Where on Earth is That?

The Betting Pools

Volcano near Gusev crater, Apollineris Patera, 120 miles northwest. Credit: NASA/JPL

So far, Spirit rover scientists and engineers have set up a number of informal betting pools. Where has the rover landed, precisely? What’s the best earth analog for the martian landing site? Where’s the best place to drive the rover first? Which theory of origin best encapsulates the diverse rocks now seen?

For the bets on location, three independent teams are debating the question of where exactly the rover is now. Their bets are happening without much exchange between teams, at least not until the first answers can be compared and debated. Two camera teams are looking at orbital pictures, the rover itself shot three quick images only seconds before first touchdown, and then finally a more indirect path is followed using what are called the rover’s inertial maps–a gauge of bumps and rolls as the airbags accelerated and decelerated during its chaotic surface trajectory.

Gusev crater from orbit, with Ma’adim Vallis entry at lower right. Crivitz crater is a smaller crater located within Gusev

There is considerable discussion also of what targets to drive towards, once the rover leaves its base petal sometime around next Wednesday or so. As new and better images come from Mars, different destinations take on more appeal. Yesterday there was considerable interest in the depression called Sleepy Hollow. Today there was more interest in driving towards the distant hills or looking at a folded patch just by the rover’s base petal. Those destinations will change again as better pictures and composition data arrive on Earth.

On the first night of elation, just after the Rover sent its first image, Dr. Charles Elachi, Center Director at JPL, pointed out one feature of a bet that is new to this mission. The rover is designed to drive up to 100 meters each Martian day. Its top speed is 5 centimeters per second on a flat surface; however, on average, it will only move at 1 centimeter per second for a maximum of 4 hours each day. Because the entire suite of instruments is able to move, pan, and dig, each day can be viewed in some ways as a ‘new landing’. Each day offers the opportunity to reach a new region on Mars and multiplies enormously the science return.

Another betting pool is to interpret the features seen so far, in particular whether the impact crater at Gusev has been formed by which element: fire (volcanism), water, wind or earth (impacts or soil). It is the latter two elements, soil and wind, that currently shape what is going on, but since the crater was formed billions of years ago, the history involves much more than what is today visible on the surface.

Liquid water may have flowed over the surface of Mars in the planet’s distant past. Artist conception of a delta filling a crater.
Credit: NASA

One of the most visually tantalizing, but potentially deceptive, betting pools involves looking for good earth analogs. What is the best description of Gusev, relative to a place one might get on a plane tomorrow and visit terrestrially?

The Cup

It is a good bet that Gusev is an impact crater (Greek, meaning ‘cup’). Unlike volcanic cones or depressions, craters are usually round. They have a characteristic depth that is around five to ten times shallower than their width (aspect ratio 0.1 to 0.2), depending on the kind of ground struck by the originating asteroid or meteorite. Softer material tends to make shallow craters with broader rims–more of a splash than a bullet-hole. So where are such craters on earth?

Compared to Mars, which has the same land surface area as Earth despite being only a third as big, most craters on Earth would hit the oceans and leave no permanent evidence. In orbital pictures of Mars, the more craters in view, the older the surface is. Counting craters can be like counting tree-rings, particularly since most of the weathering effects on Earth (other than wind) are not reshaping or filling in the craters as they age.

Filling in the crater floor could be more soil, or as some scientists speculate, water. One hypothesis that Spirit will test is whether Gusev ever was filled from the impressive delta, called Ma’adim Vallis, which may have pooled enough water to form a paleolake, or ancient lakebed.

The Cup Runneth Over

The Onyx River only flows a couple of months each year.
Image Credit: Teachers Experiencing Antarctica and the Arctic, Rice University

One obvious place to look for Gusev on earth is a crater lake. The most famous crater lake on Earth is still filled with water, unlike Gusev, and is the African crater lake, Lake Bosumtwi, in Ghana.

One of the coldest and driest places on Earth has been suggested to be most like Gusev. Astrobiologist, Dr. Chris McKay of NASA Ames has pointed to Antarctic dry valleys, in particular to Lake Vanda and the Onyx River, as useful guides for a terrestrial tourist. "I would argue that the most relevant environment [to Mars] is the dry valleys of Antarctica. This is the largest ice-free region in Antarctica. The average temperature is minus 20 C (minus 4 F). Summertime temperatures are higher. Precipitation is equivalent to one or two centimeters per year of moisture. And the pressures are well above the triple point of water. [The triple point of water is a combination of temperature and pressure that enables water to exist in all three states: solid (ice), liquid and gas (water vapor). When atmospheric pressure is too low, as on the surface of Mars today, water cannot exist in liquid form, regardless of the temperature.]"

Like what may have happened in Gusev billions of years ago, water can pool in low regions. "In the Antarctic dry valleys," said McKay, "it never rains, only snows, and yet there are large rivers and lakes, such as Lake Vanda and the Onyx River, the largest river in Antarctica. Typically the Onyx River flows a couple months a year. When the temperatures in the summer get above freezing, the glaciers melt and that water forms the Onyx River, which collects in the lake."

Exotic terrestrial landforms at Amguid Crater, Algeria
Image Credit: Impact Crater Database

While such a place seems austere or permanently locked in a frozen state, "What’s interesting about this environment," said McKay, "is that because it’s so dry, it’s dead. There’s nothing growing on the surface soil. Precipitation is too slight. It evaporates, blows away. This is one of the most lifeless places on Earth. But in the water in Lake Vanda, underneath the ice, there are thick mats of algae and bacteria. So here in this Mars-like environment, where the average temperatures are minus 20 C (minus 4 F), there’s still a hydrological cycle, based on snow, glaciers, melt and accumulation in ice-covered lakes".

Chilly in Chile

There are many famous craters on Earth to select from for betting on Gusev surrogates.

Some mission scientists have recognized landforms in the initial lower resolution images that may resemble the geology at the Haughton region of Canada.

The Barringer crater in Arizona is a perfect example of a simple crater, a straightforward bowl in the ground. To-date, over 160 impact craters have been identified on Earth. As the largest example in the US, Virginia’s Chesapeake Bay was an impact crater from 35 million years ago that is one of the best preserved ‘wet-targets’. Almost all known craters have been recognized since 1950 and several new structures are found each year.

Crater lake, Lake Bosumtwi, Ghana, as imaged by remote sensing satellite.
Image Credit: Earth Impact Database

But now that pictures are arriving from the surface, scientists can begin to look for more specific earth analogs of a dry lakebed or desert plain.

Dr. Ray Arvidson, of Washington University and deputy principal investigator for MER, offered some comparisons to desert plains that resemble what the early pictures from Gusev show. "We are looking to answer the question, what is that landform? It is a rock-strewn plain, with drifts and scour-zones. It resembles a landform called desert pavement. …Examples on Earth might be eastern and western Egypt, Antarctica, and even Mauna Kea in Hawaii. Desert pavement is also seen in the American southwest."

Among the more exotic theories being debated among mission scientist, Gusev may have been formed in three layers, a lakebed covered by a nearby volcano [Appollinaris Patera, 120 miles to the northwest of Gusev], then covered again by dust and weathered by wind. "It depends on who’s on top, the volcano or the lake."

"But we can dig into that with the rover wheels," said Arvidson, "by locking five wheels and spinning the sixth one in place. That can uncover twenty centimeters deep", or about one foot below the suface. "It could be a spill plain (or lakebed), there is speculation about the effects of liquid carbon dioxide, evidence of glaciation, and aeolian (wind) action." Arvidson was on the original Viking imaging team, and feels he has awakened thirty years later–like Rip Van Winkle–somewhere near where Viking landed, in a region called Chryse Planum. The first indication of this may be the cemented, dark surfaces that give an impression of a crust or desert pavement.

An aerial view of the lake at Licancabur’s summit.
Image Credit: Michael Endl, UTA McDonald Observatory

Another possible feature that both Mars and some parts of the Earth may share is a very dry landscape. NASA Ames geologist, Nathalie Cabrol, along with her husband, Edmond Grin, were the scientists who first proposed that Gusev would make a good MER landing site. Along with Chris McKay, she has trekked to the driest place on Earth, the Atacama desert and Licancabur region of Chile. Licancabur is the world’s highest lake.

She told Astrobiology Magazine that "We have been talking in the past couple of days about how this could look like places you would see in the Arizona desert, in the California desert, in the Atacama desert (in Chile). Some things could be the result of deposition by flow, with rocks embedded in a matrix of fine sediment, and the whole region deflated (swept clean) by wind. Volcanic plains also can be very flat with small rocks. But I’m particularly interested by the smoothness and the roundness of some of these rocks".

To investigate Earth for other exotic examples of crater lakes or desert pavements, try your own luck. Is your destination to be Algeria, Canada, Chile, Antarctica or Arizona?