MOSCOW, Sep 5 , Tatyana Pichugina. The «Pragyan» apparatus was sent «to hibernation» during the moonlit night. Before that, he transmitted to Earth data on the chemical composition of the regolith obtained by onboard instruments. What was discovered and why the sensation did not happen — in the material.
For the first time at the south pole of the moon
The Vikram module for the first time in the history of astronautics made a soft landing near the south pole Moon 23 August. The next day, the wheeled lunar rover «Pragyan», which means «knowledge», separated from it and began to explore the lunar surface.
The landing site is located at latitude 69, 600 kilometers from the south pole, between the craters Manzini and Simpelius. This is a flat little rocky area without sharp changes in relief. Approximately in the same area, one of the alternate airfields for the Russian Luna-25 station was planned.
The Pragyan Lunokhod is a six-wheeled vehicle weighing 26 kilograms, developing a speed of 36 kilometers per hour. Cameras and two scientific instruments are installed on board. The rover is powered by solar panels.
In the allotted two weeks, the rover traveled more than a hundred meters and returned back. On one of the trips, he successfully discovered a four-meter crater in front of him and turned around, choosing a safer path. Cameras recorded the surrounding surface and their own lander, and onboard instruments took measurements. ratio=»0.958250497017893″ data-crop-width=»600″ data-crop-height=»575″ data-source-sid=»» title=»Chandrayan Rover Model» class=»lazyload» lazy=»1″ />
What an Indian rover can do
One of the main tasks of the Indian lunar rover is to study the composition of the lunar soil, or regolith. For this, two spectrometers are installed on board. The first uses the method of laser-spark emission spectrometry (Laser-Induced Breakdown Spectroscopy, or LIBS), the second is an X-ray spectrometer of alpha particles (Alpha Particle X-ray Spectrometer, or APXS). In both cases, the soil is analyzed in situ, without sampling, by bombardment with charged particle beams. Both instruments are built at the Indian Space Research Organization's (ISRO) Optoelectronic Systems Laboratory in Bangalore.
LIBS fires a nanosecond high-powered laser beam at the ground. At the point he touched, soil particles literally evaporate, minerals and large molecules disintegrate, forming a cloud of luminous plasma. The optical unit collects this light and transmits it to a spectrograph, which analyzes the signal and records the data.
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In plasma, chemical elements are in the form of ions, emitting spectra characteristic only of them. It is by them that the composition is determined, and the relative concentration is estimated by their intensity. LIBS devices have proven themselves in geology, archeology, metallurgy and other fields. NASA's Curiosity rover on Mars has this tool. The Indian LIBS was originally developed for the Chandrayan 2 mission that crashed in 2019.
The APXS spectrometer bombs an area of the surface with alpha particles and X-rays, which generates a response. Unlike LIBS, it collects not optical, but X-ray radiation emitted by excited atoms.
Alpha particle bombardment has been used on lunar missions for in situ soil analysis since the late 1960s. Then, after a long lunar hiatus, APXS successfully collected data aboard China's Chang'e-3 mission's Yutu rover in the Mare Imbrium in December 2013. The device has been installed in three generations of NASA rovers, starting with the Sojourner rover, which worked on the Red Planet for several months in 1997. He was on board the British lunar rover Beagle 2, which was delivered to Mars in 2003 but never made contact.
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Mysteries of the lunar soil
ISRO announced the first experiments to study regolith on August 27. The ChaSTE instrument aboard the lander sent the first temperature readings back to Earth. They are obtained with a ten-centimeter probe, on which ten temperature sensors are installed. The graph shows a fairly large spread — from ten degrees Celsius at a depth of eight centimeters to about 50 degrees at the surface. This difference surprised even ISRO engineers.
«I explain it this way. A four-legged landing device. A probe extends from its bottom, which penetrates into the ground and measures the temperature. This place is illuminated by the Sun, the ground heats up. Under normal conditions, heat would go into space, but now the bottom of the module prevents it from above «This could increase the surface temperature. Perhaps some terrain features played a role. For example, the station hit a small crater,» says Alexander Bazilevsky, chief researcher at the Laboratory of Comparative Planetology of the Geochemical Institute of the Russian Academy of Sciences.
According to the scientist, one should trust the measurements of the Diviner radar aboard the NASA LRO orbital observatory. According to these data, on a lunar day, the surface in the region is heated to about 20-30 degrees. True, the spatial resolution is much higher — 200 meters.
The rapid decrease in soil temperature with depth confirms the hypothesis that permafrost lies somewhere below. However, the Indian mission will not be able to detect it, since neither the lander nor the rover is equipped with instruments for sampling from depth. They only get data from the surface layer of dust.
According to a preliminary analysis of data from LIBS, aluminum, sulfur, titanium, calcium, iron, and chromium were found in the regolith. With the help of APXS, the presence of lighter elements — magnesium, silicon, oxygen — was also established.
It was not without intrigue. According to ISRO, the rover unequivocally confirmed the presence of sulfur in the south pole region, which could not be done from orbit. In addition, the published spectrum plot from the LIBS instrument shows a peak in sulfur content. True, the measurements are given in relative units, the true concentration is unknown.
“Sulfur on the Moon is in the form of sulfides, primarily the mineral troilite FeS, there is not much of it — about 0.1 percent. There should be a lot of aluminum, this is a continental region, feldspars close to anorthite are common there. That (according to ISRO. – Ed.) There is a lot of sulfur, surprisingly,” notes Alexander Bazilevsky. He suggests that during the measurement, the device could accidentally get into the accumulation of sulfides.
«This is a continental place, in English — highlands, there should be predominantly anorthositic rocks, significantly enriched in aluminum and calcium. The rocks are located at a depth of several meters. Unlike the lunar seas, where the rocks are basalts, heavily cracked by meteorite impacts, breccias go under the ground on the continents, that is, rocks broken by large impacts, repeatedly processed three and a half to four billion years ago. Breccias can be very strong if they are cemented by an impact melt, or not very strong if they are held together by detrital material, » the planetologist continues.
Another feature of the regolith is the enrichment in heavier isotopes associated with the repeated processing of soil from meteorite impacts. «In this case, evaporation occurs at the point of impact, the lighter isotope flies into space faster. The isotopic composition of a number of elements in the lunar soil is shifted towards heavier ones. And not only isotopic. In the lunar soil, for example, there will be less sodium than in the underlying breed,» the researcher adds. -width=»600″ data-crop-height=»440″ data-source-sid=»» title=»The first LIBS spectra of the lunar surface on the Pragyan rover» class=»lazyload» lazy=»1″ />
Hydrogen — a marker of water or water ice — has not yet been detected by Indian instruments. And the big question is whether it will be possible to detect it.
«Water and other hydrogen compounds on the surface with the first rays of the Sun should evaporate and fly away. Water ice will be at a depth of tens of centimeters, which is confirmed by impact experiments. It is seen by the domestic LEND neutron spectrometer, developed at IKI RAS,» says the scientist. He clarifies that the sensitivity of the neutron spectrometer is higher, since it penetrates to a depth of up to two meters.
«I think that Chandrayaan-3 will not find any water or ice. Because they are at a depth. An apparatus like Luna-25, which has a bucket, can do this. So you have to repeat the mission. And then to launch Luna-27 with a drilling rig and Luna-28, which will return the frozen core to Earth. Three or four years are left to wait,» Alexander Bazilevsky concludes. which lasts two weeks. Before «sleep» rover «Pragyan» performed the trick of separation from the surface, in other words, jumped up 40 centimeters and softly landed back. As ISRO explained, the experiment is necessary for testing future missions with the delivery of lunar soil to Earth and manned flights to the Moon.
April 27, 08:00