ASTR/GEOL 3300-002
Midterm #2 – In class
Name:________________________
Directions: You have the entire 50 minutes today to finish the in class portion of the exam. When you
are done you can collect the 1 hour take home portion of the exam to be done at your leisure before
class on Wednesday, The total point total is 200, 110 for the in class portion and 90 for the take home
portion. You can see the point value for each question labeled. The short and long answer will be
graded with partial credit for work and/or partially correct answers.
This test is open book and open note. Computers, etc are allowed. BUT NO
COMMUNICATION BETWEEN STUDENTS OR WITH ANYONE OUTSIDE THE ROOM IS
STRICTLY FORBIDDEN!
By signing and turning in this test you are bound by the University of Colorado Honor Code.
“On my honor, as a University of Colorado at Boulder student, I have neither given nor received
unauthorized assistance”
Signature: ____________________________________________________________
Multiple Choice (4 pts each):
1) There is one prerequisite for life which has dominated the discussion about the existence or not of
life on Mars. What is this prerequisite?
a. CO2 atmosphere
b. sunlight
c. liquid water
d. volcanic activity
e. temperature
2) Which of the following surface conditions makes the possibility of life on Venus so remote?
a.
b.
c.
d.
e.
runaway greenhouse effect from dense ammonia (NH3) atmosphere
extreme cold from the high albedo from dense sulfuric acid clouds
thin atmosphere of hydrocarbons and nitrogen with no free oxygen
thick atmosphere of CO2 and proximity to the Sun
enhanced volcanic activity created by impacts from asteroids
3) Which best characterizes the atmospheric make-up in the Miller-Urey experiment…
a. highly reducing – H, CO2, H2O, NH3, CH4
b. highly oxidizing – H, H2O2, N2O
c. intermediate oxidation - H, CO2, H2O
d. rich in salts – NaCl, SO4
4) Apart from various forms of oxygen in the extremely thin atmosphere of Europa, what else would
you expect to be produced when water molecules are broken down by radiation at the surface?
a.
b.
c.
d.
e.
sodium
nitrogen
bismuth
chlorine
hydrogen
5) What keeps the interior of Europa warm enough to stabilize liquid (possibly salty) water?
a.
b.
c.
d.
e.
tidal interaction between Jupiter and the other Galillean satellites
induction fields from Jupiter’s magnetic dynamo
high radioactivity from the core
greater solar insolation because of its dense atmosphere
the lower freezing point of salty water in the outer solar system
6) What effect will a slowly increasing luminosity over geologic time have on a star’s circumstellar
habitable zone?
a.
b.
c.
d.
e.
No effect, because as the star ages planetary orbits increase
The habitable zone will remain the same as planets change their atmospheric compositions
high radioactivity from planetary cores will keep the surfaces warm
the habitable zone will migrate away from the star
planetary albedos will decrease to compensate
7) Of the 4 largest moons of Jupiter, which is not locked in resonance
a. Io
b. Europa
c. Gaynamede
d. Callisto
e. Titan
Multiple Choice + Explanation (9 pt each)
8) Study the figure at right. It shows that any given amount of
sunlight is spread over a larger area with increasing distance from
the Sun. As shown here, the area over which the sunlight is spread
increases with the square of the distance: For instance, at 2 AU, the
sunlight is spread over an area 22 = 4 times as large as at 1 AU (the
Earth-Sun distance, or 150 million kilometers), and at 3 AU the
sunlight is spread over an area 32 = 9 times as large as at 1 AU.
Given that the planet Saturn is approximately 10 AU from the Sun,
approximately how much more or less solar radiation does its icy
moon Titan receive relative to Earth?
a. 5 × more
b. 50 × less
c. 100 × less
d. 33 × less
e. 100 × more
What might this mean in relation to the sun as a source of energy for life on Titan?
The sun is much dimmer so it is less effective as an energy source for life
9) With the knowledge that nitrogen as N2 gas (in the same way that hydrogen, as H2 gas) has been
progressively lost from the martian atmosphere over time, how would you anticipate its atmospheric
nitrogen isotope composition has evolved over time with respect to the Earth, given that nitrogen has
two isotopes, namely the lighter isotope of nitrogen (14N) and the heavier isotope (15N)?
a.
b.
c.
d.
e.
15
N/14N on Mars is greater than Earth’s
N/14N is same as Earth’s
15
N/14N is less than Earth’s
15
N/14N =1
15
N/14N is vanishingly small (close to 0)
15
Explain your choice
15N is heavier so it is less likely to escape than 14N, so since Nitrogen can escape from the lower mass
Mars we can expect the lighter isotope to be preferentially lost
10) Consider the three theories for the origin of reduced organic molecules leading to the building
blocks of life on the primitive Earth. Which of these would be the most important for icy moons of the
outer solar system like Europa, Ganymede or Callisto?
Circle one and explain your answer.
a. synthesis in the atmosphere
b. delivery by comets and meteorites
c. hydrothermal vents
They do not have significant atmospheres and we do not believe that Gaymamede or Callisto have the
rock/water boundary capable to support hydrothermal vents (Europa might though). But we do know
that they are all impacted by comets and meteorites.
Short Answer (9 pt each)
11) What is the evolutionary advantage of DNA having two strands?
DNA is more stable than RNA. The two strands allow DNA to “spell check” (look for errors).
12) The smaller size of Mars is thought to be responsible for the fact that the Martian atmosphere is so
much thinner than that of the Earth. Why does the size of a planet affect its ability to hold on to an
atmosphere?
In order for a molecule to escape a planet its thermal velocity (just based on temperature) must be
greater than “escape velocity.” The escape velocity increases with planet mass
13) What are three techniques scientists use to determine the interior structure of planets and moons?
Which have been used to date on Europa? What have they found?
Seismology
Gravity measurements – used on Europa (found that that the moon is differentiated)
Magnetospheric Measurements – used on Europa (found that there is an induced magnetic field which
means there is a conducting fluid... thought to be liquid salty water
14) What are three properties of water that make it a good candidate for the solvent of life?
Large range in which water is liquid (high boiling point, low freezing point)
It is a polar molecule and it can make hydrogen bonds
Solid water floats on top of liquid water
15) The view of most scientists after the Viking biology experiments were performed on the surface of
Mars was that they failed to detect any positive indications of life. What argument could be used to
defend the possibility that life still might exist on Mars?
We think that there may be water underground so perhaps the life is there and the experiments didn't
get down far enough. There is known to be water at the poles and the viking landers were at lower
latitudes, so many there is live elsewhere on Mars.
16) (10 pt) Consider the moons in the outer solar system.
a) Which moon would you consider to be the most likely candidate to have life on it?
b) Defend your choice, making sure you reference using the three basic criteria of habitability we have
discussed in class.
Europa – It is known to have liquid water as a solvent for life (due to magnetospheric data & surface
observations) it is in resonance with Io and Gaynamede causing tidal heating giving a plausible energy
source. We know as an outer solar system moon pleanty of CHON rich material should have hit it and
be avaliable
Enceledus – We see liquid water geysers coming out the bottom. This means there must be (1) liquid
water to be a solvent, and (2) a source of energy behind the geysers. We know as an outer solar system
moon pleanty of CHON rich material should have hit it and be avaliable
Titan – We see liquid methane pools that might be a solvent for life. This also means there are lots of
CHON rich material available. While the main enegry source is the relatively dim sun, there is an
atmosphere to help hold in heat and it is still possible that slow life living in the cold methane seas
could survive.
Extra credit: Why have there been so many more missions (and mission attempts) to Mars than to any
other planet in the solar system? There are both scientific reasons and engineering reasons, address
both for max points. (Max 5 pt)
Mars close planet to earth.
The amount of energy to get to from earth Mars is the least of all planets.
Mars shows evidence that it may have had liquid water on the surface
Mars's atmosphere is conducive to having landers (unlike Venus)
Mars is a good candidate for possible past life or even present day life
1) (30 pt) The “RNA world” scenario is a leading theory for the origin of life on Earth
a) Why is RNA a more likely 'precursor' than DNA? Include at least 4 reasons
b) How does the RNA world connect or not with ideas about the origin of life
(i) at hydrothermal vents?
(ii) in tidal pools?
(5 pt for each justification)
DNA codes for enzymes but it needs enzymes to reproduce itself, so there is a “chicken and egg”
problem. Some RNA (ribozymes) can behave as enzymes, and some of these ribozymes can selfcatalyze their own reproduction.
All known life has similar ribosomal RNA, but DNA replication is not so universal
RNA is simpler (the backbone sugar is simpler, uracil is simple than thymine)
ATP (which all life uses) is a doubly phosphorylated piece of RNA
RNA strands has been shown to form spontaneously on an inorganic substrate (clay)
(5 pt)
While RNA would be denatured at hydrothermal vents, it is possible that organic molecules formed at
vents could fall down to the ocean floor nearby and fall on to clay substrates capable of catalyzing their
original formation. As RNA strands become more complex and possibly encased in pre-cells they
have access to a significant energy source in the temperature/chemical gradient around the vents
(5 pt)
RNA could have formed in tidal pools. There is easy access to cay and as water evaporated that easily
condenses the RNA rich fluids encouraging polymerization. The big problem would be UV light which
would likely break apart growing RNA strads
2) (30 pt) Explain how Mars has evolved over it history. You will need to comment on the following
aspects:
a.) How might an early Mars have looked - in terms of its water content and its early atmosphere?
b.) What evidence is there for past water on Mars? How much water?
c.) Mars transitioned and evolved to look like the Mars of today. What was likely the main process
behind this transition? When do we think this happened?
Mars very likely had an warmer, thicker atmosphere in the past which allowed for a significant amount
of water on the surface.
The surface of Mars shows many signatures of past water. There are branching river networks and
river “deltas” which look like a flowing liquid formed them. There are hydrated minerals and clays
which we believe are only formed in the presence of water. There is a low-lying northern region whose
boundary is possibly all at the same elevation consistent with a great northern ocean. If the water on
the polar caps melted there would be enough water for a shallow world-wide ocean (11m) and it is
likely that a significant amount of water has been lost due to Mars's small size and lack of
magnetosphere. So a northern ocean of water is certainly reasonable.
There is evidence of a significant amount of water on Mars until 2-3 Gyr ago. Early Mars would hve
outgassed an atmosphere of CO2 and H2O as well as other constituants. However, due to Mars's small
size much of this atmosphere has been lost since Mars became geologically inactive (both unable to
create a global magnetic field and unable to resupply the atmosphere)
3) (30 pt) Some scientists believe there may be another gas giant planet in the solar system called
“Nemesis” which is far out beyond the orbit of Pluto (and may be responsible for some comet
showers). Imagine that sometime in the future Nemesis is discovered and we decide to send a robotic
mission to visit the new planet. You are in charge of determining what the mission should be able to do
in order to try to find if there may be life on any possible moons of Nemesis (it is too far away to see if
it has moons from Earth). Note: A lander would be too expensive for this first mission.
a) What would you like to learn about the orbits of the moons and why?
b) How might you try to learn about the moon's internal structure and what would you be looking for?
c) What type of features would you be looking for on the moon's surface? Why?
If there are multiple moons, are they in resonance? Since the planet is so far away fro the sun there
probably would need to be another energy source, and if the moons are in resonance so they can excite
eachother's eccentricity then tidal damping might be a source of energy
Gravity data from fly-bys to learn in the moons are differentiated. Magnetic measurements to look for
either internally generate magnetic fields (indicating a liquid core and therefore internal energy) or an
induced magnetic field (assuming Nemesis has a magnetic field) indicating the possibility of liquid
water.
At this distance liquid water (or even liquid methane) on the surface are unlikely. So we probably
would be looking for a young surface (few craters) indicating that there is a liquid beneath the surface
which is resurfacing the moon. We might also look for stress cracks indicating tidal deformation
indicating a possible energy source