Rimae Sirsalis

[John]

The Rimae Sirsalis (Sirsalis Rille) is looking very nice tonight. All 400+ km of it !

I like the way that it curves and cuts right across the floor of one crater but I'm not certain of the name of that one - De Vico A or AA maybe or near that one anyway ?

This is a Lunar Orbiter 4 image:

Edited May 24, 2021 by John

[Stu]

I wonder how that works then, the Rille must have formed after the crater impact,  it surely it wouldn’t run straight through it like that would it? All quite curious.

[Peter Drew]

If rilles are collapsed lava tubes this rille might have been formed first and then impacted.  Subsequently the area could have further collapsed revealing part of the original rille again.  It does look to be of different depth. ?

[Stu]

11 minutes ago, John said:

De Vico A or AA maybe or near that one anyway

That looks right John, De Vico A

[Stu]

Just now, Peter Drew said:

If rilles are collapsed lava tubes this rille might have been formed first and then impacted.  Subsequently the area could have further collapsed revealing part of the original rille again.  It does look to be of different depth. ?

Thanks Peter. That would seem to explain it better than the rille forming afterwards. I just couldn’t get my head around how it might show up after the impact.

[Peter Drew]

Just a hunch, I wasn't there at the time!     🙂

[John]

Thanks Stu.

De Vico AA must be a more recent impact then because it partially overlays the rille or at least interferes with it a bit.

According to Wood and Collins (21st Century Atlas of the Moon) under the rille is a dike which transported lavas to the mare and created the magnetic anomoly that is found here.

Edited May 24, 2021 by John

[paulastro]

I culled from 21st Century Atlas.

 

 

[Nik271]

I found this informative article in Sky at Night website by @astroavani:

https://www.skyatnightmagazine.com/news/rimae-sirsalis/

 

I tried observing the Moon last night but there was too much high cloud and could only see the big features, Rimae Sirsalis was out of reach. I've seen it at other times and it is spectacular when traced all the way.

[Barry Fitz-Gerald]

De Vico A is older (probably Nectarian in age so about 3.85 billion yrs) than Rimae Sirsalis which formed later and cut across the older terrain including the ancient craters. De Vico AA appears to be younger than Rimae Sirsalis as has been noted, as the rim interrupts the southern margin of the rille.  Rimae Sirsalis is however older than the lavas that fill Oceanus Procellarum, as these lavas have flooded the rille beyond Sirsalis K.

Rima Sirsalis is a 'graben' where two parallel faults develop and the ground in between them collapses downwards to form a linear trough. It is thought that these faults converge as you go deeper into the crust until at some point they may meet at depth, as a result the sides of the 'graben' are not vertical but are moderate slopes. One school of thought has them developing above vertical sheets of magma ascending towards the surface (dykes or dikes) where the upwards pressure cracks the rocks above to produce the faults that bound the graben on either side. An alternative hypothesis is that they form as a result of tensional forces where the crust is being stretched (such as around the margins of the mare) and the faults are produces as a result of that stretching. There is a magnetic anomaly associated with Rima Sirsalis, the only lunar magnetic anomaly associated with a graben which is notable. There is little evidence of surface volcanism along the length of Rima Sirsalis to suggest there is a dyke underneath - and the area is crossed by many other graben that may have formed due to the presence of the Orientale Basin - so maybe a tectonic origin is more plausible?  Sinuous rilles are thought to form partially as collapsed lava tubes, but they also formed open channels as the extremely hot turbulent lava cut downwards into the pre-existing lunar surface.

 

Cheers, Barry.

[John]

1 hour ago, Barry Fitz-Gerald said:

De Vico A is older (probably Nectarian in age so about 3.85 billion yrs) than Rimae Sirsalis which formed later and cut across the older terrain including the ancient craters. De Vico AA appears to be younger than Rimae Sirsalis as has been noted, as the rim interrupts the southern margin of the rille.  Rimae Sirsalis is however older than the lavas that fill Oceanus Procellarum, as these lavas have flooded the rille beyond Sirsalis K.

Rima Sirsalis is a 'graben' where two parallel faults develop and the ground in between them collapses downwards to form a linear trough. It is thought that these faults converge as you go deeper into the crust until at some point they may meet at depth, as a result the sides of the 'graben' are not vertical but are moderate slopes. One school of thought has them developing above vertical sheets of magma ascending towards the surface (dykes or dikes) where the upwards pressure cracks the rocks above to produce the faults that bound the graben on either side. An alternative hypothesis is that they form as a result of tensional forces where the crust is being stretched (such as around the margins of the mare) and the faults are produces as a result of that stretching. There is a magnetic anomaly associated with Rima Sirsalis, the only lunar magnetic anomaly associated with a graben which is notable. There is little evidence of surface volcanism along the length of Rima Sirsalis to suggest there is a dyke underneath - and the area is crossed by many other graben that may have formed due to the presence of the Orientale Basin - so maybe a tectonic origin is more plausible?  Sinuous rilles are thought to form partially as collapsed lava tubes, but they also formed open channels as the extremely hot turbulent lava cut downwards into the pre-existing lunar surface.

 

Cheers, Barry.

Thanks Barry - very interesting

[astroavani]

Excellent explanation colleague Barry.
Grateful for the Nik271 quote, these channels had already caught my attention for a long time, so much so that I published a small article on Astrobin that can be seen here: https://www.astrobin.com/321994/?nc=&nce=
It is a very interesting region that deserves to be selected as a priority target for all who enjoy lunar observation.

[John]

6 hours ago, astroavani said:

Excellent explanation colleague Barry.
Grateful for the Nik271 quote, these channels had already caught my attention for a long time, so much so that I published a small article on Astrobin that can be seen here: https://www.astrobin.com/321994/?nc=&nce=
It is a very interesting region that deserves to be selected as a priority target for all who enjoy lunar observation.

Excellent piece - many thanks for posting that link