Today's three-sol plan was all about picking interesting targets to explore at our Thanksgiving stopover point, including setting up for our winter 'frost detection' experiments, and getting SAM ready to do some power-hungry analysis while we stay put.
We're only a few sols from southern winter solstice in Gale Crater on Mars, which means it's pretty much the coldest time of year and the best time for Curiosity to try to see water frost on the surface. If we see frost formation, this provides a lot of information for atmospheric scientists like me, who can use it to test models of when and how much frost should form on different types of surfaces, and to better understand how atmospheric water interacts with the surface and subsurface. The problem is that, even in winter, the temperatures in Gale only just dip below the frost point and then only right before dawn. Also, when we've tried looking in previous years, we seem to have been unlucky: the last time we looked for winter frost, the experiment ran on what turned out to be the warmest night of the week. But this just means we have to stay alert to have a good chance of seeing it.
We started today by picking two targets: a small, smooth-topped sand patch, "Oaktree," which sits in a kind of rock circle toward the upper right edge of this Navcam image just before the darker material begins; and a small rock with an east-facing slope, "Lebombo." The sand should have a lower thermal inertia than rock, which means that it cools down more overnight and may be more likely to form frost. But porous sand can also tend to adsorb water instead of the water freezing on its top. So we also chose a rock target with an east-facing slope so it's in shadow for as much of the afternoon as possible, which means it should be able to cool down a little more than other rocks overnight.
Because we only expect the frost layer at this location to be a few microns thick, and to vanish rapidly when temperatures start going up at dawn, it's very hard to detect with cameras. So we'll be using the ChemCam instrument and its Laser-Induced Breakdown Spectrometer (LIBS) to vaporize the top few microns of the surface at night and look for extra hydrogen in the signal, then compare this to daytime measurements of a similar location on the same target.
We'll be making the daytime hydrogen measurements first, on Sol 1879, then in the next plan we'll include nighttime measurements just before dawn on Sols 1883 and 1886, and keep our fingers crossed for seeing a big increase in the hydrogen signal on at least one of the targets!
As well as the frost preparations, our new location stood out from a distance as having lots of color variety in Mastcam images, and we were able to access both brighter and darker blocks with the arm. So in today's plan we'll also be brushing bright target "Hexriver" to remove the top dust layer with the DRT before ChemCam and APXS are done, but the dark target "Zululand" was too small so no brushing will happen first. Meanwhile, Mastcam will be providing imaging of these targets, as well as documenting more of the light-gray/blue rocks that drew us here (target "Natal") and the contrast between the bright and dark toned units on target "Kansa."
We'll be making our usual REMS, RAD, and DAN measurements of the environment, with some additional cloud and sky movies with Navcam and Mastcam just before sunset on Sol 1880 to get a better idea of the aerosols - dust and water ice - around during the frost experiments. And finally, SAM will be preconditioning overnight, preparing to analyze samples from all the way back at the Bagnold Dunes over Thanksgiving.