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VENUS, AN ACTIVE PLANET: EVIDENCE FOR RECENT VOLCANIC AND TECTONIC ACTIVITY Justin Filiberto1, Piero D’Incecco2,3, and Allan H. Treiman1

DOI: 10.2138/gselements.17.1.67

Similar in size to the Earth, Venus differs from our planet by its extreme surface temperature (470 °C), suffocating atmospheric pressure (about 92 times that of the Earth’s), and caustic atmosphere (mostly CO2, with sulfuric acid rain). Venus is Earth’s hellish twin sister. However, there are some similarities. As for the Earth, Venus has also had a very complex geologic history. During the early 1990s, NASA’s Magellan spacecraft imaged the surface of Venus with radar and gave us a panorama of a volcanic wonderland (Fig. 1). The surface of Venus is dotted with some of the largest volcanoes in the solar system, complete with summit calderas and extensive lava flows. Volcanoes on Venus resemble many of those on Earth, particularly those formed from the eruption of basaltic magma, such as Mauna Kea (Hawaii, USA) and Mount Etna (Italy). One of the biggest unresolved scientific questions about Venus concerns its style and rate of volcanism during its geologic past. Did volcanic eruptions on Color overlay of heat patterns from Idunn Mons and Imdr Regio as Figure 2 Venus occur locally and constantly in time? Or did the planet undergo derived from VIRTIS (visible and infrared thermal imaging spectrometer) surface brightness data overlain on Magellan radar data as in Figure 1. Red sporadic events of global and catastrophic volcanism which rejuvenated represents the warmest regions (emissivity ≥0.9), interpreted to be the freshest its entire crust in a short amount of time? basalts, while the blue/purple are cooler regions (emissivity ~0.5), interpreted to be altered older basalts. Image: NASA/Jet Propulsion Laboratory-Caltech/ESA, Image Number: PIA13001.

rocks. Using the VIRTIS data, Smrekar et al. (2010) first reported that some lava flows at Idunn Mons emitted so little light (at 1 µm) as to suggest that these lavas had not been oxidized by Venus’ atmosphere. The implication was that these lavas were relatively fresh and young (Fig. 2). Other lava flows from the volcano (and the surrounding region) were more consistent with oxidized and altered basalt. Smrekar et al. (2010) suggested that the youngest lava flows were about 2.5 million years old or younger (possibly even as young as 250,000 years old). However, the ages of these fresh, basaltic lava flows at Idunn Mons could not be well constrained because the rate of alteration of fresh basalt and how that alteration might affect the spectral signal was not understood. Basaltic rocks on Venus are in contact with a hot caustic environment and, because water is not stable on the surface to make clay minerals (Zolotov 2018), should alter quickly to form surface coatings mainly of hematite and sulfates. These coatings could, in theory, be used to date different lava flows, because the amount of alteration should correlate with the age of the rock exposed to the atmosphere. Recently, experimental laboratory studies investigated exactly this—how quickly altera- Three-dimensional perspective view of Maat Mons (Venus) and its tion minerals would coat the surface of basaltic rocks and minerals in Figure 1 lava flows as reconstructed fromMagellan synthetic aperture radar contact with a Venus-simulated atmosphere (Berger et al. 2019; Cutler data combined with radar altimetry. Vertical exaggeration 22.5x. Image: NASA/ et al. 2020; Filiberto et al. 2020). In order to constrain the age of the Jet Propulsion Laboratory, Image Number: PIA00106. freshest lava flows at Idunn Mons, Filiberto et al. (2020) and Cutler et al. (2020) investigated how the oxidation coatings would affect the During the 2000s, the European Space Agency’s (ESA’s) Venus Express spectral signals of the basalt. The experimental results showed that orbiter shed new light on the style and age of Venusian volcanism. alteration and oxidation minerals form on the rock and mineral surfaces Through the atmospheric measurements made by the VIRTIS (visible on laboratory timescales—olivine, basaltic glass, and alkali-basalt alter and infrared thermal imaging spectrometer) instrument mounted on on timescales of weeks to months (Fig. 3). Pyroxene alters much more the Venus Express spacecraft, it was possible to derive the amount of infrared light emitted from a part of Venus’ surface (during its night- Figure 3 time) in the 1 µm spectral band. The signal from the 1 µm band corre- Backscattered electron lates mainly with the oxidation state and amount of iron in surface image of an experimentally oxidized and altered Venus analog 1 Lunar and Planetary Institute, USRA basalt. The white specks 3600 Bay Area Blvd. throughout the image Houston, TX 77058, USA are experimentally E-mail: [email protected] produced iron-oxide hematite. The hematite 2 Dipartimento di Ingegneria e Geologia (INGEO) preferentially forms on Università d’Annunzio glass, olivine, and the Viale Pindaro 42 iron-rich rims of 65127 Pescara, Italy pyroxene. Image: Cutler 3 Arctic Planetary Science Institute et al. (2020), Figure 2E. Rovaniemi, Finland.

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slowly. The spectral signal of oxidized basalt, glass, and olivine become However, in order to constrain the style and rate of volcanic resur- dominated by hematite even before a full coating forms on the surface facing during Venus’ geologic history, new missions would be needed of the sample (Cutler et al. 2020; Filiberto et al. 2020). Applying these to study the surface, subsurface, and atmosphere of Venus. Such experimental results to Venus’ lava flows suggests that any lava flow on missions are currently under review: NASA’s DANVICI+ and VERITAS, the surface of Venus with an infrared signature of fresh lava may only ESA’s EnVision, and the Roscosmos-NASA Venera-D. Any one of these be a few years old. However, the exact age of the lava flow (whether missions alone, or better yet in concert, would shed new light on the they are years or decades old) will depend on the exact mineralogy, mysterious geologic evolution of Venus. specifically the amount of pyroxene versus olivine or glass in the flow. Lava flows at other Venusian volcanoes (e.g., Maat Mons, Theia Mons, ACKNOWLEDGMENTS Rhea Mons) have radar properties consistent with being young, but AHT and JF received partial support from NASA SSW grant their emissivity signatures have not yet been measured (Brossier et al. 80NSSC17K0766. PD thanks the European Union for financial 2020). Some of these geologically young volcanic structures are also support through the Programma Operativo Nazionale Attraction and associated with extensional rift zones, based on their radar images International Mobility grant AIM1892731. This is Lunar and Planetary and physical properties (Fig. 4). Similar volcano–rift settings are fairly Institute (LPI) contribution no. 2597; LPI is operated by the Universities common on Earth: for example, the East African Rift system and the Space Research Association (Texas, USA) under a cooperative agreement Rio Grande Rift in the USA. Geologic investigations of the volcano- with the Science Mission Directorate of the National Aeronautics and tectonic interrelationships between Idunn Mons and the rift system Space Administration. in which it sits also suggest that volcanic and tectonic activity have alternated in time during the recent geologic past. This implies that REFERENCES Venus could also be seismically active (D’Incecco et al. 2020). Further Berger G and 7 coauthors (2019) Experimental exploration of volcanic rocks- suggestions of a geologically active Venus come from a recent study atmosphere interaction under Venus surface conditions. Icarus 329: 8-23, doi: on the geomorphology of corona structures (volcano-tectonic features 10.1016/j.icarus.2019.03.033 formed by the interaction between mantle plumes and the lithosphere), Brossier JF, Gilmore MS, Toner K (2020) Low radar emissivity signatures on Venus volcanoes and coronae: new insights on relative composition and age. Icarus 343, which suggested at least 37 represent active mantle plumes (Gülcher doi: 10.1016/j.icarus.2020.113693 et al. 2020). Cutler KS, Filiberto J, Treiman AH, Trang D (2020) Experimental investigation of oxidation of pyroxene and basalt: implications for spectroscopic analyses of the surface of Venus and the ages of lava flows. Planetary Science Journal 1, doi: 10.3847/psj/ab8faf D’Incecco P, López I, Komatsu G, Ori GG, Aittola M (2020) Local stratigraphic relations at Sandel crater, Venus: possible evidence for recent volcano-tectonic activity in Imdr Regio. Earth and Planetary Science Letters 546, doi: 10.1016/j. epsl.2020.116410 Filiberto J, Trang D, Treiman AH, Gilmore MS (2020) Present-day volcanism on Venus as evidenced from weathering rates of olivine. Science Advances 6, doi: 10.1126/sciadv.aax7445 Gülcher AJP, Gerya TV, Montési LGJ, Munch J (2020) Corona structures driven by plume–lithosphere interactions and evidence for ongoing plume activity on Venus. Nature Geoscience 13: 547-554, doi: 10.1038/s41561-020-0606-1 Smrekar SE and 7 coauthors (2010) Recent hotspot volcanism on venus from VIRTIS emissivity data. Science 328: 605-608, doi: 10.1126/science.1186785 Zolotov MY (2018) Gas–solid interactions on Venus and other solar system bodies. Reviews in Mineralogy and Geochemistry 84: 351-392, doi: 10.1515/ rmg.2018.84.10 

Magellan radar image of Beta Regio (Venus), which consists of Theia Figure 4 Mons (bright feature on the center bottom of the image) and Rhea Mons (the bright feature upper right of the image) and the associated Devana Chasma, a large central rift that runs through Rhea Mons and down to the south. North is to the top of the image. Image: NASA Goddard Space Flight Center, Image number: C3-14N300.

Elements 68 February 2021