I did a post four years ago entitled Volcanoes as Heat Engines that touched on the observation that we see things that look like volcanoes on every reasonably sized body with a solid surface in the solar system. In this context, a volcano is simply a location where hot material escapes to a region that is less hot. On the larger bodies, these manifest themselves as volcanoes. We see them on all the planets (other than the gas giants), the largest asteroid Ceres, and several of the larger moons. The smaller moons and comets eject their hot material into space via geysers (Enceladus, Triton) and jets (comets).
Between our former and current blog home, a couple writers (mainly Albert and myself) have written posts about volcanic activity on most of the major bodies in the solar system. Today’s post will look at what looks like volcanic activity on the innermost planet in the solar system, Mercury.
Mercury is the smallest of the terrestrial planets and closest to the sun. It is both the hottest and one of the coldest places in the entire system. It might even have ice in permanently shadowed impact craters at its poles.
Mercury is 4,880 km in diameter, smaller than the two largest moons in the system, Titan and Ganymede. It has the most eccentric orbit of any planet, varying from 0.308 – 0.467 AU (AU is Astronomic Unit, the average distance from earth to the sun). It orbits the sun in just under 88 days. Long thought to be tidally locked to the sun, it is now known as having a 3:2 resonance. This means that it rotates three times on its axis every two trips around the sun.
It is close enough to the sun for the sun’s gravity to have a small relativistic effect on its orbit. This may impact its long-term orbital stability. A small percentage of long term orbital simulations done for billions (Ga) of years in the future predict Mercury may not remain it in its current orbit and become a threat to other planets in the inner system.
Mercury’s iron-rich core is proportionally huge, occupying at least 55% of its internal volume. In contrast, earth’s core occupies only 17% of its internal volume. Several current theories attempt to explain the overly large core. One possibility is a giant impact event during formation. Another is the very young and very hot sun boiling off the mantle shortly after formation. A final is that the formation nebula preferentially favored heavier molecules. Each theory predicts a different surface chemistry and we don’t know enough about the planet yet to figure out which one is the correct answer. The surface chemistry also dictates at some level what sort of volcanic activity Mercury will have. Work on this is only beginning.
The only remaining tectonic activity on Mercury appears to be overall shrinkage as the core and mantle cool. This creates linear scarps on the crust like we see on the moon, called rupes on Mercury. Mercury is thought to have shrunk 1 – 7 km in radius since its formation.
Impact events dominate the visible surface, with at least 15 impact basins identified so far. The largest is the Caloris Basin at 1,550 km in diameter, one of the largest in the entire system. The impact was so great that it formed hilly terrain (Weird Terrain) at the antipode on the opposite side from the impact. There are those who suggest that hot spots on earth are antipodal from large known impact sites.
Comets and asteroid impacts during the Late Heavy Bombardment and afterwards are thought to supply the suspected ice at the pole(s).
Two different spacecraft visited Mercury. The first was Mariner 10 1974 – 1975 which did two flyby passes. It took the first images of the planet getting as close as 327 km on its closest pass. Mariner 10 was supposed to map the entire planet, but due to the way their orbits timed, it saw the same face on each pass. It ran out of maneuvering fuel shortly after the second pass and was shut down. The biggest surprise from Mariner 10 was the discovery that Mercury was heavily covered with impact craters.
MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging), launched in 2004 completed 4,000 orbits around Mercury 2011 – 2015. It made three flybys 2008 – 2009 before entering orbit, finally mapping the entire planet. Its mission was to investigate Mercury’s high density, geologic history, nature of its magnetic field, core structure, look for ice at the poles, and see where its atmosphere comes from. Its mission ended shortly before it was to run out of maneuvering fuel when it was crashed into Mercury. The biggest surprise from Messenger was almost everything else, tectonics, surface geology, and most importantly (from the standpoint of this post), evidence of volcanic activity.
Science took off quickly with Messenger, via imaging of (relatively) recent volcanic vents on the rim of the Caloris Basin on its very first flyby.
European Space Agency (ESA) and Japan Space Agency launched a joint mission BepiColombo in 2018. Its two probes are intended to orbit Mercury for a year, one of which will investigate its magnetic field, and the other that will map it in a variety of wavelengths.
Like the Moon, Mercury appears to have at least a couple types of volcanic activity during its history. The oldest created rolling plains of lava flows, not unlike the basalts covering the lunar Maria. This activity is thought to have stopped some 3.5 Ga ago. Volcanic activity also resurfaced the floors of the impact basins, again like what happened on the moon.
Volcanic activity diverged from that on the moon with at least 51 suspected locations of pyroclastic volcanism dated between 3.5 – 1 Ga ago. Many of these locations are on the rims of larger impact craters. There is one feature thought to be a 60 – 100 km diameter shield volcano on the rim of the Caloris basin.
Some of the pyroclastic material, identified by a substantial difference in color from that of the surface, is more than 50 km from the suspected vent. To eject pyroclastic material this far, the magma must have been rich in volatiles, leading to an estimate that the parent magma contained nearly 1.5% volatiles which flashed to gas powering the eruptions. That these sites are often associated with impact craters suggests that the impact event either disrupted quiescent magma pockets or created them, perhaps both.
Effusive lava flows carved long channels and formed teardrop shaped hills, not unlike what we have found on both the Moon and Mars.
Volcanic craters are typically scalloped depressions. They need to be individually studied to determine whether they are volcanic in origin or eroded / ancient impact craters. Formation sequence of events is similar to what we see in Hawaii – intrusion of a shallow magma dike / sill / chamber causes inflation of the summit. Lateral intrusion and eruption cause deflation and collapse. Large craters have multiple episodes / stages of inflation and collapse. This leads to multiple intersecting depressions and scalloping of the walls and rims of the depression.
Volcanic flooding of large impact craters (50 – 200 km diameter range) is found in multiple places on Mercury. Volcanic flooding is different from impact melt, takes place later, and it shows flow fronts and wraps around internal structures (embayment) inside the crater rim. These are typically inside the crater rim.
Impacts remove surface material, inject subsurface heat and relax pre-existing stress. Basins and craters may then become preferred sites for future volcanic activity on a globally contracting planet.
I am referring to recent volcanic activity on Mercury as anything that took place after the end of flood basalts some 3.5 Ga. The flood basalts appear to stop as the interior of Mercury cooled to the point that the planet started shrinking, forming the wrinkle ridges and scarps. Pyroclastic and effusive volcanism since then appear to all be related to impact events.
What would cause this? Two things, as the planet cooled, there was less magma available to erupt and due to overall contraction, it got more difficult for those massive amounts of magma to find their way to the surface. But there were magmas still present, some distance below the surface in the mantle, cooling slowly, most likely in pockets. Impact events inject heat creating impact melt which cools relatively quickly. It also removes some of the crust above the magma and fractures the crust so that the lighter, hotter magma can rise to the surface. This mostly stopped some 1 Ga ago, though it would not be surprising to find more recent eruptive locations upon closer inspection not unlike relatively recent volcanic activity on the moon.
Mercury is similar to the moon in that it is a small body, with a single tectonic plate. It is dissimilar in that it is much closer to the sun and receives significantly more surface heat from sunlight, meaning it cools down a bit slower than the moon did. There are no major shield volcanoes like we see on Mars at Tharsis Ridge, although a few smaller ones have been identified. Shallow magma reservoirs are rare and there is little evidence of hot spot magmatism. Like the moon, volcanic vents are observed around the rims of impact basins. The floors of those basins tend to be filled with lavas erupted after the impact melt cooled.
The Caloris basin is the youngest known large impact basin on Mercury. It exposes layering in the crust and contains relatively young tectonic and volcanic features. The basin is surrounded by a raised rim and rolling ejecta deposits similar to the exterior of Orientale basin on the moon. Exterior plains to the east have pervasive wrinkle ridges due to overall planetary shrinkage. Interior plains are thought to be either effusive volcanic lava flows or impact melt, perhaps both. The interior plains have wrinkle ridges and younger extensional troughs. These are both concentric and radial to the basin, forming giant polygons in places. The interior plains are generally slightly redder than the exterior plains, once again, similar to what we see on the moon.
Initial analysis of Messenger flyby photos discovered two types of volcanic vents around the Caloris basin internal margin. Deposits around one of the vents appear to be a 100 km diameter shield. Irregularly shaped rimless depressions with scalloped walls similar to those seen on the Moon are surrounded by diffuse bright deposits that are a bit redder than those surrounding fresh impact craters. They do not have ray-like patterns typical of impact craters.
The suspected volcanic structures are found along the southern margin of the Caloris basin interior. At least ten of them were identified. All are well within the expected range of suspected volcanicly created color anomalies.
Smooth plains outside Caloris (within a basin radius of the exterior rim) are also brighter, show embayment (lava coverage of ejecta fields) and evidence of lava resurfacing in places.
Caloris also has another structure, a large radial graben swarm, Pantheon Fossae, near the center of the basin. This structure is unique on the planet. It has hundreds of individual graben segments 5 – 110 km long. At the center, the graben segments crosscut one another, creating a local polygonal pattern. Others curve from the center as they approach it. The radius of graben segments is around 175 km. A few extend farther to the N and SW, intersecting with a concentric graben on the crater floor. There is a large graben, 8 km wide that extends from the center for 100 km. Some graben walls have convex outward wall segments that resemble crater chain segments. One crater chain has distinctive raised rims that parallel nearby grabens. Craters smaller than 5 km in diameter are rarely cut by grabens. This implies that the graben swarm formed shortly after the emplacement of the basin floor. The graben swarm is thought to be the surface expression of a subsurface radial dike swarm caused by a large magma reservoir. Similar features have been found on Earth, Venus and Mars. The location of the swarm suggests that formation of the structure is linked to the evolution of the basin.
Rachmaninoff crater was first closely imaged on the third Messenger flyby prior to orbital insertion. It is a 290 km diameter peak ring impact basin. It is relatively young, perhaps as recent as 1 Ga. Age estimate is based on the lack of craters within in the basin. Generally, the more craters seen in a basin, the older it is. It is large enough to be transitional between a peak ring basin (two rings) and a multiple ring basin with a partial third ring spanning 120 degrees to the SW. The inner ring is around 130 km in diameter.
The basin is surrounded by a continuous ejecta deposit and numerous secondary craters. It has no visible rays. The ejecta deposit, wall terraces and peaks are all well preserved, indicating the impact took place well after the end of the Late Heavy Bombardment 3.8 Ga ago.
There are at least three plain types on the floor within the peak ring. There is a smooth, relatively bright plain covering the floor inside the inner ring. There are at least three lower reflectance hummocky plains covering the annulus between the two rings. The inner plains are similar to a large region of smooth plains to the NE of the basin that are younger than the crater ejecta. The annular plains are thought to be formed from impact melt.
The inner smooth plains appear to be volcanically deposited some time after the impact event. The lavas flowed across the inner peak ring to partly flood the southern part of the crater. There is a narrow region of low reflectance smooth material just inside the inner ring that is similar to lunar impact basin edges that have subsided due to subsurface cooling, volcanic flooding, lava coverage of a topographic low, and lithosphere flexure in response to the volcanic load. This could have also been created by collapse of peak ring material onto the inner smooth plain.
Later pyroclastic volcanism appears to have taken place in multiple locations along the outer ring and outside the basin itself. One is a bright patch of yellowish rough material along the SE margin of the plains in the annulus.
About 480 km to the NE, there is a similar high-reflectance halo of material some 200 km in diameter surrounding a 30 km diameter, irregularly shaped, rimless, steep sided depression. The material in this halo is among the brightest features on the entire planet. Color of the material is not thought to be due to weathering. Reflectance of the halo deposits are similar to spectrally distinct deposits elsewhere in Mercury associated with crater and basin interiors thought to be products of pyroclastic volcanism where bright material was deposited ballistically around a central source vent.
The scalloped depression NE of Rachmaninoff is larger than previously identified volcanic sources around the rim of Caloris and the halo extends twice as far as the deposit around the Caloris volcanic vent. No scalloped depression has yet been found withing the Rachmaninoff annular bright deposit.
There was also deformation, extension after volcanism ended inside the basin. The extension was likely related to uplift of the basin floor seen in other large impact basins on Mercury. The extensional troughs in Rachmaninoff is confined to the inner volcanic plains, which may mean that the volcanism and uplift are related.
Northern Smooth Plains
Widespread volcanic plains volcanism was active for the first Ga after Mercury was formed, ending around 3.6 – 3.5 Ga ago. Younger volcanism has been observed, but in significantly lower volume and only associated with impact structures. Global contraction may have interfered with the ability of magma to reach the surface. As an alternative, mantle thermochemical evolution models suggest that large volume magma generation mostly stopped around that same time.
There are “ghost craters” found all around Mercury. These are older craters that have been inundated with flood basalt flows early in Mercury’s history. We see the same sort of thing in places on the moon. Maximum submerged depth is around 2 km. Source of the flood basalts, like that of the lunar maria is unknown.
Flood volcanism around the north pole covers perhaps 6% of the total surface area of the planet. The smooth plains are homogeneous and differ in color from the more heavily cratered surrounding terrain. Ghost craters, craters that were partially to completely buried in lava give clues to the thickness of the younger lava flows. Buried craters 60 – over 100 km in diameter within the plains indicate lava thickness in the plains is locally greater than 1 – 2 km.
The plains contain wrinkle ridges and broader arches that appear to postdate the surface of the smooth plains. There is no evidence of a nearby large impact basin that the lava flows may have come from, though there may be a few very old impact basins buried by the flood eruptions.
Scientists looked at the margins of the northern plains of the for clues to describe the lava emplacement. There is an unusual grouping of pits 5 – 10 km in diameter, teardrop-shaped hills, rough plains, and lobate-margin smooth plains. The pits are interpreted as the source vents. The teardrop shaped hills are in areas where lava flows sculped the terrain similar to the way water carves terrain on other planets. This sort of sculpting requires high temperature, low viscosity fluid lavas. Flow fronts are visible as are lines and areas where flow fronts contact one another.
Counts of larger craters on the northern plains indicate the age of these plains is similar to the smooth plains that fill and surround the Caloris basin, dating it near the end of the Late Heavy Bombardment 3.7 – 3.8 Ga. Unlike Caloris, these lavas are not related to any currently identifiable impact event. This means that at that time, there was extensive partial melting of the mantle and widespread eruption of flood lavas at high effusion rates.
Polar ice was a discovery that started with ground-based radar observations of Mercury. The observations found highly reflective material in the floors of deep craters at Mercury’s north pole. This was confirmed by Messenger measuring shadow lengths to estimate the actual crater depth. Messenger found the floors of some of these craters, remain in permanent shadow, which means any ice imported during the impact event, subsequent impact events, or not driven away quickly by sunlight could be trapped in the regolith beneath the surface. There are impact craters near the south Lunar pole with permanently shadowed floors also thought to contain water ice.
There are about three dozen groups of oddly shaped cavities surrounded by bright material inside impact craters at all latitudes and terrains found on the planet. Nobody knows what these are. They may represent exposure of volatile-rich material to sunlight, which heated it up and drove off the volatiles. This would be an example of the sort of crust material that powered the pyroclastic volcanism. Areas with the most sunlight seem to be most affected. Depth estimates of 2,608 hollows is 8 – 40 m with an average around 24 m. The bright halos around the hollows may be due to condensation of sublimated materials or physical / chemical modification of the surface by redeposited sublimated materials.
Tectonics on Mercury are dominated by crustal shortening as the planet cools. This shows up as positive relief landforms globally. These have been called “wrinkle ridge” and “lobate scarp.” Ridges are generally smaller than the scarps. These are thought to involve some combination of thrust faulting and folding. Many of these are found above large surfaces between lava flows and likely along older regolith layers.
Larger shortening landforms are generally in the plains between craters; the smaller ones tend to be localized within the smooth plains. There is no global pattern, though the major shortening landforms may be slightly oriented N-S. Some smooth plains structures may be due to volcanic loads, but the overall shrinkage of the planet is the dominant force as Mercury may have shrunk by as much as 7 km in radius since it was formed.
Smaller shortening structures hundreds of meters long and tens of meters high have been identified. Their relation to craters suggests that the planet is still shrinking and may have been doing so for the last 3.6 Ga.
Major extensional tectonic landforms are much less common, occurring almost exclusively in impact craters partially or entirely filled with lava. Troughs and grabens in these craters are thought to be the result of thermal contraction of ponded lavas or caused by intrusion of magmas released by the impact event itself. Some smaller craters in the 50 km diameter range have floor fracturing, which may be due to local sill emplacement. Larger impact basins in the 250 km diameter range have concentric grabens on the floor of the peak ring. The larges impact basin, Caloris has a radial dike swarm in its center, the Pantheon Fossae. On Earth, radial dike swarms are generally exposed by erosion. They are widely observed in association with volcanic activity on Venus. Mercury only has a single example in Caloris.
Cooling of the interior dominated tectonic and volcanic activity on Mercury. Early effusive volcanism likely built much of the crust and largely stopped around 3.6 Ga ago as the cooling and shortening structures made it more difficult for magma to reach the surface in large volumes. Small volume effusive and explosive volcanism continued for the next 2.5 Ga, largely confined to pre-existing impact and tectonic structures. Volcanism also influenced where tectonic deformation occurred, generally in localized extension in lava-filled impact craters / basins.
For such a small place, Mercury has had a rich and varied volcanic history, with massive volcanic flood basalts giving way to smaller effusive eruptions and pyroclastic eruptions as the planet cooled. Over the last 3.5 Ga, volcanic activity has been confined mostly to the vicinity of impact events. I think the more we look, the more we will find. And it will all be incredibly interesting and surprising.