[X] My Assistant

[Mongo]

Given that this new nearly-Caspian-Sea-sized body of fluid is likely to be called a 'Sea' rather than a 'Lake' (quite correctly, in my opinion), has further thought been given to what surface area is to be the dividing line between 'Lake' and 'Sea'?

The largest 'Sea' discovered so far on Titan, with an area of around 350,000 km^2, would be equivalent to an area of about 2,147,000 km^2 on Earth, which would be a LARGE sea on Earth:

2,974,600 km^2 South China Sea
2,515,900 km^2 Caribbean Sea
2,510,000 km^2 Mediterranean SEa
2,261,100 km^2 Bering Sea
1,507,600 km^2 Gulf of Mexico
1,498,320 km^2 Arabian Sea
1,392,100 km^2 Sea of Okhotsk
1,012,900 km^2 Sea of Japan
730,100 km^2 Hudson Bay
664,600 km^2 East China Sea
564,900 km^2 Andaman Sea
507,900 km^2 Black Sea
453,000 km^2 Red Sea

On the other hand, we already have Ontario Lacus, which is roughly 60,000 km^2 (a bit smaller than the largest fresh-water lake, Lake Superior at 82,414 km^2), but which would be proportionally equal to a body of water with area about 368,000 km^2, given a 6.135:1 ratio between the surface areas of Earth and Titan.

On Earth, all bodies of water that size are Seas -- but all bodies of water that size are also salt water.

My own two cents would be that since the fresh-water/salt-water distinction does not apply on Titan, a convenient surface area should be chosen (greater than the surface area of Ontario Lacus, to avoid inconsistencies), and all bodies of fluid with a greater area than that size are called 'Seas'. 100,000 km^2 is a nice round number...

Bill

[Juramike]

Sounds like a good proposal for a definition. Many lakes, few seas.

[Arrrrrrr, shiver me timbers and set sail for the Seven Seas of Titan.]

[Mongo]

Two seas so far, that we know of.

The largest one on this map must be around 350,000 km^2, and the second-largest on the same map (seen here, in comparison to Lake Superior), is apparently just over 100,000 km^2 in surface area, so they both make this (admittedly arbitrary) list. Of course, a lot of the north polar region is still to be imaged, so there may be more seas to be found.

On another topic, I am fairly doubtful that SAR will show much in the way of change over time in the shorelines of these huge new lakes/seas. As I understand it, the amount of solar energy available at Titan is insufficient to evaporate more than maybe 10 to 20 metres of methane/ethane over the course of a Titan year. This was discussed here in an earlier thread, several years ago now, but I cannot find it. Allowing for (judging by the cloud coverage) two 'cloudburst' events per year, one in the summer and one in the winter (could the two rainfall events have different compositions?), I would expect, at the very most, a 10 metre change in 'water' level over the course of a Titan year -- with the most likely amount of change significantly less than that, due to possible surface 'scum', if it exists, retarding evaporation, and general cycle inefficiencies (the atmosphere over the lakes/seas cannot have low 'humidity' all the time). A 'water' level change of 1 to 4 metres over a Titan year might be more realistic.

I seem to recall from another thread that the SAR probably sees down 10 or 20 metres into the methane/ethane lakes, so that 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!), presumably created by turbidity currents and the like, rather than rivers. The darkest areas would be deeper, of course. But while there may be minor advances and retreats of the shoreline, I think that any instances of lakes evaporating we see are happening on a geologic scale, not a 'yearly' one -- perhaps due to secular changes in methane abundance over millions of years.

Bill

[edstrick]

A ***LOT*** of the terrain in the northern lake-district looks "Io'ish". That long new radar strip from Feb. really drives that point home. Scroll left to right, you get out of the low latitude dunes into rather non-descript mid-lattitude terrains, then into more and more complex pitted looking terrains like fields of calderas. Then they start to show dark bottoms in a few, then more and more and darker and darker. We really can't see enough essential details in a lot of Titanian terrain at Cassini Radar's resolution <ggrrrrrssss!> but it's what we've got.

[remcook]

if the lakes cannot be drained or evaporated within half a Titan year or less, this leaves the big question why we see far less lakes in the south... But I think we don't have radar data from the south yet, is that right? will there be?

[Mongo]

There are SAR observations of the southern polar regions coming up (in particular, one is planned to image the Ontario Lacus region), but none have yet occurred. It does look like the largest lake is Ontario Lacus (which is not small, if it is almost as big as Lake Victoria), but there also appear to be numerous 'smaller' lakes (the red cross presumably marks the south pole):



Keep in mind that any lake visible on this image would be considered a large lake on Earth. I would say that, with the exception of the huge seas recently observed in the north polar region, the coverage by smaller lakes looks similar in the two polar regions, to my eyes.

It might be significant that the distribution of lakes in the south polar region is also quite asymmetrical, the same as in the north polar region. Topography?

Bill

[ngunn]

Some thoughts on interesting recent posts.

1/ Lake and Sea distinctions and definitions.
I think using a size criterion is very dodgy since the areas involved evidently change enormously over as yet undetermined timescales. If Olvegg's conjecture is correct we may yet see Mezzoramia turning into a sea before our eyes. Conversely, at what point should a shrinking feature be demoted? I think it wise to stick to the single descriptor 'lacus', if only to avoid another unseemly professional bun-fight at some future date.

2/ The north south imbalance and the extent of seasonal changes.
Either there is always much more surface liquid in the north or the seasonal changes are indeed enormous. Hopefully we'll be around just long enough to get a hint of an answer at least. It would seem to be a major leading question now to be addressed by any extended-extended mission; maybe even important enough for mission scientists to consider a Cassini end-game with fewer maneuvers and flybys but stretched over the longest possible time. For the moment I'm leaning towards the real major seasonal changes option.

3/ Lakebed channels and liquid depth
I do not think the lakebed channels that currently appear covered by liquid were made by turbidity currents. I think they were made by ordinary surface flow when the lake levels were lower, whenever that may have been (last summer or a thousand years ago). They look exactly like the ones we now see high and dry. We know that most of Titan is very flat. I think it's quite likely that most of the lakes are really very shallow - no more than a few metres - so that a change in level of just a metre or two could result in readily observable change over wide areas.

4/ A request for the map workers:
I am having trouble comparing the scales of features seen so far in both polar regions. A pair of directly comparable polar maps extending out far enough to include Ontario Lacus and southern Mezzoramia in the south and the whole of the northern 'sea' and lake district would be extremely welcome.

5/ Naming suggestions for the biggest lake
As far as I am aware there have been four so far offered here.
Mare Boreale, Great Bear, Agassiz and another beginning with 'A' (for the huge fresh water lake that once was where the Baltic is now). Edit: Ancylus. Are there any I have missed, or has anybody spotted any good ones elsewhere? Edit: add a fifth - Hertha.

Happy times!

[nprev]

Hmm. Here's an idea/question: Who was the first researcher (if it was a solo effort, that is) who first postulated oceans on Titan? Seems that honoring this person by naming one of the seas after him or her would be very apropros.

[Mongo]

5/ Naming suggestions for the biggest lake
As far as I am aware there have been four so far offered here.
Mare Boreale, Great Bear, Agassiz and another beginning with 'A' (for the huge fresh water lake that once was where the Baltic is now). Edit: Ancylus. Are there any I have missed, or has anybody spotted any good ones elsewhere? Edit: add a fifth - Hertha.

I had also suggested 'Abzu' back when the ISS team was considering mythological oceans, although that was before this 'sea' was discovered.

Bill

[ngunn]

On the question of seasonal drying:

I too remember earlier discussion about evaporation rates on Titan but to my irritation I can't locate it so far. It was around the time of the T8 discussion and Bruce Moomaw's 'Titan conference last year' thread. There was something to the effect that Titan's atmosphere can hold methane vapour equivalent to 10m of liquid but it requires 1000 years of solar heating to evaporate this much. Since the bodies of liquid we see now are at high latitudes perhaps 1000 years is an underesimate.
On the other hand maybe we don't have to rely on direct insolation of the lakes to dry them out. If they are shallow enough they will easily reach thermal equilibrium with the atmosphere above them within a single season. We then have all the heat gained by the polar atmosphere in summer available to cause evaporation. Deeper lakes would not warm up to the ambient atmospheric temperature and hence might resist big seasonal changes in appearance - Ontario for example?
I want to sound a note of caution about calderas. Talk of calderas conjures up images of lakes bounded by steep cliffs. I would emphasise that to produce the different behaviour I describe for deeper and shallower lakes it is only necessary to postulate a beach gradient of about 1 in 100 and depths of perhaps 50 metres for the 'deep' ones, compares with a beach gradient of 1 in 10,000 and a depth of around 5 metres for the shallow (drying) ones. No cliffs required! (Of course there may be cliffs, or at least hillsides. It is tempting to read the images that way in some cases.)

[The Messenger]

Hmm. Here's an idea/question: Who was the first researcher (if it was a solo effort, that is) who first postulated oceans on Titan? Seems that honoring this person by naming one of the seas after him or her would be very apropros.

The oceans were hypothesized to explain the methane in the atmosphere. Since the lakes and seas; (if they are lakes and seas) are too small to replenish atmospheric methane, the original hypothesis was wrong. Prior to Cassini's arrival, there was considerable evidence that the surface of Titan was not dominated by liquid bodies; so it could be more appropriate to name the lakes after the scientists who made this determination.

I think Jonathan Lunine and Ralph Lorenz might be in both groups. I like the sea of Lunine, and lake Lorenze.

[Olvegg]

I think this mosaic may be useful for comparison of North and South, though I don’t know to what extent it is correct. I’ve used PIA06240 image without any changes. However, it is more or less consistent with global map. Blue contour is very approximate position of future T39 swath. Mezzoramia, zero longitude and black small piece of T7 swath (from PIA09035) are to the right, Lacus Ontario is to the left. The scale is 1 pixel: 4 km and the outer circle is 70 degrees SL.

Attached thumbnail(s)

 

[Olvegg]

And comparison on a same scale:

Attached thumbnail(s)

 

[Mongo]

Thank you Olvegg for those maps. We forumers now have a chance to wildly speculate about what the differences between the two polar regions are, and what could cause those differences.

At first glance, there does appear to be a considerable difference in the percentage of area covered in lakes/seas. The figure of 7% was bandied about for the northern polar region (NPR to save my fingers), before the discovery of the larger seas. I would guess that this figure is still good, for much of the lower half of the NPR on the chart. For the upper half, maybe 20% or so? For the SPR, the total surface area covered by methane/ethane appears lower (bearing in mind that we are looking at a blurry optical image, not SAR). If the dark grey areas are indeed lakes, then perhaps the same percentage of area (7%) is covered, but over a smaller total area. In both regions, lakes appear to be mainly confined to specific areas within the polar regions.

What mechanisms could cause this? Seasonal evaporation is perhaps the most obvious, but as I have stated earlier, I have my doubts. Landform differences? Assuming no significant differences in quantity of precipitation (not a sure thing, if the north polar and south polar Hadley cells do not interact much), then a major difference in average altitude between the two polar regions could influence the fate of any precipitation. The polar regions are apparently very close to the freezing point of methane (which is almost exactly 90K at one Earth atmosphere pressure -- and ethane is only 0.7K higher), in fact they are almost certainly below it, but Titan's average surface pressure is about 1.5 times that of Earth, resulting in a slightly lower melting point for methane or ethane. Assuming a normal tropospheric fall in temperature with altitude (and corresponding rise in methane/ethane melting point), maybe liquid methane remains liquid only at lower altitudes, with methane at higher altitudes near the poles being a solid? So the lands-o-lakes we see might be inside large topographic depressions -- and the SPR is mostly highland?

How could we test this hypothesis? Well, it would imply that the methane precipitation in the polar regions would mainly fall in solid form, or 'snow'. What would RADAR see, looking at methane snow or ice? Could we distinguish solid from liquid methane?

Any thoughts?

Bill

[TritonAntares]

Very instructive map, Olvegg. Apart from the difference in size of the new found seas on the northern and southern hemisphere, most striking is the 'tilt off' from the exact position of the poles (wonders me what is hidden behind the border of the maps...).
As Titan's rotation is bound, let me ask which longitude in your map is actually facing saturn.
Somehow there must be a reason for this off axis positioning - local topographic causes being the most likely ones to me.
I'm curious about the upcoming radar observations of the regions next to the great lakes and seas.

Bye.