What modern science knows about extraterrestrial caves

Caves from ancient times attract a person, their darkness is full of dangers, but the secret in the depths attracts both Pithecanthropus, modern scientists, and tourists. On Earth hundreds of overland and underwater caves have been studied, but a more complex goal looms ahead – caves on other planets. Blogger and popularizer of astronautics Vitaly “Green Cat” Yegorov told about what awaits future space speleologists.

Most of the known caves of the Earth were formed as a result of erosion – the destruction of the rock, usually under the influence of water and dissolved in it chemical compounds. Such caves are called karst. In volcanic regions, underground cavities having a lava origin, domes and tubes, are common. Unlike karst caves, the formation of which requires thousands or millions of years, volcanic caves are formed quite quickly, during the eruption and active outpouring of lava.

Volcanic caves

The lava tube is an extended tunnel of natural origin, sometimes up to tens of kilometers long, with a flat floor and a vaulted ceiling. The tube is formed during the eruption of a fairly liquid and viscous basalt lava. Spreading from the source, the lava flow begins to cool down, and the upper crust hardens first, under which the flow continues. By separating the volcanic gases between the “roof” and the flow, a cavity is formed, which expands as the flow depletes. The result is a real “underground”, suitable for walking. The increased pressure of volcanic gases leads to a secondary melting of the arches of the tube, so sometimes it is covered with lava stalactites.

Volcanism is also known on other planets.

On a number of indirect signs, it can be assumed that the volcanoes of Venus still continue to erupt and thanks to the local heat their lava cools down much more slowly, which means that the flows will be much wider. It is also assumed that, thanks to sulfur compounds, the melting temperature of the Venusian lava is lower than that of the earth, and this further contributes to the mobility of the lava flows.

Mars is known for its giant volcanoes – true, now they all fell asleep, but before that time they poured thousands of square kilometers of basalt lava.

The moon in its 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 the seas.

Scientists guessed that there should be lava caves on the Moon and the planets of the solar system, as early as the nineteenth century, but the first discoveries had to wait until the beginning of the space age.


The fallen lava tubes on the slopes of the Peacock Mountain volcano in the image from the Mars Express satellite.

Caves of Mars

The collapsed volcanic pipes on the slopes of the Martian volcanoes managed to detect the automatic interplanetary station Viking in the 1970s.

Thirty years later, the satellite Mars Odyssey captured the first failures, which pointed to the still existing caves waiting for their speleologists. The diameter of the gaping dips reaches 250 meters. Most of them were found on the slopes of shield volcanoes in the highlands of Tarsida. The modern device Mars Reconnaissance Orbiter with the help of a high-resolution telescope HiRISE was able to look into the depths of Mars, as far as possible from orbit.


Failure into the lava cave on the Martian volcano Mount Ascry. A photograph of the satellite MRO.

Martian caves attract scientists for several reasons. Because of the thin atmosphere, the entire surface of the planet is irradiated with solar ultraviolet and bombarded with cosmic charged particles, so the existence of microbial life or even complex organic compounds in the upper layers of the earth is unlikely. Under the protection of cave vaults, the chances of their safety are dramatically increased – even if life itself is no longer there, its remains will lie much longer. It is also possible that in the Martian caves there will be water ice and other more volatile compounds in the open area.

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 to future colonists will come in handy. Although a number of factors indicate that the volcanic caves of Mars – not the most suitable place for life. They are all on volcanic slopes at an altitude of several kilometers above the plain. Meanwhile, landing in high-altitude regions is difficult because of the too thin layer of the atmosphere. The atmosphere helps to save fuel for braking when landing, so the heaviest spacecraft try to lower them in the deepest places of Mars. Thanks to the same atmosphere, lowlands are better protected from radiation. And the water ice on the surface has already been studied, including in the vicinity of the deepest depression in Mars – the valley of Hellas. Therefore, until the presence of biogenic or other minerals in the Martian caves is confirmed, it is expedient to investigate them using robotic means.

An important factor that hinders the development of Martian speleology is the requirements of planetary security. If there is a probability of maintaining a hypothetical Martian life in caves, then the researcher must be 100 percent sterile to exclude the possibility of a fantastic scenario of the “Martian Chronicles”, where one Earthlings snake destroyed a great civilization. Today, it is not possible to ensure the complete sterility of the spacecraft on Earth, and our microbes are capable of carrying the conditions of space flight. Therefore, the Martians are not looking to not accidentally destroy them.

Caves of the Moon

But the doctrine of planetary security does not interfere with visiting the lunar caves. The Hollow Moon repeatedly became the scene of fantasy works. Although the reality is far from fiction, but also encourages romantics. The existence of the lunar caves was assumed long ago, but direct confirmation came only in 2009. The Japanese automatic station Kaguya first discovered unusual craters that did not have a circular shaft and no signs of emissions from the inside. Their diameter reached 100 meters, and the depth seemed so significant that the side sunlight simply did not reach to the bottom. The American probe Lunar Reconnaissance Orbiter was able to consider the dips much more detailed, at different times of the day, to 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 sublunar cavity in the Sea of ​​Tranquility. Survey of LRO satellite

   
 
Failure into the sublunar cavity in the Sea of ​​Tranquility. Survey of the satellite LRO.

A group of scientists from the University of Arizona developed a special algorithm PitScan, which in the semi-automatic mode was looking for cave holes on the surface of the moon and found them more than two hundred. They can be divided into three conditional groups:

failures of lava channels, which expired during the eruption of volcanoes;
Lava cavities formed by a melt that appeared in large craters from the fall of large asteroids;
cavity in the lunar seas.

The hole in the supposed lava tube could be seen on the volcanic highlands of the Marius Hills, at the equator in the west of the visible side of the Moon. From the satellites there is clearly visible the channel of the lava flow, stretching from the mouth of the volcano for tens of kilometers. Approximately 25 kilometers from the crater in the frozen stream, a hole is visible. Either it was done by a meteorite, or the “roof” failed itself, but now you 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 terrestrial standards.

Purdue University conducted numerical simulation, according to which the strength of basaltic lava and low lunar gravity allow to preserve without destruction the 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. Compare the simulation with reality helped the data on the gravitational field of the moon, obtained with the help of GRAIL probes. The scientists took the GRAIL testimony 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 actually discovered, although they do not coincide with those possible voids, which were calculated from the deviations of the gravitational field. However, one hole in the Sea of ​​Tranquility, about 400 kilometers to the northeast of the Apollo 11 landing site, is the largest and deepest viewed from the satellite. The hole has a diameter of about 100 meters and a depth of up to 100 meters. Nearby, there are no lava canals or volcanic domes that could indicate the presence of a tunnel, but such a presence can still be assumed.

This hole is interesting to scientists not only by what may be hidden on its bottom, but also by a layered structure that is visible on the steep walls of the dip. These layers suggest to scientists that the lava sea was formed due to multiple lava spills, some of which were fairly thin, up to one meter.

The hole in the Sea of ​​Tranquility remains one of the most suitable places for planting a robotic probe and for studying the cave from the inside. However, until the development of lunar robot caver, no space agency plans. The closest to the secrets of the lunar lava tubes approached the astronauts Apollo 15, exploring the slopes of the canyon Hadley Rill, which, according to one hypothesis, was once a lava tube, but later completely collapsed.

The commander of the crew of the mission “Apollo 15” David Scott in the background of the valley Hadley Rill. Pilot photo of the lunar module of James Irwin.

The future of interplanetary speleology

Meanwhile, the future exploration of the lunar and Martian caves is being prepared on Earth. On our planet for study and visit is available a lot of volcanic caves, which allow us to present all the complexities of interplanetary speleology. In Russia, lava pipes and caves are known in Kamchatka. One of the lava tubes, about 100 meters long, is accessible in the caldera of the Gorely volcano. This cave is quite ancient, which remained after the eruption of two thousand years ago. In it you can feel yourself a Martian researcher due to the close to zero temperature and a massive glacier that partially blocks the entrance.


The lava cave of the volcano Tolbachik, formed by the eruption of 2012-2013.

Several caves formed during the eruption of Tolbachik volcano in 2012-2013. These caves are more picturesque: with lava stalactites “shark tooth”, covering the ceiling, salt deposits on the ceiling and growing stalagmites on the floor. Here, the heat of the cooling lava still remains, hot tea cracks can still be boiled on the hot cracks, and some branches of caves are inaccessible for visiting because of the high temperature.
Despite the obvious scientific interest in exploring extraterrestrial caves, no space agency has yet encroached on their secrets. A serious obstacle in this direction remains the technical implementation of such a study. The probe will either need to be seated exactly on the bottom of the dip, or provide climbing equipment for descent along a vertical wall. This alone is enough to stop the entire development – too much complexity, and therefore, risk. Further, it will be necessary to ensure the power supply of the robot in conditions of cave darkness, and, most importantly, control and maintain communication without direct radio visibility.

In space research, preference is always given to projects with high reliability, promising long-term deliveries of unique data, so the rope-speleologists are still losing competition to satellites and telescopes. Only a few private teams of the participants in the Google Lunar XPRIZE competition announced that their development will allow them to study the lunar caves. The American team Astrobotic and Japanese Hakuto designated lunar caves as their targets, but while their probes remain on Earth, and on the Moon they need to walk just 500 meters to deserve victory. Given the rarity of the lunar caves and the difficulty of accurate landing, it is unlikely that the teams from the first time will be able to reach the cavities of the moon.

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