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Lecture #15: The Main Point • Mercury: • General properties. Mercury is a small, atmosphereless terrestrial planet with a large metallic core and evidence for a complex geologic history. • Surface characteristics. • Interior and evolution. • Reading: – Chapter 7.1, 9.3, 10.3. Astro 102/104 1 Basic Properties of Mercury Astro 102/104 2 Observing Mercury in the Sky • Average Distance from Sun: 58,000,000 km (0.39 AU). • But: Orbital eccentricity = 0.206. • So: Distance varies from 46,000,000 to 70,000,000 km. • Orbital period: 88 days. • Period of Spin around axis: 58.65 days (2/3 orbit period!) • Mass = 3.3x1023 kg = 0.055 ME. • Radius = 2440 km = 0.38 RE. • Density = 5.4 g/cm3 (recall that Earth ~5.5 g/cm3). • Surface Gravity = 3.7 m/sec2 (38% of Earth's). • Surface Temperatures: -185°C (night), +430°C (day) [!] • Surface superficially lunar, but important differences. • Mercury is the innermost planet. • It never gets more than ~28° from the Sun. • Hard to see! – Twilight. – Haze/Smog. – Starlike. Astro 102/104 Astro 102/104 3 4 1 What's up with that Density? Is the core molten? A 30-year old question in planetary science, with important implications for the internal structure, thermal evolution, and magnetic field generation of terrestrial planets. • Mercury's density vs. size is distinctly different from that of the other terrestrial planets. • Implies a large metallic core! Planet %Core Earth ~54% Moon ~10%? Mercury ~74% Astro 102/104 6 Astro 102/104 7 And that Weather! What's up with that Spin? Mercury rotates 3 times on its axis for every 2 revolutions around the Sun (i.e. 3 Mercury days = 2 Mercury years)! • Yet another example of a system influenced by tides. • Mercury, close to the Sun, feels strong solar tidal forces. • But Mercury's orbit is eccentric (Kepler: orbital speed varies), so Moon-like synchronous rotation is difficult. Astro 102/104 • 2 years : 3 days is the most favorable resonance. 9 Astro 102/104 10 2 Telescopic Observations Mariner 10 • Mercury studied by the Mariner 10 space mission. • Flew past Mercury 3 times in 1974 and 1975 (after gravity assists from the Earth/Moon and Venus...) • Only about 50% of the surface imaged by Mariner. • • • • Difficult because Mercury low in the sky. Also, Mercury is small. Contrasts are weak. Result: very little was known based on visible telescope data. • More later on radar... Astro 102/104 11 Astro 102/104 Geology of Mercury Mercury's Surface Properties • Crater shapes different from the Moon: • Surface appears generally MoonMoon-like, covered by dark gray regolith. • Brightness, roughness similar to lunar highlands. • No water! (but more about polar regions later) • Small, localized mare deposits. • Many craters, several large impact basins. Astro 102/104 12 • Gravity on Mercury is ~ twice that on the Moon. • Crater bowls shallower than on the Moon. • Ejecta blankets less extensive than on the Moon. 13 Astro 102/104 14 3 Geology of Mercury Planetary Evolution • Evidence of tectonic forces at work in the past. • Mercury has a weak magnetic field: – Is there a dynamo in a molten metallic core? – Or is the magnetic field frozen in the crust? • Surface evidence for compression, but no extension. • Mercury's crust appears to have shrunk from a once-larger size: – Contraction after cooling from a molten state? Lobate Scarp • Indicates compression of the planet's crust. Astro 102/104 Cliff is 2 km high and 100 km in length 15 Astro 102/104 The Incredible Shrinking Planet? 16 Radar Observations • Arecibo radio telescope used to discover the ~ 59 day spin period of Mercury (1965). • More recently: Evidence for polar ice caps found from radar data! – Something very "radar bright" near the poles. – Ice reflects radar very strongly. – But it sounds crazy--ice on such a hot planet? Astro 102/104 17 Astro 102/104 19 4 Related Issue: Ice on the Moon? • Radar-reflective deposits appear to occur in shadowed craters. • Mercury's axis tilt is ~0° (no seasons!) • Some deep craters near the poles never see sunlight! • Hypothesis: ice from cometary impacts. • Other less likely hypotheses: sulfur, silicates. • A recent mission called Lunar Prospector has reported possible evidence for ice on the Moon: – The Moon has permanently shadowed polar craters too. – Data showed slightly enhanced Hydrogen in these areas. – Not confirmed by radar images as on Mercury. • Controversial! – H could be from solar wind or some other process and might not be the "H" in H2O. • But big implications for space exploration. Harmon et al., Arecibo radar data. Astro 102/104 21 22 Summary NASA’s Return to Mercury Many questions are being answered by NASA’s MESSENGER Mercury orbiter: – – – – – Astro 102/104 • Mercury is an enigmatic terrestrial planet: – Strange relationship between spin and orbit. – Evidence for an enormous core. – Evidence for shrinking of the crust. Launched in 2004. First flyby on Jan 14, 2008. Orbit insertion in 2011. Orbits, maps for 1 Earth year. Imaging, spectroscopy, magnetic fields, altimetry. • Mariner 10 & recent MESSENGER data provide most of our detailed knowledge of Mercury's geology: – Outwardly Moon-like, but important differences. • Radar data indicate possibility of ice in shadowed craters near the poles! Astro 102/104 23 Astro 102/104 24 5 Next Lecture... • Venus: •General properties. •Telescopic observations. •Space missions. •Atmospheric characteristics. • Reading: –Chapters 7.1 (Venus), 9.5, 10.5. Astro 102/104 25 6