Image Credit: NASA
The photograph above was taken by the Opportunity rover, one of the two robotic rovers now on Mars. The NASA web page giving the photo says it was taken on Sol 3720, which corresponds to July 11, 2014.
If you do a Google search for “Celtic cross on Mars,” you will find quite a few Internet discussions about this photo. At first sight the picture seems to indicate something sensational – what looks like an image of something artificial on the surface of Mars, perhaps some artifact from long ago.
But almost as soon as the strange shape was discovered, skeptics came up with an explanation trying to explain it away. The explanation was that the “X within a circle” or “Celtic cross” shape was a screw imprint. It seems there is a scientific instrument on the Opportunity rover called the APXS, which stands for Alpha Particle X-Ray Spectrometer. On this NASA page, the APXS is described as a “contact” instrument, leading one to assume that it makes contact with the soil or rock it is analyzing. The APXS is shown below:
The APXS instrument
Now here is the ingenious explanation of the skeptics. They noticed that in the image above, there are exterior flat screws, which have an “X within a circle” shape similar to the “Celtic cross” shape discovered on Mars. The skeptics then suggested that the “Celtic cross” shape was caused by a “screw imprint” when the APXS instrument made contact with the surface of the planet Mars.
Some initial visual analysis leads one to be doubtful about this explanation. The APXS instrument has quite a few screws that might make an imprint, but we see only one “X within a circle” in the Mars image of the “Celtic cross.” Also, there seems to be a kind of handle shape attached to this “Celtic cross,” something that is not explained by the “screw imprint” hypothesis. Also, the “X” part of the “Celtic cross” appears to be higher than the nearby rock or soil, an effect one would not see if the shape was produced by a screw imprint.
The real test of the “screw imprint” hypothesis is the timing of the Opportunity rover's activities. In order for this explanation to hold up, it would have to have been that the Opportunity rover was actually using the APXS instrument at the particular spot that the photo was taken, before or during the Martian day of July 11, 2014 (3720) when the photo was taken. What do the NASA logs say about this?
The relevant log is found here, which covers the period between July 9th and July 17th. Below is a screen shot from the NASA web page:
According to the log, between July 9th and July 17th, the Opportunity rover was mainly busy traveling. During this period, there was only one day when the APSX instrument was used for in-place “contact” science:
The one sol of in-situ (contact) science was the first sol of a two-sol autonomous 'touch 'n go' where the rover used the robotic arm (the 'touch') on Sol 3720 (July 11, 2014), to collect a Microscopic Imager mosaic of the surface target 'Trebia,' followed by an overnight contact integration measurement performed by the Alpha Particle X-ray Spectrometer (APXS).
So is the skeptical explanation of “screw imprint” compatible with this history of what the Opportunity was doing? No, it is not. The record above makes clear that the chronology was as follows:
- First, during the
Martian day of Sol 3720 (July 11, 2014) the photos at the Trebia
site were taken by the Opportunity rover, including the photo of the
strange-looking “Celtic cross.”
- Then, during the
following Martian NIGHT (as indicated by the word “overnight,”)
the APXS instrument was used (presumably to make some type of
contact with the ground), which was the only time that instrument
was used to make contact with the ground during the period between
July 9th and July 17th.
So the official NASA chronology refutes the glib “screw imprint” explanation for the “Celtic cross” on Mars. Apparently the mysterious “Celtic cross” could not have been produced by the APXS instrument.
We are stuck with a big mystery. What in blazes is an X shape within a perfect circle doing on the planet Mars? In my next blog post, I will theorize about several possible explanations for this Martian surface anomaly and other strange anomalies on the Martian surface. These explanations will involve various awe-inspiring possibilities.
Postscript: If you read the Opportunity activity log here (the same link given above), it becomes clear that it is something like standard operating procedure for the Opportunity rover to travel to a spot, use the Microscopic Imager unit first to take pictures (the unit that took the "Celtic cross" picture), and then later (usually overnight) to use the APXS instrument (the unit claimed as the source of the "screw imprint") on the same spot. It's easy to see why that order is preferred. Since the APXS has to make contact with the surface, it's better to take the pictures first, to capture the natural, undisturbed look of the soil and rocks being studied (rather than take pictures of the soil after it has been touched and zapped by the APXS with its x-rays and alpha particles).