Exploring the Planets

Surface Features of Mercury

The surface of Mercury, at first glance, looks very much like that of the Moon, but in fact it is different in several ways. The MESSENGER spacecraft revealed that Mercury has some unique landforms, more smooth plains, and surface compositions (low in iron and high in sulfur) that are unlike any measured on the Moon.

Impact Basins

Caloris Basin — Impact Site

This is one of the largest impact basins in the solar system and the largest feature on Mercury. The Caloris Basin is 1300 kilometers (810 miles) in diameter. Only half of the basin was imaged by Mariner 10 in the 1970s, but the picture was completed by the MESSENGER mission. After the impact, the basin was flooded by lava. Ridges and fractures formed when the volcanic rock contracted and stretched as it settled under its own weight.

Mariner 10 image courtesy of M.S. Robinson. MESSENGER image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Caloris Basin on Mercury

Caloris Basin
Rembrandt — Young Impact Basin

The Rembrandt Basin displays a "wheel and spoke" pattern on its central floor that has never been seen on any other planet or moon.

Image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Smithsonian Institution/Carnegie Institution of Washington

Rembrandt Basin

Rembrandt

Rembrandt Size Comparison
The 715 km (444 mile) wide Rembrandt basin is large enough to cover the northeastern United States.

Image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Smithsonian Institution/Carnegie Institution of Washington

U.S. East Coast – Rembrandt Basin Overlay 2

Rembrandt Comparison

Scarps

A Shrinking Planet?

The surface of Mercury has landforms that indicate its crust may have contracted. They are long, sinuous cliffs called lobate scarps. These scarps appear to be the surface expression of thrust faults, where the crust is broken along an inclined plane and pushed upward. What caused Mercury's crust to shrink? As the interior of the planet cooled it contracted. Gravity then forced the crust to adjust to a smaller interior.

Image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution

Lobate Scarps

Discovery Rupes

Another large lobate scarp on Mercury is Discovery Rupes. (Rupes is Latin for cliff.) Discovery is about 550 kilometers (350 miles) long and up to 1.5 kilometers (1 mile) high. Notice that the walls and floors of two impact craters have been deformed by the thrust fault that formed the scarp.

Image courtesy of T.R. Watters, M.S. Robinson, and A.C. Cook

Mercury's Discovery Rupes

Rupes

Digital Elevation Model

How do we know how high Discovery Rupes is? One way is to use two images that provide a stereo (3-D) view of the feature. Using a computer, the heights (elevations) of all the points covered by a stereo pair can be calculated and a digital elevation model made. This Mariner 10 image shows the area covered by the stereo pair and the digital elevation model is color-coded to show the highest elevations in white and lowest elevations in black. The white arrows in the image indicate the location of part of Discovery Rupes.

Image courtesy of T.R. Watters, M.S. Robinson, and AC Cook

Digital Elevation of Mercury's Discovery Rupes

Digital Elevation Model.

Volcanism

Smooth Plains

Between the heavily cratered regions of Mercury's surface lie large expanses of smooth plains. Much of the north polar region of the planet is covered by these plains (outlined in yellow) which may be volcanic basalt deposits several kilometers thick.

Image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Smooth Plains

Flowing Lava

The MESSENGER mission has revealed many features that indicate the presence of volcanism on Mercury in the past. In this image a narrow channel leads into a large basin near the northern plains. Scientists propose that hot lava flowed through this channel, perhaps carving the terrain as it went. Nearby are odd-shaped pits that may be vents through which the lava flowed.

Image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Flowing Lava

Volcano

This feature is the vent of a small, young volcano located near the rim of the Caloris basin.

Image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Volcano

Hollows

The mysterious features named hollows appear on the floor of the crater Tyagaraja. A pit in the center of the crater may be a volcanic vent. The origin of the hollows is under study, but may relate to the release of gases or to sublimation of material exposed by the crater impact. (The crater Tyagaraja is named after a 19th Century composer from India. As with all the planets, crater names on Mercury are approved by the International Astronomical Union (IAU). Craters on Mercury are named after famous artists, authors, and musicians.)

Image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Hollows

Water Ice

Data from MESSENGER have strongly supported the existence of water ice in permanently shadowed craters at Mercury's poles. Earth-based radar data is superimposed on this image of Mercury's north pole. The yellow areas, where the radar signal is reflected strongly, are thought to be sites where ice is trapped in frigid dark shadows.

Image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory

Radar Bright Deposits in Mercury's Polar Craters

Water Ice

Ridges and Troughs

Ridge and trough systems unique to Mercury have been observed in MESSENGER imagery. These features form in relation to "ghost craters," impact craters that are filled and buried by volcanic deposits, but whose outline is revealed by ridges that form over the crater rim. The patterns of the ridges and troughs are caused by extension and contraction due to cooling of very thick lava flows and the planet's interior.

Image courtesy of NASA/The Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Smithsonian Institution

Graben in Goethe Basin on Mercury

Ridges and Troughs