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January 17, 2023

Are you curious about extraterrestrial caves? Do you wonder what’s inside of these cave systems and how they were formed? We’ve reached out to scientists to learn about the status of research aimed to unravel the mystery surrounding caves throughout the Solar System.

USGS Astrogeology Science Center and Northern Arizona University Department of Biological Sciences Center for Adaptable Western Landscapes are among researchers and who specialize in speleology—the study of caves. Potential cave openings have been identified millions of miles away, on virtually every planetary body examined with instruments of the appropriate spatial resolution.  We call these apparent openings “Subsurface Access Points” (SAPs), because while they appear to be cave entrances, we can’t be completely sure until we visit them and look more closely. This makes studying caves on other planets (or planetary caves as they’re called by those who study them) not only exciting, but tremendously difficult.

A cave on the Moon
The Mare Tranquillitatis, a pit crater on the Moon, captured by the Lunar Reconnaissance Orbiter. North is up. Photo credit: NASA/GSFC/Arizona State University.

Researchers are making headway revealing the secrets of these mysterious features. In collaboration with researchers around the world, the Flagstaff-based team have published papers on this topic for nearly two decades. Their recent research catalogs the possible formation mechanisms and locations of SAPs across the Solar System and outlines the engineering and research priorities to ultimately explore planetary caves, all from the comfort of their Flagstaff offices.

Fundamental Science and Engineering Questions in Planetary Cave Exploration – Wynne et al. 2022 – Journal of Geophysical Research: Planets

Planetary Caves: A Solar System View of Processes and Products – Wynne et al. 2022 – Journal of Geophysical Research: Planets

A roadmap for planetary caves science and exploration – Titus et al. 2021 – Nature Astronomy (accessible with a subscription).


To go with these publications, these scientists were willing to answer questions for us, and may be able to answer questions for you!


Here are some of the questions we asked about planetary caves:

What's the latest news about planetary caves?

  1. We have discovered these SAPs on other bodies across our Solar System and have concluded that many formed by processes similar to those on Earth. Such openings are formed by volcanoes, fracturing (in rock and ice), dissolution, sublimation, suffusion, and landslides.
  2. So far, we have discovered 3,545 SAPs on nine planetary bodies.

Why do you refer to caves as SAPs?

Subsurface Access Points (SAPs) represent holes or pits on a surface of any off-earth planetary body visible by remote sensing. A feature may ultimately be redefined as a cave once we have sufficient information to suggest it extends into the deeper subsurface via a laterally trending passageway.

Where did you get the data for your research?

From highly sophisticated cameras and radar instruments that can "see” through thick atmospheres. We know most about SAPs on the Moon and Mars. The Lunar Reconnaissance Orbiter Camera (LROC) has provided snapshots of these potential entrances on the Moon. The cameras on the Mars Reconnaissance Orbiter High Resolution Imaging Experiment (HiRISE) have captured photos of SAPs on Mars. However, we have also found intriguing features on nine additional planetary bodies across the Solar System. This consisted of looking at visible spectrum and radar images of Ceres, Enceladus, Europa, Titan, Triton, Charon, and Pluto.

Do you know if the Artemis Astronauts plan to visit a cave while on the Moon in 2028?

All 13 of the proposed Artemis landing sites are in the South Pole. Unfortunately, no SAPs have been identified in that region.

Did the Apollo astronauts visit a cave on the Moon?

They did not. However, when NASA was in the earliest stages of selecting lunar landing sites, they did identify pressure ridges. Scientists believed these features were indicative of lava tube formation. This question was never investigated during the Apollo missions.

What are the next steps in cave exploration?

We have identified SAPs and know their exact locations. We now need to look inside them to see if they are in fact entrances to caves. We are working with NASA on technology to have robots look inside the SAPs. First, additional work will need to be done to elevate a SAP to an actual cave. Once a cave is identified and deemed an exploration target, we could send in a robot. The robot could then search for signs of past and present life within the caves and potentially investigate whether they could be used to house spacefarers.  

What current missions are planned to go to caves on other planets?

While missions (like Moon Diver) and mission concepts (like MACIE) have been proposed, no cave exploration mission has yet to be selected.

Where can I find a listing of SAPs that scientists have found on other bodies?

Location of caves on Tharsis
Location of candidate caves in the Tharsis region on Mars.

The USGS Astrogeology Science Center has released locations of more than a thousand cave-entrance candidates on Mars. Featured here is the Tharsis Region. North is up.


What caves do you find most interesting and why?

I find lava tubes to be the most interesting as they are mostly likely the first type of cave we are likely to explore on another planetary body.

  • Dr. Tim Titus, USGS

Earth caves—because we can actually explore them! Hopefully, one day I can revise this to be “the most interesting caves to me are those beyond Earth that humans and robots can explore."

  • Dr. Jut Wynne, NAU

Caves containing perennial water ice deposits (Earth, Moon, and Mars). Caves can provide environments where perennial ice deposits may form, and we call these “ice caves”. We could use that ice as a resource. On Earth and Mars, the ice also may contain frozen extinct/extent life and provide clues to past climate conditions.

  • Dr. Kaj Williams, USGS

Why are caves on other planets important to research?

Caves on other planets will enable planetary scientists to look at the interior of the body without the need for drilling or digging. In and of itself, this would be a gamechanger as we have not peered beneath the deep surface of any planetary feature but Earth. In the search for life, the deep subsurface of Mars and some of the icy worlds may represent some of the best locations to search for evidence of past or perhaps current lifeforms. Caves may also support water ice on the Moon and Mars. Water ice could prove invaluable for long-term habitation as it could be used for making oxygen for breathing and hydrogen fuel for powering rockets for a return trip to Earth. Finally, caves are buffered environments that could protect astronauts from inhospitable surface conditions (specifically, on the Moon and Mars). Thus, lunar and martian caves should be considered as future astronaut shelters.

What if I think of a question later? Is there some place I can ask my question later and get it answered?

Sure! You can reach out at the links in the manuscripts or at our profile pages below. We’re also available at our social media links here:

USGS Instagram

USGS Twitter

USGS Facebook

NAU Research 

USGS Astrogeology Cave research

Jut Wynne, Planetary Caves


Explore More!

To find out more, check out this “What is inside of the Moon’s Mysterious Caves,” video. Planetary cave research may have been around for nearly 50 years, but it is ever evolving and the USGS, NAU, and collaborators have put together an inventory of research needs for future development. It will be an extraordinary adventure to formally identify caves and then peer inside. For Mars, we should all be super curious as to what a planetary cave interior may look like or if live critters or perhaps evidence of dead ones may be found within. Will we learn something new about our history and how the universe formed? Now is the time to dream, imagine, learn, grow, and explore with us.  Stay tuned!


Dr. Timothy Titus, Research Space Scientist, U.S. Geological Survey Astrogeology Science Center

Dr. Kaj Williams, Physical Scientist, U.S. Geological Survey Astrogeology Science Center

Dr. J. Judson Wynne, Assistant Research Professor, Northern Arizona University Department of Biological Sciences and Center for Adaptable Western Landscapes




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