Imaging the
Earth

As we launch our exploration of the rest of the Solar System, it is useful to recall what the Earth looks like using the various imaging techniques that we may wish to apply to distant planets and moons.

The Earth in Visible Light

We have seen a number of images of Earth in visible light, but mostly at large scale from a great distance. Here are two images from space of smaller objects on the Earth that we will be interested in looking for on other planets and moons: a canyon system, and an active volcano.

The Grand Canyon from space Mt. Etna from space (Source)


We shall find canyon systems and active volcanoes on other objects in the Solar System to are not too different from these images. Here is a set of links to satellite imaging of the Earth.

The Earth at Night

There is one aspect of the Earth's appearance that we do not expect to be repeated in the near future for other objects in the Solar System: at night the artificial light associated with human civilization is very visible from space. The following image shows the appearance of the United States at night as observed from a composite of many satellite passes.

The USA at night


The major source of light is from cities, but by looking carefully you can even see things like lights scattered in the Gulf of Mexico south of Louisiana that are from oil platforms (Ref). The photograph is from Defense Meteorological Satellite Program (DMSP) images.

Imaging in Ways other than Visible Light

Because our eyes are sensitive to visible light, our prejudice is to view things at those wavelengths. However, we now have instruments at our disposal that permit observations in many wasy other than the visible light region of the electromagnetic spectrum. These often offer considerable advantage; for example radar cuts through the ever-present thick cloud cover to give us images the surface of Venus that we could not obtain at visible wavelengths.

Radar Imaging of the Earth's Surface

The adjacent images show a comparison of the Mt. Everest region (border of Nepal and Tibet). The top image was taken through thick cloud cover with synthetic aperture radar on the space shuttle Endeavor; The bottom figure is an optical image of the same region taken from Endeavor (Ref). One can see many of the same features in the two photographs (the photographs were taken at different times of the year, so they have different snow covers).

The curving and branching features are glaciers. The radar technique used is sensitive to characteristics of the glacier surfaces such as the ice roughness and water content. Thus the glaciers show a variety of colors in the radar image but are a rather featureless gray or white in the optical photograph.

Radar (upper) and visual (lower) images of Mount Everest

Infrared and more Exotic Imaging

We have seen in the preceding sections examples of imaging the Earth in the infrared, ultraviolet, and X-ray regions of the spectrum. Here we show additional examples of IR images, and a more exotic technique combining magnetic and gravitational data that can even locate objects beneath the surface of the planet.

San Francisco Bay imaged in IR from space Fossil crater imaged with representation of gravity and magnetic field data GOES-8 IR satellite image of water vapor in Earth's atmosphere


The left figure shows the San Francisco Bay area imaged from space in the infrared (IR).

The middle figure shows a composite of local gravity and magnetic field variation data to image a 112-mile wide relic meteor crater in Yucatan that presently lies below several hundred meters of sedimentary rock. This crater, called Chicxulub, is famous because it is the leading candidate for the site of the asteroid impact that is thought to have killed the dinosaurs 65 million years ago in the K-T extinction (Ref).

The right image shows a GOES-8 weather satellite image in the 6.7 micron IR channel that is sensitive to the distribution of water vapor in Earth's atmosphere (Ref). The imager on this satellite records radiation emitted by water vapor in the upper troposphere. Regions with high concentrations of water vapor are bright, while dark spots signal lower water vapor concentrations.

Surface Temperature Maps

Infrared radiation is basically radiant heat. Therefore, IR detected from satellites can be used to determine the temperature of objects. The following image shows a color-coded map constructed from a composite of satellite data and surface observations giving surface temperatures on the Earth (Ref).

Surface temperatures on Earth for January 30, 1997


Here is a link to the current temperature map (updated on a 6 hour cycle), and here is a movie (2MB MPEG---slow download) of the temperature varation over the past week.


Sea Surface Temperature Maps

Similar methods as described above may be used to construct color coded maps of surface seawater temperatures. Here is an example (Ref):

Sea surface temperatures on Earth for January 29, 1997


Here is a link to the Current sea surface temperatures (updated daily).