Digging into the far side of the moon: Chang’E-4 probes 40 meters into lunar surface

Chinese Academy of Sciences Headquarters

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 The subsurface stratigraphy seen by Yutu-2 radar on the farside of the moon.

Credit: CLEP/CRAS/NAOC

A little over a year after landing, China’s spacecraft Chang’E-4 is continuing to unveil secrets from the far side of the Moon. The latest study, published on Feb.26 in Science Advances, reveals what lurks below the surface.

Chang’E-4 (CE-4) landed on the eastern floor of the Van Kármán crater, near the Moon’s south pole, on Jan. 3, 2019. The spacecraft immediately deployed its Yutu-2 rover, which uses Lunar Penetrating Radar (LPR) to investigate the underground it roams.

“We found that the signal penetration at the CE-4 site is much greater than that measured by the previous spacecraft, Chang’E-3, at its near-side landing site,” said paper author LI Chunlai, a research professor and deputy director-general of the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC). “The subsurface at the CE-4 landing site is much more transparent to radio waves, and this qualitative observation suggests a totally different geological context for the two landing sites.”

LI and his team used the LPR to send radio signals deep into the surface of the moon, reaching a depth of 40 meters by the high frequency channel of 500 MHz – more than three times the depth previously reached by CE-3. This data allowed the researchers to develop an approximate image of the subsurface stratigraphy.

“Despite the good quality of the radar image along the rover route at the distance of about 106 meters, the complexity of the spatial distribution and shape of the radar features make identification of the geological structures and events that generated such features quite difficult,” said SU Yan, a corresponding author who is also affiliated with NAOC.

The researchers combined the radar image with tomographic data and quantitative analysis of the subsurface. They concluded that the subsurface is essentially made by highly porous granular materials embedding boulders of different sizes. The content is likely the result of a turbulent early galaxy, when meteors and other space debris frequently struck the Moon. The impact site would eject material to other areas, creating a cratered surface atop a subsurface with varying layers.

The results of the radar data collected by the LPR during the first 2 days of lunar operation provide the first electromagnetic image of the far side subsurface structure and the first ‘ground truth’ of the stratigraphic architecture of an ejecta deposit.

“The results illustrate, in an unprecedented way, the spatial distribution of the different products that contribute to from the ejecta sequence and their geometrical characteristics,” LI said, referring to the material ejected at each impact. “This work shows the extensive use of the LPR could greatly improve our understanding of the history of lunar impact and volcanism and could shed new light on the comprehension of the geological evolution of the Moon’s far side.”

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This work was a collaboration with the Key Laboratory of Lunar and Deep Space Exploration at NAOC, the University of the Chinese Academy of Sciences, the Mathematics and Physics Department of Roma Tre University in Italy, the School of Atmospheric Sciences at the Sun Yat-sen University, and the Insituto per il Rilevamento Elettromagnetico dell’Ambiente IREA-CNR in Italy.

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48 thoughts on “Digging into the far side of the moon: Chang’E-4 probes 40 meters into lunar surface

  1. So, China, with some cooperation from Italy, sent a module to the far side of the moon, landed successfully, and sent out a rover with lunar penetrating signals and sent the data back to earth. Why? China does very little without a military objective. OK, they build some golf courses, they participate in olympics, they build things for Walmart, but they focus their main energy on controlling their population, advancing influence into developing nations, and advancing their military capability. It is difficult for me to see the return on investment value in this lunar rover mission. Unless?

      • exactly to get to mars it would be so much smarter to use min weight/fuel to get up and then fuel at the moon
        as well as being able to drop fuel down to earth maybe as well?

        looks like youd want to go past 40m down to build your abodes to be safe from incomings

        • Many of the components of your craft can be fabricated on the moon, and assembled in lunar orbit. Much cheaper to launch from the moon compared to the earth.

          • “Many of the components of your craft “could” be fabricated on the moon, and assembled in lunar orbit. yeah…if…
            “Much cheaper to launch from the moon compared to the earth”
            …not quite yet. Lets not get too ahead of ourselves.
            There are available materials, but not mined nor processed. There are no current processes for making these components. Science fiction still needs to be made into practical fact.
            Rocket fantasy is easy, rocket engineering is a bitch!
            I venture to say that many who laud these technical fancies have no idea how to build one here on earth.

          • It will never be possible unless someone starts the process.

            We don’t have facilities to refine fuel and launch it into orbit either, which was the proposal I was responding too.

            BTW, did you ever look up gravity tugs? They have nothing to do with tractor beams.

      • 2046 is the date predicated by Doug vogt for supernova . The blast should not effect the dark side. Moon base survival.

    • “It is difficult for me to see the return on investment value in this lunar rover mission.”

      You may not see it.

      But I’ll bet the Chinese see it. They look at the long term. Perhaps they see the Moon as similar to the Spratly Islands……

      • Now that the Chinese have landed a rover on the moon’s surface, they will claim that the moon was always a part of China. We should expect to see bases there soon to back up their territorial claim.

      • It was more to demonstrate what we could do if we decided to do it, and contrast that with what the Soviet Union was capable of doing. This was to prove the superiority of a free country over a tyranny. The Soviet Union never came close to landing men on the moon. We, on the other hand, showed them what we could do in ways the public never appreciated. Apollo 12 was the penultimate mission. It showed that we could 1) find a small object on the lunar surface (Surveyor 3) through the use of lunar orbiting reconnaissance satellites, and 2) target a lander (Apollo 12’s LM) to set down within 600 feet of it, after a voyage of over 250,000 miles. After that, the Soviet Union bankrupted itself by building the largest ICBM force that has ever existed.

    • Perhaps they are planning on building an island there as well and extending the South China Sea Basin to the moon

    • Propaganda is a wonderful thing for Communist governments. They are already investing in more powerful rockets. Learning to land objects on other worlds may have advantages in other areas – robotics, computers, programming AI, etc. And getting a good old sense of pride from accomplishing something is always good for a nation…the U.S. knew this at one time.

      It’s way too early for anyone to be planning on colonizing the Moon, but robots? Hey they just need energy and a mission.

    • The Chinese like to think they can plan 100 years into the future.
      Personally, I think it’s a waste of time planning 100 years into the future, but my point is they believe this {and also, many non Chinese think it’s *really good* idea to plan 100 years into the future}.
      If you think you can and should plan 100 years in to future, and such long range planning is a key element of why you imagine you will become the superpower of the world, then you would have to be quite stupid not to explore the Moon.

      The graph indicates a lot “void like” region between 12 and 20 meters.

      “The Moon regolith was formed over billions of years by constant meteorite impacts on the surface of the Moon. Scientists estimate that the lunar regolith extends down 4-5 meters in some places, and even as deep as 15 meters in the older highland areas.”
      https://www.universetoday.com/20360/lunar-regolith/

      So, though it varies, the “12 and 20 meter” depth is somewhere around “bedrock” or is below the compacted dust which has formed over billions of years or lunar impactors.

      People say the lunar surface is billions of year old, though lunar impactors
      are overturning or gardening the top meter or so of the surface in terms of millions or tens of millions of years, but the deeper below the surface, the less it’s gardened by impactors. Or say, anything below 10 meter is more likely to actually be undisturbed [or pristine] for billions of years.

      • Making detailed plans for 100 years into the future is a fools errand. However thinking about what may occur in 100 years isn’t always a waste of time.

        A smart person tries to think at least 2 or 3 steps ahead.

        Thinking that the moon will probably be a source of resources in the future is not dumb. Setting programs in place to start getting to those resources, even if your plans won’t bear fruit for decades is not dumb either.

        100 years is just one of those round numbers that roll of the tongue. Those sound more like goals, than plans.

    • “So, China, with some cooperation from Italy, sent a module to the far side of the moon, landed successfully, and sent out a rover with lunar penetrating signals and sent the data back to earth.”

      Some nice goin’ for what the UN Paris agreemeent calls a ‘developing country’.

  2. A read of Robert Heinlein’s opus, ‘The Moon Is A Harsh Mistress’ will show that being on the rim of a gravity well is a monstrous tactical advantage.

  3. Digging into the far side of the moon …

    Oh good, some subsurface temperature data and some samples. What? Not actually digging. Just radar. Darn.

    • That was my thought, too. They’re not really “penetrating” the surface, just sub-surface imaging.
      Still, it is interesting.

  4. “It is difficult for me to see the return on investment value in this lunar rover mission.”

    This is the nature of basic research, it is not necessarily done for the purpose of manufacturing something. Rather it is research done for understanding nature. It is the responsibility of the state to make sure that basic research is done. Understanding the basics, is the foundation for development products and advance our civilization.

    • Is it possible that Chinese people like Buck Rogers too?

      Sometimes people “do science” because it is interesting and answer questions “because they are there”.

      The landing and remote operations of such a craft is an amazing feat with the same PR value as other space adventures. Forty years ago China could barely heat the homes of the masses and now they are exploring the sub-surface geology of the far side of the moon. How about a little grudging respect for the thousands of scientists who accomplished this?

      India routinely had mass starvation when I was a child. Now, they have operated spacecraft in orbit around Mars. And they did it for a little over $100m. They are your global neighbours and they are not going away. Let’s celebrate the success of our neighbours. It doesn’t cost much.

      • Crispin –

        “Is it possible that Chinese people like Buck Rogers too?”

        Your question got me to thinking. I know absolutely nothing about popular literature in China. But people like to tell stories everywhere. Is it possible that there is a large science fiction fan base in China? And that popular sentiment is ” let’s go to the moon to see what’s there?” Wouldn’t surprise me.

  5. From the article: “The content is likely the result of a turbulent early galaxy, when meteors and other space debris frequently struck the Moon.”

    I’m sure the authors meant “turbulent early solar system” rather than galaxy.

    That’s a pretty serious mistake for a science reporter who covers astronomy.

  6. The Chinese ought to send a big thank you note to Bill Clinton for helping them develop their launch vehicles.

    Yeah, I know, they paid him well for that, but it would also be nice for the Chinese to acknowledge Bill’s contributions to the Chinese space program.

    Before Bill got involved, most Chinese rockets were blowing up on the launchpad. Bill fixed that. Thanks, Bill.

  7. Tom Abbott is correct, President Clinton approved the transfer of missile technology to China for launch of satellites and reported the transfer to congress so it was not done in a vacuum. Loral Corp gave China the propulsion system. China said we can’t hit anything with it and Loral said no problem and gave them the guidance system. Clinton assured congress the transfer was in the interest of the US and not a threat to the US militarily.

    China is a communist dictatorship bent on world domination. What could possibly go wrong.

  8. “The content is likely the result of a turbulent early galaxy, when meteors and other space debris frequently struck the Moon.” They mean solar system. I really dislike when these two concepts are confused. It is a very fundamental error.

  9. If you’re going to build a base on the moon, somewhere, you need to know what the foundation material is and how deep. It also seems likely that much of the lunar habitat would be underground. So you need at least an idea as to what you’re going to be digging into.

    Plus what resources are on the moon, and I have no idea of that. What could be there, what is there, and what stratification there may be on the moon.

    It looks like the entire depth to 40m is like a mud-slide. rock and rubble mixed with soil. From an engineering perspective, this isn’t a good material to be digging habitats in. Probably loose material, therefor dangerous excavation and construction.

    • “Plus what resources are on the moon, and I have no idea of that. What could be there, what is there, and what stratification there may be on the moon.

      It looks like the entire depth to 40m is like a mud-slide. rock and rubble mixed with soil. From an engineering perspective, this isn’t a good material to be digging habitats in. Probably loose material, therefor dangerous excavation and construction.”

      If think of entire lunar surface as like concrete, it’s not to far off.
      The entire surface is covered fluffy dust which is couple inches deep, and below it is compacted material. Which if you wanted to dig, would be similar to digging [or drilling} into concrete.
      But if can get under it, the material is not cemented together, like concrete is. Or say, you drill hole, put dynamite in hole, fill hole, and then blast it, then it become like gravel/sand/dust.

      The resources of the Moon which are important is lunar water. Lunar water would be worth about $500 per kg.
      If you buy water on the Moon for $500 per kg, one could make cheap lunar rocket fuel. And it is thought that lunar polar regions may have billions of tonnes of water, and millions of tonnes of water which might be mineable.
      If you buy cheap lunar water, it allows one to mine lots of stuff on the Moon- including iron, and can do lot’s of stuff on the Moon.

      The lunar polar region is good place to harvest solar energy, there place in lunar pole where you get sunlight 80% of the time, whereas with Earth it’s about 25% of the time. And one can encircle to lunar polar region and always get sunlight for power.
      It’s a like poor man’s solar power satellite. Or solar power satellite in Geostationary orbit can be source of constant power {to any one location or globally}.
      Of course another aspect about to Moon is that if lower cost of lunar operations, one can just to point where you can ship SPS to GEO from the Moon so Earthlings can get cheap electrical power from Space. Or if starting lunar water mining, tomorrow, one could get SPS for Earth in about 50 to 60 years.

  10. It was learned from Apollo that the lunar surface is primarily rubble produced by impact cratering and that it ranges over depths of tens of meters on younger surfaces to tens of kilometers on old surfaces.
    A 40 meter radar profile at only one site adds very little to lunar science. But the success of the mission adds a lot to Chinese lunar exploration.

    • “It was learned from Apollo that the lunar surface is primarily rubble produced by impact cratering and that it ranges over depths of tens of meters on younger surfaces to tens of kilometers on old surfaces.”
      Got a reference?
      “A 40 meter radar profile at only one site adds very little to lunar science.”
      Aitken basin is one oldest craters on the Moon, And Apollo only explored nearer the equator.

      And no one has explored the lunar polar region, yet.

      • Got a reference?

        In the mare regions, the regolith depth averages 2-4 m. In the highlands and other regions, it averages 6-8 m. Regolith is lunar soil, composed mostly of impact-related debris (rubble).

        https://www.lpi.usra.edu/meetings/lpsc2011/pdf/2607.pdf

        Based on the image in the article, it looks like about 30 m of regolith above crystalline rock in the Van Kármán crater.

    • Combining the information provided by the radargram, the tomographic image, and the quantitative analysis (estimated wave velocity and loss tangent), we can conclude that the subsurface internal structure at the landing site is essentially made by low-loss, highly porous granular materials embedding boulders of different sizes. Given such a strong geological constraint, the most plausible interpretation is that the sequence is made of a layer of regolith overlaying a sequence of ejecta deposits from various craters (Fig. 3), which progressively accumulated after the emplacement of the mare basalts on the floor of Von Kármán crater. The layer of regolith (unit 1) is quite thick (up to 12 m), is rather homogenous both laterally and vertically, and is mostly composed of fine materials. It developed from the uppermost portion of the ejecta deposits, which were thicker than 12 m and were delivered to this area by multiple impact craters, mostly Finsen, Von Kármán L, and Von Kármán L’ craters (table S1) (2). Unit 2 (depth, 12 to 24 m) is characterized by large rocks and boulders that are interbedded with thin layers of fine materials. Ejecta deposition never occurs via simple carpeting, but it is accompanied by horizontal shearing and mixing, excavation, and subsurface structural disturbances (28), especially in areas beyond the continuous ejecta deposits of the source crater. Fine materials can be produced by the internal shearing of ejecta deposits, which reduces particle sizes, and the fine layers can also be formed due to regolith development during the intervals between different impact events. On the other hand, the CE-4 landing site is located within a secondary crater chain linked to the Finsen crater (fig. S8C), and the depth of the secondary crater is about 16 m (fig. S8D); this is the minimum depth of the structural disturbances formed during the landing of the ejecta, as the secondary crater is highly degraded. Therefore, unit 2 is likely formed due to a combination of (i) coarse ejecta that were not mobilized during the landing of impact ejecta (28), (ii) structural disturbances in local materials caused by the landing ejecta, and (iii) fine materials generated during or after the ejecta deposition. Unit 3 (24 to 40 m) contains alternating layers of coarse and fine materials. This unit can be interpreted as a combination of ejecta deposits, which were delivered by various craters older than Finsen and regolith developed during the impact intervals. Finsen is an Eratothenian-aged crater (<3.2 Ga), and many older craters have contributed to deliver ejecta deposits on the crater floor of Von Kármán before that age (30). However, tracing the source crater for each individual layer of ejecta deposits is not possible. Last, the lack of a detectable radar signal below 40 m does not allow us to make any definite conclusion about the properties of the materials at the base of the ejecta, although we can speculate that the granular materials extend beneath such a depth; accordingly, the upper contact of the mare basal layer must be deeper than 40 m.

      https://advances.sciencemag.org/content/6/9/eaay6898

      Fig. 3 Schematic representation of the subsurface geological structure at the CE-4 landing site inferred from LPR observations.
      The subsurface can be divided into three units: Unit 1 (up to 12 m) consists of lunar regolith, unit 2 (depth range, 12 to 24 m) consists of coarser materials with embedded rocks, and unit 3 (depth range, 24 to 40 m) contains alternating layers of coarse and fine materials.

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