronomical unit (AU)
orbital eccentricity: e =
Excentricity: b a f

21. Earth and Moon - 20
Moon’s orbit
Mean orbital radius: km Angle equator – orbit: 2.6°
Sidereal period: 27.3 days Angle orbit – ecliptic: 5.1°
Synodic period: 29.5 days
(٪ Sun → phases → month)
Orbital eccentricity: e =
Elliptical orbit
+ angle equator – orbit
→ apparent oscillation (libration)
→ 59% of Moon’s surface is visible

22. Characteristics of Moon
Earth and Moon - 21
Characteristics of Moon
• No atmosphere → no erosion
• meteoritic impacts → craters
• `marias´ and `highlands´
• Fully cooled → no tectonic activity
• Mean albedo: 7 %

23. The telluric planets Planet M R g D e Tyr Tday
Mercury j j
Venus j –243d
Earth j h56
Mars j h37
M = mass R = radius g = acceleration of gravity(surface) D = mean distance to Sun (all with respect to Earth)
e = orbital eccentricity
Tyr = revolution period Tday = rotation period (sidereal day)

24. The telluric planets - 2
Mercury
No atmosphere, except H et He captured from solar wind, P ~ 10–12 bar
Orbital eccentricity: e = 0.206
Rotation : 59 d = 2/3 of year
→ gravitational resonance
1 rotation ½ over itself between 2 perihelion passages
→ always a tidal bulge towards the Sun at perihelion
→ faces alternatively hot and cold
Mercury imaged by Mariner 10

25. The telluric planets - 3
Venus (1)
Thick atmosphere, P ~ 90 bar, density ρ ~ 0.1, Tsurface ~ 480°C
CO2 (96%) – N2 (3.5%)
H2O – SO2 – H2SO4 (traces)
Greenhouse effect increases T by 500 K
SO2 → volcanic activity
Retrograde rotation
→ collision with another planet? (but, then, why e ≈ 0?)
Resonance with earth (5 orbits of Venus between each alignement)
Venus in visible light (Galileo)

26. The telluric planets - 4
Venus (2)
Opaque atmosphere → reconstruction of surface relief by radar measurements from probes orbiting Venus (Magellan, 1990)
Reconstruction of Venus surface by radar measurements (Magellan)

27. The telluric planets - 5
Mars
Tiny atmosphere, P ~ bar, Tsurface ~ –140 (night) to +20°C (day)
CO2 (95%) – N2 (3%) – Ar (2%)
H2O – O2 (traces)
g too low to effectively retain the atmosphere
Polar axis inclined (25°) → seasons
Polar caps : H2O + CO2
Weather: sandstorms
Mars seen from Earth (HST)

28. Martians 1877 : Schiaparelli sees straight lines on Mars surface
The telluric planets - 6
Martians
1877 : Schiaparelli sees straight lines on Mars surface
1894 : Lowell builds an observatory and observes the same lines
Channels built by Martians to irrigate dry lands with water from the polar caps!
1970: Mariner probes → channels don’t exist
`Channels´ on Mars and a recent picture

29. Other martian fantasmagories
The telluric planets - 7
Other martian fantasmagories
1976 (Viking 1) : structure resembling a human face (Mars Global Surveyor): where is it gone?…
… at better resolution
`Face´ on Mars…

30. The telluric planets - 8
Mars landscapes
Since 2002, `Spirit´ and `Opportunity´rovers (and `Curiosity´ since 2012) explore Mars → harvest of pictures
Mars = arid desert, with sandstorms from time to time
Mars or Southern Morocco?
Martian landscape

31. Water on Mars? No surface liquid water in present conditions
The telluric planets - 9
Water on Mars?
No surface liquid water in present conditions
Numerous gullies: depth ~ few m width ~ few 10 m length ~ few km (much too narrow to account for Sciaparelli observations)
+ remains of river systems
→ water must have flown on Mars in the past, when the atmosphere was thicker
Gullies observed by MGS

32. The telluric planets - 10
Life on Mars (1)
1976: 2 Viking probes land on Mars at median latitudes
(température from –170 to +few °C)
Soil samples → 4 experiences to detect signs of life
• no organic molecules (< 1/109)
• search for chemical changes due to living organisms (soil samples placed in nutritive environments) : slight changes observed not due to life according to specialists
Viking mission

33. The telluric planets - 11
Life on Mars (2)
1996: analysis of a meteorite found in Antarctica in 1984
• fragment of Mars crust ejected by a big meteorite impact some ~15 millions years ago, fallen on Earth some ~15000 years ago
• some scientists claim that microscopic structures in the meteoritewould be remains from a primitive life form
• it could have appeared some 3.5 billion years ago, under a thicker atmosphere plus dense and with liquid water
→ life on Mars: controversial subject
Meteorite ALH84001

34. The telluric planets - 12
Mars satellites
Phobos and Deimos (sons of Ares): 2 captured asteroids (27 and13 km)
TPhobos < Trot(Mars) → tidal effects reverse from Earth – Moon system → Rorbit decreases → Phobos will crash on Mars (in ~108 years)
Phobos (MGS)
Deimos (Viking)

35. Jovian planets Planet M R g D e Tyear Tday
Jupiter yr 9h55
Saturn yr 10h39
Uranus yr 17h
Neptune yr 16h
M = mass R = radius g = acceleration of gravity (surface) D = mean distance from Sun (all with respect to Earth)
e = orbital eccentricity
Tyear = revolution period Tday = internal rotation period

36. The jovian planets - 2
General properties
• Made of a fluid, which density increases with depth (gradual transition gas → liquid)
• Probably small core made of rocks and metals
• Differential rotation of atmosphere (vequator > vpole)
• Strong magnetic field → allows to measure internal rotation

37. The jovian planets - 3
Jupiter
Upper layers: H2 (78%) + He (20%) + CH clouds of NH3, NH4SH, H2O
Cloud color: solid particles (sulfur, methane compounds)
Great red spot: huge storm (2 × Earth size) discovered by Robert Hooke (1664) = high pressure zone
Jupiter emits more energy than it receives from the Sun (gravitational contraction)
Jupiter (Cassini)

38. Dive into Jupiter’s interior
The jovian planets - 4
Dive into Jupiter’s interior
Gradual increase of pressure and temperature
gaseous H2 et He + clouds
(2) gradual transition to liquid H2 + He (~0.75 RJ)
(3) dissociation of H2 followed b y ionization of H → metallic hydrogen → strong magnetic field (17000 × terrestrial field)
(4) Core of H2O, NH4, rocks, metals (1% of total mass)
Region of the Great Red Spot

39. The moons of Jupiter 16 moons including 12 captured asteroids
… and their moons - 5
The moons of Jupiter
16 moons including 12 captured asteroids
4 largest: discovered by Galileo in 1610
Moon M(MM) R(RM) T(d) g(ms-2) Io
Europe
Ganymede
Callisto
All in synchroneous rotation (tidal effects)
T° ~ –150 °C
The 4 galilean moons

40. … and their moons - 6
Io (1)
D = km from Jupiter
Most active volcanism in the solar system
Eruptions of S and SO2 and not H2O and CO2 as on Earth (probably exhausted)
Volcanism caused by tidal effects (perturbations from other moons → oscillations around the equilibrium position → frictions → heat)
Io (Galileo)

41. Io (2) Surface constantly renewed by volcanic ashes
… and their moons - 7
Io (2)
Surface constantly renewed by volcanic ashes
Gas ejected at v > 1 km/s, part of it escapes and forms a ring around Jupiter
Volcanic eruption on Io
Io in April and September 1997

42. Europa (1) D = 670000 km from Jupiter
… and their moons - 8
Europa (1)
D = km from Jupiter
Very smooth surface (features < 1 km) composed of ices (mainly H2O, with NH3, CO2)
Model:
• Metallic core
• Rocky mantle
• Ocean of water or mud (life?)
• Ice crust (thickness ~100 km)
Europa (Galileo)

43. … and their moons - 9
Europa (2)
Few craters → surface rapidly regenerates → crust not too thinck, nor too rigid
Covered with cracks 10 to 80 km broad, up to 1000 km long
Impact on Europa
Surface of Europa

44. Ganymede (1) D = 1070000 km from Jupiter Biggest moon in solar system
… and their moons - 10
Ganymede (1)
D = km from Jupiter
Biggest moon in solar system
Density: ρ ~ 0.5 ρMoon
→ ± 50% ice
→ prototype of `ganymedian´ objects (as all giant planet moons, except Io and Europa)
Ganymede (Galileo)

45. … and their moons - 11
Ganymede (2)
Surface partly coverded by grooves a few hundred meters deep
Current explanation:
Ganymede still cooling
→ phase transition: water → ice
→ increase of volume
→ cracks filled up by new ice
Surface of Ganymede

46. Callisto D = 1840000 km from Jupiter Ganymede `little brother´
… and their moons - 12
Callisto
D = km from Jupiter
Ganymede `little brother´
No big cracks
→ thicker crust
Craters
→ `Icy´version of our moon
Callisto (Galileo)

47. Saturn Chemical composition similar to Jupiter (1) density ρ < 1
The jovian planets - 13
Saturn
Chemical composition similar to Jupiter
(1) density ρ < 1
(2) fast rotation
→ flattening ~10 %
Emission of energy more efficient than Jupiter : lower temperature → helium droplets falling towards the core → energy from phase transition + gravity
Saturn (Voyager 2)

48. The jovian planets - 14
Seasons on Saturn
Contrary to Jupiter, Saturn’s equator is significantly inclined with respect to orbit (27°)
→ seasons
→ rings seen under different angles from year to year
→ seen by Galileo but not by Huygens, who found the correct explanation
Saturn (HST)

49. The jovian planets - 15
Saturn rings
Rings present around all jovian planets, but by far the most massive and bright around Saturn
Composed of rock and ice blocks of various sizes (from a dust grain to a few meters)
Estimated thickness ~ 10 m
Distance: to km from Saturn center
High albedo (~ 0.6)
Total mass ~ 1020 kg
Saturn rings (Cassini)

50. The jovian planets - 16
The Roche limit (1)
dmin for a satellite whose cohesion is due to its own gravity
Tidal force:
(on a mass element)
Gravitational force (cohesion) : C A d R MP RP

51. The Roche limit (2) This simple calculation neglects:
The jovian planets - 17
The Roche limit (2)
This simple calculation neglects:
– the satellite rotation
– its tidal deformation
These 2 factors weaken cohesion
A more sophisticated computation gives:
Backlighting of Saturn’s rings (Cassini)

52. The jovian planets - 18
Planetary rings
No massive satellites under Roche limit → rings composed of dust and small rocks
Rings = transient or stable structures? >Opinions diverge...
The rings of Jupiter, Uranus and Neptune are:
– much less massive (J, U, N)
– less reflective (U, N):
CH4 ice adds to H2O
→ albedo ~0.05 instead of 0.60
Jupiter ring (Galileo)

53. Titan (1) Moon M(ML) R(RL) T(j) g(ms-2) Titan 1.8 1.48 16 1.4
… and their moons - 19
Titan (1)
Moon M(ML) R(RL) T(j) g(ms-2)
Titan
Main satellite of Saturn
Dense atmosphere → awakened scientists interest
Visited by space mission Cassini – Huygens in 2005
Orbital probe + lander
Methane sea on Titan (Cassini)

54. Titan (2) Psurf ~ 1.5 bar, T ~ –170°C
… and their moons - 20
Ground of Titan (Huygens)
Titan (2)
Psurf ~ 1.5 bar, T ~ –170°C
N2 (98%) + CH4 (2%) organic compounds
Ground covered by organic precipitates + ice pebbles
Best candidate for life in our Solar System?
However temperature is very low…
River system and coast onTitan (Huygens)

55. Other moons of Saturn Hyperion: Diameter ~ 250 km
… and their moons - 21
Other moons of Saturn
Hyperion:
Diameter ~ 250 km
Too small for gravity to dominate → no spherical shape
`Sponge´ aspect
Very low density → inner caves?
Hyperion (Cassini, false colours)

56. Other moons of Saturn Dione: Diameter ~ 1100 km … and many others…
… and their moons - 22
Other moons of Saturn
Dione:
Diameter ~ 1100 km
… and many others…
Dione, Saturn and rings (Cassini)

57. Uranus Discovered by William Herschel in 1781, first named `George´
The jovian planets - 23
Uranus
Discovered by William Herschel in 1781, first named `George´
Atmosphere : H + He
+ CH4 → blueish colour
Density higher than Jupiter and Saturn → less H et He, core probably more important
Equateur at 98° from orbital plane → collision with another planet soon after its formation?
Rings and moons align with the equatorial bulge
Uranus (HST, increased contrast)

58. The jovian planets - 24
Neptune (1)
Existence predicted in 1843 by J.C. Adams on the basis of perturbations of Uranus orbit (nobody took him seriously)
Independant prediction in 1846 by Urbain Le Verrier (already well-known → taken seriously)
Discovered by J.G. Galle at the predicted position
→ triumph of Newtonian theory of gravitation
Neptune (Voyager 2)

59. Neptune (2) Slightly more massive and denser than Uranus
The jovian planets - 25
Neptune (2)
Slightly more massive and denser than Uranus
Atmosphere: H + He + CH4
Stronger meteorological phenomena despite less solar energy received
Lower T° → lower viscosity → less energy needed to activate motion
Spots = high pressure zones
Neptune (Voyager 2)

60. Triton Diameter: 2760 km T° ~ –236°C
… and their moons - 26
Triton
Diameter: 2760 km
T° ~ –236°C
Retrograde orbit with 20° inclination with respect to Neptune’s equator
→ captured TNO?
(Trans-Neptunian Object, see below)
Triton (Voyager 2)

61. The small bodies The Titius – Bode law (1)
1741: mathématicien Max Wolf discovers that distances of planets to the Sun obey a simple law
1766: Johann Daniel Titius rediscovers ans fomalizes this law
1778: Johann Elert Bode publishes the law
Titius
Bode

62. Titius – Bode law (2) Planet k dcalc dobs δd (%) Mercury 0.4 0.39 2.6
The small bodies - 2
Titius – Bode law (2)
Precision ~ 3 %
+ predictive power! (discovery of Uranus)
But: a planet is missing
Where is that 5th planet?
→ searching!
Planet k
dcalc
dobs δd
(%)
Mercury
0.4
0.39
2.6
Venus 1
0.7
0.72
2.8
Earth 2
1.0
1.00
0.0
Mars 4
1.6
1.52
5.3 ? 8
Jupiter 16
5.2
5.20
Saturn 32
10.0
9.54
4.8
Planet k
dcalc
dobs δd
(%)
Mercury
0.4
0.39
2.6
Venus 1
0.7
0.72
2.8
Earth 2
1.0
1.00
0.0
Mars 4
1.6
1.52
5.3 ? 8
Jupiter 16
5.2
5.20
Saturn 32
10.0
9.54
4.8
Uranus 64
19.6
19.2
2.1

63. The small bodies - 3
Asteroids (1)
1801 : Giuseppe Piazzi, founder of Palermo observatory, discovers a body at 2.77 UA from the Sun and names it Ceres (tutelary goddess of Sicily)
• at the predicted distance → new triumph of Titius – Bode law (but failure for Neptune)
• diameter ≈ 940 km
• M ≈ 1021 kg ≈ 1/6000 ME
Later, discovery of more bodies: Pallas, Juno, Vesta…
→ more than at present time
Ceres (Dawn)

64. Asteroids (2) Total mass ~ 1/1000 ME
The small bodies - 4
Asteroids (2)
Total mass ~ 1/1000 ME
Most orbiting in main belt, from 2.2 to 3.3 AU (in between orbits of Mars – 1.5 AU and Jupiter – 5.2 AU)
Nearly circular orbits
A dozen with size > 250 km
Large range in size
Examples: Ida (60 km)
Itokawa (500 m)
Asteroids Ida et dactyl (Galileo)
Asteroid Itokawa (Hayabusa)

65. Asteroids (3) A minority do not belong to the main belt
The small bodies - 5
Asteroids (3)
A minority do not belong to the main belt
Some have rather eccentric orbits which bring them close to Earth (as Eros) or even cross its orbit
They were probably deflected by Jupiter’s gravitational attraction
Other effect of Jupiter : Kirkwood gaps (at 2.50, 2.82, 2.96, 3.27 AU): orbits with periods T = 1/3, 2/5, 3/7 and 1/2 TJupiter
→ gravitational resonances
Asteroid Eros (NEAR)

66. The small bodies - 6
Pluto
1915: from perturbations of Neptune’s orbit, Percival Lowell computes that a planet of 6.5 ME should be found at 42 AU
Searches around the ecliptic, no success
1930: Clyde Tombaugh discovers Pluto
• eccentric orbit: e = 0.25, a = 39 UA
• inclination of 17° onto the ecliptic
• R ≈ 0.18 RE , M ≈ ME
1978: discovery of moon Charon
• d = km from Pluto
• MCharon ≈ 0.17 MPluto
Pluto (New Horizons)

67. Trans Neptunian Objects (TNOs)
The small bodies - 7
Trans Neptunian Objects (TNOs)
Since 1992: discovery of numerous objects further away than Neptune, some with sizes comparable to Pluto
→ International Astronomical Union, 2006 : new definition.
A planet is a celestial body:
• is in orbit around the Sun
• has sufficient mass to assume hydrostatic equilibrium (a nearly round shape)
• has `cleared the neighborhood´ around its orbit
→ Pluto is no more considered as a planet, but as a dwarf planet (new class of celestial bodies)

68. Oort cloud and Kuiper belt
The small bodies - 8
Oort cloud and Kuiper belt
TNOs are located in a ring from the orbit of Neptune (30 UA) up to ~50 AU: the Kuiper belt
A multitude of small bodies probably orbit even much further away, in a spherical shell at about
Gerard Kuiper
AU (a light-year) from the Sun
Its existence has been postulated in 1932 by Öpik, then in 1950 by Oort to explain the origin of long period comets
Short period comets come from Kuiper belt
Ernst Öpik
Jan Oort

69. The small bodies - 9
Comet nuclei
Comet nucleus = agregate of rocks and ices (size: a few km)
Gravitational perturbation → leaves Oort cloud
→ hyperbolic or high eccentricity elliptical orbit (→ periodic or not)
→ enters the inner solar system
When d < 3 UA → ices start to sublimate
Nucleus of comet Tempel 1 (Deep Impact)

70. The small bodies - 10
Comet nuclei & Rosetta
2014: the Rosetta mission reaches comet 67P/Churyumov-Gerasimenko and the Philea module lands on the nucleus
Nucleus of comet P67 (Rosetta)

71. Comets Sublimation of ices → `coma´ composed of gas and dust
The small bodies - 11
Comets
Sublimation of ices → `coma´ composed of gas and dust
Solar wind and radiation pressure → drag particles → tail (direction opposite to the Sun, length up to 0.5 AU)
Orbital motion of comet → the tail `trails´
Effect is stronger for dust, which moves slower → 2 tails
Light emission: fluorescence (gas) or diffusion of sunlight (dust)
Comet Hale-Bopp

72. The small bodies - 12
Halley’s comet
In 1705, Edmund Halley realizes that the comets of 1531, 1607 and 1682 are the same object, seen at succesive orbits
→ predicts it will return in 1759
Most famous of short period comets (76 years)
Last appearance to date: 1986
Visited by several space probes
Edmund Halley
Halley’s nucleus (Giotto)

73. Comets: astronomical fossils, sources of life?
The small bodies - 13
Comets: astronomical fossils, sources of life?
• Stayed in the outskirts of the solar system until recently
→ not modified by physico-chemical processes
→ chemical and isotopic composition unchanged since formation of the solar system
• Contain many simple organic molecules → did they have some influence on the emergence of life on Earth?
Comet Swan

74. The small bodies - 14
Shooting stars
Brief luminous flash produced by aerodynamic heating of a small body entering the atmosphere (typical size ~ 1 cm ; altitude ~ 100 km)
Total ~ 10 tons / day
Often cometary debris → more numerous when Earth crosses the orbit of a (past) comet
Perseids (~11 August)
Leonids (~11 November) …
Perseids 2004 (F. Bruenjes)

75. Meteors and fireballs (1)
The small bodies - 15
Meteors and fireballs (1)
Larger size bodies which burn (partially or completely) in the atmosphere
Speed ~ 30 km/s ~ km/h
Pieces of comets or asteroids
If they reach the ground → danger
1911, Egypt: dog killed by a 40 kg meteorite
1954, Alabama: woman leg hurt by a meteorite which went through the roof of her house
Meteor Perseid 2004 (C. Mouri)

76. Meteors and fireballs (2)
The small bodies - 16
Meteors and fireballs (2)
1972: a bolide ~ 1000 tons flies 60 km over North America and bounces back to space
1992, New York State: car damaged by 12 kg meteorite
No record of human being killed by meteorite in last millenium
Luckily, most recent major impacts happened in uninhabited locations
Car damaged by meteorite (1992)

77. The Tunguska impact (Siberia)
The small bodies - 17
The Tunguska impact (Siberia)
30 June 1908: asteroid or comet nucleus (mass ~ tons)
explodes in Siberia
Seismic wave recorded km away
Trees lying down in a 30 km radius
No crater → the meteor exploded several km before touching ground (power ~ 1000 times the Hiroshima bomb)
The Tunguska impact

78. The small bodies - 18
Major impacts
Frequency of Tunguska-like imapcts: ~ one every few centuries
Meteor Crater, Arizona: diameter 1.2 km; depth 200 m, happened ± years ago
Iron meteorite ~ 109 kg (60 m diameter)
Largest known crater: Chixculub, Yucatán : diameter 200 km, dated 65 millions years
= extinction of dinosaurs
Meteor Crater, Arizona