Five Against Eight
These are the roots of rhythm and the rhythm remains.
--Paul Simon, "Under African Skies."
Venus makes one trip around the Sun every 224 days. But since Earth is orbiting in the same direction, it takes a total of 584 days for Venus to go around and catch up with us again. This is how long it takes for Venus to come back to the same position relative to Earth (say, from one inferior conjunction to the next), so this is also the time period of the cycle of motions we see Venus make in our sky. This includes one complete evening appearance and one morning appearance, plus the two disappearnce intervals.
There is something strange and wonderful about the numerolgy of this particular time interval. If you multiply 584 days by 5 you get 2,920. Divide this by 8 and you get 365 days, or one Earth year. Thus, there is a simple whole number ratio, a resonance, between the orbital periods of Earth and Venus. Venus completes five cycles, morning to evening and back again, in eight of our years. In musical terms, this is a polyrhythm, with the orbits of Venus and Earth beating five against eight.
As if all that weren't enough, another reason to worship Venus and find significance in her movements is that there are numerous connections between the timing of aspects of her motions and timescales of natural interest to humans. Perhaps most strikingly, the approximate 260-day length of a Venus appearance in the morning or evening coincides closely with the average length of human gestation period.
The 260-day Venus interval and the 365-day year come into phase every 18,980 days or 52 years. This time interval represents one Calendar Round, an interval of great ritualistic significance throughout Mesoamerica. The Maya were fascinated by the mathematical interweavings of different astronomically significant time intervals, and they celebrated the passing of a Calendar Round with lavish rituals of creation and renewal. . .
Radar signals were first successfully reflected off the surface of Venus in 1958. In l962, radar measurements allowed us to resolve the centuries-long argument over the true rotation rate, which was described n Chapter 2. It turned out, once again, that everyone was wrong. Venus rotates extremely slowly, so that one day there is equal to 117 Earth days. And it turns in a backward, or "retrograde," direction compared with most other planets. If you were standing on the surface of Venus, you would sense the Sun rising in the west and setting in the east 59 (Earth) days later. I say "sense," not "see," because it is always overcast on Venus, everywhere, every day. . . . Every Venusian year the Sun rises and sets twice.
But the real surprises came when Mariner took photographs through an untraviolet filter; a dynamic and volatile new face of Venus suddenly appeared. Observers on Earth had previously seen some vague features in ultraviolet images taken with telescopes, so the researchers expected some kind of markings. But what we actually saw far exceeded anyone's expectations: a complex swirl of high-contrast features, ranging from tiny, detailed splotches to huge planetwide streaks (see Figure 3.6). And the stuff moves around like crazy.
The identity of this material, so dark in the unltraviolet that it is responsible for nearly half the solar energy absorbed by Venus, is still not known, one of the great mysteries of Venus. Whatever this "unknown ultraviolet absorber" is, its motions allow us to trace the atmospheric currents in the cloud-top region of Venus. Like ink drops in water, or smoke in a wind tunnel, the ultraviolet marking suddenly rendered the air currents visible. Meteorologists now had something to sink their teeth into, a new pattern of atmospheric circulation to model and compare with those seen on Earth, Mars, and Jupiter. Huge C- and sideways Y-shaped dark markings, symmetrical across the equator, were seen forming and dissolving. These all rushed by at a speed of nearly two hundred miles per hour, circling the whole planet in four days. This pattern of rapid planetwide rotation was dubbed the "superrotation." Finding its cause presented a major challenge for comparative planetology. Can our models of atmospheric circulation, developed on Earth but based on (hopefully) universal laws of physics, also be used to explain such an extremely different pattern of atmospheric motions on this nearby world?: another major mystery that persists to this day.
What Pioneer found, courtesy of the randomly clogging drop, was that hydrogen on Venus is extremely fractionated; the measurements indicated a phenomenally high D/H ration. Earth's oceans have an average of one deuterium atom for every six thousand hydrogen atoms. On Venus, we measure a ratio roughly 120 times greater. This means more than 2 percent of the hydrogen on Venus is of this odd, heavier variety.
How can we explain this massive buildup of hearvy hydrogen on Venus? Clearly, some highly effective process that discriminates by mass has been at work for a long time on the planet, getting rid of hydrogen and concentrating deutrium. It seemed clear that the fractionating force must be simple gravity.
All planets with atmospheres are losing gases to space all the time. This escape flow is composed of the lightest gases and so is usually mostly hydrogen. It is one of the important ways in which the composition of planetary atmospheres changes, evolves, over time.
The ease of atmospheric escape is sttongly dependent on the mass of the gravitational prisoners. The lightest atoms get accelerated to the highest speeds by the occasional collision in the low-density gas mixture of an upper atmosphere. The same amount of random collisional energy will launch a hydrogen atom to a velocity four time higher than it will an oxygen atom. In the mosh pit of the upper atmoshere, the little H dancers get thrown around a lot more that the comparatively huge O's. This is why hydrogen dominates the escape flow. Those fast-moving atoms that happen to be heading up, away from the planet, will escape into space, leaving Venus for good, to join the wispy interplanetary hydrogen winds.
A steady stream of hydrogen is escaping from th etop of Venus's atmosphere at present. Observations from the Pioneer orbiter were key in helping to pin down the rate of this escape and the physical processes responsible for it.
Since deuterium is twice as heavy as hydrogen, it has a harder time escaping. As H escapes and D is left behind, the percentage of D in the remaining atoms goes up. The more hydrogen escapes, the higher the D/H ratio. The implication was clear: a very large amount of hydrogen had escaped into space from the top of Venus's atmosphere throughtout the planet's history.
Given the dramatic contrast in water abundance between the planetary "twins" Venus and earth, the conclusion was irresistible: The discovery of the huge D/H ratio meant that Venus was once wet! As the hydrogen from Venus's vanishing water supply has escaped over the eons, it has left behind a residue of deuterium, resulting in an ever-increasing D/H ratio. This discovery was hailed as the long-sought direct evidence for the "missing" oceans of Venus.
...
The high D/H ratio is strong evidence that a lot of hydrogen has excaped over time, and it probably does mean that at times Venus had substantially more water than it does now. But saying that this observation is proof that Venus once had oceans just like Earth is a bit of a stretch.
One reason for thinking that Venus may not be losing water at present, but breaking even in the long run is the short "lifetime" of water on Venus; if you let the amount of hydrogen contained in all the water that is now on Venus escape at the currently estimated rate, you would run out of water completely in a fairly short time, 100 to 200 million years. This is the lifetime, and it's considerabley shorter than the age of the planet. It seems unlikely that we came along at just the right instant of cosmic time to observe the last few molecules of a former ocean flee into space. . . . Grinspoon 1997:108-109
The alien pattern of noble gases does suggest that the origin or early history of Venus is different from Earth in some significant way. But no one has yet come up with an explanation that has a strong enough ring of truth to be widely embraced amoug comparative planetologists. This remains an area of lively debate and no small amount of confusion.
We were especially puzzled by the isotopic composition of argon. Here on Earth the most common kind is argon-40, which is radiogenic produced by the radioactive decay of potassium. The argon thus created in Earth's interior eventually winds up in volcanic gas. Its amount in the air is believed to reflect both the terrestrial concentration of potassium (a common element in rock-forming minerals) and the total amount of gas that has been added to the atmosphere by volcanoes.
In Earth's atmosphere we find four hundred times as much argon-40 as argon-36, its nonradiogenic cousin that is presumably left over from planet formation. Thus the ratio of argon-40 to 36 is four hundred. This high relative abundance of argon-40 results from Earth's long history of volcanic activity. The ratio we measured on Venus was four hundred times lower, roughly equal to one. Argon-40 and 36 are present on Venus in equal quantities! Despite its extremely thick atmosphere and huge total amount of argon, Venus has much less radiogenic argon-40 than Earth.
ANOTHER STRANGE PENTAMETER
Recall that although Venus orbits the Sun in 225 days, she passes the Earth only once every 584 days, since we are moving on a slower orbit in the same direction. We have already discussed the strange five against eight resonance, well known to the classic Maya, between this orbital period and our terrestrial year, producing the five sky paths that repeat every eight years. Now, here is another strange fact of planetary numerology: within this time period of 584 days neatly fits 5 of these (117-day) Venusian days. MOre precisely, there are 5.001 solar days on Venus between one inferior conjunction and the next. This spin resonance is completely separate from the orbit resonance; they are two independent coincidences. Together, they ensure that Venus spins almost exactly twenty-five times over the eight-year cycle of five repeating paths so dear to the Maya. This adds an additional rhythmic counterpoint, and agogo bell ticking off a measure of five for each drumbeat of Venus's slower orbital five, which itself repeats in time with Earth's eight. It would have delighted the Mayan astronomer priests.
The many layers of superimposed faults (cracks) in the tessera give it an unruly, chaotic appearance and indicate that it has been pushed (compressed.) and pulled (extended) many times from different directions. In most places the compression seems to have come first and the extension later, but the tesserae are so jumbled that the historical sequence is often hard to unravel. One thing everyone agrees on is that the tesserae are the oldest places on the surface. We know this because they are always covered up at their edges by surrounding volcanic plains. This tells us that the tesserae were there first. In fact, being oldest and found preferentially at high elevations, the tesserae seem like islands of an older surface that survived the volcanic flooding that covered most of the planet. Does this mean that there are tesserae everywhere, underneath the plains, with the pieces we can see representing just some small vestiges of what the whole surface one looked like? Maybe the intense history of deformation seen on the tesserae means that Venus was more tectonically active in the past that it is now; or maybe these areas are just showing their age, having been around for so long that their skin has become cracked and wrinkled.
All areas of the surface have more or less the same low number of craters. There are some minor variations in crater density from place to place, but these are of the kind that we would expect to be produced by the blind marksman of impact cratering. The global map of craters on Venus (see Figure 5.15) is indistinguishable from a random pattern.
WHAT IS EATING THE CRATERS OF VENUS?
All the craters on Venus look unnaturally pristine. Instead of blending into the volcanic plains that cover most of the planet, they seem planted on top as an afterthought, as though a crew had built a cheap movie-set planet and realized at the last minute that they had better throw in some craters.
The total number of craters indicates that the planet's surface is not very old by planetary standards--a billion years, tops. Therefore, some process has removed most of the craters that ever formed on the planet. But there are no signs of any such process. The thief who stole Venus' craters has sucessfully covered his tracks. There are three suspects, but each has a good defense.
If it were erosion, we would see craters in a range of states, from pristine fresh ones to highly degraded ones that are nerly gone. We see that on Mars, where erosion by windblown dust is the dominant process.
Craters can also be destroyed by tectonic disruption, if they are simply cracked and faulted to the point where they are no longer recognizable. Here again, we would expect to see the process at work, but the number of partially disrupted craters on Venus is small.
A prime suspect is volcanism, since volcanic flows dominate the visible surface. But if craters were mostly being covered over by lava, then we should see a lot of them partially buried beneath the plains. For example, the volcanic plains of the Moon contain craters ranging from those with barely detectable circular outlines that have been almost completely covered by lava to those that are almost pristine but have lava flows lapping slightly up their flanks. On Venus only about 4 percent of the craters are partially covered by volcanic flows, and almost none are mostly buried (see Figure 5.17). This seems to rule out volcanism as a dominant process removing craters from Venus. Why, on a planet smothered with volcanic features, are the craters so untouched? Do volcanoes on Venus worship craters, as Hindus venerate cows, and thus spare them from their otherwise global carnage? This is not a favored hypotheses, but we needed a way to explain the odd, and globally pervasive, occurrence of pristine craters overlaying widespread lava plains.
It is as if some mysterious process were swallowing craters whole and leaving no trace. There are a few ways in which we can imagine this appearance coming about. If you see a road sign full of holes, it is not necessarily old. Some well-armed fool pumped up on testosterone and booze could have opened fire on it the night before last. What if all of the craters really were added as a kind of afterthought, produced by a very recent barrage of impacts? On any planet with reasonable erosion rates, you could test this idea by noticing if some of the craters were worn down, showing signs of age. (Are the bullet holes rusted?) But on Venus you can't tell, so the hypothesis is consistent with the way all the craters sit atop the "paint job" of planetwide lava flows.
Unfortunately, this scenario is only slightly more plausible that Crater-worshiping volcanoes. Everything we know about the inner solar system suggests that there has been no recent burst of cratering activity. Impacting objects do not discriminate greatly amoung the terrestrial planets. If there had been a recent shower of large meteoroids, we would also see it effects on the surfaces of the Moon, Mars, and Earth. We don't. . . .
Here is another way to explain the unusual crater population: Suppose that half a billion years ago something suddenly happened to Venus, wiping out all older craters. Vast lava flows occurring simultaneously all over the planet would do the trick. Then, if there has been relatively little surface activity since that time and Venus has been slowly collecting craters all along, things should look as they do.
This idea, called "catastrophic resurfacing," rubs many scientists the wrong way. Like the disrupted moon theory, it invokes a special event, and quite an incredible one at that. We can hear Ockham sharpening his razor. . . .
The repetitive debate seems to have largely died down, to the great relief of most Venus scientists. Although some controversy continues, the community seems to be converging toward an aceptance of something like catastrophic resurfacing (although not everyone calls it that). Roughly 600 million years ago, Venus seems to have been wiped clean of craters.* Some combination of widespread volcanic flooding and tectonic disruption crated a tablua rasa for later cratering.
*The crater counts give a surface age of between 300 and 800 million years, with a best guess of 600 million years.
Although the idea that our planetary twin had such a radical makeover as recently as 600 million years ago may be unsettling, it would be instructive to remember that at roughly the same time Earth was undergoing its own midlife crisis. For, back on Earth, this was the age of the Cambrian explosion, which dramatically transformed our planet from a world on which nothing more complex than bacteria and algae had evolved for billions of years to one teeming with an incredible diversity of animal and plant species.
The science of the Venusian surface does seem to be stuck in some sort of Newtonian quagmire where every idea is accompanied by and equal and opposite idea.
Venus, too, lost its ocean. (As I've discussed, we don't know if Venus was born as wet as Earth, but it surely had some water to begin with.) Grinspoon 1997:314