c24concept

Part A

Why are fossils of early life on Earth more rare than fossils of plants and animals from the past few hundred million years?

Hint A.1
Study Section 24.1.

ANSWER:

Fossils could not form before there was oxygen in the atmosphere. Life was far less abundant prior to a few hundred million years ago. Early organisms lacked skeletons and other hard structures that are most likely to be fossilized. We find fossils in sedimentary layers, and no sediments were deposited until just a few hundred million years ago.

Part B

Which of the following best describes natural selection?

Hint B.1
Study Section 24.1.

ANSWER:

It is the idea that the strong survive and the weak die off. It is the idea that organisms naturally increase in complexity and intelligence with time. It is a guess made by scientists about how life develops, but it has no hard evidence to support it. It is the idea that organisms with genetic traits that improve their ability to reproduce are more likely to pass those traits on to future generations.

Part C

Which of the following is NOT key evidence in support of the idea that all life today shares a common ancestor?

Hint C.1
Study Section 24.1.

ANSWER:

Mapping of gene sequences shows how life is all related. All life builds proteins from the same amino acids and uses ATP to store energy in cells. All life uses DNA and the same genetic code. We have identified fossils of the first life forms that ever existed on Earth.

Part D

Which of the following best describes the predominant scientific view of the origin of life on Earth?

Hint D.1
Study Section 24.1.

ANSWER:

We may never know precisely how life arose on Earth, but current knowledge suggests that life likely arose easily under the conditions that prevailed on the early Earth. Life arose through a series of extremely unlikely chemical coincidences, making it seem almost miraculous that life ever came to exist at all. Life probably migrated to Earth from some other world. We can describe with great certainty the precise steps by which life arose on Earth.

Part E

Why didn't oxygen begin to accumulate in the atmosphere for more than a billion years after life appeared on the Earth?

Hint E.1
Study Section 24.1.

ANSWER:

Oxygen released by life was removed from the atmosphere by dissolving in the ocean until the oceans could dissolve no more. Oxygen released by life was removed from the atmosphere by chemical reactions with surface rocks until the surface rock could absorb no more. Early forms of animal life consumed the oxygen released by plants during the first billion years of life on Earth. Early life did not release oxygen, and oxygen releasing organisms didn't evolve for a billion years after the earliest life.

Part F

When we analyze whether a world is a possible home to life, the key thing we look for is _________.

Hint F.1
Study Section 24.2.

ANSWER:

evidence of atmospheric oxygen the presence of organic molecules such as amino acids the past or present existence of liquid water surface coloration changes that could indicate vegetative growth

Part G

Which of the following best describes the current status of the debate over evidence for life in the Martian meteorite ALH84001?

Hint G.1
Study Section 24.2.

ANSWER:

Most scientists agree that the evidence would support life if the meteorite truly comes from Mars, but very few scientists accept that the meteorite is from Mars and instead think it may be an ordinary Earth rock. Most scientists find the evidence intriguing but suspect that it can be explained without requiring past life on Mars. All scientists now agree that the meteorite shows clear evidence of past life on Mars. All scientists now agree that the meteorite shows no evidence for past life on Mars.

Part H

Why is Europa considered a good candidate for the possible existence of life?

Hint H.1
Study Section 24.2.

ANSWER:

It is located within our Sun's habitable zone. The Galileo spacecraft found strange seasonal changes on its surface that look like they could be due to life. It has a thick atmosphere with a surface pressure greater than that on Earth. Strong evidence suggests that it has a deep, subsurface ocean of liquid water.

Part I

In general, how does the size and location of a star's habitable zone depend on the star's mass?

Hint I.1
Study Section 24.3.

ANSWER:

The smaller (less massive) the star, the larger and the closer-in the habitable zone. The smaller (less massive) the star, the smaller and the closer-in the habitable zone. The habitable zone is always about the same size, but its location moves inward for smaller stars. The smaller (less massive) the star, the larger and the farther-out the habitable zone.

Part J

We are not yet capable of detecting life on planets around other stars. But as our technology develops, our first real chance of detecting such life will probably come from _________.

Hint J.1
Study Section 24.3.

ANSWER:

sending spacecraft to study the planets up close examining spectral lines from the atmospheres of distant planets examining high-resolution images of the planets made by orbiting telescopes determining the orbital properties of the planets

Part K

Suppose that Jupiter had never existed, and there was no planet in our solar system between Mars and Saturn. How would we expect this to have affected Earth?

Hint K.1
Study Section 24.3.

ANSWER:

There would not have been any effect, since Jupiter is in the outer solar system and Earth is in the inner solar system. There would be no water on Earth. Earth's orbit would have been unstable, and our planet would have spiraled into the Sun. Earth would have been hit by many more comet impacts.

Part L

Which of the following best describes how the Drake equation is useful?

Hint L.1
Study Section 24.4.

ANSWER:

The Drake equation has allowed us to determine the number of civilizations in the Milky Way Galaxy. The Drake equation tells us what wavelengths of light will be most useful to examine in the search for extraterrestrial intelligence. The Drake equation allows us to calculate the masses of planets orbiting other stars. The Drake equation helps us understand what we need to know in order to determine the likelihood of finding other civilizations.

Part M

In the equation Number of Civilizations = N _HP ( f _life ( f _civ ( f _now, we expect the term f _civ to be small if:

Hint M.1
Study Section 24.4.

ANSWER:

Primitive life is common but intelligent life is rare. Most civilizations destroy themselves within just a few hundred years of arising. Most of the civilizations that have ever existed are still out and about in the galaxy. Most habitable planets never actually get life on them.

Part N

Suppose it turns out that one in 1 million stars has a planet that at some point in its history is home to an advanced civilization. Then the total number of civilizations that have arisen in our galaxy would be closest to _________.

Hint N.1
Consider the number of stars in our galaxy.

ANSWER:

40,000 400,000 4,000 400 40 4

Part O

Which of the following describes a major danger of interstellar travel at near-light speed?

Hint O.1
Study Section 24.4.

ANSWER:

Time dilation will slow the heart beats of the crew to a dangerously low rate. Atoms and ions in interstellar space will hit a fast-moving spacecraft like a flood of dangerous cosmic rays. Any interstellar journey will take much longer than the lives of the crew members. Asteroid fields floating in interstellar space will present a navigational challenge.

Part P

Which of the following statements about matter-antimatter engines is NOT true?

Hint P.1
Study Section 24.4.

ANSWER:

One of the major challenges to developing matter-antimatter engines is finding a way to store antimatter after it is produced. Matter-antimatter engines would be great in theory, but to date we have no evidence that antimatter even exists. Matter-antimatter reactions represent the most efficient possible reactions in terms of energy release. Spacecraft powered by matter-antimatter engines could probably reach speeds of more than half the speed of light.

Part Q

Which of the following is NOT considered a potential solution to the question of why we lack any evidence of a galactic civilization?

Hint Q.1
Study Section 24.5.

ANSWER:

There is no galactic civilization because we are the first species ever to achieve the ability to study the universe. The galactic civilization probably is undetectable because they operate under different laws of physics from the ones we know. The galactic civilization is deliberately avoiding contact with us. There is no galactic civilization because all civilizations destroy themselves before they achieve the ability to colonize the galaxy.