Mercury TJ McCoy and LR Nittler 2014 (2003) Why is Mercury important? Background from Mariner 10 to MESSENGER An unusual geochemistry Pre-MESSENGER models Re-evaluation of models + discussion
Mariner 10 (1974) MESSENGER (2012)
Accretion and solar system evolution Chambers (2004) Distance from Sun (AU)
MESSENGER Low FeO content of surface silicates (<2-3 wt%) Al/Si and Ca/Si lower than terrestrial and lunar basalts High volatile surface content Strange surface features - volcanism and hollows 1974 Mariner 10 Little known of Mercury s chemistry Large metallic core Low FeO on the surface (<3 wt% FeO) Na, K and Ca in the exosphere sputtering MESSENGER 2008 MESSENGER 2008
2014 James Dwight Dana Contrational theory of Earth (~1850) Drying prune Shortening at mountain belts caused by contraction
Enterprise Rupes 820 km long 3 km of relief Lobate scarps, thrust faults Shrinking caused by core cooling and contracting 5,934 scarps on Mercury Calculated diameter reduction of ~9 14 km! Fit better with predictions than Mariner 10 s observations Toasty seismics?
Hollows ranging in size from 60 feet to over a mile across and 60 to 120 feet deep Swiss cheese Caldera collapse volcanism, explosive outgassing, space weathering High reflectance - volatiles
MESSENGER Mission Timeline August 3, 2004 -- MESSENGER Launch January 2008 -- Mercury flyby October 2008 -- Mercury flyby September 2009 -- Mercury flyby March 2011 -- Yearlong science orbit of Mercury begins Gamma-Ray and Neutron Spectrometer (radioactive decay K, Th, U) Mercury LASER Altimeter Mercury Atmospheric and Surface Composition Spectrometer. Mid UV (200 nm) near Infra Red (1300 nm) X-Ray Spectrometer
Water and volatiles at poles (MLA 1064 nm wavelength) comets? UV absorption <280 nm Fe 2+ O of silicates is low Low FeO seen at depth in craters too 575 nm as red, 415 nm/750 nm as green, 310 nm/390 nm as blue (MASCS Mercury Atmosphere and Surface Composition Spectrometer)
An Unusual Geochemistry Huge metallic core 80% of radius Low FeO content on surface Surprising volatile content (S and K) Temp range 430 to -180 o C Water ice at poles in permanent shadow
Volcanism on Mercury do lava flows reflect the interior? The enhanced colour shows the smoother northern volcanic plains have a different composition to the surrounding material. Morphology and colour measurements low FeO lavas high density crust anticrust? Assumed no dramatic fractionation as magmas migrated to surface justified?
Pre-MESSENGER Models
1. Stripping of silicate post differentiation Benz et al., (1988) explain strange density by Head on collision 20 km/s Glancing blow 35 km/s Planetesimals from range of heliocentric distances FeO issues
2. High temperature evaporation and condensation T-Tauri winds from young sun heated nebula Mercury s mantle affected by high temperature Loss of silicate from mantle, remaining mantle has a more refractory composition Predicts low FeO, Na and K Enrichments in Ca, Al and Mg Increasing vaporisation
Atomization enthalpy (kj mol -1 ) Volatility Super-refractory: higher than 1700 K Refractory: 1700 1300 K Volatile: 700 1100 K Very volatile: less than 700 K ~ boiling points Compositional and oxygen fugacity effects 50% condensation temperature (K) (50% of the element will be in the form of a solid under a pressure of 10 4 bar)
3. Refractory volatile mixtures Model for Mercury s composition based on mixing of material throughout inner solar system Enriched Al/Si, Ca/Si and Ti/Si Volatile rich (FeO, FeS, H 2 O). Amount of volatile material is dependent on extent of heliocentric mixing. Fe/Si ratio increased by impact Models not mutually exclusive Goettel (1988)
4. Formation of Mercury from known chondrites CB Chondrites as building blocks of planets? Need precursor material that is reduced, metal rich, FeO poor and rich in refractory and volatile elements Bencubbinites metal and volatile rich chondrites Close to mercury but low in Ca/Si ratios and volatile elements EC Enstatite chondrites very reduced, formed close to sun Close to Mercury but low in FeO content Unique pre-cursor material likely not preserved/ sampled Enstatite Chondrites Bencubbin Chondrites Mercury FeO < 1 3.2 wt% ~3 wt% Metal ~25 wt% ~80 wt% ~70 wt% Minerals En, CaS, MgS (v. reduced) En, Ol, Plag, Sulphides En, Plag, Di, CaS and MgS
Types of chondrite
MESSENGER Results
Mercury s chemical characteristics Pre Messenger Large metallic core Low surface Fe/Si plag floatation crust? Volatile Na and K in exosphere (and on surface?) Post - Messenger Smooth volcanic plains with low FeO in surface Pyroclastic volcanic vents volatile content of magma Sublimation hollows volatile removal Volatile ions in exosphere (Na +, O +, K +, S +, H 2 S + ) High surface S content in refractory CaS and MgS? High Mg/Si Al/Si and Ca/Si lower than lunar crust no plag flotation Similar K to other planets, much higher than moon FeS anticrust and Fe-S-Si rich core?
Mercury red circles Compared to terrestrial and lunar compositions And model predictions CB: Bencubbinite chondrites EC: Enstatite chondrites MA: refractory volatile mixture Komatiites: ultra-mafic mantle derived volcanic rocks (Forsterite, Ca-Pyroxene and Anorthite) Peridotite: ultramafic igneous mantle rock (low Si, Ol, Px) Doesn t match plag floatation crust like moon
Mercury red circles Compared to terrestrial and lunar compositions And model predictions CB: Bencubbinite chondrites EC: Enstatite chondrites MA: refractory volatile mixture Komatiites: ultra-mafic mantle derived volcanic rocks (Forsterite, Ca-Pyroxene and Anorthite) Peridotite: ultramafic igneous mantle rock (low Si, Ol, Px) Doesn t match plag floatation crust like moon
Surface sulphur content Mercury much higher than Earth, Moon and Mars High S and correlation with Ca and Mg suggests Mg and Ca sulphides Refractory explains S content Oldhamite and niningerite present in highly reduced ECs
Potassium and thorium surface concentrations Gamma-ray spectrometer K volatile Th refractory Important heat producing elements Mercury similar K/Th content to Earth and Mars Reasons for shape of this plot?
Evaluating Pre MESSENGER models 1. Stripping of silicate post differentiation extreme heating and volatile loss X 2. High temperature evaporation and condensation of Mercury s precursor material in nebula X 3. Refractory volatile mixtures? 4. Chondrite proxy likely a near match but feeding zone was unique? MESSENGER results Implications Chemistry best explained by volatile and refractory rich precursor material; very reducing conditions Higher volatile content than previously thought Strange geological features reflect a dynamic past Extent of shrinking now better matches theory
Future exploration of mercury: Higher resolution spectrometric data Coupled with better interpretation of geological and colour features understand strange terrains. Vertical heterogeneities does the high volatile content reflect volcanism or crust/ interior? Investigate the bright radar deposits at the poles water ice from comets? Sulphur-rich? Mercurian sample in meteorite population? BepiColombo Launch - 2016 Mercury - 2023 Currently being built and tested Europe s first mission to Mercury 970 Million Euros
Points for debate: Does the high volatile surface content reflect the interior? What planetary formation processes led to the high metal/silicate ratio in Mercury? Convection in Mercury s mantle? FeO gradient degree of inner solar system mixing during accretion? Hypothetical seismics of the drying prune? What are the radar-reflective materials at Mercury's poles? What are the important volatile species and their sources and sinks on and near Mercury?
Transit of mercury captured by X-Ray telescope, November 2006 Next one in 2016