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Home , Atmosphere of Mercury, Late heavy bombardment

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Almost everything we know about Mercury comes from the single mission we had to that planet: ( 23 - April 3) 2000 high-res television pictures (Two more flybys: 9-21-74, and 3-16-75) mapped 45% of Mercury’s surface We’re going back: MESSENGER (2004) MErcury Surface, Space ENvironment, GEochemistry and Ranging closest planet to the Sun, very similar to the

Feature Named After Craters authors, artists, and musicians Valleys radio observatories (Arecibo and Goldstone) Scarps ships associated with scientific exploration Plainsthe word “mercury” in various languages with some exceptions: Caloris Basin (basin of heat!)

Outline of today’s presentation: I. Overall characteristics II. III. Polar Deposits IV. Magnetic field V. Terranes VI. Chemical and Mineralogical Composition VII. Geologic History

I. Overall characteristics

Characteristic Relative to Absolute Value (Earth = 1) Mass 0.0553 3.303 x 1026 g Radius 0.382 2,439 km Mean Density Earth: 5.52 g/cm3 5.43 g/cm3 Rotation Period 58.65 Earth days ---- Mean Orbital 0.3871 AU 57.91 x 106 km Distance Orbital Period around Sun 87.9 Earth days ----- Mean Orbital 1.6 47.89 km/s

1 Velocity (112,000 mi/hr) ----- 0.2056 Inclination to ----- 7.004 ecliptic (degrees) Temperature range ------360 F (-183C) to 840F (450C)

Huge eccentricity in its orbit, so orbital velocity changes a LOT 4.6 x 107 km at perihelion 6.98 x 107 km at aphelion

Rotation period = 58.646 Earth days Orbital period = 87.969 Earth days 3 rotations on its axis for every 2 orbits around the Sun http://www.windows.umich.edu/cgi- Obli bin/tour.cgi?link=windows3.html&sw=false&sn=4444&d=/&edu=mid&br=graphic& quit cd=false&tour=&fr=f y very See Mercury’s (under Atmosphere of Mercury) clos e to 0, so ther e are no seasons polar regions are always frigid (allowing ice to be preserved at crater bottoms near poles) Not the hottest planet ( is hotter), but has greatest T variation: 90-740 K

Too dense to be explained by current condensation/accretion models still a mystery... maybe lost its “crust” and got knocked into present orbit

2 II. Atmosphere very tenuous a trillion times less atmosphere than the Earth!

H, He, O, Ar detected by Mariner 10's UV spectrometer Na and K not detected until 1985, from Earth-based observations highly variable abundances on short time scales (hours or ) also variable by factor of 5 day/night! Global distribution also changes! Consistent with time scales for photoionization of : Na (90 min) and K (120 min) ? Na and K photoionization caused by 1. Interactions between surface minerals and solar radiation 2. Impact vaporization of III. Mercury’s Atmosphere Pola r Deposits

Polar ice does exist on Mercury found in 1991 by radar images Goldstone radar transmitter in Mohave sent radar pulses to Mercury Very Large Array (VLA) in New Mexico picked up the return pulses “Delay Doppler mapping” mapped surface reflectivity, and found polar ice deposits ice may be H2O, CO2 (dry ice), or other ices - controversial could also be vapor-deposited S interpreted to be ice in small, permanently shadowed craters within 6.5_ of poles with high depth:diameter ratios T = 100 to 60 K (coldest spot inside ’s orbit!) Mass of ice required to explain observations is as low as a few km3 a lot more than that would be deposited over time by Short period comets may be the source of the ice also possible, but we don’t know much about Mercury-crossing asteroids because they are so low in the sky.

IV. Magnetic Field

Mercury is the only (other than Earth) that has a significant magnetic field magnetic sphere is about 7.5 times smaller than Earth’s 3 suggests that Mercury has a fluid outer core, solid inner core Not well characterized because only 2 passes (1st and 3rd) through magnetic field 3 3 Densest known silicate is fayalite (Fe2SiO4) D = 4.2 g/cm , DMercury = 5.43 g/cm so, Mercury must have a larger fraction of Fe than anywhere else in - 70% Fe, 30% silicate we expected core to be Fe due to high density: Earth’s uncompressed density = 5.52 g/cm3 Earth’s core is 54% of diameter, 16% of volume Mercury’s uncompressed density = 5.3 g/cm3 Mercury’s core is 75% of its diameter and 42% of its volume crust is only 100 km thick Problems with an outer Fe core that’s liquid: 1. Mercury rotates too slowly to have a circulation-driven dynamo in core 2. Mercury has been geologically inactive for so long that the core should have cooled off by now! Either you have to enrich the core with radioactive elements OR put something other than pure Fe in the core that would lower its melting point Between 0.2 and 7% would keep the core liquid based on current thermal models! http://www.windows.umich.edu/cgi- bin/tour.cgi?link=windows3.html&sw=false&sn=4444&d=/&edu=mid&br=graphic& cd=false&tour=&fr=f

See structure (under Mercury’s Interior and Surface)

4 V.

1. Impact Craters and Basins We know from the Moon the time scales of bombardments in our solar system; there was particularly heavy meteoroid bombardment ending about 3.8 m.y. on Moon Bombardment was of objects left over from terrestrial planet formation OR from the formation of Uranus and DIFFER FROM IN THREE WAYS: A. For a constant rim diameter, Mercury ejecta blankets are uniformly smaller (by 0.65x) than on Moon AND maximum density of secondary impact craters is closer to crater rims than on Moon (2-2.5 crater radii on Moon vs. 1.5 crater radii on Mercury) This is due to differences in their gravity: Mercury: 3.70 m/s2 Moon: 1.62 m/s2 B. The densely cratered terrain is not saturated with craters ?different densities of at different places in the solar system - no ?maybe the crust was warmer during impacts (i.e., Mercury cooled more slowly than the Moon), so impacts were erased - maybe ?maybe this bombardment erased all the earlier craters - maybe ?early heavy bombardment craters got covered up by intercrater plains - yes! intercrater plains formed 4.0 -4.2 by ago – end of heavy bombardment C. Mercurian craters are very shallow and ill-defined - they’ve degraded, primarily from ejecta being thrown on them - ejecta doesn’t travel as far because of stronger gravity (2.5x) than on Moon

SIMILAR TO LUNAR CRATERS IN THESE WAYS: A. Small craters are all bowl-shaped B. With increasing size, interiors become flatter, craters get rims C. fresh craters all have haloes (bright or dark) and rays

Mercury has 22 multiring basins Caloris Basin is 1340 km, ridge is 2 km high ridged floor 2.5 km higher than surrounding plains interior fractures are the only extensional fault on the planet interior covered by smooth plains material Fm. is radially spreading ridges and grooves extends 1,000 km beyond Caloris Mtns. 2nd largest impact on any of the terrestrial planets 5 (S. Pole Aitken basin on Moon, 2,600 km, is biggest) 1 trillion 1-megaton bombs

http://www.windows.umich.edu/cgi- 2. bin/tour.cgi?link=windows3.html&sw=false&sn=4444&d=/&edu=mid&br=graphic&c Hill d=false&tour=&fr=f y and See Caloris Basin (under Surface Features of Mercury) Line ated Terr ain antipodal p

directly opposite Caloris Basin, 500 km across disrupt preexisting landforms such as crater rims hills are 5-10 km wide, 0.1-1.8 km tall similar, smaller features exist on the Moon opposite Orientale and Imbrium

3. Intercrater Plains either impact basin ejecta or lava plains clusters of impact craters common shallow secondary craters, often aligned in long chains topographically very complex oldest surface on Mercury; very similar to lunar highlands records the same bombardment flux as lunar suface dated by analogy with lunar samples widespread volcanism occurred at the same time as heaviest bombardment most extensive terrane on Mercury few craters older than plains flows 6 planet was resurfaced during this time fill and are superposed by craters — contemporaneous with craters dated 4-4.2 by high population of craters <15 km diameter — late emplacement source basins are lacking; believed to be fissure flows global distribution implies volcanic origin

4. Smooth Plains/Lowland Plains 40% of Mariner 10 image area associated with large impact basins heaviest concentration in northern hemisphere (like the Moon) fill and surround Caloris Basin — smooth plains are clearly younger than basins —> late volcanic eruptions dated to end of late heavy bombardment (3.8 b.y.) BUT no evidence of fissures or vents ?relatively thin flows – mysterious because no volcanic features no lobes, no domes, no cones ?could possibly be giant sheets of impact melts Compositionally similar to surrounding rocks

http://www.windows.umich.edu/cgi- 5. bin/tour.cgi?link=windows3.html&sw=false&sn=4444&d=/&edu=mid&br=graphic&c Shie d=false&tour=&fr=f ld volc See Wrinkle Ridges (under Surface Features of Mercury) ano

may also be a large shield on the unimaged side of Mercury

6. Lobate scarps unique to Mercury 20-500 km in length, 100 m - 3 km high random spatial distribution transect fresh and degraded craters extend from pole to pole, trend roughly north-south uniform distribution suggests contraction thrust faults resulting from compressional stress in the crust caused by cooling 1-2 km decrease in radius when core/mantle cooled use them to estimate amount of decrease in radius due to cooling 7 assume fault plane inclination = 25_, ave. Ht = 1 km, count up all the faults and their lengths... but mantle/core solidification should have shrunk radius by 6-10 km ?maybe some contraction happened before current features formed ?maybe core has not yet solidified (which is why it has magnetic field)

postdate intercrater plains and smooth plains roughly comparable in age to Caloris basin fm. VI. Chemical and Mineralogical Composition surface composition not well known color relatively homogeneous, suggesting homogeneous distribution of elements that reflect color suggests that Ksp is present Fe-poor pyroxene all the Fe on Mercury seems to have gone in its core implies lack of extrusive volcanism on its surface also plag (labradorite/bytownite) > highly differentiated lavas about 6.0 wt% FeO in crust

8 VII. Geologic History

1. Core formation and differentiation (4.5 by) metallic Fe is denser, moves toward center due to gravity movement toward core generates friction, heat no H2O; apparently moved further out in nebula DIFFERENTIATION occurred, crust+mantle = 600-700 km 2. Portion of mantle was stripped away by giant impact “Post-accretionary vaporization” proto-Mercury was 2.25 x present Mercury Fe-cored projectiles of 20 km size could vaporize silicates but not Fe (Fig. 21, p. 143) 3. Intercrater plains erupt (?) about 4 by Heavy bombardment 4. Caloris Basin formed hilly and lineated terrains 5. Thrust faulting due to cooling 6. Further eruption of smooth plains 3.8 by heavy bombardment decreased dramatically smooth plains form as final stage of volcanism (2-3.6 by) lithosphere cools and thickens some light cratering

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