Universe Today
Venus' atmosphere is as mysterious as it is dense and scorching. For generations, scientists have sought to study it using ground-based telescopes, orbital missions, and the occasional atmospheric probe. And in 2006, the ESA's
mission became the first probe to conduct long-term observations of the planet's atmosphere, which revealed much about its dynamics.
Using this data, a team of international scientists - led by researchers from the
Japan Aerospace and Exploration Agency
(JAXA) - recently conducted
a study
that characterized the wind and upper cloud patterns on the
night side of Venus
. In addition to being the first of its kind, this study also revealed that the atmosphere behaves differently on the night side, which was unexpected.
The study, titled "
Stationary Waves and Slowly Moving Features in the Night Upper Clouds of Venus
", recently appeared in the scientific journal
Nature Astronomy
. Led by Javier Peralta, the International Top Young Fellow of JAXA, the team consulted data obtained by
Venus Express'
suite of scientific instruments in order to study the planet's
previously-unseen cloud types, morphologies, and dynamics.
[caption id="attachment_137239" align="aligncenter" width="580"]
The atmospheric super-rotation at the upper clouds of Venus. While the super-rotation is present in both day and night sides of Venus, it seems more uniform in the day. Credits: JAXA, ESA, J. Peralta and R. Hueso.
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Whereas plenty of studies have been conducted of Venus' atmosphere from soace, this was the first time that a study was not focused on the dayside of the planet. As Dr. Peralta explained in an ESA
press statement
:
" "
Since the 1960s, astronomers have been aware that Venus' atmosphere behaves much differently that those of other terrestrial planets. Whereas Earth and Mars have atmospheres that co-rotate at approximately the same speed as the planet, Venus' atmosphere can reach speeds of more than 360 km/h (224 mph). So while the planet takes 243 days to rotate once on its axis, the atmosphere takes only 4 days.
This phenomena, known as "super-rotation", essentially means that the atmosphere moves over 60 times faster than the planet itself. In addition, measurements in the past have shown that the fastest clouds are located at the upper cloud level, 65 to 72 km (40 to 45 mi) above the surface. Despite decades of study, atmospheric models have been unable to reproduce super-rotation, which indicated that some of the mechanics were unknown.
[caption id="attachment_130521" align="aligncenter" width="580"]
Artist's impression of the atmosphere of Venus, showing its lightning storms and a volcano in the distance. Credit and ©: European Space Agency/J. Whatmore
[/caption]
As such, Peralta and his international team - which included researchers from the Universidad del País Vasco in Spain, the University of Tokyo, the Kyoto Sangyo University, the
Center for Astronomy and Astrophysics
(ZAA) at Berlin Technical University, and the
Institute of Astrophysics and Space Planetology in Rome -
chose to look at the unexplored side to see what they could find. As he described it:
This consisted of observing Venus' night side clouds with the probe's
Visible and Infrared Thermal Imaging Spectrometer
(VIRTIS). The instrument gathered hundreds of images simultaneously and different wavelengths, which the team then combined to improve the visibility of the clouds. This allowed the team to see them properly for the first time, and also revealed some unexpected things about Venus' night side atmosphere.
What they saw was that atmospheric rotation appeared to be more chaotic on the night side than what has been observed in the past on the dayside. The upper clouds also formed different shapes and morphologies - i.e. large, wavy, patchy, irregular and filament-like patterns - and were dominated by stationary waves, where two waves moving in opposite directions cancel each other out and create a static weather pattern.
[caption id="attachment_137244" align="aligncenter" width="551"]
Examples of new types of cloud morphology discovered on the night side of Venus thanks to Venus Express (ESA) and the infrared telescope IRTF (NASA). Credits: ESA/NASA/J. Peralta and R. Hueso.
[/caption]
The 3D properties of these stationary waves were also obtained by combining VIRTIS data with radio-science data from the
Venus Radio Science experiment
(VeRa). Naturally, the team was surprised to find these kinds of atmospheric behaviors since they were inconsistent with what has been routinely observed on the dayside. Moreover, they contradict the best models for explaining the dynamics of Venus' atmosphere.
Known as
Global Circulation Models
(GCMs), these models predict that on Venus, super-rotation would occur in much the same way on both the dayside and the night side. What's more, they noticed that stationary waves on the night side appeared to coincide with high-elevation features. As Agustin Sánchez-Lavega, a researcher from the University del País Vasco and a co-author on the paper,
explained
:
" "
This is not the first time that scientists have spotted a possible link between Venus' topography and its atmospheric motion. Last year, a team of European astronomers produced
a study
that showed how weather patterns and rising waves on the dayside appeared to be directly connected to topographical features. These findings were based on UV images taken by the
Venus Monitoring Camera
(VMC) on board the
Venus Express
.
[caption id="attachment_137245" align="aligncenter" width="580"]
Schematic illustration of the proposed behaviour of gravity waves in the vicinity of mountainous terrain on Venus. Credit: ESA
[/caption]
Finding something similar happening on the night side was something of a surprise, until they realized they weren't the only ones to spot them. As Peralta
indicated
:
" "
These findings also challenge existing models of stationary waves, which are expected to form from the interaction of surface wind and high-elevation surface features. However, previous measurements conducted by the Soviet-era
Venera landers
have indicated that surface winds might too weak for this to happen on Venus. In addition, the southern hemisphere, which the team observed for their study, is quite low in elevation.
And as Ricardo Hueso of the University of the Basque Country (and a co-author on the paper) indicated, they did not detect corresponding stationary waves in the lower cloud levels. "We expected to find these waves in the lower levels because we see them in the upper levels, and we thought that they rose up through the cloud from the surface," he said. "It's an unexpected result for sure, and we'll all need to revisit our models of Venus to explore its meaning."
[caption id="attachment_116726" align="aligncenter" width="580"]
Artist's impression of Venus Express performing aerobreaking maneuvers in the planet's atmosphere in June and July 2014. Credit: ESA–C. Carreau
[/caption]
From this information, it seems that topography and elevation are linked when it comes to Venus' atmospheric behavior, but not consistently. So the standing waves observed on Venus' night side may be the result of some other undetected mechanism at work. Alas, it seems that Venus' atmosphere - in particular, the key aspect of super-rotation - still has some mysteries for us.
The study also demonstrated the effectiveness of combining data from multiple sources to get a more detailed picture of a planet's dynamics. With further improvements in instrumentation and data-sharing (and perhaps another mission or two to the surface) we can expect to get a clearer picture of what is powering Venus' atmospheric dynamics before long.
With a little luck, there may yet come a day when we can model the atmosphere of Venus and predict its weather patterns as accurately as we do those of Earth.
Further Reading: ESA
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