1. Venus Winds Meeting
Finding Evidence For Giant Planetary Scale Gravity Waves at Venus’ Cloud Base
Sep 2015 Venus IRTF Images
These slides expand upon those presented at the 1/17/17 meeting, where I described the large north-south bow-like feature seen in temperature maps of the Venus clouds, as imaged by the Akatsuki 10 micron camera. The recent paper that I distributed described this as a feature that remains fixed relative to the surface, while the strong east-west winds rage across the disk at 100 meters/s. The surface only rotates at about 2 meters/second. The paper concludes that it is a stationary mountain wave, the result of wind impinging on the Aphrodite highlands from the east, flowing over the highlands, launching gravity waves are launched upwards from the surface all the way to the top of the clouds at 67 km above the surface. The highlands of Aphrodite are about 3 km high.
There is a very high probability that our telescope images of Venus’ night side clouds over this region also show a bow like feature, but we haven’t noticed it yet. Even if there is no direct visual evidence of this planetary scale wave, the slowdown and speeding up of the east-west winds are almost certainly a result of the wind being slowed down as it passes through this stationary wave. Our job is to show this quantitatively, which will add crucial information on the nature of the mountain gravity wave and its effect on Venus’ atmospheric circulation and chemistry. To begin, I will point out where Aphrodite lies under our cloud images, and the first order of business is to visually inspect these images for any north-south features downwind of Aphrodite that may be visual evidence of the mountain wave WITHIN the clouds, rather than at the top.
Mark A. Bullock
DMNS Jan 31, 2015

2. Typical Venus Express UV Image
This is an image of Venus in reflected solar light at UV wavelengths (0.35 microns). It was taken by the VMC camera on board the Venus Express spacecraft. The spacecraft is flying beneath Venus’ South pole. It is annotated with a latitude and longitude grid. The winds and clouds rotate from right to left (east to west), Note that the longitude decreases to the left, in the direction of the winds. The high continent-like feature on the equator is called Aphrodite Terra, and its highest point is at about 85 degrees longitude, near the top center of the image. The perpetualy bright clouds at south polar latitudes can be seen. What also can be seen is an increase in the brightness of the equatorial clouds right around 80 degrees latitude.

3. Cloud Top Winds and Topography
Black line: Measured cloud top east-west winds as function of longitude.
Right hand Y-axis is inverted – peaks are slower winds.
The blue line is the height of Venus surface at 10° S latitude, as a function of longitude (left hand y-axis).
If the black line is shifted 30° to the left (downwind), it correlates well with surface topography.
Winds flowing over high features will launch waves fixed relative to the surface. These waves propagate all the way to the clouds but remain stationary within the 100 meter/sec east-west flow.
East-west winds slow down when they encounter stationary (fixed to the surface) wave.
The cloud top winds on the dayside of Venus were measured by tracking cloud features in images of reflected solar UV light. The images were taken by the VMC camera on Venus Express. The black line shows the average of east-west winds at the cloud tops as a function of longitude. These winds speeds are the average of winds in a 10 degree latitude band from 5 degree S to 15 degrees S – the latitudes of Aphrodite Terra. Note that the y-axis is decreasing wind speed – therefore the ‘peak’ at 80 degrees longitude is actually the slowest east-west wind. The fastest winds are around longitude 330 degrees.
The blue line represents the height of Venus’ surface as a function of longitude at 10 degrees S latitude. Aphrodite is the double-peaked feature at 100 degrees latitude. Note that if you shift the black curve 30 degrees in longitude to the left (downwind), it correlates well with the surface topography. This is strong evidence that east-west winds flowing over the surface influence winds at the cloud tops at 67 km.

4. How Gravity Waves Work
Schematic of how these gravity waves affect winds within the clouds.
East-west surface winds hit mountains and launch mountain waves upward.
The waves break within the clouds, just like ocean waves.
The thick dark arrows show what happens to the east-west winds when they encounter the waves. The wind speed downwind of the high regions is lower.
Graphic from Bertaux et al

5. …But There’s More
When east-west winds speed up, they ‘stretch’ the atmosphere and more air from below must fill it in. This is ‘upwelling’.
When east-west winds slow down, they ‘compress’ the air, and push the excess downward: ‘downwelling’.
There is much more water vapor within the clouds than above them, so upwelling causes the atmosphere above the clouds to have more water vapor.
Similarly, downwelling ‘dries out’ the atmosphere within the clouds.
Clouds become thicker in regions of upwelling, and thinner in regions of downwelling.
Measurements of water vapor abundance above the clouds show that the atmosphere is more humid where the east-west winds speed up (upwelling) and drier where the winds slow down (downwelling).

6. …And the Mysterious UV Absorber
No one knows what makes the dark and light patches at the cloud tops in the UV (see first image).
Nevertheless, this mysterious substance is responsible for absorbing half of all the sunlight that is absorbed in the Venus clouds.
The unknown absorber thus has an enormous impact on Venus’ climate.
But we do know that it lives only in the upper cloud.
The dark patches correlate with ‘stretching’ winds (upwelling), while the light patches correlate with ‘compressing’ winds (downwelling)
This makes sense if the unknown absorber is dragged up to the top of the clouds in regions of upwelling, but pushed down and out of view in regions of upwelling (just like water vapor).
This still doesn’t tell us what this mystery substance is, but at least it fits into the emerging picture of Venus atmospheric circulation.
In spite of all the wild speculation (life, liquid metal), the dark patches are probably due to elemental sulfur chains like S3, S4, and S8.
Oxygen only exists high up in Venus’ atmosphere, where it is produced by the breakdown of CO2 due to solar UV radiation.
Regions of upwelling are more chemically reducing, favoring S3, S4, and S8 over SO2. Regions of downwelling are more oxidizing, favoring SO2 over S3, S4, and S8.

7. UV Albedo and Venus Winds
Correlation between east west winds and the brightness (albedo) of the Venus cloud tops at UV wavelengths.
Bertaux et al suggests that the unknown UV absorber is pulled out of the upper cloud and to the top of the clouds in regions of upwelling, making the cloud tops dark.
Similarly, the clouds are bright in regions of downwelling because the UV absorber sinks beneath the cloud tops.

8. Why Our Measurements Are So Important
Images from Akatsuki of the mountain gravity wave are of the cloud tops (67 km) on the day side.
Measurements of east-west wind variations are from UV images of the cloud tops (67 km) on the day side.
The waves must propagate through the cloud deck, including levels where we measure winds, near the cloud base at km on the nightside.
There may be visual evidence of the giant north-south bow of gravity waves at 80° longitude in our cloud images, but we have never noticed them.
Measuring east west winds around 80° longitude above Aphrodite should show a similar pattern of speeding up and slowing down seen at the cloud tops in UV images.
Quantitative analysis of these winds at km will determine the physical properties of the gravity waves (speed, wavelength, angular momentum) at the cloud base.
Bracketing the wind and wave measurements at the top and bottom of the clouds will provide the first-ever 3D reconstruction of the mountain waves.

9. What We Need to Do
First order of business is to identify the images where Aphrodite Terra is lurking beneath the clouds, and not on the other side of the planet.
Second order of business is to carefully inspect the region around 80° longitude in all these images to look for north-south bow-like features that indicate the gravity wave. We may have missed it.
Third order of business is to measure the east-west winds at several latitudes and longitudes in these images to look for changes in windspeed across the 80° meridian.

10. Orientation – Sep 25 2015 Images
The blue x is the center of the disk.
The red line of x’s is where the peak of Aphrodite Terra lies (80° longitude)

11. 1. Identify the images where Aphrodite Terra is lurking beneath the clouds
September 25, 2015 Venus Images
I ran the JPL HORIZONS ephemeris to determine the Venus longitude and latitude in the very center of the disk (Center Lat and Center Lon in table below).
This indicated that Aphrodite Terra (80° longitude) is halfway between the center of the disk and the right hand limb in the September 2015 images.
I also determined the latitude and longitude of the subsolar point (i.e., noon). This is Subsolar Lat and Subsolar Long.
This allowed me to determine the local time of day at the center of the disk, which was about 4:30 am. But see notes below the table. A ‘day’ on Venus is actually 217 Earth days.

12. This is your voyage of discovery. Go forth and find the wave!
2. Inspect the region around 80° longitude in all these images to look for north-south bow-like features that indicate the gravity wave
This is your voyage of discovery. Go forth and find the wave!
This is Hot, Hot, Hot! I’d like to get some real results by the next meeting. So let’s be in frequent contact over the next 2 weeks. I will work on (3) and show you some wind velocities from the Sept 2015 images then.