Mercury (Hermes : Greek)

1. Mercury(Hermes : Greek) Roman god of commerce, travel and thievery

2. General Information • Named after Roman fleet-footed messenger god • Appeared to move quickly across the sky • Closest planet to Sun • Difficult to observe from Earth • Never gets more than 28 degrees from Sun • Can only be viewed just prior to sunrise or just after sunset • Low in sky means 10 X more atmosphere to penetrate than if directly overhead • Second smallest planet (after Pluto) • Moons Ganymede and Titan are larger • Sun would appear 2 ½ X larger in sky from the surface

3. Planetary Statistics • Mean distance from Sun 57,910,000km • 0.38 A.U. • Highly eccentric (28.6 - 43.5 million mi) • Highest in SS • Orbital Period = 87.96 days • Rotational Period = 58.6 days • Spin-orbit coupling • 2 Orbits (Mercurian years) to 3 rotations (Mercurian days) • Axis Tilt = 0.01 degrees (23.5) • Smallest of any planet in our SS • Diameter = 4880km (7926km) • Mass = 3.303 x 1023 (5.97 x 1024) • (.05527 of Earth) • Density = 5.42gm/cm3 (5.515gm/cm3)

4. A typical day on Mercury • The high eccentricity of Mercury's orbit would produce very strange effects for an observer on it's surface • At some longitudes the Sun would rise then gradually increase in apparent size as it slowly moved toward the zenith • At that point the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon and decreasing in apparent size • Meanwhile, the stars would be moving three times faster across the sky • Observers at other points on Mercury's surface would see different but equally bizarre motions • Image below – Time lapse of Mercury over Leeds, England

5. Planetary Statistics • Equatorial surface gravity = 2.78m/sec2 (9.78m/sec2) • Visual albedo = 0.10 (.37) • Moons = none • Surface = heavily cratered with volcanism and some tectonics • Atmosphere = atoms blasted off surface by fierce solar wind • Very, very thin • Composition • Helium 42% • Sodium 42% • Oxygen 15% • Mean surface temperature = 170C (330F) • Max temp = 427C (870F) • Min temp = -173C (-360F) • Most extreme in our SS

6. Mercury – Surficial Features • Heavily cratered surface • Represents geological record of SS impact rates • Infers no erosional yet some depositional geologic processes throughout planets history • Infers extremely thin atmosphere • No liquid water • Fault scarps • Demonstrate vertical with small horizontal tectonic movement • Can be 100’s of kms in length and > 2mi high • Probably due to planets cooling history • Heating/expansion = normal faulting (tensional stress) • Cooling/contraction = reverse (compress. stress) • Some may be result of larger impact events • Focus of seismic waves antipodal to impact point

7. Early Investigations • Giovanni Schiaparelli • 1880’s sketched faint features from telescopic observations • Suggested Mercury tidally locked to Sun • Much like the Moon is to Earth • Pettengill and Dyce • 1965 determined Mercury’s rotational period proving it is not tidally locked • Speculation rotation was perhaps as quick as 8 hrs • Slowly despun over 109 yrs • Raised interior temp by 100K • Einstein’s Theory of Relativity • Newtonian mechanics didn’t fit orbital characteristics • Once thought another planet (Vulcan) was reason for orbital perturbations • Correctly predicted the precession (very slowly moves backward) of the perihelion of Mercury

8. Early Satellite Exploration • Mariner 10 (1973-1975) was the 7th and last successful launch in the Mariner spacecraft series (Mariner 11&12 were re-designated Voyager 1&2), and the first to use the gravitational pull of one planet (Venus) to reach another (Mercury) • 3 fly-by’s of Mercury • Problems with gyroscopes and stellar tracking • Used solar wind pressure to stabilize attitude and change trajectory • High-gain antenna to help track Earth and determine position • Instruments on board the spacecraft were designed to measure the atmospheric, surface, and physical characteristics of Mercury and Venus • Experiments included: • Television photography • Magnetic field • Plasma • Infrared radiometry • Ultraviolet spectroscopy • Radio science detectors • An experimental X-band, high-frequency transmitter was flown for the first time on the spacecraft

9. Mariner 10 (cont’d) • Mariner 10 reached Mercury on March 29, 1974, passing over the planet at 705 kms (438 mi) above the surface • Made a total of three passes during 1974-75 • Photographs revealed an intensely cratered, Moon-like surface, a faint atmosphere • Engineering tests were continued until March 24, 1975, when the supply of attitude-control gas was depleted and the mission was terminated • Hasn’t been tracked or seen since but should still be orbiting the Sun

10. Mercury – Internal structure • About 1/3 the size of Earth • Density comparable to Earth • Indicates Mercury has large core > size of Earth's Moon or about 85% of the planet's radius • Likely composed of 60- 70% Fe • Messenger measurements reveal a dipolar magnetic field • Different from any planetary core in the SS • Solid silicate crust & mantle • Overlying a solid, iron sulfide outer core layer • A deeper liquid core layer • Possibly a solid inner core • Thin silicate crust of about 100 kilometers

11. Surface Features and Processes • Craters • Range from 100m– to 1,300km in diameter • Caloris Basin • Largest multi-ring basin on Mercury • 100km diameter asteroid • Concentric mountain rings 3km high • Ejecta blanket covers 600-800km • Lava flows • Post-accretion phase volcanic activity • After crustal cooling 3rd phase of flows producing smooth plains • Scarps • 2nd phase tectonics • Thrust faulting • Contraction and shrinking of silicate crust • Water? • 1991 radio waves from Caltech scientists yield bright returns at north pole • Axial tilt low = no sun on crater interiors = temp <161C • Possibility of ice at or very near surface

12. Tectonics – Santa Maria Rupes • Sinuous dark feature running through the crater at the center of this image • Interpreted to be enormous thrust faults • Indicate that the radius of Mercury decreased by 1-2 kms after the solidification and bombardment of the surface • Volume change probably was due to the cooling of the planet, following the formation of a metallic core >¾ the size of the planet

13. Caloris Basin • Mariner 10 mosaic • Terminator enhances relief • Note at least 3 concentric rings • Many ring mountain blocks are over 3 kms high • Secondary mountain rings likely created by impact shock waves • Radiating outward are systems of valleys, hills, and other craters • Secondary craters surround the Basin • Some are in excess of 20 kms diameter • The Caloris Basin interior is filled with mostly smooth plains most probably volcanic in nature

14. Tectonics – “Weird Terrain” • Hilly, lineated region at the antipodal point from the Caloris Basin • Shock wave from the Caloris impact was reflected and focused to this antipodal point • Result : jumbled crust broken into a series of complex blocks • Note orthogonal fracture systems • The area covered is about 100 kms (62 mi) on a side

15. Planetary Geologic Evolution • Size • Mass • Radius • Chemical Composition • Volatile poor / refractory rich • Dense • Three general phases • Highly active • Crustal formation and mobility • Volcanic • Accompanying thickening sub-crustal lithosphere • Terminal quiescent • Lithosphere too thick to allow volcanism or lateral movement

16. Mercury’s Evolutionary Factors • Important reference point • Planet closest to the Sun • Solar wind does not allow substantial atmosphere to form • End-member of chemical composition • High temperature • Refractory elements • Few volatiles • Larger than Moon evolving at slightly different tempo • Mercury • ~Same mass and surface gravity as Mars • ~Same bulk density of Earth

17. Mercury’s Geological History • Stages • I. Accretion and differentiation • Formation of planet from SS material • Initially an undifferentiated internal structure • Beginning of diff. into large molten core and silicate mantle w/thin cooler crust • Expansion and tensional fracturing of thin crust • II. Intense bombardment • Large and varied impacts • Formation of inter-crater plains • Volcanism caused by large impacts of thin crust overlying hot, fluid mantle

18. Mercury’s Geological History (cont’d) • III. Excavation of Caloris • Convection in mantle causes cooling and contraction initiating compressional thrust faulting • IV. Formation of smooth plains • Continued volcanism as interior continues to cool • V. Cooling and contraction • Completion of process resulting in tectonically inactive crust • Occasional rayed cratering (recent) • Lithosphere thickened

19. Current Mission • Messenger • First portrait of our SS from the inside looking out • Why Mercury? • Smallest • Densest • Oldest surface • Largest temp variations • End-member planet • Least explored

20. Messenger Payload • Mercury Dual Imaging System (MDIS): Wide-& narrow-angle imagers • Gamma-Ray and Neutron Spectrometer (GRNS): Detect gamma rays and neutrons that are emitted by radioactive elements on Mercury's surface • X-Ray Spectrometer: Will detect Gamma rays and high-energy X-rays from the Sun, striking Mercury's surface • Magnetometer (MAG): Map Mercury's magnetic field and search for regions of magnetized rocks in the crust • Mercury Laser Altimeter (MLA): Laser that will produce highly accurate topographic data • Mercury Atmospheric and Surface Composition Spectrometer (MASCS): Infrared to the ultraviolet will measure the abundances of atmospheric gases, as well as detect minerals on the surface • Energetic Particle and Plasma Spectrometer (EPPS): Measures the composition, distribution, and energy of charged particles (electrons and various ions) in Mercury's magnetosphere • Radio Science (RS): Uses the Doppler effect to measure very slight changes in the spacecraft's velocity as it orbits Mercury • Study Mercury's mass distribution

21. Messenger’s Mission Timeline • KEY EVENTS:August 3, 2004 -- MESSENGER Launch August 2005 -- Earth flybyOctober 2006 -- Venus flybyJune 2007 -- Venus flybyJanuary 2008 -- Mercury flybyOctober 2008 -- Mercury flybySeptember 2009 -- Mercury flybyMarch 2011 – Mapping orbit initiated, yearlong science mission begins

22. Messenger False-color mosaic • Mosaic was produced using images from the color base map imaging campaign during MESSENGER's primary mission • Colors enhance the chemical, mineralogical, and physical differences that make up Mercury's surface • Young crater rays, extending radially from fresh impact craters, appear light blue or white • Medium- and dark-blue areas are a geologic unit of Mercury's crust known as the "low-reflectance material", thought to be rich in a dark, opaque mineral • Tan areas are plains formed by eruption of highly fluid lavas • Caloris basin is the large circular tan feature

23. MDIS – highest res color image even acquired • Closest approach, just 200 kms above the surface • (500 meters/pixel • 120-km diameter Rudaki Crater • Younger smooth surface on left • Older rougher, bluish terrain on right • Orange crater rim • Blue-floored crater • Dark blue material was ejected from the 105-km diameter crater on the right side • A relatively young, small crater then excavated through this blue material to reveal the smooth plains beneath

24. Messenger (New Images) • One of the highest and longest scarps (cliffs) yet seen on Mercury • (The Sun is shining low from the right, so the scarp casts a wide shadow) • Compressional tectonic forces in crust have thrust the terrain occupying the right two-thirds of the picture up and over the terrain to the left This image was taken from a distance of only 5,800 kms (3,600 mi) from surface of the planet and shows a region about 200 kms (about 125 mi) across

25. Messenger (New Images) • Mosaic of area not viewed by Mariner 10 during any of its three flybys (1974-1975) • The outer diameter of the large double ring crater at the center of the scene is about 260 km (160 mi) • Appears to be filled with smooth plains material that may be volcanic in nature • Multiple chains of smaller secondary craters are also seen extending radially outward from the double ring crater • The transition diameter at which craters begin to form rings is not the same on all bodies and, although it depends primarily on the surface gravity of the planet or moon, the transition diameter can also reveal important information about the physical characteristics of surface materials.

26. Messenger : Sullivan Crater • Multi-ring basin • 135km diameter • Several periods of basalt flooding • Structural scarps and lineaments • Ridges similar to those in previous image

27. Messenger : Machaut Crater • 100km across • High Sun angle image • 2nd flyby Oct. 6th, 2008 • Craters of two ages • Older flooded by basalt flows • Younger simple bowl shape excavated flow material • Ridges • Unknown origin

28. Kuiper & young rayed craters • Kuiper • Rayed crater form • 62km diameter • Reddish ejecta blanket indicates excavated material subjected to solar radiation caused chemical alterations • Very young rayed crater • 14km diameter • Bluish areas is melt material that flowed out of small crater after impact

29. Rudaki Crater – True & False Color Images • The plains near Rudaki Crater • Left image approximates Mercury's true-color • The right image highlights subtle color differences on the surface

30. Caloris Basin • Multi-ringed basin • ~1550km diameter • False-color image • Compositional differences between basin and surrounding crust • Dark-orange areas could be volcanic centers

31. Apollodorus Crater • Pantheon Fossae • Complex system of extensional troughs located near the center of the Caloris Basin • Although located close to the center of the Pantheon Fossae, the crater and trough system appear to be unrelated

32. Praxiteles Crater • Multi-ringed basin • 182km diameter • Irregularly shaped depressions on the floor of • Buried older crater • Possible past volcanic activity within this crater

33. Firdousi Crater • 96km diameter • Young, few & only small craters on floor • Complex ejecta blanket with various materials in evidence • Complex, slumped rim structure • Scalloped rim • Odd internal structure with anomalous blue features

34. How geologists estimate the thickness of lava flows • Left - A fresh impact crater (Hokusai, 114 km diam.) • Measure depth of its interior and height of its rim above the surrounding terrain • Right - A "ghost" crater (90 km diam.) buried preexisting impact crater • Estimate height of the rim = thickness of the lava covering the crater

35. Rembrandt Impact Basin • Newly discovered large impact basin • >700km in diameter • 3.9by in age • One of the youngest “ancient” impact basins from just after the Heavy Bombardment Period • Smooth, uplifted floor suggests lava resurfacing & exhibits old crater floor surface

36. Raditladi Basin • 290km-diam • Double-ring basin • Still-unnamed • Remarkably well preserved • Formed relatively recently

37. Hovnatanian Crater • Named for19th century Armenian painter Hakop • Formed by an object that impacted at very oblique angle • Incidence angles <15º (from the horizontal) will create elliptical craters • Note asymmetric ejecta blanket at top of crater

38. Unnamed crater • Orthogonal lines are secondary crater chains caused by ejecta from two primary impacts outside of the field of view

39. Raditladi Impact Basin • The "hollows“ • Depressions possibly formed by the removal of volatile-containing material exposed within impact craters

40. Water at poles Hidden in deep, shadowed craters H2O buried by dark, organic deposits Partially liquid core Offsets in expected map locations Imaged 100% of Mercury’s surface 1st time complete coverage has been accomplished High concentrations of magnesium and calcium on night side Magnetic field far offset northward from planetary center Not uncommon in our SS Messenger Discoveries