The latest from NASA's Mars Reconnaissance Orbiter

Six science instruments on the MRO have been examining Mars since 2006 and the craft is still going strong. Here are some of its recent discoveries.

A simulated 3-D perspective view, created from image data taken by the THEMIS instrument on NASA's Mars Odyssey spacecraft illustrates how climatic cycles of ice and dust build the Martian polar caps, season by season, year by year, and periodically whittle down their size when the climate changes.
NASA / JPL / Arizona State University, R. Luk

A radar record of Mars' polar cap

Radar images (top) hold evidence for a past ice age in the northern polar ice cap of Mars. Below, the boundary identified by scientists is shown as a blue line.
NASA / JPL-Caltech / Sapienza University of Rome

Scientists recently decoded a record of the most recent Martian ice age, using radar data from NASA's Mars Reconnaissance Orbiter (MRO) gathered around the northern polar ice cap.

The latest findings tally with previous models that suggest the glaciation ended about 400,000 years ago.

The discovery was published in the journal Science in May.

The new data gives insights into the movement of ice on the planet that will help scientists improve their modelling of past and future climate changes on the Red Planet.

On Mars, ice ages occur when the planet’s tilt increases, making the poles warmer than mid-latitudes. The polar caps retreat, and water vapour forms into ground ice and glaciers at mid-latitudes.

When this cycle reverses, polar ice begins accumulating again, while ice is lost from mid-latitudes.

This retreat and regrowth of polar ice are exactly what is shown in the images from MRO’s Shallow Subsurface Radar (SHARAD).

These take vertical slices of the polar ice deposits and scientists studied hundreds of these looking for variations in the layers.

They identified a boundary in the ice across the north polar cap. On one side of this, the layers accumulated more quickly and uniformly than on the other.

“The layers in the upper few hundred metres display features that indicate a period of erosion, followed by a period of rapid accumulation that is still occurring today,” said Isaac Smith, the study's lead author.

An image of dark dunes on Mars was taken on by the High-Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter.
NASA / JPL / University of Arizona

Morse code in the dark dunes

The MRO’s High-Resolution Imaging Science Experiment camera (HiRISE) took this image in February 2016 of these eerie dark dunes that have been influenced by local topography.

Scientists at first were puzzled by the dunes’ complex shapes that appear to tell little about the wind direction that might have shaped them.

They believe a circular depression, probably an old crater now filled in with dust and debris, has influenced local winds and reduced the amount of sand available for dunes to form.

While scientists still do not understand the process fully, they believe that winds funnelled at right angles to the dunes from two directions have formed the linear “dashes” dunes. They think the smaller “dots” occur where this process has been interrupted.

A map showing the frequency of carbon dioxide frost's presence at sunrise on Mars, as a percentage of days year-round. Carbon dioxide ice more often covers the ground at night in some mid-latitude regions than in polar regions.
NASA / JPL-Caltech

Carbon dioxide frost map

NASA scientists have drawn up a map showing the frequency of carbon dioxide frosts at sunrise on Mars.

They found that carbon dioxide ice covers the ground at night in some mid-latitude regions more often than it does in in polar regions, where it is generally absent during summer and autumn.

The colour coding in the map above is based on data from the MRO’s Mars Climate Sounder instrument.

Yellow indicates high frequencies, blue identifies areas where CO2 is rarely present and red is intermediate.

Areas without colour coding are regions where carbon dioxide frost is not detected at any time of year.

Carbon dioxide frost occurs most often in regions with a loose dust surface, which do not retain heat as well as rocky areas. On the other hand, those dusty regions also have high mid-afternoon temperatures, as the heat up as quickly as they cool down.

Blue dots on this map indicate sites of recurring slope lineae (RSL) in part of the Valles Marineris canyon network on Mars. RSL are seasonal dark streaks that may be indicators of liquid water. The area mapped here has the highest density of known RSL on Mars.
NASA / JPL-Caltech / University of Arizona

New clues about liquid water on Mars

So-called Martian recurring slope lineae, or RSL for short, were first discovered in 2011 and have been puzzling scientists ever since.

Many believe they are the strongest evidence that liquid water still exists on Mars.

The RSL are dark lines running down slopes and the sides of canyons. They appear during warm weather and disappear in winter.

Data from the MRO confirmed last year the presence of hydrated salts at some RSL sites, including in Valles Marineris, near the equator. This suggested that water, even if extremely briny, still flowed in certain seasons, on Mars.

Now, in a new study, scientists, also using MRO data, have investigated thousands of RSLs in the same region. And while the work does not come to definite conclusions about the cause of the phenomenon, it does tend to support previous theories.

Sites investigated include canyon ridges and isolated peaks – places where it is unlikely the streaks come from underground water reaching the surface.

The number of individual streaks in each site ranges from a few to more than 1,000.

“There are so many of them, it's hard to keep track," said Matthew Chojnacki of the University of Arizona, and lead author of the study published in the Journal of Geophysical Research: Planets.

“The occurrence of recurring slope lineae in these canyons is much more widespread than previously recognised.”

One possible explanation is that, as many appear on the sides of craters, an extensive underground layer holding water was punctured by the crater-forming impact and those reservoirs still feed warm-season flows.

However, at several of the RSL sites in the new study, no underground layer would be possible.

Scientists have also suggested that the RSL are caused when some types of salts so strongly pull water vapour out of the Martian atmosphere that liquid brine forms.

While the new study confirms the presence of salts at the RSL, it raises questions about the possibility of pulling enough water from the atmosphere.

Chojnacki says the amount of liquid water required each year to form the streaks in the studied portion of Valles Marineris would total up to 100,000 cubic metres – enough to fill 40 Olympic-size swimming pools.

There is enough water vapour for that, but researchers have not identified a process efficient enough to extract it as liquid water.

“There do seem to be more ways atmosphere and surface interact in the canyons than in blander topography, such as clouds trailing out of the canyons and low-lying haze in the canyons,” he says in a NASA press release.

“Perhaps the atmosphere-surface interactions in this region are associated with the high abundance of recurring slope lineae. We can’t rule that out, but a mechanism to make the connection is far from clear.”

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