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Venus as a more Earth-like planet

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Abstract

Venus is Earth's near twin in mass and radius, and our nearest planetary neighbour, yet conditions there are very different in many respects. Its atmosphere, mostly composed of carbon dioxide, has a surface temperature and pressure far higher than those of Earth. Only traces of water are found, although it is likely that there was much more present in the past, possibly forming Earth-like oceans. Here we discuss how the first year of observations by Venus Express brings into focus the evolutionary paths by which the climates of two similar planets diverged from common beginnings to such extremes. These include a CO2-driven greenhouse effect, erosion of the atmosphere by solar particles and radiation, surface-atmosphere interactions, and atmospheric circulation regimes defined by differing planetary rotation rates.
PROGRESS
Venus as a more Earth-like planet
Ha
˚
kan Svedhem
1
, Dmitry V. Titov
2,3
, Fredric W. Taylor
4
& Olivier Witasse
1
Venus is Earth’s near twin in mass and radius, and our nearest planetary neighbour, yet conditions there are very different in
many respects. Its atmosphere, mostly compos ed of carbon dioxide, has a surface temperature and pressure far higher than
those of Earth. Only traces of water are found, although it is likely that there was much more present in the past, possibly
forming Earth-like oceans. Here we discuss how the first year of observations by Venus Express brings into focus the
evolutionary paths by which the climates of two similar planets diverged from common beginnings to such extremes. These
include a CO
2
-driven greenhouse effect, erosion of the atmosphere by solar particles and radiation, surface
atmosphere
interactions, and atmospheric circulation regimes defined by differing planetary rotation rates.
V
enus, Earth and Mars—the three terrestrial planets with
atmospheres, grouped close together in the inner solar
system—have many features in common. Earth and
Venus, in particular, are nearly the same size and seem
to have been quite similar in the epoch when they formed and cooled,
probably with large inventories of CO
2
in Earth’s atmosphere and
liquid water oceans on the surfaces of Venus (and Mars). Today they
have very different conditions on their surface as a result of evolu-
tionary processes that we try to understand by measuring and
modelling the common processes, aided by data from space missions
designed to probe the planets and their environments. Earth and
Venus have roughly the same amount of CO
2
; on Earth it is bound
in carbonates in the crust, whereas on Venus it exists mostly as gas.
The extreme climate at the surface of Venus, driven by this excess of
CO
2
in the atmosphere, reminds us of pressing problems caused by
similar physics on Earth.
More than 30 spacecraft have made the trip to Venus since the
Americans sent Mariner 2 in 1962—the first successful man-made mis-
sion to another planet. The Soviet Venera and Vega and the American
Pioneer Venus missions in 1967–92 were particularly influential in
establishing a basic description of the physical and chemical conditions
prevailing in the atmosphere. They showed the venusian atmosphere to
be filled with corrosive gases and thick clouds, extraordinarily active,
with high winds and complex cloud formations sculpted by meteoro-
logical systems that seemed to defy categorization by terrestrial analogy,
and a vast double-eyed vortex over each pole. Now the European Space
Agency has sent its first mission to our nearest planetary neighbour, to
investigate how the global atmospheric circ ulation, the cloud chemis try,
surface–atmosphere physical and chemical interactions including vol-
canism, atmospheric escape processes and the global energy balance and
the ‘greenhouse’ effect at the surface all act together to produce a climate
apparently defiantly different from Earth’s
1
.
Venus Express
The Venus Express design is based on the successful Mars Express
spacecraft—a 600-kg, three-axis-stabilized platform with a body-
fixed communications antenna
2
. It was launched by the Russian
Soyuz-Fregat launcher from Baikonur, Kazakhstan, on 9 November
2005 and is the first mission dedicated to atmospheric and plasma
investigations of Venus since NASA launched its Pioneer Venus
orbiter and probes more than a quarter of a century ago. It arrived
at Venus on 11 April 2006 and became fully operational in June of
that year, deploying a new generation of instrumentation
2
and using
new modes of observation
3
. The core of the payload is composed of
optical instruments including spectrometers and spectro-imagers
(Fig. 1 and Table 1), which make the first systematic use of the
spectral windows between 1 and 3 mm for three-dimensional imaging
of the atmosphere all the way down to the surface
4
. Solar, stellar and
Earth radio occultation is used for vertical profiling of atmospheric
properties. The highly elliptical polar orbit combines global nadir
observations of extended duration of the southern hemisphere with
close-up snapshots of the equatorial and northern latitudes
3
.
Middle and lower atmosphere
These first observations covering a range of depths in Venus’s
atmosphere at high spatial resolution have revealed, in addition to
1
ESA/ESTEC, PB 299, 2200AG Noordwijk, The Netherlands.
2
Max Planck Institute for Solar System Research, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany.
3
Space
Research Institute (IKI), Profsojuznaja ul. 84/32, 117997 Moscow, Russia.
4
Department of Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford OX1 3PU, UK.
MAG
VMC
VeRa
ASPERA
VIRTIS
PFS
SPICAV/
SOIR
Figure 1
|
The Venus Express spacecraft. The inset shows the positions of the
seven scientific instruments in a semi-transparent view. The optical
instruments for remote sensing are mounted below the upper platform; all
apertures are aligned with the 1z axis (pointing towards the top of the image).
The magnetometer (MAG) on its 1-m-long boom can be seen on the upper
platform, while the two ASPERA sensors are mounted on the bottom platform
(only one can be seen in this view). PFS, Planetary Fourier Spectrometer;
VeRa, Venus Radio Science Experiment; VMC, Venus Monitoring Camera.
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localized ‘weather’ phenomena, the overall organization of the atmo-
spheric circulation. Three broad regimes are clearly present in the
middle and lower atmosphere, with convective and wave-dominated
meteorology in the lower latitudes and an abrupt transition to
smoother, banded flow at middle to high latitudes
5
. The latter ter-
minates at about 30u from the pole, where the cold polar collar dis-
covered by earlier missions lies. This encloses a vast vortex-type
structure several thousand kilometres across with a complex double
‘eye’ that rotates every 2.5–2.8 Earth days. Simultaneous observations
in the ultraviolet and thermal infrared spectral ranges show corre-
lated patterns, indicating that the contrasts at both wavelengths,
although representing different atmospheric levels, are driven by
the same circumpolar dynamical regime
5,6
. Spectroscopic observa-
tions indicate marked changes in the temperature and cloud struc-
ture in the vortex, with the cloud top in the polar collar located at an
altitude of 70–72 km, about 5 km or one scale height higher than in
the eye. Night-side observations in the transparent spectral windows
showed that the vortex structure and circulation exist at as least as
great a depth as the lower cloud deck at 50–55 km, although its
‘dipole’ appearance seems to be confined to the cloud-top region
6
.
The edge of the polar collar at 50–60u latitude apparently marks the
poleward limit of the Hadley circulation, the planet-wide overturn-
ing of the atmosphere in response to the concentration of solar heat-
ing in the equatorial zones (Fig. 2a). Indirect evidence of such
meridional circulation is provided by monitoring of the latitude
distribution of minor constituents, especially carbon monoxide, as
dynamical tracers in the lower atmosphere.
The mesopause on Venus at 100–120 km altitude marks another
transition between different global circulation regimes, this time in
the vertical. The predominance of zonal super-rotation in the lower
atmosphere below the mesopause is replaced by solar to antisolar
flow in the thermosphere above, as revealed by non-LTE (non-local
thermodynamic equilibrium) emission in the spectral band of O
2
at
1.27 mm that originates from the recombination of oxygen atoms in
descending flow on the night side (Fig. 2b). The observed emission
patterns are highly variable, with the maximum at about the anti-
solar point and the peak altitude at about the mesopause
7
. A meso-
spheric temperature maximum is observed on the night side
8
,
produced by adiabatic heating in the subsiding branch of the thermo-
spheric solar to anti-solar circulation.
Sequences of ultraviolet and infrared images have been used to
measure the wind speeds at different altitudes by tracking the
motions of contrast features in the clouds. Zonal winds at the cloud
tops (,70 km) derived from the ultraviolet imaging are in the range
100 6 10 m s
21
at latitudes below 50u (ref. 5), in good agreement with
the earlier observations
9,10
. The new data, which penetrate the bright
upper haze obscuring the main cloud at middle latitudes, find that
the cloud-top winds quickly decline poleward of 50u. The infrared
observations
6
sound the dynamics in the main cloud deck at , 50 km
altitude on the night side, finding strong vertical wind shear of about
3ms
21
km
21
below 50u, and no shear poleward of this latitude, when
compared with the higher-altitude ultraviolet-derived winds. The
wind velocity profiles on Venus are found to be roughly, although
not exactly, in agreement with those predicted by the cyclostrophic
Table 1
|
The scientific payload of Venus Express
Name (acronym) Description Measured parameters
ASPERA-4 Detection and characterization of neutral and charged particles Electrons 1 eV
20 keV; ions0.01
36 keV/q; neutral particles0.1
60 keV
MAG Dual sensor fluxgate magnetometer, one sensor on a 1-m-long boom B field 8 pT
262 nT at 128 Hz
PFS Planetary Fourier Spectrometer (currently not operating) Wavelength 0.9
45 mm; spectral resolving power about 1,200
SPICAV/SOIR Ultraviolet and infrared spectrometer for stellar and solar occultation
measurements and nadir observations
Wavelengths 110
320 nm, 0.7
1.65 mmand2.2
4.4 mm; spectral
resolving power up to 20,000
VeRa Radio Science investigation for radio-occultation and bi-static radar measurements X- and S-band Doppler shift, polarization and amplitude variations
VIRTIS Ultraviolet
visible
infrared imaging spectrometer and high-resolution infrared
spectrometer
Wavelength 0.25
5 mm for the imaging spectrometer and 2
5 mm for
the high-resolution channel; resolving power about 2,000
VMC Venus Monitoring Camera for wide-field imaging Four parallel channels at 365, 513, 965 and 1010 nm
These instruments are expected to produce more than 2 terabits of data during the design lifetime of four Venus sidereal days (about 1,000 Earth days). Venus Express is operating in an elliptical
polar orbit with a period of 24 h and an apocentre altitude of 66,000 km. The pericentre altitude is maintained between 250 and 400 km approximately over the north pole. q is elementary charge.
Polar vortex
Sub-solar to
anti-solar cell
Hadley cell
Cold
Cold
Warm Warm
Polar collar
Solar heating
EUV flux
Recombination
of O atoms
into O
2
()
Night-side
airglow
CO
2
photodissociation
a
b
Figure 2
|
Schematic view of the general circulation of Venus’s atmosphere.
a
, The main featureis a convectively driven Hadley cell, which extends fromthe
equatorial region up to about 60u of latitude in each hemisphere. The trend is
polewards at all levels that can be observed by tracking the winds (at about
50–65 km altitude above the surface), so the return branchof thecell must be in
the atmospherebelowthe clouds.A cold‘polarcollar is foundaroundeach pole
at about 70u latitude; the Hadley circulation evidently feeds a mid-latitude jet at
its poleward extreme, inside which there is a circumpolar belt characterized by
remarkably low temperatures and dense, high clouds. Inside the collar a
thinning of the upper cloud layer forms a complex and highly variable feature,
called the ‘polar dipole in earlier literature describing poorly resolved
observations, which appears bright in the thermal infrared
6
. Because in general
terms thinner-than-average or lower-than-average cloud is often associated
with a descending air mass, and vice versa, the vortex may represent a second,
high-latitude circulation cell, resembling winter hemisphere behaviour on
Earth.
b, Above about 100 km altitude the circulation regime on Venus changes
completely to a sub-solar to anti-solar pattern. Oxygen airglow emission at
1.27 mm reveals the recombination of oxygen atoms into molecular oxygen
while descendingto lower altitudes in theanti-solar region.Additional evidence
ofthiscirculationis givenby theupper-atmospheretemperatureprofiles,which
show a pronounced temperature maximum on the night side that is due to
compressional heating in the downward branch of the circulation cell
8
.
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approximation
9
, which postulates a balance of pressure gradient
and centrifugal force in a purely zonal flow. This is, as expected, in
contrast with Earth, where so-called geostrophic balance generally
applies, dominated by the Coriolis forces produced by the relatively
rapid rotation of Earth.
Figure 3 summarizes the results so far of composition measure-
ments by Venus Express
8
. In the deep atmosphere, the most remark-
able result is the distribution of CO, which shows a larger systematic
latitudinal variability than the other minor species observed, includ-
ing water vapour. With a large source high in the atmosphere from
the photolysis of CO
2
and sinks in the clouds and near the surface,
and a lifetime measured in weeks, CO turns out to be an excellent
tracer of the general circulation. VIRTIS (Visible and Infrared
Thermal Imaging Spectrometer) global maps show a well-defined
maximum in the abundance of CO at a latitude of about 60u, near
the outer edge of the cold collar, which probably marks the poleward
extent of the Hadley cell (Fig. 2a). The other species seem to be much
more uniformly distributed over the globe in the deep atmosphere
but may be variable at the #10% level. The clouds show tremendous
variability, with a variety of meteorological systems in several differ-
ent layers, and distinct regions in which different mean particle sizes
predominate. There are, for instance, distinctly larger particle sizes in
the clouds in the polar region, although no definite departure from
the composition of sulphuric acid and water has been detected.
Upper atmosphere and plasma environment
The absence of an internal magnetic field for Venus means that the
solar wind interacts directly with the upper atmosphere, leading to a
different distribution of the energies and densities of electrons, ions
and energetic neutral atoms from that around Earth
11,12
(Fig. 4).
Venus Express measurements are taken at solar minimum, thus com-
plementing the Pioneer Venus plasma studies that were acquired
during solar maximum. Photoelectrons with a typical energy of
22–28 eV are measured in situ when the satellite dips into the iono-
sphere while passing the pericentre at 250–350 km. Below this alti-
tude the vertical distribution of electron density is sounded by radio
occultation
13
, suggesting a stable bottom of the ionosphere at 120 km.
The electron density peaks at about 4 3 10
5
cm
23
at about 140 km
altitude, and a very dynamic topside ionosphere is observed.
Simultaneous measurements of the vertical profiles of hydrogen-
bearing species in the upper atmosphere and plasma in situ monitor-
ing have begun to characterize the escape processes that have been
responsible for the depletion of water on Venus over the planet’s
history. Earlier measurements established a D/H ratio ,150 times
the terrestrial value in the lower atmosphere
14
, which is consistent
with the long-term loss of much larger amounts of hydrogen—
presumably from water—from Venus compared with Earth. Still
higher values of D/H, up to twofold higher, are now being found
above the clouds by SPICAV/SOIR (Spectroscopy for Investigation
of Characteristics of the Atmosphere of Venus/Solar Occultation
at Infrared), which has also uncovered strong variability in both
H
2
O and HDO content
8
. This unexpected behaviour has been
0.001 0.01 0.1 1.0 10 100 1,000
Mixin
g
ratio (p.p.m. by volume)
Altitude (km)
100
HF
HDO
HCl
HCl
H
2
O
H
2
O
SO
2
H
2
O
CO
CO
COS
80
60
40
20
0
Clouds
Figure 3
|
Atmospheric composition from the Venus Express observations.
The colours mark different trace gases. The vertical profiles of H
2
O, HDO,
CO, HCl and HF above the clouds have been derived from SPICAV/SOIR
solar occultation measurements
8
; the abundances of H
2
O, CO, SO
2
and COS
below the clouds are derived from VIRTIS spectra. The error bars in mixing
ratios indicate the minimum and maximum detections over all latitudes, and
the error bars in altitudes for the lower atmosphere indicate the width of the
weighting functions used for deriving the altitude. The bars with arrows in
the lower atmosphere show the expected sensitivity of the Venus Express
measurements for which data analysis is still in progress.
Solar wind
Escaping ions
H
+
, O
+
, He
+
Induced magnetopause
Bow shock
H
+
He
2+
B
sw
B
ps
B
t
0
1
2
3
0
–1
1
2
3–2
–3
Distance to Venus–Sun line (R
V
)
Venus–Sun line, x (R
V
)
Figure 4
|
The plasma environment of Venus as determined by Venus
Express.
All parametersnoted in the figure are measured on a regular basis by
the magnetometer and the ASPERA instrument, in three distinctly different
regions: the unperturbed solar wind (sw), the plasma sheath (ps) and the
induced magnetosphere/tail (t). The boundaries determined by the two
instruments are shown approximately to scale. Oxygen ions are observed at
high concentrations around the terminator and at lower concentrations well
into the tail, indicating escape from a specific source region. He
1
shows
similar behaviour, whereas H
1
is observed much more evenly distributed
around the planet
11
. The figure shows cylindrical coordinates; the x axis is
aligned with the Venus–Sun line. R
V
, radius of Venus.
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tentatively explained by a combination of fractionation in the con-
densation of ice particles and atmospheric transport, which would
imply that species bearing hydrogen and deuterium are already frac-
tionated in the mesosphere right beneath the region from which
escape occurs.
ASPERA-4 (Analyser of Space Plasmas and Energetic Atoms 4) has
established for the first time the composition of the escaping plan-
etary ions, finding that, after H
1
, the main escaping ion is O
1
. This is
in contrast with Mars, where the escaping plasma consists of approxi-
mately equal amounts of O
1
,O
2
1
and CO
2
1
, and it results from the
higher gravitational acceleration at Venus, which tends to retain
heavier components such as CO
2
1
. High fluxes of escaping He
1
are also detected
11
. Oxygen and hydrogen ions are formed by the
dissociation of neutral atmospheric species, including water, by solar
ultraviolet radiation, which are then blown away towards the outer
reaches of the Solar System (Fig. 4). This happens at a faster rate on
Venus than on Earth, not just because Venus is closer to the Sun but
also because it lacks the magnetic field that protects Earth from the
flux of energetic charged particles from the Sun. These loss processes
must have removed large amounts of water from Venus during the
first billion years or so after the formation of the Solar System. A
detailed quantification of the loss rates enables a more accurate
estimate of how much water Venus has lost over its entire history,
and by the end of the mission we should know better whether the
planet once had an ocean as extensive and deep as Earth’s.
Lightning
For a long time the existence of lightning on Venus has been contro-
versial. Whistler-mode waves, which can be considered reliable evid-
ence of lightning, were detected by the Venus Express magnetometer
during more than 10% of the pericentre passes
15
. This corresponds to
a lightning rate at least half that of Earth. Frequent lightning repre-
sents a significant energy input that has important implications for
the chemistry in the lower and middle atmosphere on Earth, and this
now seems likely to be true for Venus also.
Venus is more Earth-like
The overall sense of the results from the first year of operation of
Venus Express is that the differences, particularly in climate, between
Venus and Earth are much less mysterious than previously thought
after the early phase of spacecraft exploration. They are consistent
with theoretical ideas and interpretations suggesting that the two
planets had similar surface environments in the past and that they
evolved differently, with Earth’s oceans converting most of its atmo-
spheric CO
2
to carbonate rocks, and Venus losing most of its water to
space. Both processes can now be seen to be still going on. The high
zonal winds and near-equatorial turbulence on Venus, as well of
course as the high surface temperatures, result from the depth of
the atmosphere and huge inventory of greenhouse gas retained by
Venus. The slow rotation of Venus, as well as possibly being respon-
sible for the lack of magnetic field that makes erosion of the atmo-
sphere by the solar wind so effective, permits the Earth-like Hadley
cell component of the atmospheric circulation to extend closer to the
poles, where it breaks down in spectacular fashion to form mid-
latitude jets and polar vortices that are larger and more energetic
than Earth’s but are in many respects quite similar.
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Acknowledgements We thank R. Hueso and J. Bailey for the provision of graphics
and data for Fig. 2b, and E. Marcq, C. Tsang, P. Drossart and J.-L. Bertaux for
providing data for Fig. 3.
Author Information Reprints and permissions information is available at
www.nature.com/reprints. Correspondence should be addressed to H.S.
(h.svedhem@esa.int).
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A key item of interest for planetary scientists and astronomers is the habitable zone, or the distance from a host star where a terrestrial planet can maintain necessary temperatures in order to retain liquid water on its surface. However, when observing a system's habitable zone, it is possible that one may instead observe a Venus-like planet. We define "Venus-like" as greenhouse-gas-dominated atmosphere occurring when incoming solar radiation exceeds infrared radiation emitted from the planet at the top of the atmosphere, resulting in a runaway greenhouse. Our definition of Venus-like includes both incipient and post-runaway greenhouse states. Both the possibility of observing a Venus-like world and the possibility that Venus could represent an end-state of evolution for habitable worlds, requires an improved understanding of the Venus-like planet; specifically, the distances where these planets can exist. Understanding this helps us define a "Venus zone", or the region in which Venus-like planets could exist, and assess the overlap with the aforementioned "Habitable Zone". In this study, we use a 1D radiative-convective climate model to determine the outer edge of the Venus zone for F0V, G2V, K5V, and M3V and M5V stellar spectral types. Our results show that the outer edge of the Venus zone resides at 3.01, 1.36, 0.68, 0.23, and 0.1 AU, respectively. These correspond to incident stellar fluxes of 0.8, 0.55, 0.38, 0.32, and 0.3 S, respectively, where stellar flux is relative to Earth (1.0). These results indicate that there may be considerable overlap between the habitable zone and the Venus zone.
... Venus and Earth share several similarities, yet the evolution of their planetary environments have followed distinctly different evolutionary paths(e.g. Walker, 1975;Kasting, 1988;Svedhem et al., 2007;Driscoll and Bercovici, 2013) leading to two very different environments in the context of potential habitability. The high pressure, high temperature conditions on the present-day surface of Venus are known to preclude the presence of liquid water, thus making the surface inhospitable to life as we know it. ...
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Despite the harsh conditions in the atmosphere of Venus, the possibility for an aerial habitable zone exists. A thermal habitable zone is predicted to exist at an altitude range of 62 to 48 km, above which temperatures drop below the lower thermal limit of cell growth and below which temperatures exceed the evaporation temperature. Many biocidal factors must be considered for the complete definition of an aerial habitable zone; in this study we consider the constraint specifically from the perspective of biocidal solar ultraviolet (UV) intensity in the atmosphere. We simulated the penetration of solar ultraviolet and visible light through the atmosphere using a radiative transfer model, to determine the spectral environment (and thus the UV biocidal effect) as a function of altitude in the atmosphere of Venus. At the top of the thermal aerial habitable zone (62 km) the incoming solar irradiance creates a severely challenging UV environment, with extremophiles such as Deinococcus radiodurans expected to be able to endure these UV conditions for approximately 80 s. At an altitude of around 59 km the biologically-weighted UV irradiance drops below that calculated for the Archean Earth, and continues to fall with decreasing altitude until at 54 km it is less than that found currently at the surface of Earth. Crucially, longer wavelength photosynthetically active light continues to penetrate to these altitudes and below, resulting in a solar radiation environment in the venusian atmosphere below around 54 km that screens biologically-damaging UV radiation yet permits the process of photosynthesis. Whilst not claiming to suggest the existence of an aerial habitable zone in general, by considering thermal conditions, ionising radiation and the UV flux environment of the venusian cloud deck alone, we define a potential habitable zone that extends from 59 km to 48 km. This region should form the focus of future remote and in situ astrobiological investigations of Venus.
... Unlike the Earth, Venus has no intrinsic magnetic field, but the upper ionised layers of the atmosphere provide an induced magnetosphere that protects the lower atmosphere from the Solar magnetic field. Lighter gases, such as water, are continuously lost from the atmosphere along the induced magnetotail as a result of the solar wind (Svedhem et al., 2007). ...
Thesis
The atmospheres of bodies in the Solar system display a great degree of diversity in their mass and composition. Impacts onto these bodies by smaller objects, such as asteroids, comets and planetesimals left over after terrestrial planet formation are evidenced by crater observations, and are an inevitable outcome of dynamical simulations of planet and moon formation. These impacts deliver mass and energy, and are capable of altering the atmosphere through erosion, volatile delivery and impact-triggered outgassing from the target body surface. In this thesis I investigate the effect of bombardment by these small bodies on the evolution of atmospheres on planets and moons. I first provide an introduction to the processes relevant to the formation and evolution of terrestrial planets and their atmospheres in Chapter 1, with a focus on the current state of research on the role impacts have played in shaping atmospheres. I develop in Chapter 2 an analytic method through which the characteristic stalling mass (at which impact induced erosion and accretion are balanced) can be predicted, and which can be used to predict the degree of stochastic variation expected for a given atmosphere and impactor combination. I also present a numerical model for stochastic atmosphere evolution due to bombardment, incorporating prescriptions for a range of impact outcomes (fragmentation and aerial bursts, cratering events and the non-local mass loss caused by giant impacts). These models are applied to the bombardment by asteroids, comets and left-over planetesimals on the Earth in Chapter 3 and a comparative study of the terrestrial Solar system planets (Venus and Mars) in Chapter 5, using distributions of impact velocities I calculate from the results of recent dynamical simulations. The sensitivity of these results to both the initial atmosphere conditions and the properties of the impacting populations are investigated. In Chapter 4, the numerical code and analytic predictions are also applied to cometary impacts of the atmospheres of the moons of the outer giant planets, incorporating a prescription for impact-triggered outgassing when applied to Titan. Finally in Chapter 6, I make general predictions about the influence of impacts on the atmospheres of exoplanets and hypothetical exomoons and present a simple model for the simultaneous evolution of a magma ocean and atmosphere on a terrestrial planet. The results of this dissertation are summarised in Chapter 7.
... Its value is deduced exactly from k and N A (from ( VII.16) and (VII.17)) using the relation R = N A k: (Svedhem et al., 2007). Quantities are given as the average value at the surface of each body. ...
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This thesis focuses on the mechanical coupling between a planet’s solid body and its atmosphere. We study natural and anthropogenic geophysical events under the scope of seismic waves and infrasound. These phenomena are keys to uncover the atmospheric structure of Earth, the interior of Venus, and Mars as a whole. Acoustic and seismic waveforms contain valuable information, about both the source event and the propagation medium. Our work is two-fold. Firstly, we develop a numerical simulation software for the coupled ground-atmosphere system. We rely on the linearised Navier-Stokes equations to model the atmosphere, and on visco-elastodynamics to model the sub-surface. We employ a discontinuous spectral finite elements method, allowing the simulation of full waveforms. The implementation is validated using two techniques: analytical and manufactured solutions. Our software can model all types of air-ground couplings, and accurately accounts for acoustic and seismic wave propagation. Complex topographies can be used, as well as range-dependant atmospheric models. As a result, it is particularly well suited to study most geophysical phenomena in planetary atmospheres. Example events include seismic waves, microbaroms, underground and overground explosions, or gravity waves. Secondly, we study numerous application cases related to the aforementioned planetary science objectives. With the exploration of Venus’ interior in mind, we conduct terrestrial experiments to study seismically-induced infrasound, and involve balloon-borne instruments. We show that it is possible to infer the properties and structure of the sub-surface from these infrasonic waves. These instrumented balloons also render the localisation of ground events possible, which is crucial both for planetary exploration and for the airborne monitoring of the Earth. Finally, we demonstrate that the Martian atmosphere features infrasound, establishing for the first time the existence of infrasound on another planet. This is achieved thanks to InSight’s seismometer SEIS, able to measure the faint ground motion caused by passing airwaves.
... For the high-pressure model Case4 we obtain a water mass of 1.27 × 10 18 kg at around 10 Myr. For comparison, on Venus, where the atmosphere is CO 2 -dominated and approximately 10 times denser than our high-pressure cases, the mixing ratio of water at the surface is 20 ppm (smaller than our findings in general, excluding Case3), while the total mass of water in the atmosphere is 4 × 10 15 kg (Svedhem et al., 2007). However, on Venus the temperature and the pressure measured at the surface do not allow the presence of liquid water (Way and Del Genio, 2020). ...
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A free-floating planet is a planetary-mass object that orbits around a non-stellar massive object (e.g. a brown dwarf) or around the Galactic Center. The presence of exomoons orbiting free-floating planets has been theoretically predicted by several models. Under specific conditions, these moons are able to retain an atmosphere capable of ensuring the long-term thermal stability of liquid water on their surface. We model this environment with a one-dimensional radiative-convective code coupled to a gas-phase chemical network including cosmic rays and ion-neutral reactions. We find that, under specific conditions and assuming stable orbital parameters over time, liquid water can be formed on the surface of the exomoon. The final amount of water for an Earth-mass exomonoon is smaller than the amount of water in Earth oceans, but enough to host the potential development of primordial life. The chemical equilibrium time-scale is controlled by cosmic rays, the main ionization driver in our model of the exomoon atmosphere.
Article
Described here is a concept for a variable-altitude aerobot mission to Venus developed as part of the 2020 NASA Planetary Science Summer School in collaboration with NASA Jet Propulsion Laboratory. The Venus Air and Land Expedition: a Novel Trailblazer for in situ Exploration (VALENTInE) is a long-duration New Frontiers–class mission to Venus in alignment with the goals recommended by the 2013 Planetary Science Decadal Survey. VALENTInE would have five science objectives: (1) determine the driving force of atmospheric superrotation, (2) determine the source of D/H and noble gas inventory, (3) determine the properties that govern how light is reflected within the lower cloud later, (4) determine whether the tesserae are felsic, and (5) determine whether there is evidence of a recent dynamo preserved in the rock record. The proposed mission concept has a total duration of 15 Earth days and would float at an altitude of 55 km, along with five dips to a lower altitude of 45 km to study Venus’s lower atmosphere. The instrument payload allows for measurements of the atmosphere, surface, and interior of Venus and includes six instruments: an atmospheric weather suite, a mass spectrometer, a multispectral imager, a near-infrared spectrometer, light detection and ranging, and a magnetometer. Principle challenges included a limitation caused by battery lifetime and low technology readiness levels for aerobots that can survive the harsh conditions of Venus’s atmosphere. This preliminary mission was designed to fit within an assumed New Frontiers 5 (based on inflated New Frontiers 4) cost cap.
Article
A key item of interest for planetary scientists and astronomers is the habitable zone: the distance from a host star where a terrestrial planet can maintain necessary temperatures in order to retain liquid water on its surface. However, when observing a system’s habitable zone, it is possible that one may instead observe a Venus-like planet. We define “Venus-like” as greenhouse-gas-dominated atmosphere occurring when incoming solar radiation exceeds infrared radiation emitted from the planet at the top of the atmosphere, resulting in a runaway greenhouse. Our definition of Venus-like includes both incipient and post-runaway greenhouse states. Both the possibility of observing a Venus-like world and the possibility that Venus could represent an end state of evolution for habitable worlds require an improved understanding of the Venus-like planet, specifically the distances where these planets can exist. Understanding this helps us define a “Venus zone”—the region in which Venus-like planets could exist—and assess the overlap with the aforementioned “habitable zone.” In this study, we use a 1D radiative−convective climate model to determine the outer edge of the Venus zone for F0V, G2V, K5V, and M3V and M5V stellar spectral types. Our results show that the outer edge of the Venus zone resides at 3.01, 1.36, 0.68, 0.23, and 0.1 au, respectively. These correspond to incident stellar fluxes of 0.8, 0.55, 0.38, 0.32, and 0.3 S ⊙ , respectively, where stellar flux is relative to Earth (1.0). These results indicate that there may be considerable overlap between the habitable zone and the Venus zone.
Article
To evaluate a recent hypothesis of species stratification in the Venus lower atmosphere, a time-dependent, three-dimensional model is proposed for studying turbulent mixing of many species in the Venus lower atmosphere. This model is based on fundamental physics embedding non-equilibrium thermodynamics, the Onsager classical relationship between fluxes and forces, high-pressure transport property calculation and a real-gas equation of state. The conservation equations are self-contained, with no need for coefficients based on parametrizations to match Venus observations. The rationale for solving these equations in relatively small spatial domains at different altitudes in the Venus lower atmosphere is presented. The equations are solved in a temporal mixing layer configuration that is eminently suited to investigate turbulent mixing between two streams of different composition. The adopted simulation method is Direct Numerical Simulation (DNS). A substantial database of DNS realizations is generated for mixtures of two and seven species, with different levels of initial stratification, and at the realistic conditions of pressures and temperatures in the Venus lower atmosphere at three altitudes. High density-gradient magnitude regions (HDGM) are shown to form under initial high stratification conditions, with larger gradient values obtained at low altitudes where supercritical conditions occur. Independent of the altitude lower than 50 km, under low stratification conditions, mixing produces a more uniform spatial distribution of the density than at higher initial stratification. Under low initial stratification conditions, the influence of the minor species on the global behavior of the flow seems to be negligible. At high initial stratification conditions, differences in diffusion of various species play an important role in determining the mixture characteristics. The hypothesis of chemical species separation is discussed in the light of the present results which do not support the existence of stable species separation in the lower Venus atmosphere, independent of the number of species or the altitude.
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Venus Express is the first European mission to the planet Venus. Its payload consists of seven instruments and will investigate the atmosphere, the plasma environment, and the surface of Venus from orbit. Science planning is a complex process that takes into account requests from all experiments and the operational constraints. The planning of the science operations is based on synergetic approach to provide good coverage of science themes derived from the main mission goals. Typical observations in a single orbit - so-called "science cases" are used to build the mission science activity plan. The nominal science mission (from June 4, 2006 till October 2, 2007) is divided in nine phases depending on observational conditions, occurrences of the solar and Earth occultation, and particular science goals. The observation timelines for each phase were developed in a coordinated way to optimize the payload activity, maximize the overall mission science return, and to fit into the available mission budgets.
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The occurrence of lightning in a planetary atmosphere enables chemical processes to take place that would not occur under standard temperatures and pressures. Although much evidence has been reported for lightning on Venus, some searches have been negative and the existence of lightning has remained controversial. A definitive detection would be the confirmation of electromagnetic, whistler-mode waves propagating from the atmosphere to the ionosphere. Here we report observations of Venus' ionosphere that reveal strong, circularly polarized, electromagnetic waves with frequencies near 100 Hz. The waves appear as bursts of radiation lasting 0.25 to 0.5 s, and have the expected properties of whistler-mode signals generated by lightning discharges in Venus' clouds.
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Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.
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Venus, unlike Earth, is an extremely dry planet although both began with similar masses, distances from the Sun, and presumably water inventories. The high deuterium-to-hydrogen ratio in the venusian atmosphere relative to Earth's also indicates that the atmosphere has undergone significantly different evolution over the age of the Solar System. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry, and although the bulk of O and O2 are gravitationally bound, heavy ions have been observed to escape through interaction with the solar wind. Nevertheless, their relative rates of escape, spatial distribution, and composition could not be determined from these previous measurements. Here we report Venus Express measurements showing that the dominant escaping ions are O+, He+ and H+. The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H+)/Q(O+) = 1.9; Q(He+)/Q(O+) = 0.07. The first of these implies that the escape of H+ and O+, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.
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Venus has thick clouds of H2SO4 aerosol particles extending from altitudes of 40 to 60 km. The 60-100 km region (the mesosphere) is a transition region between the 4 day retrograde superrotation at the top of the thick clouds and the solar-antisolar circulation in the thermosphere (above 100 km), which has upwelling over the subsolar point and transport to the nightside. The mesosphere has a light haze of variable optical thickness, with CO, SO2, HCl, HF, H2O and HDO as the most important minor gaseous constituents, but the vertical distribution of the haze and molecules is poorly known because previous descent probes began their measurements at or below 60 km. Here we report the detection of an extensive layer of warm air at altitudes 90-120 km on the night side that we interpret as the result of adiabatic heating during air subsidence. Such a strong temperature inversion was not expected, because the night side of Venus was otherwise so cold that it was named the 'cryosphere' above 100 km. We also measured the mesospheric distributions of HF, HCl, H2O and HDO. HCl is less abundant than reported 40 years ago. HDO/H2O is enhanced by a factor of approximately 2.5 with respect to the lower atmosphere, and there is a general depletion of H2O around 80-90 km for which we have no explanation.
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Venus Express is the first European mission to planet Venus. The mission aims at a comprehensive investigation of Venus atmosphere and plasma environment and will address some important aspects of the surface physics from orbit. In particular, Venus Express will focus on the structure, composition, and dynamics of the Venus atmosphere, escape processes and interaction of the atmosphere with the solar wind and so to provide answers to the many questions that still remain unanswered in these fields. Venus Express will enable a breakthrough in Venus science after a long period of silence since the period of intense exploration in the 1970s and the 1980s.
Article
An overview is given of current knowledge and mysteries about the planet Venus, with emphasis on those aspects that are intended to be studied with the Venus Express mission following orbit insertion at the planet in March 2006.
Article
With its comprehensive suite of near-infrared instruments, Venus Express will perform the first detailed global exploration of the depths of the thick Venusian atmosphere. Through the near-daily acquisition of Visible and Infrared maps and spectra, three infrared-sensing instruments—the Planetary Fourier Spectrometer (PFS), the Venus Monitoring Camera (VMC), and the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)—will comprehensively investigate the Thermal structure, meteorology, dynamics, chemistry, and stability of the deep Venus atmosphere. For the surface, these instruments will provide clues to the emissivity of surface materials and provide direct evidence of active volcanism. In so doing, ESA's Venus Express Mission directly addresses numerous high-priority Venus science objectives advanced by America's National Research Council (2003) decadal survey of planetary science.
Article
The global circulation of the Venus atmosphere is characterized at cloud level by a zonal super rotation studied over the years with data from a battery of spacecrafts: orbiters, balloons and probes. Among them, the Galileo spacecraft monitored the Venus atmosphere in a flyby in February 1990 in its route toward Jupiter. Since the flyby was almost equatorial, published analysis of zonal winds obtained from displacements of cloud elements on images obtained by the SSI camera [Belton, M.J.S., and 20 colleagues, 1991. Science 253, 1531–1536] stop at latitudes 50° north and south. In this paper we present new results on Venus winds based on a reanalysis of an extended set of images obtained at two wavelengths, 418 nm (violet) and 986 nm (near infrared), that sense different altitude levels in the upper cloud. Our main result is that we have been able to extend the zonal wind profile up to the polar latitudes: 70° N and 70° S at 418 nm and 70° N at 986 nm. Binned and smoothed profiles are given in tabular form. We show that the zonal winds drop in their velocity poleward of latitudes 45° N and 50° S where an intense meridional wind shear develops at the two cloud levels. Our data confirm the magnitude of this shear, retrieved previously from radio occultation data, but disagrees with it in the latitudinal location of the sheared region. The new wind data can be used to recalibrate the zonal winds retrieved from the previous measurements of the temperature field and the cyclostrophic balance assumption. The meridional profiles of the zonal winds at the two cloud levels are used to assess the vertical wind shear in the upper cloud layer as a function of latitude and locate the most unstable region.
Article
The atmosphere and ionosphere of Venus have been studied in the past by spacecraft with remote sensing or in situ techniques. These early missions, however, have left us with questions about, for example, the atmospheric structure in the transition region from the upper troposphere to the lower mesosphere (50-90 km) and the remarkably variable structure of the ionosphere. Observations become increasingly difficult within and below the global cloud deck (<50 km altitude), where strong absorption greatly limits the available investigative spectrum to a few infrared windows and the radio range. Here we report radio-sounding results from the first Venus Express Radio Science (VeRa) occultation season. We determine the fine structure in temperatures at upper cloud-deck altitudes, detect a distinct day-night temperature difference in the southern middle atmosphere, and track day-to-day changes in Venus' ionosphere.