Future Venus Missions Options

[Phil Stooke]

Oh well, might as well start that new topic since it's already well advanced in the Juno area...

My perspective on landers is as follows. All the landers we've had so far were dropped blind onto an essentially unknown surface. Any future landers can be targeted for specific terrains. It really is not true that we have had representative landings. Even a descent image or two, a panoramic photo plus a bit of surface composition, from a simple Venera-class lander just updated a bit, would be useful if we could put several down at well chosen targets. My choices would be:

Examples of the main plains units (smooth, fractured, ridged)

tesserae

high elevation radar-bright tesserae

large fresh lava flow unit ('fluctus')

crater dark parabola

crater ejecta outflow unit

dunes area.

And I have always assumed, rightly or wrongly, that it would be relatively easy to put these down, so they ought to be fairly inexpensive as planetary landers go.

Phil

[Guest_BruceMoomaw_*]

The presentations from the second VEXAG meeting have just arrived ( http://www.lpi.usra.edu/vexag/May2006/presentations.html ). In the one on the latest update of the Solar System Roadmap ( http://www.lpi.usra.edu/vexag/May2006/VEXAG_52006_ELLEN.pdf ), Ellen Stofan's group recommends that for the projected Flagship-class Venus Surface Explorer, an "air mobility platform with long traversing" is now "preferred over a surface rover" for Venus, logically enough. (Not only does it allow much longer traversing, but in the case of Venus it would also allow the vehicle to land, hastily take a look around and grab some samples for later digestion, and then take off again for the cooler upper atmosphere, thereby reducing its heat burden.)

Actually, though, the single most useful presentation from this VEXAG meeting may be Larry Esposito's summation of our current scientific knowledge of Venus ( http://www.lpi.usra.edu/vexag/May2006/Chap...ummaryVEXAG.pdf ).

Also see Emily's series of very useful blog entries on the goings-on at VEXAG ( http://planetary.org/blog/ ).

[nprev]

Mr. Esposito's presentation was indeed informative, Bruce; thanks for posting the link!

2 By of oceans, huh...hmm. Although this is wild speculation, you have to wonder if maybe the advent of photosynthetic life on Earth was what saved us from becoming Venus' slightly bigger sibling in all respects.

Still, if oceans did persist that long, why wasn't more CO2 captured as limestone to prevent a runaway greenhouse? Did Venus all of a sudden become enormously more volcanically active than Earth ever was, causing that 'global resurfacing event' and a CO2 overload in the atmosphere that the oceans just couldn't process fast enough?

Lots of interesting and potentially quite significant contingency scenarios here...

[Guest_BruceMoomaw_*]

Mr. Esposito's presentation was indeed informative, Bruce; thanks for posting the link!

2 By of oceans, huh...hmm. Although this is wild speculation, you have to wonder if maybe the advent of photosynthetic life on Earth was what saved us from becoming Venus' slightly bigger sibling in all respects.

Still, if oceans did persist that long, why wasn't more CO2 captured as limestone to prevent a runaway greenhouse? Did Venus all of a sudden become enormously more volcanically active than Earth ever was, causing that 'global resurfacing event' and a CO2 overload in the atmosphere that the oceans just couldn't process fast enough?

Lots of interesting and potentially quite significant contingency scenarios here...

The current concept of Venus is that it was never quite warm enough to develop a genuine "runaway greenhouse", in which the greenhouse effect from all the water vapor initially in its atmosphere raised its temperature by a greater enough margin to evaporate a really huge additional amount of water into the air...and so on in a self-amplifying positive feedback that took the form of a diverging series that didn't stop until ALL the planet's water was in the form of atmospheric steam, after which enough of it soared into the upper atmosphere for solar UV to get at it and break it down.

Instead, it appears that early Venus was instead a "moist greenhouse". That is, its initial warmth was greater than Earth's by a relatively modest margin, so that the amount of additional water that was evaporated into the air by that warmth was also fairly modest and so produced only a small additional greenhouse effect...and so on, in a positive-feedback effect that took the form of a converging rather than diverging series and thus finally leveled off at a certain point (as indeed our own water-vapor-generated self-amplifying greenhouse effect does after warming Earth by a total of about 33 deg C).

This stabilized level of early Venusian warmth, however, was still high enough to loft the planet's water vapor to altitudes high enough that solar UV could break it down with tremendously greater efficiency than was happening on Earth even before our photosynthetically created ozone layer appeared. Thus Venus was still stripped of ALL its water supply after (according to the majority view) a few hundred million years, at which point its "carbonate thermostat" -- which depends on the existence of liquid water -- also shut down. That is: after Venus' liquid water vanished, all the atmospheric CO2 which had been turned into carbonate minerals by that liquid water got eventually dragged back down by the planet's still-functioning plate tectonics into its semi-molten asthenosphere, where the carbonates were broken back down into CO2, which was then belched back into the atmosphere again by its volcanoes -- and this time that CO2 did not get turned back into carbonates again, so that the volcanoes eventually belched the planet's entire large CO2 supply into the air as a permanent super-thick atmosphere whose greenhouse effect (even without the assistance of water vapor) was strong enough to raise its temperature to its current roasting level and keep it there.

The planet's plate tectonics, according to this model, did shut down a billion years or so after the planet lost all its surface liquid water. This is because mixing liquid water with rock greatly lowers its melting point -- and so, without water to do this, the planet's asthenosphere solidified and permanently jammed up its plate-tectonic conveyor belt. Thus there may still be some carbonates sitting around on Venus' surface that were not taken underground and broken back down into CO2, although most of them were thus destroyed before the plate tectonics shut down completely. But at any rate, there's no evidence that the advent of photosynthetic life did anything to prevent Earth from turning into a Venus-type oven -- we were safe from that fate whether life had ever evolved on Earth or not, simply because we were far enough from the Sun for virtually all of our atmospheric water vapor to stay trapped in our dense lower atmosphere by the "cold trap" of our stratosphere and thus be safe from breakdown by solar UV.

David Grinspoon has recently proposed an interesting variant of this idea, based on the assumption that the calculations of James Kasting -- which are what have led to the rejection of the "runaway greenhouse" model of Venus and the acceptance of the "moist greenhouse" model instead -- are (by Kasting's own statement, an upper limit) which ignores the possible cooling effect of the dense high-albedo water clouds which the initial warm Venus would have had. Grinspoon thus thinks that early Venus may have been cool enough that it didn't lose all its liquid water (and thus start building up a super-dense CO2 atmosphere) for fully 2 or 3 billion years -- and therefore that its plate tectonics may not have shut down until only about 500 million years ago, so that the fact that Venus' surface (in the opinion of most geologists) suddenly started retaining impact craters at that point was not due to a separate "catastrophic resurfacing" event at that time, but just to the fact that, before then, plate tectonics had been erasing most of the planet's craters just the way it still does for Earth.

As Grinspoon points out, one astonishing side aspect of his revisionist view of Venusian history is that Venus would have had time to evolve not just microbial life (which Kasting's classic moist-greenhouse view might also allow), but photosynthetic and maybe even primitive multicellular life -- unlike Mars, Europa, or any other place in the Solar System. Ah, but is there any chance that any fossils of such Venusian life could survive to the present day in such a savage environment? Now you can see one reason why geologists are so interested (as the 2002 Decadal Survey said) in looking for any evidence at all of surviving sedimentary rocks, carbonates, or other aqueous minerals on Venus' surface.

[tty]

David Grinspoon has recently proposed an interesting variant of this idea, based on the assumption that the calculations of James Kasting -- which are what have led to the rejection of the "runaway greenhouse" model of Venus and the acceptance of the "moist greenhouse" model instead -- are (by Kasting's own statement, an upper limit) which ignores the possible cooling effect of the dense high-albedo water clouds which the initial warm Venus would have had. Grinspoon thus thinks that early Venus may have been cool enough that it didn't lose all its liquid water (and thus start building up a super-dense CO2 atmosphere) for fully 2 or 3 billion years -- and therefore that its plate tectonics may not have shut down until only about 500 million years ago, so that the fact that Venus' surface (in the opinion of most geologists) suddenly started retaining impact craters at that point was not due to a separate "catastrophic resurfacing" event at that time, but just to the fact that, before then, plate tectonics had been erasing most of the planet's craters just the way it still does for Earth.

As Grinspoon points out, one astonishing side aspect of his revisionist view of Venusian history is that Venus would have had time to evolve not just microbial life (which Kasting's classic moist-greenhouse view might also allow), but photosynthetic and maybe even primitive multicellular life -- unlike Mars, Europa, or any other place in the Solar System. Ah, but is there any chance that any fossils of such Venusian life could survive to the present day in such a savage environment? Now you can see one reason why geologists are so interested (as the 2002 Decadal Survey said) in looking for any evidence at all of surviving sedimentary rocks, carbonates, or other aqueous minerals on Venus' surface.

Plate tectonics don't renew the whole surface, only the deep ocean part. The continents (including the continental shelf) are too light to be pulled down in the subduction zones. If Venus once had plate tectonics the "continents" (highlands) should be more heavily cratered than the "oceans" (basins). Whether this also implies that fossils and carbonates should be preferentially sought for in the highland areas is uncertain. The last life would have been found in the deepest parts of the basins, but these may have been "reprocessed" before plate tectonics stopped (here on Earth the ocean bottoms are completely "reprocessed" after ca 200 million years).

Incidentally there is evidence that Earth also had a "moist hothouse", not once but three times and that it saved rather than extinguishing life here. In the Late Proterozoic (600-800 million years ago) Earth suffered a series of extreme glaciations when all, or almost all, oceans froze over and continents were glaciated even in near-equatorial areas. Such a "snowball Earth" is climatically stable since the high albedo reflects most solar radiation back into space. However volcanism continued and since the oceans were ice-covered and the continents frozen no CO2 could be absorbed, but rather kept accumulating for maybe 20-30 million years. Temperatures slowly rose until the ice finally started melting, the albedo went down, the ice melted faster etc in a runaway process that converted Earth from Super-Antarctica to Super-Tropics in just a few thousand years. In this extreme hot-wet environment chemical weathering became intense and CO2 was rapidly drawn down and vast amounts of carbonates were deposited right on top of glacial deposits - a most unusual juxtaposition.

As for whether fossils could survive such extreme conditions, the answer is probably yes. Fossils can occasionally be recognizable in rocks that have been heated to similar temperatures on Earth. However here such heating is invariably linked to great depth and extreme pressures and also not continued for such a long period (500 mya), so it is difficult to make comparisons.


tty

[Guest_BruceMoomaw_*]

Plate tectonics don't renew the whole surface, only the deep ocean part. The continents (including the continental shelf) are too light to be pulled down in the subduction zones. If Venus once had plate tectonics the "continents" (highlands) should be more heavily cratered than the "oceans" (basins). Whether this also implies that fossils and carbonates should be preferentially sought for in the highland areas is uncertain. The last life would have been found in the deepest parts of the basins, but these may have been "reprocessed" before plate tectonics stopped (here on Earth the ocean bottoms are completely "reprocessed" after ca 200 million years).

Incidentally there is evidence that Earth also had a "moist hothouse", not once but three times and that it saved rather than extinguishing life here. In the Late Proterozoic (600-800 million years ago) Earth suffered a series of extreme glaciations when all, or almost all, oceans froze over and continents were glaciated even in near-equatorial areas. Such a "snowball Earth" is climatically stable since the high albedo reflects most solar radiation back into space. However volcanism continued and since the oceans were ice-covered and the continents frozen no CO2 could be absorbed, but rather kept accumulating for maybe 20-30 million years. Temperatures slowly rose until the ice finally started melting, the albedo went down, the ice melted faster etc in a runaway process that converted Earth from Super-Antarctica to Super-Tropics in just a few thousand years. In this extreme hot-wet environment chemical weathering became intense and CO2 was rapidly drawn down and vast amounts of carbonates were deposited right on top of glacial deposits - a most unusual juxtaposition.

As for whether fossils could survive such extreme conditions, the answer is probably yes. Fossils can occasionally be recognizable in rocks that have been heated to similar temperatures on Earth. However here such heating is invariably linked to great depth and extreme pressures and also not continued for such a long period (500 mya), so it is difficult to make comparisons.
tty

This is why one of the biggest goals in Venusian exploration is whether there is anything on Venus that can be called "continents" -- that is, patches of lightweight granitic rocks floating on top of the basalt plates, and therefore resistant to being pulled down into the deep by crustal-plate subduction. Earth's continents are thought to be made of lightweight silica-rich melt rock that was produced and separated when Earth's original basalt was mixed with large amounts of liquid water while it was being pulled down into the asthenosphere and remelted -- so the existence and size of any continents on Venus is yet another factor that seems to depend on the size of any initial liquid-water oceans it had early on. (The 2002 Decadal Survey is very clear on this point -- find any large amounts of granite on Venus anywhere, and you have strong evidence that it had substantial water oceans early on.)

Thus the strong interest in using the future landers to inspect the two types of Venusian terrain suspected of being possible continents; the tesserae (which were the target of one of the twin landers in Donald Esposito's "SAGE" concept at the last New Frontiers submission), and the huge "Ishtar Terra" in Venus' north polar region.

(By the way, the JPL technical report on SAGE -- http://trs-new.jpl.nasa.gov/dspace/bitstre...4/1/03-2520.pdf -- implies (pg. 18 and 21) that the only reason it was rejected as a finalist is simply that its launch window to Venus didn't fit in with the assigned period during which NF 2 was supposed to be launched. If that's the only grudge they had with it, I think it will definitely be a front-runner for the NF 3 selection, whenever that is finally made -- espcially since the Inner Planets Subgroup of the 2002 Decadal Survey ranked a Venus lander as more important than a sample-return mission to the moon's Aitken Basin, and the only reason the Survey as a whole gave the latter such a high overall rating is that it said the automatic rendezvous and docking technology that they thought an Aitken Basin mission would need would be useful practice for a Mars sample return. As things turned out, the Aitken Basin proposal that was a finalist -- "Moonrise" -- didn't use unmanned R&D at all.)

I've read quite a bit about the two supposed "Snowball Earth" periods in the Precambrian, and the wild pogo-sticking in global temperature that's thought to have occurred during them. There are several explanations proposed for them -- ranging from continental drift breaking up an initial giant supercontinent to create a bunch of smaller continents with a larger total amount of shallow coastlines that thus pulled more CO2 out of the air by carbonate weathering, to the idea that the first photosynthetic cyanobacteria may have brought on one of the two crises themselves by producing enough oxyen to destroy the methane greenhouse that had been keeping early Earth warm. (And the latter, in turn, may perhaps have been originally created by the earlier generation of methanogenic bacteria!) There are still a few holdouts on whether the Snowball Earth episodes occurred at all, but the evidence seems to be growing steadily.

And as for the survival of fossils -- or at least microfossils -- under the savage surface conditions of Venus, there's a rather encouraging new LPSC abstract ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1028.pdf ) on the apparent ease with which biochemical fossil evidence seems to survive even in giant-meteor impact melts on Earth. But if you think a fossil hunt on Mars will be difficult, consider one on Venus...