Megameter chasm on an icy moon

By Phil Plait | July 29, 2010 1:52 pm

I know I haven’t been posting much astronomy the past few days — Comic Con, w00tstock, and "Bad Universe" have kept me hopping — so to make up for it a little bit, here’s a lovely image sent back a billion kilometers from Cassini:

cassini_tethys_canyon

This is Tethys, an ice moon of Saturn. The angle of Cassini, Tethys, and the Sun light the moon as a crescent. The most obvious feature is Ithaca Chasma, a (more than) thousand-kilometer-long gash in the side of the object. Note that Tethys is only about 1000 km in diameter, so the chasm runs along a third of the moon’s surface (circumference = diameter x π, remember).

How big is that? Stand up and take a long stride. That’s about one meter. Now do it 999,999 more times. That’s a megameter: a million meters, or 1000 kilometers. Better pack a lunch.

The chasm is billions of years old, and may have formed when water inside the moon froze, expanded, and cracked the surface open. It’s a hundred kilometers across and 3-5 km deep, too. It’s far larger than the Grand Canyon, the largest canyon on Earth.

Space is big, and weird, where even small objects have huge features. It’s surprising, but surprising things are the best things to know.

Tip o’ the dew shield to Carolyn Porco.


Related posts:

- An otherwordly eclipse
- A billion km distant ice mountain against the black


CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Tethys

Comments (38)

  1. Kareth

    Or it could have been caused by a glancing blow from a mass accelerator round of unimaginable destructive power around thirty-seven million years ago.

  2. Edward P

    …Reapers…

  3. TCs

    As far as I know, very few people use megameters (Mm) and Wikipedia seems to agree with me. Astronomers might use it or it’s a little joke on your part, I don’t know. All I know is, I’m a European geek, I wouldn’t trade SI units for anything else in the world and I’d rather express the distance as “a thousand km” instead.

  4. TCs (3): No, not a joke, just the use of a cool unit that goes along with my analogy of taking a long stride.

  5. JohnG

    Brings to mind the use of the terms “megasecond” and “kilosecond” in Vernor Vinge’s “A Deepness In The Sky”. Notions such as days, months, and years had no usefulness between various star systems. I had a hard time grasping how long things were taking in that novel. A “megasecond” (1 million seconds) is longer than a week, but less than two.

  6. Jamey

    TCs: Would a kilokilometer sound better to you?

  7. Gary

    I’m curious about the source of the name Ithaca Chasm. A Carl Sagan / Cornell University connection perhaps?

  8. BJN

    I personally think that subduction trenches around the Pacific tectonic plate are the biggest canyon on this planet. Bering Canyon in the Bering Sea is 1495 km long and has 1840 m of wall relief. Zhemchug Canyon in the Bering Sea is the largest volume discrete canyon formation on this planet.

  9. TCs @ 3: Megameter is not another unit. ‘meter’ is the unit, and ‘mega’ is the decadic prefix. It is easier to say and write than “ten to the sixth meters”. The decadic prefixes is the feature that sets SI and cgs units apart from emperial units and make the former a lot more convinient. In cgs/SI units you can talk about things from sub-atomic (e.g., pico-, femto-, attometers) and up to planetary and stellar scales (e..g, mega-, gigameters) using the same basic unit, whereas in emperial units you change units when you change scale (e.g., 1/32th of an inch, inch, foot, yard, mile…- good grief). And I for one use megameters (Mm) a lot since it is a very convienient scale for my stellar convection simulations. A convection cell at the surface of the Sun (a granule) is about 1Mm wide and the Solar convection zone is about 200Mm and the solar radius is about 700Mm. I hope that clarifies matters.
    Cheers, Regner

  10. AliCali

    Have we ever decided if it’s kill-AH-meter or if it’s KILL-o-meter? Someone pointed out that the latter sounds like a measuring device of how many people one has killed.

  11. Rae

    Beautiful stuff.

    Also, I was on the Dailymail website (right wing evil tabloid in Britain, I only read it to laugh at the comments section) this comment made me sad though: it was the story about the new images of the face of mars actually just being a hill (duh) and some guy quoted the article saying he didn’t know who the bloody hell Carl Sagan was in a very rude way, screen cap:

    http://1.bp.blogspot.com/_Vlh8RMtAr6w/TFH8H-22KlI/AAAAAAAAAHo/pR0oA0uddUI/s1600/idiot.bmp

  12. Radwaste

    “Someone pointed out that the latter sounds like a measuring device of how many people one has killed.”

    Ahh, well, there are different units for that. Examples are the “Callahan” (everyone within pistol range) and the “Norris” (everyone of whom Chuck is aware).

    Units are a fascinating study in themselves. You may be familiar with the one for feminine beauty, the “Helen”. Based on the appearance of Helen of Troy, whose visage was fine enough to inspire a thousand ships, modern beauty may be measured in “milliHelens”, to denote those for whom only a few ships are called. There is a negative scale, to describe how many ships one would scuttle to avoid the commensurate ugliness; there is considerable debate as to the identity of that unit due to the numerous examples considered suitable of the honor.

  13. cosmo_steve

    How many MegaFootballFields is that?

  14. Brian Too

    11. AliCali,

    Strictly speaking, I believe KILLO-meter is correct. The entire prefix is emphasized following the syllable boundaries. However Ki-LAU-meter pronounciation has entered common usage and it’s unlikely to die. Nor do many people get upset (or confused) about the difference.

    To understand why Ki-LAU-meter is nonstandard, compare to other combinations of SI prefixes and units. KILLO-liter is good and accepted pronounciation, but you never hear someone saying Ki-LAU-liter. It’s KILLO-Joule, not Ki-LAU-Joule.

  15. olderwithmoreinsurance

    A thousand km sounds best of all! (too cute all the time just gets boring)

  16. MoonShark

    What’s the difference between a chasm, canyon, and trench? I assummed canyons were cut by flowing material, like the Grand one we’re familiar with.

    For comparisons of large geologic gashes, of course the Mariana Trench is here and much bigger, plus there’s Valles Marineris on Mars which is the biggest of all.

  17. George Martin

    JohnG @5 says

    Brings to mind the use of the terms “megasecond” and “kilosecond” in Vernor Vinge’s “A Deepness In The Sky”.

    That struck a chord with me. While I haven’t read the Vernor Vinge novel, I have read the Joan D. Vinge novel The Outcasts of Heaven’s Gate where she used multiples of seconds for lengths of time also. Her novel was published in 1978. And yes, if you don’t already know, Joan was once married to Vernor.

    JohnG’s remark caused me to find Joan’s website. There she credits Vernor with some of the basic ideas in Outcasts including the idea of expressing the duration of time in multiples of seconds.

    Of course Joan is much better know for her novel The Snow Queen which won a Hugo award.

    George

  18. Grand Lunar

    Cassini happened to find the Grand Canyon on that moon.
    :)

  19. George Martin

    I’m STUPID! at 18. The novel I referenced should be The Outcasts of Heaven’s Belt.

    George

  20. Joe

    @ 1 and 2:

    Yes. Absolutely. This blog needs more ME references.

  21. gopher65

    Brian Too: I’m from Saskatchewan, and here most people (if not everyone) would say “kil-au-joule” rather than “kil-o-joule”. Ditto to many of the other kilo-units. It’s mostly an accent then when it comes right down to it.

  22. Twinarp

    In reply to Jamie # 6,

    Yeah but I like KarmaKarmaChameleon!

  23. ChH

    “kilometer” is pronounced “click”.
    I’m wondering if “Megameter” should therefore be pronounced “m’gick”…

  24. Messier Tidy Upper

    Nice image – Cassini never disapppoints & I believe its a birthday or anniversary of sorts for this wonderful spacecraft too. Right? :-)

    @8. BJN Says:

    I personally think that subduction trenches around the Pacific tectonic plate are the biggest canyon on this planet. Bering Canyon in the Bering Sea is 1495 km long and has 1840 m of wall relief. Zhemchug Canyon in the Bering Sea is the largest volume discrete canyon formation on this planet.

    (Emphasis added.)

    Well a subduction trench isn’t exactly a canyon in my view. You get your gullies, ravines, valleys, canyons, chasms, rift valleys and trenches. Plus we have fissures usually but not always volcanic in nature and, on worlds like Ganymede, you have grooves (Sulci?) which indeed look pretty groovy! (Sorry couldn’t resist the pun.) Vallis, eg. Vallis Marineris, means ‘valley’ so valley and Canyon at least are sort of interchangable but I think there are differences and subtle shifts of meaning between these terms. Similar landforms at different scales and with varying formation mechanisms and characteristics. Grabens fit in there too somewhere although I’m not 100% sure where – been a while since I studied geology & geomorphology. (Ie. The study of how landscapes change over time – fascinating area actually.)

    The BA wrote :

    This is Tethys, an ice moon of Saturn.

    Being very pedantic and nit-picking here but does Saturn have any primarily rock moons? Out there in the Saturnian system its mostly ice and for the planet itself gas isn’t it? I’d call Jupiters moon Io at least a rock – specifically sulphur – moon but Saturn doesn’t have anything quite like that or does it? Even Titan is mainly ice plus atmosphere and tholin “gumbo” slush right?

    Also while I’ve got pedant mode on :

    How big is that? Stand up and take a long stride. That’s about one meter. Now do it 999,999 more times. That’s a megameter: a million meters, or 1000 kilometers. Better pack a lunch.

    That’s on Earth. On Tethys with its lower gravity I’m guessing a stride would take you a *lot* further and walking vast distances would be much easier. Of course, carrying your oxygen and the cumbersomeness of a spacesuit may offset that a little. Hmm .. I vote we launch a human mission to Tethys and go find out how long it takes to walk the distance! ;-)

  25. Twinarp

    And to Cosmo_steve # 14 MegaFootballFields indeed.

    In the US 360 feet by 160 feet…. 57,600sq feet. Do you guys use square yards? Meh.
    In the Soccer playing countries… 75 yds by 120 yds. 81,000 sq feet.
    In OZ, where I’m from… 165 metres by 135 metre OVAL.
    So converted to feet. 540 feet by 442 feet.
    So if the area of an oval is 1/2A * 1/2B * Pi when A = Length and B = Width
    270* 221* 3.141 = 187,423 sq feet.
    So as folksy and nice Football fields sounds as an example, STOP IT!!!!!
    NOT A USEFUL ANALOGY!!! 8)>

  26. AJ

    @ Rae, no. 9: Wow… that guy’s comment is epically stupid.

    “So who the hell is Carl Sagan anyway – not a rocket scientist, that’s for sure.”

    MWAAHAAHAA, couldn’t be funnier if he was trying. Then there’s the strange thing which appears to be the revenge of Mother Theresa from beyond the grave…

  27. magetoo

    The chasm is billions of years old, and may have formed when water inside the moon froze, expanded, and cracked the surface open.

    Wow, that’s just insane. Thinking that a “human scale” process like water expanding when freezing could scale up to that size … it’s hard to imagine.

    Megameter is not another unit. ‘meter’ is the unit, and ‘mega’ is the decadic prefix. [...] I hope that clarifies matters.

    Thank you Captain Obvious, but we went to school too. I think there is some interesting psychology here though (thinking out loud):

    The thing is that when you use e.g. “kilograms” and “millimeters” in regular life, in your mind they become the units and you don’t think of it as applying prefixes to the base units meter and gram. It is natural to work in these units, since they are something you have direct experience with; refering back to meters and grams just gets in the way.

    And so here we have the megameter. Those of us not doing astronomy rarely work with things that size, and we have to work back to something we know. The kilometer is the closest thing we have, so “a thousand kilometers” seems, if not natural, then at least halfway comprehensible. I have no idea how much a million meters is, but a thousand kilometers is something I can at least begin to work with.

    Which I suppose is why TC assumed it was a joke (to use such an obviously ridiculous unit).

  28. Nigel Depledge

    AliCali (11) said:

    Have we ever decided if it’s kill-AH-meter or if it’s KILL-o-meter? Someone pointed out that the latter sounds like a measuring device of how many people one has killed.

    In England, it’s mostly pronounced KIL-om-etah, but in France it’s more keelo-metre (with the “r” rolled in the back of the throat and no specific emphasis). Take your pick.

  29. Steve Kernow

    It’s always been kil-OM-ettah to most people I know here in the UK.

  30. Brian Schlosser

    Hi folks, and welcome to “Bad Astronomy”! As this is evidently your first visit, please note that our host, Dr. Phil Plait, frequently uses light-hearted and humorous post titles. Some even learn to find this endearing!

    /sarcasm

    Seriously, kids, a lot of wangst going on in this thread about things that are so incredibly inconsequential. “1000 Kilometer chasm on a moon” somehow lacks a certain something. Maybe Dr. P should have used “10.7 picolightyear chasm on an icy moon”

    (People arguing about trifles online? “Hi Brian, and welcome to The Internet…”)

  31. Matt T

    @Jamey (#6)
    I’m sure Coleridge would. “And a megaslimything lived on, and so did I” just doesn’t scan as well.

    @Carolyn Porco (#10)
    Holy crap, it’s Carolyn Porco!

    Um, nice picture, BTW.

    @AliCali (#11)
    Ironically, I’d say that the *former* (stressing the second syllable) would actually mean a device for measuring deaths. That way, the word breaks up as kill-ometer. (cf thermometer, odometer, anemometer.) Stressing the first syllable, to me at least, makes it clearer that it’s kilo-meter.

  32. Gamercow

    *Beep*
    Hi, you’ve reached the Bad Astronomer’s answering service. Phil is too busy to respond to your call at this time, but please see his You Tube video on the subject of the pronunciation of the word “kilometer”.

    http://www.youtube.com/watch?v=CZVG9Ji08xg
    :)

  33. Rocco

    Now THIS is why I joined the mailing list! In between all the random information (all interesting, I must stress), a simple explanation makes the unimaginable so much closer to reality for myself, and I would imagine, any children or interested amateurs:

    “How big is that? Stand up and take a long stride. That’s about one meter. Now do it 999,999 more times. That’s a megameter: a million meters, or 1000 kilometers. Better pack a lunch.”

    Perfect :) … of course, I am currently collecting as many can pull-tabs and bottle tops as I can in my community to illustrate such an enormous number to school children, so the poetry of the explanation really hit home.

    Thanks Phil!

  34. reidh

    Looks like a piece of that planet that USED to orbit between Jup. and Marz, hit a glancing blow on the way by this Awesome Moon, and contributed to the overall content of the Oort cloud, I would wager.

  35. Mark the Sundog

    Well, I’m a long-time Alternity player, so megametres is fine with me.

    Cassini has been an incredible mission.

  36. Jim Graber

    On pronunciation:
    I’ve heard three fairly frequently, only two seem to have been mentioned;
    kih- LOM- muh-ters (where uh is the unemphasizeded schwa)
    KILL-OWE- meet-ers
    and
    KILL -uh -mee-ters with secondary emphasis on the mee.
    YMMV.
    Just my two cents worth.
    Best,
    Jim Graber

  37. Nigel Depledge

    @ Steve Kernow (30) – oh, yeah, my mistake.

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