PTYS/ASTR 206Moon and Mercury 3/8/07 The Moon and Mercury

PTYS/ASTR 206 Moon and Mercury3/8/07

The Moon and Mercury


Announcements

• Reading Assignment– Chapter 12

• 4th homework due March 20 (first class after spring break)

• Reminder about term paper – due April 17.

• Next study-group session is Monday, March 19, from 10:30AM-12:00Noon – in room 330.


Our Moon is one of seven large satellites in the solar system

(it is the 4th largest)


The Moon keeps nearly the same face to the Earth at all times

Full Moon Seen from Earth Far Side of the Moon (Apollo 16)


The Moon is in 1:1 synchronous rotation with the Earth – it rotates once on its axis every complete

orbit around the Earth


The Moon always faces the Earth because of tidal

forces due to Earth

• The tidal force is a secondary effect of the force of gravity. It arises because of the difference in gravitational force across a body

• For an orbiting body experiencing tidal forces, there is a gradual dissipation of rotational energy and the object’s rotation rate slows down– This happened to the Moon, and will

also eventually happen to Earth

• We will see tidal forces again as the course goes on

Illustration of the origin of the tidal force



Exploration of the Moon• Most heavily explored extraterrestrial

body• First flyby – 1959 (USSR)• Unmanned orbiters, landers throughout

1960s• Apollo astronaut missions, US, 1969-

1973– 6 landings, 12 astronauts – Only

extraterrestrial body visited by humans

• Luna robotic sample return, USSR, 1970-1976– Three robotic sample returns

• Clementine orbiter 1994• Lunar Prospector orbiter, 1998-1999

– Intentionally Crashed into the Moon• Lunar Reconnaissance Orbiter (fall

2008)


Visible Lunar Features

• The Earth-facing side of the Moon displays light-colored, heavily cratered highlands and dark-colored, smooth-surfaced maria

• The Moon’s far side has almost no maria


The Surface Features on the Moon

• Impact Craters– Formed by impacts from asteroids

and comets• Maria

– Younger and Darker– Fewer craters

• Highlands– Older and Brighter– Lots of craters

• Regolith– Fine powdery dust-like material– Footprints left behind by 12

astronauts• No evidence for plate tectonics or

volcanoes


The Origin of the Lunar Maria

• The maria formed after the surrounding light-colored terrain, so they have not been exposed to meteoritic bombardment for as long and have fewer craters

• They are nearly circular, which suggests an impact (a large one!)


Formation of Mare


Sea of Serenity

Wrinkled-looking edges indicate cooled and solidified ancient lava flows

Rimae and Rilles: “rivers” indicating ancient lava flows


• The close association of the observed iron content on the lunar surface with the lunar maria confirms the theory of their origin (iron-rich lavas)– This iron may be a combination of that

left by the impactor, or from deeper within the moon where there is slightly more iron than on the surface

These images were produce by the unmanned Clementine probe


Water Ice on the Moon ?

• Water evaporates in sun-exposed regions• Permanently shadowed spots cold enough

for H2O ice (from comet/asteroid impacts) to stay

• Could only happen in craters near poles• Clementine – radar reflection looks like ice• Lunar Prospector – thermal neutrons

detected (evidence of H2O ice)

• Prospector intentionally crashed into crater, but no plume of water detected

• Is there water ice?– Probably, but not definitely– If it is -- good place for lunar base

Perhaps as much as 300 million metric tons of water ice !


All of the lunar rock samples are igneous rocks formed largely of minerals found in terrestrial rocks

• The lunar rocks contain no water (distinctly different from Earth rocks)

• They generally differ from terrestrial rocks in being relatively enriched in the refractory elements and depleted in the volatile elements

Mare basaltFound in maria, similar to Earth basalts

Highlands anorthositeOldest, found in highlands

Impact breccia found in highlands (fused material from impacts)


Lunar rocks reveal a geologic history quite unlike that of Earth

• The anorthositic crust exposed in the highlands was formed between 4.0 and 4.3 billion years ago

• The mare basalts solidified between 3.1 and 3.8 billion years ago

• The Moon’s surface has undergone very little change over the past 3 billion years


Does the Moon have a Core ?

• Evidence– Density is low (matches rock,

not rock + metal)– Apollo program left

seismometers• Not many moonquakes

• Seismometers turned off in 1978 to save money

• If there’s a core, it’s small

• Lunar Prospector– Mapped gravity in detail– Concluded there’s a core, 1-

4% of total lunar mass Seismic Equipment on the Moon


The Moon has no global magnetic field but hasa small core beneath a thick mantle


The Origin of the Moon• What any theory needs to account for

– Density is nearly that of the rocks found there

• (i.e. not rock + metal, like Earth)– Composition, other than Iron, is like

the Earth

The Giant-Impact Theory

• Early earth was impacted by a large object.

• A piece of Earth broke off and eventually formed the Moon

• The Iron in Earth was already at the center (differentiation had already taken place)

• The Iron in the impactor melted and was integrated into the Earth


Mercury• The smallest planet (now that Pluto is

no longer a planet)• Avg. distance from the sun: 0.387 AU• Year: 88 days• Day: 58.6 days

3 Mercury Days = 2 Mercury Years• Eccentricity: 0.206• hot. airless, Moon-like

– Daytime temp: +662 oF– Nightime temp: -274 oF

• 1/3 size of Earth • heavily cratered • polar ices • scarps • Magnetic field and magnetosphere

(like Earth, but weaker)


• Mercury is difficult to observe from the Earth

• It is close to the Sun• Never rises more than two

hours before the Sun• Never sets more than two

hours after the Sun

Earth-based Views of Mercury


• At its greatest eastern and western elongation, Mercury is never more than 28° from the Sun

• It can be seen for only brief periods just after sunset or before sunrise

Viewing Mercury from Earth


Occasionally, Mercury moves in front of the Sun (one of only two planets that can do this!)

There was a transit on November 8, 2006

There will not be another for 10 years

Transits occur about twelve times a century when the Sun, Earth and Mercury are aligned


Mercury has a slow rotation period (long days and nights)

• Long believed (incorrectly) to be 88 days– Difficult to tell because it is so hard to observe from Earth

• Ground based radar found it to be about 59 days!– Used Doppler shift in radar signals sent from Earth

Arecibo radio telescope, Puerto Rico


Mercury’s 3:2 spin/orbit ratio

Mercury turns on its axis 1 ½ times in one complete orbit about the Sun

This is a 3:2 spin-orbit resonance


• Strong tidal effects, Mercury’s slightly elongated shape and its very eccentric orbit cause this strange 3-to-2 orbit

• A “day” of solar light on Mercury would be 88 earth days

Documents

PTYS/ASTR 206Moon and Mercury 3/8/07 The Moon and Mercury