MIT OpenCourseWarehttp://ocw.mit.edu 12.002 Physics and Chemistry of the Earth and Terrestrial PlanetsFall 2008For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.Mercury Orbital period of 80 days 1800s first detailed telescope observations close to the Sun: makes telescope observations tremendously difficult 1881 - 1889 - G. Schiaparelli -- tried to map surface of Mercury, came up with this idea that the spin period of mercury is equal to the orbital period of 88 days 1962 - microwave studies of Mercury's dark hemisphere showed that it was quite hot 1964 - Gordon Pettengill (MIT) showed using radar observations that the spin period = 2/3 orbital period Spins three times for every two orbital periods -Mercury has two hot longitudes 180 degrees apart Surface temperature on sunwards side is 740 K, slightly < Venus Dark side, 90 K 88 days of darkness Obliquity = 90.0 degrees due to tidal effects. Permanently shadowed regions inside a crater at the pole, temperatures 60-100 K Hydrogen - radar bright anomaly, trapped water ice in shadowed areas Mariner 10 only spacecraft to visit Mercury, imaged 45% of the surface Found lots of craters (heavily craters) No weathering or plate tectonics Most of the rest of the surface recently imaged by MESSENGER spacecraft. Two Terranes (geological term!) 1 Highlands (4.0 - 4.2 Ga) 2 Lowlands Plains (3.8 Ga) Caloris Basin 1340 km wide. Second largest impact crater on terrestrial planets Composition Almost no information about the composition of the crust -hard to do spectroscopy when near bright object (near the Sun) -high air mass to observe right after sunset, poor spatial resolution Nearly featureless absorption spectra Possible absorption of 0.9 microns, low iron pyroxene Weak absorption features from 8-12 microns bronzite Mg0.9Fe0.1SiO3albite (feldspar) NaAlSi3O8sodalite Na4Al3Si3O12Cl Leading idea is that Mercury's crust is predominately plagioclase feldspar – anorthosite Mercury also has an atmosphere….a very small one, originates from micrometeorite impacts(Na, K), solar wind (H, He) and planetary degassing Geophysics of Mercury High uncompressed density predicts high Fe/FeO ratio Assuming chondritic bulk composition predicts a core that is 75% of the radius of the planet! Matches spectroscopy data suggesting Fe-poor crust We have showed that the gravitational energy of formation of a spherical body This represents the minimum energy of assemblage, if we neglect energy of materials assembling body. So what is this for Mercury? Eg = 3 x 1030 J For the Earth Eg = 8 x 1030 J This energy has to be lost somehow, turned into heat: ΔQ = mCpΔT ΔT = ΔQ/mCp ΔQ = Eg = 3 x 1030 J Mass (mercury) = 3.3 x 1023 kg Cp = 750 J / (k kg) (70% metal, 30% silicate ) So ΔT = 12,000 degrees C Heat of fusion (required to go from solid to liquid state) is about equal to heat required to heat rock up to melting temperature. This means that Mercury was initially mostly molten. Do to radiative cooling, molten zone probably was only outer few hundred km: magma ocean. Would predict a crust of anorthosite on Mercury. Predicts metallic core formation should occur nearly instantaneously, crust and mantle as