Today in Astronomy 111: Earth and Mars
 Earth, life, water and ice
• Global temperature
measurements for the
last few hundred
thousand years
 Mars and its basic features
• Mars is similar to Earth
• Mars is different from
Earth
 Mars, life, water and ice
Unretouched picture of the “face on Mars,” from Mars Global Surveyor
(JPL/NASA). The modern version of Martian canals…
27 September 2011
Astronomy 111, Fall 2011
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Earth, water and ice
Earth, of course, has a lot of surface water.
 It is thought that most of what’s here was brought here by
meteorites, after Earth had cooled off enough that parts of
it were cooler than the 1 atm boiling point, T = 373 K.
 We estimate that 90% of the mass of such meteorites came
from inner solar-system planetesimals (like the main-belt
asteroids), and 10% came from comets.
• …because deuterium abundance in the oceans is a
better match for asteroids than for cometary water.
 97% of Earth’s water is in the oceans, and about 2% in the
ice caps (mostly Antarctica); most of the rest is in lakes,
rivers, and the ground.
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Ice ages
For the last two million years or so, in the era geologists call
the Pleistocene, the fraction of Earth’s water in the form of ice
has fluctuated wildly, by factors of 2-3 – that is, from less
than 1% to about 6 % of the total – in synch with annual
average ocean temperature fluctuations of ±3 C.
We know this because
 we can see widespread evidence of glacial action
requiring enormously-extended ice caps in both
hemispheres. The place you now live, for instance (the
Great Lakes, the Finger Lakes, drumlins, the moraine-like
appearance of the hills south of the Finger Lakes….).
 we can measure the ocean temperatures through the ages
from the heavy oxygen and hydrogen relative abundances
( 18 O 16 O , 2 H 1 H ) in core samples of seabed and icecaps.
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Why this works: heavy water in icecaps and plants,
and the temperature of the oceans
There are significant vapor-pressure differences between the
isotopologues of water −H 2 16 O, HDO, H 2 18 O, H 2 17 O, etc. −
due to differences in molecular mass.
 Easiest to measure: D/H from HDO H 2 O . (D = 2 H.)
 HDO is heavier than normal water, and thus compared to
normal water has a lower vapor pressure at any T, that
depends more sensitively on T. (All verified in lab.)
 Less D/H in icecaps or plants = more D/H in ocean water
= lower temperature of ocean. So more-negative values of
D H − ( D H )SMOW
SMOW = standard
δD =
mean ocean water
( D H )SMOW
means lower global temperature.
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Why this works: heavy water and oceanic T
through the ages.
Combined with timing from annual growth, a.k.a. dendrochronology, measurements of T via δD can give oceanic T(t) .
 Most familiar example: tree rings. δD
AST 111,
and thus T can be measured in indivi22 September 2011
dual tree rings, and the year that a
tree ring formed can often be
determined exactly. This is how T(t)
is best measured for the last few
thousand years, as at right.
 In ice cores, the annual layers of snow and ice are
distinguished by different purity of ice deposited in
summer and winter, as below. (GISP2/NICL/USGS)
NH mean ocean temperature - 16 C
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
200
600
1000
1400
1800
Year
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ℵD(parts
(parts per
per million)
δD
million)
-450
0
Depth (meters)
500
1000
1500
2000
2500
3000
3500
Ice ages
-400
Temperature difference from present (C)
-500
Ice ages (continued)
4
2
0
-2
-4
-6
-8
-10
0
100000
200000
300000
400000
Years before present
Temperatures for the past 420,000 years, from deuterium abundance δ D
(ppm = 10-6, relative to SMOW) in a 3.4 km (!) core sample taken at
Vostok, Antarctica (Petit et al. 1999, Jouzel et al. 1987, 1993, 1996).
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Ice ages (continued)
Many scientists have searched
for evidence of periodic,
oscillating behaviour in the
temperature record.
 And they generally find a
weak tendency for the
temperature to oscillate at
periods of 23000 years,
40000 years, and 100000
years.
 And that’s why we
mention ice ages in an
astronomy class…
27 September 2011
Figure by Ken Carslaw, Leeds U.
Astronomy 111, Fall 2011
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Ice ages (continued)
As was first pointed out in connection
with climate by Milankovitch (1920s),
there are periodic changes in Earth’s
orientation and orbit that influence
how much sunlight is received at
various parts of the Earth:
 Axis precession, which has a
synodic period of 22000 years.
 Nutation (oscillation of the tilt
of the axis between 21.5º and 24.5º),
which has a period of 41000 years.
Precession and nutation animation
 Oscillation of the eccentricity of
by Michael Gallis, Penn State U.
Earth’s orbit (due to forcing by
the Sun and the outer giant planets), which has periods of 100000 and
400000 years.
That these numbers match the peaks in the previous period, has
convinced many that these orbital fluctuations are the origin of ice ages.
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Ice ages (continued)
Here’s why the Milankovitch cycles do not explain ice ages.
 The variation in solar illumination from these cycles is
tiny compared to the temperature variation from other
sources (like the solar cycle), and to other climatechanging effects (like ocean-current configuration).
 The 23000-year precession oscillation should have
opposite effects in the northern and southern hemisphere,
contrary to the global nature of ice ages.
 The 100000-year eccentricity oscillation should be the
smallest and the 400000-year oscillation much larger.
Instead the 100000-year oscillation is largest, and no
400000-year oscillation is seen.
It’s an odd coincidence of numbers, but such coincidences are
not that rare. Read on...
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Mass
6.4185 × 10 26 gm (0.107M⊕ )
Equatorial radius
3.397 × 108 cm (0.533R⊕ )
Average density
3.933 gm cm -3
Moment of inertia
0.366MR 2
Albedo
Effective temperature
0.2
210.1 K
Orbital semimajor axis
Orbital eccentricity
Obliquity
Sidereal
revolution period
Sidereal
rotation period
Length of day
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Mars’s
vital statistics
Mars in 2003, by Jim Bell
(Cornell) with the HST
2.2792 × 1013 cm
(1.524 AU)
0.0935
25.19°
686.980 days
24.6229 hours
24.6597 hours
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Mars’s atmosphere’s vital statistics
Surface pressure: 0.006 earth
atmospheres
Average temperature: 210 K
(-60 C)
Diurnal (day-night)
temperature range: 184-242 K
Surface wind speeds: 2 - 30
m/s
Atmospheric composition
(near surface, by volume):
95.32% CO2, 2.7% N2, 1.6%Ar,
0.13% O2, 0.08% CO, 0.02%
H2O
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Mars’s moons
Viking Project/JPL/NASA
Phobos
G. Neukum et al.,
Mars Express/DLR/ESA
Deimos
Johannes Schedler (Panther Observatory)
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Interesting facts about Phobos and Deimos
 Phobos (“fear”) and Deimos (“terror”) are henchmen of
the god of war, Ares (=Mars) in ancient Greek mythology.
 They are very difficult to observe without photography.
• Discovered in 1877 by American astronomer Asaph
Hall, during a close approach of Mars to the Earth.
• More on those close approaches: see Homework #4.
 But they were first written about a century and a half
earlier: by Jonathan Swift, in Gulliver’s Travels (1726).
• Swift probably knew about Kepler’s numerological
“prediction” that, since Earth had one moon and
Jupiter had four (known to Kepler), Mars must have
two, Saturn eight, etc.
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Phobos and Deimos (continued)
• “Discovery” of the moons was a feat by the astronomers
of the advanced society of Laputa. Swift reported their
orbital periods as 10 and 21.5 hours -- they’re really 7.7
and 30.3 hours. (Amazingly close.)
 They were next discussed by Voltaire – who had certainly
read Swift and knew Kepler’s work (see quote below) – in
his short story Micromegas (1752).
• “…I am well aware that Father Castel will write, and
pleasantly enough too, against the existence of these two
moons, but I believe those who reason from analogy.”
• Micromegas marks the beginning of science fiction as a
literary genre.
An even odder coincidence of numbers, but clearly an accident.
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The similarities between Earth and Mars
For the past century, ever since it was first appreciated that
Mars has an atmosphere, this planet has been the focus of the
search for life outside Earth. Mars has:
 an atmosphere and reasonable surface gravity.
 a day length and an obliquity (seasons) almost the same
as Earth.
 terrestrial composition, even terrestrial appearance.
 not much in the way of surface impact cratering.
 strong evidence of past volcanism and some faulting and
other geological activity (though no plate tectonics).
 surface color variegation that, for a time, was thought
possibly to reveal vegetated areas, fancifully connected by
“canali” in the view of early observers.
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The similarities between Earth and Mars (cont’d)
One is of southern Morocco, the other of Mars. Which is
which? (Morocco by Filipe Alves, Mars by the Spirit rover,
MER/JPL/NASA.)
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Martian volcanism
On the same scale: the largest volcanoes on Earth and Mars.
The Big Island of Hawai’i,
with Mauna Loa, Mauna Kea,
Olympus Mons: 24 km high,
Kilauea: 10.6 km high from
base (4 km from sea level), 350 550 km across (Viking 2
Orbiter/NASA)
km across (140 km on coast)
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Mars, then and
now
HST images and
maps drawn by
Eugene
Antonaldi,
rendered and
scaled by Tom
Ruen.
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The Martian dichotomy
The northern hemisphere of Mars
looks different – far fewer
mountains and craters – than the
southern.
 Lower elevation, too, spawning
many theories that an ocean
used to occupy most of the
north.
 Wilhelms & Squyres proposed
in 1984 that this was due to the
Martian topography with (top) and
impact of a Earth’s-Moon-size
without (bottom) its volcanoes, by
object, about 4 billion years ago. Jeff Andrews-Hanna et al. 2008.
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The Martian dichotomy (continued)
This idea seems to be validated by new
(2008) simulations and results on
Martian topography and magnetism:
 Elliptical shape covering 40% of
surface consistent with oblique
impact (Andrews-Hanna et al. 2008).
 Can do with 0.1-0.3 Lunar-mass
object (Marinova et al. 2008), of
which there were enough, 4 Gyr ago.
 Could explain southern
concentration of Mars’ small
magnetic field (Stanley 2008).
27 September 2011
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Jeff Andrews-Hanna/
NASA
20
The differences between Earth and Mars
The differences outweigh the similarities; Mars is in almost
every sense intermediate between Venus/Earth and
Mercury/Moon.
 It’s low in mass and relatively undifferentiated (low
density, high moment of inertia).
 The atmosphere is thin and dominated by heavy
molecules, probably because of its small mass.
 It’s cold; too cold for liquid water on the surface.
So it has not been terribly surprising that the Viking landers
(and Pathfinder and MER rovers) found no evidence of life,
nor that the claims of fossil microorganisms in Martian
meteorites have not held up.
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Water on Mars
Still, it is clear that Mars has a little bit of water on the surface in the form
of ice near the poles, and evidence is emerging that in the distant past (a
few Gyr ago) it may have had liquid water oceans and a denser
atmosphere:
 Mars is remarkably free of impact craters, especially small ones and
especially in the low plains, but even in the southern highlands. Thus
the surface used to have more protection from impacts than it does
now (Helfer 1990).
 Erosion features can be seen scattered over the planet (albeit rarely)
that resemble terrestrial gullies and canyons in exact detail.
 Hematite – a mineral that forms in water on Earth – is very abundant
on the Martian surface.
So Mars may once have had the appropriate conditions for life, and may
still, beneath the surface.
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Water on Mars (continued)
Water ice on the
floor of a deep,
high-latitude,
Martian crater
(G. Neukum,
Mars
Express/ESA).
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Water on Mars (continued)
Canyons in
Gorgonum Chaos,
from Mars Global
Surveyor (Malin
Space Science
Systems/
JPL/NASA)
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Water on Mars
(continued)
Sedimentation (?) in
Schiaparelli Crater, from
Mars Global Surveyor (Malin
Space Science Systems/
JPL/NASA)
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Water on Mars (continued)
Canyon walls in
Gorgonum Chaos,
from Mars Global
Surveyor (Malin
Space Science
Systems/
JPL/NASA)
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Water on Mars (continued)
Walls in a highlatitude pit, from
Mars Global
Surveyor (Malin
Space Science
Systems/
JPL/NASA)
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Water on Mars
(continued)
Nirgal Vallis, southfacing wall, from Mars
Global Surveyor (Malin
Space Science Systems/
JPL/NASA)
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Water on Mars
(continued)
Best evidence for
surface water: wadis on
Mars? Here are
recurring slope lineae –
dark-looking channels –
which darken in seasons
expected to be wet, and
fade in dry seasons.
(McEwen et al. 2011)
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Water on Mars (continued)
Spherules: blueberry-size pebbles on the Martian surface,
which geologists think may have been shaped by liquid
water (Mars Exploration Rovers/JPL/NASA).
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And, finally, water under the Martian surface
Long suspected, this was finally confirmed in 2008 by the
NASA Phoenix lander, which dug several cm into the dirt,
found ice, melted it, and chemically analyzed the results.
NASA/JPL/
U. Arizona
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