Author Topic: Dark Haloed Crater Reference Resource  (Read 8073 times)

Thomas J

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Dark Haloed Crater Reference Resource
« on: July 05, 2010, 01:48:58 am »
Recently, I have become more interested in these enigmatic craters, therefore I have decided to do some research and bring it to the forum. I will lock this topic as a reference and update it, regularly.

Back in 1972 Apollo 17, the last lunar mission, landed on the Moon. One of the three mission objectives was ‘lunar geology investigation’ and part of this was to follow up telescopic observations of dark deposits in the Taurus-Littrow region (below)

20.19080° N latitude, 30.77168° E longitude

It had been suggested that these dark areas may be volcanic, pyroclastic deposits.  The Apollo geological investigation confirmed that the deposits were volcanic when small, orange and black, glass spheres were discovered in the lunar soil. These spheres were found to compose around 2 to 18% of the soil content in this region. The spheres were dated and confirmed to be related in age to the subjacent basalt.

Further observations confirmed more pyroclastic deposits within Alphonsus Crater leading to a new class of object; The Alphonsus-Type dark halo crater.

Can you find the three dark halos in this image?

Volcanic deposits can be clearly seen here in this region of Alphonsus Crater

Credit: NASA

A Dark Haloed Crater may be described, in simple terms as:

‘A crater of possible volcanic origin, surrounded by an ejecta blanket darker than the adjacent landscape material.’

*Note the use of ‘possible volcanic origin’ as there are two types of dark haloed crater:

Non-impact (internal) craters. 
These are formed due to volcanic activity and reveal the locations of lunar eruptions.  The low albedo ejecta are of pyroclastic origin, which may spread to a range of three to four times the radius of the crater.
Things to take note of when searching for this type are:

*They may not be circular, rather they are extruded and ragged looking
*They never exhibit light rays associated with impacts
*They may appear shallow
*They are often aligned with linear rilles
*The ejecta may appear smooth and untextured
* The crater may not show any sharp raised rim feature.

Impact (external) craters.
These are formed through impacts that have excavated darker mare material from below the lighter Lunar regolith.

Things to take note of when searching for this type are:
*Crater will generally be circular
*Lighter rays interspersed with the darker deposits are common
*They will often exhibit a clearly raised rim.
*May have secondary craters associated with them.

Please look out for more dark haloed craters and post them in the thread along with any questions you may have. I will record all the entries and catagorise them as ‘volcanic’ or ‘impact’.

Dark Halo thread
Of course, please continue to flag any that you find in the crater survey now you have a better idea what to look for.

Thank you,
« Last Edit: November 01, 2010, 03:18:28 pm by Thomas J »