In the past few days, Mars Climate Sounder has detected elevated temperatures in its data on Mars' atmosphere. The elevated temperatures are very likely the first warning of a significant dust event, one that may have already encircled the planet. Image data that is expected to arrive on Earth in the coming days may show the first signs of what could possibly become a global dust storm.
To review, Mars Climate Sounder works to study Mars' atmosphere by sighting across Mars' limb so that its gaze passes above Mars' surface; it sees the infrared emission of Mars' thin atmosphere against black space. Detectors are lined up vertically so that each effectively studies a 5-kilometer-thick slice of the atmosphere.
The rising dust event is best observed by zeroing in on one of the detectors, one that senses infrared light at a wavelength of 15 microns, where carbon dioxide (the dominant constituent of Mars' atmosphere) is strongly absorbing. The detector effectively measures the temperature of Mars' atmosphere high above the ground, from roughly 20 to roughly 40 kilometers' elevation.
March 24, 2009 was Spirit's sol 1,857 and Opportunity's sol 1,836. Just a week ago, on March 24, 2009, the data looked fairly typical for the season. The two plots below are maps of all of Mars. The map on the left represents Mars Climate Sounder's observations of the day side of Mars in the local afternoon, as MRO ascended in its orbit around Mars; the map on the right is a plot of observations gathered as MRO descended on the night side. Blue and red dots mark the locations of the Opportunity and Spirit rovers, respectively.
The multicolored lines sweeping generally north-south are the temperature readings from Mars Climate Sounder, plotted along Mars Reconnaissance Orbiter (MRO)'s ground track; the orbiter circles Mars about thirteen times in an Earth day, so there are thirteen tracks on each map. Occasionally there is a gap in the data, most often caused when MRO slews to one side to allow its cameras to target a spot on the ground that is off the orbital path.
The plot above shows fairly typical temperatures for the season, late southern spring. There is a strong northern polar cold region (the north polar vortex) and otherwise fairly uniform temperatures (around 180 to 190 kelvins or -90 to -80 Celsius). There are a few warmer spots near latitudes of 50 north and 50 south.
March 25 was another fairly typical day for the season, similar to March 24.
March 26, 2009 was Spirit's sol 1,859 and Opportunity's sol 1,838. The data from March 26 showed some deviations from the generally uniform temperatures, but not strong ones. The plot below shows a slight warming, of about 5 kelvins (5 Celsius) in the southern midlatitudes (between 40 and 60 degrees south) from 160 to -80 degrees east, south and southeast of Spirit's landing site. There is also a slightly more intense warming centered at 50 north and -150 east as well as a slight distortion or displacement of the north polar vortex. This pattern is relatively common for the season, late southern spring, and is presumably due to some regional dust activity in the southern hemisphere.
By March 27 (plot below) the warming of the atmosphere over the southern midlatitudes was expanding to cover a bit broader area than is usual for the season. There is warming in the south with a complex pattern. A number of regions have 5 to 7 kelvins (5 to 7 Celsius) of warming. The warming in the north is still localized in longitude at around -150 east. In the south, although the maximum warming has shifted west (to around 170 degrees east), the polar vortex displacement is still centered around -160 east.
A striking feature of the March 27 pattern -- repeated in all the plots after this date -- is that although there is some warming in both the north and the south, the southern warming appears only on the day side of Mars (left-side plot), whereas the northern warm regions are warm both during the day and at night (right-side plot). The warm during the day, cold at night pattern is a strong indicator that dust has been lofted to unusually high altitudes; dust warms rapidly in sunlight and cools rapidly after sunset. Ordinarily Mars' atmospheric dust is concentrated within about 10 kilometers of the surface; for it to show up in this detector, it must have risen to 20, 30, even 40 kilometers above the ground. Since the atmosphere in the northern hemisphere is also warm at night, the heating is probably due to atmospheric dynamics and not to dust.
March 28, 2009 was Spirit's sol 1,861 and Opportunity's sol 1,840. By March 28, there was significant unseasonal weather. The plot below contains several regions at 205 kelvins (-70 Celsius), at least 10 kelvins (10 Celsius) above normal. Most of the southern region between latitudes of 30 and 70 degrees south is affected. Most of the impact is between 150 and -110 degrees east (from southwest to southeast of Spirit's landing site), with a secondary region around 50 east. The northern warming is still mostly concentrated near 180 east (plus or minus 30 degrees).
By March 29, there was no question that the warming had become unusually severe. The plot below shows very strong warming in the southern mid-latitudes at all longitudes. The warmest regions are around 215 kelvins (-60 Celsius) and easily 20 to 25 kelvins (20 to 25 Celsius) above seasonal values. The warming is also extending both equatorward and poleward, covering nearly the entire south, from 10 to 80 south, at some longitudes. There is still a cooler region around the prime meridian (near Opportunity's landing site) covering 80 to 90 degrees of longitude. The northern warming is broader and more intense, but still shows the signatures of dynamical heating. It appears that the global circulation has been significantly affected.
March 30, 2009 was Spirit's sol 1,863 and Opportunity's sol 1,842. The data from March 30, only partially complete at the time this update was written, shows a further spread of the warmest regions. A significant portion of the atmosphere has now been affected.
How Bad Is It?
The last major dust storm season was in the summer of 2007. At the peak of the 2007 dust activity, the atmosphere measured by the same detector on Mars Climate Sounder showed temperatures up to 245 kelvins (-30 Celsius) in regions in the mid-latitudes (around 50 north and 50 south, the same latitudes where warming is now being observed). The bulk of the atmosphere, excluding the northern polar vortex, exceeded 200 kelvins (-70 Celsius), with large regions in the 210 to 220 kelvin (-65 to -55 Celsius) range. The polar vortex warmed from its normal 135 kelvins up to around 200 kelvins (from -160 to -75 Celsius) for a few days, when it appeared that the vortex was disrupted, and was mostly around 180 kelvins (-95 Celsius).
Present conditions are not -- yet -- so extreme. The current north polar vortex is showing only about 5 kelvins (5 Celsius) of warming and a bit of shrinking, but is not disrupted.
However, the warming is certainly unseasonal and unusual, indicating a planet-encircling dust event, at least in the southern hemisphere. Such an event has the potential to affect imaging and other science activities from orbit as well as power availability for the rovers, especially Spirit, which is located farther to the south than Opportunity and already suffers from low power availability due to the dust that has collected on its solar panels during five years of operations.
As it always does, the Mars Climate Sounder team will watch the weather on Mars closely and provide updates on this season's storms to all the active Mars missions.