All interstellar and interplanetary space is filled with cosmic radiation. This is the result of radiation from stars, accretion disks of black holes, neutron stars and pulsars, supernova explosions … Almost any cataclysm in the Universe is the cause of radiation emissions. Radiation is a problem for astronauts and electronics, but for scientists it is a gift to learn a lot of details about space. We continue our review of scientific instruments used to study the solar system.
Earlier we learned how planets are studied by optical means.
Gamma spectroscopy
The gamma range, in principle, is also optics, since gamma rays are high-energy photons. But gamma spectroscopy in planetary science studies not those rays that are emitted from stars and black holes, but those that illuminate planets and other non-atmospheric or weakly atmospheric cosmic bodies.
Planets and asteroids begin to emit in gamma when bombarded with more massive particles: high-energy protons, alpha-beta rays and neutrons. The charged particles hit the surface soil and it begins to emit in the gamut. And, which is typical, each chemical element emits in its own range. That is, we just need to hold a gamma spectrometer over the surface to understand what it consists of. So we will understand only the chemical composition, not the geological, but supplementing it with information, for example, from infrared spectrometers, and from cameras of the visible range, we can get a more visual picture.
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So, using gamma spectrometry, scientists learned about the relatively high concentrations of thorium, iron and titanium ores on the moon.
With the help of such a device on Mars Odyssey, it was possible to find two regions on Mars with anomalously high contents of thorium and, probably, uranium ores. It is quite possible that processes there once took place as in Africa, with the formation of a natural nuclear reactor. True, others, on the basis of the same data, are talking about a thermonuclear war … One way or another, this is an encouraging find, since it means that the nuclear power plants of future Martian settlers can work on local raw materials.
Neutron detectors
Cosmic neutrons, unlike alpha and beta particles, are not completely absorbed by the soil. Some of the neutrons are reflected from the surface of stony bodies, while they manage to sink into the ground by about half a meter. The neutrons returning from the surface, as a rule, are already moving much slower, their speed and energy depends on what they went through in the soil. More precisely, only one parameter is measured with their help - the hydrogen content.
Hydrogen, due to the lightness of atoms, effectively slows down neutrons in elastic collisions, and this efficiency directly depends on its concentration. At the same time, in free form, hydrogen will not remain in the soil, especially where atmospheric pressure tends to zero. In order to store hydrogen in the soil, it must be bound at a chemical level, and water remains the best remedy. Thus, flying over the surface and collecting data on the velocities of the "taking off" neutrons, one can determine the approximate water content in the soil. Of course, the lower we fly, the more accurate the data will be. Satellites still give an error of plus or minus a hundred kilometers.
It was with the help of the Russian LEND and HEND instruments that data on the distribution of hydrogen / water in the near-surface soils of the Moon and Mars were obtained.
And if the Martian data have already been confirmed twice, then the lunar ones are still waiting for their verification. On Mars, the Phoenix lander landed in the circumpolar region, and where HEND promised up to 70% of the water in the ground, a layer of water ice was found right under the dust. And in Gale Crater, where the Curiosity rover is operating, HEND promised 5%, the water content in the ground ranges from 3% to 5%, and only rarely come across six percent "oases".
After such a success of HEND, his brother DAN was "seated" directly on the rover, and now he collects data not from an altitude of 300 km, as its predecessor, but 0.5 m. True, the sounding depth still does not exceed 1 meter, but the spatial resolution has increased from tens of kilometers to centimeters.
However, despite the success of neutron detectors, there is no final confidence in them. Glaciers on the Moon are still waiting for their discoverer, and space agencies, like private companies, are paying more and more attention to the Moon's poles. Although the concentration of moisture there, according to satellites, is no more than 4%.
Radars
The sounding of the planets in the radio range began to be carried out from the Earth. A lot of information was given by the Arecibo radio telescope, 300 meters in diameter. For example, back in the 80s, he discovered at the poles of hot Mercury, a strange reflection that water ice could give. Scientists for a long time could not believe that glaciers could exist on the planet closest to the Sun. I had to wait for the results of the Messenger probe, which, using a neutron detector and laser ranging, was able to confirm the presence of ice.
Arecibo showed impressive pictures during the 2013 supermoon. On the Moon, he can see the consequences of catastrophic lava flows and "floods" with his help.
If these images are combined with maps of the distribution of minerals obtained from orbital spectrometers, it is possible to compile a detailed geological map of the area, and it is possible to reconstruct the evolution of the surface. Although it is strange that until now a satellite with a powerful radar has not been sent to the moon.
But three radar satellites flew to Venus. There is no other way to study the surface from the orbit of this planet. Venera-15 and -16 mapped the North Pole in the 1980s, and then, in the 1990s, Magellan made a complete map.
Now Cassini is busy with a similar business in orbit of Saturn. Here radar is used to penetrate Titan's dense atmosphere. In the course of numerous flights, the space station gradually reveals the eternal veil and reveals to science this truly amazing world, in some ways incredibly similar to the earthly one, but in some way strikingly different.
Multiple radar surveys allow not only mapping, but also observing dynamic processes. Thus, the mysteriously appeared and then disappeared island was considered a sign of ongoing seasonal changes. Perhaps it was an icy iceberg that crashed into a methane sea.
Other wavelengths and different radar designs allow you to go deeper. In the orbit of Mars, there are two spacecraft equipped with "echo sounders" that penetrate the crust of the planet for 1-3 kilometers.
The study of the European spacecraft Mars Express made it possible to obtain information on the power and structure of polar ice, distinguish carbon dioxide ice from water ice, and estimate water reserves.
His scanning also revealed ancient asteroid craters, buried by hundreds of meters of volcanic lava and sedimentary accumulations of the Martian ocean, in the northern hemisphere of the planet. Scientists have repeatedly noted the apparent difference in the number of meteorite craters in the southern and northern hemispheres of Mars, and Mars Express has solved the mystery. If anyone still had hopes for the Martians buried from vacuum, drought and frost into the sub-Martian Zion, then I have bad news for them …
The New Horizons spacecraft also has instruments for radar research, but the antenna size is inferior to many interplanetary colleagues, so the research will concentrate on finding and studying the atmosphere.
I am looking forward to the results of radar scanning of the nucleus of comet 67P / Churyumov-Gerasimenko, which was done by the Rosetta and Philae spacecraft for a couple.
The radar was even brought to the moon. The Chinese "Jade Hare" managed to walk only a hundred meters, but even on it he managed to get the most interesting profiles of the lunar surface, to a depth of about four hundred meters. In the future, such information will be vital for the construction of a lunar station, base or settlement.
Alpha proton spectroscopy
When it comes to the study of space bodies by lander, it is almost impossible to do without touching moments of alpha-proton X-ray fluorescence spectroscopy.
Devices of the APXS type (Alpha Particle X-Ray Spectrometer) were installed on all NASA Mars rovers. APXS is available on the Philae lander at the nucleus of comet 67P / Churyumov-Gerasimenko. There was a similar device (RIFMA) on the Soviet lunar rovers.
The principle of operation of the method resembles gamma spectroscopy, except that the sensor has its own source of charged particles (some kind of radioactive isotope), primarily alpha rays. The studied sample is irradiated with radiation and it starts to glow in the X-ray range.
Moreover, each chemical element glows in its own way, which makes it possible to obtain spectra of elemental composition.
This is far from an exhaustive overview of equipment for exploring the solar system. As a rule, astrophysical instruments are also installed on interplanetary vehicles to register energetic particles, interplanetary radiation, plasma and dust. Interplanetary flights also make it possible to study outer space, the relationship between the Sun, planets, and the interstellar medium, but that's another story.
