Probable Aspects of Natural Arch Formation on Mars
We start with the assumption that the feature found on HiRISE image PSP_001420_2045 is a lava natural arch. Several observed attributes of the feature support this hypothesis and none contradict it. However, since the presence of an opening is not directly observed, final confirmation awaits further evidence. Confirming evidence might result from subsequent imagery taken when the sun is at a favorable and lower aspect angle, e.g., two or three hours after local noon, during northern winter, such that a shadow cast by the underside of the lintel is visible.
Making the assumption in the meantime, what can be said about the arch? We are able to locate it fairly accurately within the currently accepted Martian latitude/longitude system. We can also get upper bounds on its span and height, a very good measurement of its width, and even make a guess about its likely thickness. However, the most interesting discussion centers on how the arch formed and how old it is.
On Earth, all the natural arches currently documented are very young geologically. None are likely to be older than about 50,000 years and most are much younger. However, this is a consequence of the processes and rates of erosion that occur on Earth. These processes and rates may not pertain to Mars. When asking how old this arch is, or how it formed, we have to account for what the characteristics of Martian erosion might be.
One obvious indication that erosion rates are slower on Mars than on Earth is the much greater number of visible impact craters on the Martian surface. This is not the result of a higher frequency of impacts on Mars. It is because the impact craters on Earth erode into obscurity very rapidly. They last longer on Mars. Similarly, slower erosion rates could mean that natural arches on Mars have a longer lifespan on average than their terrestrial cousins.
On Earth, all known natural arches, including lava natural arches, form, evolve, and collapse as the result of erosion processes that are significantly influenced by the presence of water. While water is present on Mars, it is much less abundant. It cannot act to accelerate erosion processes on Mars to anywhere near the extent it does on Earth.
In addition to water abundance, there are other differences between Earth and Mars that must influence erosion processes and rates. The pull of Martian gravity is only one third that of Earth’s. The surface temperature is several degrees cooler on average and varies over a greater range. The atmosphere is much thinner.
On the other hand, there are some important similarities. Diurnal temperature fluctuation has a similar period (24.6 hours versus 24.0 hours per day). There is clear evidence of recent and on-going volcanic activity. The chemical characteristics of basaltic rock are very similar.
With these differences and similarities in mind then, let us consider what erosion processes are likely to be involved in the formation of natural arches on Mars. The first point to make is that erosion processes that depend on abundant water are not going to be found with much frequency. Those that do not depend on water will be much more important.
Flowing water, seeping water, and cycles of freezing and thawing water drive many of the formation processes for natural arches on Earth. They also lead to the formation of specific types of natural arches, e.g., natural bridges, cave natural arches, pillar natural arches, pothole natural arches, and shelter natural arches. Thus, these types are not likely to be found on Mars. Indeed, if they are found, it would be strong evidence that at least local pockets of abundant water existed very recently on Mars.
The factors that would likely drive the arch forming processes on Mars are thermal fluctuation and gravity. Thus, we would still expect to see alcove natural arches, buttress natural arches, fin natural arches, and lava natural arches. Although water accelerates the formation of these types of arches on Earth, the absence of water would not prevent their formation on Mars.
As stated above, we are assuming the feature is a lava natural arch and hence formed as the result of roof collapse over a lava tube. Would the Martian environment make any differences in these processes? Yes, clearly so.
One consequence of the lighter gravity on Mars is that lava flowing in a tube moves more slowly than on Earth. Further, the thinner air means that surface cooling is slower. Cooling is primarily radiative rather than convective. The lava stays hotter longer and takes longer to evacuate the tube. In turn, this makes tubes longer, wider, and deeper than is typical on Earth. It also makes any tube roofs that form thinner.
Another consequence of the lighter gravity is that the upward convexity of the roof need not be as pronounced in order for the arch shape to withstand the downward pull as the tube evacuates. Flatter roofs are more probable than on Earth. Thus, one would expect to see longer, wider tubes with flatter, thinner roofs on Mars.
Thermal fluctuation is almost certainly the principle process leading to roof collapse on Mars. The tube roofs on Mars go through a daily cycle of contraction and expansion, weakening and fracturing the rock. Thin, flat roofs over wide tubes would not last for long, even in the weaker Martian gravity.
When roof collapse first begins to happen on terrestrial lava tubes it forms sequences, or series, of lava natural arches. Over time, the surviving lintels narrow in width. One by one they collapse, leaving only the most structurally sound. Finally, the roof is completely gone and no arches remain. A solitary lava natural arch on a long lava tube is rare on Earth and must be considered relatively old (although still very, very young on geologic time scales). There is no reason to suspect things would be different on Mars.
Clearly, the feature we are assuming is a lava natural arch is quite convex upward. There are series of terraces rising from the ends of the lintel toward its center. Further, the width of the lintel is about as great as its span. Both are indications of structural strength. Finally, it is the only lintel on a very long tube. It is easy to conclude, therefore, that this is a relatively old lava natural arch.
Obviously, relative age is only important if there are other examples of lava natural arches found on Mars. If this proves true, however, the relative age of these arches would be very useful in determining the relative ages of the tubes they are on, and hence the relative ages of the volcanic activity that caused the tubes. Lava natural arches, if found with any great frequency, could be a very important source of information about the recent history of geologic activity on the Martian surface.
It is even conceivable that an approximate timeline could be established to date nearby surface features based on local density counts of the lava natural arches found in a region along with other observed attributes of the natural arches surveyed. Furthermore, such a timeline, even if only relative, would cover the most recent details of the surface history. Compare this to the technique of using crater counts to date surfaces. Crater counts can only provide information over much longer time scales and with much less granularity.
A similar train of logic could be applied to other types of natural arches. A study of the counts and characteristics of Martian alcove natural arches, buttress natural arches, and fin natural arches would also yield valuable information about the characteristics of recent surface processes. Unfortunately, these types of natural arches would be very difficult to detect from orbit. Those that were detected would be biased by a selection effect, i.e., predominately oriented to cast telltale shadows of the opening.
Lava natural arches do not suffer from this limitation. They are easily detected with the HiRISE instrument and are likely to be found in abundance on most lava tubes that are narrow enough, and young enough, to have retained them. The figure below includes a detail from an image of the Pavonis Mons region taken with the High Resolution Stereo Camera (HRSC) on board ESA's Mars Express. It shows two potential targets that would benefit from HiRISE imagery. Many similar targets exist. A program to systematically discover and study these features would yield significant scientific return.