Help Shape the Future of Space Exploration

Join The Planetary Society Now  arrow.png

Join our eNewsletter for updates & action alerts

    Please leave this field empty

Headshot of Emily Lakdawalla

Curiosity sol 11 update: Decision to drive to "the high thermal inertia unit" and what that means

Posted by Emily Lakdawalla

17-08-2012 18:06 CDT

Topics: pretty pictures, mission status, geology, explaining science, Mars, Curiosity (Mars Science Laboratory)

As I post this blog entry, it's just past midnight, in the wee hours of sol 12 for Curiosity on Mars. Back on Earth, there was a phone briefing this morning. The biggest news: according to project scientist John Grotzinger, the team has "pretty well decided" on Curiosity's first science target: the "triple junction" of three different rock types located a little more than 400 meters to the east of the rover's current position. They've named the spot "Glenelg."

Here is a view of the triple junction, cropped from the fantabulous HiRISE image taken 6 days after landing. I have stretched the heck out of the color to make the distinction among the three surface units easy to see. There is the reddish one, on which Curiosity sits; a light-toned one, occupying the top right quadrant; and a bluish one occupying the lower right quadrant. This isn't true color, but the relative color is correct -- one is redder, one is brighter, one is bluer.

Curiosity's first destination: Glenelg

NASA / JPL / UA / Emily Lakdawalla

Curiosity's first destination: Glenelg
A cropped view of a HiRISE image taken 6 days after Curiosity landed includes a "triple junction" of three different rock types. The team named that spot "Glenelg" and planned to make that Curiosity's first driving destination. The rover is visible at far left, surrounded by a dark splash where its landing jets disturbed the dust.

The lightest unit in the image above is a rock type that's being referred to as "the high thermal inertia unit." I think it's time I explained what that means and why it's important.

Thermal inertia is a physical property of a material. If you know what "inertia" means, "thermal inertia" is exactly what it sounds like: a measure of the material's resistance to changes in temperature. Generally speaking, rocks and water have high thermal inertia. Dust and soil, with their large surface area relative to volume, have low thermal inertia.

Mars experiences wide swings in temperature because it lacks much of an atmosphere. The temperature at Curiosity's landing site varies by about 80 degrees Celsius or Kelvin each day-night cycle. A material that has high thermal inertia hangs on to its heat overnight, so if you look at Mars at night with a thermal imaging system, dark areas are dusty and bright areas are rocky. (This is, of course, an oversimplification, but it's a good first approximation of what nighttime thermal imaging data means.)

So here is a blink gif comparing what Gale looks like in daytime and nighttime infrared, as seen by the Thermal Emission Imaging System (THEMIS) on Mars Odyssey. The shading in daytime thermal imaging is dominated primarily by solar heating, so it looks like a regular photo in most places. It's easy to recognize the mound in the daytime infrared, but nighttime infrared is freaky-looking! It took me a while even to figure out what I was looking at -- in fact, that's why the daytime infrared image is there, to help me locate myself. I have put a little yellow square where Curiosity landed.

Blink comparison of daytime and nighttime infrared views of Gale crater

NASA / JPL / ASU / Emily Lakdawalla

Blink comparison of daytime and nighttime infrared views of Gale crater
The Curiosity landing site is marked with a yellow square, just to the south of a deposit of material that is bright at night and dark during the day. This material is referred to as the "high thermal inertia unit" by the Curiosity science team.

Okay. What does this tell us? In the nighttime infrared image you should be able to see, immediately north of Curiosity, a light-colored splash on the Gale crater floor. Notably, it's a somewhat darkish area in the daytime infrared. This is an area of high thermal inertia, resistant to changes in temperature: it's warm at night, and cool during the day.

To put it in its geological context, look to the northwest of Curiosity. You will see a beautiful branching channel system, that once collected flowing water from higher elevations and channeled it into Gale crater. Where it breaks out of the crater rim it develops a mildly sinuous pattern, and it ends in a fan-shaped sedimentary deposit. This is an "alluvial fan." The water in the channel system ran swiftly in the steeply sloped terrain on the crater rim and so was able to carry rocks along with it. As soon as the channel debouched onto the crater floor, it flowed with much less speed so couldn't carry that heavy material anymore; the sediment fell out of the water, with the biggest cobbles being dropped closest to the wall. The finest clays continued to be suspended in the water until it flowed no more, so were left at the end of the fan. That's clay-rich material deposited out of flowing water -- exactly the kind of environment Curiosity was sent to Mars to study. That high-thermal-inertia unit may be a rock formed out of those fine clays.

On the other hand, it might not be. We're going to find out one way or the other, because that's where Curiosity is headed as soon as she starts driving.

Here's a few other bullet-point updates from today's briefing.

  • They'll start the drive after finishing Commissioning Activity Period 1B, during the so-called "Intermission." That's some time after next week. If they drive straight, it'll take 3 to 4 weeks to get there. (Go here for an explanation of what the Commissioning Activity Phases are.)
  • However, they might interrupt the drive to perform Commissioning Activity Period 2 activities if they spy an area of nice fine-grained material that will be straightforward to pick up and deliver to the SAM and CheMin laboratory instruments.
  • The DAN instrument operated successfully in active mode today, firing neutrons at the ground; furthermore, the RAD instrument detected the operation of DAN as expected. Grotzinger called this "great team collaboration."
  • The REMS weather instrument is operating and Grotzinger promised that its principal investigator would present the first results from that next week. The current daily maximum temperature is 276 Kelvin (3 degrees Celsius, 37 degrees Fahrenheit).
  • Roger Wiens showed an image of a little rock that the ChemCam team has picked out for their first shot, chosen more for its suitability for target practice than for science.
  • I asked about the Mastcam-100 (the zoom one) and they do not plan to go through their initial pointing tests and so forth until after Commissioning Activity Period 1B is complete.
  • They have not yet sequenced Mastcam-34 views that reach the top of the mound.

If 3 or 4 weeks seems like a long time for a 400-meter drive, it's because you're used to Opportunity's ridiculous one-sol drives of more than 100 meters. Remember, she was not doing that until way after her primary mission had ended, after her drivers had gotten very confident handling her on the repetitive, incredibly flat terrain of Meridiani Planum. It's possible that Curiosity will never achieve drives of that length, because all her flat-terrain driving is going to be early in the primary mission, when the drivers won't have achieved that confidence yet. By the time the driving becomes truly comfortable (though not easy -- it's never easy driving a rover on Mars!), Curiosity might already be in the much steeper terrain near the mound, and such long drives may not be possible.

Of course, I could be selling the rover drivers short here. For any rover drivers reading this, yes, that's a challenge. Achieve an over 100-meter drive with Curiosity and I'll bake you some cookies!

See other posts from August 2012


Or read more blog entries about: pretty pictures, mission status, geology, explaining science, Mars, Curiosity (Mars Science Laboratory)


DonInMaine: 08/17/2012 07:25 CDT

Thank you Emily, for your thorough, detailed reporting and explanations. It's a wonderful thing to have someone who doesn't try to dumb things down or get all gee-whiz. I enjoy your enthusiasm and sense of fun with this. Because it IS fun, and I know everyone following you thinks the same. That's why we're here. I'm glad my curiosity about Curiosity brought me to the Planetary Society. And say hi to Bill Nye.... Now, to a question.. I tried to sneak this in to at the Hangout, but missed. I am also a fan of pretty pictures. With substance. I did a bit of work with some satellite data to synthesize an earth image once, but nowhere near what you and the JPL / NASA folks whip up. What is the full slate of capabilities of the camera array? Not necessarily a tour of the cameras, but a summation of capabilities. e.g. What is the full spectral range we can detect? Also wonder about video. Is it all a matter of assembling images back here on Earth? Keep up the good work.

Emily: 08/17/2012 08:24 CDT

Don, for explanations of the cameras check my previous blog posts: They contain links to papers that will describe them in more detail. I will eventually write about Mastcam and ChemCam as well, but haven't yet. Typically, spectral range for visible cameras is from near-UV (high 300s of nm) to near-IR (slightly over 1000nm).

DonInMaine: 08/18/2012 12:27 CDT


Leonidas: 08/18/2012 02:20 CDT

Emily, I agree with Don wholeheartidly! You're doing such a tremendous, detailed and thorough job! Every piece of news on 'regular' news sites on the net, just seem so trivial when it comes to covering Curiosity's mission. Not only they don't do justice to this awesome mission, but I find them misleading also. Keep up the great work! You rock! Planetary Society's folks are so top-notch! I'm so happy to be a member!

Jumper: 08/19/2012 09:42 CDT

Great job on your blog, Emily. I'm new here and I will be back often. A couple of questions came up during a discussion about Curiosity #1: Will Curiosity drive at night? I'm assuming not but it would seem since the drive is preprogrammed and the rover can avoid hazards on its own the question popped up and I couldn't answer it. #2: Given the nasty dust storms on Mars are there any provisions for protecting the optical glass (and other sensitive components) from being sandblasted and compromised? Can Curiosity lower her mast and stow her 'face' on the rover deck for protection?

Hugo: 08/19/2012 09:45 CDT

As Leonidas says, great work Emily! And it's true, it's so difficult to find in any "traditional" news media covering seriously Curiosity mission what it's worrying. But in the same time it's telling us something about the current state of the global culture, tasteless (using a polite word) shows like Jersey Shore or Showmatch here in Argentina have millions of watchers and the mainstream news media are following similar paths to win their share of the pie. We must do all in our reach to find some way to effectively turn the tides to another more positive direction.

Stardust: 08/19/2012 07:18 CDT

Thanks, Emily for all your hard work and the sol updates. Enjoy each read!!!

Jeff: 08/20/2012 08:01 CDT

Very nice reading explaining the high thermal inertia areas.. the dark mattter--including the dunes which will be towards curiositys left as it approaches the pass--certainly must be comet material--which are twice as dark as asphalt. There are also black almost coal type rocks--closeby. I am surprised I cant search out anything that the dunes--and all the dark material must certainly be the very lightweight comet material that formed the crater...I hope the next rocks that they laser--will be the black rocks very near curiosity even before it gets to the fine powder of the black dunes. thanks

Leave a Comment:

You must be logged in to submit a comment. Log in now.
Facebook Twitter Email RSS AddThis

Blog Search

Planetary Defense

An asteroid or comet headed for Earth is the only large-scale natural disaster we can prevent. Working together to fund our Shoemaker NEO Grants for astronomers, we can help save the world.


Featured Images

LightSail 2 and Prox-1
Bill Nye at LightSail 2 pre-ship review
LightSail 2 pre-ship review team photo
Swirling maelstrom
More Images

Featured Video

Class 9: Saturn, Uranus, and Neptune

Watch Now

Space in Images

Pretty pictures and
awe-inspiring science.

See More

Join The Planetary Society

Let’s explore the cosmos together!

Become a Member

Connect With Us

Facebook, Twitter, YouTube and more…
Continue the conversation with our online community!