Geophysical Research Letters
RESEARCH LETTER
10.1002/2015GL064088
Key Points:
• VMC was able to sound Venus surface
through the atmosphere transparency
window
• Transient bright phenomena were
observed in the Ganiki Chasma zone
• They are consistent with hypothesis of
lava lakes on the surface
Supporting Information:
• Readme
• Texts S1 and S2, and Table S1
• Figure S1
• Figure S2
• Figure S3
Correspondence to:
E. V. Shalygin,
[email protected]
Citation:
Shalygin, E. V., W. J. Markiewicz,
A. T. Basilevsky, D. V. Titov,
N. I. Ignatiev, and J. W. Head (2015),
Active volcanism on Venus in the
Ganiki Chasma rift zone, Geophys.
Res. Lett., 42, 4762–4769,
doi:10.1002/2015GL064088.
Received 2 APR 2015
Accepted 15 MAY 2015
Accepted article online 23 MAY 2015
Published online 17 JUN 2015
©2015. American Geophysical Union.
All Rights Reserved.
SHALYGIN ET AL.
Active volcanism on Venus in the Ganiki Chasma rift zone
E. V. Shalygin1 , W. J. Markiewicz1 , A. T. Basilevsky1,2,3 , D. V. Titov4 , N. I. Ignatiev5 , and J. W. Head3
1 Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany, 2 Vernadsky Institute, Moscow, Russian Federation,
3 Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island, USA,
4 ESA-ESTEC, Noordwijk, Netherlands, 5 Space Research Institute, Moscow, Russian Federation
Abstract Venus is known to have been volcanically resurfaced in the last third of solar system history
and to have undergone a significant decrease in volcanic activity a few hundred million years ago.
However, fundamental questions remain: Is Venus still volcanically active today, and if so, where and in
what geological and geodynamic environment? Here we show evidence from the Venus Express Venus
Monitoring Camera for transient bright spots that are consistent with the extrusion of lava flows that
locally cause significantly elevated surface temperatures. The very strong spatial correlation of the transient
bright spots with the extremely young Ganiki Chasma, their similarity to locations of rift-associated
volcanism on Earth, provide strong evidence for their volcanic origin and suggests that Venus is currently
geodynamically active.
The current surface geology, geodynamics, and atmospheric characteristics of Venus, as well as its history,
differ significantly from those of Earth [Phillips and Hansen, 1998; Smrekar and Phillips, 1991; Bullock and
Grinspoon, 2001, 2013; Baines et al., 2013]. In contrast to global plate tectonics that dominates Earth geodynamics (geologically young seafloor, ancient continents, and tectonism and volcanism concentrated at plate
boundaries), Venus is characterized by a single global lithospheric plate, like the Moon, Mars, and Mercury, but
the age of its surface is anomalously young and Earth-like [Solomon and Head, 1982; McKinnon et al., 1997].
Furthermore, the atmosphere of Venus is radically different from that of Earth (the pressure at the surface
level is almost 100 times higher, and this pressure is created almost entirely by CO2 ). The question of why the
Earth and Venus display such sharp divergence is one of the most fundamental problems in planetary science
[Phillips and Hansen, 1998; Smrekar and Phillips, 1991; Baines et al., 2013].
Analysis and geologic mapping of the surface of Venus over the course of the space age has revealed that
geological units representing the first 80% of the history of Venus are no longer exposed at the surface and
that, unlike the Earth, Venus may have undergone a geologically rapid global resurfacing within the last billion
years [Phillips and Hansen, 1998; Smrekar and Phillips, 1991]. This global resurfacing included tectonic deformation [Solomon et al., 1992], creating the deformed highlands (tesserae), followed by near-global volcanic
resurfacing [Head et al., 1992], creating the regional plains that cover more than 70% of the surface. Stratigraphic relationships and the density of superposed craters provide strong evidence that the rates and styles
of tectonism and volcanism changed significantly a few hundred million years ago [Ivanov and Head, 2011,
2013]. Broad upwellings, shield volcanoes, and a system of regional intersecting linear rift zones replaced
tesserae formation and global flood volcanism. Stratigraphic relationships show this transition clearly [Ivanov
and Head, 2011], but the exact time of its occurrence, the rates of tectonism and volcanism, and whether Venus
is still active today, are uncertain due to the small number of superposed impact craters on young terrain.
The formation and degradation of radar-dark parabolas associated with the most recent impact craters
[Izenberg et al., 1994] provides evidence that rifting has been active in the last tens of millions of years
[Basilevsky, 1993], but unknown is the presence, level, and location of any current activity. The extreme youth
of these craters was deduced from observation that only a very small fraction of the population is embayed
by volcanic lavas or fractured by tectonic faults, while for craters having no parabolas, such volcanic and
tectonic superpositions are less rare. This was demonstrated through analysis of practically complete population of Venusian craters [Izenberg et al., 1994] and geologic analysis, first, on a regional basis Basilevsky and
Head [1995] and then globally [Ivanov and Head, 2011, 2013].
Observations of changes in the composition of the atmosphere over the course of the space age have been
cited as possible evidence for surface volcanic activity [Esposito et al., 1988], with volcanic effusions and
eruptions as candidates for transient anomalies of SO2 detected in the atmosphere. Emissivity anomalies
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Figure 1. Northern Atla Regio and examples of geologically recent volcanic activity: (a) Topographic map (blue is low, red is high); red contours outline areas
where transient bright spots were identified by VMC measurements. (b) Magellan Venus Radar Mapping Mission (MGN) synthetic aperture radar (SAR) image of
the same area; radar-dark parabola associated with the crater Sitwell is seen in the center-right (black arrows indicate locations of lava flow/rift interactions
shown in Figures S1a–S1d). (c) Portion of the global geologic map of Venus by Ivanov and Head [2011] showing the study area and its surroundings. The
stratigraphically youngest units (rift zones, purple; lobate plains, lava flows, red) are contemporaneous. VMC transient bright spots (A–D, white ovals) are closely
associated with the rift zone. Inset shows global MGN SAR map with study area location.
associated with several lava flow complexes have been cited as evidence for volcanism in the last 250,000 years
[Smrekar et al., 2010], and the radar properties of one lava flow complex (which displays a significant apparent
microwave thermal emission excess, suggesting increased subsurface temperature [Bondarenko et al., 2010])
and the summit of Maat Mons (which shows the partial absence of high-reflectivity material above a critical
altitude, suggesting a shortage of time to complete typical surface alterations [Klose et al., 1992]) are consistent
with relatively recent volcanism.
The Venus Monitoring Camera (VMC) [Markiewicz et al., 2007] onboard the European Space Agency Venus
Express (VEx) spacecraft [Svedhem et al., 2007] provides the opportunity to observe changes in the thermal
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Figure 2. Maps of relative brightness in the VMC IR2 channel. Each panel shows the ratio of the mosaic composed from images obtained in the given to the
averaged VMC mosaic of the region. Orbital mosaics were obtained for specified dates in orbits: (a) 793, (b) 795, and (c) 906. Ganiki Chasma and object “A” in
orbit 795 are outlined with white lines in each panel. The grid size is 5∘ by 5∘ , that is ≈528 km at the equator.
emission [Basilevsky et al., 2012] of the surface of Venus that might be associated with current ongoing volcanic eruptions. Recent analysis of VMC images reveals several regions whose brightness changed during a
series of successive observations (Figures 1, 2, and Figure S3 in the supporting information). Careful analyses
were undertaken to demonstrate that the transient brightening events were not due to instrument artifacts
(see Text S1). The variations of the apparent brightness of these spots are interpreted to correspond to volcanic
eruptions and related changes in surface temperature due to eruption of lavas.
VMC obtains images in four spectral channels; one of these, centered at 1.01μm (IR2), registers the nightside thermal emission from the surface of Venus [Markiewicz et al., 2007] inside an atmosphere transparency
window [Allen and Crawford, 1984]. Variations of the brightness registered by the camera can be caused by
either variations of the atmospheric attenuation or by variations of the flux from the surface, which, in turn,
can be due to variations of the surface emissivity and surface temperature [Basilevsky et al., 2012]. Variations of
emissivity can change registered brightness to a limited degree (it cannot exceed that of the ideal black body
with the temperature equal to the temperature of the surface), and anomalous brightness detected above
these limits must therefore be attributed to temperature or attenuation changes.
At this wavelength, and with a mean temperature of the surface of ≈740 K [Seiff et al., 1985], the
thermal near-infrared (NIR) flux from the surface strongly depends on the surface temperature, providing the
opportunity to detect higher surface temperatures associated with volcanic eruptions. The appearance and
disappearance of such thermal anomalies (“bright spots”) in the VMC data would be strong evidence for transient volcanic events (see estimations of their visibility by VMC in Shalygin et al. [2012]). These measurements
are at the limit of VMC capability. Even at the maximum exposure of 30 s, the faintness of the surface emission
and low efficiency of the CCD detector (≈2%) at 1.01μm result in the measured signal not exceeding 200 DN
(digital numbers, which are ≈3% of the CCD full well) and the average signal-to-noise ratio (SNR) for an individual image is ∼4. However, since VMC takes many overlapping surface images (usually ∼10) the value of
SNR in mosaics is appropriately higher. To remove uncertainties of the VMC radiometric calibration [Shalygina
et al., 2014, section 3] we did not rely on radiometric brightness of the observed bright spots but instead utilized relative measurements that is possible, because in all surface observations, VMC uses the same exposure
time (30 s) and the temperature of the camera does not vary significantly during one observational session
(orbit). Therefore, the radiometric sensitivity can be assumed to be stable during an observation session.
If so then every VMC orbital mosaic is radiometrically consistent. We divide mosaics by values at some point
(or mean value in a region(s)) and use these normalized mosaics, which do not bear information about
absolute value of fluxes but contain correct contrasts.
Measurements performed by landers showed that the temperature near the surface is a stable function of
altitude. Therefore, to detect only the presence of a transient bright spot on the surface, one can compare
brightness maps obtained at different times provided that the expected brightness variations are significantly
higher than those caused by the variations of the atmospheric attenuation and emissivity. Such a detection
was made in VMC observations of a 1.44 × 106 km2 area of northern Atla Regio (5∘ N–25∘ N, 180∘ E–200∘ E),
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in a region of geologically recent volcanoes and rift zones in the western part of the geologically young
Beta-Atla-Themis (BAT) region (Figure 1).
VMC performed 316 observational sessions (37 among them are of relatively good quality) and took 2463
images in this area (among them 899 images within the location of object “A,” 770 within “B,” 726 within “C,”
and 911 within object “D”). From these data we constructed orbital mosaics, which are maps of registered
brightness in Mercator projection. During the systematic analysis of these mosaics we identified a bright feature that is present at the same location in several consequent orbits (well seen in orbits 793 and 795), but
that disappeared afterward (the next images of this location were taken in orbit 906, see Figure 2). The bright
anomaly discovered is much brighter than typical brightness variations due to changes in the atmosphere:
averaged VMC brightness in the surrounding region is 13.5 mW/m2 μm sr), which correspond to average
brightness of 1 in Figure 2 (but see Shalygina et al. [2014, section 3.2]) and its standard deviation between
orbits is ∼2 mW/m2 μm sr). This bright spot is detectable without any assumptions about the surface or the
atmosphere properties. Its key difference from other bright spots, caused by clouds (for example, ones like
the spot in the SE corner of the Figures 2a and 2b) is that it does not move in surface coordinates. Detection of
other possibly existing brightness anomalies that are fainter than the limit implied by clouds is not possible
in this way. Several assumptions and radiative transfer modeling are needed to reveal possible not so bright
objects. The basic logic remains the same: transient bright anomalies that are brighter than the limit imposed
by emissivity variations and that do not change their geographic locations from orbit to orbit are very likely
to be caused by a process on the surface, since the typical wind speeds at the level of the main cloud deck
(where attenuation of most of the surface thermal flux occurs) is ∼ 102 m/s and the VEx orbital period is 24 h.
Assuming horizontal optical homogeneity of the atmosphere on a scale of ∼ 102 km (the size of point spread
function (PSF)), we can model emission intensity at a point with horizontal coordinates (x, y) at the top of the
atmosphere:
I(x, y) =
[
]
t(x, y)𝜀(x, y)
⋅
B TS (x ′ , y′ ) ⋅ F(x − x ′ , y − y′ )dx ′ dy′
1 − (r(x, y)(1 − 𝜀(x, y))) ∫ ∫
where t(x, y) is the atmospheric transmittance, r(x, y) is the atmospheric reflectance of surface radiation in
backward direction (both depend on surface altitude), 𝜀(x, y) is the emissivity distribution of Lambertian surface, B(TS ) is the Planck function of the surface temperature TS , and F is the point spread function (PSF).
Comparing such model images I0 with the VMC one I, we compute (the method is described in our previous
works [Basilevsky et al., 2012; Shalygin et al., 2012; Shalygin, 2013]) maps of local relative surface brightness
(𝜀B)∕(𝜀0 B0 ) (index means model as before), i.e., the relative brightness that would be detected by a hypothetical observer near the surface. In these maps we found three more events that are likely to be caused by surface
processes. Examining VMC observations of the NW and SW parts of this rift system (outside of the region in
Figure 1a), we have not yet found any evidence of similar transient phenomena.
The bright spots are located at the edges of the stratigraphically recent tectonic rift zone, Ganiki Chasma
(Figures 1 and S3). The most prominent feature (“A”) is seen in mosaics from VEx orbits 793 (22 June 2008) and
795 (24 June 2008). The next good observation here was obtained from orbit 906 (13 October 2008, 111 Earth
days afterward) and showed no anomalous brightness. Bright spots “B” and “C” behave in a similar manner:
they are bright in images obtained from two and three subsequent orbits (in the second week of June 2009)
and are not visible in orbits prior to or after these detections. Object “D” was imaged under conditions that
do not permit a certain identification of change (see Figure S3 for observation dates and orbit numbers).
Among these four objects, “A” and “B” show distinct differences from the other ones and a clear difference
from the regular pattern of surface images that VMC obtains. For these two objects, VMC has obtained observational sequences that show how the objects become brighter on the time scale of days. For all four objects
we computed temperature excess out of brightness excess (brightness for all of them are above the emissivity variation limits). These results are presented as maps of temperature excess (Figure 3). We reject the
hypothesis that changes in brightness for the objects “A” and “B” might be caused by global changes in the
atmospheric transparency, because such changes should change the mean level of brightness in the VMC
mosaics, but this is not observed (and see also the supporting information).
The obtained excess temperature could be produced by areally extended sources or due to strong scattering (blurring) in the atmosphere, by smaller and much hotter sources (see Text S2 in the supporting
information) as well as by any configuration in between. Estimations were made for both extreme cases
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Figure 3. Maps of excess temperature. Each subfigure shows a map of surface temperature derived from MGN data (black and white) with and without the
retrieved excess temperature overlain (color). The grid size is 5∘ by 5∘ .
(Figures S2 and 4). VMC data do not allow to give a precedence to one of the configurations, except the
following hint: the smallest dimensions of all registered bright spots and their spatial profiles are very similar
to those of the atmosphere point spread function (PSF). Such a coincidence seems to be unlikely and gives
us reason to believe that their true dimensions are much smaller (otherwise the profiles would differ from the
point spread function (PSF) shape). We found (see Text S2, Table S1, and Figure 4) that a few hot spots with
an area of 1 km2 each and temperatures up to 1100 K can explain the brightening in VEx orbits 793 and 795
(object “A,” Figure 4); small spots with temperatures up to 950 K together with larger areas up to 200 km2 at
800 K can explain features observed in orbits 1147 and 1148 (objects “B” and “D”).
Rift zones such as Ganiki Chasma are typical of the latest stage in the history of Venus (network rifting/volcanism regime) [Ivanov and Head, 2011, 2013] and are characterized by extensive crustal and lithospheric extension and thinning, mantle upwelling, tectonic rifting, and the extrusion of numerous long lava
flows from the rift faults and fractures. The location of the transient bright spots is typically near the flanking
faults of the rift (Figure 1), that are often the sites of active eruptions in terrestrial rift zones [Franke, 2013].
These associations strengthen the interpretation that the transient bright spots represent the sites of active
volcanic eruptions.
Active lava flows and flow fields on Earth commonly display broad thermal anomalies associated with source
regions and distribution systems (source ponds and lakes, multiple channelized flows with continuously
exposed lava, pahoehoe breakouts, partly roofed lava tubes, etc. [Flynn et al., 1994]). These anomalies persist
throughout the eruption period and the subsequent cooling of the lava, periods often measured in years.
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Figure 4. Pairs of VMC to synthetic mosaics ratios showing the excess brightness modeled (top) without and (bottom)
with hot material on the surface. In the top panels the model image is calculated assuming a constant surface emissivity
and the adiabatic temperature lapse rate of −8.1 K/km. In the bottom panel of each pair, artificial hot spots are added.
The locations of hot material are marked by crosses and lines. The grid size is 1∘ by 1∘ . The hot spot parameters are
given in Table S1.
The estimated dimensions of the hot spots on Venus are similar to those of a wide variety of common active
eruption phenomena on Earth (lava flows, lava channels, ponded parts of lava flows, and lava lakes) [Pyle,
1999] and thus can readily explain the bright spot magnitudes above the ambient surface background and
their duration. Relatively short (comparing to Earth) duration of the Ganiki temperature anomalies may be
due to more effective cooling by very dense Venusian atmosphere (65 kg/m3 ) [Head III and Wilson, 1986].
Besides, posteruption thermal anomalies on Earth are often supported by circulation of ground waters which
are absent on Venus. Similar configurations are well known in areas of active volcanism on Earth [Flynn et al.,
1994; Pyle, 1999] and are observed elsewhere on Venus in older deposits [Ivanov and Head, 2013]. We considered the possibility that the bright spots might represent explosive eruptions but favor effusion because
(1) the very high atmospheric pressure significantly inhibits explosive activity [Head III and Wilson, 1986], (2)
explosive eruptions are favored from edifices, rather than rifts [Glaze, 1999], (3) candidate examples of explosive volcanic deposits are very rare in the geologic record of Venus [Ivanov and Head, 2013], and (4) the linear
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alignment with the rift is more consistent with lava flows. In summary, the characteristics and behavior of
these bright spots suggest that they represent the volcanic eruption of lava onto the surface of Venus, causing
transient thermal anomalies.
Further evidence for the extreme youth of the tectonic and volcanic activity in this area comes from relationships with the radar-dark parabola impact crater Sitwell (Figure 1). Dark parabolas are associated with
the youngest impact craters mapped on Venus and craters are known to lose their parabolas through eolian
modification over the course of a few tens of millions of years [Izenberg et al., 1994]. Parabolas have been
successfully used as a stratigraphic indicator of the youngest end member of geologic activity on Venus
[Ivanov and Head, 2011]. Thus, any geologic activity that superposes or cuts these parabolas must be among
the absolutely youngest activity on Venus [Basilevsky, 1993]. Magellan Venus Radar Mapping Mission (MGN)
synthetic aperture radar (SAR) images of the flanks and interior of Ganiki Chasma and the Sitwell Crater dark
parabola (Figure S1) in the areas of the transient bright spots show clear evidence of (1) lava flows superposed on rift fractures and faults, and flooding them, (2) fresh faults cutting very young lava flows, and (3)
lava flows superposed on the Sitwell Crater dark parabola. Together, these observations strongly support the
interpretation that the transient bright spots represent the sites of currently active volcanic eruptions.
The detection of current volcanic eruptions in VEx VMC images indicates that the Atla Regio rise area is
presently geologically and geodynamically active and that historically observed variations in atmospheric
chemistry [Esposito et al., 1988; Marcq et al., 2011] could be due to active volcanic eruptions. Atla Regio should
receive priority in terms of future Venus exploration and change detection experiments.
Acknowledgments
We acknowledge teams at the Institut
für Planetenforschung of Deutsches
Zentrum für Luft- und Raumfahrt
and the Institut für Datentechnik
und Kommunikationsnetze der
Technische Universitt Braunschweig
for their efforts in supporting the VMC
experiment. The authors are grateful
to Deutsches Zentrum für Luft- und
Raumfahrt, who provided the VMC
data processing (data are available
from ESA PSA), especially to T. Roatsch
and K. D. Matz.
The Editor thanks Robin Fergason and
Lionel Wilson for their assistance in
evaluating this paper.
SHALYGIN ET AL.
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