Comparative Planetology

The goal for this week is to see how our understanding can expand by comparing landforms on different worlds. The Moon and Earth are the places we know best, but there are fascinating examples across the solar system. This discipline is called comparative planetology but that is too long to fit on the menu, so it simply says analogs!

Lunar Features & Planetary Analogs

Here is a list of features observed on the Moon. We haven't discussed each of them, but in the pictures and text you will learn about them, and their planetary cousins.

Impact Craters

mcrater4_s.jpg Impact craters occur everywhere in the solar system because there are trillions of pieces of material orbiting the Sun that can collide with other pieces of material. Most of these projectiles are tiny - dust sized and smaller - so they have little impact (ha ha), but everything larger that collides with something else makes an impact structure. Or, if the energy is high compared to the target strength, destroys the target - making more projectiles. There are tens of thousands of lunar craters larger than 1 km in diameter, which is about the size of the best known impact crater on Earth, Meteor Crater in Arizona. Meteor Crater image from Smithsonian Institution

There are about 200 known impact craters on Earth - most are heavily eroded or buried by sediments and not easy to detect. Here is a map and database of the distribution of impact craters. Explore those in North America and other places to gain an idea of the number, diameters, ages and state of preservations.

Here is info about some well-preserved impact craters.
Meteor Crater a simple crater formed ~50,000 years ago. Here is a virtual tour.

Impact Basins

normal_Mare_Orientale_lo4_m187.jpg Because large projectiles were much more common at the end of the accretion period of the solar system about 4 billion years ago, most impact basins are ancient, and thus not preserved on the dynamic Earth. But there are few here and others on Mars, Mercury and in Titan.
Ries Basin a 23 km wide, two-ring impact basin, 14.5 million years old in southern Germany. Another Virtual Tour. Orientale Basin image from Lunar Orbiter IV


On the Moon, maria are vast piles of basaltic lavas that fill impact basins. Because Earth doesn't currently have large basins we don't have lavas in them, but that was probably common 3+ billion years ago. Earth does have vast piles of lavas - the ocean floors and so-called continental flood basalts. The Columbia River Basalts of Washington state and the Deccan Traps of India are the best known flood basalts. Both the ocean floor basalts and the flood basalts erupt at plate tectonic boundaries, where crustal rocks split apart. On the Moon, there is no plate tectonics but there is deep fracturing associated with impact basins.

Sinuous Rilles

normal_1512915-hadley-rille-270307.jpg Rilles are depressions that are much longer than they are wide. Sinuous rilles are typically narrow - 1-3 km wide, 10s to 100 km long wiggles across lunar maria. The most famous sinuous rille is Hadley Rille, and the Apollo 15 landing site was selected partially to understand what these snakelike channels are. But we already knew from comparison studies with the Earth (e.g. Cruikshank and Wood, 1972?) that they are lava channels and tubes, common features formed in flowing lava on Earth. The Moon's biggest sinuous rille - and its most unusual one - is Schröter's Valley north of the crater Aristarchus.
SInuous rilles occur on Venus and Mars, and on Earth, especially in Hawaii, Iceland and Idaho. The occurrence of sinuous rilles on lunar maria is further evidence the maria are lava flows.

Linear Rilles

Lunar rilles have multiple shapes and origins. There are perhaps a dozen or so linear rilles that are often hundreds of kilometers long. These are sometimes along the edges of maria, but more tens to be about radial (Sirsalis Rille), and the biggest (Ariadaeus) doesn't have any convincing relation to a mare basin.
Linear rilles may have multiple, related origins. Some of the lunar ones appear to be the surface expression of a dike, which is a thin vertical sheet of magma (what lava is called when it is still in the ground) that may extend hundreds of kilometers. Some lunar rilles have little volcanic cones along their lengths where the dike reached the surface. The way dikes make rilles is that they push their way through the crust the surface over the dike is pulled apart and drops down.

Concentric Rilles

Rima_Hippalus_Higgins-SMALL.jpg These are called rilles, because they are depressions with lengths much longer than their widths, but their location gives away their origins. Concentric rilles occur along the outer edges of impact basins. As the basin fills with mare lavas it sags and the edges crack. The Hippalus Rilles around the eastern shore of Humorum is the Moon's most dramatic example. Narrower ones occur around the eastern side of Serenitatis and the western edge of Tranquillitatis. (Hippalus Rilles image from Wes Higgins)


Hortensius-domes-Pau.jpg Most of the interesting little features on the Moon are volcanic, and familiar to us from very similar landforms in terrestrial volcanic regions. Lunar domes are minor hills that are found in maria. Some have a classic shape like a half a hemisphere, but most are more gentle with slopes of only a few degrees. Some have little pits at their summits. Domes are eruptions of lavas that flow out from a vent (the source hole in the ground) and travel only a few to a dozen or so kilometers before cooling and stopping. A famous cluster of six domes is just north of the crater Hortensius, which is west of Copernicus.
On Earth these little domes are called shield volcanoes and they are very common in the same places that we find lava channels and tubes. Hortensius domes image from K.C. Pau

Dark Halo Craters

LPOD-2004-05-21.jpeg A final small volcanic feature of great interest are little craters - typically 1-5 km in diameter, surrounded by dark deposits. These dark halo craters (DHC) are very well seen on the floor of Alphonsus and some are connected by little linear rilles. The dark material is volcanic ash - the more scientific word is pyroclastics. On Earth, these are cinder cones, a very common type of gas-rich eruption in which the rising magma is torn apart by the gas it contains and forms pieces of cinder or scoria. If you have been to Flagstaff, AZ you have seen hundreds of such cinder cones. But the lunar ones look very different from the terrestrial ones - find out why below!
CInder cones also occur on Mars - part of my doctoral thesis was comparison of them with ones on Earth.
Just when we thought we had nailed DHC it was realized that some aren't of volcanic origin! The crater Copernicus H is a US-standard little impact crater, but its surrounded by a halo. H formed on a the bright ejecta of Copernicus, but it excavated below it and its own ejecta is from the underlying dark mare basalt! So now that we know that some impact craters excavate buried mare lavas we use them to locate regions of otherwise unknown, buried maria. This shows that the present maria making the face of the Man in the Moon, aren't the only ones that ever existed. Alphonsus and DHC image from Bruno Daversin

Crater Chains

Most crater chains on the Moon are lines of secondary craters produced by ejecta from primary impacts. After the observation of the breakup of the Shoemaker-Levy Comet in 199x, various researchers (including me) realized that some crater chains on the Moon could be the result of aligned impacts from similarly disaggregated comets. The best example cuts the floor of the crater Davy Y. Here is a case where we figured out how to interpret peculiar features on the Moon by looking at a totally different type of feature - comets!


NEW_Rupes_Recta_A47S297_11-05-04_P5IP1ROTATE.jpg Faults are very common on Earth, because plate tectonic forces pull apart and shear plates. The Moon doesn't have plate tectonics and thus has far fewer faults. The most famous one is the straight Wall on the east shore of Mare Nubium. This xx km long fault scarp is xx m high and has a slope of about 20° - its steep. but not vertical. I think the origin of the fault is indicated by its surroundings. It cuts the middle of a xx km wide crater whose entire western side is missing. I call the crater Ancient Thebit, after the smaller, younger crater Thebit that is within it. Ancient Thebit formed on the edge of the old Numbium impact basin. Later, when the Mare Nubium lavas erupted onto the surface it filled the basin and covered the western rim of Ancient Thebit. The weight of the mare caused the basin floor to sink, and the western half of Ancient Thebit was carried downward, creating a fracture that is the Straight Wall fault. Straihgt Wall image by Wes Higgins

Polar Ices

In 1960 a CalTech scientist predicted that there are deep craters near the lunar poles that are always dark - the Sun never rises above their rims so they have perpetual darkness and are very cold. He predicted that these craters could be cold traps for comet gases (mostly water) from occasion comet impacts on the Moon. This was a sort of wild-eye theory until radar studies of the poles of Mercury discovered ice in that torridly hot planet's cold polar craters. The Clementine spacecraft team did an experiment where they tried to detect radar reflections from our Moon's cold polar craters and got ambiguous results that could be consistent with ice. The Lunar Propsector spacecraft detected hydrogen at the lunar poles - most intensely near the permanently dark craters - buttressing the interpretation that there is ice in them thar craters. But it could just be solar wind emplaced hydrogen, not H from H2O.
Both Earth and Mars have polar ice deposits, but because these planets are gravitationally large enough and cool enough for H not to easily escape to space the ice caps can seasonally tilt toward the Sun and still survive. Survival occurs because more cold moist air is carried to the poles and deposited as snow where it becomes compressed into ice. But as we are observing on Earth, changes in climate can make the poles to warm to maintain ice caps and we may be in the early stages of a capless planet - with much higher sealevel. On Mars, as the caps melt each summer - actually they sublimate: the solid turns to gas - moisture is atmospherically carried to the opposite pole which is heading into winter.

Processes that Effect How Features Look

We understand most geologic (and atmospheric, biologic and other -ogic) processes based on our experience on Earth. But different conditions on other worlds made it hard to sometimes see the similarities.

Erosion & Destruction

When astronomers started talking about the origin of lunar craters they did not know about impact craters, so they interpreted lunar craters in terms of what they did know - voclanism - so all lunar features were thought to be some odd form of volcanism. Why weren't impact craters recognized on Earth until early in the 1900s (Meteor Crater was first)? Because nearly all of the millions of impact craters that must have formed during Earth's long history were wiped away by erosion and plate tectonics. The remaining craters were mostly partially buried or so eroded that they weren't even recognized as existing. And although impact craters still form on Earth, Moon and everywhere else in the solar system, most impact craters (especially the large ones) formed during the first half billion years of solar system history, so the replenishment rate of craters is low.
Contrast that with volcanism - we see about 60 eruptions/yr on Earth and have witnessed the formation of the Earthly equivalent of dark halo craters, sinuous rilles and faults. That is why we can be confident of interpreting lunar and planetary examples.


The Moon's Dark Halo Craters are wider and flatter than terrestrial cinder cones even though we think that very similar eruptions caused both. Why? Gravity. Because of the Moon 1/6th Earth's gravity, explosive eruptions there throw material far further than they would on Earth. On Earth, erupted cinders fall down around the vent building up steep-side cones. On the Moon the same eruption makes much wider, lower rimed craters.
Another possible effect of gravity is on lava tubes and channels. With Earth's six times greater pull of gravity, the ceilings of lava tubes - especially wide ones - commonly collapse, leaving the interiors exposed to view. On the Moon we see many collapse pits that in widely spaced lines, suggesting that a lava tunnel connects them, and its roof is mostly uncollapsed. Such lava tubes may be good places for lunar habitations because of the shielding from heat and cosmic rays.


The Moon's lack of atmosphere makes it impossible to have water on the surface (no atmospheric pressure so it instantly evaporates in the Sun), so the sinuous rilles, for example, could not be stream valleys. Atmosphere's also retards the movement of projectiles through them. So the dense atmospheres of Earth, Venus and Titan frictionally burn up small projectiles, keeping them from cratering the surface. The Moon's soil - regolith - is made by the constant impacting, fragmenting, melting, and redistributing the top few meters of material. Without an atmosphere, ejecta from impact events on the Moon and Mercury travel great distances to make secondary craters and rays. On Venus, the secondaries are clumped much closer to the primary craters' rims.


H and O are very common in the universe, and water (as ice) occurs widely in the solar system. The lack of water within the Moon - due to its unusually energetic formation - explains the Moon's lack of surface features created by water (lakes, oceans, streams and rain) , and also perhaps other features. Without water (water is about 1/10% of terrestrial basalts) it has been speculated that plate tectonic sliding of layers is less likely to occur, and none does for the Moon.


Studying Saturn's moon Titan has reminded me of the importance of temperature to landforms and geologic processes. At 90 degrees above absolute zero Titan is very cold. Too cold for water to be liquid or a gas - we think the crust of Titan may contain water rocks (ice). But Titan has the equivalent of a hydrological cycle with methane as the liquid, gas and maybe solid. At Titan's frigid temperature methane can exist in multiple states. So we see rivers that were probably carved by flowing methane, there are lakes filled with it, and it is in clouds and probably in the crust.
The high temperature of the Moon means that any low density liquids on the surface would evaporate quickly - and there is no evidence of flow except for lava flows.


It is not really a characteristic of a planet, but lack of proper training delayed understanding of the nature of the Moon's terrain. Most people who studied and speculated about the Moon were astronomers. They did not know about geology, but the processes that create landforms are geologic processes. Because a telescope had to be used, the Moon belonged to astronomers, not geologists. The few trained geologists who looked at the Moon realized that lunar craters are nothing like terrestrial volcanoes so they were open to meteorite bombardment.
Also, much of the speculation about the Moon was made in the 18th and 19th centuries by amateurs who had no particular training in physical sciences. Hence, they came up with preposterous ideas because they didn't know better.

Assignment #4

What is the impact crater and lunar-like volcanic feature closest to where you live. How do those features compare to ones on the Moon? Bigger? Smaller? Different erosional state? What questions do you have about any lunar feature?

Exploring lunar craters on Earth