Oct. 30th - Impacts

Question 1

Shaping Planetary Surfaces

What processes shape planetary surfaces?

Answer 1

• Impact cratering: the creation of bowl-shaped impact craters by asteroids or comets striking a planet’s surface
• An impact crater forms when one of the leftover planetesimals from the solar system’s formation—meaning an asteroid or comet—slams into a solid surface
• At such tremendous speed, the impact releases enough energy to* vaporize solid rock and blast out a crater*
• Debris from the blast shoots high above the surface and then rains down over a large area. If the impact is large enough, some of the ejected material can escape into space.

Question 2

Crater Nature & Size

Answer 2

Small craters far outnumber large ones, demonstrating that many more small planetesimals formed during the birth of the solar system (and more small leftovers still orbit the Sun today) than large ones.
* Craters are usually circular, because an impact blasts out material in all directions, regardless of the incoming object’s direction.
* Experiments suggest that craters are typically about 10 times as wide as the objects that create them and about 10–20% as deep as they are wide.
* A large crater may have a central peak, which forms when the center rebounds after impact in much the same way that water rebounds after you drop a pebble into it

Question 3

How do details of crater shapes provide clues about geological conditions?

Answer 3

• EX: surrounded by mud flows, suggesting that underground water (or ice) melted or vaporized on impact; the muddy debris then flowed across the surface and hardened into the pattern we see today
• EX: erosion - no defined ridges or bowl shape; suggests that ancient rainfall eroded the crater and that the crater bottom was once a lake.

Question 4

How do impact craters reveal a surface’s geological age?

Answer 4

Impact cratering is from external causes: the random impacts of objects from space
* This fact leads to one of the most useful insights in planetary geology: the geological age of any surface region is from its number of impact craters, with more craters indicating an older surface.

By “geological age” we mean the age of the surface as it now appears:
* A geologically young surface is dominated by features that have formed relatively recently in the history of the solar system
* While a geologically old surface still looks about the same today as it did billions of years ago.

WHY?
* Recall that all planets were hit with impacts during the collapse of the solar nebula, FEW HAVE OCCURRED SINCE THEN
* In places where we see numerous craters, such as on much of the Moon’s surface, we must be looking at a surface that has stayed virtually unchanged for billions of years.

Question 5

The degree of crowding among craters varies greatly from place to place on the Moon.

Answer 5

In the lunar highlands (ancient and heavily cratered), craters are so crowded that we see craters on top of other craters:
* Those from the lunar highlands are about 4.4 billion years old, telling us that the heavy cratering occurred early in the solar system’s history.

In the lunar maria (huge impact basins that were flooded by lava), we see only a few craters on top of generally smooth volcanic plains:
* Rocks from the maria date to 3.0–3.9 billion years ago, telling us that the lava flows that made these volcanic plains had occurred by that time
* Formed after heavy bombardment ended

Question 6

Volcanism

Answer 6

the eruption of molten rock, or lava, from a planet’s interior onto its surface

Question 7

Tectonics

Answer 7

the disruption of a planet’s surface by internal stresses

Question 8

Erosion

Answer 8

the wearing down or building up of geological features by wind, water, ice, and other phenomena of planetary weather

Question 9

Planetary Properties Controlling Volcanism and Tectonics

Volcanism and tectonics both require internal heat, which means…

Answer 9

…they depend on planetary size
* Hence, larger planets have more internal heat and hence more volcanic and tectonic activity.

Question 10

Planets & Active Volcanism: Terrestrial

Answer 10

• The Moon and Mercury lack ongoing volcanism and tectonics because their small sizes allowed their interiors to cool long ago.
• Earth has active volcanism and tectonics because it is large enough to still have a hot interior.
• Venus, nearly the same size as Earth, must still be hot inside and probably also has active volcanism and tectonics.
• Mars, with its smaller size, has much less volcanism and tectonic activity today than it did in the distant past.

Question 11

Planetary Properties Controlling Erosion

Answer 11

• Erosion arises from weather phenomena such as wind and rain.
• Planetary size is important because erosion requires an atmosphere, and a terrestrial world can have an atmosphere only if it is large enough to have had significant volcanic outgassing and if its gravity is strong enough to have prevented the gas from escaping to space.
• Distance from the Sun is important because of its role in temperature: If all else is equal (such as planetary size), the higher temperatures on a world closer to the Sun will make it easier for atmospheric gases to escape into space, while the colder temperatures on a world farther from the Sun may cause atmospheric gases to freeze out.
• Distance is also important because water erosion is much more effective with liquid water than with water vapor or ice and therefore is strongest when moderate temperature allows water vapor to condense into liquid form.
• Rotation rate is important because it is the primary driver of winds and other weather: Faster rotation means stronger winds and storms.

Question 12

Planets & Erosion: Terrestrial

Answer 12

• The Moon and Mercury lack significant atmospheres and erosion because they lack outgassing today, and any atmospheric gases they had in the distant past have been lost to space.
• Mars has only a thin atmosphere because much of the water vapor and carbon dioxide outgassed in its past either escaped into space or lies frozen in its polar caps or beneath the surface.
• Venus and Earth probably had similar amounts of outgassing, but cooler temperatures on Earth led to condensation and the formation of oceans, allowing wind and weather to drive strong erosion.
• Most of Venus’s gas remained in its atmosphere, making its atmosphere much thicker than Earth’s, but Venus has little erosion because its slow rotation rate means that it has very little surface wind and its high temperature means that rain never falls to the surface.

Question 13

Planetary Properties Controlling Impact Cratering:

Answer 13

• Impacts are random events and therefore the creation of craters is not “controlled” by fundamental planetary properties
• However, because impact craters can be destroyed over time, the number of remaining impact craters on a world’s surface is controlled by fundamental properties
• The primary factor is size: Larger worlds have more volcanism and tectonics (and in some cases erosion), processes that tend to cover up or destroy ancient impact craters over time.

Question 14

Why did so many small bodies ended up in orbits that could send them crashing into the planets?

Answer 14

tell us that such collisions were far more common in the past

Question 15

Did an impact kill the dinosaurs?

Answer 15

K-Pg boundary layer points to impact:
* Rich in iridium - not on earth, but from meteorites
* Unusually high abundances of several other metals (including osmium, gold, and platinum) that are more consistent with what we commonly find in meteorites rather than what we find elsewhere on Earth’s surface.
* Contains grains of shocked quartz, which could have formed with impact
* quartz crystals with a distinctive structure indicating that they experienced the high-temperature and high-pressure conditions of an impact.
* Spherical rock “droplets” (tektites), forms when molten rock cools and solidifies
* Soot (the black, dusty residue of burnt organic material) - suggests that widespread fires were ignited as molten debris fell from the sky

Question 16

The fossil record suggests that…

Answer 16

…up to 99% of all living organisms died around that time and that up to 75% of all existing species were driven to extinction.

• This vast loss of life makes the event a clear example of a mass extinction—the rapid extinction of a large fraction of all living species. Could it really have been caused by an impact?

Question 17

Additional Evidence for an Impact

Answer 17

Scientists identified a large, buried impact crater with a radiometric dating age that matches that of the 66-million-year age of the iridium-rich layer.

The crater, about 200 kilometers across, is located on the coast of Mexico’s Yucatán Peninsula, about half on land and half underwater.

Question 18

But how would the impact have caused the extinction event? Based on a combination of geological evidence and modeling, the basic scenario is likely as follows:

Answer 18

1. Asteroid slammed into Mexico with the impact of a hundred million hydrogen bombs
2. Hit at an angle, sending hot debris across North America
3. Triggered a huge tsunami
   * Wiped out many
4. Hot debris set fire and killed many others
5. Dust & smoke remained in atmosphere for weeks or months, blocking sunlight and causing temperatures to fall (global winter)
   1) Reduced sunlight stopped photosynthesis for up to a year, killing large numbers of species throughout the food chain.
   2) Acid rain may have been another by-product, killing vegetation and acidifying lakes around the world.
   3) Chemical reactions in the atmosphere probably produced nitrous oxides and other compounds that dissolved in the oceans and killed marine organisms
   4) Some evidence suggests that the impact site was rich in carbonate rocks, so the impact may have released large amounts of carbon dioxide into the atmosphere
   Strengthened greenhouse effects - making long, harsh summers follow right after winters

Question 19

Controversies & other mass extinctions

Some scientists suspect that the dinosaurs were already in decline and the impact was only the last straw.

There appear to have been at least four other mass extinctions during the past 500 million years:

Answer 19

• One mass extinction was far worse than the dinosaur event: About 252 million years ago, the “great dying” (also called the “end-Permian” event) killed off close to 90% (and perhaps even more) of all species living at the time.
• However, neither this nor any of the other mass extinctions have been as closely tied to an impact as the dinosaur extinction, and none show a similar iridium layer

Question 20

How do the jovian planets affect impact rates and life on Earth?

Answer 20

We now know that planets can have a real effect on life on Earth. By shaping the orbits of asteroids and comets, the planets have contributed to cosmic collisions that helped shape our destiny.

Esp. Jupiter
□ disturbed the orbits of rocky planetesimals outside Mars’s orbit, preventing a planet from forming and creating the asteroid belt.
□ The jovian planets also ejected icy planetesimals to create the distant Oort cloud of comets, and through migration and orbital resonances they shaped the orbits of the comets in the Kuiper belt.
□ Ultimately, every asteroid or comet that has impacted Earth since the end of the heavy bombardment was in some sense sent our way by the influence of Jupiter or one of the other jovian planets.

Question 21

We therefore find a deep connection between the jovian planets and the survival of life on Earth.

Answer 21

If Jupiter did not exist, the threat from asteroids might be much smaller, since the objects that make up the asteroid belt might instead have become part of a planet.

On the other hand, the threat from comets might be much greater: Jupiter probably ejected more comets to the Oort cloud than any other jovian planet, and without Jupiter, those comets might have remained dangerously close to Earth.