Mercury. Basic Properties of Mercury Average Distance from Sun: 58,000,000 km (0.39 AU). –But: Orbital eccentricity = 0.206. –Therefore distance varies.

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 Propertiesonly about 45% of Mercury’s surface has been

seen

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 axis0.207 = eccentricity7 deg = inclination88 day = period

Mercury’s southern hemisphere

DensityMercury'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!)

-280 F (100 K) at midnight

Largest temperature range (~1100 F) in solar system

Telescopic ObservationsDifficult because:

– Mercury low in the sky.

– Mercury is small.– Contrasts are weak.

Result: Very little was known based on visible telescope data.

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!

Observations

Transit of Mercury

It's hard to view surface details

Early maps showed imagination only!

Synchronous Rotation?

1962Radio astronomers found that the backside wasn't cold.

1965Dyce & Pettengil bounced radar off Mercury and learned its rotation period = 58.7 days

Mercury’s year is about 88 days

A Solar Day on MercuryGiuseppi 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 Mission1974-1975

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 [email protected] - 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)

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).

vents

partly filled crater

Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider [email protected] - http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009

The Big Picture

• 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

Hawaii

VenusMars

Alaska

Mercury

Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider [email protected] - 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 Atmosphere10-14 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.

Mercury’s Magnetic Field

More like Earth’s than Venus, Moon, MarsOnly other terrestrial planet besides Earth with an overall magnetic field.Strength ~ 1% of Earth’sCould 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.

• 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.

Water Ice at Mercury’s Poles?

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

• 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 STS61Ahttp://tinyurl.com/maunaloashieldvolcano

• Aerial view of erupting Mauna Loa in Hawaii courtesy of HVO/USGShttp://hvo.wr.usgs.gov/

• Image of Alaska’s Redoubt Volcano courtesy of AVO/USGS, taken by Heather Bleickhttp://www.avo.alaska.edu/image.php?id=17872

• Image of Olympus Mons on Mars and Maat Mon on Venus courtesy of NASA/JPLhttp://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 [email protected] - http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009