The New Horizons Jupiter encounter provides a unique opportunity to quantitatively measure storm development. As New Horizons approaches Jupiter, the LORRI camera will spend two weeks making movies of the planet's stormy weather. By utilizing both the high spatial resolution and wide spectral complement of the New Horizons science instrument suite, atmospheric storms can also be studied in three dimensions - specifically, measuring localized cloudy regions of dramatic vertical and horizontal dynamics.

Historically, large-scale cloud development, eddies and waves have been observed across the planet. Following results from the Galileo mission, two specific regions are thought to be of special importance.

Great Red Spot Turbulent Wake

In general, the development of small-scale storms on Jupiter is unpredictable. However, we now know that the turbulent wake region to the northwest and west of the Great Red Spot (GRS) regularly spawns dramatically evolving dynamical cloud features. The explanation is uncertain, but there are some simple hypotheses. The predominant circulation of cloud-level winds on Jupiter is zonal. However, the Great Red Spot, a long-lived (more than three centuries) anti-cyclonic feature some 20,000 kilometers across, disrupts this flow of zonal air in the 10-30 degree south latitude region. Air currents divert around this impediment (much as water in a brook diverts around a rock), which causes some zonal flows to wander into regions that are normally occupied by currents flowing in the opposite direction.

Interactions between these jumbled-up air currents seem to extend far downstream (that is, to the west) of the turbulent region, with a host of whorls observed for more than a GRS-diameter to the west. Indeed, within these whorls, storm-like features can be seen developing and evolving as the eastward current carries them toward the GRS, where they are increasingly more affected by the diverted westward current interacting with it in a complex, three-dimensional manner.

This specific region of Jupiter is exceptionally active, with extreme gradients in cloud structures and associated reflectivities and large temporal variability. This region is a focus of jovian dynamical/storm studies by New Horizons.

Atmospheric Structure and Chemistry from Absorbed Starlight

The composition and structure of the upper atmosphere of Jupiter is not well known. The one in situ measurement from the Galileo probe provided a useful profile of temperature vs. pressure near the equator, with only indirect composition information. Next to in situ measurements (which would require a probe), occultations are the most robust way to "sound" an atmosphere. Earlier stellar and solar occultations (such as those from the Voyager mission's Ultraviolet [UV] Spectrometer) have been of limited use because of low instrumental spectral resolution and sensitivity. An observation by the New Horizons Alice UV spectrometer of one or more stellar occultations during the Jupiter flyby will provide abundance and scale height information for many important species (for example, hydrogen [H₂], methane [CH₄] , acetylene [C₂H₂], and higher hydrocarbons) over a wide range of pressure (10^-4to 10⁴ Microbars). The H₂ and long-lived hydrocarbon scale heights will be used to provide the bulk atmosphere temperature profile, while the profiles of more chemically active hydrocarbons will provide strong constraints on upper atmosphere dynamics and photochemical models. This experiment also provides a "dry run" for similar occultations New Horizons will perform at Pluto and Charon.

Mapping Jupiter's Clouds and Storms in 3-D

New Horizons will map the three-dimensional structure of clouds and storms in Jupiter's atmosphere. It will do this by imaging in many different colors of light simultaneously and measuring how much sunlight the clouds reflect in each of these colors. Some colors of sunlight don't travel very far into Jupiter's atmosphere. This is because methane and hydrogen gas in the atmosphere absorb these colors, destroying the light. The remaining, non-absorbed colors of light make it farther down before being absorbed. If an image shows cloud features in colors of light that are strongly absorbed by the atmosphere, that means the clouds are so high that they are able to reflect the sunlight before the atmospheric gases adsorb the color. By watching what happens to the various colors of light, an accurate determination of the cloud-top altitudes can be made.

The figure below demonstrates how New Horizons will determine the 3-D structure of clouds around the Great Red Spot. Here, in the top two rows, four black-and-white images of the Great Red Spot acquired by the Galileo Near-Infrared Mapping Spectrometer (NIMS) in four different colors of light are shown. On the right side, the topmost image (labeled "Red") is taken in a color that the atmosphere readily absorbs. The Great Red Spot is clearly seen as a large oval feature. Because the light is not being completely absorbed by the atmosphere over the Great Red Spot, we know that not much atmosphere lies above it, and therefore the Great Red Spot extends very high up, more than 10 kilometers (about 6 miles) above the neighboring clouds.

The neighboring clouds are revealed in colors that are not absorbed so much by the atmosphere, such as shown in the right-middle image (labeled "Green"). This image indeed reveals that clouds cover much of Jupiter, but that most clouds reside much deeper in the atmosphere than seen in the Great Red Spot.

Some clouds absorb sunlight in certain colors. For example, a cloud feature located just above and to the left of the Great Red Spot is seen in all but one of the black and white images. Specifically, the cloud appears to be missing in the left-middle image. At the color of light in which this image was taken, the cloud particles actually absorb the light, making the cloud disappear. However, at two nearby colors, the cloud shows up well, as shown in the two images at the top of the figure. The fact that the cloud absorbs at this one color in the set of three images means that the cloud is composed of large particles of ammonia. It is interesting to note that the Great Red Spot does not appear to change very much from color to color among this set of black and white images. This means that the Great Red Spot probably is comprised of substances other than ammonia, or has much smaller particles of ammonia that do not absorb much sunlight in the ammonia-absorbing color.

The image on the bottom left (labeled "Blue") demonstrates even more vividly the differences among the clouds near the Great Red Spot. This image was generated by subtracting the middle image on the left-side from the image above it. Here, the cloud to the northwest of the Great Red Spot appears bright, demonstrating a large difference in its reflectivity between the non-absorbing and absorbing colors of light. In contrast, the Great Red Spot appears very dark, which indicates that it reflects nearly the same amount of light in each of the two colors.

The color image on the bottom right is a composite image of the three black-and-white images labeled "Red," "Green" and "Blue." It shows in a glance which clouds are at high altitude (the reddish ones), which clouds are at low altitude (green) and which clouds are comprised of large ammonia particles at relatively high altitude (bluish-white). The fact that the bluish-white clouds are both high-altitude and comprised of large ammonia particles means that relatively violent dynamics occur here. This is because the ammonia particles are created by condensation relatively deep in the atmosphere, some 10 kilometers below the level at which they are seen here. To get to the high altitudes observed here, these large particles must have been carried in very strong updrafts, similar to what occurs in thunderstorms on Earth. During its encounter with Jupiter, New Horizons will study such features repeatedly to witness the formation and dissipation of ammonia storm systems.