Mercury

Basic Properties of Mercury Average Distance from Sun: 58,000,000 km (0.39 AU). But: Orbital eccentricity = 0.206. Therefore distance varies from 46,000,000 to 70,000,000 km. Orbital period: 88 days. Period of Spin around axis: 58.65 days Mass = 3.3x1023 kg = 0.055 M Radius = 2436 km = 0.38 R 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.

Global Properties only about 45% of Mercury’s surface has been seen Mercury’s southern hemisphere Albedo = 0.12 Mass = 0.055 of Earth’s Density = 5.43 gm/cm3 [2nd most iron-rich object in solar system] Surface gravity = 0.38 Earth’s [twice the Moon’s] Axial tilt = 0 degrees ORBIT: 0.39 AU = semimajor axis 0.207 = eccentricity 7 deg = inclination 88 day = period 0.055 equals ~1/20 No seasons!

Density 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%

800 F (700 K) at noon at perihelion (lead would melt!) Largest temperature range (~1100 F) in solar system 800 F (700 K) at noon at perihelion (lead would melt!) -280 F (100 K) at midnight

Telescopic Observations Difficult because: Mercury low in the sky. Mercury is small. Contrasts are weak. Result: Very little was known based on visible telescope data.

Observations Mercury’s small orbit causes it always to appear close to the Sun. At best you can see Mercury just after sunset or just before sunrise toward the horizon through a lot of blurring atmosphere. Early astronomers thought Mercury would be in synchronous rotation like our Moon. Thus, the Sun-facing side would always be hot! Transit of Mercury Mercury (at bottom) & Venus near top-left at twilight. The following slides begin with ground-based and extend to space-based.

It's hard to view surface details

Early maps showed imagination only!

Synchronous Rotation? 1962 Radio astronomers found that the backside wasn't cold. 1965 Dyce & Pettengil bounced radar off Mercury and learned its rotation period = 58.7 days Mercury’s year is about 88 days Emphasize “thermal emission” The faster the motion, the greater the Doppler Shift.

A Solar Day on Mercury Giuseppi Columbo (1920-1984) Mercury’s solar day is 176 days long Sun USUALLY rises in east & sets in west: UNLESS Mercury is near perihelion when it swings by Sun more quickly & Sun seems to reverse directions This “resonance” is caused by tides, just as we discussed for the Moon. Mercury rotates exactly 3 times during two orbits around Sun. RATIO = 87.97/58.65 = 3:2 [“3:2 spin-orbit resonance”]

The Sun from Mercury At 0.38 AU from Sun, Mercury is 2.5 times closer than the Earth Sunlight is (2.5)2x = 6.25x more intense

Mercury 2nd highest density of any planet. Its ancient surface records processes from the earliest part of planetary formation. Its exotic atmosphere is the thinnest among those of all the terrestrial planets. Mystery: It is the only terrestrial planet besides Earth to possess a global magnetic field.

Mariner Venus-Mercury Flyby Mission 1974-1975 The first-ever gravity-assist trajectory used Venus’s gravity & orbital motion to help Mariner 10 fall towards Mercury. Such a maneuver avoids the need for large braking rockets. Mariner 10 went into orbit around the Sun and flew by Mercury three times before exhausting its positioning fuel.

Mercury's Surface Properties Surface appears generally Moon-like, covered by dark gray regolith. Brightness, roughness similar to lunar highlands. No water! Small, localized mare deposits. Many craters, several large impact basins.

Mariner 10 Mercury studied by just one space mission: Mariner 10 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.

Geology of Mercury • Crater shapes different from the Moon: • Gravity on Mercury is ~ twice that on theMoon. • Crater bowls shallower than on the Moon. • Ejecta blankets less extensive than on the Moon.

Mercury’s Surface Features Transition from heavily cratered area to smooth lava plains. About 490 km across. “Discovery scarp” 350 km long. Intersects two craters with diameters 35 & 55 km.

Geology of Mercury Evidence of tectonic forces at work in the past. Lobate Scarp indicates compression of the planet's crust.

Volcanoes on Mercury Mercury appears to be geologically dead and is heavily cratered. There are no large volcanoes like Mars’ Olympus Mons, but there are many smooth, flat plains with few craters Scientists have debated whether these ancient plains were formed by erupting volcanoes driven by internal heat, or simple melting associated with impact processes The latest close-up images by NASA’s MESSENGER support the volcano theory MESSENGER false color image of Caloris impact basin (light orange is the basin interior). Extinct volcanoes were imaged in several of the bright orange regions just inside the southern crater rim. Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider dpsdisc@aas.org - http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009

Direct & Indirect Evidence for Volcanoes MESSENGER has found shield volcanoes and vents suggesting explosive volcanism inside the large Caloris basin The Mercury volcanoes may be similar to the Hawaiian Islands or Olympus Mons on Mars Lava appears to have partly filled impact craters both inside and far from Caloris basin (not shown) vents partly filled crater MESSENGER image (left) of a shield-like volcanic dome, multiple vents and associated bright deposits, and partially buried nearby features. Shield volcanism formed the island of Hawaii (right). Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider dpsdisc@aas.org - http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009

The Big Picture Hawaii Venus Mars Alaska Mercury Volcanism appears to be responsible for formation of Mercury’s widespread plains Mercury’s ancient plains-forming, crater-filling volcanic style was more similar to the Moon than Mars or Earth MESSENGER will enter orbit around Mercury in 2011, offering abundant opportunity to image volcanic features and place Mercury’s volcanism in a solar system context Volcanic features in the inner solar system Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider dpsdisc@aas.org - http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009

Moon or Mercury? Why would craters differ between Mercury & Moon? A large crater is shown at higher resolution. It is ~100 km diameter and shows radial ejecta, secondary craters, central peaks, etc.

Consequences of a Gravity Difference Between the Mercury & Moon

Mercury’s Atmosphere 10-14 of Earth’s pressure: an “exosphere” So thin that molecules are more likely to bounce around the surface than to hit each other. Hydrogen & helium come from solar wind. Hydrogen & oxygen may come from comets that evaporated nearby as they approached the Sun. Potassium, sodium, & oxygen may come from surface rocks that have been vaporized by impacts. Neon, too!

Mercury’s Magnetic Field More like Earth’s than Venus, Moon, Mars Only other terrestrial planet besides Earth with an overall magnetic field. Strength ~ 1% of Earth’s Could Mercury still have a molten core?

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?

Water Ice at Mercury’s Poles? Since 1992, radio astronomers have bounced radar beams off Mercury and detected possible water ice at its poles and in numerous areas with circular shapes.

Water Ice at Mercury’s Poles? 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 never evaporated (primordial!) Other hypotheses: sulfur, silicates.

Planetary Evolution Mercury has a weak magnetic field: Metallic core may still be partially molten? 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?

The Shrinking Planet

NASA’s Return to Mercury Many questions will be answered by NASA’s MESSENGER Mercury orbiter Launched in 2004. Orbit insertion in 2011. Orbits, maps for 1 Earth year. Imaging, spectroscopy, magnetic fields, altimetry. One of the new NASA “better, faster, cheaper” space missions… Hope to confirm the water ice at the poles

For more information… Press Releases Images space.com - 7/3/08 - “Volcanoes on Mercury Solve 30-year Mystery” http://www.space.com/scienceastronomy/080703-mercury-messenger.html Images Global view of Caloris basin and Mercury shield volcano courtesy of Science / AAAS http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/caloris_color_MB.jpg http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/Head_Fig1.jpg Aerial view of Hawaii courtesy of NASA/JSC STS61A http://tinyurl.com/maunaloashieldvolcano Aerial view of erupting Mauna Loa in Hawaii courtesy of HVO/USGS http://hvo.wr.usgs.gov/ Image of Alaska’s Redoubt Volcano courtesy of AVO/USGS, taken by Heather Bleick http://www.avo.alaska.edu/image.php?id=17872 Image of Olympus Mons on Mars and Maat Mon on Venus courtesy of NASA/JPL http://pds.jpl.nasa.gov/planets/captions/mars/olympus.htm http://photojournal.jpl.nasa.gov/catalog/PIA00106 Source Article (on-campus login may be required to access journals) Head et al., ‘Volcanism on Mercury: Evidence from the First MESSENGER Flyby’, Science, 321(5885), p. 69,
DOI: 10.1126/science.1159256, 2008. http://www.sciencemag.org/cgi/content/abstract/321/5885/69 Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider dpsdisc@aas.org - http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009