Lec 18: 2 November 2011 Chap 10: The Earth’s Moon Lunar ...neffj.people.cofc.edu/ASTR129/Notes/lec18.pdf · 1 Lec 18: 2 November 2011 Chap 10: The Earth’s Moon LAST - Lunar Overview

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Lec 18: 2 November 2011 Chap 10: The Earth’s Moon LAST - Lunar Overview and History of Lunar Exploration

–  global and observed properties of the Moon –  two types of surface: Highlands and Maria –  history of lunar exploration

TODAY - Cratering as a Tool; Formation of the Moon –  “ground truth” from Apollo –  the impact cratering process –  using craters as tools to understand the solar system –  formation of the Moon (giant impact?)

NEXT Week- “Earth as a Planet” (Chapter 9) –  Chapter 9 pre-Quiz on Monday

Lunar Surface

•  100% cratered •  “bright” gray •  mountains ring craters •  83% of surface •  higher elevations •  shattered crust (25 km) •  breccias & anorthosite •  4.2 billion years old

•  2% cratered •  “dark” gray •  circular •  17% of surface •  impact basins •  -- •  basaltic lava •  3.1-3.9 billion years old

HIGHLANDS MARIA

Most lunar rock samples are IGNEOUS

•  No water or volatile elements (e.g. N, C, S, Cl, K)

•  Rich in refractory elements (e.g. Al, Ca, Si, Mg)

•  soil contains glass beads; accumulates ~ 2mm/year

Mare basalt

High-lands

anorth-osite

Impact breccia

The Moon is covered with craters, but there are only about 100 large craters on Earth. Why?

Impact Craters on Earth

No. Formed about the same time, 4.6 billion years ago •  Is the Moon older?

•  Is the Moon bigger? A more “attractive” target?

•  Perhaps Earth’s atmosphere protects us from impacts ??

The Moon is covered with craters, but there are only about 100 craters on Earth. Why?

No. Earth is bigger and more massive.

Not really. It does filter out most of the small stuff, but a lot gets through (anything big enough to produce a crater we could see on the Moon would certainly get through).

Impact Craters on Earth (continued)

•  Studying the Moon allows us to measure the rate of impacts over the past 4.6 billion years –  very important for predicting current impact risk –  large impacts were much more common in the

early solar system than they are today –  planets “sweep up” debris along their orbits or on

eccentric orbits in the ecliptic plane –  current debris “scattered” from asteroid belt,

Kuiper belt, Oort cloud, from collisions, or from comet dust

•  Impact CRATERS are a very important TOOL for studying planetary surfaces

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The Impact Cratering Process

•  Impact crater caused by explosion •  1/2 sphere gouged out; debris settles back

into floor of crater •  viewed from above it is round, no matter

what shape the meteoroid had •  viewed from the side you see a raised rim •  sometimes you see a central peak •  size of crater depends on kinetic energy

– KE = 1/2 mass x velocity^2

– depends more on speed than mass or size

Side View of an Impact Crater

usually appear circular when viewed from top

raised rim and central peaks commonly seen in large lunar craters

what can you say about the relative ages of these craters?

What can we learn from IMPACT CRATERS? (volcanoes can also produce craters)

1.  ...from the details of craters? •  surface/subsurface material •  relative age (craters on craters; erosion) •  type, size, speed of impactor •  maybe direction of impactor

(not usually, though)

2.  ... from crater size distributions? •  size distribution of meteoroids near Earth

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What can we learn from impact craters?

3.  ... from the surface density distribution? •  relative age of surface (how long has it been exposed) •  rate of cratering as a function of time (is it constant?) •  rate of erosion, if any (on Moon: eroded by other

impacts, lava flows, rock slides, etc; NOT by weathering)

from crater density/current impact rate, age of maria: 3.5 to 4 billion years

highlands: older than this, but younger than 4.6 billion years

Surface Density of Craters: Highlands v. Maria

4.2 billion years old

100% cratered

3.1-3.9 billion years old

2% cratered

impact or volcanic?

•  Heavy cratering during “Terminal Bombardment” •  slow and constant rate since

How Did The Moon Form?

•  Similarities with the Earth... –  density similar to Earth’s mantle –  elemental abundances from this part of solar system

•  Differences... –  refractory/volatile > Earth’s (heated again to over 1000 K) –  isotope ratios not precisely the same (some of it, at least,

from slightly different part of solar system) •  Moon’s orbit not in Earth’s equatorial plane •  Oldest rocks on Moon younger than 4.6 billion years

FISSION? CAPTURE? COLLISION?

We don’t really know what happened, but we know what didn’t happen, or at least what doesn’t fit our observations or doesn’t make physical sense.

How Was the Moon Formed?