Hi all,

I was looking back at the stuff I have in my binders I noticed something interesting.  A few months ago, I mentioned a Europan linea-like crack running NE-SW from the NW end of Isum Patera (30 N, 207 W).  This crack runs for a 200 km and is surrounded by pyroclastic material.  This is similar to many of the cracks seen on Europa. I am not saying that Europan linea and this crack formed the same way but it is interesting to note their similarities.  In fact articles I have read and my own observations only note two similar forming structures on both Europa, mountains on Io and chaos on Europa and this crack and linea on Europa.  Anyways, I was reading the article from 1998 on the very hot lava on Io by Alfred McEwen and I noticed that in table 2, McEwen observes that this fissure was an active hotspot during orbit E11 or November 1997.  The size of this hotspot 0.0016 sq. km and the temperature was 1450 K, all consistent with a Tvashtar-type lava fountain or curtain.

Here it gets interesting.  Observations taken during orbit E14 (March 28, 1998) and I24 (October 11, 1999) show that the debris pattern surrounding the fissure has not changed. The pyroclastic material from the November 1997 eruption was no larger than any event that had occured there previously.  The same thing happened at Tvashtar after its November 1999 eruption.  Images from February 2000 show that the debris field surrounding the fissure remained the same.  The fact that debris fields remain constant probably indicated that peaks in the cycle of eruptions at calderas remain constant.  This adds to my idea that different volcanoes on Io can be catagorized by how often they erupt.

R. Lopes-Gautier et al. at last year's LPSC proposed a way to catagorize hotspots on Io.  One of the catagories they proposed was duration.  The two types of hotspots in terms of duration proposed by them were persistent and sporadic.  According to the paper, sporadic activity were events that lasted shorter than 3 months and persistent activity were events that lasted longer than a year.  Based on what has been observed a Loki and at Tvashtar and "Isum Linea" I believe that basing theis catagorization on 3-4 years of data isn't correct.  Some events last shorter than 3 months and in a predictable time period the volcano "flares up" again.  No where is this more obvious than Loki.  I could set my calender by its eruptions.  Loki erupts every 15-18 months like clockwork.  The area it erupts at on its caldera even appears constant.  What causes this periodicy I don't know but this does occur at some volcanoes.  This would be my catagorization of volcanoes on Io:

This kind of catagorization does have its flaws.  First, many volcanoes can't be labeled because the length of time in which we have observed them is too short.  It is possible that Loki was once constant but is now shutting down by erupting on a periodic basis.   Secondly, the data we do have is too sporadic.  We may be missing eruptions by sporadic volcanoes that might actually be periodic.

To stretch this even farther its is possible that a volcanoes age could be derived from what catagory it belongs to.  A constant volcano may be young because the amount of lava it has to work with from the mantle plume has to be tremendous to be able to erupt continuously for decades.  A periodic volcano is slightly older, the lava chamber has to take a certain amount of time to "fill up."  A sporadic volcano is even older.  Its lava chamber takes time to fill up and the rate of fill in the chamber is not constant.  A rare volcano only fills up its magma chamber every once in a great while to be able to erupt with the furocousness like that seen a Pillan in 1997.

It should be well noted that periodic volcanoes seem to erupt at about the same rate every time.  This explains why the fissures at Tvashtar and the one NW of Isum Patera never have different debris pattern.  Each eruption has the same power so the debris field never changes.  An paper about lava fountains on Io was done for the LPSC this year (J.W. Head et al. 2000).  I am not going to go to indepth into it but it does make the point that a lava fountain of a certain strength will make the same size debris field every time.  So lava fountains at Tvashtar and "Isum Linea" pretty much are the same size.  This same kind of eruption magnitude has also been seen at Loki.

To prove that this catagorization works, all that is needed is continued observations of Io from Earth.  John Spencer and Bob Howell have been observing Io for 15 years so this kind of monitoring hopefully will continue.  If this idea is right, then Loki's next eruption should occur between November 2000 and February 2001.