At the surface, Venus roasts at more than 800 degrees Fahrenheit under a suffocating blanket of sulfuric acid clouds and a crushing atmosphere more than 90 times the pressure of Earth's. Although Venus is often referred to as Earth's twin, since they are almost the same size, it ended up with a climate very different from Earth. A deeper understanding of Venus' atmosphere will let researchers compare it to the evolution of Earth's atmosphere, giving insight as to why Earth now teems with life while Venus suffered a hellish fate.
"Also, its orbit is even more circular than Earth's, which prevents it from getting significantly hotter or cooler by moving closer to or further away from the sun. And while you might expect things to cool down at night -- especially since Venus rotates so slowly that its night lasts almost two Earth months -- the thick atmosphere and sulfuric acid clouds act like a blanket while winds move heat around, keeping temperatures pretty even. Finally, almost all the planet's water has escaped to space, so you don't get any storms or precipitation like on Earth where water evaporates and condenses as clouds."
A set of images of the Venus south polar vortex in infrared light (at 3.8 microns) acquired by the Visible and Infrared Thermal Imaging Spectrometer instrument on ESA's Venus Express spacecraft. The images show the temperature of the cloud tops at about 65 km (40.4 miles) altitude.
A darker region corresponds to higher temperature and thus lower altitude. The center of the vortex, at a temperature of about 250K (around minus 9.7 degrees Fahrenheit), is the deepest zone, exhibiting the highest temperature. Credit: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA
However, higher up, the weather gets more interesting, according to a new study of old data by NASA and international scientists. The team detected strange things going on in data from telescopic observations of Venus in infrared light at about 68 miles (110 kilometers) above the planet's surface, in cold, clear air above the acid clouds, in two layers called the mesosphere and the thermosphere.
"Although the air over the polar regions in these upper atmospheric layers on Venus was colder than the air over the equator in most measurements, occasionally it appeared to be warmer," said Dr. Theodor Kostiuk of NASA Goddard.
"In Earth's atmosphere, a circulation pattern called a 'Hadley cell' occurs when warm air rises over the equator and flows toward the poles, where it cools and sinks. Since the atmosphere is denser closer to the surface, the descending air gets compressed and warms the upper atmosphere over Earth's poles. We saw the opposite on Venus. In addition, although the surface temperature is fairly even, we've seen substantial changes – up to 54 degrees Fahrenheit (about 30 K change) – within a few Earth days in the mesosphere – thermosphere layers over low latitudes on Venus. The poles appeared to be more stable, but we still saw changes up to 27 degrees Fahrenheit (about 15 K change)."
"The mesosphere and thermosphere of Venus are dynamically active," said lead author Dr. Guido Sonnabend of the University of Cologne, Germany. "Wind patterns resulting from solar heating and east to west zonal winds compete, possibly resulting in altered local temperatures and their variability over time."
This upper atmospheric variability could have many possible causes, according to the team. Turbulence from global air currents at different altitudes flowing at more than 200 miles per hour in opposite directions could exchange hot air from below with cold air from above to force changes in the upper atmosphere. Also, giant vortexes swirl around each pole. They, too, could generate turbulence and change the pressure, causing the temperature to vary.
Since the atmospheric layers the team observed are above the cloud blanket, they may be affected by changes in sunlight intensity as day transitions to night, or as latitude increases toward the poles. These layers are high enough that they could even be affected by solar activity (the solar cycle), such as solar explosions called flares and eruptions of solar material called coronal mass ejections.
Changes were seen over periods spanning days, to weeks, to a decade. Temperatures measured in 1990-91 are warmer than in 2009. Measurements obtained in 2007 using Goddard's Heterodyne Instrument for Planetary Wind and Composition (HIPWAC) observed warmer temperature in the equatorial region than in 2009. Having seen that the atmosphere can change, a lot more observations are needed to determine how so many phenomena can affect Venus' upper atmosphere over different intervals, according to the team.
"In addition to all these changes, we saw warmer temperatures than those predicted for this altitude by the leading accepted model, the Venus International Reference Atmosphere model," said Kostiuk. "This tells us that we have lots of work to do updating our upper atmospheric circulation model for Venus."
The team measured temperature and wind speeds in Venus' upper atmosphere by observing an infrared glow emitted by carbon dioxide (CO2) molecules when they were energized by light from the sun. Infrared light is invisible to the human eye and is perceived by us as heat, but it can be detected by special instruments. In the research, it appeared as a line on a graph from a spectrometer, an instrument that separates light into its component colors, each of which corresponds to a specific frequency. The width of the line revealed the temperature, while shifts in its frequency gave the wind speed.
The planet Venus is blanketed by high-level clouds. At visible wavelengths, individual cloud features are difficult to see, but observations made by instruments on ESA's Venus Express orbiter have revealed many small-scale wave trains. Analysis shows that the waves are mostly found at high northern latitudes, particularly above Ishtar Terra, a continent-sized region that includes the highest mountains on the planet.
Venus is a world of contrasts. On the surface, the temperature reaches 450°C (723.15 K), hot enough to melt lead, while winds in the dense atmosphere blow at a sluggish 3-4 km/h. At the cloud tops, temperatures are a frigid -70°C (203.15 K), but wind speeds reach 300-400 km/h, much faster than hurricanes on Earth.
It might be expected, therefore, that there is little connection between the baking atmosphere close to the ground and the upper atmosphere, some 60-70 km above. However, spacecraft observations over several decades indicate that the relationship more resembles an 'ocean-like' lower atmosphere, topped by an opaque cloud layer which acts like the surface of the ocean. Ripples and air currents visible at the cloud tops provide hints about processes and influences far below.
Early evidence of atmospheric waves being generated by air flowing over major topographic features came in 1985, when two Soviet Vega balloons flying at an altitude of 54 km experienced a bumpy ride above the southern uplands of Aphrodite Terra. Almost three decades later, observations made by instruments on board Venus Express provide new evidence that confirms the upward propagation of atmospheric waves from the surface to the main cloud deck and above.
These so-called gravity waves can only exist in a stably stratified atmosphere. They can be triggered, for example, by convection (the rise of lighter, warmer air) from below or by horizontal flow passing over an obstacle, such as a mountain. This is the same process that creates ripples on the surface of a river when it flows over a submerged boulder.
Gravity waves are very important since they can transport energy and momentum by propagating both vertically and horizontally through the atmosphere. They are a common feature in the upper atmospheres of terrestrial planets. On Earth, gravity waves frequently reveal their presence through cloud formations, such as in the case of waves on the lee side of mountains. They often take the form of wave trains - a series of waves travelling in the same direction and spaced at regular intervals.
Evidence for this wave formation process in the atmosphere of Venus was first reported in November 2012, when an international team, led by Silvia Tellmann of the Rheinisches Institut für Umweltforschung, University of Cologne, Germany, used the Venus Express Radio Science Experiment (VeRa) to obtain atmospheric profiles above the planet's limb at altitudes of 40-90 km. By studying changes in the frequencies of the radio signals as they were reflected and bent during their passage through the Venus atmosphere en route to Earth, the team obtained more than 500 atmospheric profiles between the spacecraft's arrival at Venus in 2006 and July 2011. The data enabled them to calculate pressure and temperature at different altitudes and locations above the planet.
These side views of the upper atmosphere made it possible to study the dependence of small-scale, vertical temperature variations on local time and latitude. Temperature differences of a few degrees Celsius and vertical wavelengths of 1-4 km were extracted from the data, revealing numerous gravity waves. These were evident as quasi-periodic disturbances on the atmospheric temperature profiles, often hundreds of kilometres across. The waves were found to be more common at latitudes 60-75 degrees, with the greatest activity on the lee side of mountains in the northern hemisphere.
"We believe that these waves are at least partly associated with atmospheric flow over Ishtar Terra, an upland region which includes the highest mountains on Venus," said Silvia Tellmann. "We don't yet fully understand how such topographic forcing can extend to high levels, but it seems likely to be one of the key processes for the generation of gravity waves at high northern latitudes on Venus. The waves may form when a stable air flow passes over the mountains."
This result has now been confirmed by a separate analysis of waves seen at the cloud tops of Venus in images taken by the Visible Monitoring Camera (VMC) on Venus Express. The new study, published in the January 2014 issue of Icarus, was carried out by an international team led by Arianna Piccialli, a postdoctoral research fellow at the Laboratoire Atmospheres, Milieux, Observations Spatiales (LATMOS-UVSQ), Guyancourt, France. She was based at ESA's ESTEC facility in the Netherlands when the research was undertaken.
The Daily Galaxy via NASA's Goddard Space Flight Center and ESA
Image credit: Layers of Venus' winds, R. Hueso (Universidad del PaÃs Vasco)