Possible Crater at Zal Montes

When looking at images of Zal Patera and Zal Montes, I noticed a bowl shaped depression in the southern part of the Zal Montes "Plateau."  Based on these images the crater is 7 km across and 1-2 km deep.  The crater appears to be a simple bowl crater.  This is the same as similar sized craters on the moon.   It now appears to be a sinkhole formed after an explosive sapping event at the cliff that borders the north part of the crater.  Here are some of my comments on it from my posts on Jupiter_List:

From May 17, 2000

"Near the eastern end of this cliff appears to be the only obvious impact crater yet found on Io. It is 5-10 km across? and appears to have a bowl shape with a flat bottom. I hope to get more size constraints later, but it is definitely larger than the 1.5 km I reported earlier. "

From May 9, 2000

"First off, none of the craters I have found are very large; the largest being 1.5-2 km across. Secondly, they all occur on
mountains, where NO caldera has ever been found. If the one on Zal Montes "plateau" is a caldera, it would throw out the idea that the mountains are not volcanic. If its not a crater, which I strongly believe it is, I do have an alternative idea on how it formed. Again, I'm just talking about the one on Zal Patera "Plateau." The "crater" is near a scarp on the plateau. The scarp may have undergone sapping though the scarp doesn't show any evidence of sapping. The NW cliff face of the plateau does show signs of sapping so its is possible that this area does too. The sapping may have left a cavity and the crater was formed when the surface collapsed to fill the void. There are many flaws to this theory. First off, why don't we see these craters elsewhere on Io like near Tvashtar Catena or Telegonus Mensa. Secondly, no one knows if the cliff has
undergone sapping. It couldn't be a caldera because it is unlike any caldera found on Io. Most Ionian calderas have at least one rectilinear side, they are flat (ie they don't look like bowls). Most active ones are dark, and none occurs on top of mountains. This is a bowl shapped crater with a flat bottom, which is consistent with other craters its size. The color on the inside of the caldera is the same as the outside, no caldera shows this relationship. All other craters are too small to tell more about their structure but one on "Monan Mensa" is 400-800 m across, two on South Zal Montes are about 300 m across, and 3-6 near a sapping cliff east of Isum are about 10-15 meters across."

From May 17, 2000

"I have a better size constraint for the crater at Zal.  It appears to be 4-6 km across, but since I have nothing to compare it too in that mosaic, it could be as large as 14 km wide."

From May 30, 2000

"Based on new info I received last night, I am beginning to doubt the identification of the bowl structure on the Zal Montes Plateau as an impact crater.  A few weeks ago when I announced my discovery of the crater, I also mentioned another possible cause of the crater.  I said since the crater was near a cliff, and other cliffs on the same mountain have features that indicate sapping has occured, I said it was possible that the crater was caused by sapping at the cliff.  This new info confirms to me that craters do form when sapping occurs rapidly.  This "explosive sapping" leaves a cavity where the SO2 once was and a sink hole forms to fill the cavity.  This forms a crater. However, there are few problems that still bother me about this crater being caused by sapping.  Even though other cliffs on the Zal Montes Plateau have undergone sapping, the cliff that the crater is near does not show any other signs of sapping.  The cliff is straight and has none of the tailtell bright SO2 frosts that often are associated with sapping cliffs on Io.  Also, no other crater like this has been found
near other sapping cliffs on Io.  Finally, this crater is much larger than craters found near sapping cliffs on Earth or Mars.   So even with this new info, I still think that the most likely explanation for this crater is that it was caused by the impact of an asteroid on Io. Its kinda weird that if this were on any other world it would have instantly been thought of as a crater, but because it is on Io we have to think about it."

From June 6, 2000:

"I was messing with the Solar System Collisions website, http://janus.astro.umd.edu/astro/impact.html , and based on what I have figured there, I believe I have determinged the size of the comet that struck Zal Montes "plateau."  Assuming an impact speed of 35 km/sec. and that the impactor is a comet, the size of hte comet would be 370 meters in diameter.  A bit smaller than I thought.

I now have a definitive size for the crater. Drumroll please...the crater on Zal Montes "plateau" is 7 km across. This is based on "counting pixels" form the two images of the crater from I25.  I then averaged the two figures and multiplied the resulting number by 260 m/pixel.  The precise figure is 7020 meters.  This size is the same as En-zu on Ganymede."

From June 9, 2000:

As most of you know, I have been harping for the past few weeks on this list about a possible impact crater on Zal Montes "plateau." Well, I got a message back from Dr. Paul Geissler today.  This was the message he sent me:

"Hi Jason,

Thanks for your message. This does seem like a reasonable candidate, especially given its location on a plateau (most of the Earth's large impact craters are found on continental cratons, simply because they represent the oldest surviving crust on our planet). But when you reproject the image as if you're looking directly down upon this feature, it loses its circular appearance and appears to be just another collapse pit, probably formed by the same sapping process as the other troughs and pits on Zal. Have a look at this rectified image at http://pirlwww.lpl.arizona.edu/hiips/jason/

Best Regards, Paul Geissler"

So the alternative theory wins the day.  My alternative theory goes like this.  This crater could be somekind of sinkhole formed when the semicircular scarp north of the crater went through an explosive sapping event.  This left a cavity where the SO2 that was sapped used to be.  A sinkhole formed when this cavity caved in.

Certainly this remains a candidate impact crater but it now seems less likely.

From September 24, 2000:

Here is a message I sent to the Outer Planets Mailing List that was directed to a newly joined member of the list, Jeffery Moore of the Ames Research Center:

"A few months ago, there was a discussion on this list about N. Zal Montes and I would like to ask what your opinion on this is. In the southern part of the plateau, there is what appears to be a crater- like feature. It appears to border a cliff and a trench. Now we came up with two explainations for it. One is that it is the only crater found on Io. It appears to be oblong but still would still be a simple bowl crater, consistent with a crater its size (~7 km). The other is that this is some kind of sinkhole formed after an explosive sapping event at the cliff north of the pit.
"What do you think it is?"

These are replies to this request for information on the Zal Montes pit. This is from Alex Blackwell, a planetary geologist at the University of Hawaii:

"FWIW, I do not think it is an impact feature. There is a plateau formation that abuts the depression and that is topographically controlled by it. IOW, the depression appears to postdate the plateau. Although our knowledge of the surface properties of Io is very limited, it seems logical that an impactor that produces a 7 km excavation would probably have produced much more extensive modification of the surrounding terrain or obliterated the plateau."

This is from Jeffery Moore, a planetary geologist at NASA's Ames Research Center:

"We have asked ourselves that question but came up with no definitive answer. If it is an impact crater, then subsequent flows have covered its ejecta facies. Sinkholes and explosion craters are equally viable and equivocal."

I have three images of the Crater at Zal: