Clark College Astronomy 101
Lab 06 - Relative Ages of Surface Features on Mars
Objective: Identification of the main topographic features on Mars and determination
their relative ages.
In this lab we use the terms “older” and “newer” (or “younger”) to describe the relative
ages of individual geographical features on the surface of Mars. A feature which formed
later in the history of the planet will be referred to as “younger” or “newer” than those
features which formed earlier.
You will use the interactive maps of Mars compiled from recent images collected by the
Mars Global Surveyor and Mars Odyssey spacecraft at Google Mars (http://
www.google.com/mars/) Turn on the computer at your station and navigate to this site.
You may also find it helpful to use the images at http://www.mapaplanet.org/explorer/
mars.html
You will use two methods (described below) to determine the sequence of formation for
adjacent geographic structures on the surface of Mars. You will work through a series
of exercises, answering questions about each feature to help make these
determinations. Your score for this assignment will be based upon the accuracy of an
ordered list of features from youngest to oldest, and the annotations you make on the
sketch map, on the last page of this handout:
Method 1 - Crater Density:
Rocks of all sizes have been falling on Mars since the planet formed. The impacts of
these rocks produced craters. If the craters are not weathered away, nor covered up by
lava or dust, then the number of craters per
unit area is so large that they generally
overlap. Large portions of Mars are covered
with such overlapping craters.
Figure 1. Lightly cratered region “B” has
been modified by some event that obscured
or obliterated the the craters evident in
region A. We say that surface of region “B”
is newer than region surface of region “A”
However, if the craters are covered by
subsequent lava flows (as on some portions
of the
Moon and Mars) or eroded away by the
action of wind and water (as on Earth) or
covered with dust (as with very small craters
on the Moon and Mars), then the number of
visible craters per unit area will be reduced.
Comparing the number of craters per unit
area in different regions allows us to
determine which region was more recently modified by some crater-obscuring event.
Method 2 - Superposition:
“Superpose” means to place above, or over top of. Whatever is on top of, or partially
obliterates, another feature may be taken to be younger (more recent) than the
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Clark College Astronomy 101
Lab 06 - Relative Ages of Surface Features on Mars
Figure 2. The later impact formed a crater “superposed” upon the crater formed by an
earlier impact.
underlying features. In the images above, the the crater labeled “B” was formed after
the crater labeled “A”
Figure 3. Use the buttons in the upper right hand corner of the map to change the view
between elevation, infrared, and visible wavelength images of mars.
EXERCISES
The colors of the elevation map correspond to distances above a mean
altitude (there is no liquid water on Mars from which to establish a “sea
level”). This elevation is designated 0 km and corresponds to a yellowgreen color on the map.
On the last page of this hand-out you will find a “sketch map”
representing the entire surface of Mars between 70°N latitude and 70°S
latitude. You will use this map to indicate the location of some of the
major features we will examine:
• four major volcanoes,
• a very large canyon
• two very large craters
• even larger roughly circular smooth regions
In the following exercises, you will identify these, and some other major
features, on the surface of Mars and determine their relative ages. You will then list all
of the features you have identified from most recent to oldest.
Your grade for this lab will be based on a correctly labeled sketch map and a
correctly ordered list.
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Clark College Astronomy 101
Lab 06 - Relative Ages of Surface Features on Mars
Procedure
1. Identify these features on the sketch map located on the last page of this write-up:
a. Label each of the four volcanoes with a V,
b. Label very large canyon with a B
c. Label each of the two very large craters marked with VLC
d. Label the VLC which has a double rim with DR (compare the VISIBLE images of
the two very large craters)
e. Label each of the two VERY large roughly "circular smooth" regions with CS
(These regions are many times lager than the objects you labeled "VLC")
Roughly half of Mars consists of plains (fairly smooth regions with a few features
such as volcanoes or the large canyon superposed), and roughly half consists of
regions where the craters generally overlap. From radar surveys of the surface, and
photographic images obtained at oblique angles, we know that the heavily cratered
region is at higher average elevation than the smoother plains. Hence, we call the
cratered region the highlands. The thick dotted line meandering across the entire
width of the sketch map marks the boundary between these two regions.
2. Based upon crater density, it is apparent that the region south of the thick dotted line
is the ________surface (circle your answer).
a. NEWER
b. OLDER
We can now begin make a more precise estimate of the relative ages of the two
regions.
3. Find at least two "very large"
craters that do not lie in the
highlands. Indicate their
locations on the sketch map
by drawing small circles.
4. The image at right is of one
of the VLC on your map.
Find this crater and zoom in
to a region about as wide as
the image. Count the
number of craters that are
at least as large as the
circled craters and which lie
entirely in the smooth
plains. Record that value in
the space below.
Nplain = ___________
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Figure 4 -- A region just past the edge of the Highlands.
The circled craters will be counted as "large craters"
Clark College Astronomy 101
Lab 06 - Relative Ages of Surface Features on Mars
5. Without changing the size of the window or the zoom level, move your field of view
to a region in the southern highlands and record the number of similarly sized large
craters.
Nhighlands = ___________
6. Compute the ratio of these two numbers and record it below
Nhighlands /Nplain = ____________
7. Suppose now that we know the absolute age for the plains of Mars to be 3 billion
years. Suppose also that the rate of cratering (number of craters formed per billion
years) is, and has been, constant for the entire age of Mars. Using the ratio you
calculated above, the age of the highlands would be roughly
______billion years.
Noting that the age of the solar system, and all of the planets in it, is approximately
4.5 billion years, explain what is wrong with this result:
___________________________________________________________________
___________________________________________________________________
What do you conclude about the rate of cratering? (circle one)
a. The rate of cratering has been constant for the last 4.5 billion years.
b. The rate of cratering has increased over the last 4.5 billion years.
c. The rate of cratering has decreased over the last 4.5 billion years.
8. From the relative ages of features on the Moon and other airless bodies, it can be
deduced that most of the really large asteroids in the solar system collided with each
other (making smaller rocks) and with planets (making very large craters) during the
first billion years after the formation of the solar system. Therefore the "very large"
craters should appear mostly on the oldest surfaces.
What does the existence of asteroids today and of "very large" craters on the plains
on Mars imply about the possibility of the formation of new large craters on Mars?
a. There is no possibility of new large craters on Mars.
b. New, large craters will continue form on Mars, only infrequently.
c. New, large craters will form on Mars frequently
Explain your reasoning:_______________________________________________
__________________________________________________________________
9. The two very large, roughly circular and smooth regions indicated on the sketch
map (The Argyre and Hellas planitias)The were likely caused by the impacts of very
large bodies. However, the smooth plains at the bottoms of these basins are lava
flows, which appear to have formed quite late, after most of the bombardment and
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Clark College Astronomy 101
Lab 06 - Relative Ages of Surface Features on Mars
cratering on Mars was over. This leaves us with a dilemma. Did these giant impacts
happen early, when most impacts
occurred; excavating large basins
with raised rims (see figure), which
filled with lava only very much later?
Or, did these impacts occur later in
the history of the planet, after most
other impacts, with lava filling the
basin at the time of the impact?
Inspect the elevation map near the two large basins and identify a rim for each, or at
least a fairly circular ring-like structure around each. Look for evidence of impact craters
interrupting the rims of these basins. Using what you have learned about
superposition, choose between:
a. impact early in the history of Mars - lava flow much later
b. impact later in the history of Mars - lava flow soon after impact
Explain your reasoning:________________________________________________
___________________________________________________________________
___________________________________________________________________
Does your choice agree with the earlier conclusion that most "very large" craters
were formed rather early in the evolution of Mars?___________________________
10. Look at the area south of the great canyon, between longitudes 45°W and 100°W.
This area is not uniformly cratered (or wrinkled). A boundary seems to occur at
about longitude ________.
The older surface is to the EAST WEST of this boundary (circle your answer).
11. Zoom in on the large canyon; compare the crater density within the canyon to the
terrain just to the north and south of the rims. From this examination, you may
concluded that:
a. the canyon formed before the surrounding plains, and the lava floods that made
the plains simply did not reach the canyon.
b. the canyon might have formed as a boundary between two plains of very
different ages
c. the canyon might have formed after the plains
Explain how you arrived at your conclusion (in terms of crater density and the
superposition of features)___________________________________
___________________________________________________________________
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Clark College Astronomy 101
Lab 06 - Relative Ages of Surface Features on Mars
12. Notice on that each of the four large volcanoes has a crater at its center (that is, on
top). Considering the crater density in the areas surrounding the volcanoes, are the
craters at the tops of the volcanoes likely to be due to impacts? __________
Explain your reasoning ________________________________________________
___________________________________________________________________
___________________________________________________________________
13. The southernmost volcano is Arsia Mons, it is surrounded by an expanse of smooth
terrain called the Tharsis Plateau.
Look at the long structures on the flanks of the volcano, extending radially away from
the crater. These are almost certainly due to eruptions of the volcano. Do these
structures extend out onto the surrounding plain without interruption? In particular,
examine the gully to the south of the crater.
Using what you have learned about the superposition of features, the appearance of
these radial features suggests that:
a. Arsia Mons is older than the surrounding terrain.
b. Arsia Mons is younger than the surrounding terrain.
Explain your reasoning ________________________________________________
___________________________________________________________________
14. At about Latitude 23° South, Longitude 217° West -- to the SW of Arsia Mons -- at
the edge of the smooth terrain surrounding the Volcano, there are three craters
which just overlap (see the image on the next page). Zoom in on this feature. Look,
in the infrared image, at the northern rim of the crater labeled “C” in the figure.
Which of these craters is the YOUNGEST feature. _______
Are any of these three craters newer (YOUNGER) than the smooth terrain to the
North and East? __________
Explain your reasoning ________________________________________________
___________________________________________________________________
___________________________________________________________________
15. If the ridges running SW to NE on the right side of the volcano were made by the
event that formed canyon, this would indicate that:
a. Arsia Mons is older than the canyon.
b. Arsia Mons is younger than the canyon.
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Clark College Astronomy 101
Your grade for this lab will be
based on a correctly labeled
sketch map and a correctly
ordered list.
Lab 06 - Relative Ages of Surface Features on Mars
Name:_____________________
Partners:_____________________
_____________________
_____________________
Rank the ages of the regions you examined in this lab from 1 (youngest) to 5
(oldest).
____ highlands ____ volcano (Arsia Mons) ____ canyon (Valles Marineris)
____ south of canyon, longitudes 45°-85°
____ south of canyon, longitudes 85°-100°
Extra Credit (5 points).
At about 14°N 178°E, there is a large (about 375 km) oblong depression called
the Orcus Patera. What kind of feature is this, and where would you place it on
the list above? Explain your answers:
________________________________________________________________
________________________________________________________________
________________________________________________________________
________________________________________________________________
________________________________________________________________
FOR QUESTION 14
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Clark College Astronomy 101
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Lab 06 - Relative Ages of Surface Features on Mars
Clark College Astronomy 101
Version
Updated
Lab 06 - Relative Ages of Surface Features on Mars
Author
NOTES
2.1
April, 2010
John LaBrasca
Revised and updated the
procedure and graphics
2.0
April, 2009
John LaBrasca
Revised and updated all parts
of the procedure to use
Google Mars in place of hard
copy prints.
1.0
August, 2002
John LaBrasca
Revised and updated graphics
0.0
??
Dick Shamrell
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Clark College Astronomy 101
Lab 06 - Relative Ages of Surface Features on Mars
Instructor's Notes
This lab was originally taught with three 8.5" x 11" glossy prints of images collected by the
Mariner 9 spacecraft, circa 1971.
These prints were the property of Clark College so, when I taught the lab at WSU in the fall of
2006, I tried to recreate them using more up-to-date images collected by later missions to Mars.
In the course of this effort it became apparent that several of the features identified in the
version of the lab I received in 2003 where, in fact, artifacts of the process by which those
images were compiled. I've eliminated several of these but two still persist, and need to be
revised: The radial features described in question 13, for example, do not disappear in recent,
high resolution, images in the way that they did in the Mariner photographs. Likewise, the
boundary at 85°W longitude, just south of Valle Marinaris is much more subtle in the more
modern images.
Both of the craters marked on the sketch map appear to be "double ringed" in the elevation
map. I considered eliminating this question, but decided that it could be used to illustrate the
way that the elevation map differs from the visible map -- and why it is important multiple views
when answering some of the questions.
The prints are also fragile, and too easily misplaced. With Astronomy 101 classes running
simultaneously on three campuses, a more portable and universally accessible data source
seemed to be required.
My first attempt to move the lab to web-based images -- for Ast135 -- used the USGS database.
This proved too slow to access to be practical for a lab.
The version of the lab here uses the Google Mars maps almost exclusively.
The sketch map at the end of the lab has been updated and rescaled to more closely match the
appearance of the Google Mars map. I have also superimposed a grid of lat. and long, since
these references are absent from the Google Mars Maps. (I've included two different schemes
for longitude, since this coordinate is reported differently by different sources).
When teaching this lab, I try to make sure that the students understand that the high crater
density within the southern highlands makes large scale application of either crater density or
superposition dating techniques difficult in that region -- which is why we date the highlands as a
single feature.
While workable in its current form, this lab badly needs reformatting. The procedure needs to
be updated; many of the questions need to be be rewritten or eliminated, and all of the
responses should be moved to the worksheet at the end of the lab.
Most importantly, it needs a new list of features to be ranked according to age for the "final
check" portion of the lab.
Note: There is no "correct answer" for the bonus question regarding Orcus Patera. I mark this
on the clarity of the argument presented by the student.
John LaBrasca
Portland, OR August 2010
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