Rotation and Interior of Terrestrial Planets

Transcription

1 Rotation and Interior of Terrestrial Planets Veronique Dehant and Tim Van Hoolst Royal Observatory of Belgium

2 introduction WHAT DO WE KNOW ABOUT THE MEAN ROTATION AND INTERIOR OF THE PLANETS?

3 Orbit, rotation and orientation characteristics of the 8 planets of the Solar System at present Planets Orbital periods (Earth days or years) Rotation periods (Earth hours or days or years) Obliquity Mercury days days s 2.0 arcmin l days Venus days o (retrograde) w days r Earth 23h56m04s or 1 year a p days 24h37min23s Mars i or ~2 years or daysd e r Jupiter years 9.55h x 3.1 tr a Saturn years 10.32h 26.7 e p Uranus years 17.24h m i e 97.8 d Neptune years 16.11h l y 28.3

4 NASA NASA Interior Structure [Courtesy: BepiColombo Study Report]

5 Terrestrial planets OBSERVATION OF THE ROTATION OF PLANETS?

6 Nutation measured by VLBI Very Long Baseline Interferometry

7 Quasar Earth s orientation in space

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11 NASA s DSN and ESA s ESTRACK networks of tracking stations ESA s 34 m. antenna at New Norcia & Cebreros (Mars Express, Venus Express) Radio-link for data & telemetry Doppler & range radio-tracking mainly at frequencies of around 8 Ghz (X band) (dual-frequency X/S bands to correct ionospheric and interplanetary plasma perturbations)

12 Mars missions MGS ODY MEX future TGO MRO MAVEN MERs MSL Phoenix ExoMars EDM InSIGHT

13 Lander driving science requirements Doppler measurements between the lander on Mars and the ground station on Earth in X-band. X-band radiolink maser Coherent transponder

14 InSIGHT: A Geophysical Mission to Mars Interior exploration using Seismic Investigations, Geodesy, and Heat Transport

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17 Terrestrial planets VARIATIONS WITH RESPECT TO THE MEAN ROTATION AND INTERIOR OF THE PLANETS?

18 Two terrestrial planets rapidly rotating flattened, inclined, thus large precession and nutation ROB

19 Terrestrial planets VARIATIONS WITH RESPECT TO THE MEAN ROTATION AND INTERIOR OF THE EARTH?

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21 Why is the length-of-day not constant? External excitation Atmosphere Ocean Earth interior (mantle, outer core, inner core)

22 Long-term Earth rotation variations Days per year 380 Hours per day TRIAS JURASSIC CRETACIAN TERTIAIRY years 370 DEVONIAN CARBONIFEROUS PERMIAN Million of years

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24 Elastic Earth Earth rotation Tidal bulge Moon Rotation axis Inelastic Earth Earth rotation Rotation axis Tidal bulge additional torque Moon

25 Days per year years Million of years Hours per day DEVONIAN CARBONIFEROUS PERMIAN TRIAS JURASSIC CRETACIAN TERTIAIRY Decadal variations

26 Decadal variations 20 years Years Variation of length of day (milliseconds)

27 Coupling at core-mantle boundary

28 years Decadal variations Variation of length of day (milliseconds) Seasonal/inter-annual Years variations

29 Seasonal variations years Variation of length of day (milliseconds) Year Drawn from

30 Length-of-day computed from atmospheric angular momentum Observed length-of-day milliseconds Drawn from

31 Length-of-day computed from atmospheric angular momentum Observed length-of-day milliseconds November 2012 November 2014 Drawn from

32 Effect of the ENSO cycle on the LOD El Nino events milliseconds La Nina events Time (year)

33 ecliptic Sun

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35 equator ecliptic Constant torque (time averaged): precession, similar to motion of a top

36 periodic torque: obliquity nutation 23 o 27 equator ecliptic

37 in obliquity Nutation in obliquity as a function of time, starting at J2000.

38 Nutation in longitude as a function of time, starting at J2000 in longitude

39 Nutations for 18.6 yrs, starting from J2000 in obliquity in longitude

40 ROB

41 rotation axis of the mantle rotation axis of the core ROB

42 rotation axis of the core rotation axis of the inner core ROB

43 ROB

44 ROB

45 Some results concerning Earth interior From nutations: flattened core is not in hydrostatic equilibrium: increase of equatorial radius of about 350m From nutations: electromagnetic field is important at CMB if ignoring viscous and topographic torques, more important than downward continuation of surface field From LOD: inner core gravitational coupling, torsional oscillations, explain decadal timescale fluctiations.

46 Earth s response to external forcing

47 Earth s response to external forcing

48 Earth s response to external forcing

49 Earth s response to external forcing

50 Terrestrial planets VARIATIONS WITH RESPECT TO THE MEAN ROTATION AND INTERIOR OF MARS?

51 Mars response to external forcing

52 Mars response to external forcing

53 Mars response to external forcing

54 Mars response to external forcing

55 Mars response to external forcing

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57 Z ecliptic Rotation Axis Precession Nutation Y ecliptic = 0 + X ecliptic 57

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59 Bratio for the nutation of Mars; FCN Measurement of nutation... prograde semi-annual nutation

60 Meter in a plane tangent to the planet Nutation for different core dimensions Measurement of nutation Residuals between solid and liquid case

61 Retrieving the core radius from the core moment of inertia Range of 300km Work of ROB Relative core moment of inertia C core /C tot 6161

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63 Mars radioscience objectives Measurement of Length of day variations

64 Some results concerning Mars interior Tides and Precession allow to conclude that core is at least partially liquid Tides and Precession allow to constrain liquid core dimension at 1800km ±150km If the light element in the core is Sulfur, there is no inner core. More to come soon InSIGHT 2016!

65 Terrestrial planets VARIATIONS WITH RESPECT TO THE MEAN ROTATION AND INTERIOR OF VENUS?

66 The twin sister of Earth?... Not for the rotation! Venus Terre

67 Retrograde rotation of Venus Possible causes? Original collision/state Slow down of the rotation until developing a retrograde rotation Slow down of the rotation and flip of the rotation axis What slows down the rotation? Tidal friction Internal friction (tends to favor 0 or 180 degree inclination) See Laskar s talk Interior? From tidal Love number: liquid core

68 Terrestrial planets VARIATIONS WITH RESPECT TO THE MEAN ROTATION AND INTERIOR OF MERCURY?

69 Sun Mercury Mercury 1 st rotation

70 2 d rotation Sun 1 st revolution Mercury 1 st rotation

71 Decelerate the rotation 2 d rotation Sun 3 d rotation 2 d revolution 1 st revolution 1 st rotation Accelerate the rotation

72 Solid core Some results for Mercury s interior 190 m Liquid core 455 m ± as ±1.6 Margot et al., 2012 The radius of the core is 2000 km ± 40km (Rivoldini and Van Hoolst, 2013) using Messenger (Smith et al. 2012) gravity field and libration and obliquity (Margot et al., 2012)

73 Conclusions Planetary geodesy is a very helpful for studying planets or moons of the solar system. In particular (this talk), for obtaining their rotation and orientation, and therewith for obtaining properties of the interior of these planets or moons.