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Snow, a Slowing Planet, and a Last Dangerous Dance with Venus

The views expressed are those of the author and are not necessarily those of Scientific American.

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Venus Express streaks across the upper atmosphere of its planetary host (Credit: ESA, artist's impression)









In about a month’s time, the European Space Agency’s (ESA) Venus Express spacecraft will adjust its orbit and dip into the outer venusian atmosphere. This hypervelocity skimming will allow scientists to not only obtain a little more data on Venus’s atmosphere, but to also learn more about the seat-of-your-pants technique called aerobraking. It’s also the mission’s swan song after an incredible array of discoveries, from snow to an unexpected planetary spindown.

An aerobraking procedure is one in which the atmosphere of a planet is used to slow a spacecraft, either for full atmospheric entry, or as a means to adjust the orbit without expending precious fuel. For example, it’s a way to circularize an elliptical orbit, by ‘braking’ a craft during its lowest pass – bleeding off orbital velocity via the drag of the atmosphere. The Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter missions all made use of aerobraking to reach their nominal orbital configurations. The Magellan mission to Venus did this too – circularizing its orbit.

But it’s a tricky business. The top of a planetary atmosphere is a somewhat fickle beast, strongly affected by solar radiation and planetary conditions. Small variations in the local density of gas can have major effects on a spacecraft plowing through at well over 10,000 mph.

So why risk Venus Express? Its science mission is effectively completed, and its fuel is almost gone, there’s little to lose by playing chicken. If the spacecraft doesn’t survive this dip into the ionosphere (at an altitude of some 130 km, more than 30 km deeper than the spacecraft has gone before) it will simply burn up. If it makes it through, it’ll live to play for another few months, gathering a little more data, before taking the ultimate dive.

This all seems more reasonable when you consider the incredible scientific bounty from the mission. After arriving at Venus back in 2006 it has gazed at this cloud-covered world in infrared, visible, and ultraviolet light, measured magnetic fields, the interaction of the solar wind with the atmosphere, and studied the atmospheric temperature structure and composition. One of many major discoveries came when the spacecraft spotted a crazy double vortex at Venus’s southern pole – shown here in the infrared.

4 shots of the southern venusian polar vortex, darker shades indicate higher temperatures/lower altitude(Credit: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA)













It also discovered an unexpected long-term climate trend. When it arrived at Venus the atmosphere at 70 kilometers altitude was circulating with an average speed of about 300 km/h. But now, some 8 years later, that powerful ‘super-rotation’ (faster than the solid body of the planet itself spins) has shot up, hitting 400 km/h.

Why is this? We don’t know.

Even more surprising was the discovery that the solid surface has altered its spin rate in the years since the 1990s when the Magellan spacecraft made measurements. Venus is a ponderous beast, it takes some 243 Earth days to spin around once (compared to 4 days for its bulk atmosphere). But 16 years ago it was spinning a little faster, by some 6.5 minutes each rotation.

That’s an incredible change, this is an entire rocky planet, with enormous inertia. One possible explanation is linked to the thick atmosphere, which can exert significant frictional drag on the surface. It may also mean that Venus has a molten core, allowing its outer crust to respond more readily to these atmospheric forces.

The list of Venus Express discoveries goes on, from an ozone layer, to the possible existence of an atmospheric ‘snow line’ at about 125 km altitude, where temperatures plummet to 175 Kelvin – allowing for carbon dioxide to freeze out.

All in all Venus Express has earned the right to one last tango, even if it’s a dangerous one.

Caleb A. Scharf About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His books include Gravity's Engines (2012) and The Copernicus Complex (2014) (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. SJCrum 5:35 pm 05/22/2014

    The science involved with all of the things described in the article is as follows.
    First, the spin of a planet, like earth for example, is that the sun’s heat causes a pull on the left side for earth, and that is the side that is coldest and which has always just come from the night temperatures that are colder. In other words, the sun pulls on the colder side more than the right, warmer one.
    For Venus, it is the exact same except that the temperatures there are enormous hotter, and that leads to the mystery as to where any cold has eve come from.
    To make a long description short, when the extreme heat event on earth occurred that killed the dinosaurs, it also caused Venus to be temporarily pulled outward into space and away from the sun. And, the exact same as occurred for earth for the Ice Age that occurred right after the heat event.
    So, for Venus, when that planet later got pulled back toward the sun, the pull caused a spin, and a cold/hot type of spin pull for Venus.
    As for why it is slowing down, there isn’t enough night time cold, and that because of the slow spin causing low cold effect.
    As for the high-speed atmospheric gases, they are much lighter and are spinning at a far greater speed around Venus, and which is causing the huge southern pole type of vortex.

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  2. 2. jtdwyer 6:14 pm 05/22/2014

    Very interesting! See also and, on another front,

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  3. 3. SJCrum 7:44 pm 05/22/2014

    The giant red spot on Jupiter is blocked for comment, so the following is why it is there, etc.
    It was caused during the enormous heat explosion that killed all of the dinosaurs and which also caused the enormous black spot on Mars. Both planets were hit by the heat and directly toward the two spot locations. They were both exactly in the center of the blast.
    As for why it spins, it is all heat-attraction of colder air near the polar area above, and the opposite of that air being pulled back toward the equator after the air cools down. So, it is then a continuous spin.
    As for why it is slowing down, that is because the original heat is disappearing and there is no longer the pulling heat, and the heat that causes the air to get hotter.
    As for why it is red, mars is red also, and that because of the extreme heat causing a magma color of red. As for why magma turns brown soil color into red, when the soil liquefies it has its atoms tilt to an angle that then has the electrons radiate a tilted direction of light that is red. As for why that occurs, the reason is because all atoms are made with enormous, molten-like heat and that heat causing a normal straight up axis of the atom to have its northern pole with a positive charge to be pulled downward toward the center of the heat below. When it cools, solidity of the soil occurs before the heat pull from below disappears. So, the color direction is locked in place.
    As for the red color of the spot also, the swirling causes a tornado type of upward pull that sucks the red dirt from below up into the spinning air clouds. As for one last item, the spinning red spot will last another five hundred and two years before it will be finally gone.

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  4. 4. SteveO 6:25 pm 05/23/2014

    SJCrum, you really have no idea what science is, do you? These and other posts of yours show a predilection for magical thinking – if only you believe hard enough, it will be true; that because something makes sense to you, in violation of all observed phenomena, it is true. It is remarkable seeing this in the modern era.

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