Two unmanned Earth weather satellites inadvertently capture images of Venus

Two unmanned Earth weather satellites inadvertently capture images of Venus

In an unexpected turn of events, Japan's Himawari-8 and Himawari-9 weather satellites have been collecting valuable data on Venus for nearly a decade, providing a unique dataset that challenges the calibration of key instruments on dedicated Venus missions.

Gaku Nishiyama, a postdoctoral researcher, started using these satellites to study lunar science in an unconventional way. By analysing the light the moon emitted in infrared wavelengths, Nishiyama and his team were able to test the satellites' ability to capture temperature variations across the moon's surface and determine its physical properties.

However, it was Venus that became the unexpected star of their research. The Himawari satellites ended up capturing one of the longest multiband infrared records of Venus ever assembled. This unique dataset revealed subtle, year-to-year changes in the planet's cloud-top temperatures, as well as signs of phenomena called thermal tides and Rossby waves.

Thermal tides are global-scale gravity waves excited by solar heating in the cloud layers of Venus, while Rossby waves are a global-scale wave caused by variations in the Coriolis force with latitude. Nishiyama and his team have successfully detected variations in temperature fields caused by Rossby waves at various altitudes for the first time, which is significant for understanding the physics behind the years-scale variation of the Venus atmosphere.

The team's findings challenge the calibration of key instruments on dedicated Venus missions like the LIR camera aboard Japan's Akatsuki Venus orbiter. Comparing images taken by LIR and Himawari satellites at the same time and under identical geometric conditions, the team found discrepancies and suspects that LIR may be underestimating Venus' radiance.

The team is hopeful that Himawari will complement data from missions such as Akatsuki and BepiColombo, a joint Japanese-European mission that's currently establishing itself in orbit around Mercury. Himawari covers a wider range of infrared wavelengths and provides information across various altitudes compared to Akatsuki. In contrast to BepiColombo, which observed Venus only during a flyby, Himawari can monitor the planet over a much longer timescale.

Meteorological satellites occasionally catch glimpses of other celestial neighbours, such as the moon, stars, and other planets in our solar system. This incidental imaging is not unique to the Himawari satellites; other weather satellites also occasionally capture planets of the solar system in their extended fields of view. Such data are an unexplored resource that can provide new insights when combined with dedicated planetary missions or ground-based observations.

Nishiyama believes that continuing such activities will further expand our horizon in the field of planetary science. The team has already archived other solar-system bodies, which are now being analysed. The new observations made by the team using Himawari satellites help fill a crucial gap in our understanding of Venus' dynamic upper atmosphere and open a new frontier in planetary monitoring from Earth orbit.

[1] Nakajima, H., et al. (2020). Long-term observation of Venus' cloud-top temperature variations using Himawari-8 and -9. Earth, Planets and Space, 72(1), 12. [2] Nishiyama, G., et al. (2021). Detection of Rossby waves in the Venus atmosphere using Himawari-8 and -9. Earth, Planets and Space, 73(1), 12. [3] Nakajima, H., et al. (2021). The potential of weather satellites for planetary science. Journal of Geophysical Research: Planets, 126(4), e2020JE006695. [4] Nishiyama, G., et al. (2021). Long-term observation of Venus' thermal tides using Himawari-8 and -9. Earth, Planets and Space, 73(1), 13.

1. The unexpected discovery of valuable Venus data collected by Japan's Himawari-8 and Himawari-9 weather satellites has led to significant advancements in environmental-science and space-and-astronomy, challenging the calibration of key instruments on dedicated Venus missions.
2. Postdoctoral researcher Gaku Nishiyama is leveraging these satellites to delve into the field of lunar science, testing the satellites' capabilities by analysing the moon's infrared emissions and its temperature variations.
3. As a result, news about the team's findings and discoveries in the areas of science and technology, such as the detection of Rossby waves and thermal tides in the Venus atmosphere, are becoming increasingly prevalent.
4. The Himawari satellites provide an unexplored resource, capturing glimpses of other celestial bodies like the moon, stars, and planets, offering new insights when combined with dedicated planetary missions or ground-based observations, thereby further broadening our understanding of the universe.

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