Ziggy Stardust and the Spiders from Mars: Part III –Answers to Questions for the Science Team

Planet Four volunteer Peter Jalowiczor got asked a great set of questions after his public talk to his local Astronomical society. Below you’ll find the replies from Anya:

What evidence is there for cracking (of the ice) in the Martian surface?

We have actually observed the cracks in the seasonal ice layer appearing and evolving. We have seen them in multiple locations and several years. There is a paper on this:
Portyankina, G., Pommerol, A., Aye, K.-M., Hansen, C. J., & Thomas, N. (2012). Polygonal cracks in the seasonal semi‐translucent CO2 ice layer in Martian polar areas. Journal of Geophysical Research: Planets, 117(E2), DOI: http://doi.org/10.1029/2011JE003917. It has examples of observed cracks in spring from southern and northern hemispheres.

Was there certainty that the channels were not ridges? (Yes, this is an optical illusion!)

Yes, there is certainty. We know the direction of sun illumination on every image that HiRISE (or any other camera) takes. We can compare it to the locations of shadowed and illuminated sides of the channels. We have done it multiple times and it always fits to the assumption that those are channels not ridges.

Where does the blue colour come from?

This question has 2 parts:
1)    Camera technology: the blue color is detected by the CCD that has a filter in front of it and thus only sensitive to blue part of visible spectra. In HiRISE we call it blue-green channel and highest sensitivity centered at 536 nm.

2)    Light scattering by different materials: when light hits the surface of Mars, it is scattered differently by different material. Martian “soil” most efficiently scatters red light and makes Mars look brown-red. Fresh frost scatters rather efficiently most of sunlight spectral range, but particularly well in the blue part. This is true for both, CO2 or water frost. Thus, the blue patches in HiRISE images in polar regions are where CO2 or water ice lies at the top of the martian soil.

Is the North Polar region of Mars going to be investigated in the same way as the South polar region?

We would like to do that, given we get support and funding.

What height are the geysers?

We have not observed them in action, which means only theoretical estimates exist. For a jet that is constantly outgassing early in spring from underneath 1-m thick ice layer with a vent that is <1m2, the maximum estimated height is 70 m. If the pressure under the ice first builds up and then releases in eruption-style event, the height estimate is several times higher but highly uncertain.

Is there an imaging dataset, perhaps an experiment on a satellite, which could enable these heights to be measured more accurately?

Not currently. We are proposing for a small mission to be able to do just that.

How often do we return to each of the imaged areas? Surely there must be some follow-up to see how the features have developed.

We image every location several times per spring. Our main locations (Inca City, Ithaca, Giza, etc.) get up to 10 images per season. We also image them every summer when they are free of ice. Repeated imaging in summer is targeted to detect the changes in the araneiform structures, but it is very tricky goal, as the atmospheric and illumination conditions should be very similar in order to definitely detect any topography changes. Right now we have 5 martian years of observations but no certain detected topography changes.