Peering through the Dust

You may have noticed that our favorite red planet, Mars, has been a popular feature in the news recently. Not that we need any special reasons to take a closer look at Mars, but a number of coincident events have brought this about:

[1] Mars has just passed through opposition, or the point in its orbit where it is closest to Earth throughout the year—see Figure 1. This particular opposition brings Mars closer to Earth than it has been in 15 years.

[2] As a result of being closer to the Sun and having more atmospheric energy, Mars has been experiencing a severe dust storm that has grown over the last two months to envelop nearly the entire planet.

[3] As if this wasn’t enough, scientists also recently announced the discovery of an underground liquid water lake below the southern polar cap of Mars!

opposition_schematic
Figure 1
Mars at opposition. The average distance from Earth to Mars is about 140 million miles (225 million km). The closest known approach was in August 2003 at 34.8 million miles (56 million km). On July 27, 2018 the distance from Earth to Mars was about 37 million miles (59 million km).

While we’re not announcing any “Mars-shattering” news like liquid water over here at Illuminated Universe, we do have new Hubble imagery and animations of Mars taken just before opposition during the global dust storm which are, in our humble opinion, pretty exciting. The storm was a growing concern to the team as the observations drew near. We speculated that the resulting images might resemble a smooth red billiard ball! Fortunately, the storm had just started dissipating as the images were taken, revealing more of the surface, and the filter selection chosen for these observations helped to pull out yet more detail on the surface of Mars. The high quality data also proved useful in creating frame-by-frame animations from the observations showing the rotation of Mars and its moons Phobos and Deimos.

Making the Red Planet Red

In order to make color images, we planned the observations to make use of some of Hubble’s many color filters (see Figure 2 below). Hubble observed Mars with filters allowing a wide range of ultraviolet light, a smaller range of blue light, a narrow range of green light, and a narrow range of red light. Most of the data showed a pretty smooth surface, obscured by dust, and punctuated by clouds and ice caps in the extreme north and south. However, the longer wavelength, narrow-band filter at 673 nanometers was able to see through the dust in the atmosphere and give us a glimpse of some of the familiar surface features usually seen in images of Mars. Longer wavelengths of light aren’t scattered by the dust as easily so we’re able to see through the dust in these wavelengths. The image below shows the raw data in each filter in the order the data were taken. This sequence of images was taken over a period of about 43 minutes and clearly shows how very different Mars looks in the different filters. The color outline around each image indicates the filter used and time increases from left to right.

mars_raw
Figure 2
The full sequence of Hubble’s observations of Mars on July 18, 2018. Time increases from left to right, with the color of each square representing its filter. Exposure times are listed for the first run of filters, and were roughly the same after that.

Creating a color image is as simple as selecting any three adjacent red, green and blue filters and combining them to make the color image.

color_v1
Figure 3
 A three-color combination of Mars using filters F410M, F502N and F673N.

This image is pretty good as-is, but we can do more to uncover the detail inherent in the data. We do this through local contrast enhancements targeting image structures between 3 and 8 pixels in size. This isolates the surface features of Mars without overdoing the rest of the image. The example below has been greatly exaggerated to illustrate how this technique targets those features. The final version of the Mars image, seen at the top, used this technique as well as some color adjustments and finally a rotation to orient the image with North up.

color_wavelets
Figure 4
This version of the image has used a technique to emphasize small-scale structural features in the image. The technique has been greatly exaggerated to illustrate the effect.

This type of color and contrast processing was also used in creating an animation so that we could use the full sequence of images to make many color images over the 43 minutes of observation time.

Mars in Motion

One of the great benefits of observing solar system targets like Mars is that we can see real-time changes in these objects over very short periods of time. Observations separated by minutes show dramatic changes and convey a sense of motion. Aligning all of the Mars images and putting them together in a time sequence shows the rotation of the planet over 43 minutes.

mars_motion_sm
Figure 5
A color GIF animation of Mars created from the data shown in Figure 2.

Also, if we do an extreme stretch of the image brightness, we can reveal the much fainter Martian moons Phobos and Deimos. The GIF animation below shows the raw data stretched to uncover the moons with no cleanup done to remove cosmic rays and other defects in the images. Even with all that extra noise, the moons are quite apparent as they persist from frame to frame.

moon-animation_sm
Figure 6
A black and white GIF animation of the full sequence of raw data. The Martian moons Phobos and Deimos can be seen marching across the upper left. Deimos starts around 11 o’clock relative to the Martian disk while Phobos appears on the left edge around 7 o’clock and disappears behind the shadow of Mars.

Mainly because of the flickering of the moons, the final version made for the press release used images of the moons taken in one frame and then interpolated the motion seen to give a smooth animation.

Hubble has taken many images of Mars over its lifetime in orbit, yet each picture has its own story to tell. Images of planets in our solar system never cease to amaze and continue to capture the imagination of the public. From dust to water and everything in between, it’s truly remarkable that we continue to learn new things about even our closest cosmic neighbors in our quest to understand the universe.

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