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.