[X] My Assistant

[nprev]

Great work, Olvegg...thank you very much!

Bill, I can't shake the impression that most of the SPR is well above Titan's MSL, while the NPR is not...yet another rough parallel to Mars. Stretching that analogy even further, Mars' permanent (EDIT: exposed) south polar cap is much less extensive than the north pole's, possibly because H2O/CO2 sublimation occurs more readily down south due to higher elevation/lower surface pressure. Perhaps we're seeing something similar in Titan's SPR if the mean elevation is enough to lower the boiling points of methane & ethane below the average surface temperature there...

[Olvegg]

As Titan's rotation is bound, let me ask which longitude in your map is actually facing saturn.

Zero longitude is 3 o'clock in NPR and 9 o'clock in SPR.

[angel1801]

It is convention for a body in syncronious rotation to have the following:

Maps centered at the north pole: Zero degrees longitude is at the 6-o-clock position, and 180 degrees longitude is at the 12-o-clock position.

Maps centered at the south pole: Zero degrees longitude is at the 12-o-clock position, and 180 degrees longitude is at the 6-o-clock position.

[Stu]

Just pointing a finger at a curiosity that caught my eye...

Looking at the recent radar swath, I got the nagging feeling I'd seen one of the features on it before, and very recently. Then it came to me... there's a "dried up lake" area at the end of a long, narrow channel that, at least to me, looks a lot like the landscape around everyone's favourite Ionian volcano...



Anyone else notice that?

[Mongo]

I thought that I would summarise what I think is controlling the lakes/seas on Titan.

I believe that there are two major controls: the atmospheric circulation pattern (Hadley cells) that determine where the precipitation falls; and the altitude and latitude of each location, which determines whether surface methane/ethane at that location exists as a solid or a liquid.

The simplest circulation pattern would consist of two hemispheric Hadley cells. 'Warm' air would rise in the equatorial region -- cooling adabiatically with altitude -- and move toward the poles, slowly cooling and descending as it progresses poleward. Eventually, it would approach the surface, as some of the methane/ethane vapour condenses out (probably as 'snow'). This would produce heat, both from the latent heat of the methane/ethane condensing, and from the adabiatic compression of the air. The warmish, relatively 'dry' air would then move equatorward close to the surface and repeat the cycle, picking up methane/ethane vapour when passing over the seas and lake districts along the way -- which would tend to cool the air -- and warming up again as it nears the equator. The result would be that most of the precipitation on Titan would be over the polar regions, probably in the form of 'snow' -- which would likely turn into rain as it descended to lower altitudes. Remember that the average surface temperature on Titan is barely above the freezing point of methane and ethane. In fact, the difference is so low that there may be some feedback mechanism keeping Titan's temperature at that value.

The second control on liquid surface methane/ethane would be latitude and altitude. At the equator, the temperature would be high enough (except for the tops of any sufficiently tall mountains) for liquid methane/ethane to exist. However, with very little precipitation, any liquid methane/ethane that does collect would evaporate away relatively quickly without being replenished. This would not preclude occasional precipitation, but any lakes would be seasonal.

As you go poleward, the 'snow line' would descend, until at the poles, only areas at low altitude (probably well below the MSL) would support liquid methane/ethane on the surface. The exact height of this snow line would depend on temperatures at a given air pressure, which may not follow a smooth slope, due to latent heat effects. There would be plenty of available precipitation, so I would expect methane/ethane glaciers at higher altitudes, and lakes/seas at lower altitudes. The existing 'lands of lakes' we see may be the interiors of very large basins, rather like Hellas on Mars (athough not nearly as deep, and they may be tectonic in origin, not impact-related). The existing surface liquid distribution would suggest that several low-lying areas exist in the north polar region, while the south polar region is higher, with fewer low-lying regions.

Titan appears to be quite 'smooth', with little variation in altitude compared to other large planetary bodies. However, since its average surface temperature is so close to the freezing points of methane and ethane, even a relatively small change in altitude might produce a dramatic change in surface appearance, with a change between liquid and solid methane/ethane.

Thoughts?

Bill

[ngunn]

Olvegg, thank you so much for those polar maps. The two regions seem very similar indeed to me, given that we are 'seeing' them in such different ways. Lakes that look very black in RADAR are merely a darker grey in infra-red. I expect that southern RADAR swath to look exactly like the northern ones, except that more of the basins may have dried out somewhat due to the season. The only thing missing in the south is the enormous northern lake imaged by ISS. If one were to extend your maps to include this, and also more of Mezzoramia, I think their similarity in size and position relative to the respective poles would be quite striking.

I don't see evidence here of any major altitude difference between the poles. If there were significant deposits of methane snow on the hills I would expect it to look dark in RADAR, and I'm not seeing that. I think we are viewing a fen-like landscape with occasional hillier patches. Some of the lakes may occupy depressions created by some form of surface subsidence, but this could be quite modest - a few metres or so. Comparison with volcanic calderas on Io or elsewhere may be fun (and, who knows, possibly even valid) but for me that's a long way from what I think is going on here.

I know too little to comment on models of the atmospheric cirulation but I'm grateful for posts on this fascinating subject. From our own world we know just how complex - indeed chaotic - such systems can be. Does Titan have an equivalent of El Nino every century or two that produces freak rainstorms in equatorial locations like eastern Adiri, or is that down to methane buildup from cryovolcanos? So many unknowns, so much to look forward to.

[nprev]

Yeah, you're probably right, ngunn...really can't tell if the Southern Hemisphere is higher overall than the NH from this data. They do look different to me, but it's very subjective until we get the same high-quality radar data on the SH that we've enjoyed thus far from the NH.

[tty]

... the channels in the dark area seen on the images are actually 'underwater' (we need a better terminology to discuss non-aqueous lakes and seas!)...

How about "submerged"?

On another topic, the Ancylus Lake stage of the Baltic was named for a freshwater mollusc, which is perhaps not ideal for a lake on Titan. As a matter of fact the largest known freshwater lake in Earth history was probably the vast ice-dammed lake that existed in Western Siberia during earlier glaciations. I've seen it called the Obik sea, but I'd rather stick with classic greek/latin names "Mare Borealis" sounds good to me.

[ngunn]

Any names are fine with me actually but I'll be surprised if they decide to create a new category - mare - for a feature that may even shrink and separate into smaller patches as the seasons change. Indeed I think SAR is required to make absolutely sure that it is even now one single lake rather than several in close proximity linked by channels. Is 'Ontario' latin? It sounds plausibly so to be sure, though most other named features on Titan definitely aren't. My preference would be to continue the North American theme for the largest lakes, in line with Ontario and in honour of the continent where these discoveries are actually being made. Thus: Superior Lacus for the one that has already attracted that comparison and Agassiz Lacus for the giant (unless Agassiz was also a freshwater mollusc).

[Mongo]

Lake Agassiz was named for Louis Agassiz, the nineteenth century zoologist/geologist/glaciologist who first proposed the existence of a former ice age. (Actually, this would make his name quite suitable for a feature on the surface of Titan.)

I have just re-read parts of Ralph Lorenz's book Lifting Titan's Veil and found the following passage about evaporation rates on Titan (page 108-109):

(he first talks about the Earth -- to produce the amount of global evaporation observed requires about 70 W/m^2 averaged over the globe, compared to a global average insolation of around 260 W/m^2, so about 1/4 of solar insolation goes into evaporation)

The same kind of calculation can be done for Titan. On Titan only around 0.4 W/m^2 of solar energy reaches the surface. Even if all of that heat went into evaporating methane, which has a latent heat smaller by a factor of five less than water and a density half that of water, it is only enough to evaporate a 5-cm layer of methane, which sets an upper limit on the amount that can fall again as rain in each (Earth) year. Since some heat must be radiated and convected away, 5 cm is a wild overestimate and 1 cm seems a more likely value for the annual rainfall on Titan.

So a total evaporation of about 20 cm per Titan year. Even allowing for possible concentration of solar energy over lakes (via dry air being warmed above land and then blowing over the lakes, picking up methane vapour), there is no way that tens of metres of methane could be evaporated over one Titan year. The seas and lakes of Titan are permanent, on less than geological timescales. The distribution of liguid methane/ethane on Titan must be related to precipitation patterns, with almost all the precipitation falling in the polar regions.

The book also contained a chart of temperature versus altitude -- the temperature in the lower troposphere falls by 2.5K per km of altitude. The global average temperature is about 94K (I could not find the expected values for the equatorial region versus the polar regions), with methane freezing at 90.65K, and ethane freezing at 89.95K, both figures at one atmosphere pressure. On average, then, methane would freeze at an altitude of about 1,340 metres, and ethane at an altitude of 1,620 metres. The altitudes at the polar regions would be lower by some unknown (to me) amount, but they probably would be low enough that methane on the highlands, and likely ethane as well, would be solid rather than liquid. I do recall that the temperature difference between equator and pole is fairly small, but even a single degree Kelvin drop in temperature would reduce the respective 'snow lines' by 400 metres.

Bill

[JRehling]

Yeah, you're probably right, ngunn...really can't tell if the Southern Hemisphere is higher overall than the NH from this data. They do look different to me, but it's very subjective until we get the same high-quality radar data on the SH that we've enjoyed thus far from the NH.

Unless there were a radical difference in altitude, the comparison seems to be irrelevant. There's probably no global drainage system, so liquid would not run from the one pole to the other, just as the Caspian Sea could not drain into Lake Victoria or vice versa. Whatever we see at each Titanian pole is the result of local meteorology filling things up to a level geoid describing the aquifer level, and there may be a fair amount of non-interconnectedness even within each region. Interchange the respective altitudes of the two regions -- if there is a difference, and I don't think we'd see any effects.

If there is a substantial difference, though, that may amount to something. Titan's temperatures are close to the boiling point of methane and ethane, so perhaps a really profound difference in altitude is making the difference between lakes and virga rains.

For now, though, a sufficient (possibly incorrect) explanation would be that a slight difference in seasonal rains makes a large difference in the areal extent of some extremely shallow lakes.

[Mongo]

Titan's temperatures are close to the boiling point of methane and ethane, so perhaps a really profound difference in altitude is making the difference between lakes and virga rains.

Actually, the temperatures are close to the freezing points of methane and ethane. Methane's boiling point (at one atmosphere pressure) is 111.6K, and ethane's boiling point is a whopping 184.5K.

The reasin that altitude in the south polar regions is important is that an increase in altitude of a few hundred metres may be enough to result in solid methane on the surface, instead of liquid.

Bill

[Juramike]

From the drainage channels in the equatorial highlands, we know that "when it rains on Titan that the rains are fierce when they come." Presumably this liquid (and transported goo and gunk) flows down into the dark basins, then what?

Is it slowly evaporated away (at 5 cm/year)?

Or does it percolate into the ice-gravel and end up in polar regions through underground movement?

[So why no equatorial lakes? Presumably if you had a deep depression in the equatorial region (Sinlap crater), it would first fill in as a lake, then evaporation would leave behind deposits that would fill in the relative depression. Evaporation at the polar zones would not proceed as quickly and lakes would hang out longer.]


I would looove to know if all the "liquid level" of the lakes are at the same relative topographical altitudes in the polar regions, or even in neighboring regions.

At least we would get a rough idea of the methanofer zones and percolation potential of the underground geology.

-Mike

[ngunn]

Lake Agassiz was named for Louis Agassiz, the nineteenth century zoologist/geologist/glaciologist who first proposed the existence of a former ice age. (Actually, this would make his name quite suitable for a feature on the surface of Titan.)

Excellent

I have just re-read parts of Ralph Lorenz's book Lifting Titan's Veil and found the following passage about evaporation rates on Titan (page 108-109):
So a total evaporation of about 20 cm per Titan year. Even allowing for possible concentration of solar energy over lakes (via dry air being warmed above land and then blowing over the lakes, picking up methane vapour), there is no way that tens of metres of methane could be evaporated over one Titan year. The seas and lakes of Titan are permanent, on less than geological timescales.

Thanks for finding the evaporation rate info. I am not so sure that this line of reasoning, though strong, is fully conclusive, however. You mention the 'concentration' of solar heating via convection. I think this could actually be a very powerful effect. Methane vapour is MUCH lighter than nitrogen and once released would be expected to rise quite quickly to great heights. Titan's air can hold a much higher fraction of methane vapour than Earth's air can hold water vapour, allowing convection to be driven by density differences independent of temperature. I think this is why the tropospheric clouds tower so high (45 km) into the atmosphere. This process could bring a large lake surface into contact with a very large amount of (relatively) warm air in the course of a Titan summer.
Also, as I pointed out earlier, 'tens of metres' of evaporation may not be required for substantial changes in lake area. If, as seems likely, the lakes contain significant amounts of polar material in emulsion or suspension form then the radar penetration could be as litle as a few metres and we would have to infer that most of the lakes are very shallow. I accept that some, like Ontario, do persit all year but I still think it's possible that most dry out or at least shrink substantially every summer.

Now suppose I'm just plain wrong - quite likely! - and there is indeed a bigger inventory of surface liquid in the north all year round. There is no shortage of possible reasons: Porosity of the ground, extent of subsurface reservoirs and liquid transport, availability and profile of suitable depressions (deeper depressions allow the same volume of liquid to be stored within smaller total area as well as reducing evaporation), thermal capacity and IR albedo of the solid surface materials - plenty of alternatives to big hemispheric asymmetries in climate or altitude.

[Mongo]

Now suppose I'm just plain wrong - quite likely! - and there is indeed a bigger inventory of surface liquid in the north all year round. There is no shortage of possible reasons: Porosity of the ground, extent of subsurface reservoirs and liquid transport, availability and profile of suitable depressions (deeper depressions allow the same volume of liquid to be stored within smaller total area as well as reducing evaporation), thermal capacity and IR albedo of the solid surface materials - plenty of alternatives to big hemispheric asymmetries in climate or altitude.

One possibility that occurs to me is that Ontario Lacus may be the equivalent of Lake Baikal on Earth, compared to Lake Michigan/Huron (Michigan/Huron has an area of 117,318 km^2 -- the largest freshwater lake on Earth -- versus 31,500 km^2 for Baikal, but a volume of only 8,456 km^3, versus 23,000 for Baikal), meaning that the total volume of liquid methane/ethane may be almost the same in both polar regions. In fact, I would expect that total precipitation in the two polar regions to be almost the same.

The question that I have is whether the total mass of liquid methane/ethane precipitation reaching the surface is the same in the two polar regions, or whether the total mass of liquid and solid methane/ethane precipitation reaching the surface is the same, but with a smaller fraction of the precipitation in the south polar region being liquid (presumably due to topographic effects).

Bill