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Headshot of Emily Lakdawalla

HiRISE image coverage of the Curiosity field site on Mars

Posted by Emily Lakdawalla

10-02-2014 14:39 CST

Topics: Mars, Curiosity (Mars Science Laboratory), explaining image processing, Mars Reconnaissance Orbiter

Edit: This post was updated on April 3, 2014 to add one image, taken February 10, to the table; however, the image has not yet been added to the coverage maps. Its color coverage is redundant with prior images.

Edit: This post was updated on May 23, 2014 to add two more photos, taken on March 14 and March 26, which (finally) covered Murray Buttes in color.

This post began with a desire to figure out which HiRISE images to use when making maps of Curiosity's traverse. 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. Just today, there's a major new story involving HiRISE and CRISM data on  possibly currently active water seeps on Mars.

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.

The HiRISE focal plane

NASA / JPL-Caltech / UA / courtesy of Timothy Reed

The HiRISE focal plane
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.

At this point in the HiRISE assembly the detector elements are bare but are correctly aligned, set to the same height, and measured so that the locations of the pixels is known to an accuracy of a few microns. Later a metal cover was installed that had spectral filters, sharp-edged rectangular apertures, and stray light baffling.

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.
  • 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 in the order that Curiosity crossed them, and tell you which sols Curiosity was in them. I'll probably repost this list from time to time as Curiosity enters new swaths.
  • 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. The light yellow part of the path shows Curiosity's completed drive. The dark yellow shows the planned route to Murray Buttes. 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.

HiRISE Digital Terrain Models (DTMs) covering the Curiosity field site

NASA / JPL / UA / Emily Lakdawalla

HiRISE Digital Terrain Models (DTMs) covering the Curiosity field site
Digital Terrain Models (DTMs) are high-resolution topographic data computed from stereo pairs of HiRISE images.

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

DTMLeft image
Right image
Anaglyph
NotesMSL presence
Possible MSL Landing Site in Gale Crater PSP_010573_1755
PSP_010639_1755
Anaglyph
Contains landing site. Covers mostly northern Gale crater plains, barely reaches into the dune field at southern end. Entered sol 0.
Exited sol 511.
Possible MSL Landing Site in Gale Crater ESP_018854_1755
ESP_018920_1755

Anaglyph
Misses the landing site to the west, but crosses the entire dune field and laps a little onto the interesting layered rocks encircling the base of Mount Sharp. Curiosity will exit this DTM on the west shortly before reaching Murray Buttes. The notional traverse up the mound drawn early in the mission does not cross back into this DTM after Curiosity exits it on its western side, for what that's worth. Entered sol 345.
Crater Floor and Central Mound in Gale Crater (MSL) PSP_009650_1755
PSP_009716_1755

Anaglyph
Contains Murray Buttes and crosses the light-toned ridge before lapping up on to the yardangs at the base of Mount Sharp. Not yet. Will enter shortly before reaching Murray Buttes.
Inverted Riverbed in Gale Crater PSP_009149_1750
PSP_009294_1750

Anaglyph
If Curiosity travels eastward after crossing the dune field, it will pass into this one. --
Lower Northwest Portion of Mound in Gale Crater ESP_019698_1750
ESP_019988_1750

Anaglyph
Substantial overlap with previous two DTMs but extends to south and west. --

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. There is still a gap over Murray Buttes, but unfortunately for rover fans, the proposed Insight landing site is near Curiosity's, and the priority for HiRISE imaging just now is to wallpaper the Insight landing site. So I am not sure when that gap will get filled in. Edit: half of the gap was filled in an image taken March 14, 2014.

Color HiRISE swaths covering the Curiosity field site

NASA / JPL / MSSS / UA / Emily Lakdawalla

Color HiRISE swaths covering the Curiosity field site

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. 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.

ImageSol / Ls
Date
Emis.
Phase
Inc.
DS&BS
LS
rover
Notes
ESP_030168_1755 Sol 145
Ls 237.0
2013-01-02
17.4
35.8
52
--
gray
gray
Includes all tracks to date in gray. Color strip is to the east of any region traversed to date by the rover; this image was probably taken for its coverage of clay-rich mineral terrain to the south of the dune field.
ESP_028190_1755 --
Ls 148.1
2012-08-01
8.1
62.1
55
-- Taken just days before landing, contains Glenelg and Yellowknife Bay in black-and-white region, extends color coverage to east of traverse route.
ESP_028269_1755 Sol 1
Ls 151.2
2012-08-07
44.9
93.1
51
gray
gray
gray
Was taken from a very high emission angle (45 degrees), so is lower-resolution than normal and has severe geometric distortion.
ESP_028335_1755 Sol 6
Ls 153.9
2012-08-12
29.5
80.4
53
gray
color
color
Was taken from a relatively high emission angle (30 degrees), so is lower-resolution than normal and has severe geometric distortion.
ESP_028401_1755 Sol 11
Ls 156.6
2012-08-17
9.6
63.4
54
gray
color
color
Color covers traverse from sol 0 to 351
ESP_032436_1755 Sol 316
Ls 342.7
2013-06-27
29.9
5.5
35
gray
color
color
Includes all tracks to date in color. Color covers traverse from sol 0 to 354. Targeted specifically to have very low phase angle (5.5 degrees). Not a great mapping product because of low phase angle and high emission angle, but interesting for subtle color.
ESP_030313_1755 Sol 157
Ls 244.1
2012-01-13
4.3
46.3
50
gray
color
color
Includes all tracks to date in color. Color covers traverse from sol 0 to 369. A shorter-than-usual strip captured along with CRISM data of blast zone.
ESP_028612_1755 Sol 27
Ls 165.3
2012-09-02
9.0
62.9
54
color
color
color
Includes all tracks to date in color. Color covers traverse from sol 0 to 38 and 343 to 403.
ESP_029957_1755 Sol 129
Ls 226.7 2012-12-16
1.8
50.7
53
color
gray
gray
Includes all tracks to date in gray. Color covers traverse from sol 345 to 406
ESP_028678_1755 Sol 32
Ls 168.1
2012-09-08
16.9
40.1
56
gray
gray
gray
Includes all tracks to date in gray. Color covers traverse from sol 371 to 454. Color strip contains some bits of backshell debris.
ESP_034572_1755 Sol 479
Ls 61.2
2013-12-11
9.9
59.9
52
gray
gray
gray
Includes all tracks to date in gray. Tracks cross entire width of color swath. Includes tracks in color from sol 371 to 453.
ESP_035350_1755 Sol 538
Ls 87.7
2014-02-10
7.7
51.5
57
gray
gray
gray
Includes all tracks to date in gray. Tracks cross entire width of color swath. Includes tracks in color from sol 371 to 439.
ESP_029746_1755 Sol 113
Ls 216.4
2012-11-30
2.9
56.0
53
gray
--
--
Color covers traverse from sol 403 to 518. Contains no tracks.

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 because there are two DTMs; Curiosity's current position at Dingo Gap is covered in color in six different HiRISE images!

ImageLs
Date
Emis.
Phase
Inc.
Notes
PSP_010573_1755 Ls 148.7
2008-10-28
22.9
77.2
56
Color covers traverse from sol 417 to... Part of a DTM with next. This image just barely crosses the traverse, containing only a little terrain to the south of the path.
PSP_010639_1755 Ls 151.3
2008-11-02
2.5
55.6
58
Color covers traverse from sol 419 to... Part of a DTM with previous.
ESP_027834_1755 Ls 134.3
2012-07-04
15.4
44.1
57
Color covers traverse from sol 436 to...
PSP_003453_1750 Ls 223.9
2007-04-22
4.2
57.6
53
Color covers traverse from sol 438 to...
ESP_018920_1755 Ls 130.6
2010-08-09
19.7
42.2
58
Color covers traverse from sol 515 to... Part of a DTM with next.
ESP_018854_1755 Ls 128.1
2010-08-04
5.9
61.1
56
Color covers traverse from sol 524 to... Part of a DTM with previous.
ESP_022111_1755 Ls 274.4
2011-04-15
2.0
42.2
44
--
ESP_033649_1750 Ls 29.1
2013-09-30
2.6
41.5
44
Taken for dune motion monitoring
ESP_021610_1755 Ls 249.7
2011-03-07
28.3
25.8
51
Considerably lower phase than usual
ESP_035917_1755 Sol 581
Ls 107.4
2014-03-26
3.8
60.3
57
Includes tracks in gray from sols 548 to rover at sol 581.
ESP_035772_1755 Ls 102.3
2014-03-14
17.1
46.2
59
--

This does not quite reach Murray Buttes. There is a gap in color coverage about the width of two HiRISE swaths right over Murray Buttes. EDIT: When I first wrote this blog post, there was no color swath over Murray Buttes, but two images have now been acquired. 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):

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

Color view Curiosity on Mars from HiRISE, sol 157

NASA / JPL / UA / Emily Lakdawalla

Color view Curiosity on Mars from HiRISE, sol 157
Mars Reconnaissance Orbiter snapped this color photo of Curiosity on the rover's sol 157 (January 14, 2013). The rover was at the "Snake River" site within Yellowknife Bay. It is rotated to place north at left in order to show it larger on the website.
 
See other posts from February 2014

 

Or read more blog entries about: Mars, Curiosity (Mars Science Laboratory), explaining image processing, Mars Reconnaissance Orbiter

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