Autumn comes to Mars' north pole

We know that Jupiter and Earth are two worlds with dynamic, cloud-filled skies. But it's easy to forget that Mars is another such world with cloudy weather and seasonally varying climate. This lovely image release from the CRISM instrument on Mars Reconnaissance Orbiter brings that point home.

CRISM is an imaging spectrometer, like VIMS on Cassini or THEMIS on Mars Odyssey or Ralph on New Horizons. Imaging spectrometers generally achieve poorer spatial resolution than straight camera systems, which is to say that they usually can't see features as small as those visible to the cameras on the same spacecraft. This particular image has a resolution of about 20 meters per pixel; the HiRISE camera on the same spacecraft can do more than 70 times better.

But what imaging spectrometers lack in spatial resolution they make up for in spectral resolution. Many spacecraft cameras are black-and-white, meaning they're totally blind to color variations. Some spacecraft cameras are "multispectral," meaning that they can see some variations in color, usually (but not always) by slipping a color filter in front of the camera optics that only lets some wavelengths of light through to the camera. These cameras can see as many colors as they have filters, usually fewer than 10 different colors, though some have more. HiRISE sees up to three different colors; Cassini's narrow-angle camera has 23 different filters. Imaging spectrometers aren't multispectral, they're "hyperspectral" -- they can separate the light that comes in to their optics into dozens, even hundreds of different colors. CRISM is the most hyper of all hyperspectral imagers ever sent to Mars; it can see in 544 different colors!