On two consecutive days, photographs from Mars seemed to show an unusual light on the horizon. Let's look at these photographs, and consider whether NASA's explanations for them are plausible.
The first photograph was taken by Navcam: Right B (NAV_RIGHT_B) onboard NASA's Mars rover Curiosity on Sol 588 (2014-04-02 09:04:28 UTC):
Below is a closeup of the unusual part of the image (which appears on the top left corner).
The second photograph was taken the next day by Navcam: Right B (NAV_RIGHT_B) onboard NASA's Mars rover Curiosity on Sol 589 (2014-04-03 10:00:03 UTC).
Both photographs show what appear to be flashes of light on the surface of the horizon, with an orientation perpendicular to the horizon. It is as if the lights were shining straight up from the surface in the direction of the sky, without any diagonal slanting.
Three explanations have been put forth for the images by Justin Maki, a NASA imaging scientist. The first explanation is that the lights are reflections from a rock on the horizon. The second explanation is that the apparent lights are caused by a vent hole in the camera apparatus. The third explanation is that the apparent lights are caused by cosmic rays.
First, let's look at the cosmic ray explanation. This was immediately seized upon with great matter-of-fact dogmatic certitude by Nancy Atkinson at the Universe Today site (www.universetoday.com). But it is easy to show that cosmic rays don't work well as an explanation for both of the photographs.
A cosmic ray is a particle from deep space that sometimes appears as a little streak on a photograph. But a cosmic ray can come with equal likelihood from any direction, as suggested by the visual below.
Cosmic rays can arrive from any direction
Also, a cosmic ray might appear as a streak anywhere on a photograph. It is therefore very unlikely that a cosmic ray would cause what looks like a streak of light that appears on the horizon directly perpendicular to the horizon.
One can calculate the chance of this with a little math. Let's divide up the area shown in the photograph into about 50 vertical slices, one of which corresponds to the horizon line. The chance of a random cosmic ray matching the horizon line is therefore roughly about 1 in 50. The chance of the ray having an orientation directly perpendicular to the horizon is about 1 in 20. So the chance of you having a random cosmic ray appearing as something on the horizon and perpendicular to the horizon is about 1 in 50 times 1 in 20, or about 1 in 1000. The chance of such a coincidence occurring on two consecutive days is less than 1 in 1000 times 1 in 1000 (in other words, less than 1 in a million). Not impossible, but very, very unlikely.
Now let's look at the vent hole explanation. This explanation suffers from exactly the same problem as the cosmic ray explanation. If a little vent hole were causing a tiny bit of sunlight to leak into the camera, it is extremely unlikely that we would coincidentally have two consecutive days in which there appeared what looks like a flash of light on the horizon and perpendicular to the horizon.
The vent hole explanation is even more implausible than the cosmic ray explanation, because once you have adopted the vent hole explanation you are then wedded to the theory that there is a camera defect. Such a camera defect would have to keep on showing up again and again in photographs, but there is apparently no evidence of such a defect in photographs after April 3, 2014.
What about the third theory, the theory of a rock that is reflecting light? There are two reasons why such a theory does not hold water. The first is that Mars is much farther away from the sun than Earth. Ask yourself: have you ever seen a rock far away on the horizon reflecting light? You probably never saw that, and your chance of seeing it on Mars is much smaller, as the sun appears as a much smaller object in the Martian sky. Rocks are dull, non-reflective objects that do not reflect light to a significant degree. Glass and metal reflect light well; rocks do not. The only type of rocks that reflect light reasonably well are wet, round rocks like the rocks at the seashore; and such rocks presumably do not exist on Mars.
Try a Google image search for “rock glint” or "rock reflection" and you will find not one single good example of one. When we don't see rocks strongly reflecting sunlight on Earth, how could they be strongly reflecting light on Mars?
Another reason why the rock reflection theory is not plausible is that glints of light caused by reflections typically appear as star-shaped glints, not a straight upward line as shown in the photographs.
In short, none of the three theories that NASA has advanced holds up very well to scrutiny. We do not currently have a good explanation for the images that appear to show lights at the horizon.
I am not suggesting that this necessarily means that the lights are caused by some alien activity. There are numerous unusual possibilities, and not all of them involve aliens. The apparent lights could have been caused by geysers or the geological phenomenon known as a dust devil (a swirling column of dust). There could be some unusual phosphorescence going on, or some unusual outgassing of chemicals. I have no idea what exactly caused these image features that look like lights, but it is clear that the explanations given thus far by NASA are not persuasive.
We have additional possible photos of lights on Mars in this photo (in the upper left corner) and this photo.