金星の火山活動 — 地球の邪悪な双子 — NASA のマゼラン データで明らかに

科学者たちは、地球の双子座に活発な火山活動の直接的な証拠を初めて見た。

最近の火山活動の直接的な地質学的証拠が地表に見られる[{” attribute=””>Venus for the first time. Scientists made the discovery after poring over archival radar images of Venus taken more than 30 years ago, in the 1990s, by NASA’s Magellan mission. The images revealed a volcanic vent changing shape and increasing significantly in size in less than a year.

Venus is sometimes called Earth’s “evil twin” because despite being similar in size and composition to Earth, its surface is incredibly hostile, with a thick atmosphere of carbon dioxide and clouds of sulfuric acid. The extreme greenhouse effect on Venus has caused its surface temperature to soar to over 900 degrees Fahrenheit (475 degrees Celsius), hot enough to melt lead. In contrast, Earth has a much milder climate and is much more hospitable to life as we know it. Therefore, Venus is often seen as Earth’s “evil twin” because it presents a stark contrast to our planet’s relatively benign conditions.

Scientists study active volcanoes to understand how a planet’s interior can shape its crust, drive its evolution, and affect its habitability. One of NASA’s new missions to Venus will do just that. Led by the agency’s Jet Propulsion Laboratory in Southern California, VERITAS – short for Venus Emissivity, Radio science, InSAR, Topography, And Spectroscopy – will launch within a decade. The orbiter will study Venus from surface to core to understand how a rocky planet about the same size as Earth took a very different path, developing into a world covered in volcanic plains and deformed terrain hidden beneath a thick, hot, toxic atmosphere.

This artist’s concept shows the proposed VERITAS spacecraft using its radar to produce high-resolution maps of Venus’ topographic and geologic features. Credit: NASA/JPL-Caltech

“NASA’s selection of the VERITAS mission inspired me to look for recent volcanic activity in Magellan data,” said Robert Herrick, a research professor at the University of Alaska Fairbanks and member of the VERITAS science team, who led the search of the archival data. “I didn’t really expect to be successful, but after about 200 hours of manually comparing the images of different Magellan orbits, I saw two images of the same region taken eight months apart exhibiting telltale geological changes caused by an eruption.”

The search and its conclusions are described in a new study published in the journal Science. Herrick also presented the findings at the 54th Lunar and Planetary Science Conference in the Woodlands, Texas, on March 15.

Maat Mons is displayed in this computer-generated, three-dimensional perspective of the surface of Venus. The viewpoint is located 634 kilometers (393 miles) north of Maat Mons at an elevation of 3 kilometers (2 miles) above the terrain. Lava flows extend for hundreds of kilometers across the fractured plains shown in the foreground, to the base of Maat Mons. NASA Magellan mission synthetic aperture radar data is combined with radar altimetry to develop a three-dimensional map of the surface. The vertical scale in this perspective has been exaggerated 10 times. Credit: NASA/JPL

Modeling a Volcano

The geological changes Herrick found occurred in Atla Regio, a vast highland region near Venus’ equator that hosts two of the planet’s largest volcanoes, Ozza Mons and Maat Mons. The region has long been thought to be volcanically active, but there was no direct evidence of recent activity. While scrutinizing Magellan radar images, Herrick identified a volcanic vent associated with Maat Mons that changed significantly between February and October 1991.

In the February image, the vent appeared nearly circular, covering an area of less than 1 square mile (2.2 square kilometers). It had steep interior sides and showed signs of drained lava down its exterior slopes, factors that hinted at activity. In radar images captured eight months later, the same vent had doubled in size and become misshapen. It also appeared to be filled to the rim with a lava lake.

Altitude data for the Maat and Ozza Mons region on the Venus surface is shown at left, with the area of study indicated by the black box. At right are the before (A) and after (B) Magellan observations of the expanded vent on Maat Mons, with possible new lava flows after an eruptive event. Credit: Robert Herrick/UAF

But because the two observations were from opposite viewing angles, they had different perspectives, which made them difficult to compare. The low resolution of the three-decade-old data only made the work more complicated.

Herrick teamed up with JPL’s Scott Hensley, the project scientist for VERITAS and a specialist in analyzing radar data like Magellan’s. The two researchers created computer models of the vent in various configurations to test different geological-event scenarios, such as landslides. From those models, they concluded that only an eruption could have caused the change.

“Only a couple of the simulations matched the imagery, and the most likely scenario is that volcanic activity occurred on Venus’ surface during Magellan’s mission,” said Hensley. “While this is just one data point for an entire planet, it confirms there is modern geological activity.”

The scientists liken the size of the lava flow generated by the Maat Mons activity to the 2018 Kilauea eruption on the Big Island of Hawaii.

This annotated, computer-simulated global map of Venus’ surface is assembled from data from NASA’s Magellan and Pioneer Venus Orbiter missions. Maat Mons, the volcano that has exhibited signs of a recent eruption, is within the black square near the planet’s equator. Credit: NASA/JPL-Caltech

Magellan’s Legacy

Herrick, Hensley, and the rest of the VERITAS team are eager to see how the mission’s suite of advanced science instruments and high-resolution data will complement Magellan’s remarkable trove of radar imagery, which transformed humanity’s knowledge of Venus.

“Venus is an enigmatic world, and Magellan teased so many possibilities,” said Jennifer Whitten, associate deputy principal investigator of VERITAS at Tulane University in New Orleans. “Now that we’re very sure the planet experienced a volcanic eruption only 30 years ago, this is a small preview for the incredible discoveries VERITAS will make.”

VERITAS will use state-of-the-art synthetic aperture radar to create 3D global maps and a near-infrared spectrometer to figure out what the surface is made of. The spacecraft will also measure the planet’s gravitational field to determine the structure of Venus’ interior. Together, the instruments will offer clues about the planet’s past and present geologic processes.

And whereas Magellan’s data was originally cumbersome to study – Herrick said that in the 1990s they relied on boxes of CDs of Venus data that were compiled by NASA and delivered in the mail – VERITAS’ data will be available online to the science community. That will enable researchers to apply cutting-edge techniques, such as machine learning, to analyze the planet and help reveal its innermost secrets.

Those studies will be complemented by EnVision, an ESA (European Space Agency) mission to Venus slated for launch in the early 2030s. The spacecraft will carry its own synthetic aperture radar (called VenSAR), which is being developed at JPL, as well as a spectrometer similar to the one VERITAS will carry. Both Hensley and Herrick are key members of the VenSAR science team.

Reference: “Surface changes observed on a Venusian volcano during the Magellan mission” by Robert R. Herrick and Scott Hensley, 15 March 2023, Science.
DOI: 10.1126/science.abm7735

More About the Mission

In 2021, NASA chose the VERITAS and DAVINCI missions as their next ventures to Venus under the Discovery Program. VERITAS has collaborated with several organizations, including Lockheed Martin Space, the Italian Space Agency, the German Aerospace Center, and France’s Centre National d’Études Spatiales. The Planetary Missions Program Office at NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Discovery Program for NASA’s Science Mission Directorate’s Planetary Science Division in Washington.