On 15 September 2021, the EXI camera system onboard the EMM mission obtained a set of multispectral images of this fully illuminated hemisphere of Mars.
At the time the images were taken, the Hope spacecraft was orbiting at an altitude of about 19,900 kilometers above the surface. This view is centered at 4.0°N latitude, 66.8°E longitude, with North toward the top of the image. The season was early winter in the southern hemisphere. The color composite presented here was assembled from images taken through EXI’s blue, green, and red filters (centered at 437, 546, and 635 nanometers).
These images have been “calibrated” to remove several types of artifacts introduced by the camera system, and the contrast has been adjusted to enhance the visibility of surface and atmospheric features. The prominent dark “shark’s fin” at the center of this view is known as Syrtis Major. In 1659, the renowned Dutch astronomer Christiaan Huygens included this dark marking in a sketch of his view of Mars through his early telescope – making this the first feature to be documented on the surface of another planet. Huygens also used repeated observations of Syrtis Major to estimate the length of the Martian day (about 24 hours).
Over the following centuries, astronomers noted dramatic changes in the size, shape, and “darkness” of this feature, and it was believed by some to be related to seasonal changes in vegetation growing in and near a shallow sea. Beginning in the early 1970’s, spacecraft observations revealed Syrtis Major to result from dark sand deposits covering the gentle slopes of a massive volcanic plain. The noted variability over the Martian year is caused by winds moving fine, bright dust and coarser, dark volcanic sand across the region.
The bright feature to the south of Syrtis Major is known as Hellas Planitia. Caused by the collision of a large asteroid with Mars about 4 billion years ago, and measuring about 2,300 km across and up to 7 km deep, Hellas is among the largest impact basins in the Solar System. The interior of Hellas is often obscured by water-ice clouds. In southern winter, deposits of water ice and frost can also mantle the basin’s surface. In this view, a dust storm (the tan clouds) is swirling over northwestern Hellas.
