Researchers from the Japan Aerospace Exploration Agency’s Institute of Space and Astronautical Science believe the phenomenon is the largest stationary “gravity wave” ever recorded in the solar system. Emanating from the mountains below, the unusual weather phenomenon is strong enough to withstand ferocious background winds, causing an enormous bow-like structure to hang in the upper atmosphere like a gigantic scar.
Venus, that cloud-covered hellhole of a planet next door, is rife with exceptionally strange atmospheric behavior. Winds in its upper atmosphere howl at 223 miles per hour (359 kilometers per hour), a speed that’s considerably faster than the slowly-rotating planet pictured below (a single Venusian day lasts longer than an entire Venusian year). Thick clouds of sulfuric acid move in a westerly direction on account of the entire upper atmosphere rotating significantly faster than the planet itself.
In late 2015, JAXA’s Akatsuki orbiter—a spacecraft designed to investigate the atmospheric dynamics and cloud physics of Venus—made some strange observations over the course of several days. As described in a new study published in the journal Nature, the probe detected a stationary, bow-shaped structure in the upper Venusian atmosphere. A research team led by Rikkyo University astronomer Makoto Taguchi spotted the anomaly by analyzing images taken by Akatsuki in the middle infrared and ultraviolet spectrums.
Parked 40 miles (65 km) above the scorched surface, the bright, unusually warm region stretched for 6,200 miles (10,000 km) across the top of Venus’ clouds (nearly the diameter of the entire planet). The structure did not move despite the background surrounding atmospheric winds, remaining stationary above a mountainous region of the planet’s surface. Prior to this observation, scientists observed a number of small-scale atmospheric features that moved either faster or slower than the prevailing winds.
This gigantic streak, and its contempt for the winds around it, caught the JAXA scientists completely off guard. Several weeks after their initial observations, the bow had disappeared, so it’s not a permanent feature. The researchers are now anxiously waiting for its possible return.
After ruling out possibilities like a thermal tide (these are similar to ocean tides, but highly unlikely given that Venus has no moon) or an instrumental error (Akatsuki is in tip-top shape), the JAXA researchers concluded that the anomaly is likely caused by a gravity wave. Not to be confused with gravitational waves, gravity waves happen where mediums, like a fluid or gas, fight for a state of equilibrium under the force of gravity. Or as Taguchi explained to Gizmodo, “it’s an oscillation of density, pressure, velocity or temperature that propagates in an atmosphere by a balance of buoyancy and gravity forces as a restoring force.” On Earth, gravity waves produce waves on the ocean and airflow over mountains.
Scientists have observed small, transient gravity waves in Venus’s atmosphere before, a sign that mountain ranges are present below, but they’ve never seen anything quite like this one. On this scorched planet, gravity waves are generated near rugged, mountainous surface areas, and then drift upwards, lifting up into the sky and growing larger and larger in amplitude until they dissipate just below the cloud-tops. As the waves break in the upper atmosphere, they push back against fast-moving atmospheric winds with tremendous force, slowing those winds down.
That’s the theory of how gravity waves typically work, at least, but this newly-observed anomaly suggests they also work on a near-planetary scale, affecting the cloud-tops for thousands of miles.
“Given the shape and the fast speed [of the surrounding winds] relative to the background super-rotation, the only reasonable interpretation of the stationary bow shape is that it is induced by an atmospheric gravity wave packet,” write the researchers. “The present study shows direct evidence of the existence of stationary gravity waves, and it further shows that such stationary gravity waves can have a very large scale—perhaps the greatest ever observed in the solar system.”
Using computer models, the researchers verified that large, stationary gravity waves are indeed possible. “Our simulations of gravity wave propagation showed a similar pattern of temperature distribution at the cloud-top altitudes as the observed one,” Taguchi told Gizmodo.
All this said, Taguchi’s team is not entirely certain if the gravity waves produced by the Venusian mountain ranges are capable of spreading upwards as far as the cloud-tops without a little help. The researchers speculate that winds in the deep atmosphere may be more variable in space and time than previously assumed, boosting the ability of gravity waves to propagate to the upper portion of Venus’s atmosphere.
Looking ahead, Taguchi would like to study variations in the atmosphere so he can compare the conditions when the bow is present to when it’s absent. “We also have to collect more data for statistical studies,” he said. “Ongoing computer simulations will be important to justify a hypothesis raised from the observational results.”
Venus, as we’re learning, is a surprisingly complicated—and unusual—place.
|A temperature map of Venus shows the unusually warm, bow-shaped structure extending over a significant portion of the planet. (Image: JAXA/Taguchi et. al., 2017)|
Giant Mystery Wave Spotted in Atmosphere of Venus
A huge wave has been spotted in the upper atmosphere of Venus, baffling scientists because it's staying so still above the planet's surface. Usually clouds in that region move at about 100 meters (328 feet) per second, whereas this cloud is stationary compared to the planet's rotation.
New work suggests that the wave was created in the lower atmosphere when it flowed over a mountain, which would be similar to a phenomenon on Earth called a "gravity wave." The feature, related to atmospheric flow over mountains, has nothing to do with the similarly named "gravitational wave," which refers to space-time ripples in the early universe.
"Although it is unclear whether gravity waves induced by mountains can readily propagate upwards to the cloud tops of Venus, the observations suggest that the atmospheric dynamics of Venus are more complex at depth than previously appreciated," the journal Nature's statement on the finding said.
The new findings came from observations by Japan's Akatsuki spacecraft, which has orbited Venus since 2015. The work was detailed yesterday (Jan. 16) in the journal Nature Geoscience, and it was led by Makoto Taguchi at Rikkyo University in Tokyo.
The standstill region is about 10,000 kilometers (6,213 miles) in diameter and hovers over mountains on the planet's surface, shaped like a bow and very bright, the study's authors said. The strange feature was observed by the Japanese Aerospace Exploration Agency's Akatsuki spacecraft, which entered Venus' orbit in late 2015.
Venus has been known for its weird cloud formations for more than 50 years. For example, a large Y-shaped feature was first observed in ultraviolet light; the Mariner 10 spacecraft in 1973 discovered that this feature was also moving independently of clouds in the same region. In 2015, scientists hypothesized it may be created by centrifugal forces, which draw things on a rotating body away from its rotating center, much like a centrifuge.
The European Space Agency also investigated the role of mountains in disturbing Venus' atmosphere in 2016. Based on observations from the now-finished Venus Express mission, scientists found that air filled with water vapor rises up from the lower atmosphere as it moves over the planet's mountains and creates gravity waves.
"Despite the [similar] name, these have nothing to do with gravitational waves, which are ripples in space-time," Jean-Loup Bertaux, a researcher from the Laboratoire Atmosphères, Milieux, Observations Spatiales in France and the lead author of the European study, said in a 2016 statement.
"Instead, gravity waves are an atmospheric phenomenon we often see in mountainous parts of Earth's surface. Crudely speaking, they form when air ripples over bumpy surfaces. The waves then propagate vertically upwards, growing larger and larger in amplitude until they break just below the cloud top, like sea waves on a shoreline."
Giant gravity wave spotted on Venus
It looks like a giant ripple across the face of Venus, and that's pretty much what it is. Not long after entering Venusian orbit in December 2015, the Japanese Akatsuki orbiter captured a strange atmospheric phenomenon: a gravity wave in the planet's upper cloud layer.
Don't confuse this with the gravitational waves caused by colliding black holes that made history last year. Gravity waves are common, and you've probably seen their effects on the clouds and oceans with your own eyes. They're caused when an atmosphere or body of water is disturbed, such as tides flowing over a sandbar, or air flowing over a mountain range. As gravity attempts to restore equilibrium, it overshoots, causing a wave effect.
This is what probably caused the 10,000-kilometre (6,214-mile) wave across the surface of Venus. As the atmosphere of thick sulphuric acid cloud rotates at a faster rate than the planet itself, details in the surface topography can cause these disturbances, resulting in large-scale gravity waves.
"Here we report the detection of an interhemispheric bow-shaped structure stretching 10,000 km across at the cloud-top level of Venus in middle infrared and ultraviolet images from the Japanese orbiter Akatsuki," the study, led by Makoto Taguchi of Tokyo's Rikkyo University and published today in the journal Nature Geoscience, reads.
"Over several days of observation, the bow-shaped structure remained relatively fixed in position above the highland on the slowly rotating surface, despite the background atmospheric super rotation. We suggest that the bow-shaped structure is the result of an atmospheric gravity wave generated in the lower atmosphere by mountain topography that then propagated upwards."
Every planet in the solar system has its own peculiarities, but Venus is a straight-up weirdo. Although it's sometimes described as Earth's "twin", being rocky like Earth and of a similar size and mass, it's also wildly different. Its incredibly thick atmosphere is over 96 percent carbon dioxide, which creates an incredible greenhouse effect, and it has a mean surface temperature of 735 Kelvin (462 decrees Celsius, or 863 degrees Fahrenheit). Surface probes sent to Venus never last longer than an hour in the extreme heat, so observations need to be made using orbiters such as Akatsuki.
The wave imaged by Akatsuki is positioned over a continent-sized rugged highland called the Aphrodite Terra. But the surprising thing is that these gravity waves can travel into the upper cloud layers -- and that there seems to be more complex dynamics at play than disturbance caused by the planet's surface topography.
The team plans to make further observations in the next few years as Akatsuki continues its mission in orbit around Venus.
"Numerical simulations provide preliminary support for this interpretation, but the formation and propagation of a mountain gravity wave remain difficult to reconcile with assumed near-surface conditions on Venus. We suggest that winds in the deep atmosphere may be spatially or temporally more variable than previously thought," the study reads.