Low radar emissivity signatures on Venus volcanoes and coronae: New insights on relative composition and age

Journal Pre-proof Low radar emissivity signatures on Venus volcanoes and coronae: New insights on relative composition and age

J.F. Brossier, M.S. Gilmore, K. Toner PII:

S0019-1035(20)30084-1

DOI:

https://doi.org/10.1016/j.icarus.2020.113693

Reference:

YICAR 113693

To appear in:

Icarus

Received date:

10 September 2019

Revised date:

31 January 2020

Accepted date:

12 February 2020

Please cite this article as: J.F. Brossier, M.S. Gilmore and K. Toner, Low radar emissivity signatures on Venus volcanoes and coronae: New insights on relative composition and age, Icarus(2020), https://doi.org/10.1016/j.icarus.2020.113693

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2020 Published by Elsevier.

Journal Pre-proof

Low radar emissivity signatures on Venus volcanoes and coronae: New insights on relative composition and age J. F. Brossier*, M. S. Gilmore, K. Toner Wesleyan University, Department of Earth and Environmental Sciences, Planetary Sciences Group, 265 Church St., Middletown, CT 06459, USA

Jo

ur

na

lP

re

-p

ro

of

*Corresponding author: Jeremy Brossier ([email protected])

1

Journal Pre-proof Keywords Venus

▪

Magellan

▪

Volcanism

▪

Composition

▪

Age

Jo

ur

na

lP

re

-p

ro

of

▪

2

Journal Pre-proof Highlights We characterize the radar emissivity of all major Venus volcanoes and coronae

▪

Three or more distinct ferroelectric or semiconductor minerals are represented

▪

Mineralogical variations could be associated with specific geodynamic environments

▪

Several edifices indicate recent volcanic activity on Venus

Jo

ur

na

lP

re

-p

ro

of

▪

3

Journal Pre-proof Abstract Multiple studies reveal that most of Venus highlands exhibit anomalously high radar reflectivity and low radar emissivity relative to the lowlands. This phenomenon is thought to be the result of atmosphere-surface interactions in the highlands, due to lower temperatures. These reactions are a function of rock composition, atmospheric composition, and degree of weathering. We examine the Magellan radar emissivity, altimetry and SAR data for all major volcanoes and coronae on Venus. We characterize and classify edifices according to the pattern of the variation of radar emissivity with altitude. The volcanic highlands can be classified into 7 distinct patterns

of

of emissivity that correspond to at least 3 discrete types of mineralogy based on the altitude

ro

(temperature) of the emissivity anomalies. The majority of emissivity anomalies support the hypothesis of a weathering phenomenon at high altitude (>6053 km), but we also find strong

-p

emissivity anomalies at lower altitudes that correspond spatially to individual lava flows,

re

indicating variations in mineralogy within an evolving volcanic system. The emissivity signature of tallest volcanoes on Venus are consistent with the presence of ferroelectric minerals in their

lP

rocks, while volcanic edifices in western Ishtar Terra and eastern Aphrodite Terra are consistent with the presence of semiconductor minerals. Sapas Mons and Pavlova Corona are also

na

consistent with ferroelectrics, but at a different Curie temperature than the other volcanoes in Atla Regio. The spatial distribution of radar emissivity classes correlates to different geologic

ur

settings indicating that different mantle source regions (deep/shallow plumes, and possible convergence zones) may contribute to differences in mineralogy for the studied edifices. Finally,

Jo

we show that the emissivity signatures of Idunn, Maat and other volcanic edifices are consistent with relatively fresh and unweathered rocks, indicating recent or possibly current volcanism on Venus.

1. INTRODUCTION NASA‟s Magellan radar images of Venus reveal a dominantly volcanic surface with an average crater retention age of around 300 Ma – 1 Ga (Strom et al., 1994; McKinnon et al., 1997) that can be broadly divided into three major types of terrain broadly spanning three chronological eras of Venus‟ history based on their stratigraphic position. First, the heavily deformed terrain named tesserae (Ivanov and Head, 1996) are considered as the oldest materials on Venus. Then, volcanic plains buried ~80% of the planet. Finally, large volcanoes forming clusters are the most 4

Journal Pre-proof recent terrains in Venus history (Price and Suppe, 1994; 1995). While the composition of the volcanic plains is inferred to be basaltic (e.g., Weitz and Basilevsky, 1993), the composition of the tesserae and the large volcanoes are still open questions. Pioneer Venus and Magellan data show that many of Venus‟ highlands display unusual increases in radar reflectivity (Ford and Pettengill 1983) and thus decreases in radar emissivity at their summits (Pettengill et al., 1992). These changes are ascribed to the presence of a high dielectric mineral at high elevation, where chemical weathering reactions between the rocks and the near-

of

surface atmosphere are facilitated by the lower temperatures. It is expected from theory that materials with elevated dielectric constants will enhance their radar reflectivity and lower their

ro

radar emissivity (Pettengill et al., 1992; Campbell, 1994). Candidate high dielectric minerals include (1) ferroelectric minerals, such as chlorapatite or perovskite, included in the rocks that

-p

become conductive at low temperature (Arvidson et al., 1994; Shepard et al., 1994; Treiman et

re

al., 2016); (2) the precipitation of the high dielectric mineral pyrite produced through sulfidation and/or oxidation of iron (Pettengill et al., 1988; Klose et al., 1992; Wood and Brett, 1997), or (3)

lP

coatings formed by condensation onto the rock as „metallic frosts‟ like lead or bismuth sulfides (Brackett et al., 1995; Pettengill et al., 1996; Schaefer and Fegley, 2004). These reactions are a

na

function of rock composition, atmospheric composition, temperature, and length of the reaction, or surface age. Because of the temperature dependence of these reactions, the detailed

ur

description of the variations in radar emissivity with altitude may yield insight into rock and

Jo

atmosphere characteristics.

Here, we characterize the variation of radar emissivity with altitude and thus temperature, in order to compare similarities and differences among the venusian highlands. Such a study was done by Klose et al. (1992) who compared selected volcanoes, mountains and tesserae and recognized differences in the style, magnitude and altitude of emissivity variations between and among these regions. Treiman et al. (2016) also notes similar differences in the radar emissivity signatures between Ovda Regio tessera and Ishtar Terra. Both studies ascribed these variations to differences in the composition, volume and type of dielectric materials in these regions. In this study, we examine all the major volcanoes and coronae on Venus. Coronae are irregular to circular structures characterized by a complex interior zone occupied by volcanic flows, domes and tectonic ridges. Like volcanic rises, they are inferred to be surface manifestations of mantle 5

Journal Pre-proof plumes (Pronin and Stofan, 1990; Stofan et al., 1992). The radiophysical behaviors of volcanoes and coronae may help to retrieve, or at least constrain, their relative composition and age. A complementary study focusing on tesserae and mountain belts is under preparation in a followup paper. The present paper is organized in the following way. We first describe the changes in radar emissivity with altitude for the studied regions in order to assess in detail the unusual radiophysical signatures seen in the high elevated areas of Venus (section 3). This helps to

of

determine whether the material measured on these regions has the behavior consistent with that of known substances, such as ferroelectric or semiconductor minerals (section 4.1). Finally, we

ro

consider whether emissivity variations are related to the age (section 4.2) or location (section

-p

4.3) of particular edifices.

re

2. DATA & METHODS

The regions of interest are selected using radar datasets compiled during the Magellan mission

lP

(frequency = 2.4 GHz, λ = 12 cm). We mapped 35 large volcanoes and 15 coronae with the Cycle 1 left-looking Synthetic Aperture Radar (SAR) images (FMAPS) produced at a resolution

na

of 75 m per pixel. All volcanic edifices are mapped by including the boundary of their major flows. The main characteristics of the selected volcanoes and coronae are reported in Tables 1

ur

and 2, respectively (name, coordinates, highest altitude, and height). Our regions have heights ranging from 1.4 km (Melia Mons) to 8.9 km (Maat Mons) for the volcanoes, and from 1.3 km

Jo

(Otygen Corona) to 5.8 km (Atahensik Corona) for the coronae. Heights given throughout this paper may differ from those found in the past literature as our values are measured from a mean planetary radius taken as 6051.80 km (MPR, Ford and Pettengill, 1992). [Insert table 1] [Insert table 2] We derived altimetry and emissivity from the Magellan global topography data records (GTDR) and global emissivity data records (GEDR). Altimetry data have resolution ranging from ~10 km at periapsis (ca. 10˚N latitude) to ~20 km near the poles (ca. 90˚N and 70˚S) where the orbiting spacecraft was high above the planet. Emissivity data were collected while the spacecraft was operating in radiometer mode. The spatial resolution of the emissivity data varies from ~20 km 6

Journal Pre-proof near periapsis to ~80 km at high latitudes (Pettengill et al., 1991). Near-global mosaics are produced in the GTDR data product [footnote 1] and the GEDR data product [footnote 2] that are publicly available through the USGS „Map-a-Planet‟ websites. The two mosaics have a spatial resolution of 4.6 km per pixel (scale of 22.7 pixel per degree). Altimetry and emissivity data are extracted from these mosaics to return scatterplots of the variation of emissivity with altitude for each region, as previously done in Klose et al. (1992). Our study includes all volcanic features that reach above 6053 km altitude, or 1.2 km above the

of

MPR. Interpretation of the SAR and emissivity data for regions below this altitude is made difficult by the complex effects of surface roughness and dielectric constant on emissivity and

ro

reflectivity (Campbell, 1994). Above 6053 km, we assume that very low emissivity signatures account for a signal dominated by dielectric constant, not surface roughness (Plaut, 1993).

-p

Moreover, because surface roughness is predicted to have a much greater influence on

re

reflectivity values than on emissivity (Robinson and Wood, 1993), we use emissivity data rather than reflectivity data to characterize the changes in dielectric properties across our regions of

lP

interest as a guide to the bulk composition and degree of weathering of the surface materials. Mapping and extraction processes are performed using the ArcGIS 10.6 (ESRI) software

na

package, while the plots are produced with RStudio software. We also calculate temperatures as a function of elevation using the Vega 2 lander data (Seiff et al., 1987; Lorenz et al., 2018). To

ur

date, the Vega 2 lander is the only probe that has returned in-situ measurements of the

Jo

temperature below 12 km down to the surface. For the first 12 km above MPR we find a best fit of the planetary radius (PR, km) vs. temperature (T, K) dependence of PR = - 0.1192 × T + 6139.5 with R2 above 0.99 (see Table S1 in the supplemental material). These temperatures are essential to test the stability and reactivity of candidate minerals under Venus conditions. 3. RESULTS 3.1. Emissivity variations with altitude and classification scheme As reported in Klose et al. (1992), the radar emissivity of many highland regions decreases with altitude from a global mean value of ~0.8 to values as low as 0.3. We define an emissivity excursion as the region on an emissivity – elevation plot (Figures 1 and 2) where radar emissivity declines and becomes distinct from values seen at lower elevation that cluster at the global plains 7

Journal Pre-proof average (0.8 – 0.9). In our study, almost all the regions of interest show such excursions: 28/35 volcanoes and 10/14 coronae. The elevations and magnitudes of the excursions are variable from one region to another (Figure 3). We observe different magnitudes and behaviors of the excursions (last column in Tables 1 and 2): (i) a strong decrease where emissivity declines to low values ranging from 0.7 to 0.3, (ii) a subtle decrease where emissivity reaches slightly lower values but remains above 0.7, or (iii) no changes where emissivity is nearly constant with elevation.

of

[Insert figure 1]

ro

[Insert figure 2]

-p

[Insert figure 3]

The relationship between emissivity and elevation allows us to group the regions sharing similar

re

patterns. Figure 4 is a flow chart illustrating our classification process. This classification is primarily based on the presence of an emissivity excursion, its magnitude, its elevation and the

lP

elevation of the volcano or corona itself. Each volcanic feature is therefore classified as one of 7 groups, each of which refers to a unique pattern, as summarized in Tables 3 and 4. Edifices in

na

groups 1 to 5 have an emissivity decrease with elevation starting below 6053 km. In groups 6 and 7, no excursions are observed and the emissivity stays relatively constant from the lowlands

ur

to an altitude of 6055 km, marking the transition to the highlands. Groups 1 to 3 include regions having strong excursions with emissivity values dropping below 0.7, while groups 4 and 5

Jo

consist of regions with only weak reductions where the lowest emissivity remains between 0.7 and 0.8. The emissivity excursion is gradual in groups 1 and 2, while it is more precipitous in group 3. For the groups 4 and 5 with subtle excursions, we subdivide the volcanoes and coronae that reach an altitude over 6055 km (group 4) from the others standing below (group 5). We make a similar distinction for group 6 having altitude over 6055 km and group 7 being below 6055 km; this altitude marks the beginning of the sharp decline of emissivity seen in Maxwell Montes (Klose et al., 1992; Treiman et al., 2016). Features that lie above 6055 can be compared to Maxwell Montes, which warrants this distinction. Figure 5 shows the distribution of the regions of interest, where the volcanoes and coronae are color-coded according to our

8

Journal Pre-proof classification. The location of major mountain belts and tesserae are included as mapped by Ivanov and Head (1996) for reference purposes. [Insert figure 4] [Insert table 3] [Insert table 4] [Insert figure 5]

of

The tallest volcanoes on Venus are included in group 1 (dark blue in Figure 5): Maat and Ozza montes (Atla Regio), Theia and Polik-mana montes (Beta Regio), Yunya-mana Mons (Phoebe

ro

Regio), and Tepev Mons (Bell Regio). Their emissivity decreases smoothly with elevation to a

-p

minimum value at ~6056-6057 km, and then increases abruptly at their highest elevations (Figure 1). This pattern is fainter for Polik-mana, Yunya-mana and Tepev montes due to fewer

re

data points at the summit. Mapping of these volcanoes reveals that they may have multiple emissivity excursions. Ozza Mons has an emissivity high of 0.88 at its summit and exhibits three

lP

emissivity excursions (e) at lower elevations: an excursion of 0.36 at 6055.7 – 6056.7 km (e1-2) and 0.53 at 6052.5 km (e3) (Figure 1). Maat Mons, Ozza Mons‟s neighbor, also has multiple

ur

(Figure 1).

na

emissivity lows with values varying from 0.53 at 6056.2 km (e1) to 0.69 at 6052.7 km (e4)

Volcanoes and coronae from group 2 (light blue in Figure 5) also have emissivity decreasing

Jo

gradually with elevation, but unlike group 1 they do not reach altitudes of 6057 km. Group 2 includes Gula montes (Figure 1) and Maram Corona (Figure 2). Gula Mons in western Eistla Regio (Figure 1) displays a strong excursion where emissivity gently decreases with elevation to values as low as 0.55 at an altitude of 6055.4 km. Maram Corona located near Parga Chasma (Figure 2) shows a gradual decline to emissivity values of 0.51 at an altitude of 6055.4 km. Unlike the gradual decreases seen in the groups 1 and 2, the volcanoes and coronae from group 3 (green in Figure 5) have sharper decreases with elevation, examples include Sapas Mons (Figure 1) and Pavlova Corona (Figure 2). Sapas Mons located near Atla Regio exhibits a sharp decline to emissivity values of 0.40 at an altitude of 6054.6 km. Similarly, Pavlova Corona from eastern Eistla Regio has an excursion as low as 0.47 at an altitude of 6053.8 km. An approach to

9

Journal Pre-proof quantitatively distinguish group 2 (gradual decline) from group 3 (sharp decline) is to determine the minimum elevation at which emissivity reaches 0.7 (Table 5). In group 2, an emissivity low of 0.7 is reached at an elevation of 6054.2 km (± 0.2 km), while group 3 reaches this value at 6053.4 km (± 0.3 km). [Insert table 5] Groups 4 and 5 include volcanoes and coronae that show slight drops in emissivity with elevation (purple and pink in Figure 5, respectively). Idunn and Otafuku montes, forming group

of

4, reach elevations over 6055 km and have emissivity excursions starting at ~6053 km (Figures 1 and 2). For instance, Idunn Mons from Imdr Regio displays a subtle drop in emissivity with

ro

values as low as 0.77 at an altitude of 6055.2 km. Unlike group 4, regions from group 5 stand

-p

below 6055 km, such as Sif and Tuulikki montes (Figure 1). Sif Mons from western Eistla Regio has a subtle decrease with low emissivity of 0.73 reached at 6054.4 km. Tuulikki Mons located

re

between Beta and Phoebe regiones also has a subtle drop in emissivity of 0.76 peaking at 6053.4

lP

km.

Group 6 comprises all volcanoes and coronae having emissivity values that remain constant with

na

elevation until altitudes of ~6055 km (red in Figure 5). Some of these landforms have a decline in emissivity values above this altitude and are classified as group 6a. Emissivity starts to drop

ur

for Atahensik Corona (in Diana-Dali chasmata) with values reaching 0.56 at an elevation of 6056.6 km (Figure 2). Artemis, Ceres and Miralaidji coronae show subtle excursions with values

Jo

as low as 0.76-0.78 (Figure 2). Colette and Sacajawea paterae from Lakshmi Planum do not show any excursions where emissivity remains above 0.8, even after reaching 6055 km of altitude, and thus constitute group 6b (Figure 1). Finally, Group 7 contains all volcanoes and coronae that are standing at elevations below 6055 km and show no changes with elevation (yellow in Figure 5), such as Kunapipi Mons (Figure 1) and Eve Corona (Figure 2). Tables 3 and 4 report the lowest emissivity with corresponding mean elevation and temperature for every excursion observed; these values are plotted in Figure 3. We also plotted the minimum emissivity and maximum elevation values reached for all regions with no excursions below 0.8 (groups 6b and 7). 3.2. Maps of the emissivity excursions 10

Journal Pre-proof 3.2.1. Volcanoes Figures 6 and 8 are maps of volcanoes where emissivity is overlain onto SAR images. In these maps, bluish patches represent low emissivity excursions with values below 0.7. The majority of volcanoes (20/35) generally have the lowest emissivity values at their summits (Figures 6-8). This supports the temperature-based reactions proposed in previous studies (e.g., Klose et al., 1992; Pettengill et al., 1992). There are several exceptions to this behavior where the emissivity excursions occur at lower elevations and correspond to mappable geologic units.

of

The tallest volcanoes (group 1) are Maat, Ozza, and Theia montes (Figure 6), as well as Yunyamana and Tepev montes (Figure 7). Their summit regions have high emissivity values

ro

comparable with the global plains average of ~0.85 (e.g., Pettengill et al., 1992). They have

-p

relatively low SAR backscatter (i.e. appear dark in SAR images) and are found at elevations above 6057 km. In Figure 6, Maat Mons displays extensive lava flows overlying almost all of its

re

flanks and its summit region, with emissivity values reaching 0.97. Ozza Mons has a summit plateau, while Theia Mons has a summit caldera (Senske et al., 1992; Robinson and Wood,

lP

1993). These two summits have emissivity values as high as 0.88 and 0.83, respectively (Figure

na

6).

Yunya-mana and Tepev montes (also in group 1) barely reach 6057 km of altitude and only

ur

display subtle returns to higher emissivity at their summits (Figure 7). Tepev Mons has two summits topping at different altitudes (Campbell and Rogers, 1994). Its western summit stands

Jo

below 6057 km, while its eastern summit stands at ~6057 km and shows high emissivity value of 0.77 (Figure 7, also in Robinson and Wood, 1993). The summit of Yunya-mana Mons is a plateau standing slightly below 6057 km that has an emissivity maximum of 0.68 (Figure 7). Similar high emissivity areas are seen at high elevations, and are comparable to the summits of Ozza and Theia montes (yellow arrows in Figure 6). They are both located along the crest of Dali Chasma, one of the rift zones converging into Atla Regio (Campbell et al., 1999). Sapas Mons (group 3) and Ushas Mons (group 5) also undergo a similar upturn to near-normal emissivity values on their summit regions (Figures 7), as seen in the scatterplots (Figure 1). The summit region of Sapas Mons consists of two scalloped-margin collapsed domes (Guest et al., 1992), and they both present a slight increase in emissivity with values reaching 0.69 in the northern dome and 0.71 in the southern dome (Figures 7). As for Ushas Mons, the summit is a 11

Journal Pre-proof steep-sided dome that culminates at 6053.7 km, and shows a maximum emissivity of 0.87 (Figure 7). Low SAR backscatter values (and high emissivity values) observed at the peaks of the volcanoes in group 1 could be explained by several reasons: (1) variations in compositional properties of the rock at the summits related to temperature (elevation) (e.g., Palazzari et al., 1995; Campbell et al., 1999); or (2) mass wasting from areas of steep domes resulting in fragmental material that reflects radar poorly (e.g., Keddie and Head, 1994; 1995). The dark areas associated with the group 1 have an emissivity pattern with altitude that is consistent with the temperature-based

of

weathering reactions. Conversely, Sapas and Ushas montes have emissivity highs associated

ro

with fine-grained deposits that have flowed or fallen as debris from their summit domes (Keddie

-p

and Head, 1994; 1995).

Maat and Ozza montes display multiple emissivity excursions at low elevations that correspond

re

to mappable flows with a particular morphology (Figure 6). These excursions are labeled e1 to e4 in Table 3 and in the scatterplots from Figure 1. On Maat Mons, three low emissivity

lP

excursions (e1-e3) are found within a lava field lying on the southwest flank of the volcano, and another excursion (e4) at the end of a lava flow to the northwest of Maat Mons. On Ozza Mons,

na

two excursions (e1-e2) are located near the summit and correspond to a lava complex covering the east and northeast flank of the volcano, and another excursion (e3) at the end of a lava flow

ur

to the north of Ozza Mons. A single excursion is found on Theia, Yunya-mana and Tepev

Jo

montes, and is located near their summits. All these low emissivity excursions correspond to mottled bright flows, while the surrounding flows with higher emissivity values have moderate to low SAR backscatter (Senske et al., 1992). [Insert figure 6] [Insert figure 7] [Insert figure 8] 3.2.2. Coronae The coronae also have emissivity excursions found at their highest elevations (e.g., Stofan et al., 1992; Stofan, 1995). Most of the emissivity excursions found on the coronae occur on their highest elevation ridges (annuli), such as Maram, Didilia, Pavlova, and Atahensik coronae in 12

Journal Pre-proof Figure 9. Maram Corona (group 2) has an excursion of 0.51 on its northern rim, a topographic ridge rising to 6056 km. Didilia and Pavlova coronae (group 3) have annular rims that also display low emissivity values of 0.64 and 0.47 on their most elevated ridges, with altitudes of 6052.6 km and 6052.9 km, respectively. Atahensik Corona (group 6) has an emissivity low of 0.56 on its northern rim, a topographic ridge rising above 6057 km. Kaltash Corona (group 2) is a 2 km-deep depression marking the boundary between Manatum and Ovda tesserae. This region displays a low emissivity excursion of 0.45 located on the

of

western and eastern flows extending from the corona annulus (Figure 9). Some of the selected coronae have radially fractured domes in their interiors, such as Maram,

ro

Didilia and Pavlova coronae. These domes are also called novae, and display subtle emissivity

-p

declines (red arrows in Figure 9, and Table 4). Maram‟s nova is found close to the northern rim of the corona annulus, and it reaches an altitude of 6054 km (Krassilnikov and Head, 2003) with

re

a minimum emissivity of 0.75. In eastern Eistla Regio, the novae located within Didilia and Pavlova coronae reach altitudes of 6054.0 km and 6054.7 km, respectively. They are more

lP

elevated than the highest ridges of the corona annuli (Aittola and Kostama, 2002). They both

na

display subtle declines in emissivity at their peaks, with values as low as 0.76 – 0.77. [Insert figure 9]

ur

4. DISCUSSION

Jo

4.1. Diverse mineralogy

The anomalous radar emissivity signatures seen on Venus highlands were first recognized in the Pioneer Venus radar data (Ford and Pettengill, 1983), and then confirmed by the data returned by the Magellan spacecraft (Tyler et al., 1991; Klose et al., 1992; Pettengill et al., 1992). There is general consensus that high radar reflectivity and low radar emissivity signatures are caused by the presence of high dielectric minerals, but the specific mineral(s) remain unclear. One of the first strong contenders is the semiconductor mineral pyrite FeS2 as a weathering product of basaltic rock interacting with the Venus atmosphere (Pettengill et al., 1988; Klose et al., 1992; Wood, 1997). Pyrite has a high enough dielectric constant to explain the signatures detected in the highlands (Pettengill et al., 1988). Pyrite is predicted to be stable at cooler high elevations, while iron oxides (e.g., magnetite Fe3O4 or hematite Fe2O3) with higher radar emissivity are 13

Journal Pre-proof predicted to be stable at warmer low elevations (Wood, 1997). In this model, the transition from iron oxide to pyrite would result in an abrupt change in emissivity. However, Fegley and colleagues argued against the stability of pyrite on the venusian surface (Fegley et al., 1995a;b; Fegley, 1997). Based on their thermochemical calculations and experiments, they claimed that iron sulfides present in fresh volcanic rock would be unstable under Venus conditions and should decompose rapidly to iron oxides and sulfur vapor. More recent studies tend to reconsider the stability of pyrite under highlands conditions (Wood and Brett, 1997; Hashimoto and Abe, 2005; Kohler et al., 2014; 2015; Port et al., 2016; Berger et al., 2019). The disparity between these

of

works reflects the poorly constrained composition of Venus‟s deep atmosphere (e.g., abundances

ro

of gases or oxidation rates). Alternatively, some studies propose other plausible candidates, such as ferroelectric compounds (Arvidson et al., 1994; Shepard et al., 1994), or a coating of volatile

-p

metallic vapors halides and sulfides emitted from volcanoes also called „metal frosts‟ (Brackett et al., 1995; Pettengill et al., 1996; Schaefer and Fegley, 2004). All of the elements and

re

compounds involved in the metallic frost model are semiconductors and have high dielectric

lP

constants (e.g., elemental tellurium Te, lead Pb, galena PbS, and bismuth Bi sulfides). Thus, the best candidates responsible for the low emissivity observed on most venusian summits

na

are likely to be ferroelectric and/or semiconductor minerals derived from surface – atmosphere interactions. In the following sections we discuss the elevation – emissivity patterns obtained in

ur

the different groups described in section 3.1, and determine whether they are consistent with the

Jo

behavior of known materials.

4.1.1 Strong emissivity excursions (groups 1-3) With the exception of Maat Mons, the tallest volcanoes on Venus (group 1) have a steady, gradual trend of decreasing emissivity with increasing elevation, at elevations above ~6056 km the emissivity jumps sharply to higher values characteristic of the plains (Figures 1 and 6). This behavior has been noted in studies of Ovda Regio (Arvidson et al., 1994; Shepard et al., 1994; Harrington and Treiman, 2015; Treiman et al., 2016). According to these studies, the variations in radar properties in Ovda Regio, and hence the tallest volcanoes in our study, are consistent with the presence of ferroelectric minerals in the rocks. A ferroelectric mineral is a substance that undergoes a phase transition when it reaches a certain temperature, also called Curie temperature, where its dielectric constant increases strongly. As the temperature rises above the 14

Journal Pre-proof Curie temperature (lower elevation on Venus), its dielectric constant gradually declines to normal values (Shepard et al., 1994). On the tallest volcanoes, the transition occurs at around 6056 km, and hence the Curie temperature would be ~700 K (Table 2), as derived from the temperature profile of the Vega 2 lander (Seiff et al., 1987; Lorenz et al., 2018, see Table S1). The temperature of the emissivity excursion is a function of composition, while the magnitude of the excursion is a function of volume of the ferroelectric (Shepard et al., 1994). Ferroelectric compounds are good candidates for these regions as they are expected to be SAR-bright at the critical altitude and to become SAR-dark at the highest elevations (Arvidson et al., 1994;

of

Brackett et al., 1995). This is coherent with the clear transition in SAR backscatter seen on Ozza,

ro

Theia, Yunya-mana and Tepev montes (Figures 6 and 7, see also Palazzari et al., 1995; Campbell et al., 1999). The lack of low emissivity, SAR-bright flows on Maat Mons, except for

-p

the stratigraphically older lava field on the southern flank, has been ascribed to young, unweathered extensive lava flows covering most of its flanks and summit region (Senske et al.,

re

1992; Robinson and Wood, 1993). More details about the possible recent activities of Maat Mons

lP

is discussed in section 4.2.

A good ferroelectric candidate would be chlorapatite, Ca5(PO4)3Cl (Treiman et al., 2016), as it is

na

a common mineral whose transition from ferro- to paraelectric occurs at temperatures found on the surface of Venus. This phase transition is predicted to happen at a temperature between 675

ur

and 775 K (Rausch, 1976), or around 695 ± 4 K (Hitmi et al., 1984). Additionally, the perovskites and pyrochlores were proposed by Shepard et al. (1994) as potential ferroelectric

Jo

candidates, since they have Curie temperatures ranging from 0 to 760 K (Lines and Glass, 1977). These values are comparable with the temperatures obtained for our regions (Table 2). Edifices in group 2 also show a gradual decline in emissivity with elevation, however, we do not observe any return to high emissivity values at their summits. We suggest that these volcanoes and coronae are also consistent with the ferroelectric behavior and mineralogy of the first group, although they are not tall enough to reach the Curie temperature (~700 K) of that mineral. Unlike the gradual decline seen in the groups 1 and 2, edifices in group 3 have a more precipitous drop in emissivity with elevation. Their excursions mainly peak at lower elevations, below 6055 km (Figure 3), and like group 2, lack a return to high emissivity at high elevation. Because of this, we cannot rule out that the rocks in groups 2 and 3 may be comprised of high 15

Journal Pre-proof dielectric (low emissivity) minerals, other than ferroelectrics. However, the shape of the emissivity excursion in both groups is compatible with the presence of ferroelectric compounds: group 2 matches the pattern seen in group 1 at similar elevations and the sharpness of the excursion in group 3 is typical of ferroelectric behavior. If ferroelectric, group 3 materials correspond to Curie temperatures significantly higher than that modeled for group 1, reaching ~720 K (Figure 3). This pattern could be explained by several reasons: (1) group 3 rocks may contain a different mineral than groups 1 and 2 that has a higher Curie temperature, and hence a transition occurring at lower elevation; (2) group 3 rocks may contain the same mineral as in

of

groups 1 and 2 but with subtle differences in composition, and hence varying the transition

ro

temperature; or (3) groups 1-3 rocks have the same mineralogy, but underwent different atmospheric conditions during the surface weathering (e.g., a colder environment would lower

-p

the transition elevation).

re

These possible scenarios can also account for the multiple excursions observed at low elevations on Ozza and Maat montes (group 1). It is possible that the mineralogy of each flow is different

lP

either because it contains a different mineral or a single mineral with slightly different composition. For example, Shepard et al. (1994) report that minor change of the Pb abundance in

na

a (Pb, Ca)TiO3 perovskite can increase/decrease the Curie temperature (Rupprecht and Bell, 1964). More precisely, a 1% change in the Pb abundance changes the Curie temperature of ~8 K,

ur

corresponding to a 1 km change in the transition altitude. Treiman et al. (2016) posits that differences in anion composition (proportions of F-, OH-, Cl-) or cation composition

Jo

(substitution of Sr or rate Earth elements for Ca) in a Ca5(PO4)3(OH, F, Cl) apatite can also change the Curie temperature. Importantly, they point out that chlorapatite is ferroelectric and thus the F:Cl ratio will control the Curie temperature where apatite with a larger F:Cl ratio would require higher temperatures (lower elevations) to exhibit a high dielectric constant (Rausch, 1976). The association of the high dielectric compounds with individual lava flows suggest that minerals in the rocks control the emissivity signature, as opposed to changes in atmospheric conditions. 4.1.2. Subtle emissivity excursions (groups 4-5) Volcanoes from group 4 are tall enough to record the strong decrease in emissivity seen in groups 1-3: Idunn Mons reaches 6055.3 km, while Otafuku summits at 6056.4 km. The gentle 16

Journal Pre-proof decrease of emissivity observed in both volcanoes (Figure 1) relative to the other regions of the same elevation requires that they contain a lower volume of high dielectric material than groups 1-3. This can be because they comprise lavas with lower amounts of dielectric material, or because they have relatively young surface, where the fresh rocks have not had time to react with the atmosphere to produce high dielectric products. Based on independent evidence of young volcanism at Idunn (Smrekar et al., 2010), we favor the latter interpretation as discussed in the next section (see section 4.2). Like group 4, edifices in group 5 also display subtle decline in emissivity with increasing

of

elevation. Most of them have emissivity excursions at low elevations (≤6053 km) (Figure 3),

ro

where emissivity is more likely attributable to variations in surface roughness changes rather than dielectric properties (e.g., Pettengill et al., 1992; Campbell, 1994). Only Sif and Kali

-p

montes have gentle declines in emissivity occurring at sufficiently high elevation, above 6054

re

km (Figure 3). Both volcanoes could have young, unweathered lava flows, like Idunn and Otafuku montes (group 4). It seems unlikely to be the case because of the occurrence of impact

lP

craters superimposed onto their flanks (Mc Gill, 2000; Copp and Guest, 2007), indicating that they are rather old and presumably dormant (Senske et al., 1992). We therefore propose that Sif

na

and Kali montes are comparable with the edifices from groups 1 and 2, but their heights are too low to reach through their altitude that corresponds to the Curie temperature for those groups.

ur

This interpretation was also suggested for Sif Mons by Klose et al. (1992).

Jo

Novae found within Maram, Pavlova and Didilia coronae share similar emissivity patterns as Sif and Kali montes. Indeed, they reach altitudes above 6054 km and only present subtle declines in emissivity with values down to 0.75-0.77 (Table 4 and Figure 3). This indicates that the nova materials in these regions could be made of a different composition from that of the „hosting‟ coronae (groups 2 and 3). Based on the stratigraphic and topographic relationships of these novae and the annuli of Pavlova and Didilia coronae, as well as the model of evolution predicted for such corona-nova regions (Aittola and Kostama, 2002), we suggest that Pavlova‟s and Didilia‟s novae are relatively young. Like Maat, Idunn and Otafuku montes, the possible youth of these two novae is discussed in the next section (section 4.2). 4.1.3. No emissivity changes below 6055 km (groups 6-7)

17

Journal Pre-proof Edifices in group 6 show no emissivity variations below 6055 km. Different emissivity behaviors are observed above this altitude (see scatterplots in Figures 1 and 2). Colette and Sacajawea paterae rise above 6055 km but barely reach 6056 km, whereas Artemis, Ceres and Miralaidji coronae stand slightly higher (6056 - 6057 km) and have subtle variations in emissivity. Only Atahensik Corona reaches an altitude (above 6057 km) that is high enough and exhibits a large decline in emissivity. Atahensik‟s pattern is similar to that of Maxwell Montes at comparable elevation range (Klose et al., 1992; Treiman et al., 2016). Klose et al. (1992) and Treiman et al. (2016) showed that Maxwell‟s emissivity remains constant with altitude up to 6055 km, and then

of

sharply declines at its highest elevations. Although the coronae from this group reach elevations

ro

above 6056 km, they do not display increases in emissivity at the highest elevations, unlike the volcanoes from the group 1. This behavior does not seem in agreement with that of a

-p

ferroelectric substance. This is rather consistent with the presence of a semiconducting material. The semiconductor materials could be inherent to the rock like pyrite (Pettengill et al., 1988;

re

Klose et al., 1992; Wood, 1997), or they could be deposited from the atmosphere as „frosts‟ like

lP

lead, lead-bismuth sulfosalts or tellurium (Brackett et al., 1995; Pettengill et al., 1996; Schaefer and Fegley, 2004). Presence of semiconductors on Venus has already been suggested from radar

na

bistatic observations conducted during Magellan and Venus Express missions over Maxwell Montes in Ishtar Terra (Pettengill et al., 1996; Simpson et al., 2009). Alternatively, the lack of change in emissivity found on Colette and Sacajawea paterae could be explained by the presence

ur

of young, unweathered lava flows and/or by the absence of high dielectric minerals in their

Jo

rocks. It has been suggested that both edifices might be formed during the latest stage of Lakshmi Planum evolution (e.g., Roberts and Head, 1990a,b; Hansen, 1995; Ivanov and Head, 2008).

Volcanoes and coronae of group 7 do not display an emissivity excursion, but their elevations are comparable to regions in groups 2 and 3 reaching elevations between 6053 and 6055 km (Figure 3). This pattern could be explained by several reasons: (1) group 7 rocks may contain the same mineral as group 3 but have young and unweathered rocks; (2) group 7 rocks may contain the same mineral than groups 1 and 2 but the edifices are too short to have a strong decline in emissivity; or (3) they may have the same composition as group 6, but their elevation is too low. 4.1.3. Summary 18

Journal Pre-proof The changes in the emissivity with altitude for volcanic systems on Venus indicate the presence of at least three different high dielectric minerals: (1) those with ferroelectric behavior at a Curie temperature of ~700 K and altitude of ~6056 km (groups 1 & 2), (2) those with ferroelectric behavior at a Curie temperature of ~720 K and altitude of ~6054 km (group 3), (3) those with semiconductor behavior (group 6a). Most edifices of the remaining groups (groups 6b and 7) are tall enough to not be in groups 1-2 or 3, while some (e.g., Melia and Otygen) are too short to determine to which of these (or other) classes they may belong. Because these classes span a range of locations, these differences are more likely to be related to differences in rock

of

mineralogy rather than to atmospheric variability.

ro

Whatever the high dielectric mineral(s) are, it is clear from this survey that they are common to volcanic systems on Venus. If the composition and temperature of the atmosphere of Venus has

-p

been constant over the timescale of the surface-atmosphere reaction, this requires that the

re

minerals involved in the reaction are ubiquitous. This is further supported by the fact that similar emissivity excursions are found on tesserae and mountain belts (Treiman et al., 2016; Gilmore et

lP

al., 2019). The fact that some excursions are clearly spatially related to individual flows favors models where the minerals are included in the rocks themselves, as opposed to formation via

ur

4.2. Relative age

na

vapor deposition at a common altitude (temperature), at least in these cases.

Maat, Otafuku, and Idunn montes, as well as the novae within Pavlova and Didilia coronae have

Jo

higher emissivity values at elevations where other edifices have low emissivity. This pattern is consistent with the presence of young and unweathered materials relative to the other regions studied here.

4.2.1. Maat, Otafuku and Idunn montes The extensive occurrence of high emissivity values on the summit of Maat Mons and on most of its flanks (Figures 1 and 6) has been ascribed to lava flows that are so young that their exposure was insufficient to produce the high dielectric material (Klose et al., 1992). Maat Mons was therefore interpreted as one of the most recent edifices in Atla Regio, and presumably the youngest volcano on Venus. Figure 10 (left panel) combines a sketch map and an elevationemissivity plot for Maat Mons allowing the correlation of emissivity patterns with geological 19

Journal Pre-proof landforms and their stratigraphic relationships as seen in SAR images. Maat Mons has been subdivided into 3 major flow units, simplified from Keddie and Head (1994): the early-stage flows (in orange), followed by flows of an intermediate age (in blue), and finally the late stage flows extending from the summit (in red). The left side of the plot (in blue) matches the decline in emissivity seen in other group 1 volcanoes, in addition to the multiple excursions similar to those seen on Ozza Mons (Figure 1). These emissivity values correspond to the SAR-bright lava flows that have weathered to low emissivity materials. The right side of the plot (in orange and red) is different and shows a more distributed pattern of data points climbing to an altitude of 9

of

km with an average emissivity of ~0.85, as also noted in Klose et al. (1992). Unlike the blue

ro

region, the orange and red regions – in plot and sketch map – correspond to lava flows with moderate to low backscatter. The lower part of the right side is associated to the early-stage

-p

flows (in orange), while the upper part correlates with the late-stage flows postdating all previous flows (in red). The first and second excursions (e1 and e2, Figure 10) seen in the blue region

re

would correspond to group 1, while the third excursion (e3, Figure 10) would be associated with

lP

group 3. The orange region (early stage flows) has an excursion below 6053 km (e4, Figure 10) that could be related to surface roughness or ferroelectric behavior at high temperature. Here,

na

emissivity drops below 0.7 and hence the excursion is too strong to be considered as due only to surface roughness. Finally, the red region (late stage flows) would be classified as young flows

[Insert figure 10]

ur

like volcanoes in groups 4 or 6.

Jo

Additionally, several morphological observations provide further evidence for the relative youthfulness of Maat flows. Lava flows of Maat Mons overlie all other landforms in their paths, notably the rift zone (Dali Chasma) located to the east which cuts the flanks of Ozza Mons (Senske et al., 1992; Robinson and Wood, 1993). This implies that the deposition of Maat flows postdate the rifting activity of Dali Chasma. No impact craters are found on its flanks (Robinson et al., 1995), unlike other large volcanoes like Sif and Kali montes, which are considered to be volcanically dormant. Maat flows embay the impact craters surrounding the volcano. Basilevsky (1993) noted that the easternmost lava flows of Maat Mons appear to overlap the crater Uvaysi located on Ozza‟s southwest flank, which has an extended SAR-dark parabola (Campbell et al., 1992). Presence of a visible parabola indicates that the crater is likely a few tens of millions years old and hence the flows that cover it should be even younger (Basilevsky and Head, 20

Journal Pre-proof 2002a;b). Finally, Klose et al. (1992) reported that Maat Mons is unusually high (9 km) relative to other volcanoes on Venus, and they suggest that it could indicate the presence of an active mantle plume beneath the volcano. An active plume is corroborated by analysis of the gravity and altimetry data returned by Magellan (Phillips, 1994; Smrekar, 1994). Two other volcanoes are tall enough to display strong emissivity excursions, but do not: Idunn and Otafuku montes (6055.3 km and 6056.4 km, respectively). Otafuku Mons has a series of SAR-bright lava flows that are superposed on those of Tepev and Nyx montes, where they encircle the summit of Tepev Mons which is 0.6 km higher (Figure 7). The stratigraphic

of

relationship of these volcanic flows implies that Otafuku Mons postdates all major edifices of

ro

Bell Regio, as reported by previous mapping (Campbell and Rogers, 1994; Campbell and

-p

Campbell, 2002).

Idunn Mons has already been considered as a young and possibly active volcano on the basis of

re

its thermal emission at 1 µm (Smrekar et al., 2010). Infrared (1-µm) emissivity was derived from the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS) images returned by the ESA

lP

Venus Express (VEx) mission (Helbert et al., 2008; Mueller et al., 2008). In their work, Smrekar et al. (2010) suggest that the surface of Idunn Mons is made of fresh and unweathered lava flows

na

causing the volcano to display a high 1-µm emissivity anomaly relative to the plains in the VIRTIS images. Detailed investigation performed by D’Incecco et al. (2017) indicates that the

Jo

emissivity anomaly.

ur

young lava flows found on the eastern flanks of the volcano are likely responsible for the 1-µm

Only areas in the southern hemisphere of Venus have been sufficiently imaged by the VIRTIS instrument to produce a map of thermal emission at 1 µm (Mueller et al., 2008). Hathor and Innini montes in Dione Regio are among them and they both show a high 1-µm emissivity relative to the plains, but lower than Idunn Mons. This suggests that they are quite young, although they are older than Idunn Mons. This is consistent with our radar observations; Hathor and Innini peaks have moderately low emissivity values of 0.68 and 0.66, respectively. Other volcanic edifices mentioned in Smrekar et al. (2010) have a high 1-µm emissivity, notably Mielikki Mons, Shulamite and Shiwanokia coronae (group 7) in Themis Regio (Stofan et al., 2009; 2016). Such regions are relatively short and do not display radar emissivity excursions. 4.2.2. Pavlova and Didilia novae 21

Journal Pre-proof Pavlova and Didilia coronae host novae in their interiors (Figure 9) that are more elevated than their highest rims. The Pavlova nova is 0.9 km higher than its rim, while the Didilia nova is 0.6 km higher (Table 4). As mentioned in section 4.1, the high emissivity values at high elevations at these novae suggest they could be made of young and unweathered materials. Figure 10 (right panel) illustrates the different structures composing the Pavlova and Didilia coronae: the corona annuli (brown), their extensive flows (orange), and the novae (purple). It is shown from geological mapping that Pavlova and Didilia coronae and their novae (Campbell and Clark, 2006) represent the youngest volcanic edifices in east Eistla Regio. Interestingly, Aittola and

of

Kostama (2002) show that stratigraphic relationships between the coronae and novae indicate

ro

that novae formation has taken place during the latest stage of the coronae development. Indeed, the radial structures of the novae cut through the corona annuli suggesting that the novae are

-p

younger than the coronae. The elevated topography of the novae compared to the low elevated coronae is also characteristic of novae forming after the relaxation of the coronae, which is the

re

latest stage of a corona evolution (Squyres et al., 1992; Roberts and Head, 1993).

lP

Venusian hotspots were first proposed by examination of Pioneer Venus data (e.g., McGill et al., 1981; Phillips and Malin, 1983; Smrekar and Phillips, 1991) and later from Magellan data (e.g.,

na

Smrekar, 1994; Stofan et al., 1995), based on their distinctive topography, associated volcanism and gravity anomalies. These hotspots are among the most likely sites for recent or current

ur

volcanic activity on Venus (Smrekar, 1994; Stofan et al., 1995). Interestingly, 4 of the 10 hotspots identified on Venus are represented in our study: Maat Mons in Atla Regio, Otafuku

Jo

Mons in Bell Regio, Idunn Mons in Imdr Regio, and the novae in eastern Eistla Regio (Figure 5). Atla and Imdr regiones are assumed to be underlain by active plumes, whereas Bell and eastern Eistla regiones are suggested to be underlain by waning plumes, possibly recently extinct (Smrekar, 1994; Smrekar and Stofan, 1997). The radar emissivity data provide an independent constraint on the degree of weathering and therefore surface age of volcanic systems. The fact the emissivity data are in agreement with the 1 µm emissivity data, gravity data and stratigraphic position powerfully supports the idea that Venus is volcanically active. The terms „young‟ and „recent‟ used throughout this paper means that the lava flows are young relative to the duration of their weathering into high dielectric surface materials. Although the 22

Journal Pre-proof timescale for chemical weathering to high dielectric minerals in the near-surface environment of Venus is fundamentally unknown, other data roughly constrain the time frame. The VIRTIS 1 µm emissivity data are thought to be controlled by the oxidation of ferrous iron in basalts to hematite (Fe2O3), where the high emissivity of Idunn Mons is thought to represent lower degrees of weathering; Smrekar et al (2010) estimate that Idunn is on the order of 250,000 yrs. and no older than 2.5 Ma based on this reaction. More recent experimental work under Venus conditions suggests that olivine is stable from oxidation on much shorter time frames, on the order of years (Filiberto et al., 2019). Another constraint is provided by the aforementioned embayment

of

relationships seen at Maat Mons, where the high radar emissivity summit flows are seen to cover

ro

deposits from a crater parabola. A variety of degradation states of these parabolas is consistent with their being weathered away by aeolian, chemical or physical processes over time (Izenberg

-p

et al., 1994; Basilevsky et al., 2004). The 49 craters with visible parabolas (Campbell et al., 1992) represent ~5% of the total number of craters on Venus and thus are geologically young, ca.

re

25-50 Ma if the average age of the surface is indeed 500 Ma – 1 Ga. Thus, the range of age for

lP

the start of the weathering reaction to produce high dielectric minerals is broadly constrained to 1-10s Ma. If the volume of reactants is similar for all volcanic rocks on Venus, this would

na

represent a minimum age for this reaction to go to completion (e.g., for Idunn to display the same pattern as Ozza). This further predict that volcanoes with strong emissivity anomalies such as Ozza and Sapas are older than this time frame. However, we cannot know that the volume of

ur

dielectric minerals is constant, and indeed volcanoes with a strong dielectric signature may

Jo

indicate a greater volume of reactants. 5. DISTINCT SOURCE REGIONS? In this section we attempt to consider the differences in our classification with known characteristics of potential mantle source regions for the studied edifices. All volcanoes from the groups 1 and 2 are located on volcanic swells (topographic rises). Maram and Zisa coronae (group 2) are found in rift valleys converging to Atla Regio, the highest volcanic rise on Venus. Idunn and Otafuku montes (group 4) are also located in volcanic swells, Imdr and Bell regiones, respectively. Volcanic swells (i.e., Atla, Beta, Bell, western Eistla, Phoebe, Dione and Idunn regiones) are suggested to be possible hotspot sites associated to deep mantle plumes ascending from the core – mantle boundary (Figure 11A, Stofan and Smrekar, 2005). Alternatively, 23

Journal Pre-proof volcanoes from the group 3 are isolated from the volcanic swells. Irnini, Anala and Tefnut montes are three volcanoes located in areas dominated by coronae, forming clusters: Irnini and Anala montes in central Eistla Regio and Tefnut Mons in Themis Regio. Didilia and Pavlova coronae (group 3) are also found in a cluster of coronae, in eastern Eistla Regio. Volcanoes from the group 5 are mostly isolated from the topographic rises (only exception is Sif Mons in western Eistla Regio), while volcanoes and coronae from the group 7 are either isolated or located in coronae clusters. Coronae clusters (i.e., Themis, central and eastern Eistla regiones) are interpreted as secondary hotspot sites (Smrekar and Stofan, 1999) related to small-scale plumes

of

or diapirs originating from shallower depths (Janes et al., 1992; Stofan et al., 1992). Secondary plumes are proposed to spawn off of larger-scale convective upwelling when a superplume

ro

impinges on the upper – lower mantle boundary (Figure 11B, Courtillot et al., 2003; Stofan and

-p

Smrekar, 2005). The large volcanoes that occur away from the volcanic swells possibly form by a number of mechanisms, with some associated with secondary plumes, like the coronae, and

re

some related to tertiary hotspots (Figure 11C, Stofan and Smrekar, 2005). Tertiary hotspots are

lP

proposed to be related to lithospheric extension accompanied by decompression melting producing large bodies of magma (Stofan and Smrekar, 2005), as probably do a majority of large

[Insert figure 11]

na

flow fields on Venus (Magee and Head, 2001).

ur

Group 6 comprises the four coronae in the Diana – Dali chasmata area (eastern Aphrodite Terra) and the two paterae in Lakshmi Planum (western Ishtar Terra), the mantle dynamics responsible

Jo

for their origin are thought to be different from the ones proposed above. Coronae in the Diana – Dali chasmata area have long been identified as possible sites of subduction on Venus based on the similarity of the deep trenches and high outer rises occurring at the margins of the large coronae to features observed at terrestrial subduction zones (Sandwell and Schubert, 1992a,b; Schubert and Sandwell, 1995; Davaille et al., 2017). Recent laboratory experiments done by Davaille et al. (2017) tend to support the occurrence of localized retrograde subduction induced by mantle plumes at Artemis and Quetzpoptlatl coronae (Figure 11D). Additionally, a detailed geological investigation of western Ishtar Terra performed by Ivanov and Head (2008) tends to favor convergent models for the formation and evolution of Lakshmi Planum that seem to reasonably explain the topographic and morphologic characteristics of the region. A plausible model involves the collision and subsequent underthrusting of lower-lying plains against an 24

Journal Pre-proof ancient elevated and rigid tessera massif (Figure 11E). This could account for the formation of mountain belts surrounding the plateau, while the late-stage volcanism forming Colette and Sacajawea paterae within the plateau could be the result of the melting at the base of the subducted crust (e.g., Roberts and Head, 1990a,b). Through this section, we hypothesize that the volcanic edifices selected in our study have distinct compositional properties (distinct mineralogy) related to different mantle dynamics at the origin of their formation. Volcanic swells (groups 1, 2 and 4), coronae clusters and isolated large

of

volcanoes (groups 3, 5 and 7) would be linked to mantle upwellings occurring at different depths and scales. Conversely, the regions in group 6 would be associated with possible

ro

subduction/underthrusting processes. The reader is referred to the aforementioned studies for

-p

further explanation, more details, and additional references on the proposed models.

re

6. CONCLUSION

In this investigation we describe the variations of emissivity with elevation on the major

lP

volcanoes and coronae on Venus and better constrain the range of composition and degree of weathering of the surface materials. We find that high dielectric minerals are a common feature

na

on both the volcanoes and coronae at altitudes >6053 km. This implies that the minerals involved in this reaction are widespread and common in volcanic materials. The emissivity signatures can

ur

be divided into 7 classes which represent at least three high dielectric minerals with distinct excursion temperatures. These differences in mineralogy may be related to different geologic

Jo

settings including deep and shallow plumes, as well as potential convergence zones which could result in differences in lava composition. We propose that most of the volcanoes and coronae on Venus are compatible with the presence of ferroelectric compounds in their rocks, particularly the tallest ones (e.g., Maat and Ozza Mons). Low emissivity signatures also occur at low elevations and are found to correlate with individual lava flows indicating differences in the mineralogy within the volcanic system. This suggests that the emissivity excursions are controlled by variations in rock chemistry as opposed to the deposition of atmospheric precipitates. Elevation and shape of the emissivity excursions indicate the presence of at least two ferroelectrics with distinct Curie temperatures, at ~700 K (~6056 km altitude) and ~720 K (~6054 km altitude). Conversely, the emissivity pattern of

25

Journal Pre-proof volcanoes in western Ishtar Terra and the coronae in eastern Aphrodite Terra (Diana – Dali chasmata) are most likely consistent with the presence of a semiconducting materials rather than a ferroelectric. The varying “critical altitude” reported in Klose et al. (1992) and seen here could be due to different minerals, or local differences in the atmospheric composition or temperature. We find, in addition to Maat Mons, the emissivity signatures of Idunn and Otafuku montes and the novae within Pavlova and Didilia coronae are consistent with relatively recent and less weathered lava flows. These volcanic edifices are associated with 4 of the 10 presumably active hotspots that are among the most likely sites for recent or current volcanic activity on Venus

of

based on geophysical data (Smrekar, 1994; Stofan et al., 1995). A less weathered surface has

ro

also been suggested for Idunn based on 1-µm VIRTIS data (Smrekar et al, 2010). Thus, the radar emissivity data provide an independent constraint on recent volcanic activity on Venus in

-p

agreement with both 1-µm emissivity and gravity signatures.

re

To date, we cannot provide exact composition or age for the probed surface materials as we know only little about the presence, stability and reaction rates of possible candidate compounds

lP

(e.g., ferroelectrics, semiconductors). Further experimental investigations are needed to test for stability and reactivity of candidate compounds under the near-surface conditions of Venus. New

na

chemical and mineralogical analyses at the Venus surface are critical to constrain the poorly known surface environment, notably the gas and rock composition and abundance, temperature

Jo

Acknowledgments

ur

regimes and oxidation state.

This research has been carried out at the Wesleyan University and supported by NASA Solar System Workings Grant 80NSSC19K0549 to MSG. We greatly acknowledge the teams responsible for the Magellan data (SAR, altimetry and emissivity) accessible via the USGS „Map-a-Planet‟ websites. We give special thanks to Sébastien Lebonnois for providing us the Vega 2 lander data profiles (see Table S1). We are also grateful to Misha Ivanov for sharing with us his GIS map of tesserae. We appreciate stimulating discussions with Allan Treiman and Suzanne Smrekar. References Aittola, M., 2001. Age relations between coronae and novae. 32th LPSC abstracts, 1503. 26

Journal Pre-proof Aittola, M., Kostama, V.-P., 2002. Chronology of the formation process of Venusian novae and the associated coronae. JGR 107, 5112. https://doi.org/10.1029/2001JE001528 Arvidson, R. E., Brackett, R. A., Shepard, M. K., Izenberg, N. R., Fegley Jr, B., 1994. Microwave signatures and surface properties of Ovda Regio and surroundings, Venus. Icarus 112, 171 – 186. https://doi.org/10.1006/icar.1994.1176 Basilevsky, A. T., 1993. Age of rifting and associated volcanism in Atla Regio, Venus. GRL 20, 883 – 886. https://doi.org/10.1029/93GL00736

of

Basilevsky, A. T., Head, J. W., 2002a. Venus: Analysis of the degree of impact crater deposit

5061. https://doi.org/10.1029/2001JE001584

ro

degradation and assessment of its use for dating geological units and features. JGR 107,

-p

Basilevsky, A. T., Head, J. W., 2002b. On rates and styles of late volcanism and rifting on

re

Venus. JGR 107, 5041. https://doi.org/10.1029/2000JE001471

lP

Basilevsky A. T., Head, J. W., Abdrakhimov A. M., 2004. Impact crater airfall deposits on the surface of Venus: Areal distribution estimated thickness, recognition in surface panoramas, and implications for provenance of sampled surface materials, JGR 109, E12003.

na

https://doi.org/10.1029/2004JE002307

under

Venus

ur

Berger, G., et al., 2019. Experimental exploration of volcanic rocks-atmosphere interactions surface

conditions.

Icarus

329,

–

8

23.

Jo

https://doi.org/10.1016/j.icarus.2019.03.033 Brackett, R. A., Fegley Jr., B., Arvidson, R. E., 1995. Volatile transport on Venus and implications for surface geochemistry and geology. JGR 100, 1553 – 1563. https://doi.org/10.1029/94JE02708 Campbell, B. A., 1994. Merging Magellan emissivity and SAR data for analysis of Venus surface dielectric properties. Icarus 112, 187 – 203. https://doi.org/10.1006/icar.1994.1177 Campbell, B. A., Rogers, P. G., 1994. Bell Regio, Venus: Integration of remote sensing data and terrestrial

analogs

for

geologic

analysis.

JGR

99,

21153

–

21171.

https://doi.org/10.1029/94JE01862

27

Journal Pre-proof Campbell, B. A., Campbell, D. B., DeVries, C. H., 1999. Surface processes in the Venus highlands: Results from analysis of Magellan and Arecibo data. JGR 104, 1897 – 1916. https://doi.org/10.1029/1998JE900022 Campbell, B. A., Campbell, P. G., 2002. Geologic map of the Bell Regio quadrangle (V-9), Venus. U.S. Geological Survey Sci. Inv. Map I-2743. Campbell, B. A., Clark, D. A., 2006. Geologic map of the Mead quadrangle (V-21), Venus. U.S. Geological Survey Sci. Inv. Map 2897.

of

Campbell, D. B., et al., 1992. Magellan observations of extended impact crater related features

ro

on the surface of Venus. JGR 97, 16249 – 16277. https://doi.org/10.1029/92JE01634 Copp, D. L., Guest, J. E., 2007. Geologic map of the Sif Mons quadrangle (V-31), Venus. U.S.

-p

Geological Survey Sci. Inv. Map 2898.

re

Courtillot, V., Davaille, A., Besse, J., Stock, J., 2003. Three distinct types of hotspots in the

lP

Earth‟s mantle. EPSL 205, 295 – 308. https://doi.org/10.1016/S0012-821X(02)01048-8 Davaille, A., Smrekar, S. E., Tomlinson, S., 2017. Experimental and observational evidence for subduction

on

Venus.

Nature

Geoscience

10,

349

–

355.

na

plume-induced

https://doi.org/10.1038/ngeo2928

of

recently

active

lava

flows.

PSS

136,

25

–

33.

Jo

extent

ur

D‟Incecco, P., Mueller, N., Helbert, J., D‟Amore, M., 2017. Idunn Mons on Venus: Location and

https://doi.org/10.1016/j.pss.2016.12.002 Fegley Jr., B., Lodders, K., Treiman, A. H., Klingelhöfer, G., 1995. The rate of pyrite decomposition

on

the

surface

of

Venus.

Icarus

115,

159

–

180.

https://doi.org/10.1006/icar.1995.1086 Fegley Jr., B., 1997. Why pyrite is unstable on the surface of Venus. Icarus 128, 474 – 479. https://doi.org/10.1006/icar.1997.5744 Filiberto J., Trang D., Treiman A. H. and Gilmore M. S., 2019, Present-day volcanism on Venus as evidenced from weathering rates of olivine, Science Advances, in press.

28

Journal Pre-proof Ford, P. G., Pettengill, G. H., 1983. Venus: Global surface radio emissivity. Science 220, 1379 – 1381. https://doi.org/10.1126/science.220.4604.1379 Ford, P. G., Pettengill, G. H., 1992. Venus topography and kilometer-scale slopes. JGR 97, 13103 – 13114. https://doi.org/10.1029/92JE01085 Gilmore M. S., Brossier J. F., Zalewski N., 2019. Variations in the radiophysical properties of Venus tesserae; Could be the rocks? Could be the climate? 50th LPSC abstracts, 2632.

15949 – 15966. https://doi.org/10.1029/92JE01438

of

Guest, J. E., et al., 1992. Small volcanic edifices and volcanism in the plains of Venus. JGR 97,

ro

Hansen, V. L., Phillips, R. J., 1995. Formation of Ishtar Terra, Venus: Surface and gravity

-p

constraints. Geology 23(4), 292 – 296.

Harrington, E., Treiman, A. H., 2015. High radar reflectivity on Venus‟ highlands: Different

re

signatures on Ovda Regio and Maxwell Montes. 46th LPSC abstracts, 2713.

lP

Hashimoto, G. L., Abe, Y., 2005. Climate control on Venus: Comparison of the carbonate and pyrite models. PSS 53, 839 – 848. https://doi.org/10.1016/j.pss.2005.01.005

na

Helbert, J., Mueller, N., Kostama, P., Marinangeli, L., Piccioni, G., Drossart, P., 2008. Surface brightness variations seen by VIRTIS on Venus Express and implications for the evolution

ur

of the Lada Terra region, Venus. GRL 35, L11201. https://doi.org/10.1029/2008GL033609

Jo

Hitmi, N., Lacabanne, C., Young, R., 1984. TSC study of electric dipole relaxations in chlorapatite. Journal of Physics and Chemistry of Solids 45(6), 701 – 708. https://doi.org/10.1016/0022-3697(84)90066-0 Ivanov, M. A., Head, J. W., 1996. Tessera terrain on Venus: A survey of the global distribution, characteristics, and relation to surrounding units from Magellan data. JGR 101, 14861 – 14908. https://doi.org/10.1029/96JE01245 Ivanov, M. A., Head, J. W., 2008. Formation and evolution of Lakshmi Planum, Venus: Assessment of models using observations from geological mapping. PSS 56, 1949 – 1966. https://doi.org/10.1016/j.pss.2008.09.003

29

Journal Pre-proof Izenberg N. R., Arvidson, R.E., Phillips R.J., 1994. Impact crater degradation on venusian plains, GRL 21, 289–292. https://doi.org/10.1029/94GL00080. Janes, D. M., et al., 1992. Geophysical models for the formation and evolution of coronae on Venus. JGR 97, 16055 – 16067. https://doi.org/10.1029/92JE01689 Keddie, S. T., Head, J. W., 1994. Sapas Mons, Venus: Evolution of a large shield volcano. Earth, Moon and Planets 65, 129 – 190. https://doi.org/10.1007/BF00644896 Keddie, S. T., Head, J. W., 1995. Formation and evolution of volcanic edifices on the Dione

of

Regio rise, Venus. JGR 100, 11729 – 11754. https://doi.org/10.1029/95JE00822

reflectivity

of

Venus

mountaintops.

JGR

97,

16353

–

16369.

-p

https://doi.org/10.1029/92JE01865

ro

Klose, K. B., Wood, J. A., Hashimoto, A., 1992. Mineral equilibria and the high radar

re

Kohler, E., Chevrier, V., Johnson, N., Craig, P., Lacy, C., 2014. Proposed radar-reflective

lP

minerals tested under Venus surface and atmospheric conditions. 45th LPSC abstracts, 2321. Kohler, E., Port, S., Chevrier, V., Johnson, N., Lacy, C., 2015. Radar-reflective minerals

na

investigated under Venus near-surface conditions. 46th LPSC abstracts, 2563. Krassilnikov, A. S., Head, J. W., 2003. Novae on Venus: Geology, classification, and evolution.

ur

JGR 108, 5108. https://doi.org/10.1029/2002JE001983

Jo

Lines, M. E., Glass, A. M., 1977. Principles and applications of ferroelectrics and related materials. Clarendon Press, Oxford, 680 pages. Lorenz, R. D., Crisp, D., Huber, L., 2018. Venus atmospheric structure and dynamics from the VEGA lander and balloons: New results and PDS archive. Icarus 305, 277 – 283. https://doi.org/10.1016/j.icarus.2017.12.044 Magee, K. P., Head, J. W., 2001. Large flow fields on Venus: Implications for plumes, rift associations, and resurfacing. GSA special paper 352, 81 – 101. https://doi.org/10.1130/08137-2352-3.81 McGill, G. E., Steenstrup, S. J., Barton, C., Ford, P. G., 1981. Continental rifting and the origin of Beta Regio. GRL 8, 737 – 740. https://doi.org/10.1029/GL008i007p00737 30

Journal Pre-proof McGill, G. E., 2000. Geologic map of the Sappho Patera quadrangle (V-20), Venus. U.S. Geological Survey Sci. Inv. Map I-2637. McKinnon, W. B., Zahnle, K. J., Ivanov, B. I., Melosh, H. J., 1997. Cratering on Venus: Models and observations. In Bougher, S. W., Hunten, D. M., Phillips, R. J. (Eds.), Venus II. Tucson, Univ. of Arizona Press, 969 – 1014. Mueller, N., et al., 2008. Venus surface thermal emission at 1 µm in VIRTIS imaging observations: Evidence for variation of crust and mantle differentiation conditions. JGR 113,

of

E00B17. https://doi.org/10.1029/2008JE003118

ro

Palazzari, G. M., Brackett, R. A., Arvidson, R. E., 1995. Nature of the low emissivity highlands on Venus inferred from the analysis of Magellan data: Sapas, Maat, and Ozza montes. 26th

-p

LPSC abstracts, 1089.

re

Pettengill, G. H., Ford, P. G., Chapman, B. D., 1988. Venus: Electromagnetic properties. JGR

lP

93, 14881 – 14892. https://doi.org/10.1029/JB093iB12p14881 Pettengill, G. H., Ford, P. G., Johnson, W. T. K., Raney, R. K., Soderblom, L. A., 1991. Magellan:

Radar

performance

and

data

products.

Science

252,

260

–

265.

na

https://doi.org/10.1126/science.252.5003.260

ur

Pettengill, G. H., Ford, P. G., Wilt, R. J., 1992. Venus surface radiothermal emission as observed

Jo

by Magellan. JGR 97, 13091 – 13102. https://doi.org/10.1029/92JE01356 Pettengill, G. H., Ford, P. G., Simpson, R. A., 1996. Electrical properties of the Venus surface from

bistatic

radar

observations.

Science

272,

1628

–

1631.

https://doi.org/10.1126/science.272.5268.1628 Phillips, R. J., Malin, M. C., 1983. The interior of Venus and tectonic implications. In Hunten, D. M., Colin, L., Donahue, T. M., et al. (Eds.), Venus. Tucson, Univ. of Arizona Press, 159 – 214. Phillips, R. J., 1994. Estimating lithospheric properties at Atla Regio, Venus. Icarus 112, 147 – 170. https://doi.org/10.1006/icar.1994.1175

31

Journal Pre-proof Plaut, J. J., 1993, The non-SAR experiments, in Ford et al., (Eds.), J. P. Guide to Magellan Image Interpretation, NASA, Jet Propulsion Laboratory Publication 93-24, 19-32. Port, S., Kohler, E., Craig, P. I., Chevrier, V., 2016. Stability of pyrite under Venusian surface conditions. 47th LPSC abstracts, 2144. Price, M., Suppe, J., 1994. Mean age of rifting and volcanism on Venus deduced from impact crater densities. Nature 372, 756 – 759. https://doi.org/10.1038/372756a0 Price, M., 1995. Resurfacing history of the Venusian plains based on distribution of impact

of

craters. 26th LPSC abstracts, 1143.

ro

Pronin, A. A., Stofan, E. R., 1990. Coronae on Venus: Morphology, classification, and

-p

distribution. Icarus 87, 452 – 474. https://doi.org/10.1016/0019-1035(90)90148-3 Rausch, E. O., 1976. Dielectric properties of chlorapatite. Georgia Institute of Technology, p.

re

268.

lP

Roberts, K. M., Head, J. W., 1990a. Lakshmi Planum, Venus: Characteristics and models of origin. Earth, Moon, and Planets 50, 193 – 249. https://doi.org/10.1007/BF00142395

of

na

Roberts, K. M., Head, J. W., 1990b. Western Ishtar Terra and Lakshmi Planum, Venus: Models formation

and

evolution.

GRL

17,

–

1341

1344.

ur

https://doi.org/10.1029/GL017i009p01341

Implications

Jo

Roberts, K. M., Head, J. W., 1993. Large-scale volcanism associated with coronae on Venus: for

formation

and

evolution.

GRL

20,

1111

–

1114.

https://doi.org/10.1029/93GL01484 Robinson, C. A., Wood, J. A., 1993. Recent volcanic activity on Venus: Evidence from radiothermal

emissivity

measurements.

Icarus

102,

26

–

39.

https://doi.org/10.1006/icar.1993.1030 Robinson, C. A., Thornhill, G. D., Parfitt, E. A., 1995. Large-scale volcanic activity at Maat Mons: Can this explain fluctuations in atmospheric chemistry observed by Pioneer Venus? JGR 100, 11755 – 11763. https://doi.org/10.1029/95JE00147

32

Journal Pre-proof Rupprecht, G., Bell, R. O., 1964. Dielectric constant in paraelectric perovskites. Physical Review 135, 748 – 752. https://doi.org/10.1103/PhysRev.135.A748 Sandwell, D. T., Schubert, G., 1992a. Evidence for retrograde lithospheric subduction on Venus. Science 257, 766 – 770. https://doi.org/10.1126/science.257.5071.766 Sandwell, D. T., Schubert, G., 1992b. Flexural ridges, trenches, and outer rises around coronae on Venus. JGR 97, 16069 – 16083. https://doi.org/10.1029/92JE01274 Schaefer, L., Fegley Jr., B., 2004. Heavy metal frost on Venus. Icarus 168, 215 – 219.

of

https://doi.org/10.1016/j.icarus.2003.11.023

ro

Schubert, G., Sandwell, D. T., 1995. A global survey of possible subduction sites on Venus.

-p

Icarus 117, 173 – 196. https://doi.org/10.1006/icar.1995.1150 Senske, D., Schaber, G., Stofan, E., 1992. Regional topographic rises on Venus: Geology of

re

western Eistla Regio and comparison to Beta Regio and Atla Regio. JGR 97, 13395 –

lP

13420. https://doi.org/10.1029/92JE01167

Seiff, A., and the VEGA Balloon Science Team, 1987. Further information on structure of the

na

atmosphere of Venus derived from the VEGA Venus Balloon and Lander mission. Adv. Space Res. 7, 323 – 328. https://doi.org/10.1016/0273-1177(87)90239-0

low

emissivity

highlands

on

Venus.

GRL

21,

469

–

472.

Jo

the

ur

Shepard, M. K., Arvidson, R. E., Brackett, R. A., Fegley Jr., B., 1994. A ferroelectric model for

https://doi.org/10.1029/94GL00392 Simpson, R. A., Tyler, G. L., Häusler, B., Mattei, R., Pätzold, M., 2009. Venus Express bistatic radar:

High-elevation

anomalous

reflectivity.

JGR

114,

E00B41.

https://doi.org/10.1029/2008JE003156 Smrekar, S. E., Phillips, R. J., 1991. Venusian highlands: Geoid to topography ratios and their implications. EPSL 107, 582 – 597. https://doi.org/10.1016/0012-821X(91)90103-O Smrekar, S. E., 1994. Evidence for active hotspots on Venus from Analysis of Magellan gravity data. Icarus 112, 2 – 26. https://doi.org/10.1006/icar.1994.1166

33

Journal Pre-proof Smrekar, S. E., Stofan, E. R., 1999. Origin of corona-dominated topographic rises on Venus. Icarus 139. 100 – 115. https://doi.org/10.1006/icar.1999.6090 Smrekar, S. E., et al., 2010. Recent hotspot volcanism on Venus from VIRTIS emissivity data. Science 328. 605 – 608. https://doi.org/10.1126/science.1186785 Squyres, S. W., et al., 1992. The morphology and evolution of coronae on Venus. JGR 97, 13611 – 13634. https://doi.org/10.1029/92JE01213 Stofan, E. R., et al., 1992. Global distribution and characteristics of coronae and related features

of

on Venus: Implications for origin and relation to mantle processes. JGR 97, 347 – 13378.

ro

https://doi.org/10.1029/92JE01314

Stofan, E. R., Smrekar, S. E., Bindschadler, D. L., Senske, D. A., 1995. Large topographic rises Venus:

implications

for

mantle

JGR

327.

317

–

323.

re

https://doi.org/10.1029/95JE01834

upwellings.

-p

on

Stofan, E. R., 1995. Coronae on Venus: Topographic variations and correlations between

lP

morphology and regional setting. 26th LPSC abstracts, 1361.

na

Stofan, E. R., Smrekar, S. E., 2005. Large topographic rises, coronae, large flow fields and large volcanoes on Venus: Evidence for mantle plumes? In Foulger, G. R., Natland, J. H.,

ur

Presnall, D. C., et al. (Eds.), Plates, Plumes and Paradigms. Geol. Soc. Am. 388, 861 – 885.

Jo

https://doi.org/10.1130/0-8137-2388-4.841 Stofan, E. R., Smrekar, S. E., Helbert, J., Martin, P., Mueller, N., 2009. Coronae and large volcanoes on Venus with unusual emissivity signatures in VIRTIS – Venus Express data. 40th LPSC abstracts, 1033. Stofan, E. R., Smrekar, S. E., Mueller, N., Helbert, J., 2016. Themis Regio, Venus: Evidence for recent

(?)

volcanism

from

VIRTIS

data.

Icarus

271,

375

–

386.

http://dx.doi.org/10.1016/j.icarus.2016.01.034 Strom, R. G., Schaber, G. G., Dawson, D. D., 1994. The global resurfacing of Venus. JGR 99, 10899 – 10926. https://doi.org/10.1029/94JE00388

34

Journal Pre-proof Treiman, A., Harrington, E., Sharpton, V., 2016. Venus‟ radar-bright highlands: Different signatures and materials on Ovda Regio and on Maxwell Montes. Icarus 280, 172 – 182. http://dx.doi.org/10.1016/j.icarus.2016.07.001 Tyler, G. L., Ford, P. G., Campbell, D. B., Elachi, C., Pettengill, G. H., Simpson, R. A., 1991. Magellan: Electrical and physical properties of Venus‟ surface. Science 252, 265 – 270. https://doi.org/10.1126/science.252.5003.265 Weitz, C. M., Basilevsky, A. T., 1993. Magellan observations of the Venera and Vega landing

of

site regions. JGR 98, 17069 – 17097. https://doi.org/10.1029/93JE01776

ro

Wood, J. A., Brett, R., 1997. Comment on “The rate of pyrite decomposition on the surface of Venus”. Icarus 128, 472 – 473. https://doi.org/10.1006/icar.1997.5743

-p

Wood, J. A., 1997. Rock weathering on the surface of Venus. In Bougher, S. W., Hunten, D. M.,

re

Phillips, R. J. (Eds.), Venus II. Tucson, Univ. of Arizona Press, 637 – 665.

lP

Footnotes: Footnote 1. Link to GTDR:

Footnote 2. Link to GEDR:

na

https://planetarymaps.usgs.gov/mosaic/Venus_Magellan_Topography_Global_4641m_v02.tif

Jo

if

ur

https://planetarymaps.usgs.gov/mosaic/Venus_Magellan_MicrowaveEmissivity_Global_4641m.t

35

Journal Pre-proof Table 1. Volcanoes selected in our study.

Jo

ur

na

lP

re

-p

ro

of

Name Long. (˚E) Lat. (˚N) Planetary radius (km) Height (km) Anala 14.0 10.4 6054.3 2.5 Colette 324.6 65.1 6055.7 3.9 Copacati 276.8 34.6 6054.7 2.9 Dzalarhons 34.1 0.4 6055.2 3.4 Gula 357.7 20.7 6055.7 3.9 Hathor 323.3 -38.4 6054.2 2.4 Idunn 215.3 -46.8 6055.3 3.5 Innini 328.1 -34.6 6054.3 2.5 Irnini 16.1 14.8 6054.0 2.2 Kali 29.0 9.4 6054.3 2.5 Kunapipi 85.5 -33.7 6054.1 2.3 Maat 194.0 1.1 6060.7 8.9 Melia 119.7 62.8 6053.2 1.4 Metis 254.8 71.4 6053.9 2.1 Mielikki 280.8 -27.4 6053.7 1.9 Nyx 50.1 29.9 6054.7 2.9 Otafuku 45.1 30.1 6056.4 4.6 Ozza 201.6 3.0 6058.5 6.7 Polik-mana 264.1 24.9 6056.8 5.0 Ptesanwi 46.2 2.8 6053.9 2.1 Renpet 233.7 75.6 6053.8 2.0 Rhpisunt 301.6 2.6 6053.7 1.9 Sacajawea 334.4 63.0 6056.0 4.2 Sapas 187.9 8.3 6055.7 3.9 Sekmet 241.7 44.9 6054.4 2.6 Sif 351.7 21.6 6054.6 2.8 Tefnut 304.1 -38.2 6054.9 3.1 Tepev 44.6 29.6 6057.0 5.2 Theia 280.1 24.1 6057.5 5.7 Tuulikki 274.8 10.2 6053.5 1.7 Ushas 323.2 -24.7 6053.7 1.9 Uti Hiata 69.6 16.3 6053.4 1.6 Vostrukha 299.8 -6.6 6054.7 2.9 Xochiquetzal 269.7 3.3 6054.3 2.5 Yunya-mana 286.0 -19.2 6056.8 5.0 Notes: Heights are taken above a mean planetary radius of 6051.8 km Magnitude of emissivity decline with altitude ▪ Strong: emissivity decreases below 0.7 ▪ Subtle: emissivity gently decreases below 0.8 but remains above 0.7 ▪ None: emissivity stays constant with values above 0.8

Magnitudes Strong None Strong Strong Strong Strong Subtle Strong Subtle Subtle None Strong None None None Strong Subtle Strong Strong Strong None Strong None Strong Strong Subtle Strong Strong Strong Subtle Subtle Subtle Strong Subtle Strong

36

Journal Pre-proof Table 2. Coronae selected in our study. Height (km) 4.9 5.8 4.6 2.2 1.9 2.4 4.2 4.3 5.0 1.3 2.9 2.5 2.4 2.5 5.2

ro

of

Planetary radius (km) 6056.7 6057.6 6056.4 6054.0 6053.7 6054.2 6056.0 6056.1 6056.8 6053.1 6054.7 6054.3 6054.2 6054.3 6057.0

-p

Lat. (˚N) -34.0 -20.4 -15.0 18.7 -58.0 -31.9 0.0 -7.8 -15.6 -57.2 15.1 -42.4 -38.8 -39.6 11.5

Magnitudes Subtle Strong Subtle Strong None None Strong Strong Subtle None Strong None None None Strong

ur

na

lP

re

Long. (˚E) 133.1 171 152 37.4 9 359.2 74 221.6 161.1 31.1 40.6 280.4 284.2 297.1 220.4

Jo

Name Artemis Atahensik Ceres Didilia Eithinoha Eve Kaltash Maram Miralaidji Otygen Pavlova Shiwanokia Shulamite Ukemochi Zisa

37

Journal Pre-proof Table 3. Elevation (as planetary radius) and temperature of low emissivity excursions seen on volcanoes.

Jo

ur

na

lP

re

-p

ro

of

Name Lowest emissivity Planetary radius (km) Temperature (K) Anala 0.628 6054.1 716.3 Copacati 0.695 6054.6 712.2 Dzalarhons 0.431 6055.0 708.4 Gula 0.554 6055.4 704.6 Hathor 0.679 6053.8 719.6 Idunn 0.769 6055.2 707.0 Innini 0.664 6054.2 715.6 Irnini 0.691 6053.8 719.6 Kali 0.750 6054.2 715.6 Maat (e1) (1) 0.527 6056.2 696.5 Maat (e2) 0.500 6055.4 704.6 Maat (e3) 0.582 6053.9 718.7 Maat (e4) 0.688 6052.7 729.6 Nyx 0.681 6053.9 718.7 Otafuku 0.742 6056.1 697.8 Ozza (e1) 0.358 6056.7 692.7 Ozza (e2) 0.359 6055.7 701.5 Ozza (e3) 0.529 6052.5 730.8 Polik-mana (e1) 0.564 6056.2 696.5 Polik-mana (e2) 0.585 6053.2 724.8 Ptesanwi 0.515 6053.7 720.3 Rhpisunt 0.611 6053.6 721.2 Sapas 0.395 6054.6 712.2 Sekmet 0.611 6054.2 715.6 Sif 0.734 6054.4 713.7 Tefnut 0.680 6054.4 713.7 Tepev 0.497 6056.4 695.5 Theia 0.343 6056.1 697.8 Tuulikki 0.758 6053.4 723.0 Ushas 0.733 6052.5 730.8 Uti Hiata 0.773 6052.8 728.5 Vostrukha 0.569 6053.6 721.2 Xochiquetzal 0.788 6053.0 726.3 Yunya-mana 0.530 6055.1 707.8 (1) Notes: Multiple excursions: from e1 (highest altitude) to e4 (lowest altitude) (2) Emissivity patterns with altitude (details in text)

Patterns (2) 3 2 3 2 2 4 2 3 5 1 2 4 1 1 3 3 3 3 5 3 1 1 5 5 5 3 5 1

38

Journal Pre-proof Table 4. Elevation (as planetary radius) and temperature of low emissivity excursions seen on coronae. Temperature (K) 696.5 693.6 703.7 723.0 717.2 708.4 704.6 717.2 704.6 719.6 712.2 705.8

of

Planetary radius (km) 6056.2 6056.6 6055.5 6053.4 6054.0 6055.0 6055.4 6054.0 6055.4 6053.8 6054.7 6055.3

Patterns 6a 6a 6a 3 5 2 2 5 6a 3 5 2

ur

na

lP

re

-p

ro

Lowest emissivity 0.758 0.560 0.776 0.641 0.770 0.450 0.510 0.750 0.771 0.465 0.760 0.504

Jo

Name Artemis Atahensik Ceres Didilia (rim) Didilia (nova) Kaltash Maram (rim) Maram (nova) Miralaidji Pavlova (rim) Pavlova (nova) Zisa

39

Journal Pre-proof Table 5. Lowest altitude where emissivity (E) reaches 0.7.

of

Group 3 Altitude for E = 0.7 (km) Anala 6053.7 Didilia 6053.2 Dzalarhons 6053.1 Irnini 6053.6 Pavlova 6053.1 Ptesanwi 6053.1 Rhpisunt 6053.3 Sapas 6053.1 Sekmet 6053.5 Tefnut 6053.7 Vostrukha 6053.7 Average: 6053.4 ± 0.3 km

Jo

ur

na

lP

re

-p

ro

Group 2 Altitude for E = 0.7 (km) Copacati 6054.5 Gula 6053.9 Hathor 6053.9 Innini 6054.0 Kaltash 6054.0 Maram 6054.1 Nyx 6054.3 Zisa 6054.0 Average: 6054.2 ± 0.2 km

40

Journal Pre-proof Figure captions [2-column for all figures, and color should be used online only] Figure 1. Elevation – emissivity plots obtained for the selected volcanoes. Color-coded lines in plots are reference values of elevation and emissivity: (dashed gray) the mean planetary radius of 6051.8 km, elevations at (blue) 6053 km and (red) 6055 km, as well as (green) emissivity of 0.7. Temperatures are given by the Vega 2 lander data (Lorenz et al., 2018). Figure 2. Elevation – emissivity plots obtained for the selected coronae. Annotations are the same as in Figure 1.

of

Figure 3. Lowest emissivity versus corresponding mean elevation for each region having subtle

ro

to strong excursion(s). Data points are color-coded according to our classification (see details in text). Note that we plot the highest elevation for all regions having no excursions: Colette and

-p

Sacajawea paterae from group 6b (empty red circles) and all landforms from group 7 (yellow

re

filled circles).

Figure 4. Flow chart of the classification of the emissivity patterns observed in the selected

lP

volcanoes and coronae. Note (1) in addition to group 1, two other volcanoes also exhibit a slight return in emissivity at higher elevations: Sapas (group 3) and Ushas (group 5).

na

Figure 5. Global distribution of the regions of interest after classification. Mountain belts (dark

reference purposes.

ur

grey) and tesserae (light grey) are mapped and provided by Ivanov and Head (1996) for

Jo

Figure 6. Emissivity maps overlapping SAR images of the three tallest volcanoes on Venus: Maat and Ozza (198˚E, 4˚N), and Theia (280˚E, 24˚N). Blue arrows show the locations of the emissivity lows, while the red arrows show the high emissivity summits. Yellow arrows denote other high emissivity areas located above 6057 km, comparable with the high emissivity summits of the tallest volcanoes. North is on top for all images. Figure 7. Emissivity maps overlapping SAR images of a few selected volcanoes: Yunya-mana (286˚E, 19˚S), Sapas (188˚E, 8˚N), Tepev, Otafuku, and Nyx (49˚E, 30˚N), Ushas (323˚E, 25˚S), Rhpisunt (302˚E, 3˚N), and Sekmet (242˚E, 45˚N). Annotations are the same as in Figure 6. Figure 8. Emissivity maps overlapping SAR images of a few selected volcanoes: Dzalarhons (34˚E, 0˚N), Sif and Gula (355˚E, 22˚N), Tuulikki (275˚E, 10˚N), Kali (29˚E, 9˚N), Colette and 41

Journal Pre-proof Sacajawea (330˚E, 64˚N), Idunn (215˚E, 47˚S), Kunapipi (86˚E, 34˚S), and Melia (120˚E, 63˚N). Annotations are the same as in Figure 6. Figure 9. Emissivity maps overlapping SAR images of a few selected coronae: Maram (222˚E, 8˚S), Kaltash (74˚E, 0˚N), Didilia and Pavlova (39˚E, 17˚N), Atahensik (171˚E, 20˚S), and Eve (359˚E, 32˚S). Radially fractured domes (or novae) are present in Maram, Didilia and Pavlova coronae, and they also have emissivity lows but with higher values. Annotations are the same as in Figure 6.

of

Figure 10. Top: Sketch maps of Maat Mons after Keddie and Head (1994) (left), as well as Pavlova and Didilia coronae after Aittola and Kostama (2012) (right). The regions are

ro

subdivided into major flow units. Bottom: Elevation – emissivity plots for major flow units.

-p

Figure 11. Schematic cross-section of Venus outlining the potential sources of the different types

re

of mantle dynamics responsible for the volcanic edifices observed at the surface: (A) volcanic swells, (B) coronae clusters, (C) isolated large volcanoes adapted from Courtillot et al. (2003);

lP

Stofan and Smrekar (2005), (D) eastern Aphrodite Terra (from Artemis Corona to the coronae in Diana – Dali chasmata) adapted from Sandwell and Schubert (1992); Davaille et al. (2017), and

Jo

ur

na

finally (E) western Ishtar Terra (Lakshmi Planum) adapted from Ivanov and Head (2008).

42

Jo

ur

na

lP

re

-p

ro

of

Journal Pre-proof

Figure 1. Elevation – emissivity plots for the volcanoes (1). 43

Jo

ur

na

lP

re

-p

ro

of

Journal Pre-proof

Figure 1. Elevation – emissivity plots for the volcanoes (2). 44

re

-p

ro

of

Journal Pre-proof

Jo

ur

na

lP

Figure 1. Elevation – emissivity plots for the volcanoes (3).

45

Jo

ur

na

lP

re

-p

ro

of

Journal Pre-proof

Figure 2. Elevation – emissivity plots for the coronae. 46

-p

ro

of

Journal Pre-proof

Jo

ur

na

lP

re

Figure 3. Excursion plot.

47

Jo

ur

na

lP

re

-p

ro

of

Journal Pre-proof

Figure 4. Flow chart.

48

-p

ro

of

Journal Pre-proof

Jo

ur

na

lP

re

Figure 5. Distribution after classification.

49

Jo

ur

na

lP

re

-p

ro

of

Journal Pre-proof

Figure 6. Emissivity/SAR maps of tallest volcanoes.

50

Jo

ur

na

lP

re

-p

ro

of

Journal Pre-proof

Figure 7. Emissivity/SAR maps of volcanoes (1).

51

ur

na

lP

re

-p

ro

of

Journal Pre-proof

Jo

Figure 8. Emissivity/SAR maps of volcanoes (2).

52

Jo

ur

na

lP

re

-p

ro

of

Journal Pre-proof

Figure 9. Emissivity/SAR of coronae.

53

ur

na

lP

re

-p

ro

of

Journal Pre-proof

Jo

Figure 10. Sketch maps of Maat Mons and east Eistla Regio (subunits).

54

ur

na

lP

re

-p

ro

of

Journal Pre-proof

Jo

Figure 11. Venus cross-section and mantle dynamics

55