BAN #464: JWST sees the heat from Mars

[Hubble image of NGC 3603. Credit: NASA, ESA, R. O'Connell (UVa), F. Paresce (NIA, Bologna, Italy), E. Young (USRA/Ames Research Center), the WFC3 Science Oversight Committee, and the Hubble Heritage Team (STScI/AURA)]

Pic o’ the Letter

A cool or lovely or mind-bending astronomical image/video with a short description so you can grok it

If you go outside around midnight local time and set your gaze to the east, you’ll see a flaming orange dot low to the horizon. That angrily glaring orb is the planet Mars, currently about 125 million kilometers from Earth.

On September 5, 2022, JWST set its gaze that way as well, targeting Mars for the first time. What it saw is… well, maybe not exactly pretty, but certainly pretty danged cool.

[Mars seen by JWST. See text for description. Credit: NASA, ESA, CSA, STScI, Mars JWST/GTO team]

On the left is a simulated map of Mars depicting it at the time of the JWST observation, based on a topographical map made using the Mars Orbiter Laser Altimeter instrument on the Mars Global Surveyor orbiter. The white square shows the footprint of JWST’s Near-Infrared Camera (NIRCam) image taken at 4.3 microns (shown lower right), and the blue square is the footprint of the 2.1-micron image.

At a wavelength of 2.1 microns — roughly three times the reddest wavelength the human eye can see — what JWST sees is reflected sunlight, so in that sense it’s a lot like looking at an image of Mars in visible light. Three features are noted: Syrtis Major, a dark triangular plain covered in basaltic rock (and pretty easy to see in even small telescopes); Huygens Crater, an impact crater 470 kilometers across (YEGADS what an impact, way bigger than the dinosaur killer); and part of Hellas Basin, an immense depression caused by an impact that makes Huygens look like a caress. Hellas is 2,300 kilometers across. Oof.

The 4.3-micron shot is very interesting. At that wavelength the Sun doesn’t put out much light, and you’re starting to see the thermal glow of Mars itself, literally the heat it emits by dint of being warmer than absolute zero. Anything warm emits light, which might be in the radio wavelengths, or infrared, or in the case of the Sun in visible light; it depends on how warm the object is.

The subsolar point is the spot on Mars directly under the Sun; if you connect a line from the center of Mars to the Sun it passes through the subsolar point. Another way to think of it is that if you stood there the Sun would be directly overhead, on your zenith. That is likely to be the warmest point on the planet, and you can see it glowing strongly with heat. Well, “heat”; Mars is still pretty cold in human terms.

The fact that Hellas Basin is darker is not for the reason you might think: It’s not necessarily cooler, it’s that the air is thicker there. The atmosphere of Mars is mostly carbon dioxide, and that gas is very good at absorbing light at 4.3 microns. So Hellas Basin isn’t physically darker at that wavelength, it’s the air absorbing the light coming from it, so we don’t see as much light from it. Think of it like fog dimming a streetlight.

These observations are useful in mapping things like clouds (which Mars has!), surface features, dust storms, and the like, and to see how all this affects the surface conditions and temperature. JWST can get an overall view of Mars that can be compared to the more narrow views from orbiters and landers and rovers that are on or above the planet.

JWST also took a spectrum, and it’s pretty amazing:

[A spectrum of Mars’s atmosphere, graphing brightness versus wavelength. Explanation, once again, in the text. Credit: NASA, ESA, CSA, STScI, Mars JWST/GTO team]

A spectrum breaks up the light from an object into specific wavelengths (think of them as colors), and you can then graph the brightness versus wavelength. Different atoms and molecules absorb light at different wavelengths, and you can see the sudden dips caused by carbon dioxide (like at 4.3 microns, as stated above), carbon monoxide, and water. The amount of the dip tells you how much of the substance is in the air, too.

But what I think is cool is the overall shape of the graph. It starts off high on the left and drops down in brightness until about 3 microns, then starts going up again. That’s because to the shorter wavelengths (to the left) of 3 microns the light we see from Mars is reflected sunlight, and that drops with wavelength. But at around 3 microns the surface heat of Mars starts to become important, so at longer wavelengths (to the right) it gets brighter!

We see Mars because it’s sitting in sunlight and reflecting it, but also because it’s warm from that light as well, and glows in the infrared due to that heat. I mentioned this talking about the 4.3-micron image above, but in the spectrum you can see it happening.

Spectra are crucial to astronomy! Spectra give us the key to the physics of an object. It’s not too much of an exaggeration to say spectra are what transform astronomy into astrophysics.

I’ll add these observations were part of the Guaranteed Time Observations program, where astronomers involved with the design, construction, and shakedown of JWST got time on it to observe their favorite objects. They still have to propose the programs and make sure there’s lots of science, but it’s a nice perq (I was part of the GTO team for Hubble’s camera STIS back in the late 90s and we got very cool observations from it). I was delighted to see that these observations were taken in part by my long-time Hubble friend Heidi Hammel, who had the GTO time to get these JWST data! It’s always great to see excellent scientists get the observations they need and want.

By the way, I was thinking about the position of Mars in space when I saw this image, because I know that it rises late at night. I had a sneaky suspicion that was confirmed when I looked at the positions of Earth and Mars compared to the Sun:

[Mars (red dot), Earth (blue dot) and the Sun on September 5, 2022. Credit: NASA/JPL-Caltech, annotated with lines by me]

JWST uses a huge sunshield to block the light and heat from the Sun, which is always perpendicular to the Sun. The telescope sits on the other side of the sunshield, and the shield is like its horizon: It can look anywhere in the sky parallel to the sunshield up to 90° from it, the direction opposite the Sun in a limited range of angles away from the Sun, but never closer than 90° on the sky from the Sun*. That means the angle between the Sun, Earth, and Mars has to be more than 90° or else the sunshield blocks it. And yup, when JWST took those images Mars was just above its “horizon”.

This tweet shows the JWST configuration which will hopefully help you understand what I mean. If not, leave a comment below!

That was fun to figure out. Sometimes being familiar with physics and astronomy and trig coupled with being pretty good at picturing angles in your head comes in handy.

Incidentally, part of the JWST design was to be able to track solar system objects as they move around the Sun, and Mars is literally the fastest thing it can keep up with. These observations are even more amazing due to that fact.

And here’s more fun for you: You can look at Mars, and JWST can look at Mars, but on September 16, 2022, Gianluca Masi of the Virtual Telescope Project looked at JWST!

[JWST (the dot in the center) among a field of trailed stars. Credit: Gianluca Masi]

How awesome is that? He used a 44-centimeter telescope to spot the observatory, which is in a stable orbit about 1.5 million kilometers from Earth (it was somewhat closer than that when the image was taken). He used the JPL Horizons software to get the coordinates for JWST at the telescope’s location, and tracked the motion of the observatory so that the stars appear as streaks over the short 5-minute exposure.

So now we’ve seen Mars and Jupiter via JWST (and Neptune, which I’ll have on the blog tomorrow). I know Saturn’s on the list, as is Uranus. I can’t wait to see those too!

Stay Tuned.

* [CORRECTION (added Sep. 22 at 23:50 UTC): I originally wrote that JWST can see the entire hemisphere of the sky centered on the point opposite the Sun, but that turns out not to be the case, so I corrected the text and added a link to what JWST can see. My thanks to Kevin Parker for pointing that out!

Heh. “Pointing”. Heh.]

Et alia

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