NASA's latest Solar System Exploration Roadmap calls for a "Venus Mobile Explorer" (VME) to be the third of the coming Flagship missions to Solar System targets other than Mars -- after the Europa Explorer to orbit that Jovian moon, and the Titan Explorer to examine Titan (and maybe Enceladus as well) in detail.

Venus' ferocious 460-480 deg. C surface temperature, however, is an entirely new technology challenge for any mission intended to poke around on or near its surface for weeks or months. For this reason, the VME is scheduled to fly only after the Europa and Titan missions -- that is, around 2027 at the absolute earliest.

The Roadmap states that the most urgent task for this mission is to look for evidence that Venus -- during its earliest days -- might have been hospitable to the point of having extensive seas or oceans of liquid water, providing an environment in which microscopic life could perhaps have begun evolving. If Venus did have such seas, they would have been gradually destroyed by the "moist greenhouse effect".

Because Venus' surface temperature at that time was fairly close to the boiling point, large amounts of water vapor could have risen to a high enough altitude in its atmosphere to be broken down by solar ultraviolet light into hydrogen (which would have escaped from the planet) and oxygen (which would have quickly reacted with its surface rocks).

More humidity would then evaporate into Venus' air from its seas to replace this lost oxygen, and would be destroyed itself. This process didn't happen on Earth because our stratosphere is cold enough to keep any large amount of water vapor from rising that high above Earth's surface.

Pennsylvania State University's James Kasting has calculated that this slow leak could have completely destroyed Venus' stock of surface water in about 600 million years, after which the carbon dioxide that was continuing to be vented from Venus' volcanoes would no longer be mostly dissolved in the planet's liquid water and then react with its silicate rocks to form carbonate minerals, as is the case on Earth.

And so Venus' large volcanic outpouring of CO2 -- instead of being mostly changed at any one time into solid carbonates, which would later be recycled by the volcanoes to re-emit CO2 -- would have then built up into a suffocating blanket almost a hundred times denser than Earth's entire atmosphere, capable of generating enough greenhouse effect to heat Venus' surface to its present-day inferno.

But 600 million years might very well be long enough for bacterial life to start evolving on Venus -- just as such life might have had time to begin evolving in surface water on ancient Mars before that planet also became uninhabitable for entirely different reasons.

Indeed, the University of Colorado's David Grinspoon has pointed out that Kasting's calculations ignore the fact that early Venus would have been covered by a dense layer of liquid-water cumulus clouds that might have reflected enough sunlight, and thus cooled the planet enough, to extend the lifetime of its initial oceans for as much as 2 billion years. That is, conceivably, long enough even for primitive multicellar organisms to start tentatively evolving on the planet before it turned into Hell.

That is a very long shot, but the chances that single-celled life might have appeared on Venus is not. However, hunting for fossil evidence of microscopic life in Earth's most ancient rocks is itself difficult. It will be harder on Mars, and maybe flat-out impossible on Venus.

It's also possible that Venus -- because of its different conditions of formation -- never did have any large amount of liquid water on its surface. So looking for geological evidence that ancient Venus did have liquid-water seas is obviously the necessary first step before we decide to ever undertake such a gargantuan task as hunting for Venusian fossils.

The Venus Mobile Explorer would prowl across Venus' surface looking for such evidence -- in the form of granite (which, unlike basalt, can only form when volcanic lava is exposed to large amounts of liquid water), hydrated silicate minerals, or highly oxidized iron.

It would also, of course, examine the other aspects of Venus' geology, including the second big puzzle about the planet's surface: the fact that the Magellan orbiter's radar maps show the distribution of meteor craters on its surface to be very uniform -- suggesting that most of the planet's surface may have been blanketed by a gigantic flood of molten lava about half a billion years ago, with the planet undergoing relatively little geological activity ever since. There are several different theories as to why this might have happened -- and, indeed, there are still some doubts that it actually did happen.

VME would probably be a strange craft resembling a cross between a balloon and a research submarine, with an instrumented gondola hanging beneath a small "balloon" that would be an accordion-like bellows made of thin steel.

In Venus' highly compressed near-surface atmosphere, even such a small, heavy balloon, if pressurized with helium, could produce enough buoyancy to lift the craft off the surface. By controlling its buoyancy, the VME could hang just a few meters above the surface, cruising along either with propellers or by using Venus' gentle near-surface breezes, periodically lowering itself to gather and analyze surface samples.

The devilish problem for a VME, of course, is arranging for its electronics to work at Venus' savage temperatures. This would probably require a mixture of some electronics -- only now starting to be developed -- that actually can work at 500 deg C., and some more complex and delicate electronics that can only work up to about 250 deg C.

The latter would have to be encased in small containers cooled by a powerful active refrigeration system on the craft. (I've already mentioned NASA's plans to develop a more efficient "Stirling-based" version of the plutonium-fueled RTGs that its outer Solar System craft use to supply their electricity. Such a Stirling-based RTG would need only about one quarter as much plutonium as current-day RTGs.

And it would be an absolute necessity for VME -- both because of the greater difficulty of using the plutonium's heat to generate electricity on a world where the RTG's exterior would also be very hot, and because such a rotating "Stirling engine" could not only generate electricity but could directly power the refrigeration system's pump much more efficiently than if that pump used an electric motor.)

Clearly VME will itself be a major undertaking -- especially when you also consider such other problems as keeping its sealed compartments free of high-pressure leaks for months, and the fact that Venus' air seems to contain some corrosive trace gas that has already caused serious malfunctions on both American and Soviet Venus entry probes when they were near the surface. NASA at first hoped to fly this mission for no more than about $1.5 billion, but it almost immediately became clear that it will be much more difficult and expensive than that.

So, before we build it, it would seem wise to first make an overall examination of Venus' surface to see if it does possess any significant patches of granite or other minerals that might possibly be water-related. Only then should we fly VME to examine such areas in finer detail, to see if they are indeed aqueous in origin.

As one might expect, carrying out such a preliminary inspection of Venus' overall surface will be a lot harder than for, say, Mars. Europe's current Venus Express orbiter will use its main instrument -- the near-infrared mapping spectrometer "VIRTIS", which is mostly for atmospheric and cloud studies -- to try to examine a few limited spectral windows that might allow it to peer through the planet's atmosphere and clouds and do some very tentative mapping of its actual surface composition, in particular a search for granite. (It will take some time to analyze VIRTIS' data enough to reach any conclusions about this.)

The proposed New Frontiers-class "Venus In-Situ Explorer" mission would land small, short-lived stationary landers on two or three spots to analyze Venus' surface mineralogy in detail for the first time -- and it's a safe bet that one of those landers will be aimed for one of Venus' small patches of finely cracked "tessera" terrain, which seem to be the only remaining spots on its surface older than that half-billion-year-old global lava flood, and which may possibly be remnants of ancient Venusian granite continents.

But there's another possible mission design in which a single lander could touch down briefly at dozens of different spots all across Venus' surface to make such preliminary analyses of its surface minerals -- the "reversible-fluid balloon".

Drop a regular plastic balloon with an instrumented gondola into the cool upper sulfuric-acid cloud layer of Venus, and inflate it with helium -- but only partially, not enough to keep it from continuing to drop. Have a small tank of liquid water linked to its bottom. When the balloon drops down below 42 km altitude, it will pass the point at which the growing temperature of Venus' lower atmosphere makes water boil -- and the resultant steam will inflate the balloon and give it enough further buoyancy to make it rise back into the upper cloud layer until it rises above the boiling-point altitude.

At that point, the steam will start cooling off enough to recondense back into liquid water and trickle back into the water tank, at which the balloon will lose buoyancy and start dropping and the whole cycle will repeat.

Left to itself, such a balloon would bob up and down in ever-decreasing cycles until it finally leveled off at the 42-km altitude.

But if you have a motorized valve linking the water tank to the balloon -- and close that valve as soon as the steam has all condensed and the balloon is starting downward again -- the balloon will retain negative buoyancy and continue to sink steadily downwards until whatever point you choose to command the valve open again, at which point the water will flash into steam and send the balloon soaring back upwards to about 60 km altitude before the steam recondenses and the balloon starts down again, at which point you can close the valve again and so repeat the up-down bobbing cycle indefinitely.

And if you do this, then you can leave the steam valve closed until any point in the balloon's descent -- even letting it drop all the way down to Venus' roasting surface if you choose, where its insulated gondola can hover a short distance above the surface on a guide weight for an hour or so, making observations, until its regular electronics are starting to dangerously overheat.

At that point you simply open the valve and let the balloon soar all the way back up into Venus' cloud layer to cool down again. Such a balloon, blown along by the powerful 180 to 360 km/hour winds in Venus' cloud-level air, can cruise all the way around the planet repeatedly, making dozens of landings to analyze the Venusian surface with only a single vehicle.

Of course, to do this, you have to have some plastic which is lightweight enough to make up the very thin-walled, big standard-model balloon envelope -- but which can also endure Venus' 480 deg C surface temperatures. Amazingly, such a substance does seem to exist -- "polybenzoxasole" -- and it may indeed be possible to mold and sew it into a thin balloon envelope.

You'll also have navigational problems. This mission resembles the proposed "Titan Explorer", a hot-air balloon that would be blown along a few kilometers above Titan's landscape, with its Earth controllers commanding it to reduce its buoyancy whenever it arrives at an interesting place so that it can drop down to the surface and collect samples.

But Titan's winds, at that altitude, will blow the balloon along at just a few km/hour whereas the Venus R.F. Balloon, at its necessary cruise altitude in Venus' cloud layer, will be blown along at hundreds of kilometers per hour, making it much harder for its ground controllers to command it to touch down at a precise chosen location.

Still, a few weeks of initial cruising at various altitudes within Venus' cloud layer will give Earth controllers a much more precise idea of the wind speeds and directions at different altitudes (including some north-south winds), greatly increasing their ability to make it touch down within a few dozen kilometers of any chosen spot on the surface -- which is good enough for an initial geological survey of Venus as a whole. By contrast, the Venus Mobile Explorer's steel bellows-balloon wouldn't allow it to lift more than about 10 km off the ground, letting it be blown along only slowly to map a much smaller individual site in far more detail.

One other problem exists with such a balloon: whenever it reaches the surface, it will have to hover several meters above the ground on its guide weight, to remove any danger of its big, fragile plastic envelope hanging up or ripping on surface obstacles -- and this will make it hard for such a balloon to collect surface samples.

But there's a possible solution for this too. Some instruments exist which can do a surprisingly detailed, sensitive analysis of both the elements and minerals making up a rock from a distance of 10 meters or more. Besides near-infrared spectrometers that could detect many of Venus' minerals, there are "LIBS" and Raman spectrometers which use lasers to enable them to analyze most of the important elements and minerals (respectively) in rocks.

The big 2009 "Mars Surface Laboratory" rover will carry a LIBS spectrometer, which will use a small laser cannon to heat pinpoints on such distant rocks to brief incandescence and take spectra of the glow to identify the elements in that sample within a split-second.

And lab tests have also shown that such LIBS and Raman spectrometers can work well at a range of meters even through Venus' very dense surface air -- especially since the Venus R.F. Balloon could carry its cameras and spectrometers inside part of the jointed, snag-proof metal "snake" that it will drop onto the surface as a guide weight.

They could peep and fire their lasers through little portholes in the sides of a capsule incorporated into such a snake to study the immediate surface -- just as they could do inside tiny, durable hard-lander capsules suggested for Europa and other worlds.

Indeed, the two short-lived Venus landers included in the New Frontiers "VISE" concept already officially proposed by the University of Colorado's Larry Esposito -- which would actually drill up a sample of Venusian soil and stuff it through a complex little airlock to be analyzed by X-ray instruments -- might instead use such infrared, Raman and LIBS instruments to analyze Venus' surface far more simply and easily, thus lightening the landers to the point that the mission might be able to carry more than two of them.

At any rate, the idea of exploring Venus in this way has already been studied in some detail by the Jet Propulsion Laboratory in the 1990s -- and, if the Venus Mobile Explorer that NASA currently desires instead does indeed turn out to be as difficult and expensive as it now looks, the R.F. Venus Balloon's time may yet come.

In this series, I've described a set of possible missions that might serve as an alternative to the big, expensive Flagship missions that NASA currently wants to fly to Solar System targets over the next few decades.

Unlike the case with the Pluto flyby probe concept that NASA originally rejected until Congress ordered it to be flown simply because the scientific case for it was so solid (and which has now just finished passing Jupiter), we can't say that any of the concepts I've mentioned definitely should be flown -- they would certainly turn out a good deal less science than the alternative Flagship missions to their targets. But they would still turn out a lot of science for a much lower cost than the Flagship alternatives.

And -- given the fact that NASA's space science programs will unquestionably be seriously strapped for cash for the foreseeable future, regardless of what happens to America's manned space program -- I think that they are worthy of serious consideration.

Bruce Moomaw is our first "Space Blogger" at www.spaceblogger.com Feel free to create an account on SpaceBlogger and discuss this issue and more with Bruce and friends.

Email This Article

Related Links
Lots of Space For Opinion
Space Blogs at SpaceBlogger.com