To Understand Habitability, We Need to Return to Venus

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This column was written by my colleague, Elizabeth Tasker.  Based in Tokyo, she is a scientist and communicator at the Japanese Space Agency JAXA and the Earth-Life Science Institute (ELSI).  Her book, “The Planet Factory,” was published last fall.


This image shows the night side of Venus in thermal infrared. It is a false-color image using data from the Japanese spacecraft Akatsuki’s IR2 camera in two wavelengths, 1.74 and 2.26 microns. Darker regions denote thicker clouds, but changes in color can also denote differences in cloud particle size or composition from place to place.  JAXA / ISAS / DARTS / Damia Bouic

“You can feel what it’s like on Venus here on Earth,” said Kevin McGouldrick from the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder. “Heat a hot plate until it glows red, place your palm on its surface and then run over that hand with a truck.”

The surface of Venus is a hellish place. Suffocated by a thick atmosphere, pressure on the Venusian surface is 92 times greater than on the surface of Earth. Temperatures sit at a staggering 863°F (462°C), which is sufficient to melt lead.

The longest a spacecraft has survived in these conditions is a mere 127 minutes; a record set by the Russian Venera 13 mission over 35 years ago.

As the brightest planet in the night sky, Venus allured ancient astronomers into naming the world after the Roman mythological goddess of love and beauty. This now seems an ironic choice, but the contrast between distant observation and surface conditions produces an apt juxtaposition for exoplanets.

The comparison has led to an article in Nature Geoscience by McGouldrick and a nine author white paper advising on astrobiology strategy for the National Science Foundation. The conclusion of both publications echoes the irony of Venus’s name: we need to return to the inferno of Venus to understand habitable worlds.

 

A portion of western Eistla Regio is displayed in this three-dimensional perspective view of the surface of Venus. Synthetic aperture radar data from the spacecraft Magellan is combined with radar altimetry to develop a three-dimensional map of the surface. Rays cast in a computer intersect the surface to create a three-dimensional perspective view.  The simulated hues are based on color images recorded by the Soviet Venera 13 and 14 spacecraft. The image, a frame from a video released in 1991, was produced at NASA’s JPL Multimission Image Processing Laboratory.

In the last 25 years, scientists have discovered over 3,500 extrasolar planets. The vast majority of these worlds have not been imaged directly, but are detected by tiny influences on their host star. These observations provide a measurement of the planet’s size and the average energy received from the star, but no details of the conditions at the planet surface. This leaves modern astronomers as blind to exoplanets as the ancient Romans were to the worlds of our solar system.

While we cannot visit the surface of exoplanets, our knowledge may be about to take a major leap forward. New instruments such as NASA’s James Webb Space Telescope (launch date 2019) and ESA’s Ariel mission (launch 2026) are aiming to detect the atmospheres of these distant worlds. The enveloping gases are a product of the planet’s geology, chemistry and biology, producing a direct indication of what is occurring on the surface.

However, this signature is hard to measure for terrestrial worlds with thin atmospheres, requiring a large number of precious observing hours. This means we need to select our telescope targets carefully. An ideal choice would be a planet writhing with geological and biological activity that is imprinting its presence in the atmosphere.

In short, what we want is an exo-Earth. What we don’t want is an exo-Venus.

Surface photographs from the former Soviet Union’s Venera 13 spacecraft, which touched down in March 1982. Ten probes from the Venera series successfully landed on Venus and transmitted data from the surface of the planet between 1961 and 1984. In addition, thirteen Venera probes successfully transmitted data from the atmosphere of Venus.

At first glance, a tantalizingly simple way to distinguish these two planets is the circumstellar habitable zone. Broadly defined, this is the region around a star where the radiation levels are right to support liquid water on a planet’s surface. At the inner edge of the habitable zone, water evaporates and is rapidly lost from the planet in a process known as ‘runaway greenhouse’. At the outer edge, carbon dioxide condenses into clouds and is unable to trap sufficient heat to prevent global freezing.

With a thick atmosphere lacking in water, Venus shows signs of having undergone a runaway greenhouse phenomenon that would seem to support the habitable zone edges. But digging a little deeper reveals more complex story. The habitable zone edges are traditionally calculated via climate models for the Earth. Yet, there is evidence that Venus was never that similar to the home planet we know.

While the Earth’s crust is broken into plates, Venus is thought to have a ‘stagnant lid’; a non-mobile crust that does not cycle material and nutrients up from the planet’s interior. Possibly linked with that —or maybe not— Venus has no magnetic field and would likely have suffered a similar fate to Mars and lost most of its atmosphere if it did not have such a thick reservoir of gases. There are almost certainly other differences; we don’t know because we haven’t been back to the Venusian surface since the 1980s.

What this means is that the path that led Venus to be an unimaginable inferno likely started a long time before a runaway greenhouse could occur.

If the deviation from Earth were initially driven by geodynamics, formation or other non-climate differences (such as the failure to form crustal plates, or poor internal heat circulation preventing a magnetic field) then the boundary between Earth and Venus is likely unrelated to the climate-based habitable zone. Without understanding the history of this second Earth-sized planet in our own system, we have no hope of differentiating between a Venus or Earth around another star.

A topographic map of Venus, with sinusoidal projection, based on data from the Magellan spacecraft, which was one of the few missions launched from the space shuttle.  It left for Venus in 1989 and mapped the planet until 1994.  (NASA)

But is this concern unnecessary? While an exo-Venus would not be an interesting place to hunt for signatures of life, we might learn about the formation of our two worlds by exploring the atmosphere of similar planets around other stars.

And this is where we hit a second problem; we don’t understand the atmosphere of Venus.

Models predict that planets that have undergone a runaway greenhouse should have an oxygen rich atmosphere. This is from evaporated water molecules that are broken apart by the ultraviolet radiation from the Sun, causing the constituent hydrogen atoms to escape the planet’s gravity while the heavier oxygen is left behind.  Venus’s atmosphere isn’t oxygen rich. It’s full of carbon dioxide and clouds of sulphuric acid.

This leaves us trying to understand the distant signature of alien atmospheres without a good comprehension of the examples we have in our own Solar System.

These arguments present a compelling case for Venus. Yet although three out of twelve proposals for NASA’s New Frontiers Program were for Venus missions, none were selected as finalists last December. The same was true for the last call for the Discovery Program; two out of five finalists for this lower cost mission category were for Venus, but neither were selected despite Discovery proposals being typically more focussed on the inner Solar System.

Kevin McGouldrick of the University of Colorado, Boulder, says the primary focus of his research is the nature and evolution of the clouds of Venus. Some 35 to 55 miles above the surface of Venus,  temperatures and pressures resemble those at the surface of the Earth.

Is there a reluctance to go to Venus?

Undoubtedly, planet surface conditions that can melt a spacecraft curbs enthusiasm. The insulation required to protect a probe on the Venusian surface or conduct a short mission will drive costs skyward and may seem a poor return compared to other projects.

Missions to study the Venusian atmosphere would be substantially less risky. The only mission currently operating around Venus is Japan’s Akatsuki orbiter, which last year returned data suggesting mountainous terrain on Venus was sufficient to drive waves through the huge weather system.

In his Nature Geoscience article, McGouldrick proposes that detailed monitoring of Venus’s atmosphere is needed to understand the planet’s history. “To find out why Venus is how it is now, we need to know how it used to be,” he pointed out.

This information could be found in the abundances and isotopes (different variations of the same element) of the noble gases on Venus. These unreactive elements are acquired during planet formation, so changes in their quantities indicate losses in Venus’s past of strippable quantities such as atmosphere and water. Comparison with Earth can also indicate if the two planets formed from similar materials, or if part of the dichotomy between these Earth-sized worlds can be laid at the door of compositional differences.

But another orbiter loses out on basic appeal. NASA planetary exploration typically follows the pattern of fly-by spacecraft, orbiters, landers and rovers and then missions to return samples to Earth. Mars exploration is following this list, with the planned Mars 2020 mission collecting samples for possible later collection. Missions to Europa and Mercury are doing the same, albeit at an earlier stage.

But the hellish surface conditions of Venus make this pattern difficult, and the prospect of repeating the orbiter step with better instruments is significantly less enticing.

The solution might be a combined orbiter and lander mission, with data from the orbiter mitigating the risk associated with the lander. Alternatively, an aeroplane or balloon that travels through the upper parts of the Venusian atmosphere might combine originality with data that cannot be achieved in orbit. Designs like these have been proposed for a Russia-led mission with a contribution from NASA, known as Venera-D, but funding remains uncertain.


This image of the equatorial region of Venus taken by the Japanese Akatsuki probe provides striking detail of the equatorial, tropical, and extra-tropical clouds of the planet. Color changes indicate local variations in the amounts of a little-understood ultraviolet absorber and sulfur dioxide in the atmosphere. JAXA / ISAS / DARTS / Damia Bouic

While a Venus mission did not make into the finalists for the New Frontiers program, funding was given to develop a camera that could measure the mineralogy and composition of Venus’s rocks. This demonstrates a continual interest by mission experts in Venus, but publications by the community suggest this needs to be higher up the priority list.

The bottom line is that visiting our neighbor may present one of the biggest challenges in the Solar System. But exoplanet research may be lost without it.

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Red Dwarf Stars and the Planets Around Them

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Artist rendering of a red dwarf or M star, with three exoplanets orbiting. About 75 percent of all stars in the sky are the cooler, smaller red dwarfs. (NASA)

It’s tempting to look for habitable planets around red dwarf stars, which put out far less luminosity and so are less blinding.  But is it wise?

That question has been near the top of the list for many exoplanet scientists, especially those involved in the search for habitable worlds.

Red dwarfs are plentiful (about three-quarters of all the stars out there) and the planets orbiting them are easier to observe because the stars are so small compared to our Sun and so an Earth-sized planet blocks a greater fraction of starlight.  Because planets orbiting red dwarfs are much closer in to their host stars, the observing geometry favors detecting more transits.

A potentially rich target, but with some drawbacks that have become better understood in recent years.  Not only are most planets orbiting these red dwarf stars tidally locked, with one side always facing the sun and the other in darkness, but the life history of red dwarfs is problematic.  They start out with powerful flares that many scientists say would sterilize the close-in planets forever.

Also, they are theorized to be prone to losing whatever water remains even if the stellar flares don’t do it. Originally, it was thought that this would happen because of a “runaway greenhouse,” where a warming planet under a brightening star would evaporate enough water from its oceans to create a thick blanket of H2O vapor at high altitudes and block the escape of radiation, leading to further warming and the eventual loss of all the planet’s water.

The parching CO2 greenhouse of a planet like Venus may be the result of that.  Later it was realized that on many planets, another mechanism called the “moist greenhouse” might create a similar thick blanket of water vapor at high altitudes long before a planet ever got to the runaway greenhouse stage.

Finally now has come some better news about red dwarf exoplanets.  Using 3-D models that characterize atmospheres going back, forward and to the sides, researchers found atmospheric conditions quite different from those predicted by 1-D models that capture changes only going from the surface straight up.

One paper found that using some pretty simple observations and calculations, scientists could determine the bottom line likelihood of whether or not the planet would be undone by a moist greenhouse effect.  The other found that these red dwarf exoplanets could have atmospheres that are always heavily clouded, but could still have surface temperatures that are moderate.

The new studies also enlarge the size of the habitable zones in which exoplanets could be orbiting a red dwarf or other “cool” star, making more of them potentially habitable.

The green sections are the habitable zones surround the different star types.  The term refers to the region around a star where water on a planet could remain liquid at least part of the time.  The term does not mean the planets in the zone are necessarily habitable, but that they make it past one particular large hurdle.  (NASA)

 

“This is good news for those of us hoping to find habitable planets,” said Anthony Del Genio, a senior research scientist at NASA’s Goddard Institute for Space Studies (GISS) in New York, and co-author of a new paper in The Astrophysical Journal.

“These studies show that a broader range of planets could have stable climates than we thought.  This is a broadening of the width of the habitable zone by showing that we can get closer to a star and still have a potentially habitable planet.”

Yuka Fujii, author of the Astrophysical Journal article, specializes in exoplanet characterization, planetary atmospheres, planet formation, and origin of life issues. (Nerissa Escanlar)

In a NASA release, the paper’s lead author, Yuka Fujii, said this: “Using a model that more realistically simulates atmospheric conditions, we discovered a new process that controls the habitability of exoplanets and will guide us in identifying candidates for further study.” Fujii was formerly at NASA GISS and now is a project associate professor  at the Earth-Life Science Institute in Tokyo.

Since telescope time available for exoplanets will be quite limited on observatories such as the James Webb Space Telescope — which has many astronomical tasks to accomplish — the Earth-sized exoplanets around red dwarfs seem to be the more technologically feasible target to observe.

Scientists have to observe Earth-size planets for a long time and for many transits in front of the star to get a good enough signal to interpret. So given that, it will be impossible to observe all, or even many, of the candidate Earth-size planets discovered so far or will be discovered.  Tough choices have to be made.

What the group found using their 3-D models is that unlike the predictions from 1-D models, this moist greenhouse effect does not set in immediately for a particular luminosity of the star. Rather, it occurs more gradually as the star becomes brighter.

That fact, Del Genio said, makes the findings from the new 3-D modeling studies additionally important because they can help observers determine which small, rocky exoplanets might be most promising in terms of habitability.

They do this by identifying — and then eliminating — exoplanets that have undergone what is called a “moist greenhouse” transformation.

Anthony Del Genio, leader of the GISS team using cutting edge Earth climate models to better understand conditions on exoplanets.

A moist greenhouse occurs when a watery exoplanet orbits too close to its host star. Light from the star will then heat the oceans until they begin to evaporate and are lost to space.

This happens when water vapor rises to a layer in the upper atmosphere called the stratosphere and gets broken into its elemental components (hydrogen and oxygen) by ultraviolet light from the star.
The extremely light hydrogen atoms can then escape to space. Planets in the process of losing their oceans this way are said to have entered a “moist greenhouse” state because of their humid stratospheres.

What the group found using their 3-D models is that unlike the runaway greenhouse effect this moist greenhouse effect does not set it immediately at a particular temperature threshold.  Rather, it occurs more gradually, even over eons.

They came to this conclusion because the upper atmosphere heating turned out to be a function of the infrared radiation coming from the stars rather than from turbulent convective activity (as in massive thunderstorms) from the surface, as earlier believed.

The infrared radiation (which is at wavelengths slightly longer than the visible wavelength area of the spectrum) will warm the planet and cause what water is present to eventually. evaporate.

 

This is a plot of what the sea ice distribution could look like on a tidally locked ocean world. The star would be off to the right, blue is where there is open ocean, and white is where there is sea ice.  (NASA/GISS/Anthony Del Genio)

This paper comes on the heels of a related one in the August edition of  The Astrophysical Journal.

Ravi Kopparapu, a research scientist at NASA Goddard and Eric Wolf of the University of Colorado, Boulder came to a similar conclusion about surfaces on exoplanets orbiting red dwarfs. As they wrote in their abstract, the modeling  “implies that some planets around low mass (red dwarf) stars can simultaneously undergo water-loss and remain habitable.”

They also reported general circulation model 3-D modeling that showed moist greenhouse scenarios around red dwarfs were slow moving and took place at relatively low temperatures. As a result, oceans could remain for a long time — even billions of years — as they slowly evaporated.

Both groups use general circulation models (GCM), though different ones.  GCMs are an advanced type of climate model that looks at the general circulation patterns of planetary atmospheres and oceans.  They were initially designed to model Earth’s climate patterns, but now are used for exoplanets as well.

The original theory of the moist greenhouse scenario was put forward in the 1980s by James Kasting of Pennsylvania State University, who also did much original work on the concept of a habitable zone and helped popularize the concept.  Both the runaway greenhouse and the moist greenhouse have become important factors in exoplanet study.

 

 

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