PYTS/ASTR 206 – Mercury1z AnnouncementsHk#2ltbiidHomework #2 late submissions due now 50% credit Mid-term #1 on Thursday Based on the first 10 lectures (incl. this one) Multiple choice - bring #2 pencils Lasts 1 hour and starts on time You won’t need a calculator Any questions on formulas will include the formulaPractice exam available on class websitePractice exam available on class website Don’t memorize numbers…• …but understand quantities relative to each other •e g IR wavelength longer than UV wavelength•e.g. IR wavelength longer than UV wavelength• 1AU much bigger than the Earth-Moon distancePYTS/ASTR 206 – Mercury2MercuryPTYS/ASTR 206 – The Golden Age of Planetary ExplorationShane Byrne – [email protected]/ASTR 206 – Mercury3In this lecture…In this lecture…M’tbitzMercury’s strange orbit Extreme temperatures Hot and cold longitudesz Mercury’s even stranger interior Giant core Magnetic fieldzMercury’ssurfacezMercury s surface Like the Moon – but not quite A planet that shrunkNewly found VolcanoesNewly found Volcanoes Caloris basin and the ‘weird’ terrainPYTS/ASTR 206 – Mercury4Mercury’s OrbitMercury’s Orbitz Closest to the Sun Average distance 0.39 AUM ’ bit i dd dPlanet Inclination EccentricityMercury 7° 0.21zMercury’s orbit is odd compared to other planets Both eccentric and inclinedVenus 3.4° 0.01Earth 0° 0.02Mars 1.9° 0.09Jupiter 1.3° 0.05Saturn25°006Saturn2.50.06Uranus 0.8° 0.05Neptune 1.8° 0.01pPYTS/ASTR 206 – Mercury5z Mercury’s surface gets extremely hotPihlilttthPerihelion - closest to the sun Mercury is 0.31 AU from the sunAhliFthtf thAphelion –Furthest from the sun Mercury is 0.47 AU from the sunz Remember this?Solar power = 1367 W m-2/ R2 R is the solar distance in AUMercury at perihelion 14 225 W m-2Mercury at perihelion 14,225 W m Temperatures as high at 700K Mercury at aphelion 6188 W m-2 Temperatures as high as 570K Night-time temperatures on Mercury as low as 100KPYTS/ASTR 206 – Mercury6z Mercury is hard to observe from the Earth with telescopesEarth with telescopes Always visible for less than 2 hours Either just before dawn or just after Sunset Always close to the horizonPYTS/ASTR 206 – Mercury7z We have other tools available (I)Rd b tiAibRadar observations -Arecibo With radar we supply the ‘light’ and wait for the reflectionPYTS/ASTR 206 – Mercury8z Radar observations tell us about Mercury’s spin periodMercury’s spin period Doppler shift in wavelength – caused by motionWavelength is shorter when sourceWavelength is shorter when source and observer approach Wavelength is shorter when source and observer recedez The amount of wavelength shift of the radar tells us the rotation speedPYTS/ASTR 206 – Mercury9z Another example of the Doppler shiftCtillthShConvection cells on the Sun have.. Portions that rise towards the surface – approach observers on the Earth Portions that sink away from the surface – recede from observers on the Earth Wavelength of the emitted radiation is shifted slightlya e e gt o t e e tted ad at o s s ted s g t yPYTS/ASTR 206 – Mercury10z Back to Mercury…D l hift f th d fl tiDoppler shift of the radar reflections Mercury rotates every 59 daysObitl i d 88dOrbital period was 88 days Ratio is 3:2 i.e. three rotations every 2 orbitsz This was a surprise People expected a ratio of 1:1 Synchronous rotation Like Earth’s MoonNOTTHETHECASEFORMERCURYPYTS/ASTR 206 – Mercury11z Mercury does 1½ rotations every orbitzAt perihelion positionAt perihelion position Sub-solar longitude is 0°, then 180°, then 0°, then 180° then 0° etc… These are the ‘hot’ parts of Mercuryz At aphelion position Sub-solar longitude is then 90°, then 270°, then 90°, then 270° then 90° etc…These are the‘cold’parts of MercuryThese are the cold parts of MercuryRadio emission from MercurySenses temperature from the upper fewmeters of the surfaceShth‘h t’litd180°tShowsthe‘hot’longitudes180°apart100 K differencePYTS/ASTR 206 – Mercury12z We have other tools available (II)SftMi 10Spacecraft -Mariner 10 3 flybys in 1974 & 1975 Only mapped 50% of the planet Messenger 3 flybys in 2008 & 2009 Enters orbit around Mercury - 2011 Already confirmed all the Mariner 10 results Filled in the gaps in image coveragePYTS/ASTR 206 – Mercury13z Mariner 10 imageryMost of our Mercury results still come from this missionMost of our Mercury results still come from this missionPYTS/ASTR 206 – Mercury14Mercury’s InteriorMercury’s Interiorz Mercury is an anomaly among the terrestrial planetsUlll lt dUsually larger planets are denser High gravity compresses materialz Mercury’s density 5430 kg m-3 Almost as dense as the Earth 3 Rock density ~3000 Kg m-3zMercury’s radius is 2440 kmzMercury s radius is 2440 km Only 40% of Earth’s radius Only 6% of Earth’s volumePYTS/ASTR 206 – Mercury15z What’s going on? Mercury’s iron core is enormous compared to the planet Core radius Earth 54% of planet Mercury 75% of Planet Core Volume Earth 16% of the planet Mercury 42% of the Planet Mercury has a thin mantle Probably a very thin (~100km) crustPYTS/ASTR 206 – Mercury16zHow did this happen?zHow did this happen? Maybe Mercury formed that way Maybe rock boils away in hot early phase Maybe a giant impact occurred like on the Earthz Giant impact theory is most popular Large object hits an already gj ydifferentiated Mercury Iron core is protected Mantle of rocks is stripped offPYTS/ASTR 206 – Mercury17z Another variation of the giant impact theoryimpact theory Mercury is completely destroyed Reforms from iron rich debrisRock debris lostRocky debris lostz Giant impacts in the very early solar system could be commonplace Earth – large Moon Mercury – large iron core Venus – retrograde spin Mars – topographic dichotomy Uranus – spin axis in orbital plane Probably not all of these were ca sed b giant impactscaused by giant impactsPYTS/ASTR 206 – Mercury18z Mariner 10 also discovered a dipole magnetic fieldWeaker version of Earth’sWeaker version of Earth s field, ~1% as strong Implies part of Mercury’s core is still molten A surprise
View Full Document
Unlocking...