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Using ESO’s Very Large Telescope (VLT), astronomers have observed a large dark spot in Neptune’s atmosphere, alongside an unsuspected smaller bright spot. It is the first time that a dark spot on the planet has been observed with a telescope on Earth. These intermittent features in Neptune’s blue atmosphere have puzzled astronomers, but the new results provide further insight into their nature and origin.
Large spots can often be seen in the atmospheres of giant planets. The most famous is Jupiter’s Great Red Spot. A dark spot on Neptune was first observed in 1989 by NASA’s Voyager 2 space probe, but it disappeared a few years later. “Since that first discovery of a dark spot, I’ve always wondered what these ephemeral and elusive dark features actually are,” says Patrick Irwin, a professor at the University of Oxford (UK) and principal investigator on the study, published today in Nature Astronomy. published.
Irwin and his team have used data from ESO’s VLT to rule out dark spots caused by a ‘brightening’ in the cloud cover. The new observations indicate that they are instead likely due to a deep, darkening layer of aerosols below the most prominent vapor layer, where mixing of ice and vapor occurs. It was not easy to come to this conclusion, because dark spots are not always visible in Neptune’s atmosphere and astronomers have never had the opportunity to study them in detail. That changed when the NASA/ESA Hubble Space Telescope discovered several dark spots in Neptune’s atmosphere, including one in the planet’s northern hemisphere that was first spotted in 2018. Irwin and his team immediately set to work studying this spot from the ground – with an instrument ideally suited to these challenging observations.
This image shows Neptune observed with the MUSE instrument
van ESO’s Very Large Telescope – Photo: ESO/P. Irwin et al.
Using the VLT’s Multi Unit Spectroscopic Explorer (MUSE), the researchers were able to split the reflected sunlight from Neptune and its spot into their constituent colors, or wavelengths, and obtain a 3D spectrum [1]. This meant they could study the stain in greater detail than previously possible. “I’m thrilled that not only was I able to observe a dark spot from the ground for the first time, but I was also able to capture a reflection spectrum from such a spot for the first time,” says Irwin. Because you actually ‘scan’ different depths in Neptune’s atmosphere at different wavelengths, this spectrum allowed the astronomers to more accurately determine the height of the dark spot in the planet’s atmosphere. The spectrum also provided information about the chemical composition of the different layers of the atmosphere, which in turn provided clues as to why the spot appeared dark.
The sightings also yielded a surprise. “In doing so, we discovered a rare deep, bright cloud that had never been seen before, even from space,” said Michael Wong, a study co-author and a researcher at the University of California, Berkeley, USA. This rare type of cloud appeared as a bright spot right next to the larger main dark spot. The VLT data showed that the new “deep bright cloud” was at the same level in the atmosphere as the main dark spot. This means that this structure is completely different from the small “companion” clouds of high-altitude methane ice previously observed.
With the help of ESO’s VLT, astronomers can now study these kinds of spots from Earth. “This is a huge leap forward. At first, we could only observe these spots by sending a space probe, such as Voyager, to them. Then we got the opportunity to observe them remotely with Hubble. And now the technology is so advanced that we can do this from the ground up,” Wong concludes, jokingly adding, “This could make me obsolete as a Hubble observer!”
Nuts
MUSE is a 3D spectrograph that allows astronomers to observe an astronomical object, such as Neptune, in its entirety. At each pixel, the instrument measures the intensity of the light as a function of color or wavelength. The resulting data forms a 3D sequence in which each pixel of the image is provided with a full spectrum of light. In total, MUSE measures more than 3500 colors. The instrument is designed to use adaptive optics, which counteracts the effect of turbulence in the Earth’s atmosphere, resulting in sharper images. Without this combination of properties, it would not have been possible to study a dark spot of Neptune from the ground.
Bron: THAT
2023-08-24 16:15:28
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