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Nereid
2012-Sep-18, 08:22 AM
When is a crater not a crater? When it's a basin! :D

Or, perhaps, a depression.

In the scientific study of planetary surfaces - possibly including the Earth - what is the distinction between a 'basin' and a 'crater'? From my (admittedly limited) reading, it seems that an impact basin is 'just' a particularly big impact crater.

Ditto, re 'depression': is this a consistently used term, to refer to a 'geological' (I guess this word applies to rocky and icy bodies, not just to those on the Earth) feature? If so, what, and what distinguishes a 'depression' from a crater?

Finally, are craters 'impact craters' by default? Rather than, say 'volcanic craters' or 'no particular cause/origin is implied' or ...?

astrostu
2012-Sep-18, 10:48 AM
I can answer the first and last question. There is no agreed-upon distinction between basin and large crater. In my Mars crater database, I have a break between 500 km and 900 km craters, so I just call 900+ a basin and 500- a crater. In my searching, I've never really found a good distinction, though some of the old-timers may know. I head to my crater conference in literally 20 minutes, first day's tomorrow, I can try to remember to ask.

Last question - in planetary science for those of us who don't study volcanos, yes. But in papers or at the beginning of a discussion, we should always preface it with "impact" crater and then a parenthetical (hereafter "crater") or some such thing. Usually you can tell through context if someone isn't OCD about doing that, but the distinction can be important -- there are caldera craters (volcanic), pit craters (volcanic), and impact craters.

With that in mind, perhaps you could slightly modify the wording of the second question to help?

Nereid
2012-Sep-18, 11:30 AM
Thanks!

So, when rocky and/or icy solar system bodies smaller than the Moon, but larger than Vesta and Ceres, get 'imaged all over', will they be found to have any 'basins'? For example, are there any basins on Iapetus? Rhea? Dione? Tethys? IIRC, the feature which makes Mimas look like the Death Star is called a crater, not a basin. Also - again IIRC - Vesta does not have any basins.

For the second question ("re 'depression': is this a consistently used term, to refer to a 'geological' (I guess this word applies to rocky and icy bodies, not just to those on the Earth) feature? If so, what, and what distinguishes a 'depression' from a crater?"): from the distant mists of time, my brain reminded me that I'd read about how some lunar crater chains and/or collapsed lava tubes looked like 'depressions', and were hard to distinguish from rifts (maybe it was about Mercury?). Anyway, much more recently, I read this*: "I read some articles which said that at first Himeros was considered as probably a depression and not an impact crater. It was speculated that the depression was formed either due to a collisions between Eros & other body of the same size or the Eros was separated from a much bigger object(at Himeros) when a projectile collided with the bigger object. But later(in other sites) it was told that after some observations of Himeros, it has been said that the findings are consistent with the impact theory. It might be that the earlier speculations on Himeros were more accurate." Myself, I don't recall reading any such, but it also raises the question of how and when there are distinctions made between form ('morphology', to galaxy nuts like me) and (likely) cause - a 'crater' has a certain shape (etc), irrespective of how it was formed (which may not be known), but ... (you get the idea, I hope).

* it's elsewhere in this forum

astrostu
2012-Sep-18, 01:25 PM
I was thinking a bit more on this during my 5AM ride to the airport. I'm thinking perhaps an objective criterion may be that a basin would be when the curvature of the object affects the impact "depression"'s shape. For example, Mare Crisium on the moon is a basin, is ~555 km across, but is affected by the Moon's curvature.

As opposed to Herschel crater on Mimas. Herschel is 1/3 the diameter of Mimas, but even at 139 km across, the crater's shape doesn't appear to be strongly influenced by the moon's curvature (I'm saying this just by looking - I haven't searched for any papers).

Meanwhile, the two south polar basins on Vesta are half its size and curved, conforming somewhat to the asteroid's own curvature.

This is just me musing, but it seems like at least a maximum-diameter criterion you could use for when something is definitely a basin versus a crater.

Down the hall from this crater meeting that starts tomorrow is Jenny Blue at USGS who is one of the reps to the IAU on nomenclature. I can ask her if there's something official she knows of, too.


Regarding the term "depression," eh, I'm not that pedantic. "Crater" or "hole" or whatever is fine for me until you can classify as impact or volcanic. For fresh ones, impact craters have ejecta and a raised rim while volcanic pit craters and calderas do not (though lava I suppose might look like an impact crater ejecta blanket at times).

dgavin
2012-Sep-18, 07:03 PM
In reference to Volcanic terminology.

A Crater Typically means an area that had material blown out of it leaving a crater like formation. Mt. St. Helens has a crater.

A Caldera is where a portion of the crust (or moutain top) has collapsed down into/towards the magma pool that was emptied during an eruption. Yellowstone, Long Valley, Newberry, Crater Lake, Sea Mt. Axial are all Calderas.

Crater forming eruptions have an upper limit of VEI 5 to a low 6. Caldera forming eruptions are always VEI 6 or stronger.

Ara Pacis
2012-Sep-18, 07:13 PM
I'm not a geologist, but I'd suggest these possible distinctions:
A crater could be a feature that has been excavated by an explosive process, such as impact, volcano or bomb.
A basin could be a large scale feature that would be expected to retain water long term were precipitation events to occur and/or, as Astrostu suggests, has a bottom that is convex due to curvature. (I wonder how the Basin and Range part of the US Rockies fits into this description.)
A depression could be a large scale feature that is below datum (sea level on Earth).

BTW, I think some depressions are caused by stretching of the crust, so not all are caused by impacts, if we were to allow impact craters to also be called depressions if they are below datum.

astrostu
2012-Sep-19, 03:12 AM
Alrighty, talked with Jenny this afternoon 'cause my hotel wouldn't let me in 'til 3. She says that the strict definition of a crater is a circular depression. As opposed to a patera which is an irregular depression. So, think the volcanic calderae of Olympus mons -- that'd be a patera.

So, I guess the hierarchy would be depression, and then crater and patera are types of depressions, and then pit craters and impact craters are types of craters. If the volcanic opening is circular, then that would be a caldera crater.

She also said there is NO official definition of a basin. It has been brought up before their IAU nomenclature group before, but they never decided on anything. When papers at USGS (Flagstaff) are going to be submitted to a journal, they have to go through internal review and if they have nomenclature, she generally will be one of the people to review it. If someone uses the term "basin," she always recommends they lower-case the "b" to indicate it's not the official term.

She also indicated that the descriptor is not part of the name. So if I talk about Lowell Crater, I should actually have written Lowell crater, and I don't have to use the word "crater" at all because it's not actually part of the feature's name.

My immediate thought was, "What about the South Pole-Aitken Basin?" That's not an officially recognized name. Everyone calls it that, but the nomenclature is supposed to help clear up confusion about something, and since everyone calls it that already, there's zero confusion, so no one has actually submitted that as a name for that feature. If it were submitted, they'd probably have to revisit the idea of "basin."

I'm going to ask that we discuss it at this meeting on Friday. Not in any official way, but I want to see what a room of experts in cratering think in terms of a "gut feeling" of when a crater should be called a basin.

Nereid
2012-Sep-19, 09:22 AM
Thanks! :)


Alrighty, talked with Jenny this afternoon 'cause my hotel wouldn't let me in 'til 3. She says that the strict definition of a crater is a circular depression. As opposed to a patera which is an irregular depression. So, think the volcanic calderae of Olympus mons -- that'd be a patera.

OK, so elongated craters - think Messier on the Moon - are not craters, but pateras (paterae?)?


So, I guess the hierarchy would be depression, and then crater and patera are types of depressions, and then pit craters and impact craters are types of craters. If the volcanic opening is circular, then that would be a caldera crater.

And there are many other kinds of depression, depending on their shape?

Like valleys, rilles, collapsed lava tubes (though they'd be one kind of rille, wouldn't they?), and just plain holes and pits (like a 'skylight' in an otherwise enclosed lava tube)?

Does that also mean that if a very large feature were found to be significantly lower than the surroundings, it would also be a patera, if it were irregularly shaped?


She also said there is NO official definition of a basin. It has been brought up before their IAU nomenclature group before, but they never decided on anything. When papers at USGS (Flagstaff) are going to be submitted to a journal, they have to go through internal review and if they have nomenclature, she generally will be one of the people to review it. If someone uses the term "basin," she always recommends they lower-case the "b" to indicate it's not the official term.

So Caloris (Mercury), Hellas (Mars), Valhalla (Callisto), Gilgamesh (Ganymede), Odysseus (Tethys), Mimaldi (Rhea), ... are all craters? Or, if sufficiently non-circular, paterae?


She also indicated that the descriptor is not part of the name. So if I talk about Lowell Crater, I should actually have written Lowell crater, and I don't have to use the word "crater" at all because it's not actually part of the feature's name.

My immediate thought was, "What about the South Pole-Aitken Basin?" That's not an officially recognized name. Everyone calls it that, but the nomenclature is supposed to help clear up confusion about something, and since everyone calls it that already, there's zero confusion, so no one has actually submitted that as a name for that feature. If it were submitted, they'd probably have to revisit the idea of "basin."

I'm going to ask that we discuss it at this meeting on Friday. Not in any official way, but I want to see what a room of experts in cratering think in terms of a "gut feeling" of when a crater should be called a basin.

Looking forward to hearing what they had to say!

astrostu
2012-Sep-20, 04:11 AM
- Elongated depressions would still be craters. It's when they're irregular and scalloped that they're not ... but don't confuse that with a simple scalloped rim crater (think Victoria crater versus the caldera complex on Olympus mons).

- Yes, there are other kinds of depressions. But since this post was focusing on craters, I left it at that ;).

- Giant depressions would still likely be called craters if they're circular and regular and we know they're depressions. Historically, historic names will stick. Like Hellas is still technically a planitia 'cause it was an albedo feature before we knew it was a negative topographic feature.

- Yes, those giant craters are technically still craters. Everyone may call them a "basin," but my understanding is that according to the IAU they'd still be craters.


We talked a bit about this this morning when I brought it up as a topic for discussion for Friday morning. One person said that she always considered anything larger than "around" 100-200 km to be a basin. Another guy said he considered anything with a peak ring as opposed to central peak to be a basin. But then just one, or does it need two? And what about things like Mare Imbrium -- most certainly large enough to have had 2+ peak rings, but they're no longer there.

I put in my idea about it being when the shape of the object affects the shape of the crater.

It was really just three of us who partook in that mini-discussion, but I think from it, it made everyone think more about what they really would consider a basin and realize that their gut idea is less black-and-white than they thought.

Since we talked about it this morning, I'm not sure if we will again on Friday, so I thought I'd post now.

Nereid
2012-Sep-20, 10:38 AM
Thanks!


- Elongated depressions would still be craters. It's when they're irregular and scalloped that they're not ... but don't confuse that with a simple scalloped rim crater (think Victoria crater versus the caldera complex on Olympus mons).

Hmm, what did you say Jenny Blue said?


Alrighty, talked with Jenny this afternoon 'cause my hotel wouldn't let me in 'til 3. She says that the strict definition of a crater is a circular depression. As opposed to a patera which is an irregular depression. (bold added)

So what's a poor machine to do, when it finds a depression that's elongated - (more or less) elliptical (e ~ 0.6, say) - but not "irregular and scalloped" (also not irregular OR scalloped ;) )? :confused:

It's not a crater (per Jenny), 'cause it's (clearly) not circular; but it's also not a patera, 'cause it's not irregular. :p


- Yes, there are other kinds of depressions. But since this post was focusing on craters, I left it at that ;).

OK, but ... where does one find a taxonomy of depressions, as used by planetary geologists (or whatever you guys call yourselves)?


- Giant depressions would still likely be called craters if they're circular and regular and we know they're depressions. Historically, historic names will stick. Like Hellas is still technically a planitia 'cause it was an albedo feature before we knew it was a negative topographic feature.

Yes, of course; I hadn't thought of that. So Crisium is still a Mare, even though it contains no liquid water!


- Yes, those giant craters are technically still craters. Everyone may call them a "basin," but my understanding is that according to the IAU they'd still be craters.

We talked a bit about this this morning when I brought it up as a topic for discussion for Friday morning. One person said that she always considered anything larger than "around" 100-200 km to be a basin. Another guy said he considered anything with a peak ring as opposed to central peak to be a basin. But then just one, or does it need two? And what about things like Mare Imbrium -- most certainly large enough to have had 2+ peak rings, but they're no longer there.

I put in my idea about it being when the shape of the object affects the shape of the crater.

It was really just three of us who partook in that mini-discussion, but I think from it, it made everyone think more about what they really would consider a basin and realize that their gut idea is less black-and-white than they thought.

Since we talked about it this morning, I'm not sure if we will again on Friday, so I thought I'd post now.
Again, thanks. Very interesting.

astrostu
2012-Sep-20, 03:07 PM
- "Circular" just means not irregular while still, you know, "circular." :) Rahe (http://en.wikipedia.org/wiki/Rahe_(crater)) is highly elliptical, but still a crater 'cause it's regular and circular.

- I do not know where the taxonomy would be, though my go-to place for names and nomenclature is is the Gazetteer (which Jenny maintains) (http://planetarynames.wr.usgs.gov/).

- Maria fall into the historic stuff that doesn't get new names. The areas on the moon were generally named by an Italian monk, Giovanni Riccioli, in the mid-1600s. For Mars, many large feature names still go with the historic names from Schiaparelli and Lowell from the late 1800s to early 1900s. Before we knew what these really were or had much (moon) or any (Mars) sense of topography.

publiusr
2012-Sep-22, 07:43 PM
Arizon'a meteor crater was wrongly thought to be a Maar http://en.wikipedia.org/wiki/Maar

PS recent activity at Thera? http://www.spacedaily.com/reports/TerraSAR_X_image_of_the_month___the_Santorini_volc ano_expands_999.html

cran
2012-Oct-17, 08:21 PM
When is a crater not a crater? When it's a basin! :D

Or, perhaps, a depression.

In the scientific study of planetary surfaces - possibly including the Earth - what is the distinction between a 'basin' and a 'crater'? From my (admittedly limited) reading, it seems that an impact basin is 'just' a particularly big impact crater.

Ditto, re 'depression': is this a consistently used term, to refer to a 'geological' (I guess this word applies to rocky and icy bodies, not just to those on the Earth) feature? If so, what, and what distinguishes a 'depression' from a crater?

Finally, are craters 'impact craters' by default? Rather than, say 'volcanic craters' or 'no particular cause/origin is implied' or ...?
Any crater can also be a basin and a depression.

A crater is a solid surface (ie, continental or submarine) depression, formed rapidly (explosively) and usually distinguished by origin - impact or volcanic.

A depression is a solid surface feature with a lower than mean elevation for its setting. In other words, it is considered more in relative rather than absolute terms.

A basin is a solid surface feature which can retain or has retained accumulated sediments. Even filled, upwarped and/or subsequently eroded, it remains a basin in the geological sense. Basins are often called sedimentary basins for this reason.

None of these have size criteria, and neither depressions nor basins have lateral shape criteria.

IreneAnt
2012-Oct-23, 06:34 PM
Alrighty, talked with Jenny this afternoon 'cause my hotel wouldn't let me in 'til 3. She says that the strict definition of a crater is a circular depression. As opposed to a patera which is an irregular depression. So, think the volcanic calderae of Olympus mons -- that'd be a patera.


Hmm. I have never heard the term patera applied to anything other than a volcanic construct. So I interpret Jenny's comments about craters being circular while patera being irregular to apply ONLY to volcanic craters. Impact craters can most certainly be irregular in shape.

Any thoughts on this astrostu?

astrostu
2012-Oct-24, 02:21 AM
We were pretty much just talking about volcanoes at that point because we had drifted somewhat into Alba Patera -> renamed Alba Mons recently. That said, Jenny emphasized that the nomenclature's entire purpose was to be as non-specific as possible with respect to origins so that if, say, we think a feature formed from water and so gave it a name based on that but later found it formed by wind, we wouldn't ahve to rename it. So while the context of our discussion at that time was about volcanoes, I would guess that she really was talking generically.

IreneAnt
2012-Oct-24, 05:23 PM
So, does that mean we can look forward to "impact pateras" in the future?

astrostu
2012-Oct-24, 09:21 PM
I suppose if there were an impact "crater" that had a highly irregular rim, then yes?

IreneAnt
2012-Oct-25, 08:54 PM
Ah! So it's the rim that needs to be irregular for it to be a patera, not the shape? We already have irregular craters, most notably Messier and Schiller on the Moon. But their rims are fairly standard, even if a bit eroded in the case of Schiller.