Since ancient times, caves have attracted people, their darkness is full of dangers, but the secret in the depths attracts both Pithecanthropus, modern scientists and tourists. Hundreds of land and underwater caves have been studied on Earth, but a more difficult goal looms ahead - caves on other planets. Blogger and popularizer of cosmonautics Vitaly "Green Cat" Yegorov spoke about what awaits future space cavers.
Most of the known caves of the Earth were formed as a result of erosion - the destruction of rocks, usually under the influence of water and chemical compounds dissolved in it. Such caves are called karst caves. In volcanic regions, underground cavities of lava origin are widespread - domes and pipes. Unlike karst caves, which take thousands or millions of years to form, volcanic caves form rather quickly, during a period of eruption and active outpouring of lava.
Volcanic caves
A lava tube is a long, naturally occurring tunnel, sometimes up to tens of kilometers in length, with a flat floor and a vaulted ceiling. The tube is formed during the eruption of rather liquid and viscous basaltic lava. Spreading from the source, the lava flow begins to cool, and the upper crust solidifies first, under which the flow continues. Due to the release of volcanic gases between the "roof" and the stream, a cavity is formed, which expands as the stream dries up. The result is a real "subway" suitable for walking. The increased pressure of volcanic gases leads to the secondary melting of the vaults of the pipe, so sometimes it is covered with lava stalactites.
Volcanism is also known on other planets.
According to a number of indirect signs, it can be assumed that the volcanoes of Venus are still erupting and, due to the heat there, their lava cools much more slowly, which means that the flows will be much wider. It is also assumed that, due to sulfur compounds, the melting point of Venusian lava is lower than Earth's, and this further contributes to the mobility of lava flows.
Mars is known for its giant volcanoes - however, now they all fell asleep, but before that they managed to flood thousands of square kilometers of the surface with basaltic lava.
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The moon at one time, too, experienced an active period associated with both asteroid bombardment and internal tectonic activity. The vast expanses of the Moon are flooded with lava flows, which we call seas.
Scientists guessed that there should be lava caves on the Moon and the planets of the solar system back in the 19th century, but the first discoveries had to wait until the beginning of the era of cosmonautics.
Collapsed lava tubes on the slopes of Mount Pavlina volcano in the image from the Mars Express satellite.
Caves of Mars
Collapsed volcanic pipes on the slopes of Martian volcanoes were discovered by the automatic interplanetary station Viking in the 1970s.
Thirty years later, the Mars Odyssey satellite captured the first sinkholes, which indicated still existing caves waiting for their cavers. The diameter of the gaping dips reaches 250 meters. Most of them were found on the slopes of shield volcanoes in the Tarsis Highlands. The modern Mars Reconnaissance Orbiter, using a high-resolution HiRISE telescope, was able to look into the bowels of Mars, as far as possible from orbit.
A sinkhole into a lava cave on the Martian volcano Mount Askriyskaya. MRO satellite image.
Martian caves attract scientists for several reasons. Due to the thin atmosphere, the entire surface of the planet is irradiated by solar ultraviolet light and bombarded by cosmic charged particles, so the existence of microbial life or even complex organic compounds in the upper layers of the soil is unlikely. Under the protection of cave vaults, the chances of their preservation increase dramatically - even if life itself is no longer found there, its remains will lie much longer. It is also possible that water ice and other compounds that are more volatile in open terrain will be found in the Martian caves.
The most daring dreamers suggest that the Martian caves can become a haven for the first human bases and settlements - protection from radiation and water supplies will come in handy for future colonists. Although a number of factors indicate that the volcanic caves of Mars are not the most suitable place to live. All of them are located on volcanic slopes at an altitude of several kilometers above the plain. Meanwhile, landing in high-altitude regions is difficult due to the too thin layer of the atmosphere. The atmosphere helps to save fuel for braking during landing, so the heaviest spacecraft try to lower in the deepest places on Mars. Thanks to the same atmosphere, lowlands are better protected from radiation. And deposits of water ice on the surface have also been studied, including in the vicinity of the deepest depression of Mars - the valley of Hellas. Therefore,Until the presence of biogenic or other minerals in the Martian caves is confirmed, it is advisable to explore them using robotic means.
An important factor that hinders the development of Martian speleology is the requirements of planetary security. If there is a likelihood of preserving hypothetical Martian life in caves, then the researcher must be 100 percent sterile to exclude the likelihood of the fantastic scenario of the "Martian Chronicles", where one earthling sneeze killed a great civilization. Today, it is not possible to ensure the complete sterility of the spacecraft on Earth, and our microbes are able to withstand the conditions of space flight. Therefore, the Martians are not looking so as not to accidentally destroy them.
Moon caves
But the doctrine of planetary security does not interfere with visiting the lunar caves. The Hollow Moon has repeatedly become the scene of fantastic works. Although the reality is far from fiction, it is also encouraging for romantics. The existence of lunar caves was assumed for a long time, but direct confirmation came only in 2009. For the first time, the Japanese robotic station Kaguya discovered unusual craters that did not have a circular shaft and no signs of ejection from the inside. Their diameter reached 100 meters, and the depth seemed so significant that lateral sunlight simply did not reach the bottom. The American probe Lunar Reconnaissance Orbiter was able to examine the dips in much more detail, at different times of the day, estimate not only the depth of the bottom and its contents, but also the structure of the side walls, and even look under the arches.
Failure into the sublunary cavity in the Sea of Tranquility. LRO satellite survey.
Failure into the sublunary cavity in the Sea of Tranquility. LRO satellite survey.
A group of scientists from the University of Arizona has developed a special PitScan algorithm, which, in a semi-automatic mode, looked for holes in caves on the lunar surface and found more than two hundred of them. They can be divided into three conditional groups:
- failures of lava channels, which flowed out during volcanic eruptions;
lava cavities formed by the melt formed in large craters from the fall of large asteroids;
- cavities in the lunar seas.
A hole in the alleged lava tube could be seen on the volcanic highlands of Marius Hills, at the equator in the west of the visible side of the moon. From the satellites, a lava flow channel is clearly visible there, stretching from the mouth of the volcano for tens of kilometers. A hole is visible in the frozen stream about 25 kilometers from the crater. Either a meteorite made it, or the "roof" collapsed on its own, but now you can see a hole 80 meters wide and 45 meters deep. The width of the stream in the place of the hole reaches 800 meters, and upstream it reaches up to one kilometer, so there may be a giant tunnel by earthly standards.
At Purdue University, numerical simulations were carried out, according to which the strength of basaltic lava and low lunar gravity make it possible to preserve vaults in tunnels up to a kilometer wide at the surface and halls up to five kilometers wide at a depth of several hundred meters without destruction. The data on the gravitational field of the Moon, obtained using the GRAIL probes, helped to compare the simulation with reality. Scientists took GRAIL readings over a possible cavity in the Marius Hills and tried to find similar data obtained elsewhere. So it was possible to find up to ten "signatures" of possible lunar cavities, some of which are 100 kilometers long and several kilometers wide. Most of them are found under the lunar seas.
Several holes in the lunar seas were indeed discovered, however, they do not coincide with those possible voids that were calculated from the deviations of the gravitational field. However, one hole in the Sea of Tranquility, about 400 kilometers northeast of the Apollo 11 landing site, is the largest and deepest surveyed by satellite. The hole has a diameter of about 100 meters and a depth of up to 100 meters. There are no lava channels or volcanic domes nearby that could indicate the presence of a tunnel, but such a presence can still be assumed.
This hole is interesting for scientists not only because of what may be hidden at its bottom, but also because of its layered structure, which is visible on the steep walls of the hole. These layers suggest to scientists that the lava sea was formed as a result of multiple lava spills, some of which were quite thin, up to one meter.
The hole in the Sea of Tranquility remains one of the most suitable places to land a robotic probe and explore the cave from the inside. However, so far not a single space agency is planning the development of lunar caving robots. Apollo 15 astronauts who explored the slopes of Hadley Rill Canyon, which, according to one hypothesis, was once a lava tube, but later completely collapsed, came closest to the secrets of lunar lava tubes.
Apollo 15 crew commander David Scott against the backdrop of the Hadley Rill Valley. Photo of lunar module pilot James Irwin.
The future of interplanetary speleology
In the meantime, future exploration of lunar and Martian caves is being prepared on Earth. On our planet, many volcanic caves are available for exploring and visiting, which allow one to present all the complexities of interplanetary speleology. In Russia, lava tubes and caves are known in Kamchatka. One of the lava tubes, about 100 meters long, is available in the caldera of Gorely volcano. This cave is quite ancient, left after an eruption two thousand years ago. In it you can feel like a Martian explorer thanks to the temperature close to zero and a massive glacier that partially blocks the entrance.
Lava cave of Tolbachik volcano, formed by the eruption of 2012-2013.
Several caves were formed during the eruption of Tolbachik volcano in 2012–2013. These caves are more picturesque, with sharktooth lava stalactites covering the ceiling, salt drips on the ceiling, and growing stalagmites on the floor. Here the heat of cooling lava is still preserved, tea can still be boiled on hot cracks, and some branches of the caves are inaccessible to visitors due to the high temperature.
Despite the obvious scientific interest in exploring alien caves, no space agency has yet infringed on their secrets. The technical implementation of such a study remains a serious obstacle on this path. The probe will either need to be planted exactly at the bottom of the hole, or provided with climbing equipment for descending a vertical wall. This alone is enough to stop all development - the complexity is too high, and therefore the risk. Next, you will need to provide power to the robot in the darkness of the cave, and most importantly, control and maintain communication without direct radio visibility.
In space exploration, preference is always given to projects with high reliability, promising long-term supplies of unique data, so caving robots are still losing competition to satellites and telescopes. Only a few private teams of participants in the Google Lunar XPRIZE competition announced that their developments will allow the study of lunar caves. The American Astrobotic team and the Japanese Hakuto have designated lunar caves as their targets, but as long as their probes remain on Earth, and on the Moon they will need to walk only 500 meters to deserve victory. Given the rarity of lunar caves and the difficulty of accurate landing, it is unlikely that teams will be able to reach the lunar cavities the first time.