1 / 46

ESS 250: MARS

ESS 250: MARS. Dave Paige / Francis Nimmo. Lecture Outline. Bulk structure and composition Geochemical constraints Geophysical constraints Properties of the crust and lithosphere Density, thickness, rigidity Magnetic properties Evolution of Mars through time.

kayla
Download Presentation

ESS 250: MARS

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. ESS 250: MARS Dave Paige / Francis Nimmo ESS 250 Winter 2003

  2. Lecture Outline • Bulk structure and composition • Geochemical constraints • Geophysical constraints • Properties of the crust and lithosphere • Density, thickness, rigidity • Magnetic properties • Evolution of Mars through time ESS 250 Winter 2003

  3. Bulk Structure and Composition • Cosmochemical arguments • SNC meteorites • Remote sensing • In situ measurements • Mass and Moment of inertia • (Seismology) ESS 250 Winter 2003

  4. Cosmochemical constraints • The solar photosphere and carbonaceous chondrites (C-I) have essentially the same composition • In the absence of any other information, we can assume that all solar system bodies started with roughly this composition Basaltic Volcanism Terrestrial Planets, 1981 ESS 250 Winter 2003

  5. SNC meteorites • Shergotty, Nakhla, Chassigny (plus others) • What are they? • Mafic rocks, often cumulates • How do we know they’re from Mars? • Timing – most are 1.3 Gyr old • Trapped gases are identical in composition to atmosphere measured by Viking. QED. 2.3mm McSween, Meteoritics, 1994 ESS 250 Winter 2003

  6. What do the SNC’s tell us? Mars • Assume the abundances of a few key elements e.g. Mg, Si, Al (using geophysical, cosmochemical and observational constraints) • Then use ratios to infer abundances of other elements • Without the SNC’s, we would have to simply assume an Earth-like or CI-like mantle Earth Dreibus and Wanke, Meteoritics, 1985 ESS 250 Winter 2003

  7. What do the SNC’s tell us? (cont’d) Estimated bulk Martian mantle • Mars is iron-rich relative to Earth • Mars is volatile-rich relative to Earth • Mars is depleted in chalcophile (sulphur-loving) elements volatiles iron Note that Al,Mg,Mn are assumed chondritic chalcophiles Dreibus and Wanke, Meteoritics, 1985 These observations provide information about the manner in which Mars accreted, and the likely composition of the core ESS 250 Winter 2003

  8. Remote Sensing Observations (1) • K,U,Th are naturally radioactive and emit gamma rays as they decay • Other elements may emit gamma rays due to excitation by incoming cosmic rays • The energy of these gamma rays is characteristic of the element emitting them • So an orbiting gamma ray spectrometer can estimate the abundances of near-surface elements Initial results from Odyssey GRS showing variations in near- surface potassium concentrations ESS 250 Winter 2003 http://photojournal.jpl.nasa.gov/catalog/PIA04255

  9. Remote Sensing (2) “basalt” • Different minerals have different thermal infra-red spectra, controlled mainly by the characteristic bond energies • These energies can be measured remotely by an infra-red spectrometer, and the surface mineralogy determined • Doing so is very tricky, mainly because the effects of the CO2 atmosphere and atmospheric dust have to be subtracted • There is some ground-truth from Earth which suggests that the technique can work • It has been proposed that there are two types of mineralogy present: a “basaltic” type and an “andesitic” type. The latter is significant because andesites on Earth are associated with subduction zones. • One problem with the data is that IR emissions come from the top few microns, so it is not clear what the subsurface is doing “andesite” Bandfield et al, Science 2000 Using MGS-TES data ESS 250 Winter 2003

  10. In Situ Measurements • Pathfinder measured rock and soil compositions using an Alpha Proton X-Ray Spectrometer (APXS) • This works by irradiating a sample with Alpha particles and detecting the particles/radiation given off • The instrument appears not to have been properly calibrated (!), so the results have been subject to revision APXS Mars Pathfinder, 1997 • The two Viking landers (1976) employed similar technology but only on soil samples (not rocks). ESS 250 Winter 2003

  11. Results As well as the calibration problems, the rock surfaces appear to be sulphur- rich due to a kind of “desert varnish”, so the surface does not reflect the interior composition From Wanke et al., Space Sci. Rev., 2001 The “soil-free rock” may be enriched in silica relative to basalts?? The soil looks like a mixture of basaltic (shergotty-like) material plus this more silica-rich end-member Silica-rich rocks are important because on Earth they are often (but not always) associated with subduction (e.g. andesites) ESS 250 Winter 2003

  12. Moment of Inertia • I = S m r2 • r is perpendicular distance from axis • I of sphere = 0.4 MR2 • Objects with mass concentrated in centre have I/MR2 < 0.4 • So I tells us about internal structure • Normally, we measure C, the maximum MoI r m C = max. moment of inertia A = min. moment of inertia I = (A + B + C)/3 How do we measure it? ESS 250 Winter 2003

  13. Obtaining the MoI How do we get moment of inertia (C)? Three steps: 1) Obtain (C-A) from observations of orbiting spacecraft 2) Obtain (C-A)/C from observations of planetary precession 3) Use 1) and 2) to obtain C 1) Gravitational potential of a planet is given by r f where R So observations of the spacecraft orbit* give us J2 *The rate at which the node precesses ESS 250 Winter 2003

  14. Obtaining the MoI (cont’d) 2) Rate of planetary precession depends on (C-A)/C Spin axis wobbles (precesses) with time This is due to gravitational torques on a non-spherical (flattened) object The magnitude of the torque depends on C-A (larger flattening = bigger torque) But the response to the torque goes as 1/C (larger MoI=smaller response to torque) So the period of the precession is proportional to (C-A)/C Axis precesses Sun torques 3) Precession gives us (C-A)/C, measuring J2 gives us (C-A) So we can obtain C directly ESS 250 Winter 2003

  15. MoI of Mars • Inferred value of C is 0.3662 +/- 0.0017 • This is less than 0.4, so Mars has its mass concentrated towards the centre – suggests an iron core • Details of the structure depend on what elements are present, but suggest a core radius 1300km-2000km Models of Martian interior from Bertka and Fei, Science 1998. They use various different core compositions ESS 250 Winter 2003

  16. A liquid core (?) • Just like the Earth, Mars experiences solid-body tides due to the gravitational attraction of the Sun • The response to these tides depends on the interior structure of the planet • In particular, a planet with a (partially) liquid core will deform more than a planet with a solid, rigid core • Because the shape of the planet affects its gravitational potential, the tidal deformation of the planet can be measured by observing the changes in spacecraft orbit • The dimensionless response of the gravity field to the tides is given by the tidal Love number k2. Sun Liquid core (large k2) Solid core (small k2) ESS 250 Winter 2003

  17. A liquid core (?) cont’d • Yoder et al. (Science 2003) have obtained a Love number which is only consistent with an at least partially molten liquid core Edge-on orbit Love Number Angle to Sun Face-on orbit • Is this surprising? Maybe not, since the SNC evidence suggests that there is a lot of sulphur in the core, and sulphur is a very good antifreeze • What does this mean about the Martian geodynamo? (see later) ESS 250 Winter 2003

  18. Line-of-sight (LOS) to Earth spacecraft z surface Gravity • Variations in surface gravity cause spacecraft orbit to vary • Orbit variations cause doppler shift in radio signals to Earth • Doppler shift can be used to infer line-of-sight (LOS) velocity changes • With enough observations, the LOS velocity changes can be used to reconstruct the gravity field The importance of gravity measurements is that they provide one of the few ways of inferring the subsurface properties of Mars. Seismology is another (better) way, but it is not a priority for current NASA Mars exploration ESS 250 Winter 2003

  19. Gravity (cont’d) • Requires enormous precision (detect mm/s variations in velocity, orbital velocity is km/s) • Main limitation is spacecraft altitude (determined by atmosphere, ~200km for Mars) • Gravity signals are attenuated by a factor of exp (- k z) • where k is the wavenumber (=2p/l) and z is the spacecraft altitude • So it is hard to detect gravity signals with wavelengths shorter than the spacecraft altitude • Same goes for magnetic signals ESS 250 Winter 2003

  20. Gravity Map topography • Tharsis Montes have obvious signals • Big basins show small signals (compensated) • Utopia & Isidis Basins are mascons (see later) • Dichotomy is not obvious • Anomalies in S highlands are small Free-air gravity ESS 250 Winter 2003 From Zuber et al., Science 2000

  21. Gravity Signals • There are two end-member cases which are easy: • 1) Crust/lithosphere has no strength (isostatic compensation). Here surface loads are supported by lateral variations in crustal thickness or density “Airy Isostasy” r1 > r2 “Pratt Isostasy” crust mantle • To a first approximation, isostatically compensated terrain produces no gravity anomaly at all, Dg~0 • 2) Completely rigid crust. Here the surface load gives rise to a gravity anomaly Dg = 2 Gp rh where G is the gravitational constant, r is density and h is thickness h r ESS 250 Winter 2003

  22. Gravity Signals (cont’d) • A rigid load of thickness 1km and density contrast 1000 kg/m3 produces a gravity anomaly of 42 mGal • 1 mGal = 10-5 m s-2. MGS gravity accuracy is a few mGal. • So large gravity anomalies (100’s mGal) imply rigidly supported loads, small gravity anomalies imply isostatically supported loads • So we can use gravity to infer how loads are supported on other planets • In order to understand this use of gravity, we need to understand how loads are supported by the lithosphere . . . load lithosphere ESS 250 Winter 2003

  23. How are loads supported? • A very useful model for how loads are supported assumes that the lithosphere is elastic • Elastic materials deform according to the following equation: (A) • Here D is the rigidity, w is the deflection, r is the density contrast between mantle and surface and L is the load • The rigidity can be expressed as an effective elastic thickness Te • Here E is Young’s modulus and s is Poisson’s ratio • Te is measured in km and is a more intuitive measure than D ESS 250 Winter 2003

  24. How are loads supported? (2) If you assume a sinusoidal load, then you can use equation (A) to infer whether a load is closer to isostatic or rigid support. If the rigidity is very low or the load wavelength is long, you get (Airy) isostatic support; if the rigidity is very large or the load wavelength short, you get rigid support. The crossover from rigidly-supported to isostatically-supported depends on the quantity where D is the rigidity (see above), k is the load wavenumber, Dris the density contrast between mantle and crust and g is acceleration due to gravity. How do we derive this? This quantity gives the deflection due to a load, relative to the isostatic deflection (it is small for large D, =1 if D=0). ESS 250 Winter 2003

  25. Examples gravity • Large impact basins are generally isostatically compensated – low rigidity • Big volcanoes are mainly flexurally supported sediments • Some impact basins were flooded with dense lavas after the lithosphere cooled and strengthened, resulting in large positive gravity anomalies over negative topography - mascons dense lavas ESS 250 Winter 2003

  26. Back to Gravity . . . • Isostatic loads produce small gravity anomalies; rigidly supported loads produce large gravity anomalies • Short wavelength loads are rigidly supported; long wavelength loads are isostatically supported • The crossover wavelength depends on the rigidity of the lithosphere • So the gravity as a function of wavelength contains information on how loads are supported • We form a quantity called the admittance, Z(k), which is the ratio of the gravity to the topography at a particular wavelength: Z(k) = g(k) / h(k) • We can plot the calculated admittance from real data and compare it to theoretical predictions to infer the rigidity, density and thickness of the crust/lithosphere ESS 250 Winter 2003

  27. Admittance depends on crustal thickness and Te Admittance depends on crustal thickness, membrane stresses and convection From McKenzie et al. EPSL 2002 At this wavelength, Dk4/Drg=1, giving the crossover from isostatically-supported to rigidly supported Theoretical Admittance • At very short wavelengths, the admittance will be 2pGr mGal/km (why?). So the short-wavelength admittance gives us the surface density rdirectly. • The crossover wavelength from rigidly-supported to isostatically-supported depends on the rigidity D. Admittance constant, depends on crustal density (2prG) ESS 250 Winter 2003

  28. Theoretical Admittance (cont’d) • The admittance changes slightly with crustal thickness: a larger crustal thickness gives rise to a slightly higher admittance (why?) • There are tradeoffs betweenTe, crustal thickness and r which make it difficult to estimate all three properties (although r can be uniquely determined using short-wavelength data). Why might this not be possible in practice? • If there are subsurface loads, the non-uniqueness problem becomes worse. The presence of subsurface loads gives rise to incoherence between the gravity and topography. • On a small planet such as Mars, the sphericity of the body is important in the elastic deformation equations • Finally, convection can be an important source of topography at long wavelengths (and has a characteristic admittance). ESS 250 Winter 2003

  29. Admittance results (1) We calculate the admittance by taking the ratio of the observed gravity to topography as a function of wavelength: Z(k)=g(k)/h(k) We compare the results with theoretical predictions to infer Te, r etc. McKenzie et al., EPSL 2002 McGovern et al., JGR 2002 Spherical harmonic degree Different approaches produce broadly similar results, giving us confidence in the underlying technique ESS 250 Winter 2003

  30. Admittance results (2) Large Te Small Te • Young features (e.g. Olympus Mons) have very large elastic thicknesses (Te > 100 km) • Older features have lower elastic thicknesses • This may reflect a decline in heat flux with time, like the increase in rigidity with age for oceanic plates on Earth • The very longest wavelength behaviour is difficult to interpret e.g. we can’t tell whether Tharsis is supported by convection or by variations in crustal thickness Decreasing age McGovern et al., JGR 2002 ESS 250 Winter 2003

  31. Te and heat flux • Terrestrial oceans show a (rough) correlation between plate age and Te • This is (presumably) because older plates are colder and more rigid • A good rule of thumb is that the depth to the 600oC isotherm gives you Te • So by measuring Te we can infer the heat flux • The method works much less well on terrestrial continents, because of the thick crust Watts & Zhong GJI, 2000 ESS 250 Winter 2003

  32. Observed gravity Observed topography Assumed mean crustal thickness Inferred crustal thickness variation Crustal thickness • We can use the admittance approach to infer the crustal thickness, but there are tradeoffs with Te and density which means the uncertainties are large • An alternative is to assume a mean crustal thickness and use gravity and topography to calculate the crustal thickness variations. This is the approach of Zuber et al., Science, 2000 and does not require any assumptions about rigidity. • They assume a mean value of 50km, so that the largest impact basins would not penetrate to the mantle ESS 250 Winter 2003

  33. Crustal thickness (2) • If there are crustal thickness variations, these produce pressure gradients which drive flow in the lower crust • The rate of flow depends on the crustal rheology, temperature and thickness • To preserve the observed Martian topography, crustal flow must not have been significant over 4 Gyr Nimmo & Stevenson (2001) • Given estimates of crustal heat flux and rheology, this approach places upper bounds on the crustal thickness of ~100km (thicker crust produces more rapid flow) Observed topo. Model topo. (decays with time) ESS 250 Winter 2003

  34. Magnetism • Spatial resolution limited in same way as gravity (e-kz term) • E.g. terrestrial mid-ocean ridge stripe width ~10km, so they cannot be detected by spacecraft at altitude ~400km • Magnetic fields are similar to gravity fields, but interpretation is complicated by fact that magnetization is a vector quantity (while mass is a scalar) • Magnetic field (vector) is measured in units of nT (nano-Tesla) • The Earth’s magnetic field intensity (at the surface) is appx. 40,000 nT (varies spatially, slowly varies in time) • Crustal magnetism is due to induced magnetization (from background field) and remanent magnetization (acquired when the rock last cooled below its Curie temperature) • Typical crustal magnetic anomalies are ~100 nT ESS 250 Winter 2003

  35. Magnetism (cont’d) • Gravitational and magnetic potentials differ because magnetic fields are generated by dipoles q Magnetic r Gravity +p/2 r d -p/2 M Magnetic dipole moment m=dp m0 is a constant (but depends on which units you use . . .) Note lack of directional term Magnetization of a material is its magnetic moment per unit volume (SI units A/m) Note that dipole mag. field (=dW/dr) falls off as the cube of distance ESS 250 Winter 2003

  36. Magnetic Observations • From the early Mars spacecraft, we know that Mars does not have a global magnetic field like the Earth’s • A big surprise of MGS was that there are large, local magnetic anomalies present From Acuna et al, Science 1999 Note the peculiar projection Largest terrestrial magnetic anomalies are ~100nT at similar altitudes ESS 250 Winter 2003

  37. Comparison with Earth • Amplitude of crustal magnetic anomalies is ~10 times bigger on Mars than Earth. Why? • Power spectra give information on wavelength-dependence of field • Slope of crustal component of power spectrum for Mars suggests mean depth of magnetization ~50km • Young MOR basalts have magnetizations up to ~30 A/m, but with age this reduces to ~5 A/m • These magnetizations require layer thicknesses of 15-100 km to explain the observed anomalies Magnetic power spectra for Earth and Mars Slope gives depth to top of Earth’s core Mars Mars crustal anomalies bigger Earth Mars lacks core field ESS 250 Winter 2003 Voorhies et al., JGR 2002

  38. Stripes? • A potentially very significant discovery – magnetic stripes on Earth are the best signature of plate tectonics • Did Mars have ancient plate tectonics? • NB the map projection accentuates the stripeyness! • They look rather different from terrestrial stripes – larger wavelength, larger amplitude • If there were shorter-wavelength stripes, we couldn’t see them (limitations of spacecraft altitude again). Connerney et al., Science 1999 ESS 250 Winter 2003

  39. What use are they? • The fact that the large impact basins do not show any magnetic anomalies suggests that the Martian dynamo was not operating at the time these basins formed (~4 Gyr B.P.) • The ancient Martian meteorite ALH84001 does show magnetization, suggesting that the Martian dynamo operated early in Mars’ history, and then stopped (see later) • The depth of the anomalies tells us something about the temperature structure of the crust (parts of the crust that exceed the Curie Temperature will not retain magnetisation) • How do we estimate the depth of the anomaliess?: • Power spectral approaches (see before) ~50km • Effects of impact craters (big ones show demagnetization, small ones don’t – critical size depends on thickness of magnetized layer) ~50km • Layer must be thick enough to produce correct amplitude – depends on the magnetization of the material (see before) ~15-100km ESS 250 Winter 2003

  40. Evolution of Mars • What constraints do we have? • Te varies with age – can be linked to changing heat flux • Magnetic field operated early on, then stopped • Present-day state of core (at least partially molten) • Present-day crustal thickness (but how and when it formed is uncertain) • Volcanism – voluminous early on, but has carried on at a reduced rate to the present day (difficult to quantify) • Atmospheric isotopes – 40Ar contains degassing record, integrated over Martian history ESS 250 Winter 2003

  41. Modelling Martian Evolution • Convecting system consists of adiabatic interior and conductive boundary layers • Heat transported across conductive boundary layers (e.g. core-mantle boundary (CMB), lithosphere) • Thickness of boundary layers depends on the (T-dependent) mantle viscosity • Radioactive heat sources decay with time • Mantle and core thermal evolution can be tracked temperature top b.l. depth adiabat bottom b.l. CMB this region determines the core heat flow ESS 250 Winter 2003

  42. Core Behaviour (1) • The core needs to be convecting in order to produce a dynamo • The maximum heat flux it can get rid of without convection occurring is given by the adiabat: • Here k is conductivity, a is thermal expansivity, g is acceleration due to gravity, T is temperature and Cp is specific heat capacity. Typical Martian value ~15 mW m-2 • The heat flux out of the core is controlled by the mantle’s ability to remove heat • If the heat flux out of the core drops below the critical value F then core convection will stop and the dynamo will cease ESS 250 Winter 2003

  43. Pure iron melting adiabat Fe+14 wt% S melting Figure courtesy Pierre Williams Core Behaviour (2) • If there is a solid inner core, convection can also be driven by latent heat and chemical buoyancy as the inner core freezes • For Mars, we know that the core is at least partly liquid • Under Martian conditions, the adiabat and iron melting curve are almost parallel (see below) • So it is likely that the entire core is liquid • Note the effect that sulphur has on the core melting temperature Iron melting curves and adiabat. Solid inner core arises when the adiabat and the melting curve cross. ESS 250 Winter 2003

  44. Typical Martian Evolution • Convection without plate tectonics (“stagnant lid”) is rather inefficient at getting rid of heat – Mars cools slowly • Surface heat flux tracks heat production by radiogenic elements • Core heat flux drops rapidly – difficult to sustain a geodynamo for ~0.5 Gyr Adiabatic heat flux Core heat flux Nimmo and Stevenson, JGR 2000 ESS 250 Winter 2003

  45. Sustaining a geodynamo? • To sustain a geodynamo we need some way of increasing the heat flux out of the core. Several possibilities: • 1) Plate tectonics – mantle cools faster, which means that more heat is extracted from the core. When plate tectonics stops, the geodynamo will also cease. Unfortunately, not clear that plate tectonics ever happened . . . • 2) Potassium in the core – acts as an additional heat source. But the half-life of potassium is long (1.25 Gyr) – means that the dynamo lasts too long • 3) A hot core – when the core differentiates it is likely to end up hotter than the mantle (gravitational energy). A hot core will increase the core heat flux temporarily, and can sustain a geodynamo for 0.5 Gyr. ESS 250 Winter 2003

  46. Summary • SNC chemistry and surface measurements can be used to infer bulk and surface composition of Mars • Observations of spacecraft orbit and planetary precession can be used to derive moment of inertia and thus internal structure • Gravity and topography can be used to infer lithospheric rigidity, density and crustal thickness (admittance technique) • Variations in lithospheric rigidity with time can be used to infer change in heat flux • Magnetic observations give indication of depth of magnetized layer and suggest dynamo only lasted for first ~0.5 Gyr • Sustaining a dynamo for this length of time may require either a hot core or an episode of plate tectonics • Both gravity and magnetic resolution are limited by the altitude of the spacecraft ESS 250 Winter 2003

More Related