impact melta b post-impact volcanic fill

?

c

?

late-stage edifice d

?

explosive volcanism5. Conclusions

Fig. 13. Model for CV1. (a) Heaney formed when magmagenesis was ongoing but large-scale eruptions had ceased due to global contraction. (b) Impact fractures facilitate small-volume, post-impact effusions. (c) Contraction focusses eruptions to single location, causing edifice construction. (d) Terminal explosive eruptions when magma supply wanes.

Acknowledgements: JW is funded by UK Science and Technology Facilities Council (STFC) training grant ST/N50421X/1. JW is grateful to the Royal Astronomical Society (RAS) for awarding him a RAS Small Grant to attend this meeting. JW is also grateful to the UK Remote Sensing Society and the British Society for Geomorphology for additional funding provided during the course of this work.

References: [1] Denevi B. W. et al. (2013), J. Geophys. Res. Planets, 118, 891–907. [2] Wright J. et al. (2018) J. Geophys. Res. Planets., DOI:10.1002/2017JE005450. [3] Bryan S. E. and Ernst R. E. (2008) Earth-Sci. Rev., 86, 175–202. [4] Blewett D. T. et al. (2009) Earth Planet. Sci. Lett., 285, 272–282. [5] Head J. W. et al. (2008) Science, 321, 69–72. [6] Rothery D. A. et al. (2014) Earth Planet. Sci Lett., 385, 59–67. [7] Weider S. Z. et al. (2016) Geophys. Res. Lett., 43, 3653–3661. [8] Baker D. M. H. and Head J. W. (2013) Planet. Space Sci., 86, 91–116. [9] Byrne P. K. et al. (2016) Geophys. Res. Lett., 43, 7408–7416. [10] Wöhler C. et al. (2006) Icarus, 183, 237–264.

4. Comparative planetology

Skjaldbreiður

CV2

Hortensius 54°

7°

8°

Marius Hills cone

16°

0

1

km

1 2 3 4 5 6 7 8 9 10km

flank slope

CV110°

0

F F'

E

D

C

E'

D'

C'

5km

±a 20.7° W20.9° W

64.4

°N

5km

±

heig

ht(m

)

1000

0

1300!!D

!!D'

b 20.7° W20.9° W

64.4

°N

5km

±c 28° W

7°N

5km

±

!!E

!!E'

d 28° W

7°N

heig

ht(m

) 500

0

500m

±e 53.25° W

13.1

5°N

1km

±f 53.2° W53.3° W

13.2

°N

13.1

°N

heig

ht(m

)

0

475!!F

!!F'

Fig. 11. Small volcanoes (a,c,e) and their topography (b,d,f). (a,b) Skjaldbreiður, Earth. (c,d) Lunar mare domes. (e,f) Marius Hills cone, Moon. Profiles D–D', E–E' and F–F' are shown in Fig. 12.

Even if CV1 and CV2 prove not to be volcanoes, the fact that edifices are rare on Mercury is important. Early eruptions generated LIP-like plains. This eruption style does not typically construct edifices. Large-scale effusions waned abruptly due to global contraction [9]. The lack of edifices may be because Mercury mostly lacked a longer 'waning' stage of volcanism (Fig. 13), thought to have been important for construction of lunar edifices [10].

3. Candidate Volcano #2 ('CV2')

10km

!!C

!!C'

±b

height (m)9000

145° E144.5° E

44.5

°N

20km

±a 146°E144°E

44°N

Pla

nitia

Cal

oris

Fig. 9. (a) CV2 lies in the northwestern rim of Caloris. (b) MLA data used in the topographic profile C–C' shown in Fig. 10 and Fig. 12.

Fig. 10. MLA topographic profile of CV2. The feature has a shield-like shape. Profile shown without vertical exaggeration in Fig. 12.

●●● ●●●● ●●● ● ● ● ● ●

● ● ● ● ●

●●

●

●●

● ●●

●

●

●

●

●●●

●●● ●

●● ● ● ● ● ●

●●

●

0 5 10 15 20 25

040

080

0

heig

ht (

m)

distance along profile (km)

C C'

x10 vert. ex.

Fig. 12. Comparison of Mercury candidate volcanoes with small volcanoes on Earth and the Moon. The Mercury landforms have sizes comparable to Skjaldbreiður, an Icelandic shield volcano, and slopes intermediate between shield volcanoes (e.g. Skjaldbreiður and Hortensius 5) and lunar cinder cones (e.g. Marius Hills cones). Arrows indicate summit crater extents when unresolved in topography.

50km

±a 124°E122°E

34°S

2. Candidate Volcano #1 ('CV1')

Fig. 5. (a) CV1. A peak-like landform with a shallow summit crater. (b) A diffuse, red color anomaly coincides with CV1.

5km

±a 123°E

34°S

5km

±b 123°E

34°S

50km

b

34°S

122°E

±124°E

mosaicseam

Fig. 6. (a) Heaney crater. Arrow indicates CV1. (b) Heaney's ejecta contains low-reflectance material, which is thought to contain volatiles [7].

Fig. 7. Heaney has a textured ejecta blanket and secondary field (arrows) superposing nearby smooth plains, suggesting it formed after local volcanic plains emplacement. The topographic profile B–B' is shown in Fig. 8.

100km

±!!B

!!B'

125°E120°E115°E

35°S

Fig. 8. Profile from stereo-topography [7]. Heaney's original crater floor is buried by volcanic plains. Heaney's present depth is greater than that of a peak-ring basin with the same diameter [8].

0 50 100 150

−2

−1

01

elev

atio

n (k

m)

distance along profile (km)

B B'

present Heaney floor

max.depth of 125 km peak-ring basin

plains

x20 vertical exaggeration

Is it a peak-ring element? Is it a volcano?

Heaney too deep to be peak-ring basinno evidence of the rest of a peak-ring

~correct locationsmooth plains volcanism in Heaneypyroclastics on Mercury are red

~correct shape (Fig. 12)

no corroborating features (e.g. flows) volatiles available for pyroclastics

0°30°N

60°N60°S

30°S

30°E 60°E 90°E 120°E 150°E 180°E 210°E 240°E 270°E 300°E 330°E0°

90°N

Borealis Planitia Borealis Planitia

CalorisPlanitia

circu

m-Caloris plains

circum-Caloris p

lain

s

smooth plainsafter Denevi et al. (2013) J. Geophys. Res.: Planets

Smooth plains, broadly interpreted as volcanic in origin, cover ~26% of Mercury's surface [1] (Fig.1). Despite widespread volcanism, no constructional volcanic edifices have been robustly identified on Mercury to date [2] (Fig. 2–4). Here, we describe two candidates that we have found (Fig. 5) and discuss how they might have formed.

Fig.3. Detrended Agwo Facula flank slopes (dotted lines) are very shallow for a shield volcano. It is more likely a shallow ramp of pyroclasts than an edifice constucted from lavas [6]. Vent extent (gray box).

A

A'

●

●

●

●

●●●

●●●

●

●

●

●●

●●

●

●●●●

●

●

●

●

●

●

●

●●●

●

●●

●

●●

●

●

●

●

●●

●

●

●●

●●●

●

●

●●●● ●

●

● ●

●

●

●

●

●

●

●

0 20 40 60 80

250

300

350

400

450

500

detr

ende

d el

evat

ion

(m)

distance along profile (km)

0.07°

0.14

°

!

!

CV2

CV1

ele

vatio

n(k

m)

+4.

5-5

.3

150°E

120°

E

0°

30°N

30°S

Fig. 2. Agwo Facula ('Red Spot 3' [4]). This bright spot is centered on the 'kidney-shaped' pyroclastic vent [5]. Red dots show locations of Mercury Laser Altimeter (MLA) data used to make the profile A–A' in Fig. 3.

Fig. 1. Smooth plains on Mercury [1]. Plains are akin to large igneous provinces (LIPs) on Earth, which typically lack volcanic edifices due to high eruption volumes from multiple fissures [3].

!(

!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(

!(!(!(!(!(

!(!(!(!(!(!(!(

!(

!(!(!(!(!(!(!(!(!(!(

!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(

!(!(

!(!(!(!(!(!(!(!(

±

20km

!!A

!!A'

146°E

22°N

Fig. 4. Topography of Mercury. The locations of Candidate Volcano #1 ('CV1') and Candidate Volcano #2 ('CV2'), discussed here, are shown. Caloris basin (dotted line).

jack.wright@open.ac.uk@wrightplanet

1School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK2CNRS, Laboratoire de Planétologie et Géodynamique, Université de Nantes, France

Jack Wright1, David A. Rothery1, Matt R. Balme1, Susan J. Conway2

Candidate constructional volcanic edifices on Mercury

1. Mercury's missing volcanoes?