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AST 105: Introduction to the Solar System
HOMEWORK # 11 SOLUTIONS
1. Suppose all the time elapsed on the Earth was compressed into
a 24 hour clock, so the Earth began at midnight and the present
time is 24 hours later. The earliest evidence for oxygen in our
atmosphere would have occurred around noon on this clock. On
this clock, at what time did the dinosaurs disappear, and at what
time did homo sapiens first appear?
The dinosaurs became extince 65 million years ago. 24 hours
= 4.55 billion years, so 65 million years /4.55 billion years × 24
hours × 60 minutes = 20.57 minutes = 20 minutes and 34 seconds.
Subtracting this from 24:00:00 gives 23:39:26.
The oldest homo sapiens fossils are 195,000 years old. 24 hours
= 4.55 billion years, so 195,000 years/4.55 billion years × 24 hours
× 60 minutes × 60 seconds= 0.15 seconds. Subtracting this from
24:00:00 gives 23:59:45.
2. How do carbon isotope measurements in rocks provide evidence for
the existence of life?
The ratio of C12 /C13 is higher in living material than in ordinary matter on Earth, so rocks with elevated C12 /C13 ratios
imply the rock’s matter was once in living organisms and that life
is older than the age of the rock.
3. Name three common features shared by all forms of life: not only
life present today but all kinds of life detected in fossils.
Among these include: use of DNA, RNA and proteins; same 20
left-handed amino acids construct proteins; same genetic code for
triplet base sequences to code an amino acid; use of ATP/ADP
for energy storage; same methods for protein transcription.
4. How do we know that life evolved through time and that all species
presently in existence weren’t created at the same time?
Evidence comes from fossil record, embryology, genetic studies
5. What evidence supports the notion that the first forms of life on the
Earth formed on the ocean floor near deep ocean volcanic vents?
6.
7.
8.
9.
10.
Life in the vicinity of these vents is dominated by the extremophile bacteria known as thermophiles, members of the Archaea family that are the most far-removed genetically from eucaryotes and humans.
What experiment first showed that the building block molecules of
life could have been formed spontaneously in Earth’s early atmosphere?
The Miller-Urey experiment
What evidence, announced in 1984, suggested that life had been
found on Mars?
The discovery of small micro-fossil-like structures in a meteorite believed to come from Mars, ALH84001, in conjunction with
the finding of biominerals and other evidence for ancient life.
Compare a 2 M⊙ star with the Sun. Which star has a larger
habitable zone?
The size of the habitable zone correlates with the power output
of the star: the more power the larger the habitable zone. So the
more massive star has the larger habitable zone.
Which star has a longer lifetime?
The lifetime of a star anti-correlates with its mass, so the Sun
has the longer lifetime.
Why is it suspected that there is a preferred location within our
Galaxy for habitable planets to form (i.e., not too close to the
center and not too far out from the center, either)?
There are two factors: the supernova rate and the metallicity.
Close to the galaxy’s center, the supernova rate is too high (which
would destroy life) and the heavy element abundance is too large
(which would make many hot Jupiters which could destroy Earthlike planets). Too far from the galaxy’s center, the heavy element
abundance is too small to form many Earth-like planets.
If plate tectonics are necessary for life to develop and remain viable for billions of years, what constraints on habitable planets are
implied?
Plate tectonics requires a planet large enough to have sufficient radioactive heating to create convective motions in the upper
mantle, and large enough to retain sufficient water in the crust for
lubricating the motion of plates.
11. Why are radio frequencies preferred for interstellar communication?
Radio waves suffer less interference and background noise than
other wavelengths, so they will penetrate farther through interstellar space. Because their frequencies are relatively small, the cost
per photon is also much less than for visible or infrared light so
the energy cost of messages is smaller.