Terrestrial Planet Exploration: Earth as an example



Surface features


Chemistry: ocean crust chemically different than continental crust; hydrated minerals (evidence of water); evidence of life (analysis by in situ measurements, returned samples; or remote sensing from orbiting satellite)


Cratering: craters rare on earth due to tectonism and erosion; craters most likely to be found on oldest crust (continental shields up to 2 billion years old)


Crustal ages: ocean crust youngest (less than 200 million years old and still forming by sea floor spreading); continental crust up to 2 billion years old). Dating transportable to other terrestrial planets  include:


                        Isotopic ratios

Crater density (theoretical relationship between age and crater density; calibrated by lunar rocks).


Bimodal surface: stripping off the water from oceans, leaves a lower lying oceanic crust, suggesting fundamental density and chemical differences between ocean and continental crust


Linear features: mountain belts, faults, morphology of sea-floor spreading and transform faults in oceans observable from both direct bathemetry measurements as well as remotely from measuring gravity variations from satellites.






Temperature profile (convecting (windy) regions have higher dT/dz)


Scale height (height at which pressure is exp(-1) x surface pressure (about 1/3 surface P)




Mass and bulk density


For planets less 3000 km or less in radius, bulk densities on the order of 3 gm/cm**3 consistent with silicates.  Larger values suggest iron enrichment.  For Earth size planets (radius 6000 km or larger) compression due to pressure needs to be factored into the analysis. 


Gravity and topography


Gravity potential: Potential shape and precession rate give moment of inertia (constraint on existence of dense core or internal homogeneity (I = 0.4 MR**2 for homogeneous sphere).  Ellipticity of potential can also be used to measure amount of “tidal despinning” (slowing down of rotation due to tidal friction).


Wavelength at which gravity correlates or  decorrelates with topography: Can measure thickness of mechanically more rigid surface layer (plate or lithosphere).


Spatial variation of gravity:  Can locate density anomalies near the surface, e.g., the masscons of Earth’s moon.




Radial discontinuities in chemistry and solid/liquid phase:  discontinuities an be identified with high precision, e.g., Earth’s crust/mantle, mantle/outer-core, and inner core/outer core boundaries.


Smaller spatial scale variations of seismic velocities: Seismic tomography (3-D image of seismic velocities) can be used to identify temperature and chemical anomalies).


Temperature and heat flow


T with depth: Characteristic temperature versus depth profile of a terrestrial planet has a step gradient (dT/dz) near the surface, which flattens when it approaches the melting temperature of the planet.  The point where this transition occurs can be used to estimate plate thickness (100-200 km on Earth).  Transition marks a change to lower viscosity, more plastic mantle.





Curie temperature: Temperature at which rocks can preserve a magnetic field is on the order of 500 deg C.  This temperature is exceeded at about 10 km beneath Earth’s surface.


Paleomagnetism: Spatial variations of the magnetic field preserved in the upper 10 km of the Earth  confirms existence of active plate tectonics, its unique form and morphology on Earth, and can map the history of plate movements.


Evidence of active dynamo: Earth has the strongest magnetic field (primarily dipole measured at surface) of any terrestrial planet.  Since temperature exceeds 500 deg C. below 10 km from surface, this field could only be the result of an internal dynamo.


Mechanism of internal : An electrically conducting fluid in motion due to either heat transport by convection due to cooling of liquid metallic outer core, radiogenic heat in the outer core, and/or solidification of Earth’s inner core. In gas giants this moving metal is hydrogen (H is a metal with free electrons at high pressure).


Planet rotation: Important in orienting the magnetic dipole nearly conincident with the geographic (rotational or “true” north/south pole).


Venus vs. Earth: Venus, in contrast to Earth, may have a weak magnetic field because it slightly smaller radius may put iron in the P-T (pressure temperature) field of solid iron throughout its core (no fluid motion in a metallic core).