HiRISE image coverage of the Curiosity field site on Mars, Version 2.0

NOTE: THIS BLOG IS NOW OUTDATED. AN UPDATED VERSION, CURRENT AS OF FEBRUARY 5, 2016, IS AVAILABLE HERE.

Edited February 4, 2015 to add a new HiRISE image including the rover in color, taken on sol 836.

This long post is an attempt to answer the question: Which HiRISE image should I use as a base map for such-and-such a part of Curiosity's traverse? I originally published this article on Feburary 10, 2014, but have revised and updated it four times since then, so I am now re-publishing it as version 2.0.

HiRISE is the high-resolution camera on Mars Reconnaissance Orbiter that's charged with the task of scoping out the future path for our intrepid rover while also doing awesome geology research. HiRISE acquires images in long, skinny strips, like most Mars orbiting cameras. To get those strips, HiRISE's detector plane has 10 linear arrays, each of which normally covers a strip about half a kilometer wide, so full-width HiRISE images are 5 kilometers wide. Then HiRISE as two extra pairs of detectors on the two middle strips to get color data, so there's a central color swath about 1 kilometer wide. Curiosity will drive many, many kilometers before the mission is over, which means that they needed to grab a lot of HiRISE images to do reconnaissance. It has taken quite a lot of work for me to find, locate, and catalogue them. This post is a summary of what I've found. It's meant to serve as an index to HiRISE image coverage of the Curiosity landing site, so it's a bit on the technical and less on the "wow" side than my usual posts about HiRISE imagery.

Which image to use as the base for a map? It depends. Here are the rules of thumb that I'm using.

• If your map is going to be for a fancy purpose (i.e. science or a published product), begin with an orthorectified image from a Digital Terrain Model. (For more information about what's involved in the creation of Digital Terrain Models, read this explanation on the HiRISE website.) There are not very many of those and there's not much overlap between them, so it's usually obvious which one you need. I list those first. Those are mostly grayscale and were all acquired before landing.
• If your map is going to be in an online article, a color image is usually a nice way to go. Since color swaths are only 1 kilometer wide, your choice of image will depend on which part of the traverse you want to show. I list the color swaths below more or less in the order that Curiosity crossed them, and tell you which sols Curiosity was in them.
• If you're writing about something that happened a while ago, odds are good that there is a HiRISE image that contains Curiosity parked near the place you're writing about. If not, there will, at least, be a photo containing the tracks Curiosity made at the time, though not necessarily in color. I indicate the sol of each photo that contains Curiosity hardware, and comment about which hardware/tracks are in color and which are in the grayscale portions of the swath.

A few things to keep in mind about HiRISE:

• The HiRISE camera has 14 detectors, lined up on a large focal plane. Ten of the detectors see red wavelengths and form a staggered line that allow HiRISE to capture grayscale images 20,000 pixels across. At the middle of the focal plane are two more pairs of detectors that see blue-green and infrared light, allowing the HiRISE team to show the center 20% of any image swath in color.
• If HiRISE is looking straight down or nearly so (emission angle near zero), it can achieve resolutions close to 25 centimeters per pixel. So a full swath is roughly 5 kilometers wide, and the color strip roughly 1 kilometer wide.
• However, targeted images often require slews to one side or the other of the ground track, which decreases the resolution, but increases the swath width. When the emission angle isn't zero and there there is topography, there will be distortion caused by the oblique view. Map-projected image products correct for large-scale topography (like Gale crater's central mound) but not small-scale topography (like individual buttes and canyons). The only image products that are corrected for small-scale topography are the orthorectified images from digital terrain models.
• No two images have quite the same lighting geometry. HiRISE operates on a nearly sun-synchronous orbit, so it always passes over spots on Mars in the afternoon. But if it slews to the east, it sees the ground at an earlier time of day, and if it slews to the west, it sees the ground at a later time of day. And Mars has seasons, so the directions and lengths of shadows are different on different days, even if taken at the same local time of day.

Digital Terrain Models over the Curiosity Field Site

Understanding the local topography and slopes at potential landing sites was crucially important to establishing the safety of a proposed landing site, so imaging necessary to create good-quality digital terrain models (DTMs) over potential landing sites was a high priority for Mars Reconnaissance Orbiter's prime mission long before Curiosity landed. DTMs are the preferred products to use for mapping the rover traverse because they have been orthorectified -- that is, the images have been corrected for geometric distortion caused by the shape of the landscape and HiRISE's oblique perspective onto it. Orthophotos from DTMs are as close to maps as photos get. However, it's true that images taken after landing that actually feature rover tracks (see below) make it much easier to map the rover's course; anybody who produces a map product relating to the Curiosity mission should use orthophotos as a base map but post-landing images to establish the precise location of the rover with respect to surface features.

First, a location map. The base image is a 10-meter-per-pixel Context Camera photo, colorized with a Mars Express HRSC image. The light yellow part of the path shows Curiosity's completed drive. The orange path should not be taken overly seriously -- it's a notional traverse developed in 2011 by Ryan Anderson, Dawn Sumner, and Jim Bell as they were trying to characterize what Curiosity might find in the field. It could represent where Curiosity will go, but it's not a "plan" as such and the future rover path will likely deviate from it at least a little and probably a lot. Also please note that the lat/lon lines drawn on here are intended as a guide but may not be perfectly precise -- I extrapolated them from a map covering a smaller area.

The DTMs are listed in the order in which Curiosity crossed/will cross them.

Color coverage of the traverse path

The DTMs are nice products but only contain color information at their centers. HiRISE has wallpapered most of the future traverse path with skinny (1-kilometer-wide) color swaths. These are generally not orthorectified. It is not straightforward to mosaic these together because lighting conditions and geometry vary from image to image. But there is enough overlap that an appropriate color image can usually be found to provide a color base for a context map of any particular section of the rover traverse. Note that "merged" map-projected color image products on the HiRISE website are generally provided at 50 centimeters per pixel, in contrast to the 25-centimeter-per-pixel resolution of the simple grayscale or color image products. Here's a map to show you how thoroughly HiRISE has blanketed the Curiosity landing site with color coverage. This is really, really unusual for its density; HiRISE technical staff like to joke that "MRO" stands for "Mars Rover Observer" instead of Mars Reconnaissance Orbiter.

In the table below I list three different angles relevant to each image. The first is the emission angle -- this is the angle of the observation with respect to the surface normal. If HiRISE is looking straight down, the emission angle is zero and the image looks like a map. The higher the emission angle, the more oblique the view and also the lower-resolution the image. The second is the phase angle -- this is the angle from the Sun, to the surface, to HiRISE. The higher the phase angle, the longer the shadows appear. High-phase images are good for seeing shape from shading. Higher phase also brings out differences among surfaces with different textures, with rough or blocky surfaces appearing darker than smooth surfaces. Lower-phase images, lacking shadows, are best for seeing subtle color variations across the surface. Finally, there is the solar incidence angle, which is a measure of the time of day.

They are listed here in east-to-west order, because this is the order in which Curiosity crossed them. I have split them into two groups. The first, eastern group contains landing site and lander hardware. With few exceptions, the images covering the initial part of the traverse in color were taken after landing. I list the sol that they were taken on, as well as the season (Ls). Another column tells you whether the swaths contain rover hardware, and whether the hardware is only in the grayscale or if it's covered in the color part of the swath. ("DS&BS" refers to descent stage and backshell/parchute; "LS" refers to landing site; and then there's the rover.) In the notes I point out which ones contain tracks, and the specific sols covered by each color swath.

The following images mostly do not contain rover hardware, either because they were taken prior to landing or were to the west of Curiosity's landing and initial traverse areas. (If the sol number is listed, the rover is present.) There is a great deal of overlap in this region; Dingo Gap is covered in color in eight different HiRISE images!

There are color strips to the west (PSP_009650_1755, ESP_028823_1755) but if Curiosity follows the notional traverse, it will not ever enter these color strips.

There are some color strips that cover areas to the south of the ones I've listed already, which Curiosity will reach much, much later in the mission, assuming (and this is a big assumption) that it follows the notional traverse planed long before landing. These include, from east to west to match the list above (but which probably puts them in reverse order for the future traverse):

• ESP_025935_1750
• PSP_009294_1750 (This one actually just nicks the likely future traverse path but in that region it's redundant)
• ESP_029034_1750 (ditto)
• ESP_019988_1750
• ESP_026924_1750

I hope this post is useful to people! I know it will be useful to me :)