Translartion. Region: Russians Fedetion –
Source: Novosibirsk State University – Novosibirsk State University –
Astronomers from the Vega observatory of NSU managed to record a rather rare astronomical phenomenon — a comet-like sodium tail of Mercury. The photo was taken in March and is the first in our country. According to NSU astronomers, the length of the tail in the photo is about 150 thousand km. The comet-like sodium tail was discovered, by astronomical standards, relatively recently — in 2001. At the moment, astronomers from all over the world have taken several dozen similar photos.
The difficulty of photographing Mercury’s tail is that a combination of several factors is required: Mercury reaching its maximum radial velocity relative to the Sun, Mercury’s significant angular distance from the Sun (elongation), and suitable meteorological conditions.
Everyone knows that a tail can be seen in comets – these are small bodies of the Solar System, which consist mainly of ice and dust. When a comet approaches the Sun, the sublimation of matter (water, volatile gases and dust) from the surface of its nucleus occurs, forming a coma and a tail. Dust particles scatter sunlight, so the tail of comets can be seen from Earth.
However, not only comets have tails, but also some larger objects in the Solar System. For example, Mercury.
— The mechanism of formation of Mercury’s tail is similar to the principle of formation of comet tails. This planet is characterized by a thin and weak atmosphere, which consists of helium, sodium, hydrogen, potassium and oxygen. Mercury has weak gravity and a weak magnetic field, so the atoms of chemical elements that make up its atmosphere are constantly blown away by the solar wind due to its proximity to the Sun. Therefore, a comet-like sodium tail of Mercury is formed, it is called sodium, since it is most clearly visible in the sodium line – at a wavelength of 589 nanometers, — said Yegor Konyaev, engineer of the Vega observatory of NSU.
Mercury’s comet-like sodium tail was discovered in 2001, when its first photograph was taken. There were already guesses that something like this should exist in the 1980s. However, because the tail is rather dim and a special filter must be used to capture it, it was impossible to photograph it for a long time. However, this became possible with the advent of highly sensitive digital cameras. In 2001, the length of Mercury’s tail was estimated at no more than 40 thousand kilometers based on the photograph. Later, in 2008, photographs were obtained from which it became clear that the length of the tail exceeds 2 million kilometers.
— Taking a photo of Mercury’s sodium tail is not the easiest task, but with a special filter it becomes feasible for almost every amateur astronomer. I first learned about this phenomenon about a year ago, when I saw impressive photos by German astrophotographer Sebastian Voltmer. Later it became clear that only a few dozen such photos have been taken, and not a single one in our country. This prompted me to try to photograph Mercury’s tail, — Yegor said.
The photograph was taken using a special filter, for a specific wavelength – 589 nanometers, with a transmission width of 10 nanometers. This is necessary to isolate the spectrum band in which the Mercury tail has the greatest brightness. It was also necessary to shoot with long exposures and high sensitivity, the total signal accumulation time was 8 minutes.
The problem is further complicated by the fact that Mercury’s tail is brightest at ±16 days from perihelion. Mercury has a fairly elongated elliptical orbit, so at perihelion it comes quite close to the Sun.
— It is clear that the Sun’s influence on Mercury will be greatest at the moment the planet passes perihelion. Therefore, it seems logical that this is the best moment to film the tail. However, for reasons related to the Doppler shift of sodium absorption lines, Mercury’s tail is brightest at moments of ±16 days from perihelion, since the planet reaches its maximum radial velocity relative to the Sun at this time, — Yegor explained.
This year, the most favorable period for observing Mercury was the end of February and the second half of March. Mercury was photographed when it was very low above the horizon — the planet’s altitude was only 3 degrees. The shooting was done from the Vega observatory of NSU. According to NSU astronomers, the tail in the photograph is about 150 thousand km long.
Other objects in the Solar System that have comet-like tails include the Moon and Jupiter’s satellite Io. The Moon has a very weak atmosphere, it is extremely rarefied. The Moon is also exposed to the Sun, and sodium atoms leave the satellite’s atmosphere, forming a tail. The Moon’s tail can only be observed during the new moon, when the satellite is between the Sun and the Earth. At this moment, the beam of tail particles is focused under the influence of Earth’s gravity. The lunar tail looks like a weak spot in the part of the sky diametrically opposite the Sun. A special filter is also needed to photograph it.
The future plans of NSU astronomers include recording the tail of Jupiter’s closest satellite, Io.
— The peculiarity of this space object is that it is quite close to Jupiter, which causes significant tidal forces that generate high geological activity. Io’s volcanic emissions also contain sodium, which can be recorded using a filter. The difficulty in photographing Io’s tail is the proximity of Jupiter. Its relatively high brightness does not allow photographing Io’s tail. To solve this complex problem, it will be necessary to upgrade the telescope and make it a coronagraph, which will allow cutting off light from the bright planet. After that, it will be possible to try to take a picture of Io’s tail. Even fewer such photographs have been taken in the world. This is a task for the future that we set for ourselves, — Yegor said about his plans.
The next time favorable conditions for observing Mercury’s sodium tail in our latitudes will occur in the spring of 2027. With due patience and persistence and the availability of special equipment, any amateur astronomer can photograph it.
Photo of Mercury’s tail, author – Egor Konyaev, Vega Observatory of NSU. Photo of Mercury’s transit across the Sun’s disk; also taken by astronomers of Vega Observatory of NSU.
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