Homework #4 Due Wednesday, February 28, 11:59PM Covers Chapters 6 and 7 Estimated time to complete: 1 hour Read chapters, review notes before starting

Why Does Lack of Heat Matter? Once the internal heat of a world is gone, the once-soft hot rock cools off  lithosphere grows thicker and thicker. Cold rock will not flow and rise to the surface through cracks in the lithosphere  no more geological activity such as volcanoes This is what has already happened on Mercury and the Moon long ago, and is happening to Mars now.

Role of Size Smaller worlds cool off faster and solidify earlier. Since internal heat drives geological activity, a total loss of heat leads to a total loss of geological activity. Smaller worlds cool off faster and solidify earlier. The Moon and Mercury are now geologically “dead” because they have already lost all their internal heat. Important!

Surface Area–to–Volume Ratio Heat content depends on volume. Loss of heat through radiation depends on surface area. Cooling rate depends on surface area divided by volume: Surface area–to–volume ratio = 4 p r 2 3 Bigger objects cool more slowly 3 Easy to remember: a hot pea cools off much faster than a hot potato at the same initial temperature

What key quality is responsible for the Moon and Mercury being geologically dead? A) their distance from the Sun B) their chemical composition C) their size D) their lack of atmosphere

What key quality is responsible for the Moon and Mercury being geologically dead? A) their distance from the Sun B) their chemical composition C) their size D) their lack of atmosphere Because of their small size, they have higher surface area-to-volume ratios  cooled off faster (think hot pea vs. hot baked potato)

What processes shape planetary surfaces?

Processes That Shape Surfaces Impact cratering Impacts by asteroids or comets Volcanism Eruption of molten rock onto surface Tectonics Disruption of a planet’s surface by internal stresses Erosion Surface changes made by wind, water, or ice Last 3 processes stop once the internal heat is gone.

Impact Cratering Most cratering happened soon after the solar system formed (“Period of Heavy Bombardment”). Craters are about 10 times wider than object that made them, and 10-20% as deep as they are wide (impact at 40,000-250,000 km/hr). Small craters greatly outnumber large ones.

Impact Craters Meteor Crater (Arizona) Tycho Crater (Moon) (Note central rebound peak)

Volcanism Volcanism happens when molten rock (magma) finds a path through crack in the lithosphere to the surface. Molten rock is called lava after it reaches the surface. Lava can fill in/erase craters  younger surfaces show less cratering

Outgassing Volcano National Park Mount St. Helen’s Volcanism also releases gases from Earth’s interior into the atmosphere (outgassing). This is where the terrestrial planets got their atmospheres.  key Atmospheres diffuse away if not continuously replenished  explains lack of atmospheres on dead planets

Tectonics Convection of the mantle creates stresses in the crust called tectonic forces. Compression of crust creates mountain ranges. Valley can form where crust is pulled apart.

Plate Tectonics on Earth Earth’s continents slide around on separate plates of crust. Lithosphere is fractured into more than a dozen plates  continents sit on some of these plates Apparently unique to Earth in the Solar System today – no other terrestrial world has plate tectonics.

Continental Motion The idea of continental drift was inspired by the puzzle-like fit of the continents. Mantle material erupts where the seafloor spreads, causing plates to slowly drift across the globe.

Continental Motion Plates are pieces of Earth’s fractured lithosphere Proposed by Alfred Wegener in 1912 Motion of the continents (on the plates) can be measured with GPS – few centimeters/year.

Surface Features The Himalayas formed from a collision between plates.

Surface Features The Red Sea is formed where plates are pulling apart.

Rifts, Faults, Earthquakes The San Andreas fault in California is a plate boundary. Sideways motion of plates can cause earthquakes. In 20 million years, SF and LA will be near each other.

Plate Motions Measurements of plate motions tell us past and future layout of the continents. Pangaea – 250 million years ago Rodinia – 750 million year ago

http://pubs.usgs.gov/publications/text/dynamic.html

Cryolophosaurus ellioti found in Antarctica

Erosion Erosion is a blanket term for weather-driven processes that break down or transport rock. Processes that cause erosion include: glaciers rivers wind All dependent on the presence of an atmosphere.

Erosion by Ice, Water, Wind Glaciers carved the Yosemite Valley. The Colorado River continues to carve Grand Canyon Wind wears away rock and builds up sand dunes.

Erosional Debris Erosion can create new features such as deltas by depositing debris. Sediment is transformed onto sedimentary rock – most common type of rock on Earth Erosion also builds, not always destroys

Which of the following is the only process capable of shaping the surface of a dead world? A) volcanic activity B) plate tectonics C) erosion D) impact cratering

First three depend (ultimately) on internal heat. Which of the following is the only process capable of shaping the surface of a dead world? A) volcanic activity B) plate tectonics C) erosion D) impact cratering First three depend (ultimately) on internal heat.

What is an atmosphere? An atmosphere is a layer of gas that surrounds a world. Source: outgassing from volcanic activity

Earth’s Atmosphere About 10 kilometers thick Analogy: as thick as a dollar bill wrapped around a standard globe. Consists of: Nitrogen: 77% Oxygen: 21% Argon: 1% H2O, CO2: trace

Effects of Atmosphere on Earth 1) Erosion - rain, wind, ice shapes surface 2) Radiation protection - harmful X-rays, gamma-rays blocked 3) Greenhouse effect - keeps Earth warm (too warm?) 4) Makes the sky blue!

Radiation Protection All X-ray light is absorbed very high in the atmosphere. Ultraviolet light is absorbed by ozone (O3) – explains concern about holes in the ozone.

The Greenhouse Effect Visible (yellow) light passes through the atmosphere easily and warms a planet’s surface. Planet absorbs optical light and emits thermal blackbody infrared (red) radiation. The atmosphere absorbs infrared radiation from the surface, trapping heat like a blanket.

(H2O, CO2, CH4)  greenhouse gases Greenhouse effect: Certain molecules let sunlight through but trap escaping infrared photons. (H2O, CO2, CH4)  greenhouse gases The Greenhouse Effect

Greenhouse Effect: Good or Bad? Because of the greenhouse effect, Earth is considerably warmer than it would be without an atmosphere (above the freezing point of water). Without any greenhouse effect, Earth would be too cold for liquid water to exist on its surface  very bad for life! But we are pumping a lot of CO2 into the Earth’s atmosphere by burning fossil fuels. What’s going to happen to all this excess greenhouse gas?

Why the sky is blue Atmosphere scatters blue light from the Sun more than red light, making blue light appear to come from different directions. Sunsets are red because less of the red light from the Sun is scattered.

Was there ever geological activity on the Moon or Mercury Was there ever geological activity on the Moon or Mercury? (Yes, but long ago)

Formation of Lunar Maria Very large, deep crater formed by asteroid impact Runny lava seeps through lithosphere, filling crater Lava hardens to form darker-colored maria Heavily-cratered early lunar surface Some volcanic activity 3-3.5 billion years ago must have flooded lunar craters, creating lunar maria (dark areas on Moon). Maria have few craters  formed after Period of Heavy Bombardment ended.

Geologically Dead Moon is considered geologically “dead” now because geological processes have virtually stopped (no more internal heat). Lack of outgassing  no more atmosphere on the Moon. Micrometeorites pulverize rock, creating fine dust.

Cratering of Mercury Mercury has a mixture of heavily cratered (oldest) and smooth regions (somewhat younger) like the Moon. The smooth regions are likely ancient lava flows.

Tectonics on Mercury Mercury is also now geologically dead. Lack of outgassing  no atmosphere Long cliffs (hundreds of miles long, 2 miles high) indicate that Mercury shrank early in its history.

Mars

“Canals” on Mars In 1877, astronomer Giovanni Schiaparelli used the Italian word ‘canali’ (channels) to describe features on Mars. Unfortunately, this was mis-translated as “canals” in English, leading people to believe there were structures built on Mars by Martians. Percival Lowell misinterpreted surface features seen in telescopic images of Mars.

Polar Ice Caps of Mars CO2 + H2O CO2 Late winter Mid-spring Early summer Carbon dioxide (CO2) ice (“dry ice”) of polar cap sublimates as summer approaches and condenses at opposite pole.

Polar Ice Caps of Mars Residual ice of the polar cap remaining during summer is primarily water ice – water has a higher sublimation temperature than carbon dioxide. If all water ice melted, an ocean 11-meters deep over the entire planet could be formed.

Past tectonic activity… Volcanoes…as recently as 180 million years ago, but no volcanoes are known to be active today. Geological activity is probably ending right about now.

What geological evidence tells us that water once flowed on Mars?

Dry Riverbeds? Close-up photos of Mars show what appear to be dried-up riverbeds. Crater counts imply river beds are ~3 billion years old.

Erosion of Craters Details of some craters suggest they were once filled with swirling liquid water. Sedimentary rock laid down when crater was filled with water?

Eroded crater (from rainfall?) Impact Craters on Mars “Standard” crater Impact into icy ground Eroded crater (from rainfall?)

Which is Mars? Left (A) or Right (B)? Martian Rocks Which is Mars? Left (A) or Right (B)?

Martian Rocks jarosite Jarosite and hematite form in water on Earth. Mars rovers have found rocks that appear to have formed in water  hematite “blueberries”.

Climate Change on Mars Mars has not had widespread surface water for 3 billion years, but most likely had water before that. Greenhouse effect probably kept the surface warmer before that  lots of liquid water? (extremely likely). Because of its smallish size, Mars lost most of its atmosphere - low mass of Mars allowed gases to diffuse away from atmosphere. - little/no additional gas replenishment from outgassing once volcanism diminished

If Mars Had Only Been Bigger… The gravity of a larger Mars would have retained its atmosphere more easily A larger Mars would have continued volcanic/outgassing activity until today (slower cooling) A larger Mars would have a thicker atmosphere, more greenhouse effect, warmer surface  liquid water today? Would these have been our neighbors?

Which of the following is not evidence that liquid water existed on the surface of Mars in the distant past? A) Hematite “blueberries” B) Weathered craters C) Fossilized coral reefs D) Features that look like dried-up river beds

Which of the following is not evidence that liquid water existed on the surface of Mars in the distant past? A) Hematite “blueberries” B) Weathered craters C) Fossilized coral reefs D) Features that look like dried-up river beds No evidence of any fossil life on Mars (so far).