Since the day of their conception, billions of years ago, all planets in the solar system
have been subject to a similar bombardment of meteorites as our planet, Earth, has been.
As such, many worlds, and especially the inner, terrestrial worlds, bear the scars and reminders
of a tumultuous past etched across their surface… but these intriguing cosmic landmarks are
also waiting to be explored.
This is Unveiled, and today we're investigating four unexplained craters on solar system planets.
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Formed after an impact on a planetary surface, almost every surface crater shares the same
basic components; they all have a floor at the bottom, surrounding walls, an upper rim
atop those walls, and a scrawl of displaced material all around the site known as ejecta.
Yet, despite these shared components, every impact site, like a fingerprint, is unique.
And astronomers occasionally come across some that defy explanation.
Our first mysterious impact formation is found on the sulphur-rich surface of Mercury.
Like Earth's moon, Mercury has no atmosphere and therefore can’t repair its collision
sites.
This means that the various impact craters picked up throughout the planet's 4.5-billion-year
history are near-perfectly preserved.
However, out of the many hundreds of recorded cavities on Mercury's surface, the crater
Apollodorus makes our list as one of the most mysterious… thanks to the enigmatic trough
formation that surrounds it, known as the Pantheon Fossae.
When NASA's MESSENGER spacecraft made its first flyby of Mercury on January 14th, 2008,
it discovered a whole series of new craters within the wider Caloris Basin.
Among this new crop of collision scars, they discovered Apollodorus… a 25-mile-wide crater
positioned in the centre of a wider landform; a bizarre series of long, creeping troughs
projecting outwards now known as the Pantheon Fossae.
It quickly became one of the most unique crater formations in the solar system and was affectionately
nicknamed "The Spider”, due to its visual resemblance to the Earth-bound arachnid.
Despite Apollodorus' position in the centre of the Pantheon Fossae formation, however,
not everyone believes that the two landforms were formed by the same impact event.
The more common suggestion is that this curious corner of Mercury could be the result of two
impacts, with one happening on top of where another had before.
Another possibility is that the crater and troughs could be the result of some other
planetary function that has yet to be confirmed.
For now, it forever marks the surface of the closest planet to the Sun, and it remains
unexplained.
Like Apollodorus the Spider, today’s second crater is also arguably more famous for the
land fixtures that surround it.
At just over 100 miles in diameter, Isabella is the second-largest impact site on Venus,
but what sets it apart more than anything else are the two extensive flow-like structures
that span to the South and Southeast of it.
Venus is of course known for its volcanoes, and these flows might look like leftovers
from past eruptions… but scientists don’t generally believe that this is the case.
Instead, there are two alternative theories.
The first is that they’re caused by a process called impact melt… which is where, thanks
to the planet's thick atmosphere and high surface temperature plus the thermal energy
of the impact itself, large pieces of rock melt upon impact to become free-flowing lava.
The second theory is similar, but instead involves the release of hot gas clouds after
an impact that may have caused rock to melt and form the bizarre ridges south of Isabella
that way.
Whatever the cause, similar flow structures can be found in and around several craters
on the Venusian surface - it’s just that none are quite as prominent as those seen
around Isabella.
In fact, due to its high temperature, high pressure, and generally hellish conditions,
Venus' craters all tend to differ in composition from impact sites found on other planets.
For example, since the atmosphere is so thick on Venus, most meteorites break up before
impacting the planet itself.
And those that do break through usually hit in clusters rather than via one big collision.
The release of ejecta debris is also typically more contained on Venus, and less spread out.
This means that large deposits form closer to the crater itself, along with the flow-like
structures as seen on Isabella.
There’s something else that makes these stranger than most craters in the solar system,
though: their apparent age.
According to some studies, the craters on Venus are all relatively young… with the
Venusian surface (craters included) today estimated to be only about 500 million years
old.
The flow formations around Isabella, then, have emerged quite recently, cosmologically
speaking.
And when we look at Venus, we’re looking at a truly unique environment in the solar
system.
But, now let’s skip over Earth and onto our third crater, which brings us to the next
stop in the planetary line, finding itself stretched across the surface of Mars.
With an area of around 240 miles by 85 miles, this impact site is definitely huge… and
it’s known as Orcus Patera.
More than its size, however, Orcus Patera is an oddity amongst craters due to its oblong
shape.
Unlike other large craters on Mars that maintain the standard circular condition, with debris
flung out quite predictably in all directions…
Orcus Patera offers an unpredictable shape.
Its rim rises more than a mile above the surrounding plains, too, and its floor plummets as far
as 1,800 feet below the surface.
Located between two volcanoes, Olympus Mons and Elysium Mons, this abnormal landform was
initially believed to be a result of volcanism itself.
However, contemporary scientists no longer think this to be the case, and alternative
theories have developed to suggest that Orcus Patera is, in fact, the result of an impact.
The first theory is that Orcus Patera was at one point a regular round impact crater
but that, over time, planetary compressional forces may have caused it to stretch out and
shift into the irregular shape we see today.
Another and more widely supported theory suggests that the crater was caused by a rare impact
where the collision struck at a tiny angle - perhaps of five degrees, or less.
A collision like this may have caused the meteorite to skid along the Martian surface,
digging out the uniquely shaped crater in the process.
Unfortunately, we so far have no way to conclusively prove one theory or the other, and so Orcus
Patera, again, remains unexplained.
For today’s final crater it’s back to where we started, on Mercury.
Because, during the same flyby that discovered Apollodorus the Spider, the MESSENGER probe
also snapped an image of two impact cavities (again inside the Caloris Basin), but this
time surrounded by dark halo rings.
One of these sites offered another curiosity, too, due to it having an unusually shiny crater
floor.
While these locations have so far remained unnamed, there are two main theories to explain
the dark rings that surround them.
The first is called the Layered Cake Theory, proposing that there must be a layer of an
unknown material lying beneath the Caloris Basin, that when struck at the right depth
is revealed amongst the impact debris… and this is what we can see today bordering some
of the craters on Mercury.
The second theory, known as the Impact Glass Theory, is a variation of other impact melt
theories… suggesting that the crash may have melted some of Mercury's surface, causing
it to reform as glass on the crater's edge.
However, while either of these theories could explain the presence of the dark halos around
the craters, neither of them explains the inexplicable shine on the crater's floor - which
was described at the time by a MESSENGER team member, Clark Chapman, as though like ice
shining in the sun.
But, ultimately, Mercury’s lack of atmosphere combined with a scorching surface temperature
of more than 400 degrees Celsius, should make the presence of ice here impossible.
So, what could have caused this seeming anomaly?
For now, it’s another crater story that researchers can’t quite complete.
As these four examples show, remnants of planetary collision sites can be found throughout the
solar system.
And, often, these sites help us to discover the ancient history of our astronomical neighbours.
But there are plenty of questions that remain unanswered, as well.
What caused the Pantheon Fossae to stretch outwards from Mercury's Apollodorus crater?
Why does Isabella on Venus have such distinct, flowing structures snaking away from it?
What could have caused the irregular shape of Orcus Patera on Mars?
And, again on Mercury, what could two distinctly haloed spots mean for the planet as a whole?
Because those are four unexplained craters on solar system planets.
What do you think?
Is there anything we missed?
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