This evening’s announcements
Homework 3 is graded and available for pickup at entry
Quiz 4 will be held this Wednesday, March 12. Coverage:
• Feb. 25: origin of the solar system (chapter 6)
• Feb. 27: Earth, Moon, Mercury (chapter 7)
• Mar. 10: Mars, Venus as time permits (chapter 7)
Homework 4 will be distributed March 12, due March 19
Frequently missed questions from
Homework 3
5. Age of solar system: 4.5 billion years (no problem
here). Evidence: Radiometric ages of meteorites
This evening’s topics
Terrestrial worlds, continued
1. Moon (continued)
2. Mercury
3. Mars
4. Venus (if time permits)
Moon, continued
Rule of thumb: crater’s diameter approx 10 times that of
meteorite
Interpretation of maria as impact basins
• Typical diameter about 1000 kilometers; implies dug by
100-kilometer meteorites
• Later filled with smooth, dark lava
• Lava “wrinkles” visible on surface
• Also some volcanic domes, which probably were the last
vestiges of the activity that produced the lava
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Age of lunar surface
From radiometric dating of rock samples returned by Apollo
astronauts
• Highlands: various samples range from 3.8 to 4.4 billion
years
• Maria: 3.2 to 3.8 billion years, depending on location
All lunar rocks are older than almost every known Earth rock
Significance of ages of lunar rocks
• Highlands have about 10 times more large craters than do
the maria
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• Many fewer craters have formed in last 3.8 billion years
(since maria were resurfaced) than in the first 0.7 billion
• This means that most large craters formed within first
billion years: a heavy early bombardment by meteorites
The Moon has many old rocks and no known young rocks
because most surface modification ceased about 3 billion
Picture is near bottom
years ago.
of linked page.
Reason: being small, the Moon lost its internal heat rapidly,
volcanoes could no longer be active.
The only surface modification going on today is meteorite
bombardment: continual impacts by small particles, formation
of a kilometer-size crater every million years or so.
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Mercury
The least well-studied of the terrestrial planets
• Difficult to study with telescopes from the ground because
it is always close to the Sun
• Until recently, little visited by spacecraft, only Mariner 10
flyby; Messenger in late 2007
Bulk density 5.5 grams per cubic centimeter: Earthlike
But surface is Moonlike
• Impact craters & basins
• Lava flows, but without the strong dark coloration of lunar
lava
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New feature: scarps
• Typical example: crosses a crater
• No radioisotope dates for this planet because no surface
samples returned to Earth; must guess ages of scarps
Tentative explanation for scarps in terms of planetary
processes
• Initial heating by radioactivity, true of all terrestrial planets
• Small ones (Moon, Mercury) cooled rapidly
• Formed thick, rigid crust: a lithosphere
• When the metallic core cooled, it shrank a lot: lithosphere
shrank accordingly, causing scarps to form
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Heavily cratered surface suggests that all the terrestrial
planets underwent an early heavy bombardment, more
support for the nebular theory.
Sequence of events for Mercury, from stratigraphic sequence
(layering of features on surface):
1. Formation of numerous large craters in early heavy
bombardment
2. Lava flows, formation of lava plains
3. Cooling of planet, formation of scarps
4. Formation of numerous smaller impact craters
No surface samples available yet, therefore no dates
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Mars
Bulk density about 4 grams per cubic centimeter
Smaller in size than Earth but larger than Moon, Mercury
Rotation period about 24 hours; length of day similar to
Earth’s
Rotation axis tilted much as Earth’s, so seasons are similar to
Earth’s
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Major surface markings
• Polar caps (change with the seasons)
• Reddish color
• Light & dark areas
Recent exploration
Link to PDF fact sheet, 1.5 MB.
Image used is on page 2.
• Mars Exploration Rover Mission: Spirit and Opportunity
robotic rovers have been on Mars since January 2004,
taking pictures, studying surface & atmosphere by remote
control from Earth
• Mars Global Surveyor orbiter made imaging and other
studies 1999–2006
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• Mars Reconaissance Orbiter is currently operating.
Camera can distinguish features as small as 1/2 meter on
the surface.
Atmosphere
Mainly carbon dioxide
Surface pressure
• Less than 1% of Earth’s
• Equivalent to pressure at high altitude on Earth, about
130,000 feet
• Provides little shielding from solar ultraviolet
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• Liquid water cannot exist; it would either vaporize or
freeze
Wind: 100 mph not uncommon; blowing sand; dust storms;
dunes; wind erosion
Daily and seasonal temperature changes are strong but,
generally, it’s cold
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Polar caps
[Forgot to show this
one!]
Composition
• Contain water (always frozen) and carbon dioxide snow
and ice
• Seasonal vaporization & freezing of carbon dioxide
component
Act as cold storage for water and carbon dioxide
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Other surface features
Impact craters & basins
Canyons
See image labeled, "Valles Marineris"
Shield volcanoes
Shield volcanoes on Earth
• Theory: caused by a hot spot, a convective plume
originating at the outer boundary of the core
• Liquid rock breaks through & forms volcanoes
• “Shield volcano” appearance caused by more-liquid lava,
compared to steeper-sided, cone-shaped volcanoes
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• Crustal plate moving over hot spot causes chain of
volcanoes, example: Hawaiian Islands
On Mars
• Typical in shape for shield volcanoes, but very large. No
plate movement, so volcanoes build up in one place.
• Likely age (rough estimate!) 2 billion years from number
of impact craters on slopes
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Permafrost, a permanent layer of subsurface ice
Evidence: peculiar craters with flow patterns around them
probably result from impact in frozen soil
Dry river beds or channels or arroyos
• Generally quite old; craters on top of them
• “Outflow channels” are the result of flash flooding —
wider at beginning, narrower at end.
• Detailed examination by Mars Reconaissance Orbiter
shows some are typically filled with lava, although
originally carved by flowing water.
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Some gullies have been found by Mars Global Surveyor and
confirmed by Mars Reconaissance Orbiter.
• Locations have very few impact craters, hence recently
formed
• Appear to be fed by melting subsurface ice
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Evidence for long-term climate change on Mars: the channels.
• Water could not flow extensively today
• Higher atmospheric pressure in past (2 billion years ago?)
• More greenhouse warming as a result, warmer also
• Or perhaps Mars is now in an ice age
• Recent MRO results indicate the warm period was less
extensive than previously thought. They cast doubt on
claims of possible early rainfall, for example.
• Flash flooding could have been caused by an impact
melting subsurface ice.
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What happened to Mars’ atmosphere?
• Escape: because of its relatively weak gravitational pull,
Mars may have lost a significant part of its atmosphere
over time
• Freezeout: a lot of water and carbon dioxide could be
stored in permafrost and the polar caps.
Cessation of volcanic activity around 2 billion years ago
• Planet’s interior cooled due to small size
• Then atmosphere no longer replenished by gases emerging
from volcanoes
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• Fits in with pattern: small size goes with less surface
modification
– Mercury, Moon: smallest, most craters, least modified
surface
– Mars: intermediate in all respects
– Venus, Earth: largest, fewest craters, most surface
modification
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Has there ever been life on Mars?
Evidence for ancient water from Spirit and Opportunity
• Erosional, depositional landforms
2nd on linked page
• “Blueberries” - round stones that formed in water
• Other minerals that form in the presence of water
Implies life could have arisen there.
Life now? Maybe where there is underground liquid water.
Why we want to know: life on Earth is all the same
(DNA-based); can’t draw reliable conclusions about origins
from only one example.
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Venus
Similar to Earth in size, mass, density
Major difference is atmosphere: surface invisible from Earth or
space because of thick cloud layer
Except: radio waves can travel through atmosphere
• Receive radio signals from a lander
• Radar: send radio signal, receive echo with information
about surface roughness, elevation
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Atmosphere of Venus
landers
Pressure at surface: 90 Earth atmospheres (equivalent to 1/2
Image is about
mile deep under water on Earth)
• Venera 9 lander from former Soviet Union
halfway down linked
page.
• Images show red color from light filtered through clouds
Venus’s gravitational pull is similar to that of the Earth, so its
atmosphere is much more massive than the Earth’s.
Average temperature at surface: 750 Kelvin
• Hottest planetary surface, hot enough to melt lead
• Rock generally melts above 1000 K
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Composition: mostly carbon dioxide (CO2)
Explanation for the high surface temperature: the
“Greenhouse Effect” of its very massive carbon dioxide
atmosphere
Venus may have started out with a transparent atmosphere
like Earth’s but the greenhouse effect “ran away.”
Now that the surface is hot, it produces a lot of thermal
radiation, mostly infrared.
The atmosphere trapping the infrared is enough to keep the
planet hot, even though almost all the sunlight is reflected
from the cloud layer.
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Surface features
Source of best information is the Magellan orbiter, which
carried out 4 years of radar mapping (early 1990’s)
• Impact craters — more than on Earth but far fewer than
on Moon
• Lava flows covering much of surface. May have
occasional, planetwide bursts of volcanism.
• Volcanoes similar to shield volcanoes on Earth and Mars
(Note yellow false color.)
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Summary of terrestrial planets
All formed by same process: solid-body accretion in the early
solar nebula.
All subject to similar processes during their history.
Differences between them are largely explained by their
different sizes and distances from the Sun.
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