Available online at www.sciencedirect.com
Advances in Space Research 50 (2012) 293–310
www.elsevier.com/locate/asr
New results on planetary lightning
Yoav Yair ⇑
Department of Life and Natural Sciences, The Open University of Israel, Israel
Received 16 January 2012; received in revised form 11 April 2012; accepted 13 April 2012
Available online 26 April 2012
Abstract
We present the latest observations from spacecraft and ground-based instruments in search for lightning activity in the atmospheres of
planets in the solar system, and put them in context of previous research. Since the comprehensive book on planetary atmospheric electricity compiled by Leblanc et al. (2008), advances in remote sensing technology and telescopic optics enable detection of additional and
new electromagnetic and optical emissions, respectively. Orbiting spacecraft such as Mars Express, Venus Express and Cassini yield new
results, and we highlight the giant storm on Saturn of 2010/2011 that was probably the single most powerful thunderstorm ever observed
in the solar system. We also describe theoretical models, laboratory spark experiments simulating conditions in planetary mixtures and
map open issues.
Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Lightning; Electromagnetic radiation; Sprites; Planetary atmospheres; Discharge processes
1. Introduction
Lightning on Earth and other planets are markers of
dynamical and microphysical processes taking place within
condensate clouds, from charging to breakdown, to propagation and emission of radiation in various bands of the
electromagnetic spectrum. While the specific nature of the
charging processes taking place within the particular
clouds residing in the different planetary atmospheres
may differ, the over-all physics is similar, and entails the
generation of large scale electric fields strong enough to
surpass the local breakdown value, leading to generation
of streamers, leaders and finally discharge channels. The
importance of electrical processes and lightning is obvious,
due to the ability of the electrical current to break molecular bonds and induce various chemical reactions within the
discharge channel. This leads to the emergence of new,
non-equilibrium, chemical compounds, some of which
⇑ Address: Development and Learning Technologies, The Open
University of Israel, 108 Ravutzki Street, Raanana 43107, Israel. Tel.:
+972 9 7781044, mobile: +972 525415091; fax: +972 9 7781046.
E-mail addresses: [email protected], [email protected].
may be of biological importance (Miller, 1957). Several
review papers of planetary atmospheric electricity have
already been published (Levin et al., 1983; Rinnert, 1985;
Desch et al., 2002; Aplin, 2006), the most updated and
extensive one as a book in the framework of the ISSI Space
Science Series (Leblanc et al., 2008). That book gave a
thorough review of the myriad electrical processes occurring within the atmospheres of solar system planets, conducive to the generation and maintenance of a global
planetary electrical circuit, where conditions allow it to
exist. The present paper aims to present recent updates
on several key discoveries made since the publication of
the book, with specific focus on results from spacecraft
observations, laboratory results and numerical models.
Although the Moon and Mercury have interesting surface
processes involving charging and electrical dust lifting
(Renno and Kok, 2008), we adhere to a somewhat traditional approach of describing the existing knowledge
according to atmosphere-bearing planets (including Titan),
focusing on processes above the surface and below the
planets’ ionospheres. Thus we shall strive to describe the
state of knowledge on lightning and other discharge phenomena occurring in the atmosphere and up to the base
0273-1177/$36.00 Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.asr.2012.04.013
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Y. Yair / Advances in Space Research 50 (2012) 293–310
of the ionosphere, excluding (for example) planetary aurorae and airglow phenomena. We will focus on the newest
results obtained since the publication of the ISSI book
(which was written in 2007) but will strive to place these
results in the context of earlier findings which were
reviewed thoroughly by Yair et al. (2008). Finally, we will
point out the needed research to answer existing issues and
suggest new avenues for progress.
2. Earth
At any given moment, there are approximately 1800
active thunderstorms around the planet with a flash rate
estimated 100 s 1. That “classical” view had been challenged recently by Price et al. (2011) who computed only
750 storms at any given time, based on data from the
World Wide Lightning Location Network (WWLLN;
http:www.wwlln.net) and a lightning clustering scheme.
Based on satellite optical data, global thunderstorms produce an average flash rate of 44 ± 5 s 1 (Christian
et al., 2003), mainly above the continents in the tropical
regions, and diminishing towards higher latitudes. There
is little lightning activity above latitude 60°, and none at
all in the Polar Regions. Lightning discharges emit a broad
electromagnetic spectrum, which at specific frequencies
known as Schumann Resonances (SR, and see: Pechony
and Price, 2004), are capped in the surface-ionosphere
waveguide propagating from the source point around the
globe with very little attenuation. Monitoring these frequencies by ground networks allows climatic studies of
lightning distributions (Williams, 1992; Williams et al.,
2000; Williams, 2005). In a new study, Simões et al.
(2011) showed that under some conditions, SR waves can
penetrate the ionosphere to be detected by an orbiting
satellite. They used the C/NOFS satellite and identified
night-time ELF signals in the predicted SR frequencies at
altitudes between 400 and 850 km suggesting that the ionosphere is a leaky cavity. This fact allows detection of global planetary lightning activity in the extremely low
frequency range, traditionally considered to be effective
only for in-situ ground-based measurements. The importance of this new result for future planetary probes is
evident.
This flash rate is considered to act as a generator that
maintains a global atmospheric electrical circuit (GEC)
with an ionospheric potential of 250 kV producing a downward pointing electrical field with an average strength of
130 V m 1 at the surface, which shows a marked diurnal
cycle (Williams, 2009; Liu et al., 2010). Had these thunderstorms ceased to generate lightning, the potential difference
between the Earth and the ionosphere would have been discharged in several minutes by the constant flow of atmospheric ions, with an average current density of
2 pA m 2. The inter-tropical convergence zone (ITCZ)
migrates slightly with the seasons but exhibits a daily cycle
of activity over the three major “chimneys” (Williams,
2005) in Africa, South-East Asia (sometimes referred to
as “The maritime continent”) and South America. Africa
was shown to dominate the planetary lightning activity
by a factor 2.8 relative to the Amazon basin in South
America (“The green ocean”; Williams and Satori, 2004).
The intensity of lightning activity in deep continental convection is 7–10 times stronger compared to maritime deep
convective storms (Ávila et al., 2010). There is also a climatic pattern in global lightning related to the ENSO
cycles (El-Nino Southern Oscillation; Satori et al., 2009).
Moist convection produces the clouds holding the
majority of lightning on Earth: deep cumulonimbus clouds
(Zipser, 1994), which develop under unstable conditions
and attain large vertical extent, of the order of 12–17 km
in summer and 4–8 km in winter. There are almost no
reports on lightning in stratiform clouds, and very few rare
events of thundersnow occurrence (heavy snow accompanied by lightning, hinting at different electrification
conditions than usual; Crowe et al., 2006). Mesoscale convective systems (MCS) and complexes (MCC) excel in
lightning production (Mattos and Machado, 2009), as well
as hurricanes (Cecil et al., 2002) and other types of tropical
storms. The microphysical processes responsible for the
generation of electric fields within these clouds rely on
the coexistence of super-cooled liquid water and graupel
particles, which frequently collide with ice particles. This
non-inductive process (the interested reader is referred to
the extensive review by Saunders (2008)) is extremely
effective in separating charge, such that it is capable of
re-generating strong enough electrical fields within the
thunderstorm to support huge flash rates, sometimes of
the order of 20 s 1 (Fig. 1; Zipser et al., 2006).
In rare occasions lightning discharges appear within volcanic plumes, where complex charging processes occur,
unlike those in regular clouds (McNutt and Williams,
2010). Although the volume of the plume directly exposed
to lightning is less than 10 4, it has significant chemical
effects on the on radical production by reactions with magmatic gases, resulting in production of fixed nitrogen species (Martin and Ilyinskaya, 2011).
Clearly, lightning has significant chemical effects on the
surrounding air and is the largest natural source for the
production of nitrogen oxides (Lightning-produced Nitrogen Oxides, or LtNOx) in the troposphere. General assessments of the global amount of NOX produced by lightning
were recently presented by Beirle et al. (2010) and Ott et al.
(2010), and this topic is considered of importance critical to
the mapping NOx sources in climate models. This problem
requires the knowledge of 3 factors: the amount of NOx
production per joule of lightning energy (known as PE or
production efficiency), the average global value of energy
per flash, and the global lightning flash rate. In a seminal
paper, Price et al. (1997) presented the first global and
seasonal distributions of LtNOX based on multiplying the
amount of NOx produced per joule of energy with the
amount of lightning energy. They used approximated
global lightning densities derived from OTD (Optical Transient Detector on board the Microlab-1 satellite, launched
Y. Yair / Advances in Space Research 50 (2012) 293–310
295
Fig. 1. Seasonal variation of the most active thunderstorms on Earth, based on satellite observations; the colors represent different three-month periods
(DJF stands for December, January and February, and so forth). The upper panel shows only clouds where the height of the 40 dBZ reflectivity exceeds
14.2 km. Lower panel shows the location of the strongest flash rates, above 126.7 per minute. [Taken from: Zipser et al., 2006, American Meteorological
Society; Used with permission].
in 1995) and computed an annual mean production rate of
12.2 (5–20) Tg N/yr. More directly, Höller et al. (1999)
measured NOX concentrations during the field experiment
LINOX performed in southern Germany in July 1996.
NOX concentrations in thunderstorm anvils were found
to typically range from 1 to 4 ppb. Their estimation, using
a global lightning frequency of 100 s 1 and a mean channel
length of 5 km, yields an annual global production of
5 Tg N/yr. A new study by Ott et al. (2010) found that
on average, a single flash in mid-latitudes and in
subtropical thunderstorms turned 7 kgs of nitrogen into
chemically reactive NOX. Indeed, the fact that lightning
leaves “fingerprints” in the chemistry of the atmosphere,
which can be detected remotely by spacecraft and
ground-based instruments, was utilized in the early
speculations on the existence of lightning on Jupiter
(Bar-Nun, 1975) and is considered a marker for presentday searches for planetary lightning (to be reviewed in
the following sections).
Despite being limited to the troposphere, lightning has
marked effects high above the cloud tops, through the operation of the EMP (electromagnetic pulse) and Quasi-electrostatic (QE) fields, which induce local and temporary
breakdown at stratospheric and mesospheric altitudes
between 45 and 95 km. The appearance of transient luminous events (TLEs, such as Sprites, Elves, Halos, Blue Jets
etc.; Lyons et al., 2000) is complex and spans large volumes
of air above the thunderstorm, where characteristic emissions of different excited nitrogen bands are observed
(Pasko et al., 2012). The vast majority of Sprites and Halos
are produced by positive cloud-to ground flashes, with only
few and rare events by negative or intra-cloud flashes. Elves
do not show any polarity preference. The global rate of
TLEs was deduced from satellite observations (Chen
et al., 2008) and found to be 3.3 per minute for Elves,
0.5 min 1 for Sprites and 0.39 min 1 for Halos. Occasionally, storms can be prolific sprite producers with a rate
far exceeding the global average, producing a concentrated
local effect (São Sabbas et al., 2010). Although lasting for
several milliseconds only, these elusive and beautiful phenomena have chemical effects in the mesosphere which
are discernable and may last for longer time scales than
the optical phase (Arnone et al., 2008; Arnone et al.,
2009). This fact suggests an indirect method to discern
planetary lightning activity when it is hidden by overlaying
cloud layers by looking for residual chemical effects of their
induced TLEs (Yair et al., 2009).
In recent years, the detection of Terrestrial Gamma-Ray
Flashes (TGFs) had attracted considerable attention, and
although they were initially thought to be related to
Sprites, it now seems that they are caused by intra-cloud
lightning discharge processes (Stanley et al., 2006; Carlson
et al., 2010; Smith et al., 2010; Lu et al., 2011). TGFs are
typically short (1 ms) bursts of gamma-ray photons with
a broad energy spectrum between 20 keV and 40 MeV (or
higher, Marisaldi et al., 2010), with a total gamma ray
energy of 1–10 kJ. They occur within 3 ms or less from
electrical activity in thunderstorms, and the source altitude
was found to be at 15–20 km (Dwyer and Smith, 2005).
Satellite observations pick up the signal only within a cone
of 30° from the source, and show a relative density of
TGFs in the equatorial regions, in vicinity to the large
“chimneys” where convective clouds reach large vertical
dimensions due to the higher tropospheric height (postulated to be easier for TGF propagation). The physical
mechanism responsible for the generation of TGFs is
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Y. Yair / Advances in Space Research 50 (2012) 293–310
thought to be the acceleration of ambient electrons to relativistic energies by the strong electric fields in the thunderstorm such that they overcome the frictional losses induced
by the dense air at lower altitudes, producing runaway avalanche as they propagate to higher altitudes (the so-called
Relativistic Runaway Electron Avalanche (RREA) process), and emitting gamma-rays via Bremsstrahlung.
Dwyer (2008) used data from the Compton Gamma-Ray
Observatory (CGRO/BATSE) to suggest that electron
and positron beams, resulting from Compton scattering
and pair-production by TGFs, propagate along magnetic
field lines and populate the inner magnetosphere. They
concluded that 17% of events previously identified as TGFs
are in reality such electron and positron beams. More
recently, Briggs et al. (2011) reported the detection of positrons in electron beams of bright TGFs. They analyzed the
spectrum of 3 events observed by the Fermi GBM
(Gamma-ray Burst Monitor) satellite and found strong
511 keV annihilation lines. Their interpretation suggests
that pair-production accompanies lightning flashes and
that TGFs, although rare, are injecting electron–positron
beams into Earth’s atmosphere. Recent airborne measurements conducted by Smith et al. (2011) show that only 1
out of 1213 lightning discharges were associated with a
TGF event, suggesting the TGF to flash ratio to be on
the order of 10 2–10 3. Predicting which thunderstorm will
be effective in producing TGFs is still beyond our present
understanding.
3. Venus
Of the four terrestrial planets, Venus has the most
intriguing atmosphere that exhibits complex dynamics
and chemistry, as it super-rotates 60 times faster with
respect to the solid planet and exhibits a Hadley-cell global
circulation between the equatorial region and 45° latitude. It is totally covered by thick layers of stratiform
clouds that prohibit direct optical observations of the planet’s surface. The clouds of Venus are composed of small
droplets or ice crystals of sulfuric acid, and reside in three
distinct layers between 45 and 75 km above the surface.
Most of these cloud particles are believed to by charged
by GCR-induced ions (Michael et al., 2009). McGouldric
et al. (2011) compared the 3-layer cloud system on Venus
to the marine-stratocumulus system on Earth (Fig. 2).
A thick H2SO4 haze lies above the cloud tops. Above the
clouds at altitudes 80–110 km, observations reveal a permanent nightglow of O2 that emits in the visible and
infra-red (Slanger et al., 2006; Garcı́a Muñoz et al., 2009).
There had been several spacecraft that landed on Venus
and others that observed it from orbit or during fly-by during a gravity-assist maneuver towards other targets, the
most recent one being ESA’s Venus Express that entered
Venus orbit in April 2006 (the unfortunate malfunction
of the Japanese spacecraft Akatsuki in December 2010 prevented data from a newer platform). From early on there
was an expectation that the clouds of Venus should harbor
lightning activity, partly due to the similarity to Earth (in
size, mass and composition). Indeed, there were several
separate indications for the existence of electrical activity
in the clouds of Venus, obtained by instruments on-board
the Pioneer Venus Orbiter (PVO), the Venera orbiters
and balloon penetrators and from ground based telescopic
observations (see appropriate references in Yair et al.,
2008). A comparison between lightning on Venus and
Earth was recently given by Russell et al. (2011) suggesting
that the evidence for lightning is “overwhelming” but
points to the existence of a theoretical controversy, mainly
on the ability of the clouds to support a robust electrical
activity on a planetary scale. Still, there were some conflicting observational results, for example from instruments onboard the Galileo and Cassini spacecraft and from
repeated attempts by large-mirror ground telescopes to
repeat the Hansell et al. (1995) observations (D. Sentman,
personal communication) and so the issue is not resolved.
For the sake of brevity this section will not re-describe
the already presented evidence, and below we shall review
the latest results obtained by spacecraft and ground-based
Fig. 2. A schematic comparison of the similarities and differences between the Venus cloud system and terrestrial marine stratocumulus clouds. [Taken
from: McGouldric et al., 2011. Used with permission].
Y. Yair / Advances in Space Research 50 (2012) 293–310
telescopic and spectroscopic observations. The key questions concerning the nature of lightning activity in Venus
still remain open: what is the charging mechanism operating within the clouds? How effective it is when convective
motions are limited? Are lightning discharge characteristics
in terms of current flow, electromagnetic signature, energy
deposition and optical emission similar to terrestrial
flashes? If the global rate is indeed comparable to Earth,
why is there such a lack of optical evidence?
3.1. Spacecraft observations
The most consistent evidence for electrical activity in
Venus was presented by Russell et al. (2008, 2010) based
on data obtained by the fluxgate magnetometer on board
Venus Express (VEX) near the spacecraft periapsis, which
is close to the planet’s North Pole. That instrument samples the Venusian magnetic field with rates up to 128 Hz
and has detected repeated bursts with a persistent righthanded circular polarization. The bursts exhibit high
amplitudes up to 1 nT peak to peak (much stronger than
the 30 pT observed on Earth) and are interpreted as whistler-mode waves, propagating parallel to the magnetic field
lines and reaching the spacecraft when the local magnetic
field dips into the ionosphere at an angle greater than
17°. The inferred rate of electric discharges in the Venusian
atmosphere, based on the 2006 data, was about 18 strokes
per second or 20% of the terrestrial flash rate (conservatively postulated to be 100 s 1). It should be noted that
the extrapolated planetary rate assumes an equal activity
everywhere, a fact that may push the computed rate to
an unreasonably high value. Russell et al. (2010) showed
that the burst rate exhibits a complex dependence on height
above the surface and latitude. The maximum rate was
detected at polar latitudes, where the spacecraft is closest
to the planet.
In 2009 the VEX orbit was lowered to 200 km at
periapsis, yielding new data. Based on the 2009 results,
Russell et al. (2011) reported a diurnal pattern with a
maximum burst rate between 08 and 12 local time of
0.06 s 1, suggesting an increased dayside activity compared to the night side (that was sampled by PVO). They
show a control of the propagating whistler-mode waves by
the magnetic field of the ionosphere, and compute equal
source strengths for Venus and Earth. The burst rate at
low altitudes shows increase with altitude from 0.8 per
1000 s below 220 km to more than 1.8 between 280 and
310 km. While it is certain that the VEX magnetometer
detects natural signals, there is still an unexplained
absence of optical emissions that should accompany the
detected bursts. If intracloud flashes do occur within the
layer clouds of Venus, and if they are indeed produced
by processes similar to terrestrial flashes, then light emitted by the channels of these discharges should emanate
through the relatively dilute clouds and reach the onboard cameras of VEX. Still, not a single image from
the nightside of Venus was obtained showing clearly and
297
unambiguously such emissions. Williams et al. (1982) used
Monte-Carlo computation to calculate the fraction of
lightning light that should emanate through the Venusian
cloud layers into space. They found that for lightning
within or just below the lower cloud deck at around
55 km, the fraction of photons escaping to space should
be between 0.1 and 0.4. For flashes at lower altitudes,
for example near the surface, only 10 4 of the emitted
photons would escape. The numbers were later corrected
by a factor 2 (Williams and Thomason, 1983). The lack
of visible lightning may be partially understood based
on the laboratory experiments simulating Venus lightning
discharges by Robledo-Martinez et al. (2011), that investigated the impact of space charge on the discharge and
concluded that if the clouds on Venus are negatively
charged, lightning generation will be hampered.
If lightning do occur and are obscured by clouds or are
too dim, one can perhaps observe their induced fingerprint
by looking for Sprites, as suggested by Yair et al. (2009)
who calculated the potential of lightning discharges to generate Sprites on other planets. For Venus, an intra-cloud
flash between the two lower-most cloud layers with a
charge-moment change of 100 C 5 km = 500 C km will
trigger a sprite at 90 km above ground, 20 km above
the tops of the uppermost cloud layer. The morphology
of streamers and the expected emissions of these hypothetical Sprites were studied in laboratory experiments by
Dubrovin et al. (2010). Fig. 3 shows a comparison between
terrestrial streamers and their equivalent in CO2–N2 and
H2–He mixtures (representing Venus and Jupiter–Saturn).
The expected emission for Venus Sprites is found in the
UV range between 300 and 400 nm, but the overall radiance is much dimmer compared to terrestrial Sprites. They
have a very feathery look, hinting at recurrent splitting of
the streamer head as they propagate.
Conceivably, if the observation methodology from orbit
that was developed by Yair et al. (2004) is used for Venus
with the appropriate space-borne camera, Sprites could be
detected above the night-side limb (Takahashi et al., 2008).
A recent paper by Oberst et al. (2011) shows a new camera
design specialized for tracking transient luminous events in
planetary atmospheres such as lightning, Sprites, auroras
and meteors. Future mission for studying lightning from
orbit can utilize this technology.
3.2. Ground-based telescopic observations
Indirect support for the existence of lightning activity
may come from observations of chemical compounds that
exceed their expected equilibrium concentrations. Krasnopolsky (2006) suggested that the observed enhancement of
NO concentration below 60 km is a result of lightning
activity. This altitude range would match well with the
lower cloud level and the presence of solid sulfuric acid
particles (ice), which are considered a key ingredient for
charge separation (as discussed above). However, when
assuming an identical energy output of 109 J per flash,
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Y. Yair / Advances in Space Research 50 (2012) 293–310
Fig. 3. Different shapes and luminosity for various gas mixtures, representing the atmospheres of Earth, Venus and Jupiter. The measured brightness is
indicated by the multiplication factor MF which is a measure of the gain of the imaging system (Nijdam et al., 2009). An image with a high MF value, is in
reality much dimmer than an image with similar coloring, but with a lower MF value. The MF values a normalized such that for the brightest image
presented here MF = 1 [images courtesy D. Dubrovin].
the derived rate needed to account for the observed concentrations is 90 s 1, double that of the Earth’s, and as mentioned above, cannot be supported by any known charge
separation mechanism.
In an attempt to replicate the Hansell et al. (1995) telescopic observations, Garcı́a Muñoz et al. (2011) utilized
narrowband fast imaging of the Venus disk from an array
of ground-based telescopes. The search targeted the oxygen
emission line at 777 nm which should be a prominent line
in lightning (as on Earth). Two sites (Calar Alto and
Observatorio del Teide), and three instruments (AstraLux,
FastCam and Wide FastCam) were used for observations
during November 2010–January 2011. The analysis of the
night-side imaging of Venus showed no signal of optical
emissions. This negative result undermines the high flashrate that is suggested by Russell et al. (2010) and suggests
that lightning on Venus may be rare, or else too weak to
produce significant optical signatures that can be observed
from Earth, even with large-mirror telescopes and fastimaging techniques.
4. Mars
The planet Mars today is mostly an arid, cold and dry
desert, not very different from terrestrial polar deserts. Its
thin dynamical atmosphere and active boundary layer are
conducive to erosion by eolian processes which include
sand particle saltation and dust lifting. Dust storms are
very frequent and range from local (>102 km2) to regional
(>1.6 106 km2) and sometimes global scales. For example, Cantor et al. (2001) reported 783 storms in just
9 months during the 1999 dust season, based on Mars Global Surveyor images. Observations show that dust storms
occur at the two polar cap edges, at the base of high elevation regions in the northern hemisphere, near the polar
hood during northern fall, and at mid-latitudes in both
hemispheres. Global dust storms are rarer and may last
for months and encircle the entire planet, but their occurrence is not fully understood. Haberle (1986) used Viking
data and suggested that the inter-annual variability may
be a result of a competition between circulations in the
Y. Yair / Advances in Space Research 50 (2012) 293–310
northern and southern hemispheres, or is the result of
cycling of dust between hemispheres and related to the
properties of the dust storms themselves. A climatological
overview of the starting areas for large storms was presented by Newman et al. (2002). The last largest planetscale storm was in 2001 and was shown to be moving
and sedimenting dust particles across hemispheres and far
from their source regions (Cantor, 2007; Martinez-Alvarado et al., 2009).
Much smaller and more frequent phenomena are dust
devils, which are local convective events that occur daily
in dry and arid regions on Earth and on Mars (Fig. 4).
These swirling gusts of wind form in conditions of high
instability, due to strong solar heating of the surface and
the subsequent convective rise of hot air accompanied by
the lowering of colder air. The column of rising hot air is
surrounded by a vortex, which draws surface air into the
center in spiraling trajectories (Renno et al., 1998). If the
surface is sandy and dry, and the devil is sufficiently developed to lift dust particles, the convergence into the vortex
raises dust particles vertically, which makes it visible. Smaller grains rise higher and fall down slower than larger ones,
due to the smaller effect of gravity versus air drag. The
vertical dimension of most dust devils is at least 5 times
larger than their horizontal diameter but they can be
extremely tall and thin or wider than they are tall. Martian
dust devils were first detected from Viking orbiter images
(Thomas and Gierasch, 1985) and later deduced based on
their ground tracks (Grant and Schultz, 1987). Recent
missions offer excellent close-range videos of such dust
devils, as they travel close to the Spirit exploration
rover (http://marsrovers.jpl.nasa.gov/gallery/press/spirit/
20050819a.html). These small vortex-like dust columns
are larger than their terrestrial counterparts, extending to
1–6 km in height with widths of hundreds of meters
(compared to 1–15 m on Earth). Comparisons between
Martian and terrestrial saltation and dust lifting processes
was conducted by Rennó et al. (2000), Balme and Greeley
(2006) and Kok (2010).
Dust devil activity on Mars is a daytime affair with clear
diurnal activity, and they never occur at night. Ringrose
et al. (2003) showed that activity peaks around 13:00 local
time, when solar insolation is maximal. The fact that Mars
299
harbors a continuous haze layer suggests the existence of a
mechanism to replenish dust amounts against the sedimentation, but it is still unclear if dust devils are frequent
enough to supply this amount of airborne particles (Balme
and Greeley, 2006).
4.1. Charging of dust storms and devils
Small airborne dust particles collide and rub against
other suspended particles or against the surface in a dust
cloud. Upon contact, the particles redistribute charge
between them, and once separated, one of them is left with
extra negative charge and the other, positive. This process
if often named “tribo-electric charging” (Yair, 2008). The
efficiency of the process depends on the area of contact,
duration, conductivity of both particles, and their shape.
Particles composed of materials which have a higher surface work function will tend to acquire electrons from
those who are composed of materials with lower work
functions, thus charging the two particles negatively and
positively, accordingly (Sharma et al., 2008). Triboelectric
charging may be the dominant mechanism for charging
of volcanic ash clouds, which are known to exhibit vigorous electrical activity (James et al., 2008). While triboelectric charging is considered the best candidate for
electrification of dust on Mars, other processes may be of
potential importance. For example, charging due to ion
attachment is prevalent on Earth and most particles suspended in the atmosphere carry charge. Yair and Levin
(1989) showed that due to differences in small-ion mobilities, a polydispered size distribution will exhibit opposite
polarities for small and large particles. Different settling
velocities in vertically lifted dust layers may lead to the generation of an electric field that can potentially discharge. A
different charging process, not relying on tribo-electricity
but rather similar to the collision-based processes which
are believed to be dominant in tropospheric thunderstorms
(see review by Yair (2008)) may occur in the Martian polar
regions where sand and ice particles collide frequently,
especially during dust storms occurring in the seasonal
thawing-freezing cycle onset. Jayaratne (1991) showed that
non-sublimating ice particles are charged positively by
impacts of sand grains. The charge is transferred from
Fig. 4. (Left) a diffuse large dust devil in the northern Negev desert, Israel (photo by Shy Halatzi). (Center) a Martian dust devil as photographed by
NASA’s Spirit robotic rover. A second, more distant devil is seen to the right of it (NASA). (Right) a dust devil and its shadow from above, taken by the
Hi-RISE Camera aboard NASA’s Mars Reconnaissance Orbiter (NASA).
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Y. Yair / Advances in Space Research 50 (2012) 293–310
the ice surface to the dust grains, which carry a net negative
charge. This process may complement the triboelectric
charging that would be dominant in drier regions.
4.1.1. Laboratory experiments
In early laboratory experiments, Eden and Vonnegut
(1973) observed agitated 50 g of dried well-dispersed sand
particles in a low-pressure 1-liter glass flask with a 10 mb
atmosphere of pure CO2. They reported (based on eyesight,
after being dark-adapted) several types of brief discharges
occurring inside: (a) bright small sparks (b) longer, several
cm long and 1 cm wide sparks and (c) faint extensive glow.
Emissions were in the red and blue parts of the spectrum.
They repeated the experiment with an external potential
of 1200 V and noted that the discharges occurred without
the needed dust agitation. Eden and Vonnegut (1973) state
that “this experiment is not intended to be an analog of the
Martian atmosphere”, and concluded that the average
charge carried by the particles was 104 e. Conceptually
similar experiments were conducted by Gross et al.
(2001), Sternovsky et al. (2002) and Krauss et al. (2003),
to study the relative importance of particle size-distribution
and horizontal mixing. “Planetary” dust particles were
dropped off a thin metal disk with a small central hole that
was agitated by an electromagnet. A Faraday cup was
attached to a sensitive electrometer and the height of the
output pulse indicated the charge on the grain. A Martian
dust mixture of 100 lm JSC-Mars-1 particles and 53 lm
glass microballoons (beads) was used. At low pressures
(1–8 mb), the electrical field generated through triboelectric
charging between the dust and glass particles suffices to
surpass the breakdown threshold and a discharge occurs,
which was visually observed and electrically measured.
Krauss et al. (2003) reported that above a threshold wind
of 2.0 m s 1, tens of discharges per second were detected.
Maximum discharge rate of 14–15 per second was achieved
at lower pressures around 1–2 mb, and a mixture of small
and large particles was required in order to produce a significant number of discharges. However, a recent study by
Goodman et al. (2011) that attempted to replicate this setting showed that the discharge could be achieved only
when the Mars-sand particles were mixed with glass
micro-balloons. In pure Mars-dust experiments there was
no discharge, a fact that suggests that the earlier reported
discharges by Krauss et al. (2003) and Eden and Vonnegut
(1973) may have been wall-effects, caused by discharging of
the particles due to collisions with the wall of the flask.
Kok and Renno (2006) applied electric fields to samples
of desert dust in a glass vial and showed that electric forces
enhance the emission rate of particles from the surface,
suggesting a reduced critical wind velocity for the uplift
of dust from the ground. The results imply a possible positive feedback mechanism by which dust-devil-induced Efields will facilitate the uplifting of additional mineral dust,
which in turn would serve to amplify the charging within
the devil and strengthen Ez (the vertical component of
the electric field).
4.1.2. Field observations
A comprehensive observational experiment carried out
to examine the electrification of dust devils, was MATADOR (Martian ATmosphere and Dust in the Optical and
Radio) which was conducted in Nevada and Arizona.
(Renno et al., 2004; Farrell et al., 2003, 2004). The dust
devils were observed simultaneously by optical, electric,
magnetic and lidar equipment while continuously recording meteorological observations. The Nevada/Arizona dust
devils were shown to produce heat and dust fluxes 2–5
orders of magnitude larger than the ambient background
values. The charge separation within terrestrial dust devils
produces strong electric fields that might play a significant
role in dust sourcing. Results showed strong electric fields
in the dust devils (10–30 m wide) with values close to the
breakdown field of 20 kV/m. The total electrostatic
energy in such localized storms can exceed 10 4 J m 3. Farrell et al. (2004) state that “The dust devil behaves as a natural electric generator, tribo-charging and separating grains
on the basis of their mass, generating voltages possibly in
excess of 0.5 MV”, and thus it is a potential source for
arc discharges to surface objects. More recently, Sow
et al. (2011) conducted field measurements in Niger, where
mesoscale convective systems often generate “walls” of
dust that move and lift large amounts of soil and dust particles. They measured the polarity of the soil before and
after the passage of such storms and reported that small
particles are positively charged, while the larger ones are
negative. Although Sow et al. (2011) measured the charge
on the surface (and not on airborne particles) this finding
contradicts earlier results and the authors explain that the
finer fraction remains aloft and may be negatively charged,
as expected.
4.2. Models of electrical discharges on Mars
From terrestrial analogy dust on Mars will be charged
to some degree, and some form of discharge can conceivably occur there. Mills (1977) suggested that the “cleanliness” of Martian soils, as evident from the lack of any
organic or carbonaceous materials is a results of glow discharges occurring within the lifted dust. Farrell et al. (1999)
suggested different types of discharges to be possible on
Mars. The larger is on the macroscopic level, a filamentary
discharge similar to lightning on Earth, between oppositely
charged centers within the dust storm. A second type is
“glow”-like atmospheric discharge of individual dust
grains. Since particles with larger radii receive an average
net positive charge and smaller radii a negative charge,
the uneven gravity and air resistance forces acting on these
particles will result in mass stratification, which inevitably
leads to charge separation between two regions of the dust
cloud, and an electric field. If the electric field is stronger
than the atmospheric breakdown field, discharge may
occur. Farrell et al. (1999) showed that there is a maximum
possible charge density within the dust cloud, beyond
which an induced charge corona will surround it,
Y. Yair / Advances in Space Research 50 (2012) 293–310
neutralizing/shielding the electric field, limiting the electric
fields. Coronal currents may discharge individual grains of
dust and therefore create a light “glow” around the dust
cloud. The glow discharge is expected to produce HF fields
of the scale of 0.01 V m 1 for intense dust devils, at a distance of 50 km. Given the possibility that the Martian
ground has low conductivity while the dust devils/storms
have high tops, sprite-like discharges may occur. This
was suggested by Farrell et al. (1999) who concluded that
even for the largest charges, discharges will preferably happen inside the dust cloud but not above it. Recent work by
Barth et al. (2011) showed that fields 10 mV/m are generated for realistic values of dust concentrations, and that
only increasing the particle number by a factor 10 leads
to E kV/m, still below discharge levels. The field is
restricted to very shallow altitudes and diminishes above
2 km.
4.3. Radiation from electrical activity in dust storms
Often when particles collide some electrical charge is
transferred during impact. If significant ensembles of
charged particles are spatially separated, large scale discharges may occur with ensuing electromagnetic radiation.
Acceleration of charged particles and microdischarges
between colliding sand and dust grains emit electromagnetic radiation. The signals can be remotely-sensed from
large ranges and serve to deduce the source intensity and
properties. For Mars, Cummer and Farrell (1999) calculated the expected emission and propagation of very low
frequency (VLF; 3–30 kHz) and extremely low frequency
(ELF; 3–300 Hz) electromagnetic radiation from a Martian
dust storm. They assumed (somewhat unrealistically) that
the discharge current was similar to that in a terrestrial
lightning discharge, and computed radio atmospheric spectra and waveforms for a variety of discharge orientations.
The ELF and VLF waves propagating in the cavity were
shown to be different compared to terrestrial sferics, and
can be used to study the Martian surface and sub-surface
properties. Indeed, Farrell et al. (2004) measured radio
emission by terrestrial dust devils in the ULF/ELF
(ULF: 300 Hz–3 kHz) ranges. Renno et al (2003) modeled
such collision discharges as micro flat-plate capacitor, and
calculated the total emission expected. Results showed that
dust devils produce a negligible, undetectable effect on the
planet’s observed brightness temperature, whereas large
dust storms may produce a measureable temperature
change of 10 K. According to Renno et al. (2003), the
theory is supported by microwave observations performed
in the late 1970s which showed a significant increase in
microwave emissions from areas of intense dust storm
activity. Houser et al. (2003) performed AC magnetic measurements during the passage of an intense dust devil 10m wide, several hundred-meters high near and directly
above a mobile station. The devil-related magnetic activity
appeared in two forms: impulsive ELF static discharges to
the instrument as the sensor became immersed directly into
301
the devil’s electrified dust and ULF continuous emissions
that were sensed remotely as the storm approached and
receded from the station. These measurements show that
individual dust grains within the devil are charged, and that
when they are transported in bulk, the collisions and resultant discharge processes may generate an extended, coherent radiation source.
Attempts to remotely-sense the electromagnetic signature of the postulated electrical activity on Mars have been
attempted from Mars orbit and from Earth-based instruments. Ruf et al. (2009) conducted daily 5-h measurements
toward Mars between 22 May and 16 June 2006 using a
new instrument on the DSN (Deep Space Network)
radio-telescope, and reported the detection of non-thermal
radiation for a few hours that coincided with the occurrence of a deep dust storm on Mars. The spectrum of the
non-thermal radiation showed significant peaks around
predicted values of the lowest three modes of the Martian
Schumann Resonance. Since SR radiation is formed by discharges exciting the surface-ionosphere cavity, Ruf et al.
(2009) interpreted their observations as indicative for the
occurrence of lightning within the dust storm. This interpretation was challenged by Anderson et al. (2012) who
used the Allen Telescope Array for a similar survey totaling
30 h between March 9th and June 2nd, 2010. They concluded that the resonances detected in the power spectrum
of the kurtosis of non-thermal microwave radiation are due
to narrowband radio frequency interferences, and not due
to electrostatic discharges within Martian dust storms. It
should be noted that during that period there was no
large-scale dust storms on Mars, only smaller events, a fact
that may explain the difference from the Ruf et al. (2009)
result.
Additionally, Gurnett et al. (2010) used the Mars
Express MARSIS (Mars Advanced Radar for Subsurface
and Ionosphere Sounding) instrument to look for impulsive radio signals from lightning discharges of Martian dust
storms and reported negative results. The search covered
5 years of data and spanned altitudes from 275 km to
1400 km and frequencies from 4.0 to 5.5 MHz, with a time
resolution of 91.4 ls and a detection threshold of
2.8 10 18 Watts m 2 Hz 1. At comparable altitudes the
intensity of terrestrial lightning is several orders of magnitude above this threshold. Although two major dust storms
and many small storms occurred during the search period,
no credible detections of radio signals from lightning were
observed.
While dust storms and dust devils on Earth are known
to be electrified (Farrell et al., 2004; Houser et al., 2003),
there are no direct evidence that their counterparts on
Mars are indeed charged. By analogy, they should be,
and this would have important implications for lander
operations, orbiter-lander communication and in the
future, for human missions to the planet. It is therefore
highly important to demonstrate the adverse impacts of
dust storms on the propagation of radio waves and on
the performance of radio equipment. Additionally, dust
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Y. Yair / Advances in Space Research 50 (2012) 293–310
attachment due to electrostatic forces to optical lenses and
to solar-panels may degrade performance and adversely
impact mission duration and capability. Obviously, conducting measurements and evaluating the properties of
electrified dust storms on Earth can serve as benchmark
for evaluating the processes occurring on Mars. Apart
from the MATADOR campaigns mentioned, there were
no parallel efforts to conduct field campaigns which simultaneously observe dust particles and their electrical effects.
Another aspect of electrical activity within dust events on
the surface of Mars is the chemical effects, with potential
importance for astrobiology (Kok and Renno, 2009;
Jackson et al., 2010; Delory et al., 2006). The presence of
electrified dust clouds is bound to have important implications for the existence and properties of a global electrical
circuit on Mars (Aplin, 2006), as they are considered to be
the generators (like thunderstorms on Earth). If the actual
charging of dust clouds is far less than postulated, then the
existence of such a circuit cannot be easily maintained.
Balme and Greeley (2006) state several major challenges
still open in assessing the role of dust devils on Mars and
Earth, and emphasize the importance of additional
ground-based measurements of critical meteorological
parameters, in order to understand the role of atmospheric
conditions in dust devil formation. No doubt that these
requirements apply to larger dust storms as well. Furthermore, models of dust devil charging do not consider the
effects of free electrons and atmospheric ions on Mars
and rely solely on tribolelectric charging. This assumption
can lead to underestimation of the resultant electrical field
and to lower induced radiation in various bands due to the
potential discharges.
5. Titan
Titan was always considered a viable location for lightning activity, based on the general similarities to the pristine Earth: a dense nitrogen-based atmosphere (1.5 bars
at the surface), with frequent methane and ethane clouds
and shrouded by a thick organic haze. The implication of
electrical activity in Titan is of special interest due to its
exobiological implications (Sagan et al., 1993). Although
convective motions were thought to be subdued because
of the reduced solar radiation flux (due to the thick upper
haze), it was argued that there should be enough ionization
due to Galactic Cosmic Rays and Solar EUV radiation that
the drops within the clouds would be sufficiently charged to
support gravitational charge separation and eventually,
lightning. A simple 1D numerical model of an equatorial
methane cloud in Titan showed that free electrons attach
to cloud particles and build-up of negative space charge,
culminating in sufficiently strong fields (2 106 V m 1)
to reach breakdown and initiate cloud-to-ground lightning
with a 20 km long channel (Tokano et al., 2001). A more
sophisticated 3D model by Hueso and Sanchez-Lavega
(2006) predicted the formation of large convective clouds
when the relative humidity of methane in the middle
atmosphere exceeds 80%. These clouds should produce
updrafts as large as 20 m s 1 and reach up to 30 km
altitude. Having a life time of 5–8 h, they are capable of
producing heavy rainfall with values on the surface that
are comparable to flash floods on Earth. Such hypothetical
convective thunderstorms should be detectable from orbit
although the relatively rapid formation of precipitation
may lead to a situation of “rain without clouds” (Toon
et al., 1988) such that charging processes may not necessarily be co-located with optically visible clouds. Results from
Cassini suggest the existence of very shallow methane
clouds that produce a drizzle that is of major importance
for shaping the moon’s surface (Tokano et al., 2006) but
clearly such clouds cannot produce lightning activity. A
thorough review of storms and clouds on Titan is provided
by Griffith (2009), who named Titan’s weather “a deranged
version of the Earth’s”. Observations suggest a hemispheric
asymmetry in high-latitude ethane and methane clouds, as
detected by the Visual and Infrared Mapping Spectrometer
(VIMS) instrument (Brown et al., 2010) and the presence of
convective methane clouds in the tropics (Schaller et al.,
2009; Turtle et al., 2011). The lightning in convective
clouds, if it exists, should have enough power to drive
chemical reactions producing new compounds. Indeed,
numerous laboratory discharge experiments were conducted in simulated Titan gas mixtures in preparation for
the Cassini/Huygens mission in order to predict the consequences of the postulated lightning activity, pointing to the
formation of simple compounds such as acetylene, ammonia, hydrogen as well as of complex pre-biotic substances
collectively known as tholins (Sarker et al., 2003; Ehrenfreund et al., 1995; Navarro-Gonzales and Ramirez, 1997;
Coll et al., 1999; Ramı́rez et al., 2001; Navarro-González
et al., 2001). Such discharge experiments had been repeated
considering new constraints (Fischer et al., 2004) with generally similar results (Plankensteiner et al., 2007). Although
the Cassini orbiter and the Huygens probe have not found
any evidence for electrical activity in Titan, speculation on
the existence and nature of Titan’s lightning continues.
Here we will review briefly the latest observations and
theoretical work on this topic.
5.1. Spacecraft observations
Fischer et al. (2007a,b) reported the non-detection of
signals from lightning activity in Titan after the first 35
fly-bys by Cassini. In a follow-up paper, Fischer and Gurnett (2011) further enlarged the scope of search for any
lightning-related electromagnetic signature and covered
72 fly-bys, still with negative results. The Radio and Plasma
Wave Science Instrument (RPWS) instrument on board did
detect radio bursts when flying near Titan, but these were
either the radio emission from Saturn’s lightning activity
(SEDs, or Saturn Electrostatic Discharge, a somewhat historic misnomer considering the fact that they come from
lightning discharges; and see Section 7) or signals emanating from spacecraft interference or other sources of
Y. Yair / Advances in Space Research 50 (2012) 293–310
radiation such as Jupiter or the sun. The authors were careful to distinguish between the SED signals and any potential emission from the postulated lightning discharges in
Titan and concluded that lightning is either rare or nonexistent. So far, no positive result from the RPWS to indicate the existence of lightning had been reported. An additional source for information on lightning occurrence was
the Huygens probe that parachuted through Titan’s atmosphere in January 2005 (Lebreton et al., 2005). The Permittivity Wave and Altimetry (PWA) instrument on-board
was part of the Huygens Atmospheric Structure Instrument (HASI). Two different sensors were designed to monitor the AC and DC electric fields, detect the presence of
lightning and measure the atmospheric conductivity. An
acoustic sensor with a 10 mPa threshold was to register
storm signals and thunder. The results (Fig. 5; Béghin
et al., 2007) showed that the PWA recorded bursts of electrical noise during the descent, and although only half of
the data in the ELF range was obtained, there was an
enhanced signal of the 36 Hz line between 105 and
70 km, a frequency that conforms with the second eigenvalue of the expected SR resonance in the surface-ionosphere cavity of Titan (Pechony and Price, 2004; Yang
et al., 2006). The amplitude of the received quasi-horizontal electric field component signal seems too high to be
caused by lightning activity triggering that mode, and other
sources for this signal were sought (Béghin et al., 2007).
While later analysis suggested that these signals were a
result of lightning activity (Morente et al., 2009, 2008),
other works refuted this conclusion by showing that the
analysis was flawed, suggesting that the signals are in
essence just an artifact (Béghin et al., 2009; Grard et al.,
2011; Hamelin et al., 2011).
303
There is no agreement on the nature of the observed signals by HASI and whether they can indeed be related to a
(very rare) lightning discharge. Such a possibility cannot be
ruled out completely but seems highly unlikely, because the
chances that in only one specific time frame (of the descent)
there was lightning is confronted by a complete lack of
such flashes during the numerous fly-bys of the orbiter. It
may be argued that repeated observations from outside
the cavity may miss weak lightning, but this argument cannot be reconciled with the values assigned to the postulated
flash believed to have been detected by Huygens.
5.2. Models and laboratory experiments
While the existence of lightning in Titan is yet to be discovered, theoretical models are put forth to evaluate the
consequences of such activity. Due to the lack of observational data on lightning characteristics, it is common to
assume that Titan’s lightning resemble the energies and
duration of terrestrial flashes. This will dictate the temperature and pressure profiles within the channel, but because
the conditions inside thunderclouds in Earth and Titan are
remarkably different, it may also lead to an unrealistic
depiction of the physical and chemical processes taking
place. For example, Achi and Petculesco (2010) computed
the propagation of thunder in Titan’s atmosphere, based
on the assumption that the discharge channel is 20 km
long; needless to say that no such discharges had been
detected so far, nor can they be expected in light of the
observed cloud properties. Similarly, Kovács and Turanyi
(2010) used a detailed chemical model containing 1829
chemical reactions and 185 species, at an altitude of
10 km above the surface to describe the products of a
Fig. 5. Spectrogram of the ELF signal received with the electric dipole during the descent in Titan’s atmosphere between 140 and 62 km. Top: spectrogram
in the bandwidth 6–99 Hz; bottom: integrated field amplitude within the 99 Hz bandwidth (blue) and spectral level of the “36 Hz” line (red). [Taken from
Béghin et al., 2007; used with permission]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of
this article.)
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Y. Yair / Advances in Space Research 50 (2012) 293–310
lightning flash. They assume a channel temperature of
5100 K in an ambient tropospheric temperature of 81 K,
corresponding to an altitude of 10 km, and compute concentrations for the first 50 s. They find that the important
products are H2, HCN, C2N2, C2H2, C2H4, C2H6, NH3
and H2CN. Laboratory spark experiments for a 98:2 nitrogen–methane composition conducted by Horvath et al.
(2009) showed that the main product of the discharge is
C2H2 with trace amounts of HCN. When the spark was
allowed to keep discharging, it was finally covered by the
yellow residue of tholins thought to be representative of
the aerosol haze that is found in Titan’s atmosphere.
Ruiz-Bermejo et al. (2008, 2009) also conducted spark discharge experiments in prebiotic conditions with a H2, N2,
CH4 atmosphere and found two different types of tholins
(a hydrophobic and a hydrophilic type). While the results
of these discharge experiments match the observed chemical composition of substances found in Titan’s atmosphere,
the initiating agent, which is electrical activity, seems to be
missing from the clouds. Other processes are necessarily
responsible for the production of tholins in the upper
atmosphere, involving cosmic rays and electrons from Saturn’s magnetosphere, forming the high-altitude haze and
leading to slow sedimentation processes terminating in
their accumulation on the surface (Kobayashi et al., 2012).
6. Jupiter and Saturn
Undoubtedly, the most spectacular lightning activity in
the solar system was observed on the giant planets. The
existence of lightning on both Jupiter and Saturn was predicted based on chemical considerations and cloud models
and was clearly observed in optical images and electromagnetic data such as whistlers and sferics (Yair et al., 2008;
Fischer et al., 2008; and references therein). Based on the
assumed location of lightning in the deep water cloud
around 5 bars (Jupiter) and 10 bars (Saturn), the total flash
energy in both planets should be 102 stronger than
Earth’s (the breakdown field E scales with pressure and
the total energy stored is proportional to the volume integral of E2). A comparison between the properties of lightning on Saturn and Earth is presented in Table 2 of Fischer
et al. (2008). Initial estimates of SEDs’ power as observed
by Cassini showed a lightning intensity stronger by a factor
of 103–104 compared to Earth, possessing a total energy of
1012–1013 J (Gurnett et al., 2005). Later, Farrell et al.
(2007) suggested that SEDs are much shorter in duration
compared to terrestrial discharges (1 ls vs. 50 ls) and
hence the flash should possess less energy, of the order of
107 J. The real power is likely to be distributed between
these values.
While images from Voyager, Galileo and New Horizon
spacecraft showed clear optical signatures of lightning in
Jupiter, such an image was not obtained from Saturn until
Dyudina et al. (2010) reported the first detection of optical
light emanating through the clouds of what was determined
to be the location for radio emissions detected by Cassini’s
RPWS instrument (Fischer et al., 2006). Previous efforts to
correlate visible cloud features with the source of SEDs
showed that erupting bright cloud features can be traced
as the source of electrical activity, pointing to a deep source
below (Porco et al., 2005; Dyudina et al., 2007). These transient cloud features first appear as compact spots but
evolve within days-to-weeks into 2000 km long bright
plumes at high altitudes. The growth coincides with maximum SED rate, a clear manifestation of deep convective
processes involving condensation, precipitation and lightning activity. Based on the images of the 2009 storm, Dyudina et al. (2010) concluded that the lightning activity takes
place 125–250 km below the visible cloud layer, matching
the location of the deep mixed-phase H2O cloud (which
probably contains dissolved ammonia) or the NH4SH
intermediate cloud layer (Fig. 6). These observations agree
well with cloud models for moist convective processes in
the hydrogen–helium atmospheres of the Jupiter and
Saturn (Stoker, 1986; Yair et al., 1995; Hueso and
Sánchez-Lavega, 2004). It should be pointed out that
flashes dimmer than 108 J are below the camera detection
limit and hence we probably see only the stronger ones,
while others are missed (Dyudina et al., 2010).
The observed clouds and deduced lightning activity were
confined to a “storm alley” around 35°S and appear dark
in the near infra-red spectrum, suggesting the transport
of chemical species produced by the lightning in the deep
water-clouds at the 10 bar level (Baines et al., 2009). The
dark material can be elemental carbon, which would be
the product of methane and other hydrocarbons decomposition by the heat within the lightning channel, or other
compounds involving sulfur and phosphorus (Podolak
and Bar-Nun, 1988).
The emissions from lightning activity in Jupiter and Saturn can potentially be monitored from Earth (Zarka et al.,
2004; Zarka et al., 2008). Advances in radio technology
and signal processing software allow detection of SEDs
in Saturn also from Earth using ground-based radio telescopes such as the LOFAR array (Grießmeier et al.,
2011) or the UTR2 radio-telescope (Zakharenko et al.,
2012). Thermal-infrared imaging of both Jupiter and
Saturn has been conducted successfully using the
NASA/IRTF and Subaru observatories, enabling retrievals
of zonal mean properties of both planets (Fletcher et al.,
2009). Saturn is constantly observed by ground-based telescopes as part of the International Outer Planet Watch
(IOPW), and it was two amateur observers that first
reported a small white spot in the northern hemisphere in
December 2010. That was recognized as cloud eruption
of an atmospheric storm that was located at 35°N that
within a week of discovery started producing prodigious
amounts of lightning, with rates exceeding 10 per second
(Fischer et al., 2011). The average rate was maintained
for several months, in defiance of the strongest storms on
Earth which can maintain such a rate for tens of minutes
at most. The storm grew rapidly from a size of 10,000 km
and spread at a rate of 2.8° per day westward to circle
Y. Yair / Advances in Space Research 50 (2012) 293–310
305
Fig. 6. SED spectral source power, optical flash energy and visible images of lightning in Saturn, assuming they have a common source and location. The
gaps between the images indicate periods when the camera was not observing. The absence of SEDs during some optical flashes is due to the fact that the
RPWS instrument scans in frequency and detects only 1/3 of the SEDs [Taken from Dyudina et al., 2010; American Geophysical Union. Reproduced
with permission].
Fig. 7. Storm evolution from December 17th 2010 through February 19th 2011, based on multiple images from various sources. The storm head moves
west (left in the images). [Image from Sanchez-Lavega et al., 2011. Nature Publishing Group. Used with permission].
the entire visible disk of the planet in 55 days (by February
2011; Fig. 7) – almost 140,000 km at 35 N latitude – thus
“biting its own tail”. The growth rate was shown to
explode from an initial 20–212 km2 s 1, indicating an
updraft of 150 m s 1 (Sanchez-Lavega et al., 2011). The
evolution of this storm can be explained by moist convection driven by the condensation of water and ammonia,
at the 10–12 bar and 1–2 bar levels, accordingly.
Using a combination of high-resolution ground-based
thermal imaging data from the Very Large Telescope
(VLT) together with Cassini imaging Fletcher et al.
(2011) showed two “hot beacons” at the edge of the stratospheric disturbance, that later merged into a single beacon
in the stratosphere above the storm. The detection of significant enhancement of trace compound concentrations
such as C2H2, C2H4 and C2H6 in that region (Bjoraker
et al., 2011; Hesman et al., 2011) may be attributed to
the vertical transport of materials produced directly by
lightning in the deep atmosphere, or else can be the chemical product of Sprites occurring above the lightning, in a
parallel manner to what is detected on Earth (Arnone
et al., 2008).
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Y. Yair / Advances in Space Research 50 (2012) 293–310
7. Summary
Lightning is prevalent in the solar system, and is a marker of moist convective processes that results from large
scale instabilities and dynamics within the atmosphere. It
can appear in volcanic plumes or within dust and snow
storms, but most likely in the presence of precipitation processes. It is detectable from space platforms and groundbased instruments due to a variety of direct and indirect
effects it has on its surroundings. These include electromagnetic emissions in various wavelengths (from gamma and
X-ray to radio and VLF, ELF and Schumann resonances),
acoustic waves, chemical reactions and new non-equilibrium materials as well as transient-luminous events. Electrical discharge processes in a planet’s atmosphere or near the
surface have significant effects on the nature of primordial
conditions, and thus lightning offers a special tool to investigate the origins of biological life. Although Mars, Titan
and Venus to some degree still pose open questions on
the nature of their electrical activity, it is an existing challenge to decipher what is the nature of lightning on these
planets. On Earth, Jupiter and Saturn global lightning
activity offers a tool for monitoring large-scale weather systems and offers insight into the complex interactions
accompanying seasonal- and climatic-scale changes. The
continued observations by Cassini at Saturn, and VEX
and Mars Express, as well as missions en-route to Mars
(Curiosity) and Jupiter (Juno) will get new data and are
bound to enhance our understanding of the electrical activity occurring on these planets. Future ones still in the planning stage as part of ESA’s “Cosmic Vision” and NASA’s
roadmap should consider lightning as a priority for
research in the design of instruments and flight profiles.
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