Friday, December 28, 2012

Did Solar Storms Blow Away the Atmosphere of Mars?

Janet Luhmann, MAVEN Science team
Dr. Janet Luhmann is a Senior Fellow at the Space Sciences Laboratory of the University of California Berkeley and the Principal Investigator of the IMPACT suite of instruments on the twin spacecraft STEREO mission. Currently, in addition to STEREO and the Cassini mission, she is studying Venus Express observations and helping prepare MAVEN for its upcoming investigation of the Martian upper atmosphere as one of the mission science leads. (Courtesy Janet Luhmann)

Humans tend to think that what is happening now represents the way things have always been. However, we know from our own experience and from investigations of the Earth’s past that there have been, and may still be, episodes of extremes, such as ice ages, major volcanic eruptions and earthquakes, catastrophic tsunamis and floods—some of which can happen several times in a short human lifetime. Cosmically, there have been asteroid impacts that changed Earth’s landscape and climate, and comet impacts that altered landscapes and brought materials to Earth while blasting others away. These episodes can individually and collectively alter the course of life on Earth or change the planet itself.

Mars is exposed to many of the same types of events, and like those on Earth, the details of their historical impacts are speculative and debatable. It is hard to be a history “detective” when the evidence is often hidden or destroyed over time. As a result, questions related to Mars’ current frigid climate and related thin atmosphere are the focus of space missions and theoretical thinking and modeling. We do know that Mars lacks a planetary magnetic field like Earth’s, and, as a result, has been exposed to additional natural assaults.

All of the planets in the solar system are submerged in a nearly continuous outflow of ionized hydrogen gas from the Sun. This “solar wind” is essentially the Sun’s outermost atmosphere. It flows outward because it is heated at its base and because the surrounding space is a relative vacuum. It is ionized because it is so hot—over a million degrees Kelvin—that the nuclei of the hydrogen atoms (protons) and their electrons cannot stay bound together. The resulting ionized gas, or plasma, also carries the solar magnetic field with it as it expands outward. We thus literally live in the solar atmosphere, but because Earth has a fairly strong magnetic field of its own, it is usually shielded from direct impact within a magnetic bubble called the magnetosphere. The magnetosphere deflects most of the solar wind around the Earth well above its atmosphere. Mars, however, is another story. 

The 2001 Mars Global Surveyor mission proved what earlier Russian missions hinted at—that the Martian magnetic field was too weak to prevent solar wind from penetrating its upper atmosphere. These Russian missions also found evidence that ionized elements from the Martian atmosphere were being carried away into the more distant reaches of the Solar System by solar wind. This “erosion” of atmosphere is currently being measured on the European Space Agency Mars Express spacecraft.

While it is apparent that atmosphere escape is occurring at Mars, the historical importance of that process is not as clear. In particular, if the rates of atmosphere escape measured by Mars Express occurred throughout Mars’ four billion year history, they would not have been enough to wipe away the early substantial Martian atmosphere.

For decades, other space missions have studied how solar activity related to sunspots and geomagnetic storms at Earth affect conditions in interplanetary space. Today we know much more about how variable the solar wind can be, especially around the maximum of the Sun’s activity cycle. For example, we know that coronal mass ejections (CMEs), or large eruptions of material from the Sun’s corona, can enter the solar wind. There, CMEs can produce disturbances that cause extreme solar wind behavior by increasing its velocity, density, and magnetic field. (While normal solar wind flows outward from the Sun at about 350 km/s, the disturbed solar wind may flow out at more than one thousand km/s.)

MAVEN will investigate what these extreme conditions do to Mars’ atmosphere erosion rates. The Mars Express measurements hint at increased rates, but energetic particle impacts on the spacecraft systems during storm events can compromise the measurements. The experiments on MAVEN have been specifically designed to definitively answer whether nature’s space weather storms increase atmosphere escape rates to historically important levels.

Of course, MAVEN will need major solar activity in order to do its job. MAVEN should arrive at Mars at a time in the solar cycle that is traditionally rich in major events. Our MAVEN team will keep watching both the Sun and Mars, making use of the great resources for space weather observations currently at our disposal. We are also particularly lucky that the STEREO mission is now observing conditions on the far side of the Sun, giving us the unique opportunity to effectively monitor space weather far from Earth—where Mars usually is and where MAVEN will be.

SOHO real time solar activity
SOHO, the Solar & Heliospheric Observatory, is a project of international collaboration between ESA and NASA to study the Sun from its deep core to the outer corona and the solar wind. The latest 48 hours worth of data are available as animated GIF movies (click image for current animation). The movies are updated every hour that scientists are in real-time contact with the satellite. (Courtesy LASCO/NRL/SOHO)