A periodic variation in the atmosphere of Venus

Hi there! Today’ post will be my first on Venus. More precisely, it deals with its atmosphere. As you may know, the planet Venus is known for its very thick atmosphere, which precludes optical observations of its surface. The study I present today is entitled “Discovery of a 150 day period in the Venus condensational clouds”, by Kevin McGouldrick and Constantine Tsang, who work in the city of Boulder, CO (I love this place). This study has been recently accepted for publication in Icarus.

Some Venus facts

Venus is the second innermost planet of the Solar System, which means that its orbit is interior to the one of the Earth. It is sometimes said to be a twin sister of the Earth because its diameter is 95% the diameter of the Earth. However, the meteorological conditions make it a very hostile place for life. The surface pressure is ~93 times the one of the Earth, the temperature is about 470˚C, and the atmosphere is essentially made of carbon dioxide.

Its rotation is very interesting, since it rotates very slowly, and in the retrograde direction. It has a rotation period of 245 days, while its orbital period around the Sun is only 225 days. This means that a Venusian day is longer than a Venusian year. This peculiar rotational state could result from the atmospheric tides, i.e. the way the dense atmosphere interacts with the gravitational forcing of the Sun, loses some energy, and also interacts with the surface. However, the atmosphere moves much faster, with a period of about 4.2 days.

The exploration of Venus

As a putative twin sister of the Earth and a nearby planet, Venus has been a priority target of the Space Race. This is why several American and Soviet probes reached it between 1962 and 1984, allowing major progress in our knowledge of the planet. Here are the probes:

  • 1962: Mariner 2 (USA). This probe was the first one to perform successfully a flyby of another planet than the Earth. It proved that the surface was hot, detected no magnetic field, and it improved our knowledge of the mass of the planet. Beside these results of Venus, it made measurements of the Solar wind and allowed many technological improvements in space navigation and telecommunication.
  • 1965: Venera 4 (USSR), Mariner 5 (USA). Venera 4 crashed on Venus after a fall in the atmosphere with a parachute, permitting the first in situ measurements of its chemical composition, and detection of a weak magnetic field, which Mariner 2 could not have detected. Mariner 5 made a flyby of Venus and analyzed its outer atmosphere.
  • 1969: Venera 5 & 6 (USSR) were technologically similar to Venera 4, but with specific improvements of the analysis of the atmosphere, based on the results of Venera 4.
  • 1970: Venera 7 (USSR) was the first probe to land on another planet than the Earth and to transmit data from the surface. It made the first accurate measurement of the temperature and the pressure at the surface.
  • 1972: Venera 8 (USSR) showed that the atmosphere of Venus was pretty clear below 50 km, meaning that the clouds had a higher altitude.
  • 1975: Venera 9 & 10 (USSR). These two probes were the first ones to send images of the surface of another planet than the Earth. Moreover, Venera 10 measured the velocity of the wind.
  • 1978: Venera 11 & 12 (USSR), Pioneer Venus Multiprobe (USA). Venera 11 & 12 made more accurate measurements of the composition of the atmosphere, and detected lightning and thunder. Pioneer Venus Multiprobe launched 4 probes to the surface of the planet, to analyse the atmosphere during their fall. One of these probes survived the impact, but did not have any imaging instrument. These probes identified 3 layers of clouds in the atmosphere.
  • 1978-1992: Pioneer Venus Orbiter (USA). This spacecraft was the companion of Pioneer Venus Multiprobe, and was inserted into orbit on Dec 4th 1978. Its orbit was very eccentric (0.8), and it contained 17 instruments, allowing to study the magnetic field of Venus, its gravity field, its atmosphere… It also monitored the water loss of the Halley’s comet in 1986.
  • 1981: Venera 13 & 14 (USSR) were landers, they made measurements of the atmosphere during the fall and took images of the surface.
  • 1983: Venera 15 & 16 (USSR). These probes were orbiters equipped with radars. They mapped ~25% of the surface.
  • 1984: Vega 1 & 2 (USSR + Europa). These two probes made a flyby of Venus to launch a lander devoted to make measurements of the atmosphere. After the flyby, the probes approached Halley’s comet and took ~1,500 images of it.
  • 1990: Flyby by Galileo (USA). Galileo was sent to Jupiter, but used the gravitational assistance of Venus on its way. This was the opportunity to study the composition of the clouds of Venus, in comparing the measurements at 1.74 and 2.30 μm, i.e. in the infrared. These two bandwidths correspond to minimal absorption by carbon dioxide and by water, so they can be used not only to detect a signal from the surface of Venus, i.e. the Solar light reflected by the surface, but also to estimate the temporal evolution and the composition of the clouds.
  • 1990-1994: Magellan (USA). This orbiter studied the gravity field of the planet, and also provided a detailed map. It particularly revealed the presence of many volcanoes, few impact craters and large lava plains, meaning that the surface is geologically young, and evidence of some tectonic activity, which is pretty different than the terrestrial one. It was revealed by low domical structures called coronae, produced by the upwelling and subsidence of magma from the mantle.
  • 1998-1999: 2 flybys by Cassini (USA), on its way to Saturn.
  • 2006-2015: Venus Express (Europa), see next paragraph.
  • Since 2015: Akatsuki (Japan). This spacecraft should have orbited Venus since 2010, but that maneuver failed. It then orbited the Sun during 5 years in safe mode before succeeding another orbital insertion in December 2015. This spacecraft essentially studies the dynamics of the atmosphere of Venus during a 2 year regular scientific mission, which has started in May 2016.

Venus Express (VEX)

This ESA spacecraft has been launched in November 2005, and was inserted in orbit in April 2006, originally for a 2-year mission… which was completed 9 years later! The main objective of that mission was to understand the dynamics of the atmosphere of Venus, with the hope of a better understanding of the atmospheric evolution in general. It contained 7 instruments, 3 of them being devoted to spectrometry (VIRTIS, SPICAV and PFS), one to radioscience (VeRa, for Venus Radioscience), one was a magnometer (MAG), one for imaging (VMC, for Venus Monitoring Camera), and the last one, ASPERA-4, investigated the interaction between the Solar wind and the Venusian atmosphere. We are today particularly interested by VIRTIS, for Visible and Infrared Thermal Imaging Spectrometer, which measured the emitted radiance in 1.74 μm and 2.30 μm of the night-side of Venus.
Venus Express had a polar and highly eccentric orbit. Its high eccentricity resulted in a large variation of the distance between the probe and the planet, i.e. from 460 to 63,000 km, with a period of 24 hours. As a consequence, the field of view and resolution of the measurements experienced large variations.
An interesting thing to notice is the fact that Venus Express reused some technologies designed for Mars Express and Rosetta.

This paper

The authors analyzed the emitted radiance in the infrared at different latitudes, for the two wavelengths 1.74 μm and 2.30 μm. Unfortunately, they do not have measurements later than 2008 October 27, because of the failure of the instrument’s cooling system (keep in mind that infrared is very sensitive to the temperature). Moreover, they used only data taken at a distance larger than 10,000 km. The variation of this radiance characterizes the dynamics of the lower region of the clouds, at an altitude between 50 and 55 km. Observing at these two wavelengths permits to draw conclusions on the size of the particles constituting the clouds. Actually, 4 sizes of particles are expected in the clouds of Venus, and in this specific region:

  • Mode 1 particles: they have an average diameter of 0.6 μm, and are expected in the upper region,
  • Mode 2 particles: they have an average diameter of 2 μm, and are expected in the upper region as well,
  • Mode 2′ particles: they have an average diameter of 3 μm, and are expected in the lower and middle regions,
  • Mode 3 particles, with a diameter of 7 μm, are expectd in the lower region.

So, for our lower clouds, we expect only Mode 2′ and Mode 3 particles.

The authors used VIRTIS data, and after denoising they averaged the measurements over 7 days, since they are interested only in the long-term dynamics. Since the atmosphere is rotating, the authors could thus only detect variations in time and in latitude, but not in longitude.

And the results are these: the radiance steadily increases at mid-latitudes, while it decreases near the poles, which could reveal a circulation of clouds over a very-long term. This long-term variation should be a periodic effect, which future measurements by Akatsuki should help to understand.
Moreover, the authors noticed a 150-day periodic variation in the cloud coverage, especially in the 1.74 μm radiance data, at mid-latitude. This is an unexpected result, which had already been hinted by the same authors 4 years before, with less data. The cause of this periodicity still needs to be elucidated. The authors notice that this period is almost two thirds of the rotation period of Venus, but this may be by chance. This could be the manifestation of a Hadley-like circulation, i.e. a kind of circular motion of the atmosphere driven by variations of its temperature, itself controlled by the latitude and the altitude.

Some links

That’s it for today! As usual, I am interested in your feedback. Let me know what you think about this article, what kind of articles you are interested in, if you have specific questions on the science behind…
 

Thanks!

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