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One of the most productive probes ever launched, JPL’s Magellan was launched in 1989 and arrived to orbit around Venus in 1990. Its primary sensor was an imaging radar that over time provided views of nearly the entire surface at resolutions ranging between 120 and 360 m. It also could map elevations to +/- 50 meters and had a radiometer that determined brightness temperatures to +/- 20° K. This page elaborates on the first-order observations from the mission and shows some full views of Venus and a Venus surface map. It also describes the abundant examples of volcanism and volcanic features that dominate the venusian surface.


The Magellan Mission

One of the supreme triumphs of planetary exploration was the Magellan program, developed and run by the Jet Propulsion Laboratory, to study Venus close-up, by penetrating its cloud cover. This was the first spacecraft ever launched from a Space Shuttle (Atlantis), on May 4, 1989, to another planet.

Magellan almost didn't happen! The original concept for a spacecraft to Venus was VOIR - Venus Orbiting Imaging Radar. After a design was completed and a go-ahead from NASA Headquarters was sought (meaning "funding approved"), a budget squeeze during the Reagan administration forced its seeming cancellation. However, the JPL Venus team was undaunted. They began to search for already existing spacecraft components, some of which were found, that cut costs. Their next plan submission was also denied. They persisted and made further cost savings, so that finally the plan fell within current budgetary constraints and Magellan was approved. This superb spacecraft was built.

Here is Magellan at the Kennedy Space Center awaiting loading into the Shuttle:

The Magellan spacecraft at KSC, prior to launch from the Shuttle.

Magellan's primary instrument was a multimode Radar Mapper (2.385 Ghz, or 12.6 cm wavelength). In the SAR imaging mode, looking between 18° and 50° off-nadir, it could capture scenes with resolutions between 360 m and 120 m (1181-394 ft), depending on its position within its elliptical, near-polar orbit (at altitudes between 275 and 8,443 km (171 to 5,246 mi above a mean venusian radius of 6,051 km [3,760 mi] ). It first established orbit on August 10, 1990 after 1 1/2 loops around the Sun. Its altimeter mode achieved a vertical accuracy of better than 50 m (164 ft) within a ground cell of 10 km (6.2 mi) diameter. Operating in a radiometer mode, the radar could sense surface radio-emission, whose signals can be converted to brightness temperatures with an absolute accuracy of ±20° K. Investigators gleaned information on mass distribution (causing gravitational anomalies) from Doppler frequency variations due to gravity effects that varied orbital speeds. Even as Venus rotated slowly beneath (one complete day every 243 Earth days), during stages of its orbit closer to the planet, the radar imaged surface swaths between 17 and 28 km (10.6-17.4 mi) wide. Through the first cycle lasting 8 months, it mapped 84% of the surface. In the next 16 months, that percentage rose to 98%. Additional coverage provided repeat looks in search for possible transient or short-term changes. After several adjustments to lower orbits, the spacecraft finally burned up in the venusian atmosphere, in mid-October, 1994.

Image strips covering thousands of kilometers, especially after being joined as mosaics, provide stunning views of a fascinating venusian surface that is still undergoing thoughtful interpretations. Although Venus no doubt formed concurrently with Earth, its surface today is largely younger than one half billion years (Earth has some surficial regions older than 2 billion years). Planetologists base this estimate on venusian crater frequencies. Even though not uncommon, the numbers of resolvable impact structures are consistent with 1) destruction of the much larger numbers from the first 4 billion years, most probably by active processes that removed them by lava overplating (resurfacing) and by still arcane erosional actions, and 2) asteroidal flux rates for the last 5 hundred million years, in line with estimates from other planetary surfaces. Effects of volcanism are conspicuous, with thousands of small volcanoes detected, along with many lava channels. Although fracture zones and sets of close-spaced ridges are evident, no direct indications of terrestrial-like plate tectonics are discernible. Surface water, if ever present, left no signs of stream or ocean activities, and would have escaped from the planet (traces are present in its atmosphere) as Venus heated up, until a massive "runaway greenhouse" warming effect overwhelmed the planet. The slowly rotating atmosphere seems to have caused some wind streaks and dune-like deposits on the surface.

The gallery of Magellan images is vast. We show only a select few here but you can access more at JPL's Magellan Home Page (http://www.jpl.nasa.gov/magellan/).

To familiarize you with some of the major features and their locations on Venus, look at this shadowed relief map of the non-polar regions of the planet with the key geomorphic features labelled, as prepared by the U.S. Geological Survey:

Shaded relief depiction of the venusian surface with principal localities named.

Next, consider this color-coded relief map of nearly all of Venus, on a Mercator-like projection, derived by integrating imaging and altimeter data.

Color-coded Magellan relief map of nearly all of the surface of Venus.

Blues represent the lowest surfaces followed by greens, then yellows and oranges with red being highest. The greatest elevations are within Maxwell Montes (top left), that includes the high point of the uplands known as Ishtar Terra. Another high region, near the equatorial center, is called Aphrodite Terra. Beta Regio, near the central left, is also elevated. Two blue regions in the north are low plains, called Sedna Planitia, below Ishtar, and Atalanta Planitia, well to its east. A large curved channel south of Aphrodite is known as Artemis Chasma.

Now look at a hemispherical projection that lies within this full map. Try to identify the high central region (in pink); refer to the first relief map above (hint: think of a lovely goddess).

Hemispherical projection of the relevant part of the previous Magellan relief map of Venus.

Here is a second Magellan topographic map, now centered on Venus's North Pole. A small part of the first map is present in the second; find it.

Another Magellan radar image of venusian topography; center is at Venus' North Pole.

19-28: Try to identify the high central region (in pink), enlarged from the first map (hint: think of a lovely goddess). ANSWER

We can also display (below) this same hemispherical segment as a quasi-natural, color image of a mosaic of rectified Magellan scenes. There is no direct proof that Venus has this much red (the choice of assigned colors was a rather arbitrary, best guess) but, if so, the presence of oxidized iron could account for such tones. The dark, blackish low areas are presumably basalts.

 

Quasi-natural color image of Venus's surface matched to the previous hemispherical projection.

19-29: Once again, try to orient yourself in this image relative to the shaded relief map and localities described on that map. ANSWER

Magellan carried a microwave experiment (managed at MIT) from which a map of thermal emissivity (see Section 8) could be derived, as shown here. Note that the lowest emissivities (in blue) are found in the highest parts of the venusian surface, implying that the rock types there were other than basalt.

Thermal emissivity (determined by the microwave radiometer on Magellan) of the venusian surface.

Magellan has confirmed that the dominant process affect Venus' surface and upper crust is volcanism. On the rest of this page, we will examine the evidence, leaving other types of surface features for the next page.

Magellan's greatest revelations were a wide variety of volcanic features. It is not an exaggeration to refer to Venus as the "Volcanic Planet". Most of the planetary surface has been judged by geoscientists to be less than a half billion years old. Some volcanism even appears to be recent and the possibility that there is even now some activity cannot be dismissed (although no changes were observed during the mission which last until 1992). Impact craters (next page), while present, are uncommon, with perhaps 1000 large enough to be resolved by Magellan's radar; this is consistent with the presumption that the present venusian surface is relatively youthful, as the crater flux by then would have greatly diminished (as extrapolated from terrestrial crater frequency since the end of the Precambrian). There have been older surfaces, some parts of which may still persist at the surface, but these are largely "paved over" (resurfacing) by the continuing activity.

This (cluttered) map shows the major volcanic features and their locations on the venusian surface

Volcanic features on Venus.

The first volcanic feature we will look at is characteristic of the plains regions. Here at Lakshmi Planum are several light and dark surfaces that are interpreted to be equivalent to the basalt flow types known as pahoehoe (smooth lavas; somewhat specular surfaces) and known as pahoehoe (smooth lavas; somewhat specular surfaces) and aa (chunky lavas, better backscatterers) - counterparts to common Hawaiian lavas.

Multiple lava flows on Lakshmi Planum.

A series of Lava flows emanate from the Sils Mons volcanic source:

Multiple lava flows emanating from Sils Mons (off the picture).

This radar image shows a long channel filled with volcanic flow material, over which a younger flow has straddled; note volcanic material on the right. This is the Ammavaru flow sequence in the Lada region. The scene's dimensions are 450 by 630 km:

Ammavaru flows in the Lada region.

One of the longest flows (1000 km long) occurs as Myletta Fluctus in Lavinia Planitia.

Part of the Myletta Fluctus flow on Venus.

Like the Moon, thin channels and sinuous rilles have been found on Venus. Here are three examples (check captions for description):

Part (200 km length) of the 1200 km channel in the Helen Planitia region of Venus.

Rilles and pits in the Ammavaru region of Venus; the principal channel, shown here is a 250 km segment, is almost 1200 km long.

The longest channel on Venus, some 6800 km [4200 miles], part of which appears here.

Flows can sometimes be traced to shield volcanoes (with central calderas) as exemplified by Theia Mons, 4 km high, with a central caldera measuring 75 by 50 km and surrounded by a lava field reaching 800 km in maximum dimension.

Theia Mons.

Another major volcano, seen in this colorized rendition, is Sapus Mons (1.5 km high; 120 km at its base, on an upwelled domical surface 1000 km in diameter) in the Alta Regio region.

Sapus Mons (1.5 km high), artificially colorized to approximate the general colors of an oxidized venusian surface.

Perspective color views of this type of volcanic structure, made by applying altimetry data to the radar image, show it to be much like a shield volcano, with a broad base and often a central depression. This rendition of Sif Mons, about 2 km (1.2 miles) high and covering an area of nearly 300 km (200 miles) in diameter, illustrates this:

Sif Mons (2 km high), reconfigured into a perspective view; the shape is intermediate between a Mauna Loa type of shield volcano and some low stratovolcanoes.

One of the highest mountains on Venus is Maats Mons which reaches to 8 km (5 miles) above the mean venusian elevation; its shape is transitional to a stratocone, suggesting its lava may have differentiated into that of an intermediate silica content. A younger lava flow from the main volcano appears as bright flow:

Maats Mons; darker brown denotes older lava flows

Topographic maps made from Mariner data can bring out structures that are large volcanoes:

Venus Marine radar imagery colored to indicate topography using elevation differences as determined by altimetry; three volcanoes are shown.

Some of the larger volcanoes have summit calderas. The largest on Venus is Sacajawea Patera (140 km [89 miles] in long dimension:

Sacajawea caldera.

Similar to that is this 30 km wide caldera

A volcanic caldera on Venus.

Such calderas often look almost identical to large impact structures (discussed below). A case in point is the circular depression below which could have been identified as such except for the prominent lava flow emanating from its side. (Note: another interpretation considers this to be a genuine impact crater filled post-impact with shock-generated melt that leaked out.)

A volcanic caldera with impact crater-like appearance but showing a lava flowcoming from its base.

A variant of the caldera type has been given the descriptive name of "tick volcano" because of its resemblance to the insect of that name. Emanating radially from the crater walls are ridges that form the "tick legs".

A tick volcano in Alpha Regio, with ridges stemming from the 31 km wide caldera rim.

While these large shield volcanoes are uncommon on Venus, there is a much larger number of smaller volcanic structures (those that rise up from the surface). This map locates most of these by category, and also includes the principal volcanic fields which contain features like those shone above:

Global map of most of the areas of volcanic structures, with types indicated.

Cinder cones and stratocones (Vesuvius-like) are rare on Venus. Here is one example of a swarm of cones (each about 2 km wide) on the plains that are larger than terrestrial cinder cones but not typical of stratocones.

Small volcanoes, possible large cinder cones.

19-30: If large volcanic edifices like, say, Mount Rainier or Fujiyama were to occur on Venus, what would that imply? ANSWER

Typical small shield volcanoes occur in swarms such as is evident in these views:

Small shield volcano field in fractured venusian crust.

A swarm of small shield volcanoes.

Irregular flattened shield types, called fan volcanoes, are built up by several overlapping outpourings are illustrated by this example:

Fan Volcano.

Domical hills (Tholi), shieldlike in structure, as much as 25 km (15.5 mi) wide and up to 750 m (2,460 ft) high, dot the plains of Alpha Regio. Astrogeologists believe these pancake-shaped features result from upwelling at tubular vents of lavas that spread uniformly in all directions.

Pancake domes in the Alpha Regio plains on Venus.

Here is a closer look at two such domes in Tinatin Planitia; the larger is 65 km wide:

A pair of large pancake domes.

Using laser data, a pancake dome in Alpha Regio is shown in a perspective view, being colored such that it reminds the writer of the Devil's Platform in Hell (or more realistic like an upwelled lava dome in a lava lake).

Computer-generated view of a pancake dome in Alpha Regio.

Some of the small domes develop distinctive flows around them that have reminded some venutian planetologists of "sea anemones", to which that name is colloquially applied.

An anemone-like volcanic structure.

For a good summary of venusian volcanic activity, check out theVenus Volcano page produced by the University of Notre Dame.

On the next page, we finish our tour of Venus now with examples of some other landforms, some having a volcanic connections, others of different nature and origin.

 

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Primary Author: Nicholas M. Short, Sr. email: nmshort@ptd.net