# Venus: Jet-setting atmosphere

Alain Hauchecorne

► To cite this version:

Alain Hauchecorne. Venus: Jet-setting atmosphere. Nature Geoscience, 2017, 10 (9), pp.622 - 623.
10.1038/ngeo3022. insu-01599342

HAL Id: insu-01599342

https://insu.hal.science/insu-01599342v1

Submitted on 3 Oct 2025

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
sustainable responses in a region that is
understood to be particularly vulnerable.

Daniel Farinotti is at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) of ETH Zurich, Hönggerbergring 26, CH-8093 Zürich, Switzerland and the Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Zürcherstrasse 111,

CH-8903 Birmensdorf, Switzerland.

e-mail: daniel.farinotti@ethz.ch

References

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Published online: 7 August 2017

VENUS

Jet-setting atmosphere

A fast equatorial jet in the Venusian cloud layer has been revealed by the Akatsuki orbiter by tracking cloud movement in near-infrared images. The findings suggest that the Venusian atmosphere is more variable than previously thought.

Alain Hauchecorne

Venus is permanently covered by cloud layers composed of sulfuric acid and water-liquid droplets that extend from 45 to 70 km in altitude. These cloud layers prevent us from directly observing the planet's surface. Venus rotates very slowly in the westward direction: a Venus day lasts 243 Earth days. Its atmosphere rotates in the same direction, but much faster; it completes a rotation in roughly only four Earth days. Why the Venusian atmosphere rotates so much faster than its planet — an unusual situation called superrotation — remains debated, but it is thought that abundant atmospheric waves may play a role in maintaining the fast atmospheric flow. Writing in *Nature Geoscience*, Horinouchi et al.¹ report the presence of an equatorial jet that suggests unexpected variability deep in the cloud layer that could also be linked to interactions with atmospheric waves.

The upper cloud layer above 57 km in
altitude lies in stable stratified atmosphere.
The wind speed at the top of the clouds has
been estimated since the 1960s by tracking
the displacement of identifiable features in
the upper cloud layer². The fast rotation of
the Venusian atmosphere relative to that of
its planet has also been confirmed by probes
that measured a nearly linear decrease of the
zonal wind with depth³.

Below 57 km, the lower and middle cloud layers lie in convective unstable atmosphere and are denser and thus more difficult to study. Fortunately, the atmosphere of Venus is not totally opaque at near-infrared wavelengths. Some thermal emission at these wavelengths can escape from the hot lower atmosphere and this near-infrared radiation can be used to track the motion of clouds in the lower and middle layers³. Wind speed estimates from spacecraft data have ranged

from 50 to 70 ms⁻¹, either constant at low- to
mid-latitudes or with a slight maximum at
mid-latitude⁴.

Horinouchi and co-workers¹ used
new night-side images of Venus from
the Akatsuki orbiter to track small-scale
structures observable at near-infrared
wavelengths in the Venus cloud layers.
They detected strong winds in the low and
middle cloud region that exceeded 80 ms⁻¹

during July and August 2016 — faster than
previously reported for this altitude. The
wind speed peaked near the equator and
they determined that the anomalous high
velocities represented a continuous horizontal
feature, rather than representing changes in
the clouds altitude. The evidence is consistent
with an equatorial jet.

The equatorial jet was present in July and
August 2016, but not in earlier observations

<figure><img src="image_2.png" /></figure>

**Figure 1** | Variable cloud motions in the atmosphere of Venus. Using images from the Akatsuki spacecraft, Horinouchi et al.¹ tracked the motion of the Venusian atmosphere from near-infrared emissions through holes in westward-moving clouds, revealing the appearance and disappearance of an equatorial jet. One possible explanation for the jet is that gravity waves, triggered by the vertical displacement of atmospheric flow (small arrows) over a mountain below, propagate upwards (blue wiggly line) and transfer their momentum upon reaching the unstable middle clouds.
in March 2016, indicating large variability in the zonal flow. The appearance and disappearance of the jet suggests that it may be a periodic feature, which Horinouchi and co-authors hypothesize could be the result of zonal wind variability driven by interaction with atmospheric waves.

High topography on Venus may generate a type of atmospheric wave called a gravity wave, for which the buoyancy force acts to restore a disturbance. Gravity waves are generated when horizontal atmospheric flow passes above topographic relief and then propagate vertically through stable atmosphere. When a gravity wave reaches unstable (or convecting) atmosphere at higher altitude and breaks, it transfers its momentum and acts to accelerate the wind in the direction of propagation⁵.

A huge bow-shaped structure attributed to a gravity wave was observed at the top of the upper cloud layer in images from Akatsuki⁶ on the west side of the Aphrodite Terra, a mountainous region close to the equator. Potential atmospheric waves detected in night-time thermal infrared images obtained with the Venus Express orbiter also correlate

to high-elevation regions⁷. Furthermore, wind speeds at the top of the cloud layer measured from Venus Express observations at low latitudes correlate with the underlying relief of Aphrodite Terra and have been interpreted as the consequence of gravity wave breaking⁸. In principle, gravity waves should not be able to propagate through the convective zone encountered in the middle cloud layer between 52 and 57 km altitude. However, temperature profiles derived from Venus Express⁹ suggest that the convective zone is occasionally sufficiently stable in the equatorial region for the waves to propagate.

It is unclear how an equatorial jet can form at the middle cloud level of Venus, but one explanation for the temporal variability displayed by the jet could be the intermittent momentum transfer from atmospheric waves — some of them triggered by mountainous topography near the equator — during periods when they can propagate, break and accelerate the surrounding air (Fig. 1).

The findings extracted from Akatsuki and Venus Express observations by Horinouchi *et al.*¹ and others⁶,⁷,⁸ suggest that the Venusian atmosphere is more variable

than previously appreciated, although the underlying mechanisms are not entirely clear at this point. From the discovery of a transient equatorial jet in the lower cloud layer to spotting an orographic gravity wave that forces it way up to the cloud tops, these observations reveal atmospheric dynamics that are much more active than the prevailing view of a largely uniform state of superrotation on Venus. ☐

Alain Hauchecorne is at LATMOS/IPSL, UVSQ Université Paris-Saclay, UPMC University Paris 06, CNRS, 78280 Guyancourt, France.

e-mail: alain.hauchecorne@latmos.ipsl.fr

## References

1. Horinouchi, T. et al. Nat. Geosci. 10, 646–651 (2017).

2. Boyer, C. & Camichel, H. Ann. Astrophys. 24, 531–535 (1961).

3. Allen, D. A. & Crawford, J. W. Nature 307, 222–224 (1984).

4. Schubert, G. et al. J. Geophys. Res. 85, 8007–8025 (1980).

5. Lindzen, R. S. J. Geophys. Res. 86, 9707–9714 (1981).

6. Fukuhara, T. et al. Nat. Geosci. 10, 85–89 (2017).

7. Peralta, J. et al. Nat. Astron. 1, 0187 (2017).

8. Bertaux, J.-L. et al. J. Geophys. Res. 121, 1087–1101 (2016).

9. Tellmann, S., Pätzold, M., Häusler, B., Bird, M. K. & Tyler, G. L. J. Geophys. Res. 114, E00B36 (2009).

Published online: 28 August 2017

## PLATE TECTONICS

# Crustal recycling evolution

The processes that form and recycle continental crust have changed through time. Numerical models reveal an evolution from extensive recycling on early Earth as the lower crust peeled away, to limited recycling via slab break-off today.

### Valentina Magni

Continental crust has been created and destroyed throughout Earth's history. Estimates of the balance between this loss and gain are vigorously debated, but there is a mounting consensus that a large portion of continental crust formed early in Earth's history and has since been partly recycled back into the mantle¹,². Determining which processes were responsible for crustal recycling in the past is a difficult task because we have very little information on the dynamics and style of plate tectonics on early Earth. Writing in *Nature Geoscience* Chowdhury *et al.*³ use numerical simulations to show that when Earth was hotter — during the late Archaean and early Proterozoic — crustal recycling was probably much more extensive and rapid than today, and characterized by the peeling-off of dense lower continental crust into the mantle.

Today, subduction zones are sites of continental crust formation and destruction²,⁴. Continental crust is mainly created in volcanic arcs above subduction zones, where

basaltic magma generates and subsequently differentiates to more felsic compositions. Relatively small amounts of continental crust are recycled back into the mantle as the tectonic plates collide, through subduction and erosion of continental material. Subduction is also a driver for plate tectonics. It is therefore natural that in past decades there has been considerable focus on understanding how subduction dynamics and, more generally, plate tectonic style has evolved. Observations of the geochemical signature of volcanic-arc magmas found in the felsic parts of Archaean continents has led to the idea that continental crust formed via subduction during the Archaean, too⁵,⁶. Yet some suggest that modern-style plate tectonics did not begin on Earth until later⁷ and that early continental crust might have formed by different mechanisms. For example, foundering of thick, dense mafic crust into the mantle followed by mantle melting and consequent fractionation of mafic magmas to create more felsic crust⁸. Although

there is no agreement on the processes leading to its formation, we know that early continental crust was on average more mafic compared to the present-day continents that are more silica-rich⁹.

Recent continental growth models suggest that 60 to 70% of the present-day volume of continental crust was already present about three billion years ago¹. However, only less than 10% of crust of that age is preserved today, implying that a large portion of the early continental crust has been lost back into the mantle.

Chowdhury and colleagues³ propose a mechanism that can explain both the extensive crustal recycling and the formation of a more felsic continental crust during the late Archaean and early Proterozoic. They use numerical modelling to simulate continental collision at different mantle temperatures; that is, at different times in Earth's history. Their simulations show that under the much hotter mantle conditions that characterized the younger Earth, delamination would be