Venus rotation shortened for 6.5 minutes. What’s causing the planet to slow down?

Using the VIRTIS instrument at infrared wavelengths to penetrate the thick cloud cover, scientists studied surface features and discovered that some were displaced by up to 20 km from where they should be given the accepted rotation rate as measured by NASA’s Magellan orbiter in the early 1990s. In the 1980s and 1990s, the Venera and Magellan orbiters made radar maps of the surface of Venus which  gave us first detailed global view of the planet. Magellan’s observations...
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Using the VIRTIS instrument at infrared wavelengths to penetrate the thick cloud cover, scientists studied surface features and discovered that some were displaced by up to 20 km from where they should be given the accepted rotation rate as measured by NASA’s Magellan orbiter in the early 1990s. In the 1980s and 1990s, the Venera and Magellan orbiters made radar maps of the surface of Venus which  gave us first detailed global view of the planet.

Magellan’s observations allowed scientists to determine the length of the day on Venus as being equal to 243.0185 Earth days. And 16 years later, ESA’s Venus Express observed unusual situation – some surface features could only be lined up with those observed by Magellan if the length of the Venus day is on average 6.5 minutes longer than Magellan measured.

“When the two maps did not align, I first thought there was a mistake in my calculations as Magellan measured the value very accurately, but we have checked every possible error we could think of,” said Nils Müller, a planetary scientist at the DLR German Aerospace Centre, lead author of a research paper investigating the rotation.

Magellan spacecraft deployed from the Space Shuttle Atlantis (Credit: NASA)

What could cause the planet to slow down? One possibility may be the raging weather on Venus. Earth experiences a similar effect, where it is largely caused by wind and tides. The length of an Earth day can change by roughly a millisecond and depends seasonally with wind patterns and temperatures over the course of a year.

But a change of 6.5 minutes over a little more than a decade is a huge variation.

Venus Express was able to peer at infrared wavelengths through the dense atmosphere and map surface features observed from both Earth-based radar imaging and with the Russian Venera 15 and 16 missions as well as NASA’s Magellan spacecraft. Topographic maps from Magellan and Venus Express show the shift in surface features. By comparing the currently accepted rotation rate value for the planet from the Magellan mission, scientists discovered a ‘shift’ in surface features of up to 20 km caused by what they believe is a change in the rotation rate of the planet. And the most recent long-duration radar measurements from Earth indicates the same situation.

Other recent atmospheric models have shown that the planet could have weather cycles stretching over decades, which could lead to equally long-term changes in the rotation period. Some scientists suggest that  exchanges of angular momentum between Venus and the Earth when the two planets are relatively close to each other could influence on current observed surface features.

The United States, Soviet Union, and European Space Agency have sent many spacecraft to Venus. Some flew by the planet, some orbited it, some descended through the atmosphere and struck the surface (hard-landed), and a few soft-landed on the surface. Future missions could include long-lived landers, specialized descent probes, aerobots (controllable balloons), and long-term orbital atmospheric monitoring satellites.

Venus Express will keep monitoring the planet to determine if the rate of rotation continues to change.

Venera 13 images of the surface of Venus (Credit: USSR Academy of Sciences/Brown University)

For those that are not so familiar with Venus and it’s characteristic, we compiled some information for better understanding of the subject.

Introducing Venus

Venus has a similar size, mass, density and chemical composition as Earth. That’s why its often called our “twin”. Some even call it our “evil twin“. Of course, its high temperatures and extreme atmospheric pressure (93 bar or 9.3 MPa) make Venus much different than Earth. The atmospheric pressure at the planet’s surface is 92 times that of the Earth (a pressure equivalent to that at a depth of nearly 1 kilometer under Earth’s oceans). At its closest, Venus is only 42 million kilometers (26 million miles) from Earth. Venus has the densest atmosphere of all the terrestrial planets in the Solar System, consisting mostly of carbon dioxide  and a small amount of nitrogen. The CO2-rich atmosphere, along with thick clouds of sulfur dioxide, generates the strongest greenhouse effect in the Solar System, creating surface temperatures of over 460 °C (860 °F). This makes Venus the hottest planet in the solar system.

Using an ultraviolet filter of imaging system, the photo has been color-enhanced to simulate Venus’s natural color as the human eye would see it. Venus is perpetually blanketed by a thick veil of clouds high in carbon dioxide and its surface temperature approaches 900 degrees Fahrenheit. Launched on 3 Nov. 1973 atop an Atlas-Centaur rocket, Mariner 10 flew by Venus in 1974. (Credit: NASA)

Venus: Facts & Figures

Venus Fact Sheet

Venus is always brighter than any star outside our solar system. The greatest luminosity, apparent magnitude −4.9, occurs during crescent phase when it is near the Earth. Venus fades to about magnitude −3 when it is backlit by the Sun. The planet is bright enough to be seen in the middle of the day when the sky is very clear, and the planet can be easy to see when the Sun is low on the horizon. As an inferior planet, it always lies within about 47° of the Sun. 

The clouds of Venus are capable of producing lightning much like the clouds on Earth. The existence of lightning had been controversial since the first suspected bursts were detected by the Soviet Venera probes. In 2006–07 Venus Express clearly detected whistler mode waves, the signatures of lightning. Their intermittent appearance indicates a pattern associated with weather activity. The lightning rate is at least half of that on Earth.  In 2007 the Venus Express probe discovered that a huge double atmospheric vortex exists at the south pole of the planet.  Another discovery made by the Venus Express probe in 2011 is that an ozone layer exists high in the atmosphere of Venus.

The hemispheric view of Venus, as revealed by more than a decade of radar investigations culminating in the 1990-1994 Magellan mission, is centered at 180 degrees east longitude. The Magellan spacecraft imaged more than 98 percent of Venus at a resolution of about 100 m; the effective resolution of this image is about 3 km. (Credit: NASA/JPL/USGS )

Hubble’s observations of Venus show that the atmosphere continues to recover from an intense bout of sulfuric “acid rain,” triggered by the suspected eruption of a volcano in the late 1970′s. This is similar to what happens on Earth when sulfur dioxide emissions from coal power plants are broken apart in the atmosphere to make acid rain. On Venus, this effect takes place on a planetary scale. About 80% of the Venusian surface is covered by smooth volcanic plains, consisting of 70% plains with wrinkle ridges and 10% smooth or lobate plains. Venus has several times as many volcanoes as Earth, and it possesses some 167 large volcanoes that are over 100 km across. The absence of evidence of lava flow accompanying any of the visible caldera remains an enigma.

The planet has few impact craters, demonstrating that the surface is relatively young, approximately 300–600 million years old. Earth’s crust is in continuous motion, but it is thought that Venus cannot sustain such a process. Without plate tectonics to dissipate heat from its mantle, Venus instead undergoes a cyclical process in which mantle temperatures rise until they reach a critical level that weakens the crust. Then, over a period of about 100 million years, subduction occurs on an enormous scale, completely recycling the crust.

The terrestrial planets are the four innermost planets in the solar system; Mercury, Venus, Earth, and Mars. They are called terrestrial because they have a compact, rocky surface like the Earth’s. The planets Venus, Earth, and Mars have significant atmospheres, while Mercury has almost none. This diagram shows the approximate relative sizes of the terrestrial planets. Distances are not to scale. (Credit: Lunar and Planetary Institute)

Discovered By
Known by the Ancients
Known by the Ancients
Date of Discovery
Unknown
Unknown
Average Orbit Distance
108,209,475 km
149,598,262 km
Perihelion (closest)
107,476,170 km
147,098,291 km
Aphelion (farthest)
108,942,780 km
152,098,233 km
Equatorial Radius
6,051.8 km
6,371.00 km
Equatorial Circumference
38,024.6 km
40,030.2 km
Volume
928,415,345,893 km3
1,083,206,916,846 km3
Mass
4,867,320,000,000,000,000,000,000 kg
5,972,190,000,000,000,000,000,000 kg
Density
5.243 g/cm3
5.513 g/cm3
Surface Area
460,234,317 km2
510,064,472 km2
Equatorial Surface Gravity
8.87 m/s2
9.80665 m/s2
Escape Velocity
37,296 km/h
40,284 km/h
Rotation Period (Length of Day)
-243.018 sidereal days (retrograde)
0.99726968 sidereal days
Orbit Period (Length of Year)
0.61519726 sidereal years
1.0000174 sidereal years
Mean Orbit Velocity
126,074 km/h
107,218 km/h
Orbit Eccentricity
0.00677672
0.01671123
Orbit Inclination to Ecliptic
3.39 degrees
0.00005 degrees
Equatorial Inclination to Orbit
177.3 degrees (retrograde rotation)
23.4393 degrees
Orbit Circumference
679,892,378 km
939,887,974 km
Minimum/Maximum Surface Temperature
462 °C
-88/58 (min/max) °C
Atmospheric Constituents
Carbon Dioxide, Nitrogen
Nitrogen, Oxygen

Compare All Eight Planets

Venus rotation

The Earth takes one day to rotate on its axis, and it takes one year to revolve around the Sun in orbit. But Venus takes 243 days to turn once on its axis, and it takes almost 225 days to travel once around the Sun in orbit. As you can see, a day on Venus is longer than its year. If you were standing on the surface of Venus you would see the Sun rise in the west and then take 117 days to travel across the sky and then set in the east.

~Animation of the rotation of the Venus~

And there’s another aspect of Venus that’s different from Earth; Venus rotates backwards. All the planets of the Solar System orbit in a counter-clockwise direction as viewed from above the Sun’s north pole, but Venus rotates clockwise. This is called ”retrograde” rotation. Astronomers think that Venus was impacted by another large planet early in its history, billions of years ago. The combined momentum between the two objects averaged out to the current rotational speed and direction. There are suggestions that Venus may have formed from the solar nebula with a different rotation period and obliquity, reaching to its current state because of chaotic spin changes caused by planetary perturbations and tidal effects on its dense atmosphere, a change that would have occurred over the course of billions of years.

When Venus lies between the Earth and the Sun, a position known as “inferior conjunction”, it makes the closest approach to Earth of any planet, lying at an average distance of 41 million km during inferior conjunction.  The planet reaches inferior conjunction every 584 days, on average. Due to the decreasing eccentricity of Earth, the minimum distances will become greater. From the year 1 to 5383, there are 526 approaches less than 40 million km; then there are none for about 60,200 years. During periods of greater eccentricity Venus can come as close as 38.2 million km. 

Winds near the surface are only strong enough to move sand grains and dust particles, but the upper layers of the atmosphere move very fast. They circle the planet every four days, a pattern called super-rotation. Venus also has atmospheric circulation patterns between the equatorial and polar areas, similar to those on Earth.

Venus and Sun interaction

The diameter of Venus is 12,103 km, while the diameter of the Sun is 1.4 million km. In other words, the Sun is 115 times larger than Venus. You could fit about 1.5 million planets the size of Venus inside the Sun. The Sun has an enormous impact on Venus. The radiation from the Sun is trapped by the thick atmosphere of Venus, raising average temperatures across the planet to around 460 °C. In fact, this makes Venus the hottest planet in the Solar System.

Both the Sun and Venus formed at the same time, some of our scientists say, 4.6 billion years ago, with the rest of the Solar System. They formed out of the solar nebula, a cloud of gas and dust that collapsed down to become the Sun and planets. Because Venus orbits closer to the Sun than the Earth, we always see it close to the Sun in the sky. Venus is either trailing the Sun or leading it across the sky. The best times to see Venus are just before sunrise or just after sunset.

The sun with the path of Venus during the transit superimposed (Credit: SOHO)

The Venusian magnetic field is much weaker than that of the Earth. This magnetic field is induced by an interaction between the ionosphere and the solar wind, rather than by an internal dynamo in the core like the one inside the Earth. Venus’ small induced magnetosphere provides negligible protection to the atmosphere against cosmic radiation. This radiation may result in cloud-to-cloud lightning discharges. The weak magnetosphere around Venus means that the solar wind is interacting directly with the outer atmosphere of the planet. Here, ions of hydrogen and oxygen are being created by the dissociation of neutral molecules from ultraviolet radiation.

Venus-directed CME (Click on the image to start the movie)

The solar wind than supplies energy that gives some of these ions sufficient velocity to escape the planet’s gravity field. This erosion process results in a steady loss of low mass hydrogen, helium, and oxygen ions, while higher mass molecules such as carbon dioxide are more likely to be retained. Atmospheric erosion by the solar wind most likely led to the loss of most of the planet’s water during the first billion years after it formed. The erosion has increased the ratio of higher mass deuterium to lower mass hydrogen in the upper atmosphere by a multiple of 150 times the ratio in the lower atmosphere.

Anyhow, with recent mysteries of giant storm on Saturn and missing spot on Jupiter, now we got new mystery related to Venus. Our planetary system is changing, there is no doubt about it. Is our planet next on the list of changing planetary environment? Well, we shall see…

Featured image: Global radar view of the surface from Magellan radar imaging (Credit:NASA/JPL)

http://thewatchers.adorraeli.com/2012/02/16/venus-rotation-shortened-for-65-minutes-whats-causing-the-planet-to-slow-down/