Breaking Space news! I just heard a short item on the BBC news broadcast (ok, podcast). It said that a JAXA lunar orbiter has determined that a previously identified cave is quite extensive - perhaps 50 km long!

What spacecraft could they be talking about, and how does a spacecraft determine the length of sub-surface caves?

So far I've only found holes. See for example LRO Takes a Closer Look at Moon Caves and also Russia Eyes Caves on Moon for Setting Up a Lunar Base where dimensions of 50 to 400 meters (not kilometers) are mentioned.

edit: I have found a ~50 km collapsed cave identified by surface features in this NASA LRO page, but I think the breaking news is that a new, different, not-collapsed sub-surface cave has been identified.

below: Previously identified collapsed cave from LRO imagery, captioned:

This section of WAC frame M117773324 shows the area where the feature transitions from a chain of collapse pits to a continuous uncollapsed segment. A large depression at the northern tip of the chain maybe a possible source region for the flow of lava across this region. The chain is approximately 50 km long. Image resolution 58.9 m/pixel [NASA/GSFC/Arizona State University].

enter image description here


The mission is SELENE (Kaguya), and they used radar to detect a void under the surface:

In 2009, the Kaguya probe found a large shaft with an opening about 50 meters in diameter in the Marius Hills area. The shaft descends about 50 meters beneath the surface.

The JAXA team analyzed data obtained from a lunar radar sounder on the probe that indicated an underground structure extended west from the shaft.

LRS is the radar instrument:

LRS is designed for sounding the surface and subsurface structures of the Moon by using HF radar technique with the frequency of 5 MHz. The low frequency radar method makes it possible to realize the mapping of the subsurface structure within a depth of several km with a range resolution of less than 100 m for a region with a horizontal scale of several tens of km.

  • $\begingroup$ 5 MHz radar! OK that makes sense for penetration. I wonder if the subsequent seven or eight years of analysis was necessary to fish out a small reflection from the rest of the geography; with a wavelength of 60 meters the signal must have been fairly isotropic and reflections were coming from all over the place. There's probably an interesting story. Thanks! $\endgroup$ – uhoh Oct 18 '17 at 12:18
  • $\begingroup$ It hasn't made it to the English page yet: isas.jaxa.jp/topics/001156.html and here's the paper: onlinelibrary.wiley.com/wol1/doi/10.1002/2017GL074998/abstract it looks like the text is not open access though. $\endgroup$ – uhoh Oct 18 '17 at 13:42
  • $\begingroup$ I've just asked What conditions could produce 50 km long lava tubes?. $\endgroup$ – uhoh Oct 18 '17 at 14:28

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