Planetary and Space Science 59 (2011) 889–899
Contents lists available at ScienceDirect
Planetary and Space Science
journal homepage: www.elsevier.com/locate/pss
Comparative planetology, climatology and biology of Venus, Earth and Mars$
F.W. Taylor n
Atmospheric Oceanic and Planetary Physics, University of Oxford, Oxford OX1 3PU, England
a r t i c l e in f o
abstract
Article history:
Received 16 October 2009
Received in revised form
10 September 2010
Accepted 18 November 2010
Available online 26 November 2010
Spacecraft studies of the three terrestrial planets with atmospheres have made it possible to make
meaningful comparisons that shed light on their common origin and divergent evolutionary paths. Early
in their histories, all three apparently had oceans and extensive volcanism; Mars and Earth, at least, had
magnetic fields, and Earth, at least, had life. All three currently have climates determined by energy
balance relationships involving carbon dioxide, water and aerosols, regulated by solar energy deposition,
atmospheric and ocean circulation, composition, and cloud physics and chemistry.
This paper addresses the extent to which current knowledge allows us to explain the observed state of
each planet, its planetology, climatology and biology, within a common framework. Areas of ignorance
and mysteries are explored, and prospects for advances in resolving these with missions within the
present planning horizon of the space agencies are considered and assessed.
& 2011 Published by Elsevier Ltd.
Keywords:
Planetary atmospheres
Venus
Mars
Climate
Space missions
1. Introduction
Comparative planetology is a very old topic in one sense,
because observers of the heavens have always contemplated what
conditions might be like on other worlds, especially those that are
closest to Earth and therefore likely to be the most Earth-like. In an
address to the Royal Society in 1784, William Herschel said: ‘‘Mars
has a considerable but modest atmosphere, so that its inhabitants
probably enjoy a situation in many respects similar to ours.’’
Mitchel (1848), who made regular observations of Mars from
Cincinnati and discovered the ‘mountains’ near the south pole that
still bear his name, wrote: ’’the reddish tint which marks the light of
Mars has been attributed by Sir John Herschel to the prevailing
colour of its soil, which he considers the greenish hue of certain
tracts to distinguish them as covered with water. This is all pure
conjecture, based upon analogy and derived from our knowledge of
what exists in our own planet.’’
The best known early Mars studies were by Lowell (1895), who
wrote (in Mars) ‘‘yMars is blissfully destitute of weather. Unlike
New Englandy. What takes its place is a perpetual serenity, such as
we can scarcely conceive.’’ This misconception, like many of
Lowell’s assertions, was based on an over-interpretation of the
available data. Although he deduced correctly that Mars had a
much lower surface pressure than Earth, he proceeded to the
conclusion that the pressure differences in the atmosphere must be
$
An introductory overview presented at the International Conference on
Comparative Planetology: Venus–Earth–Mars, 11–15 May 2009, ESA-ESTEC
Noordwijk, The Netherlands.
n
Fax: + 44 1865 272923.
E-mail address: [email protected]
0032-0633/$ - see front matter & 2011 Published by Elsevier Ltd.
doi:10.1016/j.pss.2010.11.009
so low that they could only support ‘mild-mannered’ storms that
‘might as well not be’. He later had to revise this view, when he
observed dust storms on Mars from Earth through his telescope in
Arizona (Lowell, 1903), but still did not realize that his value for the
pressure was still more than ten times too high.
Lowell’s estimate was still being used in the 1950s, most notably
by Werner von Braun in his Marsproject that was designed to land
crew on Mars using a winged spacecraft. Earth-based observers
started to derive lower values in the 1960s, but did not determine
the correct value until 1971, by which time the exploration of Mars
by spacecraft was well under way (Taylor, 2009).
Despite these and many other early comparisons, as a rigid
scientific discipline comparative planetology is quite new since it is
only recently that extensive data has been acquired, even from
nearby Venus and Mars, which has the reliability and detail
required for comparability with the sophisticated data set available
for Earth. Measurements of surface mineralogy, atmospheric
composition and metallic core radius, for instance, now allow us
to trace the evolution of the terrestrial planets from their common
origin in the protosolar cloud around 4.6 billion years ago. Features
like the very hot climate on Venus, the anomalously large and
persistent internally generated magnetic field on Earth and the
water-eroded canyons on Mars require interpretation in terms of
divergent evolutionary processes (Fig. 1).
Very recently, comparative planetology has started to contribute to
the debate on the near-term evolution of Earth’s climate as a unique
opportunity to test energy balance models against hot and cold earthlike climates. This is extremely important, since difficult and expensive
decisions affecting the habitability of the planet as a whole have to be
made based on models that have been developed for a single example
and ‘tuned’ to current conditions. Some crucial physical processes, such
890
F.W. Taylor / Planetary and Space Science 59 (2011) 889–899
Fig. 1. Artists’ impressions of imaginary scenes on Venus, Earth and Mars, and their current mean surface pressures and temperatures. We seek to elucidate their common
features and important differences in terms of their formation and evolution in a framework of universal physics.
as cloud formation and the resulting microphysics, are very sensitive
non-linear functions of variables like atmospheric composition and
solar intensity. Others, such as atmospheric (and oceanic) general
circulation regimes, have features that models may omit entirely
without the effect being apparent until the model is stretched into
unfamiliar parameter space.
2. Common origin, current similarities and differences
A cornerstone of comparative planetology is the belief that the
planets had a common origin in the protosolar cloud that condensed to form the Sun around 4.6 Ga ago. Most currently accepted
theories of the origin of the Solar System follow the progression
shown in Fig. 2, in which a cloud of dust and gas roughly a light-year
across undergoes gravitational collapse to form a central mass
surrounded by a flat disc of material that coagulates to form
planetesimals and eventually the planets themselves. The details
are less well known and various models exist that seek to explain
the observed differences in size, mass, composition and rotational
angular momentum as well as distance from the Sun of the solid
planets (Woolfson, 2001). Further clues can be extracted from the
core size, composition and state, as determined primarily from
measurements of magnetic and gravitational fields, and from
studies of elemental abundances and mineralogy of the lithosphere. The incomplete nature of existing data sets exacerbates one
of the challenges that modern planetary science faces, which is to
discriminate between differences produced during the formation
of the planets and those due to subsequent evolution.
The most accessible parts of each planet for quantitative
measurements are the atmospheres. As well as providing some of
the most definitive data on the evolutionary history of the planet,
the composition and physical state of the atmosphere at the surface
defines the climate and habitability in general terms. For evolutionary studies, measurements of the abundances of trace constituents (methane on Mars and sulphur dioxide on Venus, for
example) are very valuable. So are the abundances and isotopic
ratios of the noble gases, which record changes that are generally
independent of chemical reactions. Climate variations require an
understanding of the processes that control the balance between
sources and sinks of CO2, H2O and other constituents, including
first-order questions of how the large differences in surface pressure
between Venus and Earth (a factor of about 100) and Earth and Mars
(another factor of about 100) arose and are currently maintained.
Evidence for liquid water on early Mars – and an understanding
of what ‘early’ means in this context – comes from studies of fluvial
F.W. Taylor / Planetary and Space Science 59 (2011) 889–899
891
FIELD
large, diffuse gas cloud
mass concentration
at centre
10 - 20 BAR
CO2
gas &
dust
cloud
rocky particles
VOLCANO
OCEAN
4 Ga
Divergent Evolution
icy particles
rotating disk
flattens
protosun
planetesimals aggregate
and find stable orbits
icy
planetesimals
particles form larger
planetesimals
rocky planets
rocky planetesimals
100 BAR
CO2
1 BAR
N2
0.01 BAR
CO2
VENUS
EARTH
MARS
gas planets
Hot young Sun clears
remaining gas and dust
Fig. 2. The formation of the Solar System from a cloud of dust and gas involved
collapse into a central mass surrounded by a disc of material that formed selfgravitating planetesimals and finally the eight planets. Their masses and orbits, and
probably other key attributes such as obliquity and rotation rate, were largely
determined at formation although evolutionary changes, particularly in their
atmospheres, followed later.
features seen on the surface of Mars, putative ‘coastlines’ identified
in global altitude maps of the Martian surface by laser altimeter
observations from orbit, and ‘blueberries’ and other sedimentary
features exposed on the surface traversed by the Mars Exploration
Rovers. For early Venus there is no direct observational evidence,
but several indirect inferences, including the fact that no current
theory of planetary formation in the solar system could produce
Venus with the small amount of water we observe at present while
nearby Earth is so wet, leading to the cautious assumption that
Venus too once had much more water than can be found at present.
3. Divergent evolutionary paths
Data such as that summarised above and discussed in more
detail below have led to the general postulate that Mars, Venus and
Earth were all more similar when they formed than they are today.
In this concept, 4 Gyr ago they all had dense CO2 atmospheres, all
had oceans, all had internally generated magnetic fields and all had
active volcanism (Fig. 3). Theories of Solar System formation help to
provide concepts for the degree to which they were the same and
which differences are to be expected as a result of forming at a given
planet’s distance from the Sun, for instance. Contrasting these with
the currently observed states sets the boundary conditions for
evolutionary models that attempt to describe what changed.
Fig. 3. Solar system formation models may suggest that Venus, Earth and Mars were
once much more similar, in terms of surface environment, than they are at present.
In this scenario, evolutionary processes removed the ocean from Venus and most of
the atmosphere from Mars at an early stage, while volcanism subsided on Earth and
Mars but remains strong on Venus, and Venus and Mars lost their magnetic fields.
These trends could have been primarily responsible for the radically different
conditions found today.
Of the three Earth-like planets, evidence for evolution is most
dramatically etched on the surface of Mars. Massive volcanic and
tectonic features, such as Olympus Mons (27 km high, compared to
11 km for Everest), and Mariner Valley (100 km wide, 10 km deep
and 4800 km long), suggest early geophysical activity, now dormant. Such activity must have been associated with a high level of
outgassing from the interior of the planet, perhaps at a level high
enough to maintain a thick atmosphere and a warm surface despite
several competing loss processes.
Mars’ residual magnetic field measured from the orbit shows it
once had a global field similar in strength or greater to Earth’s,
which has since subsided (Mitchell et al., 2007). The decline of the
field, perhaps in the same epoch as the fall-off in volcanic input of
gases to the atmosphere, may have led to an accelerated rate of loss
by solar wind erosion in the upper atmosphere. The actual role of a
field in moderating the loss process is still being debated, as further
discussed below.
Very large impact features still visible on the surface, like the
Hellas basin (more than 2,000 km wide and nearly 10 km deep),
and the fact that even large pieces of solid debris escaped to space,
some of which has reached Earth in the form of the SNC meteorites,
record a heavy early bombardment. Some estimates suggest that
collisions stripped away most of an early atmosphere with a surface
pressure as high as 1 bar (Melosh and Vickery, 1989), although
others find that the observed cratering record is more consistent
with the removal of the equivalent of only about 60 mb of CO2 by
this process (Brain and Jakosky, 1997).
892
F.W. Taylor / Planetary and Space Science 59 (2011) 889–899
The possible ineffectiveness of collisional erosion, in the face of
evidence for a thick early atmosphere, suggests that chemical
removal of atmospheric gases, especially at the surface, such as the
conversion of CO2 to carbonates as happened on a huge scale on the
Earth, may have been more important than collisions for depleting
the atmosphere on Mars. The fact that the current surface pressure
sits at the triple point for water may be evidence for this; if the
pressure were higher, liquid water could occur, which would
accelerate carbonate formation and pull the pressure back down
until the process stopped at around 6.5 mb.
The astronomical or Milanković cycles are large for Mars due to
large eccentricity and obliquity, and together with precessions
cause variations in solar forcing (Laskar et al., 2002). The result is
the layered terrain that is seen all over Mars, and this indicates that
a periodic climate change has occurred in response to the changes
in eccentricity, obliquity and phasing of the seasons. There may also
be a secular component to these cycles—for instance, the loss of
atmospheric gases other than water vapour – CO2 is again the
obvious example – during warmer, wetter phases would probably
not be reversible, resulting in a downward trend in surface pressure
superimposed on cycles with periods that typically are measured in
tens or hundreds of thousands of years.
Earth too has lost a large fraction of its early atmosphere but in
this case the process responsible left very clear indications. Huge
carbonate deposits in places as diverse as the Great Coral Reef in
Australia and the white cliffs of Dover are relicts of Earth’s early
atmosphere, which may have been equal in terms of CO2 content to
present-day Venus (Kasting, 1988).
the climate system as well. If we now further extend this in space to
include all three rather than just one planet we get a diagram like
that in Fig. 5, which is again quite complex despite the fact that not
just individual processes but entire disciplines have been collapsed
into each box. For instance, a box labelled ‘atmosphere’ must as a
minimum embrace the dynamical, chemical and radiative properties of the atmosphere as a function of time. All of these must be
built into global climate models (GCMs), which include every
important process from the interior to the space environment and
SPACE ENVIRONMENT
Solar UV flux, solar wind, magnetic field
ATMOSPHERE
Radiation, chemistry dynamics
Mars
Earth’s
Past
Climate
Future
Venus
SURFACE
Topography, mineralogy, volcanism
4. Key processes and models
On Earth, where data and models representing the geosystem
are naturally far more advanced, diagrams such as that in Fig. 4 are
commonly used to illustrate the complexity of the key processes
and their interactions and feedbacks. This is particularly so for the
changing climate, and the task for the comparative climatologist is
to develop similar charts for other Earth-like planets using the
same physical principles, chemical reactions and computer models
originally developed for Earth. In a recent review of Venus climate
history, Taylor and Grinspoon (2009) drew a simple version that
was easy to extend in time to include the past and future states of
INTERIOR
Composition, core, dynamics
Fig. 5. A process diagram in which the complexities are suppressed to show just the
basic interactions between surface and atmosphere, atmosphere and the space
environment, surface and interior. When the time dimension, represented simply as
past and future, and three different planets with similar interactions are added the
diagram gets complicated again, but stresses the links through common physics
between all of these aspects.
Fig. 4. The Earth and its environment represented by a diagram that summarises the key processes and their interactions that dominate the evolution of the climate, including
solar and solar system contributions. Despite concealing most of the physics and chemistry within summary boxes, such diagrams give an idea of the overall complexity of the
problem of understanding the system and reconstructing or predicting its behaviour.
F.W. Taylor / Planetary and Space Science 59 (2011) 889–899
that, in principal at least, apply to all three planets when the right
boundary conditions are applied (Forget et al., this issue). The
crucial and saving grace that makes this possible eventually is, of
course, that the physics which apply to each key process, for
instance exosphere–solar wind interactions, remain the same for
all time and for all three planets.
A different and also valuable approach is to construct analogues
of atmospheric circulation in the laboratory and gain insights that
are presently not possible by calculation from first principles (see
paper by Read et al. (this issue)). These too provide vital tests of
model formulations and suggest simplifying principles that are
valid in appropriate cases, despite the large difference in physical
scale between the atmosphere and the laboratory. The same can be
true, again up to a point, of controlled experiments on the radiative
properties of atmospheric constituents, the properties of core
materials under high pressures and temperatures, and cloudforming photochemical reactions.
5. Experiments with model temperature profiles
While full planetary GCMs are still in early stages of development, and can be expensive to run and difficult to understand, it is
still possible for simple radiative–convective models based on basic
hydrostatics and thermodynamics to explain gross features of the
mean vertical temperature profile on each of the three planets. We
can then go on to calculate the change in surface temperature
expected as a result of variations in the atmospheric composition
and other parameters (Taylor, 2005, 2010). Fig. 6 shows models for
present-day Venus, compared to a measured profile from the
Magellan radio occultation experiment. In the future, declining
volcanic activity may lead to a fall in the surface pressure and a
change in the cloud regime on Venus. The second model in Fig. 6 is
for a hypothetical future Venus, where the surface pressure and the
albedo are arbitrarily taken to both be the same as current Earth.
Although the climate in this case is similar to that envisaged by
Arrhenius and others before the true state of conditions on Venus
was discovered, it is not very likely to develop, since in the absence
of liquid water there is no highly efficient removal mechanism for
the large amounts of carbon dioxide still present. Also, Venus has
nearly 3 bars of nitrogen and other chemically inactive gases like
argon that will be even harder to remove by any conceivable
present or future process.
893
Fig. 7 shows the terrestrial ‘global warming’ scenario in which
warming due to greenhouse gas emissions is nearly balanced by
albedo increases due to enhanced cloud and aerosol formation. This
is essentially the situation that prevails on Earth at the present
time, with concern being expressed by experts (Philipona et al.,
2009), which further increases in gaseous opacity are being
accompanied by reductions in sulphate emissions, so that the
two dominant effects begin to reinforce instead of cancel, leading
to more rapid warming than by greenhouse gases alone. The
prospect for Earth, in the short term at least, is to become more
like Venus.
Fig. 8 shows Mars model temperature profiles with and without
dust at the current surface pressure, and two hypothetical past
scenarios where the surface pressure was 1 and 3 bar, respectively.
Both of these find a surface temperature above the melting point of
water, but only just in the 1-bar case. In both of the high-pressure cases,
the albedo is much higher than today (assumed to be similar to present
Venus), because of the denser atmosphere and the expected presence
of thick clouds. Even without cloud, the albedo of the planet could not
be as small as it is now due to increased Rayleigh scattering in a thick
atmosphere. Kasting (1991) estimates that the albedo doubles, halving
the solar heating, when the surface pressure goes from 0.006 to about
2 bar. In addition to this, scattering by water, CO2 or sulphuric acid
clouds will usually increase the albedo and cool the planet, although
Fig. 7. Models for Earth showing schematically the counteracting effects in current
global warming scenarios of (a) greenhouse gas enhancements, which increase the
optical depth down to a given pressure level and induce warming, and (b) growth in
the level of sulphate aerosol, which increases the planetary albedo and produce
cooling.
100
90
80
Altitude (km)
70
60
50
40
30
20
10
Surface
100
200
300
400
500
600
700
800
Temperature (K)
Fig. 6. Models for the temperature profile in Venus’ atmosphere (a) under present-day conditions (grey line), (b) for an Earthlike albedo without sulphuric acid clouds, and a
surface pressure of 1 bar (dashed line). A measured profile is shown for comparison (thin black line, labelled); the deviation of the model from this in the 45–75 km range is
mainly due to the heating effect of absorption in the cloud layers, which is not incorporated in the simple model.
894
F.W. Taylor / Planetary and Space Science 59 (2011) 889–899
100
Present Mars
Height (km)
80
60
A=0.8
S=0.7
=1 bar
A=0.9
S=0.7
=3 bars
40
dusty
20
clear
0
100
150
200
250
Temperature (K)
300
350
Fig. 8. Simple models of the mean temperature profile for present-day Mars with a
clear and a dusty atmosphere. Also shown is a hypothetical situation in which the
surface pressure is similar to modern Earth, with a Venus-like albedo (A ¼0.8) due to
condensate cloud formation, and one with a surface pressure of 3 bars and A ¼0.9,
both plausible extremes for early Mars. In both models of the paleoclimate the Sun is
30% less intense than today (S¼ 0.7).
Forget and Pierrehumbert (1997) examined the radiative effects of
clouds containing large CO2 particles and concluded that under some
conditions they could produce a significant warming effect at the
surface.
Dust and condensates in the atmosphere not only affect the
planetary albedo, they also modify the vertical temperature
gradient by affecting the radiative balance and the exchange of
latent heat. Gierasch and Goody (1972) examined the effect of
absorption by airborne dust after the first systematic survey of
atmospheric temperatures on Mars by Mariner 9 and found that
dust radically alters the lapse rate, while warming the middle
atmosphere by a large factor.
It is also necessary to allow for long-term changes in the
luminosity of the Sun. Most researchers believe that the solar
irradiance at Mars was smaller by perhaps 30% at the time when the
fluvial features were formed on the surface, although others
dispute this. Also controversial is the possible warming role of
various other greenhouse gases, including CH4, NH3 and H2S (Sagan
and Mullen, 1972) and (perhaps most convincingly) SO2 (Yung
et al., 1997), which are rare now but were probably more abundant
in the young Martian atmosphere.
While it remains quite uncertain what values for these and other
parameters should be used in calculations for early Mars, model
experiments both simple and complex do show that a range of
plausible greenhouse scenarios can exist for a warm, wet climate
long ago.
6. Observations: current missions and experiments
The Upper Atmosphere Research Satellite (Fig. 9, centre) is an
example of the size and sophistication now achieved by scientific
satellites in Earth orbit. The problems addressed by the instruments on
board were focussed on the terrestrial ozone depletion problem,
seeking in essence to understand the coupled radiation, chemistry
and dynamics cycle shown schematically in Fig. 10. The figure also
shows similar cycles thought to control the cloud chemistry on Venus
Fig. 9. Modern spacecraft for climate research. The Upper Atmosphere Research
Satellite (centre), launched into Earth orbit in 1991, was about 10 m long and had a
mass of nearly 8 tonnes. For comparison, Venus Express (top) and Mars Reconnaissance Orbiter (bottom) are about 1 and 2 tonnes, respectively, the exact mass in
each case depending on the amount of fuel for manoeuvres that remains on board.
and the CO–CO2 cycle on Mars. The measurements made by UARS have
gone a long way towards elucidating the ozone cycle in Earth’s
stratosphere and suggesting solutions to the depletion problem that
was of great concern in the 1990s—comparable progress on the
chemical cycles that are a key part of the climate system on Venus
and Mars is naturally taking somewhat longer.
Spacecraft and instruments of almost comparable size and
sophistication to those used for Earth observation now also operate
at Mars. Currently, these are the European Mars Express and NASA’s
Mars Reconnaissance Orbiter—the latter carries the Mars Climate
Sounder (McCleese et al., 2007) intended to observe Martian
meteorology and climate with sufficient resolution to permit
comparisons with Earth. The instrument measures temperature
F.W. Taylor / Planetary and Space Science 59 (2011) 889–899
hv
+O2
895
H2SO4
+H2O
hv
+S
S8
-H2O
hv
hv
hv
+S
S4
+CO
+O2
SO2
+CO
COS
H 2S
S3
SO3
+O2
+CO
+H2
+CaCO3
+O2
+H2O
+CO2
S2
+FeO
FeS2
CaSO4
+CO2
Ultraviolet (UV) radiation
Stratosphere
Temperature
at i
ng
He
at
tra
ns
p
po
e
er
p
i
rt
e
a
v
d
t
a
u
y
r
t
wi re
e istr
r
Ul
n
- fo
tu m
d
a
s
rc e
er he
d
mp c
Te
Ozone Transport
e
th
ole
O3
Ca
t al
y ti
ca
nd
ph
oto
e
nd
ch
em
i st
nt
ry
Te
m
Tra
NO2
HO2
ClO2
s
ce
pe
c ie
ra
st
ns
Winds
po
rt
Troposphere
Tropospheric source gases
OH
CO2
CO
H2
hv
HO2
H
hv
O(1D)
O
hv
OH
hv
O3
O2
H+M
HO
HO2
H2O2
hv
OH
H2O
HO2
O
O+M
H
Fig. 10. Reaction schemes for key climate processes. Top: A possible scheme for
cloud chemistry on Venus (Lewis, 1995). Centre: The processes studied by the Upper
Atmosphere Research Satellite, mainly related to ozone depletion in the stratosphere (NASA). Bottom: Chemical cycles in the Martian atmosphere, according to
Atreya and Gu (1994).
profiles with vertical and global coverage and resolution as good or
(in the troposphere) better than typical Earth meteorological
satellite sounders. The examples in Fig. 11 demonstrate the vertical
coverage and resolution being gained for the specific example of
the south polar winter, where the effect of CO2 sublimation effects
in confining the near-surface temperature in the polar night to a
small range of values near the frost point can be clearly seen, with
profiles measured outside the limits of the ice cap showing much
more variability, as well as high mean values. Another prominent
feature is the strong polar warming in the stratosphere, which,
Fig. 11. Examples of key results. Top: Vertical temperature profiles above the south
pole in the Martian winter of 2006 as measured by the Mars Climate Sounder
(McCleese et al. (2007, 2008)). The blue profiles are those polewards of 601S, which
was the approximate limit of the polar ice cap at the time. Centre: the vertical
profiles of ordinary and heavy water on Venus by the SPICAV/SOIR experiment on
Venus Express show a D/H ratio that is more than 200 times higher than that found
in Earth’s oceans and about 20 times that measured on Mars (Belyaev et al., 2008).
Bottom: Data for sulphur dioxide on Venus show that the declining trend seen by
Esposito (1984) seems to have reversed (Fedorova et al., 2008), corresponding to a
fluctuation of more than an order of magnitude over a time period of roughly a
decade, probably an indicator of variable but powerful volcanic activity. (for
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
since solar radiative heating is completely absent in this region at
this time of year, must have a dynamical origin. Preliminary
interpretations (McCleese et al., 2008) suggest that the general
circulation of the Martian atmosphere must be significantly more
896
F.W. Taylor / Planetary and Space Science 59 (2011) 889–899
vigorous than models had predicted to account for these new
observations. The instrument also observes and maps airborne
dust, water vapour, and water and CO2 clouds by measuring limb
brightness temperature profiles in nine wavelengths bands. Clouds
and associated precipitation of condensed CO2 are detected over
the winter pole. This is the process by which the seasonal ice cap
forms, trapping nearly a third of the mass of the global atmosphere
before subliming back into the gaseous form the following spring.
Venus has been less well served with space missions in the last
twenty years but has now been studied for more than three years
by ESA’s Venus Express (Svedhem et al., 2009) and in December
2010 the Japanese Venus Climate Orbiter should begin operations
there (Nakamura et al., 2007). Among the many achievements by
Venus Express are improved observations of the D/H ratio in water
vapour above the clouds. The new value for Venus is 240+ 25 times
that in Earth’s oceans (Fig. 11), with implications for the water
budget of the planet over time. Venus Express has also observed
SO2 in the upper atmosphere, confirming the very high mean value
(some five orders of magnitude relative to Earth and even higher
relative to Mars) and large fluctuations on time scales of a few
years, both of which seem to be related to a high level of active
volcanism (Fig. 11). These and other new data relevant to climate
are discussed in the papers to be found in the present volume and
references therein, including special sections in Nature (2007) and
Journal of Geophysical Research (2008, 2009).
7. Outstanding mysteries to be addressed by future missions
In order to look forward and help to define future missions we
can collect and discuss the leading areas of uncertainty that remain
and that are susceptible to being directly addressed with measurements that can be made in the foreseeable future. To focus the
discussion, the biggest questions in most of the current research on
the inner planets are collected into ten main issues, leading finally
to two ‘glittering prizes’ that are of almost unimaginable importance, and yet are also within current reach.
Problem I: orbital dynamics and associated questions related to
formation and early history. The history of collisions in the early
Solar System is slowly being reconstructed from surface records on
the Moon and Mars, as well as from the need to account plausibly
for macroscopic phenomena including the origin of Earth’s moon
(Stevenson, 1987); the cause of Venus’ slow retrograde rotation
(Correia and Laskar, 2001); and the bombardment history of Mars,
loss of an early atmosphere and creation of the giant basins (Brain
and Jakosky, 1997). Collisions of very large objects with all three
protoplanets may be required to account for their currently
observed states.
Problem II: determine the size, composition and state of the metallic
core and the processes responsible for the genesis of the magnetic field.
Why does the Earth, but not currently Mars or Venus, generate a
substantial magnetic field? Mars currently has a partially liquid
core, but the small size of the planet means it may have cooled
sufficiently to suppress the convective activity needed to generate a
field quite early in its history. The remnant crustal magnetism
recently detected by Mars Global Surveyor suggests that the
internal field generated on Mars was once considerably larger
than the present-day terrestrial field (Mitchell et al., 2007). Once
the magnetic properties of a sufficiently large number of samples
on the surface has been obtained and accurately dated, it should be
possible to reconstruct the history of the Martian field in considerable detail. Seismic and heat flow measurements will clarify the
current state of the core.
Venus is nearly as large as Earth so different factors must
account for the current absence of a planetary field there. Possibly
we happen to be investigating Venus at a time when the field is
close to zero during a reversal, as frequently happens on Earth.
Alternatively, models have been proposed (Stevenson et al., 1983)
that explain the difference between Venus and Earth in terms of the
lack of formation of a solid core and a more rigid outer crust on
Venus, so that convection in the interior is suppressed. These ideas
are much harder to test on Venus than on Mars, since even very
basic seismological experiments would have to be carried out in a
very hostile surface environment. Also, Venus may not have
retained the remnant crustal magnetic fields from any early period
of dynamo activity, since the surface temperature is above the Curie
point for the expected composition of the crust (Luhmann and
Russell, 1997).
Problem III: elemental abundances and mineralogy in the lithosphere. These remain poorly known for Mars and, especially, for
Venus (see the review by Basilevsky and Head (this issue)). These
require extensive surface and sub-surface sampling techniques
that are just now being developed for Mars; the high temperatures
on Venus will require additional technology not yet available.
Problem IV: history and nature of plate tectonics, weathering and
lava deposition and their links to the surface evolution on all three
planets. The link between plate tectonics, magnetic field generation
and atmospheric protection from solar wind erosion is an intriguing unknown that comparative planetary studies can potentially
resolve (see e.g. van Thienen et al. (2004)). Measurements of the
properties of the deep interior are likely to be crucial, for instance
using seismological sensors on multiple surface stations. Again, the
deployment of these devices on Venus will require new technology
developments, including electronic systems that can survive the
high temperatures for extended periods.
Problem V: gradients in composition especially isotopes of noble
gases. Noble gas abundances and their isotopic ratios are some of
the best-preserved records of the materials that formed the planets,
since they generally do not participate in chemical processes and
the non-primordial sources, essentially radioactive decay, are well
understood. The relative contributions of outgassing from the
interior and the infall of cometary material in forming the present
atmosphere can be evaluated, for instance, as can the depletion
of the Martian atmosphere by meteoritic bombardment. To obtain
the full range of possible insights requires a wide range of
measurements including the relatively rare isotopes of krypton
and xenon is required, with high precision and accuracy (Baines
et al., 2007).
Problem VI: what controls the total mass of each atmosphere?
Table 1 summarises the main components of the three atmospheres and also their total mass as reflected in the surface
pressure. There is roughly a factor of 100 between Venus and
Earth, and another 100 between Earth and Mars, before allowing for
the differences in gravity. The similarity between the total amounts
of nitrogen on Earth and Venus and the large inventory of carbonate
minerals on Earth suggests that the two atmospheres may have
been quite similar in mass and composition originally. Mars also
apparently had a more massive early atmosphere that may have
been equivalent to the other two after allowing for the smaller
planetary mass (i.e. around 3 bar surface pressure). The popular
theory that Earth’s atmospheric pressure was reduced mainly by
aqueous processes and that of Mars by impacts, while that of Venus
stayed roughly the same, has a lot of uncertainties that need new
measurements related to surface chemistry. Amongst these is the
search for substantial deposits of carbonate on Mars and for direct
evidence of paleo-oceans on Venus.
Problem VII: water budgets and ancient oceans. The three planets
may also have had roughly equal proportions of water. It would be
hard to reconcile the radically different distribution of water
between their orbits that the apparent dryness of present Venus
and Mars would require with any accepted theory of the formation
of the Solar System (Grinspoon, this issue). The evidence from the
F.W. Taylor / Planetary and Space Science 59 (2011) 889–899
Table 1
Data relevant to climate on three inner planets with Earth-like atmospheres (from
Taylor (2010)). The atmospheric composition is given as mole fractions, with
ppm meaning parts per million, ppb parts per billion and 0 meaning undetermined but very small.
Venus
Earth
8
Mars
8
Mean distance from Sun
Eccentricity
Obliquity (deg.)
Year
Rotational period (h)
Solar day (days)
Solar constant (kW m 2)
Net heat input (kW m 2)
1.082 10
0.0068
177
0.615
5832.24
117
2.62
0.367
1.496 10
0.0167
23.45
1
23.9345
1
1.38
0.842
1.524 108
0.0934
23.98
1.88
24.6229
1.0287
0.594
0.499
Atmospheric
Molecular weight (g)
Surface temperature (K)
Surface pressure (bar)
Mass (kg)
43.44
730
92
4.77 1020
28.98 (dry)
288
1
5.30 1018
43.49
220
0.007
1016
Composition
Carbon dioxide
Nitrogen
Argon
Water vapour
Oxygen
Sulphur dioxide
Carbon monoxide
Neon
0.96
0.035
0.00007
0.0001
0
150 ppm
40 ppm
5 ppm
0.0003
0.770
0.0093
0.01
0.21
0.2 ppb
0.12 ppm
18 ppm
0.95
0.027
0.016
0.0003
0.0013
0
700 ppm
2.5 ppm
897
least in part, by volcanic emissions and that therefore it may evolve
over time. Models of Venus with different cloud scenarios have been
used by Bullock and Grinspoon (2001) to investigate the effect on the
surface temperature, and shown that global climate change on a large
scale is possible due to this effect alone. Venus Express, Venus Climate
Orbiter and future in situ experiments can all be expected to produce
better data on cloud properties so that such possible trends can be
better investigated.
Mars has only thin clouds of water and carbon dioxide ices,
leaving particles of airborne dust as the main aerosol contributing
to climate in the present epoch. However, as discussed above,
clouds would have been a very important, possibly even essential,
component of the early climate that apparently led to warm, wet
conditions on the surface. Experimental studies of present cloud
and dust behaviour and their role in the atmosphere, like those by
the Mars Climate Sounder discussed above, is a prerequisite for
developing detailed models that can form a basis for understanding
possible paleoclimates on Mars.
On Earth, water clouds and sulphate aerosols are the most
important of a wide range of aerosol phenomena that participate in
the global change models that predict damaging levels of global
warming in the relatively near future (see e.g. Taylor (2005)). The
parallels between all three planets and the common aerosol physics
and radiative balance equations involved is a serious spur to
comparative planetary studies.
8. Conclusion: ‘glittering prizes’
D/H ratio and current magnetospheric measurements that Venus
lost massive amounts of H and O that used to form an ocean on the
surface needs substantiation, as does the combination of impact
erosion and sub-surface freezing that might account for the
putative Martian ocean.
Problem VIII: solar wind interaction and mechanisms for atmospheric escape. The stream of energetic changed particles from the
Sun interacts very differently with Earth than with Mars and Venus
because of the strong terrestrial magnetic field. It is often suggested
that the net effect of this has been to allow the Earth to retain its
atmosphere more effectively, and that the loss of an ocean of water
from Venus is attributable in part to this difference. Recently,
however, the role of any magnetic field (past or present) in the loss
process has been questioned, to the extent that it may not be clear
whether a field actually accelerates the loss (Barabash et al., this
issue).
Problem IX: volcanic production rate of atmospheric gases. The
high abundance and variability of SO2 and the geological evidence
for a huge number of volcanic constructs and fresh-looking lava
flows suggests active volcanism on Venus. However, searches for
thermal anomalies or plumes of volcanic gases by Venus Express
have so far proved inconclusive. New investigations will be
required in order to establish not just the existence but also the
magnitude of the current outgassing rate on Venus. Plumes of a
highly significant trace gas – methane, CH4 – have been detected on
Mars, although not on a scale that is directly relevant to climate
change. Rather, they may be ‘biomarkers’, or at least linked to
geothermal activity, and this makes them high priority targets for
heavily equipped roving vehicles.
Problem X: cloud physics and chemistry. The properties of the cloud
cover are the most vagarious, and therefore difficult to understand and
model, of any of the major features of the planets. Venus has a thick
layer of brilliantly reflective clouds, apparently with a high sulphur
content although its composition is not known throughout, which
plays a major role in the energy balance of the surface and atmosphere
(Titov et al., 2007). It seems likely that this cloud system is sustained, at
Although a new discipline, comparative planetology has within
its reach at least two far-reaching goals of great significance. One
belongs exclusively within this discipline—the detection, or not, of
life forms (and/or evidence for past life forms) in water-rich, or
formerly water-rich, environments on Mars, and the implications
for life as a common or uncommon phenomenon in the Universe.
Venus is a less favourable, but still not entirely negligible, site for
related investigations that will, like Mars, probably yield some of its
secrets to investigations now in the planning cycle. The other ‘prize’
is the contribution mentioned above that comparative planetology
may make to the struggle to understand and ameliorate harmful
climate change on the Earth. This is already underway with
missions like the Mars and Venus Expresses, Mars Reconnaissance
Orbiter, and the Exploration Rovers on the surface of Mars.
8.1. Life as persistent phenomenon in environments with liquid water
We have seen that Mars, and probably Venus, had habitable
environments on their surfaces in the past, as defined by the presence
of large bodies of standing, liquid water, but the precise nature and
duration of this epoch and the biological response are unknown. Even
today, Mars may have liquid water or brine below the surface in areas of
remnant geothermal activity, while Venus has water in the cloud
droplets in the clouds high in the atmosphere some 50 km or so above
the surface. If, as now seems likely for Mars at least, a nearby
extraterrestrial domain was habitable, did life of any kind arise and
does anything survive? If yes, what resemblances and differences does
it have to Earth life, and can we account for these?
The excitement questions like these, which generate in almost
everyone, and not just scientists, is partly responsible for the
programmes of exploration that have led us close to the first
answers. Observations from orbit of gullies on Mars with flow
patterns apparently recently produced by flowing water point the
way to regions of hydrothermal activity, where future landed robot
missions, possibly capable of climbing cliffs and equipped with
diggers and drills, can search for biomarkers (Taylor, 2009). The
traces of methane in the atmosphere, apparently from localised
898
F.W. Taylor / Planetary and Space Science 59 (2011) 889–899
10-4
10-3
Pressure (mb)
10-2
10-1
Earth
100
101
Venus
102
103
104
105
100
200
300
400
500
Temperature (K)
600
700
800
Fig. 12. Representative temperature profiles for Venus and Earth, measured by Pioneer Venus (Taylor et al., 1980) and Nimbus 7 (Taylor, 1987), respectively. Plotting these on a
log pressure scale demonstrates that the regimes are similar at pressures corresponding to the inhabited troposphere on Earth.
sources that may indicate organic chemical activity (presumably
also below the surface), provide further clues that can be followed
to the source.
Schulze-Makuch et al. (2004) have developed the idea that
Venus might be habitable at the cloud top level. The suggestion has
not been uniformly supported among planetary scientists, but has
achieved considerable coverage in the media. Certainly, the region
in question (Fig. 12) has Earth-like temperature and pressure,
impure liquid water in the form of cloud droplets, and plentiful
supply of solar energy. It is also the case, as we have seen, that
Venus, like Mars, may have had a period during its early evolution
when more Earth-like conditions prevailed at the surface, possibly
including lower temperatures and a deep ocean. If organisms
developed then, they could conceivably be hanging on in the
clouds if they have developed a defence against concentrated
sulphuric acid and powerful fluxes of ultraviolet radiation. It seems
well established now that any primordial life that may have existed
on, as opposed to beneath, the surface on Mars has not survived the
less intense flux at that planet.
8.2. Integrated and validated climate models
A unified climate model of Venus, Earth and Mars would deliver a
more reliable representation of all three planets. Specifically, predictions of climate change on Earth would benefit. The models used for
this purpose are constrained to match observations of the present-day
climate and are not necessarily reliable when applied to conditions that
evolve outside the envelope where they can be ‘tuned’ (Forget et al., this
issue). At present, policy makers have to rely on results like the
‘assessment reports’ published by the Intergovernmental Panel on
Climate Change (Solomon et al., 2007) for developing strategies to
avoid catastrophic change in the future, assuming implicitly that
scientists have already sufficiently understood the complex situation
represented in the systems diagram of Fig. 4. This will not really be the
case until the extended version shown schematically in Fig. 5 has also
been resolved and tested, the task which is the domain of comparative
planetology.
References
Atreya, S.K., Gu, Z.G., 1994. Stability of the Martian atmosphere—is heterogeneous
catalysis essential? J. Geophys. Res. 99, 13133.
Baines, K.H., Atreya, S.K., Carlson, R.W., Crisp, D., Grinspoon, D., Russell, C.T., Schubert, G.,
Zahnle, K., 2007. Experiencing Venus: clues to the origin, evolution, and chemistry of
terrestrial planets via in-situ exploration of our sister world. In: Esposito, L.W., Stofan,
E.R., Cravens, T.E. (Eds.), Exploring Venus as a Terrestrial Planet. American
Geophysical Union, Washington, DC, pp. 171–189 Geophysical Monograph 176.
Barabash et al., this issue.
Basilevsky, Head, this issue.
Brain, D.A., Jakosky, B.M., 1997. Loss of Martian atmosphere by impacts since the
formation of the Martian geologic record. Lunar Planet. Sci. XXVIII.
Bullock, M.A., Grinspoon, D.H., 2001. The recent evolution of climate on Venus.
Icarus 150, 19–37.
Correia, A.C.M., Laskar, J., 2001. The four final rotation states of Venus. Nature 411,
767–770.
Esposito, L.W., 1984. Sulfur dioxide: episodic injection shows evidence for active
Venus volcanism. Science 223, 1072–1074.
Forget, F., Pierrehumbert, R.T., 1997. Warming early Mars with carbon dioxide
clouds that scatter infrared radiation. Science 278, 1273–1276.
Forget et al., this issue.
Gierasch, P.J., Goody., R.M., 1972. The effect of dust on the temperature of the
Martian atmosphere. J. Atmos. Sci. 29, 400–402.
Grinspoon, D.H., this issue.
Kasting, J.F., 1988. Runaway and moist greenhouse atmospheres and the evolution
of Earth and Venus. Icarus 74, 472–494.
Kasting, J.F., 1991. CO2 condensation and the climate of early Mars. Icarus 94, 1–13.
Laskar, J., Leverard, B., Mustard, J., 2002. Orbital forcing of the Martian polar layered
deposits. Nature 419, 375–377.
Lowell, P., 1895. Mars. Houghton Mifflin, Boston.
Lowell, P., 1903. A dust storm on Mars. Lowell Obs. Bull. 1.
Luhmann, J.G., Russell, C.T., 1997. Venus: magnetic field and magnetosphere. In:
Shirley, J.H., Fainbridge, R.W. (Eds.), Encyclopedia of Planetary Sciences.
Chapman and Hall, New York, pp. 905–907.
McCleese, D.J., Schofield, J., Calcutt, S., Foote, M., Kass, D., Leovy, C., Paige, D., Read, P.,
Richardson, M., Taylor, F., Zurek., R., 2007. Mars Climate Sounder: an investigation of thermal and water vapor structure, dust and condensate distributions in
the atmosphere, and energy balance of the polar regions. J. Geophys. Res. 112,
E05S06. doi:10.1029/2006JE002790.
McCleese, D.J., Schofield, J.T., Taylor, F.W., Abdou1, W.A., Aharonson, O., Banfield, D.,
Calcutt, S.B., Heavens, N.G., Irwin, P.G.J., Kass, D.M., Kleinböhl, A., Lawson, W.G.,
Leovy, C.B., Lewis, S.R., Paige, D.A., Read, P.L., Richardson, M.I., Teanby, N., Zurek,
R.W., 2008. Intense polar temperature inversion in the middle atmosphere on
Mars. Nat. Geosci. 1, 745–749.
Melosh, H.J., Vickery, A.M., 1989. Impact erosion of the primordial atmosphere of
Mars. Nature 338 (6125), 487–489.
Mitchel, O.M., 1848. The Planetary and Stellar Worlds. W.L. Allison, New York.
Mitchell, D.L., Lillis, R.J., Lin, R.P., Connerney, J.E.P., Acuña, M.H., 2007. A global map of
Mars’ crustal magnetic field based on electron reflectometry. J. Geophys. Res.
112 (E1), E01002. doi:10.1029/2005JE002564 CiteID, 09 January 2007.
Nakamura, M., Imamura, T., Ueno, M., et al., 2007. Planet-C: Venus Climate Orbiter
mission of Japan. Planet. Space Sci. 55 (12), 1831.
Philipona, R., Behrens, K., Ruckstuhl, C., 2009. How declining aerosols and rising
greenhouse gases forced rapid warming in Europe since the 1980s. Geophys.
Res. Lett. 36, L02806. doi:10.1029/2008GL036350.
Read, P.L., et al., this issue.
Sagan, C., Mullen, G., 1972. Earth and Mars: evolution of atmospheres and surface
temperatures. Science 177, 52–56.
Schulze-Makuch, D., Grinspoon, D.H., Abbas, O., Irwin, L.N., Bullock, M., 2004. A
sulfur-based UV adaptation strategy for putative phototrophic life in the
Venusian atmosphere. Astrobiology 4 (1), 11–18.
Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M.,
Miller, H.L. (Eds.), 2007. Climate Change 2007: The Physical Science Basis.
Cambridge University Press.
F.W. Taylor / Planetary and Space Science 59 (2011) 889–899
Stevenson, D.J., 1987. Origin of the moon—the collision hypothesis. Annu. Rev. Earth
Planet. Sci. 15, 271–315.
Stevenson, D.J., Spohn, T., Schubert, G., 1983. Magnetism and thermal evolution of
the terrestrial planets. Icarus 54, 466.
Svedhem, H., Titov, D., Taylor, F., Witasse, O., 2009. Venus Express mission.
J. Geophys. Res. 114, E00B33. doi:10.1029/2008JE003290.
Taylor, F.W., Beer, R., Chahine, M.T., Diner, D.J., Elson, L.S., Haskins, R.D., McCleese,
D.J., Martonchik, J.V., Reichley, P.E., Bradley, S.P., Delderfield, J., Schofield, J.T.,
Farmer, C.B., Froidevaux, L., Leung, J., Coffey, M.T., Gille, J.C., 1980. Structure and
meteorology of the middle atmosphere of Venus: infrared remote sounding
from the Pioneer Orbiter. J. Geophys. Res. 85, 7963–8006.
Taylor, F.W., 1987. Remote sounding of the middle atmosphere from satellites:
the Stratospheric and Mesospheric Sounder experiment on Nimbus 7. Surv.
Geophys. 9, 123–148.
Taylor, F.W., 2005. Elementary Climate Physics. Oxford University Press.
899
Taylor, F.W., 2009. The Scientific Exploration of Mars. Cambridge University Press.
Taylor, F.W., 2010. Planetary Atmospheres. Oxford University Press.
Taylor, F., Grinspoon, D., 2009. Climate evolution of Venus. J. Geophys. Res. 114,
E00B40. doi:10.1029/2008JE003316.
Titov, D., Bullock, M.A., Crisp, D., Renno, N.O., Taylor, F.W., Zasova, L.V., 2007.
Radiation in the atmosphere of Venus. In: Esposito, L.W., Stofan, E.R., Cravens.,
T.E. (Eds.), Exploring Venus as a Terrestrial Planet. American Geophysical Union,
pp. 1121–1138 Geophysical Monograph no. 176.
van Thienen, P., Vlaar, N.J., van den Berg, A.P., 2004. Plate tectonics on the terrestrial
planets. Phys. Earth Planet. Inter. 142, 61–74.
Woolfson, M.M., 2001. The origin and evolution of the solar system. Astron.
Geophys. 41 (1), 12–19.
Yung, Y.L., Nair, H., Gerstell., M.F., 1997. CO2 greenhouse in the early Martian
atmosphere: SO2 inhibits condensation. Icarus 130, 222–224.