LRO Mini-RF data reveals extensive cave network beneath lunar surface

Evidence is mounting that an intricate, winding network of channels exist just below the surface of the Moon. These “lava tubes" are produced by underground flowing magma from ancient volcanoes. Credit: NASA

Summary

Here's a summary of some key points about detection of Lunar Caves by the NASA LRO Mini-RF dual band polarometric SAR radar instrument:

1. NASA's Lunar Reconnaissance Orbiter (LRO) has found new evidence of caves beneath the Moon's surface using its Mini-RF (Miniature Radio-Frequency) instrument.

2. Analysis of 2010 radar data revealed a cave extending over 200 feet from the base of a pit located 230 miles northeast of the Apollo landing site in Mare Tranquillitatis.

3. These lunar caves, similar to "lava tubes" on Earth, likely formed from ancient volcanic activity and could potentially extend for miles under the lunar surface.

4. The Mini-RF instrument on LRO is a synthetic aperture radar that operates at wavelengths of 4.2 and 12.6 cm, capable of sensing properties of lunar regolith to depths of meters.

5. Mini-RF uses a hybrid polarimetric architecture, transmitting circular polarization and receiving on two orthogonal linear polarizations, allowing it to calculate Stokes parameters and circular polarization ratios (CPR).

6. The instrument is particularly useful for searching for water ice near the lunar poles and characterizing surface roughness.

7. In Shackleton crater near the lunar south pole, Mini-RF data suggests that 5-10% of the materials in the crater walls could be water-ice.

8. While Mini-RF wasn't specifically designed to detect underground tunnels or lava tubes, its data could provide indirect evidence of such features through its penetration depth, polarimetric information, and high-resolution imaging capabilities.

9. The discovery of these potential cave systems could have significant implications for future lunar exploration and potential human habitation on the Moon.

LRO Mini-RF data reveals extensive cave network beneath lunar surface

by Clarence Oxford
Los Angeles CA (SPX) Jul 19, 2024

An international team of scientists has uncovered new evidence of caves beneath the Moon's surface using data from NASA's Lunar Reconnaissance Orbiter (LRO).

The re-analysis of radar data from LRO's Mini-RF (Miniature Radio-Frequency) instrument, collected in 2010, revealed a cave extending over 200 feet from the base of a pit. This pit is situated 230 miles northeast of the historic Apollo landing site in Mare Tranquillitatis. While the full length of the cave remains undetermined, it potentially spans miles under the lunar mare.

For decades, scientists have theorized the existence of subsurface caves on the Moon, akin to those on Earth. Initial images from NASA's lunar orbiters, which mapped the Moon's surface before the Apollo missions, suggested the presence of pits that could lead to caves. This theory was confirmed in 2009 when JAXA's (Japan Aerospace Exploration Agency) Kaguya orbiter captured images of a pit. Since then, numerous pits have been identified on the Moon through LRO's imaging and thermal measurements.

"Now the analysis of the Mini-RF radar data tells us how far these caves might extend," said Noah Petro, LRO project scientist based at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

Similar to "lava tubes" on Earth, lunar caves likely formed when molten lava flowed beneath a field of cooled lava, or when a crust formed over a lava river, creating a long, hollow tunnel. If the ceiling of a solidified lava tube collapses, it exposes a pit, akin to a skylight, that can lead into the cave-like tube.

Mini-RF

jhuapl.edu

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Mapping the Moon’s Poles

Flying on two spacecraft, Mini-RF instruments have observed the Moon at radar wavelengths to characterize a variety of important processes, search for water ice at the lunar poles, and characterize the safety of potential landing sites for human exploration.

About the Instrument

The Mini-RF project put two synthetic aperture radars in orbit around the Moon to map its surface at wavelengths of 4.2 and 12.6 cm, measure its surface roughness, search for water ice near the poles, and demonstrate NASA communications technologies. The first instrument launched in October 2008 on the Indian Space Research Organisation’s Chandrayaan-1 spacecraft and collected data of both polar regions through August 2009. The second instrument, currently flying on NASA’s Lunar Reconnaissance Orbiter (LRO), launched in June 2009 and has collected radar data of the lunar surface for more than a decade.

A detailed, 3D rending of the Mini-RF instrument that rides on the Lunar Reconnaissance Orbiter.

Credit: Johns Hopkins APL

Mini-RF can sense the properties of lunar regolith — the dusty layer covering the Moon’s rocky surface — to depths of meters, and has provided unique insight regarding the potential for water ice in permanently shadowed regions of the Moon. Combined with other data acquired by the LRO spacecraft, Mini-RF has also contributed to our understanding of impact melts, volcanic deposits, lunar swirls, and many other unique and fundamental processes observed on the Moon.

LRO

NASA’s Lunar Reconnaissance Orbiter (LRO) set off in June 2009 on a robotic mission to map the moon’s surface and has since provided observations that spawned numerous groundbreaking discoveries. It has created a new picture of the moon as a dynamic, complex place, and has provided essential data for future robotic and human exploration of our nearest celestial neighbor.

 

New Evidence Adds to Findings Hinting at Network of Caves on Moon

Lonnie Shekhtman

Jul 18, 2024

These images from NASA’s LRO spacecraft show a collection of pits detected on the Moon. Each image covers an area about 728 feet wide. NASA/GSFC/Arizona State University

An international team of scientists using data from NASA’s LRO (Lunar Reconnaissance Orbiter) has discovered evidence of caves beneath the Moon’s surface.

In re-analyzing radar data collected by LRO’s Mini-RF (Miniature Radio-Frequency) instrument in 2010, the team found evidence of a cave extending more than 200 feet from the base of a pit. The pit is located 230 miles northeast of the first human landing site on the Moon in Mare Tranquillitatis. The full extent of the cave is unknown, but it could stretch for miles beneath the mare.

Scientists have suspected for decades that there are subsurface caves on the Moon, just like there are on Earth. Pits that may lead to caves were suggested in images from NASA’s lunar orbiters that mapped the Moon’s surface before NASA’s Apollo human landings. A pit was then confirmed in 2009 from images taken by JAXA’s (Japan Aerospace Exploration Agency) Kaguya orbiter, and many have since been found across the Moon through images and thermal measurements of the surface taken by LRO. 

NASA’s LRO Finds Lunar Pits Harbor Comfortable Temperatures

“Now the analysis of the Mini-RF radar data tells us how far these caves might extend,” said Noah Petro, LRO project scientist based at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Lunar Pits Could Shelter Astronauts, Reveal Details of How ‘Man in the Moon’ Formed

Like “lava tubes” found here on Earth, scientists suspect that lunar caves formed when molten lava flowed beneath a field of cooled lava, or a crust formed over a river of lava, leaving a long, hollow tunnel. If the ceiling of a solidified lava tube collapses, it opens a pit, like a skylight, that can lead into the rest of the cave-like tube.

Mini-RF is operated by The Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. LRO is managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland, for the Science Mission Directorate at NASA Headquarters in Washington. Launched on June 18, 2009, LRO has collected a treasure trove of data with its seven powerful instruments, making an invaluable contribution to our knowledge about the Moon. NASA is returning to the Moon with commercial and international partners to expand human presence in space and bring back new knowledge and opportunities.

By Lonnie Shekhtman

NASA’s Goddard Space Flight Center, Greenbelt, Md.

Lunar Crater Has Water Inside Its Walls

Tudor Vieru

news.softpedia.com

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NASA scientists say that their latest study of permanently-shadowed craters near the lunar south pole has revealed a location where water-ice may exist in abundance. They say that between 5 and 10 percent of the materials making up Shackleton crater's walls could be accounted for by ice.

The findings were made using the Mini-RF radar instrument aboard the NASA Lunar Reconnaissance Orbiter (LRO). The mission launched towards the Moon aboard an Atlas V delivery system, on June 18, 2009, from the Cape Canaveral Air Force Station's Space Launch Complex 41.

The Miniature Radio Frequency radar was the first instrument of its kind to demonstrate a new type of synthetic aperture radar technology, as well as to showcase new communications technologies. In addition to Mini-RF, the orbiter carries a suite of six scientific instruments.

Shortly after it launched, LRO was able to confirm the existence of water-ice at the lunar south pole, by investigating the ejecta plume produced when the Lunar Crater Observation and Sensing Satellite (LCROSS) dropped a spent Centaurus upper stage into Cabeus Crater.

After scientists confirmed that the material exists on the Moon, LRO began a campaign to investigate the locations where higher concentrations of water-ice are present. If people are to live on the Moon, then they will need access to this previous resource.

Water-ice can be used to manufacture oxygen for breathing, hydrogen fuel for rocket, and water.

Upon investigating Shackleton crater, experts from Boston University Center for Remote Sensing and the NASA Goddard Space Flight Center (GSFC), in Greenbelt, Maryland, found that 5 to 10 percent of the materials making up its wall was water-ice.

“These terrific results from the Mini-RF team contribute to the evolving story of water on the Moon. Several of the instruments on LRO have made unique contributions to this story, but only the radar penetrates beneath the surface to look for signatures of blocky ice deposits,” says John Keller.

The expert is the deputy project scientist for the LRO mission at Goddard. He says that Shackleton crater is currently classified as a high-priority science target for future explorations and studies. Details of the Mini-RF results appear in the latest issue of the journal Geophysical Research Letters.

“The interior of this crater lies in permanent shadow and is a 'cold trap' – a place cold enough to permit ice to accumulate. The radar results are consistent with the interior of Shackleton containing a few percent ice mixed into the dry lunar soil,” says Ben Bussey.

The expert is the principal investigator for the Mini-RF instrument, and is based at the Johns Hopkins University (JHU) Applied Physics Laboratory (APL), in Laurel, Maryland.

Lunar Mini-RF Radar Specifications

The Lunar Mini-RF Radars: Hybrid Polarimetric Architecture and Initial Results

R. K. Raney et al., "The Lunar Mini-RF Radars: Hybrid Polarimetric Architecture and Initial Results," in Proceedings of the IEEE, vol. 99, no. 5, pp. 808-823, May 2011, doi: 10.1109/JPROC.2010.2084970. 

Abstract: The two mini-radio-frequency (mini-RF) radars flown in near-polar lunar orbits (on Chandrayaan-1 and the Lunar Reconnaissance Orbiter) were the first of their kind, hybrid–polarimetric. This new paradigm transmits circular polarization, and receives coherently on orthogonal linear polarizations. The resulting data support calculation of the 2 × 2 covariance matrix of the backscattered field, from which follow the four Stokes parameters. These are the basis of science products from the observations, which include images that are traditional in radar astronomy, as well as polarimetric decompositions. The instruments all have mass less than 15 kg, antenna areas of about 1 m 2, and modest power and spacecraft accommodation requirements. Data quality and instrument characteristics suggest that hybrid polarity is highly desirable for future exploratory radar missions in the Solar system. 

keywords: {Radar imaging;Radar polarimetry;Moon;Polarization;Spaceborne radar;Radar antennas;Space exploration;Circular polarization ratio;polarimetric radar;radar astronomy;Stokes parameters;synthetic aperture radar}, 

URL: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5648440&isnumber=5753253

Summary

Based on the documents, here are some key characteristics of the LRO Mini-RF instrument and how it measures surface roughness and potential ice content on the Moon:

Characteristics of LRO Mini-RF:

• - Hybrid dual-polarimetric radar architecture - transmits circular polarization and receives on two orthogonal linear polarizations (H and V)
• - Two frequencies: S-band (12.6 cm wavelength) and X-band (4 cm wavelength)
• - Two resolution modes: Baseline (150 m resolution) and Zoom (15 m x 30 m resolution)
• - Antenna size: ~1.6 m long "egg crate" design
• - Mass: <15 kg total
• - Uses burst mode transmission
• - Produces Stokes parameter data products

Measuring Roughness and Ice Content:

• - Calculates the circular polarization ratio (CPR) from the Stokes parameters
• - CPR is the ratio of same-sense to opposite-sense circular polarization backscatter
• - Higher CPR indicates rougher surfaces at the wavelength scale
• - Very high CPR (>1) can indicate the presence of water ice
• - Looks for elevated CPR values inside permanently shadowed craters near the poles, which could indicate water ice deposits
• - Uses degree of polarization (m) parameter - lower values indicate more complex/random surfaces like regolith
• - Compares CPR inside vs outside craters to look for anomalous high values inside that could indicate ice
• - Produces calibrated color-coded CPR maps to highlight areas of interest

The hybrid polarimetric architecture allows Mini-RF to obtain polarimetric information similar to full polarimetric systems, but with a simpler and lighter instrument design suitable for a lunar orbiter. The CPR and other polarimetric parameters derived from the Stokes vector data are used to characterize surface roughness and search for potential ice deposits, especially in permanently shadowed polar craters. 

Roughness and Ice

Here's an explanation of CPR, Stokes parameters, and how they indicate roughness and ice:

Circular Polarization Ratio (CPR):

CPR is defined as the ratio of power received in the same sense circular polarization (SC) to that received in the opposite sense circular polarization (OC), relative to the transmitted circular polarization. It can be calculated from the Stokes parameters as:

CPR = (S1 - S4) / (S1 + S4)

Stokes Parameters:

The Stokes parameters (S1, S2, S3, S4) are four real numbers that fully characterize the polarization state of electromagnetic waves. For the Mini-RF radar:

S1 = |EH|^2 + |EV|^2 (total backscattered power)
S2 = |EH|^2 - |EV|^2
S3 = 2Re(EH * EV*)
S4 = -2Im(EH * EV*)

Where EH and EV are the complex voltages received in horizontal and vertical polarizations.

Indicating Roughness and Ice:

1. Roughness:
- Higher CPR values generally indicate rougher surfaces at the scale of the radar wavelength.
- Smooth surfaces tend to have low CPR values (near 0), while very rough surfaces can have CPR values approaching or exceeding 1.
- The degree of polarization 'm' (calculated from Stokes parameters) is also used. Lower 'm' values indicate more complex/random surfaces like regolith.

2. Ice Detection:
- Very high CPR values (>1) can be indicative of water ice deposits due to the coherent backscatter effect in pure ice.
- The Mini-RF looks for anomalously high CPR values inside permanently shadowed craters near the lunar poles, compared to surrounding areas.
- However, high CPR alone is not definitive proof of ice, as extremely rough surfaces can also produce high CPR values.

The documents emphasize that while these parameters are useful for identifying potential ice deposits, they must be interpreted carefully in conjunction with other data sources and geologic context. The hybrid polarimetric architecture of Mini-RF allows it to gather this polarimetric information efficiently, enabling global mapping of lunar surface properties and the search for potential ice deposits.

Hunting Tunnels

The Mini-RF instrument does not have a direct capability to detect underground tunnels or volcanic tubes. However, some aspects of its design and data products can provide indirect evidence or indications of such features:

1. Penetration depth: The S-band (12.6 cm wavelength) radar can penetrate somewhat into the lunar regolith, potentially revealing subsurface structures or variations in composition.
2. Polarimetric information: The degree of linear polarization (mL) and other Stokes parameters can provide information about subsurface scattering."If this wave penetrates the surface, the V polarization will be preferentially transmitted, and the polarization state will change from circular to elliptical."
3. Volumetric scattering: The documents mention that low values of the degree of polarization (m) can indicate volumetric scattering in the regolith. Variations in this parameter might hint at subsurface structures.
4. Interferometric capabilities: The LRO Mini-RF includes a mode that maintains a uniform pulse-repetition frequency for an entire pass, which could support interferometric SAR analysis. This technique might be able to detect subtle surface deformations associated with subsurface structures.
5. High-resolution imaging: The zoom mode (15 m x 30 m resolution) could potentially reveal surface expressions of underground features.

To definitively detect such features, you would likely need:

1. A radar system with greater penetration depth (longer wavelength)
2. Specific data processing techniques designed to highlight subsurface voids
3. Complementary data from other instruments (e.g., gravity measurements)

The Mini-RF was primarily designed for surface characterization and ice detection in permanently shadowed regions. While its data might provide some clues about subsurface structures, direct detection of underground tunnels and volcanic tubes was not a stated capability of this instrument based on the provided documents.