Sebuah penemuan baru mengungkapkan alasan perbedaan warna Uranus dan Neptunus

Pesawat ruang angkasa Voyager 2 NASA menangkap pemandangan Uranus (kiri) dan Neptunus (kanan) ini selama terbang lintas planet pada 1980-an. Kredit: NASA/JPL-Caltech/B. Johnson

Pengamatan dari Observatorium Gemini dan teleskop lainnya mengungkapkan kabut asap yang berlebihan[{” attribute=””>Uranus makes it paler than Neptune.

Astronomers may now understand why the similar planets Uranus and Neptune have distinctive hues. Researchers constructed a single atmospheric model that matches observations of both planets using observations from the Gemini North telescope, the NASA Infrared Telescope Facility, and the Hubble Space Telescope. The model reveals that excess haze on Uranus accumulates in the planet’s stagnant, sluggish atmosphere, giving it a lighter hue than Neptune.

Planet Neptunus dan Uranus memiliki banyak kesamaan – mereka memiliki massa, ukuran, dan komposisi atmosfer yang serupa – namun penampilan mereka sangat berbeda. Pada panjang gelombang yang terlihat, Neptunus tampak lebih biru dalam warna sementara Uranus adalah warna pucat cyan. Para astronom sekarang memiliki penjelasan mengapa kedua planet ini memiliki warna yang sangat berbeda.

Penelitian baru menunjukkan bahwa lapisan kabut pekat yang ditemukan di kedua planet lebih tebal di Uranus daripada lapisan serupa di Neptunus dan “memutihkan” penampakan Uranus lebih banyak daripada di Neptunus.^[1] Jika tidak ada kabut di suasana Dari Neptunus dan Uranus, keduanya akan tampak hampir sama dalam warna biru.^[2]

Kesimpulan ini datang dari model^[3] bahwa tim internasional yang dipimpin oleh Patrick Irwin, profesor fisika planet di Universitas Oxford, telah mengembangkan untuk menggambarkan lapisan aerosol di atmosfer Neptunus dan Uranus.^[4] Penyelidikan sebelumnya dari atmosfer atas planet-planet ini telah difokuskan pada penampilan atmosfer pada panjang gelombang tertentu saja. Namun, model baru ini, yang terdiri dari beberapa lapisan atmosfer, cocok dengan pengamatan dari kedua planet di berbagai panjang gelombang. Model baru ini juga mencakup partikel kabur di dalam lapisan yang lebih dalam yang sebelumnya dianggap hanya berisi awan metana dan es hidrogen sulfida.

Diagram ini menunjukkan tiga lapisan aerosol di atmosfer Uranus dan Neptunus, seperti yang dirancang oleh tim ilmuwan yang dipimpin oleh Patrick Irwin. Altimeter pada grafik mewakili tekanan di atas 10 bar.
Lapisan terdalam (lapisan aerosol-1) tebal dan terdiri dari campuran es hidrogen sulfida dan partikel dari interaksi atmosfer planet dengan sinar matahari.
Lapisan utama yang mempengaruhi warna adalah lapisan tengah, yang merupakan lapisan partikel kabut (disebut di kertas sebagai lapisan aerosol-2) yang lebih tebal di Uranus daripada di Neptunus. Tim menduga bahwa di kedua planet, es metana mengembun pada partikel di lapisan ini, menarik partikel lebih dalam ke atmosfer saat salju metana turun. Karena atmosfer Neptunus lebih aktif dan bergejolak daripada Uranus, tim percaya bahwa atmosfer Neptunus lebih efisien dalam mendorong partikel metana ke lapisan kabut dan menghasilkan salju itu. Ini menghilangkan lebih banyak kabut dan membuat lapisan kabut Neptunus lebih tipis daripada di Uranus, yang berarti bahwa warna biru Neptunus tampak lebih kuat.
Di atas kedua lapisan tersebut terdapat lapisan kabut yang memanjang (lapisan aerosol 3) mirip dengan lapisan di bawahnya tetapi lebih rapuh. Di Neptunus, partikel es metana besar juga terbentuk di atas lapisan ini.
Kredit: Observatorium Internasional Gemini/NOIRLab/NSF/AURA, J. da Silva/NASA/JPL-Caltech/B. Johnson

“Ini adalah model pertama yang secara sinkron sesuai dengan pengamatan sinar matahari yang dipantulkan dari ultraviolet ke inframerah dekat,” jelas Irwin, penulis utama makalah penelitian yang mempresentasikan temuan ini di Journal of Geophysical Research: Planets. “Dia juga yang pertama menjelaskan perbedaan warna yang terlihat antara Uranus dan Neptunus.”

Model tim terdiri dari tiga lapisan aerosol pada ketinggian yang berbeda.^[5] Lapisan utama yang mempengaruhi warna adalah lapisan tengah, yang merupakan lapisan partikel kabut (disebut di kertas sebagai lapisan aerosol-2) yang lebih tebal di atas Uranus Dari Neptunus. Tim menduga bahwa di kedua planet, es metana mengembun pada partikel di lapisan ini, menarik partikel lebih dalam ke atmosfer saat salju metana turun. Karena atmosfer Neptunus lebih aktif dan bergejolak daripada Uranus, tim percaya bahwa atmosfer Neptunus lebih efisien dalam mendorong partikel metana ke lapisan kabut dan menghasilkan salju itu. Ini menghilangkan lebih banyak kabut dan membuat lapisan kabut Neptunus lebih tipis daripada di Uranus, yang berarti bahwa warna biru Neptunus tampak lebih kuat.

Mike Wong, seorang astronom di[{” attribute=””>University of California, Berkeley, and a member of the team behind this result. “Explaining the difference in color between Uranus and Neptune was an unexpected bonus!”

To create this model, Irwin’s team analyzed a set of observations of the planets encompassing ultraviolet, visible, and near-infrared wavelengths (from 0.3 to 2.5 micrometers) taken with the Near-Infrared Integral Field Spectrometer (NIFS) on the Gemini North telescope near the summit of Maunakea in Hawai‘i — which is part of the international Gemini Observatory, a Program of NSF’s NOIRLab — as well as archival data from the NASA Infrared Telescope Facility, also located in Hawai‘i, and the NASA/ESA Hubble Space Telescope.

The NIFS instrument on Gemini North was particularly important to this result as it is able to provide spectra — measurements of how bright an object is at different wavelengths — for every point in its field of view. This provided the team with detailed measurements of how reflective both planets’ atmospheres are across both the full disk of the planet and across a range of near-infrared wavelengths.

“The Gemini observatories continue to deliver new insights into the nature of our planetary neighbors,” said Martin Still, Gemini Program Officer at the National Science Foundation. “In this experiment, Gemini North provided a component within a suite of ground- and space-based facilities critical to the detection and characterization of atmospheric hazes.”

The model also helps explain the dark spots that are occasionally visible on Neptune and less commonly detected on Uranus. While astronomers were already aware of the presence of dark spots in the atmospheres of both planets, they didn’t know which aerosol layer was causing these dark spots or why the aerosols at those layers were less reflective. The team’s research sheds light on these questions by showing that a darkening of the deepest layer of their model would produce dark spots similar to those seen on Neptune and perhaps Uranus.

Notes

1. This whitening effect is similar to how clouds in exoplanet atmospheres dull or ‘flatten’ features in the spectra of exoplanets.
2. The red colors of the sunlight scattered from the haze and air molecules are more absorbed by methane molecules in the atmosphere of the planets. This process — referred to as Rayleigh scattering — is what makes skies blue here on Earth (though in Earth’s atmosphere sunlight is mostly scattered by nitrogen molecules rather than hydrogen molecules). Rayleigh scattering occurs predominantly at shorter, bluer wavelengths.
3. An aerosol is a suspension of fine droplets or particles in a gas. Common examples on Earth include mist, soot, smoke, and fog. On Neptune and Uranus, particles produced by sunlight interacting with elements in the atmosphere (photochemical reactions) are responsible for aerosol hazes in these planets’ atmospheres.
4. A scientific model is a computational tool used by scientists to test predictions about a phenomena that would be impossible to do in the real world.
5. The deepest layer (referred to in the paper as the Aerosol-1 layer) is thick and is composed of a mixture of hydrogen sulfide ice and particles produced by the interaction of the planets’ atmospheres with sunlight. The top layer is an extended layer of haze (the Aerosol-3 layer) similar to the middle layer but more tenuous. On Neptune, large methane ice particles also form above this layer.

More information

This research was presented in the paper “Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots” to appear in the Journal of Geophysical Research: Planets.

The team is composed of P.G.J. Irwin (Department of Physics, University of Oxford, UK), N.A. Teanby (School of Earth Sciences, University of Bristol, UK), L.N. Fletcher (School of Physics & Astronomy, University of Leicester, UK), D. Toledo (Instituto Nacional de Tecnica Aeroespacial, Spain), G.S. Orton (Jet Propulsion Laboratory, California Institute of Technology, USA), M.H. Wong (Center for Integrative Planetary Science, University of California, Berkeley, USA), M.T. Roman (School of Physics & Astronomy, University of Leicester, UK), S. Perez-Hoyos (University of the Basque Country, Spain), A. James (Department of Physics, University of Oxford, UK), J. Dobinson (Department of Physics, University of Oxford, UK).

NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory), the US center for ground-based optical-infrared astronomy, operates the international Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (operated in cooperation with the Department of Energy’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have for the Tohono O’odham Nation, the Native Hawaiian community, and the local communities in Chile, respectively.