U NIVERSITY OF WASHINGTON , A STRONOMY D EPARTMENT
Astr150 Spring 2015 : section notes
Chris Suberlak
April 8, 2015
S ECTION 3 : C RATER C OUNTING
From lunar mapping exercise we see that a surface of a celestial body can convey a lot of useful information about its past, unless modified by surface processes. Erosion and weathering
removed much of the cratering record from the surface of the Earth. Moon however does not
have an atmosphere, which meant that most of the impacts are left intact, and thus we can
decompose its history by looking at crater count. If we assume that meteors hit bodies in the
solar system uniformly in time , i.e. at roughly the same rate now as 100 mln years ago or
1.5 bln years ago, the more craters there are, the older the body is. This makes sense thinking for instance about the analogy of a surface constantly undergoing bombing - if bombs
are dropped at a constant rate, a surface with more bomb-craters would be older. This is the
premise of this exercise, that employs crater counting as a measure of age. Even if there is
some atmosphere, as in the case of Mars, there is no preferred location to be affected more
by the presence of the atmosphere on the entire planet, so that wherever the meteor falls on
Mars, if it is too small it will burn in the atmosphere, just like in the case of Earth. Dust devils, and other phenomena of the interaction of atmosphere with the surface on Mars have a
negligible weathering effect on larger craters, certainly insignificant for craters with diameter
bigger than few km.
Since craters come in a variety of sizes, it is reasonable to count their number from some
threshold crater diameter to be consistent with our estimate. Cumulative Crater Density is
the number of craters N larger than a certain diameter D km, denoted as N (D). The most
often employed threshold is D = 10 km , so that by N (10) we mean the number of craters with
a diameter bigger than 10 km.
On a young surface, such as that which has undergone some volcanic activity, the crater count
age is reset - from the moment a surface was covered by the lava, it is plain, and we call this
1
1118
T. Morota et al.
Fig. 3. Cumulative size-frequency distribution of craters counted on
Fig. 4. Lunar cratering chronolo
Figure 0.1: Cumulative
craterejecta
density
plot. Crater
density
used
to determine
the age
of
the continuous
of Giordano
Bruno.
Errorplot
barsisare
calculated
by
1983;
Neukum
and Ivanov 199
(N ±surface.
N1/2)/A, where
N is
the cumulative
number ofage
craters
andlunar
A is the
between absolute ages
a given
From T.
Morota
et al. ’Formation
of the
craterrelations
Giordano
counted
area. Theand
isochrons
are Science
calculated
Neukum’s
lunar
andAlso
Cone
Bruno
’ Meteoritics
Planetary
44,from
Nr 8,
1115-1120
(2009).
seecraters and small-crat
standard size-frequency distribution and cratering chronology curve
ejecta
blankets
(Neukum 1983; Ne
http://www.psrd.hawaii.edu/Feb10/GiordanoBrunoCrater.html
(Neukum 1983; Neukum and Ivanov 1994). The grey zone indicates
the terrestrial Phanerozoic craterin
a saturation equilibrium level (e.g., Gault 1970), corresponding to 3
recalculated to lunar impact condi
to 7% of the geometric saturation. Crater frequencies on continuous
of Giordano Bruno in this diagram
ejecta of other craters classified into the youngest group (Grier et al.
its ejecta blanket and assuming th
2001) are also shown for comparison.
and cratering frequencies by Neu
well as the young age hypothesi
gives the error (1σ) of about 0.8 Ma due to statistic fluctuation craters >1 km in diameter is ext
for an area of 294 km2 and an age of 4 Ma. Therefore, it is (Fig. 3).
extremely unlikely for a unit with an age of 1 ka to exhibit a
crater density as old as 4 Ma.
The existence of small craters on Giordano Bruno could
not be the result of a recent increase in crater production rate
for three reasons. First, the rate required to yield an age of
800 yr becomes too large. For craters larger than 1 km in
diameter, the rate would be 4.0 × 10−9 km−2 yr−1 (= 3.2 × 10−6
km−2/800 yr), which is about 4,800 times higher than the
constant rate (8.4 × 10−13 km−2 yr−1) (Fig. 4). Second, there is
no report of such an extremely high production rate on Earth.
Third, previous studies of terrestrial and lunar cratering
records have argued for a constant rate (within a factor of 2)
(e.g., Neukum et al. 2001; Hartmann et al. 2007). For
example, the relations between absolute ages of young lunar
(Fig. 3). Such a deflection on
possible resurfacing event (e
Hiesinger et al. 2002). Howev
fit the CSFD within the unce
consider 2the deflection to
resurfacing.
We also performed the cra
of three young craters, Byrg
which are summarized in Ta
craters were estimated to be 4
Moore F, and 80 Ma for Nec
Giordano Bruno and the th
Copernican system, which
Fortunately, most of
the debris that
crosses the Earth’s orbit is far smaller
than the Tunguska
body, so such major
impacts on the
Earth are very rare
– truly catastrophic
impacts only occur
every hundred
million years or so
on average.
Figure 0.2: The frequency of impacts depends on the size of the impactor. Larger meteors hit
more rarely than the smaller ones.
© 2010 Pearson Education, Inc.
surface a young surface. As time goes by it will be hit by more and more meteorites, creating
more and more craters, so that the crater counting will give us the age from the moment of
the most recent volcanic activity. Thus even though all locations on the surface of the planet
(such as Mars) are being hit at constant rate (eg. one 10 km crater per 1000 years), some of
them were reset by geologic or tectonic activity. Even though Mars is not geologically active
now, it used to have some activity, which is witnessed by the presence of some of the largest
extinct volcanoes in the Solar System , with Olympus Mons, the tallest one (27 km above the
surrounding terrain). See http://mars.jpl.nasa.gov/gallery/atlas/olympus-mons.
html and http://hyperphysics.phy-astr.gsu.edu/hbase/solar/marsoly.html for more
stunning images of this grand volcano. Mt Rainier seems really puny compared to this 88000
ft monster (yes, 4 times the height of Mt Rainier! ).
The way the surface looks changes not only with regards to the number of craters bigger than
10 km, but also with regards to their distribution. The older the surface, the more big craters
there is, because big impacts happen much more rarely than small impacts. On Fig. 0.1 we
can see that different regions of the Moon have different crater populations, and the older the
region the more craters it has. The type of craters changes with time - the older the surface the
larger of its portion is covered by the biggest craters, because there was enough time for the
rarely happening big impacts to occur (see Fig. 0.3). Those different crater populations are
illustrated by an R-plot - a plot of the fraction of the total area of a considered region covered
by craters of given diameter. Some guideline ages for the Moon:
3
The cratering population of the Moon
has changed over time!
Old - ancient
Young - recent
Figure 0.3: The R-plot shows different populations of craters on surfaces of different age
• Late Heavy Bombardment craters 4.6 Gy,
• Light-toned Terrae (highlands) , plagioclase feldspar 4.5 Gy,
• Dark-toned Mare, volcanic basalts 3.1-3.8 Gy
(Maria have 200 times fewer craters than highlands)
R=
Area covered by crates of diameter D
Total Area
4